ALBERT

Description: The thinning of the crust and the exhumation of subcontinental mantle in magma-poor rifted margins is accompanied by a series of extensional detachment faults. We show that exhumation along these detachments is intimately related to migration of fluids leading to changes in mineralogy and chemistry of the mantle, crustal and sedimentary rocks. Using field observation and analytical methods, we investigate the role of fluids in the fossil distal margins of the Alpine Tethys. Using Cr-Ni-V, Fe and Mn as tracers, we show that fluids used detachment faults as pathways and interacted with the overlying crust and sediments. These observations allow us to discuss when, where and how this interaction happened during the formation of the rifted margin. The results show that: (i) serpentinization of mantle rocks during their exhumation results in the depletion of elements and migration of mantle-reacted fluids that are channeled along active detachment system; (ii) in earlier-stages, these fluids affected the overlying syn-tectonic sediments by direct migration from the underlying detachments; (iii) in later-stages, these fluids arrived at the seafloor, were introduced into, or “polluted” the seawater and were absorbed by post-tectonic sediments. We conclude that a significant amount of serpentinization occurred underneath the hyperextended continental crust, and that the mantle-reacted fluids might have modified the chemical composition of the sediments and seawater. We propose that the chemical signature of serpentinization related to mantle exhumation is recorded in the sediments and may serve as a proxy to date serpentinization and mantle exhumation at present-day magma-poor rifted margins. This article is protected by copyright. All rights reserved.

Description: The East China Sea is characterized by wide continental shelf receiving a huge input of terrigenous matter from both large rivers and mountainous rivers, which makes it an ideal natural laboratory for studying sediment source-to-sink transport processes. This paper presents mineralogical and geochemical data of the clays and bulk sediments from the rivers entering the East China Sea, aiming to investigate the general driving mechanism of silicate weathering and sediment transport processes in East Asian continental margin. Two types of river systems, tectonically-stable continental rivers and tectonically-active mountainous rivers, co-exist in East Asia. As the direct weathering products, clays can better reflect the silicate weathering regimes within the two river systems. Provenance rock types are not the dominant factor causing silicate weathering intensity difference existed in the East Asian rivers. The silicate weathering intensity of tectonically-stable river basins is primarily driven by monsoon climate, and the sediment transfer is relatively slow because of natural trapping process and increasing damming effect. The geochemistry of these river-borne sediments can thus indicate paleo-weathering intensities in East Asian continent. In contrast, silicate weathering intensity in tectonically-active mountainous rivers is greatly limited by strong physical erosion despite the high temperature and highest monsoon rainfall. The factors controlling silicate weathering in tectonically-active catchments are complex and thus, it should be prudent to use river sediment records to decipher paleoclimate change. These two different silicate weathering regimes and sediment transport processes are manifestations of the landscape evolution and overall dominate the sedimentation in Asian continental margin. This article is protected by copyright. All rights reserved.

Description: Diopside phenocrysts of the Louisville Seamount Trail show an increase in Ti, Al, and Na with decreasing Mg/(Mg+Fe) as is typical for clinopyroxene in alkalic basalts. Chondrite-normalized REE patterns of calculated liquids from LA-ICPMS analyses are comparable to whole-rock and glass values. Exceptions are clinopyroxene crystals from the Rigil Seamount, the second oldest seamount drilled at the northern end of the chain. Some crystals from this site are strongly zoned with distinct compositional boundaries between cores and mantles. The cores have high Mg/(Mg+Fe) and low Al and Ti concentrations compared to the mantles and phenocrysts. Major element, clinopyroxene discrimination diagrams indicate that the clinopyroxene mantles and phenocrysts crystallized from alkalic basalts. In contrast, the Mg-rich cores have tholeiitic affinities. The REE abundances of the cores are similar to that of clinopyroxene from transitional tholeiites of the Kerguelen Archipelago. Calculated liquid La/Yb values for the cores have ratios that are similar to transitional tholeiites in Hawaii, whereas the mantles have higher La/Yb values similar to Hawaiian alkalic basalts. The major and trace element compositions of clinopyroxene cores from the Rigil seamount suggest that a transitional tholeiitic magma was present, but no evidence for Hawaiian shield-type tholeiites was found. Plagioclase crystals from the Rigil seamount have 86 Sr/ 86 Sr from 0.70306 to 0.70363, within the range of FOZO. The transitional tholeiitic signature of the Mg-rich clinopyroxene cores probably did not have a distinct source compared to other Louisville magmas, but more likely indicates a higher degree of partial melting (2-5%) of that FOZO source. This article is protected by copyright. All rights reserved.

Description: Understanding intraplate volcanism is a key to deciphering the Earth's magmatic history. One of the largest intraplate volcanic events occurred during the mid Cretaceous, roughly 75 to 125 Ma in the western Pacific. To investigate the origin and effects of this volcanism on various Earth systems, we present the first comprehensive study of volcanism in the Pigafetta Basin using seismic surveys, magnetic and gravity modeling, and Ocean Drilling Program drill core and well log data from Site 801. Our results show that intraplate volcanism in the Pigafetta Basin coincides with the rest of the western Pacific seamount provinces, supporting the previously suggested plumelets scenario for the origin of intraplate volcanism during the mid Cretaceous volcanic events. We also discover that the late stage volcanism does not overprint the remanant magnetization acquired by the Jurassic ocean crust in the Pigafetta Basin, and hence, marine magnetic anomalies recorded in the Jurassic basement are preserved. Also, the formerly identified Rough Smooth Boundary (RSB) is indistinguishable from any other rough-smooth topographic boundaries throughout the survey area suggesting that the RSB is unlikely to be a Cretaceous sill-Jurassic basement boundary. Lastly, the apparent ages and spatial distribution of volcanic features suggests a dynamic history of hydrothermal circulation in the Pigafetta Basin, indicating that hydrothermal circulation was ongoing well past 100 Ma. This article is protected by copyright. All rights reserved.

Description: The Australian-Antarctic Ridge (AAR) is one of the largest unexplored regions of the global mid-ocean ridge system. Here, we report a multi-year effort to locate and characterize hydrothermal activity on two 1 st -order segments of the AAR: KR1 and KR2. To locate vent sites on each segment, we used profiles collected by Miniature Autonomous Plume Recorders on rock corers during R/V Araon cruises in March and December of 2011. Optical and oxidation-reduction-potential anomalies indicate multiple active sites on both segments. Seven profiles on KR2 found 3 sites, each separated by ∼25 km. Forty profiles on KR1 identified 13 sites, some within a few km of each other. The spatial density of hydrothermal activity along KR1 and KR2 (plume incidence of 0.34) is consistent with the global trend for a spreading rate of ∼70 mm/yr. The densest area of hydrothermal activity, named “Mujin”, occurred along the 20-km-long inflated section near the segment center of KR1. Continuous plume surveys conducted in January-February of 2013 on R/V Araon found CH 4 / 3 He (1-15 × 10 6 ) and CH 4 /Mn (0.01-0.5) ratios in the plume samples, consistent with a basaltic-hosted system and typical of ridges with intermediate spreading rates. Additionally, some of the plume samples exhibited slightly higher ratios of H 2 / 3 He and Fe/Mn than others, suggesting that those plumes are supported by a younger hydrothermal system that may have experienced a recent eruption. The Mujin-field was populated by Kiwa crabs and seven-armed Paulasterias starfish previously recorded on the East Scotia Ridge, raising the possibility of circum-Antarctic biogeographic connections of vent fauna. This article is protected by copyright. All rights reserved.

Description: Porous sedimentary rocks fail in a variety of modes ranging from localized, brittle deformation to pervasive, cataclastic flow. To improve our understanding of this transition and its affect on fluid flow and permeability, we investigated the mechanical behavior of a siltstone unit within the Marcellus Formation, PA USA, characterized by an initial porosity ranging from 41 to 45%. We explored both hydrostatic loading paths (σ 1 =σ 2 =σ 3 ) and triaxial loading paths (σ 1 〉σ 2 =σ 3 ) while maintaining constant effective pressure (P e =P c -P p ). Samples were deformed with an axial displacement rate of 0.1 μm/s (strain rate of 2x10 −6 s −1 ). Changes in pore water volume were monitored (drained conditions) to measure the evolution of porosity. Permeability was measured at several stages of each experiment. Under hydrostatic loading, we find the onset of macroscropic grain crushing (P*) at 39 MPa. Triaxial loading experiments show a transition from brittle behavior with shear localization and compaction to cataclastic-flow as confining pressure increases. When samples fail by shear localization, permeability decreases abruptly without significant changes in porosity. Conversely, for cataclastic deformation, permeability reduction is associated with significant porosity reduction. Post-experiment observation of brittle samples show localized shear zones characterized by grain comminution. Our data show how zones of shear localization can act as barriers to fluid flow and thus modify the hydrological and mechanical properties of the surrounding rocks. Our results have important implications for deformation behavior and permeability evolution in sedimentary systems, and in particular where the stress field is influenced by injection or pumping. This article is protected by copyright. All rights reserved.

Description: Lake sediments can provide high-quality information about human activities. In this study, we investigate a sediment core from Lake Xiaolongwan using magnetic and geochemical methods. The dominant magnetic minerals of this sediment core are stable single domain (SSD) and superparamagnetic (SP) magnetite particles. The increasing amount of SP particles reflected by the rise of magnetic susceptibility and frequency dependent magnetic susceptibility since AD 1500 can be attributed to an increasing influx in pedogenic soil, which is related to a regional-scale increase in the intensity of human activity in Northeastern China. This extends the timing of human activities, which is independent from climate changes and its effects on local ecosystems in Northeastern China significantly. This article is protected by copyright. All rights reserved.

Description: Ocean island basalts (OIB) with extremely radiogenic Pb-isotopic signatures are melts of a mantle component called HIMU (high µ, high 238 U/ 204 Pb). Until now, deeply-dredged submarine HIMU glasses have not been available, which has inhibited complete geochemical (in particular, volatile element) characterization of the HIMU mantle. We report major, trace and volatile element abundances in a suite of deeply-dredged glasses from the Tuvalu Islands. Three Tuvalu glasses with the most extreme HIMU signatures have F/Nd ratios (35.6±3.6) that are higher than the ratio (∼21) for global OIB and MORB, consistent with elevated F/Nd ratios in endmember HIMU Mangaia melt inclusions. The Tuvalu glasses with the most extreme HIMU composition have Cl/K (0.11 to 0.12), Br/Cl (0.0024) and I/Cl (5-6 × 10 −5 ) ratios that preclude significant assimilation of seawater-derived Cl. The new HIMU glasses that are least degassed for H 2 O have low H 2 O/Ce ratios (75-84), similar to ratios identified in endmember OIB glasses with EM1 and EM2 signatures, but significantly lower than H 2 O/Ce ratios (119 to 245) previously measured in melt inclusions from Mangaia. CO 2 -H 2 O equilibrium solubility models suggest that these HIMU glasses (recovered in two different dredges at 2,500 to 3,600 meters water depth) have eruption pressures of 295 to 400 bars. We argue that degassing is unlikely to significantly reduce the primary melt H 2 O. Thus, the lower H 2 O/Ce in the HIMU Tuvalu glasses is a mantle signature. We explore oceanic crust recycling as the origin of the low H 2 O/Ce (∼50 to 80) in the EM1, EM2 and HIMU mantle domains. This article is protected by copyright. All rights reserved.

Description: High-resolution 2-D multi-channel seismic data, collected during the 2012 UTIG-USGS National Earthquake Hazards Reduction Program survey of Disenchantment and Yakutat Bays in southeast Alaska, provide insight into their glacial history. These data show evidence of two unconformities, appearing in the form of channels, and are interpreted to be advance pathways for Hubbard Glacier. The youngest observable channel, thought to have culminated near the main phase of the Little Ice Age (LIA), is imaged in Disenchantment Bay and ends at a terminal moraine near Blizhni Point. An older channel, thought to be from an advance that culminated in the early phase of the LIA, extends from Disenchantment Bay into the northeastern edge of Yakutat Bay, turning southward at Knight Island and terminating on the southeastern edge of Yakutat Bay. Our interpretation is that Hubbard Glacier has repeatedly advanced around the east side of Yakutat Bay in Knight Island Channel, possibly due to the presence of Malaspina Glacier cutting off access to central Yakutat Bay during times of mutual advance. We observe two distinct erosional surfaces and retreat sequences of Hubbard Glacier in Yakutat Bay, supporting the hypothesis that minor glacial advances in fjords do not erode all prior sediment accumulations. Interpretation of chaotic seismic facies between these two unconformities suggests that Hubbard Glacier exhibits rapid retreats and that Disenchantment Bay is subject to numerous episodes of outburst flooding and morainal bank collapse. These findings also suggest that tidewater glaciers preferentially reoccupy the same channels in bay and marine settings during advances. This article is protected by copyright. All rights reserved.

Description: Within the Sea of Marmara, the highly active North Anatolian Fault (NAF) is responsible for major earthquakes (Mw 〉=7), and acts as a pathway for fluid migration from deep sources to the seafloor. This work reports on pore water geochemistry from three sediment cores collected in the gulfs of Izmit and Gemlik, along the Northern and the Middle strands of the NAF, respectively. The resulting dataset shows that anaerobic oxidation of methane (AOM) is the major process responsible for sulfate depletion in the shallow sediment. In the Gulf of Gemlik, depth concentration profiles of both sulfate and alkalinity exhibit a kink-type profile. The Sulfate Methane Transition Zone (SMTZ) is located at moderate depth in the area. In the Gulf of Izmit, the low concentrations observed near the seawater-sediment interface for sulfate, calcium, strontium and magnesium results from rapid geochemical processes, AOM and carbonate precipitation, occurring in the uppermost part of the sedimentary column and sustained by free methane accumulation. Barite dissolution and carbonate recrystallization have also been identified at deeper depth at the easternmost basin of the Gulf of Izmit. This is supported by the profile of the strontium isotope ratios ( 87 Sr/ 86 Sr) as a function of depth which exhibits negative anomalies compared to the modern seawater value. The strontium isotopic signature also shows that these carbonates had precipitated during the reconnection of the Sea of Marmara with the Mediterranean Sea. Finally, a first attempt to interpret the sulfate profiles observed in the light of the seismic activity at both sites is presented. We propose the hypothesis that seismic activity in the areas is responsible for the transient sulfate profile, and that the very shallow SMTZ depths observed in the Gulf of Izmit is likely due to episodic release of significant amount of methane. This article is protected by copyright. All rights reserved.

Description: The transition zone at the downdip end of seismic coupling along subduction interfaces is often the site of megathrust earthquake nucleation and concentrated postseismic afterslip, as well as the focus site of episodic tremor and slip features. Exhumed remnants of the former Alpine subduction zone found in the Swiss Alps allow analyzing fluid and deformation processes near the transition zone region (30-40 km paleodepth). The Dent Blanche Thrust (DBT) is a lower blueschist-facies shear zone interpreted as a fossilized subduction interface where granitic mylonites overlie a metamorphosed accretionary wedge. We report field observations from the DBT region where multiple, several tens of meters-thick foliated cataclastic networks are interlayered within the basal DBT mylonites. Petrological results and microstructural observations indicate that the various cataclasis events took place at near peak metamorphic conditions (400-500°C, 1.1-1.3 GPa) during subduction of the Tethyan seafloor in Eocene times (42-48 Ma). Some of these networks exhibit mutual cross-cutting relationships between mylonites, foliated cataclasites and vein systems indicating mutual overprinting between brittle deformation and ductile creep. Whole-rock chemical compositions, in situ 40 Ar- 39 Ar age data of recrystallized phengite, and Sr isotopic signatures reveal that DBT rocks also underwent multiple hydrofracturing and metasomatic events via the infiltration of fluids mainly derived from the oceanic metasediments underneath the DBT. From the rock fabrics we infer strain rate fluctuations of several orders of magnitude beyond subduction strain rates (c. 10 −12 s −1 ) accompanied by fluctuation of supra-lithostatic and quasi-lithostatic fluid pressures (1≥λ〉0.95). DBT brittle-plastic deformation switches highlight the diversity of deformation processes and fluid-rock interactions in the transition zone region of the subduction interface. This article is protected by copyright. All rights reserved.

Description: Water is a key parameter in magma genesis, magma evolution, and resulting eruption styles, because it controls the density, the viscosity, as well as the melting and crystallization behavior of a melt. The parental water content of a magma is usually measured through melt inclusions in minerals such as olivine, a method which may be hampered, however, by the lack of melt inclusions suitable for analysis, or post-entrapment changes in their water content. An alternative way to reconstruct the water content of a magma is to use nominally anhydrous minerals (NAMs), such as pyroxene, which take up low concentrations of hydrogen as a function of the magma's water content. During magma degassing and eruption, however, NAMs may dehydrate. We therefore tested a method to reconstruct the water contents of dehydrated clinopyroxene phenocrysts from the Western Canary islands (n=28) through re-hydration experiments followed by infrared and Mössbauer spectroscopy. Employing currently available crystal/melt partitioning data, the results of the experiments were used to calculate parental water contents of 0.71 ±0.07 to 1.49 ±0.15 wt. % H 2 O for Western Canary magmas during clinopyroxene crystallization at upper mantle conditions. This H 2 O range is in agreement with calculated water contents using plagioclase-liquid-hygrometry, and with previously published data for mafic lavas from the Canary Islands and comparable ocean island systems elsewhere. Utilizing NAMs in combination with hydrogen treatment can therefore serve as a proxy for pre-eruptive H 2 O contents, which we anticipate becoming a useful method applicable to mafic rocks where pyroxene is the main phenocryst phase. This article is protected by copyright. All rights reserved.

Description: We present a new approach to identifying the source and age of paleofluids associated with low-temperature deformation in the brittle crust, using hydrogen isotopic compositions (δD) and 40 Ar/ 39 Ar geochronology of authigenic illite in clay gouge-bearing fault zones. The procedure involves grain size separation, polytype modeling, and isotopic analysis, creating a mixing line that is used to extrapolate to δD and age of pure authigenic and detrital material. We use this method on samples collected along the surface trace of today's North Anatolian Fault (NAF). δD values of the authigenic illite population, obtained by extrapolation, are -89±3‰, -90±2‰, and -97±2‰ (VSMOW) for samples KSL, RES4-1, and G1G2, respectively. These correspond to δD fluid values of -62‰ to -85‰ for the temperature range of 125°C ±25°, indistinguishable from present-day precipitation values. δD values of the detrital illite population are -45±13‰, -60±6‰, and -64±6‰ for samples KSL, G1G2, and RES4-1, respectively. Corresponding δD fluid values at 300°C are -26 to -45‰ and match values from adjacent metamorphic terranes. Corresponding clay gouge ages are 41.4 ±3.4 Ma (authigenic) and 95.8±7.7 Ma (detrital) for sample G2 and 24.6 ±1.6 Ma (authigenic) and 96.5±3.8 Ma (detrital) for sample RES4-1, demonstrating a long history of meteoric fluid infiltration in the area. We conclude that today's NAF incorporated pre-existing, weak clay-rich rocks that represent earlier mineralizing fluid events. The samples preserve at least three fluid flow pulses since the Eocene and indicate that meteoric fluid has been circulating in the upper crust in the North Anatolian Keirogen since that time. This article is protected by copyright. All rights reserved.

Description: We reconstruct the history of the mode of accretion of an area of the Mid-Atlantic Ridge south of the Kane fracture zone using bathymetric morphology. The area includes 200 km of the spreading axis and reaches to 10 Ma on either side. We distinguish three tectonic styles: 1) volcanic construction with eruption and intrusion of magma coupled with minor faulting, 2) extended terrain with abundant large-offset faults, 3) detachment faulting marked by extension on single long-lived faults. Over 40% of the seafloor is made of extended terrain and detachment faults. The area includes products of seven spreading segments. The spreading axis has had detachment faulting or extended terrain on one or both sides for 70% of the last 10 Ma. In some parts of the area, regions of detachment faulting and extended terrain lie close to segment boundaries. Regions of detachment faulting initiated at 10 Ma close to the adjacent fracture zones to the north and south, and then expanded away from them. We discuss the complex evidence from gravity, seismic surveys and bathymetry for the role of magma supply in generating tectonic style. Overall we conclude that input of magma at the spreading axis has a general control on the development of detachment faulting, but the relationship is not strong. Other factors may include a positive feedback that stabilizes detachment faulting at the expense of volcanic extension, perhaps through the lubrication of active detachment faults by the formation of low friction materials (talc, serpentine) on detachment fault surfaces. This article is protected by copyright. All rights reserved.

Description: We present a new method for measuring wettability or contact angle of minerals at reservoir pressure-temperature conditions using high-resolution X-ray computed tomography (HRXCT) and radiography. In this method, a capillary or a narrow slot is constructed from a mineral or a rock sample of interest wherein two fluids are allowed to form an interface that is imaged using X-rays. After some validation measurements at room pressure-temperature conditions, we illustrate this method by measuring the contact angle of CO 2 –brine on quartz, muscovite, shale, borosilicate glass, polytetrafluoroethylene (PTFE or Teflon), and polyether ether ketone (PEEK) surfaces at 60-71°C and 13.8 – 22.8 MPa. At reservoir conditions, PTFE and PEEK surfaces were found to be CO 2 –wet with contact angles of 140° and 127°, respectively. Quartz and muscovite were found to be water–wet with contact angles of 26° and 58°, respectively under similar conditions. Borosilicate glass–air–brine at room conditions showed strong water-wet characteristics with a contact angle of 9°, whereas borosilicate glass-CO 2 –brine at 13.8 MPa and 60°C showed a decrease in its water-wetness with contact angle of 54°. This method provides a new application for X-ray imaging and an alternative to other methods. This article is protected by copyright. All rights reserved.

Description: Local and regional S-wave splitting in the offshore South Island of the New Zealand plate-boundary zone provides constraints on the spatial and depth extent of the anisotropic structure with an enhanced resolution relative to land-based and SKS studies. The combined analysis of offshore and land measurements using splitting tomography suggests plate-boundary shear dominates in the central and northern South Island. The width of this shear zone in the central South Island is about 200 km, but is complicated by stress-controlled anisotropy at shallow levels. In northern South Island, a broader (〉200 km) zone of plate-boundary parallel anisotropy is associated with the transitional faulting between the Alpine fault and Hikurangi subduction and the Hikurangi subduction zone itself. These results suggest S-phases of deep events (∼ 90 km) in the central South Island are sensitive to plate-boundary derived NE-SW aligned anisotropic media in the upper-lithosphere, supporting a “thin viscous sheet” deformation model. This article is protected by copyright. All rights reserved.

Description: In the present study, we have investigated the C-S-Fe systematics in a sediment core (MD161-13) from the Krishna-Godavari (K-G) basin, Bay of Bengal. The core covers the late Holocene period with high overall sedimentation rate of ∼573 cm ky −1 . Pore fluid chemical analyses indicate that the depth of the present sulfate methane transition zone (SMTZ) is at ∼6 mbsf. The (ΔTA+ΔCa+ΔMg)/ΔSO 4 2- ratios suggest that both organoclastic degradation and anaerobic oxidation of methane (AOM) drive sulfate reduction at the study site. The positive correlation between total organic carbon content (TOC) and chromium reducible sulfur (CRS) content indicates marked influence of organoclastic sulfate reduction on sulfidization. Coupled occurrence of 34 S enriched iron sulfide (pyrite) with 12 C enriched authigenic carbonate zones are the possible records of paleo-sulfate methane transition zones where AOM driven focused sulfate reduction was likely fueled by sustained high methane flux from underlying gas rich zone. Aluminium normalized poorly reactive iron (Fe PR /Al) and La/Yb ratios suggest increasing contribution from Deccan basalts relative to that of Archean- Proterozoic granitic complex in sediment flux of Krishna-Godavari basin during the last 4 ky. This article is protected by copyright. All rights reserved.

Description: At slow-spreading mid-ocean ridges, crustal accretion style can vary significantly along and across ridge segments. In magma-poor regions, seafloor spreading can be accommodated largely by tectonic processes, however, the internal structure and formation mechanism of such highly tectonized crust are not fully understood. We analyze multi-beam bathymetry and potential field data from the Rainbow area of the Mid-Atlantic Ridge (35º40'N-37º40'N), a section of the ridge that shows diverse accretion styles. We identify volcanic, tectonized and sedimented terrain and measure exposed fault area to estimate the tectonic strain, T , and the fraction of magmatic accretion, M . Estimated T values range from 0.2-0.4 on ridge segments to 0.6-0.8 at the Rainbow non-transform discontinuity (NTD). At segment ends T is asymmetric, reflecting asymmetries in accretion rate, topography and faulting between inside and outside offset corners. Detachment faults have formed preferentially at inside corners, where tectonic strain is higher. We identify at least two oceanic core complexes on the fossil trace of the NTD, in addition to the Rainbow massif, which occupies the offset today. A gravity high and low magnetization suggest that the Rainbow massif, which hosts a high-temperature hydrothermal system, was uplifted by a west dipping detachment fault. Asymmetric plate ages indicate localization of tectonic strain at the inside corners and migration of the detachment towards and across the ridge axis, which may have caused emplacement of magma into the footwall. Hydrothermal circulation and heat extraction is possibly favored by increased permeability generated by fracturing of the footwall and deep-penetrating second-generation faults. This article is protected by copyright. All rights reserved.

Description: We describe a quantitative magnetic unmixing method based on principal component analysis (PCA) of first-order reversal curve (FORC) diagrams. For PCA we resample FORC distributions on grids that capture diagnostic signatures of single-domain (SD), pseudo-single-domain (PSD), and multi-domain (MD) magnetite, as well as of minerals such as hematite. Individual FORC diagrams are recast as linear combinations of end-member (EM) FORC diagrams, located at user-defined positions in PCA space. The EM selection is guided by constraints derived from physical modeling and imposed by data scatter. We investigate temporal variations of two EMs in bulk North Atlantic sediment cores collected from the Rockall Trough and the Iberian Continental Margin. Sediments from each site contain a mixture of magnetosomes and granulometrically distinct detrital magnetite. We also quantify the spatial variation of three EM components (a coarse silt-sized MD component, a fine silt-sized PSD component, and a mixed clay-sized component containing both SD magnetite and hematite) in surficial sediments along the flow path of the North Atlantic Deep Water (NADW). These samples were separated into granulometric fractions, which helped constrain EM definition. PCA-based unmixing reveals systematic variations in EM relative abundance as a function of distance along NADW flow. Finally, we apply PCA to the combined dataset of Rockall Trough and NADW sediments, which can be recast as a four-EM mixture, providing enhanced discrimination between components. Our method forms the foundation of a general solution to the problem of unmixing multi-component magnetic mixtures, a fundamental task of rock magnetic studies. This article is protected by copyright. All rights reserved.

Description: The abundance of microbial life and the sources of energy necessary for deep subsurface microbial communities remain enigmatic. Here we investigate deep microbial processes and their potential relationships to tectonic events in sediments from the Nankai Trough offshore Japan, drilled and sampled during IODP (Integrated Ocean Drilling Program) Expedition 316. Observed methane isotope profiles indicate that microbially mediated methane production occurs at Sites C0006 and C0007 in sediments below ∼450 meters below seafloor (mbsf) and ∼425 mbsf, respectively. The active carbon cycling in these deep subsurface sediments is likely related to the highly dynamic tectonic regime at Nankai Trough. We propose that transient increases in temperature have re-stimulated organic matter degradation at these distinct depths and explore several candidate processes for transient heating. Our favored hypothesis is frictional heating associated with earthquakes. In concert with transient heating leading to the reactivation of recalcitrant organic matter, the heterogeneous sedimentary system provides niches for microbial life. The newly available/accessible organic carbon compounds fuel the microbial community – resulting in an onset of methanogenesis several hundred meters below the seafloor. This process is captured in the methane C-isotope signal, showing the efficacy of methane C-isotopes for delineating locations of active microbial processes in deeply buried sediments. Additionally, simple model approaches applied to observed chemical pore water profiles can potentially constrain timing relationships, which can then be linked to causative tectonic events. Our results suggest the occurrence of slip-to-the-trench earthquake(s) 200-400 year ago, which could relate to historical earthquakes (1707 Hoei and/or 1605 Keicho earthquakes). This article is protected by copyright. All rights reserved.

Description: The source mantle of the basaltic ocean crust on the western half of the Pacific Plate was examined using Pb–Nd–Hf isotopes. The results showed that the subducted Izanagi–Pacific Ridge (IPR) formed from both Pacific (180–∼80 Ma) and Indian (∼80–70 Ma) mantles. The western Pacific Plate becomes younger westward and is thought to have formed from the IPR. The ridge was subducted along the Kurile–Japan–Nankai–Ryukyu (KJNR) Trench at 60–55 Ma and leading edge of the Pacific Plate is currently stagnated in the mantle transition zone. Conversely, the entire eastern half of the Pacific Plate, formed from isotopically distinct Pacific mantle along the East Pacific Rise and the Juan de Fuca Ridge, largely remains on the seafloor. The subducted IPR is inaccessible; therefore, questions regarding which mantle might be responsible for the formation of the western half of the Pacific Plate remain controversial. Knowing the source of the IPR basalts provides insight into the Indian–Pacific mantle boundary before the Cenozoic. Isotopic compositions of the basalts from borehole cores (165–130 Ma) in the western Pacific show that the surface oceanic crust is of Pacific mantle origin. However, the accreted ocean floor basalts (∼80–70 Ma) in the accretionary prism along the KJNR Trench have Indian mantle signatures. This indicates the younger western Pacific Plate of IPR origin formed partly from Indian mantle and that the Indian–Pacific mantle boundary has been stationary in the western Pacific at least since the Cretaceous. This article is protected by copyright. All rights reserved.

Description: Mid-Carboniferous carbonates in the western United States have undergone Pleistocene Bahamas-style meteoric diagenesis that may be associated with expanding late Paleozoic ice sheets. Fourteen stratigraphic sections from carbonate platforms illustrate the regional distribution and variable intensity of physical and chemical diagenesis just below the mid-Carboniferous unconformity. Each section contains top-negative carbon isotope excursions that terminate in regional exposure surfaces that are associated with some combination of karst towers, desiccation cracks, fabric destructive recrystallization, or extensive root systems. The timing of the diagenesis is synchronous with similarly-scaled top-negative carbon isotope excursions observed by others in England, Kazakhstan, and China. The mass flux of negative carbon required to generate similar isotopic profiles across the areal extent of middle Carboniferous platform carbonates is a significant component of the global carbon cycle. We present a simple carbon box model to illustrate that the δ 13 C of dissolved inorganic carbon in the ocean could be elevated by ∼1.4 textperthousand∼as isotopically lower carbon from the terrestrial organic weathering reacts with exposed platforms before reaching the ocean and atmosphere. These results represent an improvement on global biogeochemical models that have struggled to provide a congruent solution to the high δ 13 C of the late Paleozoic icehouse. This article is protected by copyright. All rights reserved.

Description: The coarseness of the 10-63 µm terrigenous silt (i.e. sortable silt) fraction tends to vary independently of sediment supply in current-sorted muds in the world's oceans, with coarser sediments representing relatively greater near-bottom flow speeds. Traditionally the coarseness of this size fraction is described using an index called sortable-silt mean size ( ), which is an arithmetic average calculated from the differential volume or mass distribution of grains within the 10-63 µm terrigenous silt fraction, where the relative weights of the individual size bins become increasingly disproportionate, with respect to the actual number of grains within those size bins, towards the coarse end of the size range. This not only increases the absolute value of the apparent “mean size” within the 10-63 μm terrigenous silt fraction, but it may also affect the apparent pattern of relative changes in the coarseness of the sortable-silt fraction along the core. In addition, it makes more prone to biases due to, for example, analytical errors. Here we present a detailed analysis of grain-size distributions over three selected Holocene time intervals from two complementary sediment cores (JM97-948/2A, MD95-2011), extracted from the center of a high-accumulation area along the flow path of the main branch of the Atlantic Inflow into the Nordic Seas and show that differential-number-based statistics, which likely better describes variations in the actual coarseness of the sortable-silt fraction, may provide a more robust alternative to . This article is protected by copyright. All rights reserved.

Description: Slow slip events (SSEs) are observed worldwide and often coincide with tectonic tremor. Notable examples of SSEs lacking observed tectonic tremor, however, occur beneath Kīlauea Volcano, Hawaii, the Boso Peninsula, Japan, {near San Juan Bautista on the San Andreas Fault, California, and recently in Central Ecuador. These SSEs are similar to other worldwide SSEs in many ways (e.g., size or duration), but lack the concurrent tectonic tremor observed elsewhere; instead they trigger swarms of regular earthquakes. We investigate the physical conditions that may distinguish these non-tremor-genic SSEs from those associated with tectonic tremor including: slip velocity, pressure, temperature, fluids and fault asperities, although we cannot eliminate the possibility that tectonic tremor may be obscured in highly attenuating regions. Slip velocities of SSEs at Kīlauea Volcano (∼10 −6 m/s) and Boso Peninsula (∼10 −7 m/s) are among the fastest SSEs worldwide. Kīlauea Volcano, the Boso Peninsula and Central Ecuador are also among the shallowest SSEs worldwide, and thus have lower confining pressures and cooler temperatures in their respective slow slip zones. {Fluids also likely contribute to tremor generation, and no corresponding zone of high v p /v s has been noted at Kīlauea or Boso. We suggest that the relatively faster slip velocities at Kīlauea Volcano and the Boso Peninsula result from specific physical conditions that may also be responsible for triggering swarms of regular earthquakes adjacent to the slow slip, while different conditions produce slower SSE velocities elsewhere and trigger tectonic tremor. This article is protected by copyright. All rights reserved.

Description: We present a flexible, general and efficient approach for implementing thermodynamic phase equilibria information (in the form of sets of physical parameters) into geophysical and geodynamic studies. The approach is based on tensor rank decomposition methods, which transform the original multi-dimensional discrete information into a separated representation that contains significantly fewer terms, thus drastically reducing the amount of information to be stored in memory during a numerical simulation or geophysical inversion. Accordingly, the amount and resolution of the thermodynamic information that can be used in a simulation or inversion increases substantially. In addition, the method is independent of the actual software used to obtain the primary thermodynamic information, and therefore it can be used in conjunction with any thermodynamic modeling program and/or database. Also, the errors associated with the decomposition procedure are readily controlled by the user, depending on her/his actual needs (e.g. preliminary runs vs full resolution runs). We illustrate the benefits, generality and applicability of our approach with several examples of practical interest for both geodynamic modeling and geophysical inversion/modeling. Our results demonstrate that the proposed method is a competitive and attractive candidate for implementing thermodynamic constraints into a broad range of geophysical and geodynamic studies. MATLAB implementations of the method and examples are provided as supplementary material and can be downloaded from the journal's website. This article is protected by copyright. All rights reserved.

Description: Magnetic properties and the anisotropy of magnetic susceptibility (AMS) present promising methods to track mineral orientation and petrofabric in rocks that have undergone partial melting. In order to better understand the source of the magnetic signal in these types of rocks, the interpretation of field observations may be integrated with laboratory experiments, designed to re-create conditions of partial melting. A set of experiments is presented in this study, where synthetic foliated quartz-muscovite aggregates undergo partial melting at 300 MPa hydrostatic confining pressure and 750°C. Magnetic properties and AMS are measured before and after partial melting. Prior to partial melting, the synthetic aggregate shows a compaction-related oblate magnetic fabric, dominated by paramagnetic muscovite that contains small amounts of iron. Post-experiment samples show neoblasts that crystallize from incongruent melt reactions. Most notably for the magnetic fabric, the breakdown of muscovite results in growth of secondary phases of Fe-bearing spinel and biotite. Isothermal remanence acquisition and temperature-dependence of susceptibility indicate that the spinel is magnetite. The degree of magnetic anisotropy reduces significantly after partial melting, but notably the orientation of the principal axes of susceptibility mimic the AMS of the original quartz-muscovite aggregate. Additionally, the post-experiment samples show a relationship between the amount of sample shortening (compaction) and the degree of magnetic anisotropy and susceptibility ellipsoid shape factor. These results suggest that petrofabrics in rocks that undergo partial melting at near hydrostatic pressure conditions may in part be inherited, or mimic, the original petrofabric of a sedimentary or metasedimentary rock. This article is protected by copyright. All rights reserved.

Description: Overflow of Northern Component Water, the precursor of North Atlantic Deep Water, appears to have varied during Neogene times. It has been suggested that this variation is moderated by transient behavior of the Icelandic mantle plume, which has influenced North Atlantic bathymetry through time. Thus pathways and intensities of bottom currents that control deposition of contourite drifts could be affected by mantle processes. Here, we present regional seismic reflection profiles that cross sedimentary accumulations (Björn, Gardar, Eirik and Hatton Drifts). Prominent reflections were mapped and calibrated using a combination of boreholes and legacy seismic profiles. Interpreted seismic profiles were used to reconstruct solid sedimentation rates. Björn Drift began to accumulate in late Miocene times. Its average sedimentation rate decreased at ∼2.5 Ma and increased again at ∼0.75 Ma. In contrast, Eirik Drift started to accumulate in early Miocene times. Its average sedimentation rate increased at ∼5.5 Ma and decreased at ∼2.2 Ma. In both cases, there is a good correlation between sedimentation rates, inferred Northern Component Water overflow, and the variation of Icelandic plume temperature independently obtained from the geometry of diachronous V-shaped ridges. Between 5.5 and 2.5 Ma, the plume cooled, which probably caused subsidence of the Greenland-Iceland-Scotland Ridge, allowing drift accumulation to increase. When the plume became hotter at 2.5 Ma, drift accumulation rate fell. We infer that deep-water current strength is modulated by fluctuating dynamic support of the Greenland-Scotland Ridge. Our results highlight the potential link between mantle convective processes and ocean circulation. This article is protected by copyright. All rights reserved.

Description: Mineralogical reactions which generate or consume fluids play a key role during fluid flow in porous media. Such reactions are linked to changes in density, porosity, permeability and fluid pressure which influence fluid flow and rock deformation. To understand such a coupled system equations were derived from mass conservation and local thermodynamic equilibrium. The presented mass conservative modelling approach describes the relationships between evolving fluid pressure, porosity, fluid and solid density, and devolatilization reactions in multi-component systems with solid solutions. This first step serves as a framework for future models including aqueous speciation and transport. The complexity of univariant and multi-variant reactions is treated by calculating look-up tables from thermodynamic equilibrium calculations. Simplified cases were also investigated to understand previously studied formulations. For non-deforming systems or systems divided into phases of constant density the equations can be reduced to porosity wave equations with addition of a reactive term taking the volume change of reaction into account. For closed systems an expression for the volume change of reaction and the associated pressure increase can be obtained. The key equations were solved numerically for the case of devolatilization of three different rock types that may enter a subduction zone. Reactions with positive Clapeyron slope lead to increase in porosity and permeability with decreasing fluid pressure resulting in sharp fluid pressure gradients around a negative pressure anomaly. The opposite trend is obtained for reactions having a negative Clapeyron slope during which sharp fluid pressure gradients were only generated around a positive pressure anomaly. Coupling of reaction with elastic deformation induces a more efficient fluid flow for reactions with negative Clapeyron slope than for reactions with positive Clapeyron slope. This article is protected by copyright. All rights reserved.

Description: We present experimental data on the thermodynamics and kinetics of bubble nucleation and growth in weakly H 2 O-oversaturated rhyolitic melts. The high-temperature (900-1100°C) experiments involve heating of rhyolitic obsidian from Hrafntinnuhryggur, Krafla, Iceland to above their glass transition temperature ( Tg ∼ 690°C) at 0.1 MPa for times of 0.25-24 hours. During experiments, the rhyolite cores increase in volume as H 2 O vapour-filled bubbles nucleate and expand. The extent of vesiculation, as tracked by porosity, is mapped in temperature-time ( T - t ) space. At constant temperature and for a characteristic dwell time, the rhyolite cores achieve a maximum volume where the T - t conditions reach thermochemical equilibrium. For each T-t snapshot of vesiculation, we use 3D analysis of X-ray computed tomographic (XCT) images of the quenched cores to obtain the bubble number density (BND) and bubble size distribution (BSD). BNDs for the experimental cores are insensitive to T and t , indicating a single nucleation event. All BSDs converge to a common distribution, independent of T , melt viscosity (η), or initial degree of saturation, suggesting a common growth process. We use these data to calibrate an empirical model for predicting the rates and amounts of vesiculation in rhyolitic melts as a function of η and thermochemical affinity ( A ): two computable parameters that are dependent on T , pressure and H 2 O content. The model reproduces the experimental dataset and data from the literature to within experimental error, and has application to natural volcanic systems where bubble formation and growth are not diffusion limited (e.g., lavas, domes, ignimbrites, conduit infill). This article is protected by copyright. All rights reserved.

Description: This study examined dissolved Mo and sedimentary Mo along with hydro-chemical parameters in the western Taiwan Strait (WTS) in May and August 2012. The results demonstrate that dissolved Mo could be depleted of as high as 10-20 nM during our May sampling period when the nutrient-enriched Min-Zhe coastal current ceased and spring blooms developed. The negative correlation between Chl-a and dissolved Mo suggests of the possible involvement of high algal productivity into removing dissolved Mo out of the water column. Specific oceanographic settings (little currents) permitted a high sedimentary enrichment of Mo (〉6 µg/g Mo) within the highly productive waters outside the Jiulong River mouth. Possibly, the high algal productivities and consequent organic matter sinks provide a pathway of Mo burial from water columns into sediments. Dissolved Mo was relatively high in groundwater samples, but we observed that submarine groundwater discharges (SGDs) only contributed to a relatively small percentage of the total dissolved Mo pool in WTS. It is probably attributable to the immediate removal of SGD-released Mo ions via adsorption onto newly formed Mn oxides once exposed to oxygenated seawater, followed by an elevated sedimentary Mo accumulation near the SGDs (∼5 µg/g). In addition to metal oxide particle scavenging and sulfide precipitation, we estimated that biological uptake along with Mo adsorption onto organic matter carriers could finally provide more than 10% of the annual sedimentary Mo accumulation in WTS. This article is protected by copyright. All rights reserved.

Description: The shallow water benthic foraminifer Amphistegina lessonii was grown in seawater of variable Li and Ca concentration and shell Li/Ca was determined by means of LA-ICPMS. Shell Li/Ca is positively correlated to seawater Li/Ca only when the Li concentration of seawater is changed. If the seawater Ca concentration is changed, shell Li/Ca remains constant. This indicates that Li does not compete with Ca for incorporation in the shell of A. lessonii . A recently proposed calcification model can be applied to divalent cations (e.g. Mg and Sr), which compete for binding sites of ion-transporters and positions in the calcite lattice. By contrast, the transport pathway of monovalent cations such as Li is probably diffusion-based (e.g. ion-channels), and monovalent cations do not compete with Ca for a position in the calcite lattice. Here, we present a new model for Li partitioning into foraminiferal calcite which predicts our experimental results and should also be applicable to other alkali metals. This article is protected by copyright. All rights reserved.

Description: Grain size distribution (GSD) data are widely used in Earth sciences and although large data sets are regularly generated, detailed numerical analyses are not routine. Unmixing GSDs into components can help understand sediment provenance and depositional regimes/processes. End member analysis (EMA), which fits one set of end members to a given data set, is a powerful way to unmix GSDs into geologically meaningful parts. EMA estimates end members based on co-variability within a data set and can be considered as a non-parametric approach. Available EMA algorithms, however, either produce sub-optimal solutions, or are time consuming. We introduce unmixing algorithms inspired by hyperspectral image analysis that can be applied to GSD data and which provide an improvement over current techniques. Non-parametric EMA is often unable to identify unimodal grain size sub-populations that correspond to single sediment sources. An alternative approach is single specimen unmixing (SSU), which unmixes individual GSDs into unimodal parametric distributions (e.g., lognormal). We demonstrate that the inherent non-uniqueness of SSU solutions renders this approach unviable for estimating underlying mixing processes. To overcome this, we develop a new algorithm to perform parametric EMA, whereby an entire data set can be unmixed into unimodal parametric end members (e.g., Weibull distributions). This makes it easier to identify individual grain size sub-populations in highly mixed data sets. To aid investigators in applying these methods, all of the new algorithms are available in AnalySize, which is GUI software for processing and unmixing grain size data. This article is protected by copyright. All rights reserved.

Description: Landslides are common features in the vicinity of volcanic islands. In this contribution, we investigate landslides emplacement and dynamics around the volcanic island of Martinique based on the first scientific drilling of such deposits. The evolution of the active Montagne Pelée volcano on this island has been marked by three major flank-collapses that removed much of the western flank of the volcano. Subaerial collapse volumes vary from 2 to 25 km 3 and debris avalanches flowed into the Grenada Basin. High-resolution seismic data (AGUADOMAR – 1999, CARAVAL – 2002 and GWADASEIS – 2009) is combined with new drill cores that penetrate up to 430 m through the three submarine landslide deposits previously associated to the aerial flank-collapses (Site U1399, Site U1400, Site U1401, IODP Expedition 340, Joides Resolution, March-April 2012). This combined geophysical and core data provide an improved understanding of landslide processes offshore a volcanic island. The integrated analysis shows a large submarine landslide deposit, without debris avalanche deposits coming from the volcano, comprising up to 300 km 3 of remobilized seafloor sediment that extends for 70 km away from the coast and covers an area of 2100 km 2 . Our new data suggest that the aerial debris avalanche deposit enter the sea but stop at the base of submarine flank. We propose a new model dealing with seafloor sediment failures and landslide propagation mechanisms, triggered by volcanic flank-collapse events affecting Montagne Pelée volcano. Newly recognized landslide deposits occur deeper in the stratigraphy, suggesting the recurrence of large-scale mass-wasting processes offshore the island and thus, the necessity to better assess the associated tsunami hazards in the region. This article is protected by copyright. All rights reserved.

Description: Ventilation and dissolved oxygen in Lake Superior are key factors that determine the fate of various natural and anthropogenic inputs to the lake. We employ an idealized age tracer and biogeochemical tracers in a realistically configured numerical model of Lake Superior to characterize its ventilation and dissolved O 2 cycle. Our results indicate that Lake Superior is preferentially ventilated over rough bathymetry and that spring overturning following a very cold winter does not completely ventilate the lake interior. While this is unexpected for a dimictic lake, no part of the lake remains isolated from the atmosphere for more than 300 days. Our results also show that Lake Superior's oxygen cycle is dominated by solubility changes; as a result, the expected relationship between biological consumption of dissolved O 2 and ventilation age does not manifest. This article is protected by copyright. All rights reserved.

Description: Paleoclimate reconstructions based on reef corals require precise detection of diagenetic alteration. Secondary calcite can significantly affect paleotemperature reconstructions at very low amounts of ∼ 1%. X-ray powder diffraction is routinely used to detect diagenetic calcite in aragonitic corals. This procedure has its limitations as single powder samples might not represent the entire coral heterogeneity. A conventional and a 2D X-ray diffractometer were calibrated with gravimetric powder standards of high- and low-magnesium calcite (0.3% to 25% calcite). Calcite contents 〈 1% can be recognized with both diffractometer setups based on the peak area of the calcite [104] reflection. An advantage of 2D-XRD over convenient 1D-XRD methods is the non-destructive and rapid detection of calcite with relatively high spatial resolution directly on coral slabs. The calcite detection performance of the 2D-XRD setup was tested on thin-sections from fossil Porites sp . samples that, based on powder XRD measurements, showed 〈 1% calcite. Quantification of calcite contents for these thin-sections based on 2D-XRD and digital image analysis showed very similar results. This enables spot-measurements with diameters of ∼ 4 mm, as well as systematic line-scans along potential tracks previous to geochemical proxy sampling. In this way areas affected by diagenetic calcite can be avoided and alternative sampling tracks can be defined. Alternatively, individual sampling positions that show dubious proxy results can later be checked for the presence of calcite. The presented calibration and quantification method can be transferred to any 2D X-Ray diffractometer. This article is protected by copyright. All rights reserved.

Description: The determination of in situ stress states is vital in understanding the behavior of faults and subsequent seismogenesis of accretionary prisms. In this paper, a high quality 3D seismic volume is used to map the depth of the extensional-compressional decoupling (ECD) boundary in the accretionary prism of Nankai, with the prior knowledge that strike-slip and compressional stresses occur deeper than 1250 meters below seafloor (mbsf) in the Kumano Basin, changing to extension towards the seafloor. A total of 1108 faults from the accretionary prism are analyzed to estimate paleostresses via fault inversion and slip tendency techniques. A key result is this paper is that the ECD boundary can be used as a proxy to identify active structures on accretionary prisms as its depth depends on: a) local tectonic uplift in areas adjacent to active faults, and b) on the thickness of sediment accumulated above active thrust anticlines. The depth of the ECD boundary ranges from 0 to ∼650 mbsf, being notably shallower than in the Kumano Basin. In Nankai, frontal regions of the imbricate thrust zone, and the megasplay fault zone, reveal the shallower ECD depths and correlate with the regions where faulting is most active. As a corollary, this work confirms that estimates of stress state variability based on the analysis of 3D seismic data are vital to understand the behavior of faults and potential seismogenic regions on convergent margins. This article is protected by copyright. All rights reserved.

Description: ABSTRACT Quartz is one of the most common minerals on the surface of the earth, and is a primary rock-forming mineral across the rock cycle. These two factors make quartz an obvious target for sediment provenance studies. Observations from experimental and natural samples demonstrate that the trace-element content of quartz often reflects the conditions of quartz formation. When quartz is weathered from its primary crystallization setting (i.e., quartz from a granitoid) it can retain many chemical signatures of formation throughout the sedimentation processes. These geochemical signatures can be used to understand the primary source of individual quartz grains within a sediment. Here we present a case study from the Bega River catchment to demonstrate that quartz grains in sediments at the mouth of the Bega River are sourced from granitoids within the drainage basin. Data presented here also indicate that a portion of the beach sediment is also derived from either – (i) sedimentary rocks within the basin or; (ii) mixing with sediments at the mouth of the river. The Bega River catchment was selected for this study because it is both small and has a well-constrained bedrock lithology, making it an ideal location to test the utility of this provenance technique. However, quartz trace-element provenance has broad applications to modern and ancient sediments and can be used in lieu of, or in conjunction with, other provenance techniques to elucidate sediment transport through time. This article is protected by copyright. All rights reserved.

Description: The Nicoya peninsula in northwest Costa Rica overlies a section of the subduction megathrust along the Middle America Trench. On September 5, 2012, a moment magnitude 7.6 megathrust earthquake occurred beneath a dense network of continuous GPS and seismic stations. Many of the GPS stations recorded the event at high rate, 1 Hz or better. We analyze the temporal and spatial evolution of surface deformation after the earthquake. Our results show that the main rupture was followed by significant afterslip within the first 3 hours following the main event. The behavior of the surface displacement can be represented by relaxation processes with three characteristic times: 7, 70 and more than 400 days. We assume that the long relaxation time corresponds to viscoelastic relaxation and the intermediate relaxation time corresponds to afterslip on the main fault. The short relaxation time may represent a combination of rapid afterslip, poroelastic adjustment in the upper crust, or other processes. During the first few months that followed the earthquake, afterslip likely released a significant amount of slip deficit still present following the coseismic rupture, in particular up-dip of the rupture. Afterslip seems to be bounded up-dip by regions affected by slow slip events prior to the earthquake, suggesting that the two processes are influenced by different frictional properties. This article is protected by copyright. All rights reserved.

Description: There is a lack of consensus regarding the roles of sulfide saturation versus volatile degassing on the partitioning of Cu and Ag during differentiation and eruption of convergent margin magmas. Because of their oxidized character, volatile-rich magmas from the Eastern Manus Backarc Basin (EMBB) only reach sulfide saturation following magnetite-driven reduction of the melt: the so-called “magnetite crisis”. If sulfide saturation typically precedes volatile saturation, the magnetite crisis will limit the proportion of Cu and Ag that can partition from the melt into an exsolving volatile-rich phase, which may contribute to the sporadic occurrence of magmatic-hydrothermal ore deposits at convergent margins. However, it is unclear whether the magnetite crisis is a common or rare event during differentiation of volatile-rich magmas. We report major and trace element data for submarine volcanic glasses from the Tonga Arc-proximal Valu Fa Ridge (VFR; SW Pacific). Cu-Se-Ag systematics of samples erupting at the southern VFR suggest magnetite fractionation triggered sulfide saturation. The similarity in chalcophile element systematics of the southern VFR and EMBB samples is unlikely to be coincidental, and may indicate the magnetite crisis is a common event during differentiation of hydrous melts. However, unlike many convergent margin magmas, it is unlikely the evolving VFR and EMBB were saturated in a S-bearing volatile phase prior to magnetite fractionation. Hence, the metal-depleting magnetite crisis may be restricted to backarc basin magmas that do not degas volatiles prior to magnetite fractionation and potentially convergent margin magmas fractionating at high-pressures in the continental crust. This article is protected by copyright. All rights reserved.

Description: Nitrogen isotopic ratios of nitrate (δ 15 N–NO 3 − ) were analyzed above 1000 m water depth along 17°S in the subtropical South Pacific during the revisit WOCE P21 cruise in 2009. The δ 15 N–NO 3 − and N* values were as high as 17‰ and as low as −18 μmol N L −1 , respectively, at depths around 250 m east of 115°W, but were as low as 5‰ and as high as +1 μmol N L −1 , respectively, in subsurface waters west of 170°W. The relationships among NO 3 − concentrations, N* values, δ 15 N–NO 3 − values, and oxygen and nitrite concentrations suggest that a few samples east of 90°W were from suboxic and nitrite-accumulated conditions and were possibly affected by in situ water column denitrification. Most of the high-δ 15 N–NO 3 − and negative-N* waters were probably generated by mixing between Subantarctic Mode Water from the Southern Ocean and Oxygen Deficit Zone Water from the eastern tropical South Pacific, with remineralization of organic matter occurring during transportation. Moreover, the relationship between δ 15 N–NO 3 − and N* values, as well as Trichodesmium abundances and size-specific nitrogen fixation rates at the surface, suggest that the low-δ 15 N–NO 3 − and positive-N* subsurface waters between 160°E and 170°W were generated by the input of remineralized particles created by in situ nitrogen fixation, mainly by Trichodesmium spp. Therefore, the δ 15 N values of sediments in this region are expected to reveal past changes in nitrogen fixation or denitrification rates in the subtropical South Pacific. This article is protected by copyright. All rights reserved.

Description: The age of the subducting Nazca Plate off Chile increases northwards from 0 Ma at the Chile Triple Junction (46°S) to 37 Ma at the latitude of Valparaíso (32°S). Age-related variations in the thermal state of the subducting plate impact on (a) the water influx to the subduction zone, as well as on (b) the volumes of water that are released under the continental forearc or, alternatively, carried beyond the arc. Southern Central Chile is an ideal setting to study this effect, because other factors for the subduction zone water budget appear constant. We determine the water influx by calculating the crustal water uptake and by modeling the upper mantle serpentinization at the outer rise of the Chile Trench. The water release under forearc and arc is determined by coupling FEM thermal models of the subducting plate with stability fields of water-releasing mineral reactions for upper and lower crust and hydrated mantle. Results show that both the influx of water stored in, and the outflux of water released from upper crust, lower crust and mantle vary drastically over segment boundaries. In particular, the oldest and coldest segments carry roughly twice as much water into the subduction zone as the youngest and hottest segments, but their release flux to the forearc is only about one fourth of the latter. This high variability over a subduction zone of 〈 1500 km length shows that it is insufficient to consider subduction zones as uniform entities in global estimates of subduction zone fluxes. This article is protected by copyright. All rights reserved.

Description: The formation process of gas hydrates in sedimentary matrices is of crucial importance for the physical and transport properties of the resulting aggregates. This process has never been observed in-situ at sub-micron resolution. Here, we report on synchrotron-based micro-tomographic studies by which the nucleation and growth processes of gas hydrate were observed at 276 K in various sedimentary matrices such as natural quartz (with and without admixtures of montmorillonite type clay) or glass beads with different surface properties, at varying water saturation. Both juvenile water and metastably gas-enriched water obtained from gas hydrate decomposition was used. Xenon gas was employed to enhance the density contrast between gas hydrate and the fluid phases involved. The nucleation sites can be easily identified and the various growth patterns are clearly established. In sediments under-saturated with juvenile water, nucleation starts at the water-gas interface resulting in an initially several micrometer thick gas hydrate film; further growth proceeds to form isometric single crystals of 10-20 μm size. The growth of gas hydrate from gas-enriched water follows a different pattern, via the nucleation in the bulk of liquid producing polyhedral single crystals. A striking feature in both cases is the systematic appearance of a fluid phase film of up to several µm thickness between gas hydrates and the surface of the quartz grains. These micro-structural findings are relevant for future efforts of quantitative rock physics modeling of gas hydrates in sedimentary matrices and explain the anomalous attenuation of seismic/sonic waves. This article is protected by copyright. All rights reserved.

Description: The very low B/Ca ratios characteristic of some natural biogenic carbonates, are of interest for research in ocean acidification but represent an analytical challenge. We describe a method using a novel instrument configuration (ICP-QQQ), for which we are not aware of any previously published geological applications, and for coccoliths, a sample type unique in its low B content and organic phases. Detection limits as low as 0.41 µmol/mol were achieved. Isobaric interferences, out of the reach even for SF-ICP-MS, can be solved using this instrument, which permits the safe measurement of the lowest abundance Ca isotope ( 46 Ca). This allows maximizing the B concentration measured (matrix concentration up to 800 ppm Ca) while maintaining both B and Ca signals in counting mode. More significantly for low B samples, the ICP-QQQ is also able to overcome the interference of the ubiquitous 12 C tail on the 11 B mass, which otherwise leads to significant overestimates at very low B concentrations. This could be a reason for the significantly lower B/Ca ratios observed for the low B content interlaboratory calibration standards (Carrara and OKA), while matching for the high B content standards was good. Finally, results obtained in the analysis of coccoliths grown in laboratory culture seems to corroborate that SIMS analysis of the samples mounted in Indium leads also to B/Ca overestimates due to porosity effects, as previously observed using LA-ICP-MS. This approach also permits the interference-free measurement of P/Ca and S/Ca ratios, which could be used as indicators of the complete removal of the organic matter from the samples. This article is protected by copyright. All rights reserved.

Description: Since the first discovery of ultra-high pressure (UHP) rocks 30 years ago in the Western Alps, the mechanisms for exhumation of (U)HP terranes worldwide are still debated. In the Western Mediterranean, the presently accepted model of synconvergent exhumation (e.g., the channel-flow model) is in conflict with parts of the geologic record. We synthesize regional geologic data and present alternative exhumation mechanisms that consider the role of divergence within subduction zones. These mechanisms, i.e., (i) the motion of the upper plate away from the trench and (ii) the rollback of the lower plate, are discussed in detail with particular reference to the Cenozoic Adria-Europe plate boundary, and along three different transects (Western Alps, Calabria-Sardinia, and Corsica - Northern Apennines). In the Western Alps, (U)HP rocks were exhumed from the greatest depth at the rear of the accretionary wedge during motion of the upper plate away from the trench. Exhumation was extremely fast, and associated with very low geothermal gradients. In Calabria, HP rocks were exhumed from shallower depths and at lower rates during rollback of the Adriatic plate, with repeated exhumation pulses progressively younging towards the foreland. Both mechanisms were active to create boundary divergence along the Corsica – Northern Apennines transect, where European southeastward subduction was progressively replaced along-strike by Adriatic northwestward subduction. The tectonic scenario depicted for the Western Alps trench during Eocene exhumation of (U)HP rocks correlates well with present day eastern Papua New Guinea, which is presented as a modern analogue of the Paleogene Adria-Europe plate boundary. This article is protected by copyright. All rights reserved.

Description: Analyzing subduction initiation is key for understanding the coupling between plate tectonics and underlying mantle. Here we focus on supra-subduction zone (SSZ) ophiolites and how their formation links to intra-oceanic subduction initiation in an absolute plate motion frame. SSZ ophiolites form the majority of exposed oceanic lithosphere fragments and are widely recognized to have formed during intra-oceanic subduction initiation. Structural, petrological, geochemical, and plate kinematic constraints on their kinematic evolution show that SSZ crust forms at forearc spreading centers at the expense of a mantle wedge, thereby flattening the nascent slab. This leads to the typical inverted pressure gradients found in metamorphic soles that form at the subduction plate contact below and during SSZ crust crystallization. Former spreading centers are preserved in forearcs when subduction initiates along transform faults or off-ridge oceanic detachments. We show how these are reactivated when subduction initiates in the absolute plate motion direction of the inverting weakness zone. Upon inception of slab-pull due to e.g. eclogitization, the sole is separated from the slab, remains welded to the thinned overriding plate lithosphere and can become intruded by mafic dikes upon asthenospheric influx into the mantle wedge. We propound that most ophiolites thus formed under special geodynamic circumstances and may not be representative of normal oceanic crust. Our study highlights how far-field geodynamic processes and absolute plate motions may force intra-oceanic subduction initiation as key towards advancing our understanding of the entire plate tectonic cycle. This article is protected by copyright. All rights reserved.

Description: The zones of overlap between adjacent Synthetic Aperture Radar (SAR) satellite tracks are illuminated twice more frequently than elsewhere in the SAR scene. Here, an alternative approach is presented to combine the overlapping segments of SAR images acquired at adjacent tracks and generate accurate and high spatiotemporal resolution map of the surface deformation field. To this end, a new approach is developed to unify the datums. Effects due to the difference in look angle between two overlapping tracks and atmospheric delay are estimated and removed using Kalman and wavelet based filters. This approach is first tested at Hawaii Island, where tracks 200 and 429 of Envisat C-band satellite overlap over the Kilauea south flank. The obtained time series improves the temporal sampling rate by a factor of two and comparison with GPS time series demonstrates that the presented method accurately measures the nonlinear deformation field. The advantages of this method are further demonstrated by combining SAR data sets acquired by Envisat C-band and ALOS L-band satellites over the San Francisco Bay Area, California. The validation test shows that the seamless combination of C- and L-Band time series accurately measures the surface deformation at higher resolution. This article is protected by copyright. All rights reserved.

Description: Strong tidal stresses brought much of the icy shell of Enceladus into frictional failure at past times of high orbital eccentricity. The frictional behavior of shallow terrestrial rock exposed to repeated episodes of strong seismic waves provides analogy. Frictional failure produces cracks that lower the shear modulus. Seismic regolith develops where the shear modulus increases linearly with depth. Imposed peak strains barely cause frictional failure within self-organized regolith. With regard to Enceladus, eccentricity could continue to build up in the past since little anelastic strain and hence tidal dissipation occurred within the self-organized regolith and within the underlying cold ice. A frictional instability analogous to the formation of weak major faults on the Earth likely occurred once the regolith was many kilometers thick. The effective coefficient of friction dropped to low levels along major faults within the deep cold ice. Tidal dissipation on these faults heated the ice starting thermal convention within the South Polar Terrain. Once thermal buoyancy produced stresses, the oscillating stresses from tides nonlinearly enhanced the rate of tectonic convection. Warm ice that dissipates tides now exists within Enceladus. The eccentricity will likely decrease and the object will then freeze. This article is protected by copyright. All rights reserved.

Description: Water and other trace element concentrations in olivine (1-39 ppm H 2 O), orthopyroxene (10-150 ppm H 2 O), and clinopyroxene (16-340 ppm H 2 O) of mantle xenoliths from the Labait volcano, located on the edge of the Tanzanian craton along the eastern branch of the East African Rift, record melting and subsequent refertilization by plume magmas in a stratified lithosphere. These water contents are at the lower end of the range observed in other cratonic mantle lithospheres. Despite correlations between water content and indices of melting in orthopyroxene from the shallow peridotites, and in both olivine and orthopyroxene from the deep peridotites, water concentrations are too high for the peridotites to be simple residues. Instead, the Labait water contents are best explained as reflecting interaction between residual peridotite with a melt having relatively low water content (〈1 wt.% H 2 O). Plume-derived melts are the likely source of water and other trace element enrichments in the Labait peridotites. Only garnet may have undergone addition of water from the host magma as evidenced by water content increasing towards the kelyphite rim in one otherwise homogeneous garnet. Based on modeling of the diffusion profile, magma ascent occurred at 4 – 28 m/s. In summary, plume-craton interaction appears to result in only moderate water enrichment of the lithosphere. This article is protected by copyright. All rights reserved.

Description: An upgrade of the PRIMELT algorithm [ Herzberg and O'Hara , 2002] for calculating primary magma composition is given together with its implementation in PRIMELT3 MEGA.xlsm software. It supersedes PRIMELT2.xls [ Herzberg and Asimow , 2008] in correcting minor mistakes in melt fraction and computed Ni content of olivine, it identifies residuum mineralogy, and it provides a thorough analysis of uncertainties in mantle potential temperature and olivine liquidus temperature. The uncertainty analysis was made tractable by the computation of olivine liquidus temperatures as functions of pressure and partial melt MgO content between the liquidus and solidus. We present a computed anhydrous peridotite solidus in T-P space using relations amongst MgO, T and P along the solidus; it compares well with experiments on the solidus. Results of the application of PRIMELT3 to a wide range of basalts shows that the mantle sources of ocean islands and large igneous provinces were hotter than oceanic spreading centers, consistent with earlier studies and expectations of the mantle plume model. This article is protected by copyright. All rights reserved.

Description: Co-seismic surface deformation in large earthquakes is typically measured using field mapping and with a range of geodetic methods (e.g., InSAR, lidar differencing, and GPS). Current methods, however, either fail to capture patterns of near-field co-seismic surface deformation or lack pre-event data. Consequently, the characteristics of off-fault deformation and the parameters that control it remain poorly understood. We develop a standardized method to fully measure the surface, near-field, co-seismic deformation patterns at high-resolution using the COSI-Corr program by correlating pairs of aerial photographs taken before and after the 1992 M w 7.3 Landers earthquake. COSI-Corr offers the advantage of measuring displacement across the entire zone of surface deformation and over a wider aperture than that available to field geologists. For the Landers earthquake, our measured displacements are systematically larger than the field measurements, indicating the presence of off-fault deformation. We show that 46 % of the total surface displacement occurred as off-fault deformation, over a mean deformation width of 154 m. The magnitude and width of off-fault deformation along the rupture is primarily controlled by the macroscopic structural complexity of the fault system, with a weak correlation with the type of near-surface materials through which the rupture propagated. Both the magnitude and width of distributed deformation are largest in stepovers, bends, and at the southern termination of the surface rupture. We find that slip along the surface rupture exhibits a consistent degree of variability at all observable length scales and that the slip distribution is self-affine fractal with dimension of 1.56. This article is protected by copyright. All rights reserved.

Description: Shelves surrounding reefless volcanic ocean islands are formed by surf erosion of their slopes during changing sea levels. Post-erosional lava flows, if abundant, can cross the coastal cliffs and fill partially or completely the accommodation space left by erosion. In this study, multibeam bathymetry, high-resolution seismic reflection profiles and sediment samples are used to characterize the morphology of the insular shelves adjacent to Faial and Pico islands. The data shows offshore fresh lava flow morphologies, as well as an irregular basement beneath shelf sedimentary bodies and reduced shelf width adjacent to older volcanic edifices. These observations suggest that the shelves have been significantly filled by volcanic progradation and can thus be classified as “rejuvenated”. Despite the general volcanic infilling of the shelves around the islands, most of their edges are below the depth of the Last Glacial Maximum, revealing that that at least parts of the island have subsided after the shelves formed by surf erosion. Prograding lava deltas reached the shelf edge in some areas triggering small slope failures, locally decreasing the shelf width and depth of their edges. These areas can represent a significant risk for the local population; hence their identification can be useful for hazard assessment and contribute to wiser land-use planning. Shelf morphology, subaerial geomorphology, magnetic anomalies and crustal structure data were also interpreted to reconstruct the longer term combined onshore and offshore evolution of the Faial-Pico ridge. The subaerial emergence of this ridge is apparently older than previously thought, i.e., before ∼850 ka. This article is protected by copyright. All rights reserved.

Description: Identification of Andosols is primarily based upon the content of their colloidal constituents – clay and metal-humus complexes – and on the determining of andic properties. This needs time and cost-consuming geochemical analyses. Our primary aim of this study is to describe the magnetic and geochemical properties of soils rich in iron oxides derived from strongly magnetic volcanic basement (in this case Andosols). Secondary aim is to explore links between magnetic and chemical parameters of Andic soils with respect to genesis factors: parent material age, precipitation and thickness of the soil profile. Six pedons of andic properties, developed on basaltic lavas, were analysed down to parent rock by a set of magnetic and geochemical methods. Magnetic data of soil and rock samples reflect the type, concentration and particle-size distribution of ferrimagnetic minerals. Geochemical data include soil reaction (pH in H 2 O), cation exchange capacity, organic carbon and different forms of extractable iron and aluminum content. Our results suggest the following: (1) Magnetic measurements of low-field mass-specific magnetic susceptibility can be a reliable indicator for estimating andic properties, and in combination with thermomagnetic curves may be suitable for discriminating between alu-andic and sil-andic subtypes. (2) In the studied Andosols, strong relationships were found between (a) magnetic grain-size parameters, precipitation and exchangeable bases; (b) concentration of ferrimagnetic particles and degree of crystallization of free iron; and (c) parameters reflecting changes in magneto-mineralogy and soil genesis (parent material age + soil depth). This article is protected by copyright. All rights reserved.

Description: Inversion of NASA Operation IceBridge airborne gravity over the Abbot Ice Shelf in West Antarctica for sub-ice bathymetry defines an extensional terrain made up of east-west trending rift basins formed during the early stages of Antarctica/Zealandia rifting. Extension is minor, as rifting jumped north of Thurston Island early in the rifting process. The Amundsen Sea Embayment continental shelf west of the rifted terrain is underlain by a deeper, more extensive sedimentary basin also formed during rifting between Antarctica and Zealandia. A well-defined boundary zone separates the mildly extended Abbot extensional terrain from the deeper Amundsen Embayment shelf basin. The shelf basin has an extension factor, β, of 1.5-1.7 with 80-100 km of extension occurring across an area now 250 km wide. Following this extension, rifting centered north of the present shelf edge and proceeded to continental rupture. Since then, the Amundsen Embayment continental shelf appears to have been tectonically quiescent and shaped by subsidence, sedimentation and the advance and retreat of the West Antarctic Ice Sheet. The Bellingshausen Plate was located seaward of the Amundsen Sea margin prior to incorporation into the Antarctic Plate at about 62 Ma. During the latter part of its independent existence, Bellingshausen plate motion had a clockwise rotational component relative to Antarctica producing convergence across the north-south trending Bellingshausen Gravity Anomaly structure at 94°W and compressive deformation on the continental slope between 94°W and 102°W. Farther west, the relative motion was extensional along an east-west trending zone occupied by the Marie Byrd Seamounts. This article is protected by copyright. All rights reserved.

Description: The Yamato Basin in the Japan Sea is a back-arc basin characterized by basaltic oceanic crust that is twice as thick as typical oceanic crust. Two types of ocean floor basalts, formed during the opening of the Japan Sea in the Middle Miocene, were recovered from the Yamato Basin during Ocean Drilling Program Legs 127/128. These can be considered as depleted (D-type) and enriched (E-type) basalts based on their incompatible trace element and Sr–Nd–Pb–Hf isotopic compositions. Both types of basalts plot along a common mixing array drawn between depleted mantle and slab sediment represented by a sand-rich turbidite on the Pacific Plate in the NE Japan forearc. The depleted nature of the D-type basalts suggests that the slab sediment component is nil to minor relative to the dominant mantle component, whereas the enrichment of all incompatible elements in the E-type basalts was likely caused by a large contribution of bulk slab sediment in the source. The results of forward model calculations using adiabatic melting of a hydrous mantle with sediment flux indicate that the melting conditions of the source mantle for the D-type basalts are deeper and hotter than those for the E-type basalts, which appear to have formed under conditions hotter than those of normal mid-oceanic ridge basalts (MORB). These results suggest that the thicker oceanic crust was formed by greater degrees of melting of a hydrous metasomatized mantle source at unusually high mantle potential temperature during the opening of the Japan Sea. This article is protected by copyright. All rights reserved.

Description: Hydration and metasomatism of the lithospheric mantle potentially influences both the magmatic and tectonic evolution of southwestern North America. Prior studies have suggested that volatile enrichments to the mantle underlying western North America resulted from shallow subduction of the Farallon Plate during the Laramide (∼74-40 Ma). This study examines temporal and spatial variations in volatile elements (H 2 O, Cl, F, S) determined from olivine- and orthopyroxene-hosted melt inclusions along and across the Rio Grande Rift, the easternmost extent of Laramide shallow subduction. Maximum chlorine enrichments are observed in the southern rift with a Cl/Nb of ∼210 and reduce with time to MORB-OIB levels (∼5-17). Measured water abundances are 〈0.8 wt% in rehomogenized inclusions, however calculated H 2 O, based on Cl/Nb systematics, primarily varies from 0.5-2 wt% H 2 O. Sulphur abundances (〈 0.61 wt%), and calculated sulphide saturation, indicate magmas with high Cl/Nb also contain oxidized sulphur. The abundance of fluorine in melt inclusions (up to 0.2 wt%) is not correlated to other volatile elements. Temporal variations in melt inclusion volatile abundances coupled with varying isotopic (Sr-Nd-Pb) whole rock systematics suggests a transition from lithospheric to asthenospheric melt generation in the southern RGR and potential lithosphere-asthenosphere melt mixing in the central RGR. East to west decrease in volatile enrichment likely reflects a combination of varying mantle sources and early removal of metasomatised lithospheric mantle underlying regional extension. Results indicate, from multiple causes, subduction-related volatile enrichment to the lithospheric mantle is ephemeral, and strong enrichments in volatiles are not preserved in active magmatic-tectonic provenances. This article is protected by copyright. All rights reserved.

Description: The Costa Rica Seismogenesis Project (CRISP) drilled the Pacific margin of the Middle America Trench just north of where the Cocos Ridge enters the subduction zone, resulting in basal erosion of the upper plate. Here we report the orientations of the maximum horizontal principal stress ( S Hmax ) from borehole breakouts detected by logging-while-drilling and wireline downhole measurements. All S Hmax directions were estimated in the sediment cover of the margin, above the deeper rocks of the deformed margin wedge. We observe three overall S Hmax orientations: NNE-SSW (25° azimuth) in the deepest interval drilled at the upper slope Site U1379; ENE-WSW (82°) in the rest of Site U1379 and in Site U1413, also drilled in the upper slope; and NNW-SSE (157°) in the mid-slope Site U1378. Our preferred interpretation is that the deepest interval of Site U1379 records the stress conditions in the underlying margin wedge, as S Hmax is parallel to the direction of the Cocos-Caribbean plate convergence and of the compressional axes of plate boundary fault earthquakes. The variable S Hmax directions observed elsewhere are likely due to the effect of a network of normal faults that subdivide the sediment cover into a number of independently deforming blocks. In addition, the observed S Hmax directions may be influenced by the subducting Cocos Ridge, which acts as an indenter causing oblique deformation, and by the transition to seismogenic subduction along the plate boundary fault. This article is protected by copyright. All rights reserved.

Description: Coastal wetlands store vast amounts of organic carbon, globally, and are becoming increasingly vulnerable to the effects of anthropogenic sea-level rise. To understand the effect of sea-level rise on organic carbon fate and preservation in this global sink, it is necessary to characterize differences in the biogeochemical stability of coastal wetland soil organic carbon (SOC). Here, we use ramped pyrolysis/oxidation decomposition characteristics as proxies for SOC stability to understand the fate of carbon storage in coastal wetlands comprising the Mississippi River deltaic plain, undergoing rapid rates of local sea level rise. Soils from three wetland types (fresh, brackish, and salt marshes) along a salinity gradient were subjected to ramped pyrolysis analysis to evaluate decomposition characteristics related to thermochemical stability of SOC. At equivalent soil depths, we observed that fresh marsh SOC was more stable than brackish and salt marsh SOC. Depth, isotopic, elemental, and chemical compositions, bulk density, and water content of SOC all exhibited different relationships with SOC stability across the marsh salinity gradient, indicative of different controls on SOC stability within each marsh type. The differences in stability imply stronger preservation potential of fresh marsh soil carbon, compared to that of salt and brackish marshes. Considering projected marsh ecosystem responses to sea-level rise, these observed stability differences are important in planning and implementing coastal wetland carbon-focused remediation and improving climate model feedbacks with the carbon cycle. Specifically, our results imply that ecosystem changes associated with sea-level rise will initiate the accumulation of less stable carbon in coastal wetlands. This article is protected by copyright. All rights reserved.

Description: We evaluate the effect of temperature variations on strength and effective elastic thickness ( T e ) of the lithosphere of the North American (NA) continent. To this purpose, we use two thermal models that are corrected for compositional variations and anelasticity effects in the upper mantle. These thermal models are obtained from a joint inversion of gravity data and two recent seismic tomography models (NA07 and SL2013sv). The crustal rheology was defined using NACr14, the most recent NA crustal model. This model specifies seismic velocities and thickness for a three-layer model of the crystalline crust. Strength in the lithosphere and in the crust has similar distributions, indicating that local geotherms play a dominant role in determining strength rather than crustal composition. A pronounced contrast is present in strength between cratonic and off-cratonic regions. Lithospheric strength in the off-cratonic regions is prevalently localized within the crust and T e shows low values (〈20 km), while the inner part of the cratons is characterized by a strong lithosphere with large T e (〉150 km). In contrast to previous results, our models indicate that Phanerozoic regions located close to the edge of the cratons, as the Appalachians, are characterized by low strength. We also find that locally weak zones exist within the cratons (e.g., beneath the intracratonic Illinois Basin and Midcontinent rift). Seismic tomography models NA07 and SL2013sv differ mainly in some peripheral parts of the cratons, as the Proterozoic Canadian Platform, the Grenville and the western part of the Yavapai-Mazatzal province, where the integrated strength for the model NA07 is ten times larger than in model SL2013sv due to a temperature difference (〉200˚C) in the uppermost mantle. The differences in T e between the two models are less pronounced. In both models, Proterozoic regions reactivated by Meso-Cenozoic tectonics (e.g., Rocky Mountains and the Mississippi Embayment) are characterized by a weak lithosphere due to the absence of the mechanically strong part of the mantle lithospheric layer. Intraplate earthquakes are distributed along the edges of the cratons, demonstrating that tectonic stress accumulates there, while the cores of the cratons remain undeformed. In both models intraplate earthquakes occur in weak lithosphere (∼0.5 x10^ 13 Pa s, T e ∼15 km) or near the edges of strong cratonic blocks, characterized by pronounced contrasts of strength and T e . This article is protected by copyright. All rights reserved.

Description: Numerical simulations of thermal convection in the Earth's mantle often employ a pseudo-plastic rheology in order to mimic the plate-like behavior of the lithosphere. Yet the benchmark tests available in the literature are largely based on simple linear rheologies in which the viscosity is either assumed to be constant or weakly dependent on temperature. Here we present a suite of simple tests based on non-linear rheologies featuring temperature-, pressure-, and strain rate dependent viscosity. Eleven different codes based on the finite volume, finite element, or spectral methods have been used to run five benchmark cases leading to stagnant lid, mobile lid, and periodic convection in a 2-D square box. For two of these cases, we also show resolution tests from all contributing codes. In addition, we present a bifurcation analysis, describing the transition from a mobile lid regime to a periodic regime, and from a periodic regime to a stagnant lid regime, as a function of the yield stress. At a resolution of around 100 cells or elements in both vertical and horizontal directions, all codes reproduce the required diagnostic quantities with a discrepancy of at most ∼ 3% in the presence of both linear and non-linear rheologies. Furthermore they consistently predict the critical value of the yield stress at which the transition between different regimes occurs. As the most recent mantle convection codes can handle a number of different geometries within a single solution framework, this benchmark will also prove useful when validating viscoplastic thermal convection simulations in such geometries. This article is protected by copyright. All rights reserved.

Description: Rayleigh and Love wave phase velocities in the northern part of the North China are obtained from ambient noise tomography in the period range of 8 to 35 s and two plane wave earthquake tomography at periods of 20 to 91 s using data recorded at 222 broadband seismic stations from the temporary North China Seismic Array and permanent China Digital Seismic Array. The dispersion curves of Rayleigh and Love wave from 8 to 91 s are jointly inverted for the 3-D shear wave structure and radial anisotropy in the lithosphere to 140 km depth. Distinct seismic structure are observed from the Fenhe Graben and Taihang Mountain to North China Basin. The North China Basin from the lower crust to the depth of 140 km is characterized by high velocity anomaly, reflecting mafic intrusion and residual materials after the extraction of melt, and by strong radial anisotropy with Vsh 〉 Vsv implying horizontal layering of intrusion and alignment of minerals due to vigorous extensional deformation and subsequent thermal annealing. However, low velocity anomaly and positive radial anisotropy are observed in the Fenhe Graben and Taihang Mountain, suggesting the presence of partial melt in the lithosphere due to the mantle upwelling and horizontal flow pull. This article is protected by copyright. All rights reserved.

Description: Strong seismic waves bring rock into frictional failure at the uppermost few hundred meters. Numerous small fractures slip with the cumulative effect of anelastic strain and nonlinear attenuation; these fractures should not distinguish between remote sources of stress. Still, frictional failure criteria are not evident especially when seismic waves change the normal traction on fractures. We identify three earthquakes as examples where consideration of interaction among dynamic stresses from different wave types and ambient tectonic stress provides theoretical predictions of nonlinear attenuation that are potentially testable with single station seismograms. For example, because Rayleigh waves produce shallow horizontal dynamic tension and compression, frictional failure should preferentially occur on the tensile half-cycle if no shallow tectonic stress is present and on the compressional half-cycle if the tectonic stress is already near thrust-faulting failure. We observed neither effect on records from the 2011 M w 9.0 Great Tohoku earthquake. However, Rayleigh waves from this event appear to have brought rock beneath MYGH05 station into frictional failure at ∼10 m depth and thus suppressed high-frequency S-waves. The tensile half-cycle of high frequency P-waves reduced normal traction on horizontal planes beneath station IWTH25 during the 2008 M w 6.9 Iwate-Miyagi earthquake, weakening the rock in shear and suppressing high-frequency S-waves. The near-field velocity pulse from the 1992 M w 7.3 Landers earthquake brought the uppermost few hundred meters of granite beneath Lucerne station into frictional failure, suppressing high frequency S-waves. These mildly positive examples support the reality of nonlinear wave interaction, warranting study future strong ground motions. This article is protected by copyright. All rights reserved.

Description: ABSTRACT Mantle helium is a significant component of the helium gas from deep oil wells along the Newport-Inglewood fault zone (NIFZ) in the Los Angeles (LA) basin. Helium isotope ratios are as high as 5.3 Ra (Ra= 3 He/ 4 He ratio of air) indicating 66% mantle contribution, (assuming R/Ra = 8 for mantle), and most values are higher than 1.0 Ra. Other samples from basin margin faults and from within the basin have much lower values (R/Ra 〈 1.0). The 3 He enrichment inversely correlates with CO 2 , a potential magmatic carrier gas. The δ 13 C of the CO 2 in the 3 He rich samples is between 0 and -10 ‰, suggesting a mantle influence. The strong mantle helium signal along the NIFZ is surprising considering that the fault is currently in a transpressional rather than extensional stress regime, lacks either recent magma emplacement or high geothermal gradients, and is modeled as truncated by a proposed major, potentially seismically active, décollement beneath the LA basin. Our results demonstrate that the NIFZ is a deep-seated fault directly or indirectly connected with the mantle. Based on a 1-D model, we calculate a maximum Darcy flow rate q ∼ 2.2 cm/yr and a fault permeability k ∼ 6 × 10 −17 m 2 (60 microdarcys), but the flow rates are too low to create a geothermal anomaly. The mantle leakage may be a result of the NIFZ being a former Mesozoic subduction zone in spite of being located 70 km west of the current plate boundary at the San Andreas fault. This article is protected by copyright. All rights reserved.

Description: The effect of maghemization on the magnetic properties of magnetite (Fe 3 O 4 ) grains in the pseudo-single-domain (PSD) size range is investigated as a function of annealing temperature. X-ray diffraction and transmission electron microscopy confirms the precursor grains as Fe 3 O 4 ranging from ∼ 150 nm to ∼ 250 nm in diameter, whilst Mössbauer spectrometry suggests the grains are initially near-stoichiometric. The Fe 3 O 4 grains are heated to increasing reaction temperatures of 120 – 220 ºC to investigate their oxidation to maghemite (γ-Fe 2 O 3 ). High-angle annular dark field imaging and localized electron energy-loss spectroscopy reveals slightly oxidized Fe 3 O 4 grains, heated to 140 ºC, exhibit higher oxygen content at the surface. Off-axis electron holography allows for construction of magnetic induction maps of individual Fe 3 O 4 and γ-Fe 2 O 3 grains, revealing their PSD (vortex) nature, which is supported by magnetic hysteresis measurements, including first-order reversal curve analysis. The coercivity of the grains is shown to increase with reaction temperature up to 180 ºC, but subsequently decreases after heating above 200 ºC; this magnetic behavior is attributed to the growth of a γ-Fe 2 O 3 shell with magnetic properties distinct from the Fe 3 O 4 core. It is suggested there is exchange coupling between these separate components that results in a vortex state with reduced vorticity. Once fully oxidized to γ-Fe 2 O 3 , the domain states revert back to vortices with slightly reduced coercivity. It is argued that due to a core/shell coupling mechanism during maghemization, the directional magnetic information will still be correct, however, the intensity information will not be retained. This article is protected by copyright. All rights reserved.

Description: Several mechanisms have been suggested for the formation of flat slabs including buoyant features on the subducting plate, trenchward motion and thermal or cratonic structure of the overriding plate. Analysis of episodes of flat subduction indicate that not all flat slabs can be attributed to only one of these mechanisms and it is likely that multiple mechanisms work together to create the necessary conditions for flat slab subduction. In this study we examine the role of localized regions of cratonic lithosphere in the overriding plate in the formation and evolution of flat slabs. We explicitly build on previous models, by using time-dependent simulations with three-dimensional variation in overriding plate structure. We find that there are two modes of flat subduction: permanent underplating occurs when the slab is more buoyant (shorter or younger), while transient flattening occurs when there is more negative buoyancy (longer or older slabs). Our models show how regions of the slab adjacent to the sub-cratonic flat portion continue to pull the slab into the mantle leading to highly contorted slab shapes with apparent slab gaps beneath the craton. These results show how the interpretation of seismic images of subduction zones can be complicated by the occurrence of either permanent or transient flattening of the slab, and how the signature of a recent flat slab episode may persist as the slab resumes normal subduction. Our models suggest that permanent underplating of slabs may preferentially occur below thick and cold lithosphere providing a built-in mechanism for regeneration of cratons. This article is protected by copyright. All rights reserved.

Description: The controversy over the exhumation of ultra-high pressure (UHP) rocks centers on whether it involves rising of pieces of crust detached from subducted continental lithosphere or an entire subducted plate that undergoes “eduction”, i.e. reverse subduction. We present a new thermomechanical model of continental subduction showing that these apparently contrasting mechanisms can occur together: Crust subducted deep enough is heated and weakened, causing limited diapiric rise, while crust subducted to shallower depths retains strength and is exhumed only by eduction. The model also shows for the first time how eduction followed by seafloor spreading can occur in a zone of regional convergence. This occurs spontaneously when subduction of buoyant crust causes a subduction zone to “lock up” in one place causing a new subduction zone to form in another. The model is consistent with many features of the youngest region of UHP rock exhumation on earth: the D'Entrecasteaux Islands. UHP exhumation and the amount of regional extension, as well as the seismic structure around the islands, can be explained by eduction. Ductile flow fabrics, seen on the islands, would result from exhumation of the most deeply subducted crust heated enough to undergo partial melting. Reversal of motion on the north-dipping continental subduction zone, required by this model, was likely triggered by initiation of the New Britain Trench, as suggested previously. Our model implies that the crust of Goodenough Basin, south of the islands, was exhumed by eduction in the last 5 Ma and this hypothesis can be tested by drilling. This article is protected by copyright. All rights reserved.

Description: The Comment by Pineda-Velasco et al . [2015] examined Pb isotope data presented by Kimura et al . [2014]. The authors' points are that (1) there is uncertainty in the analytical results of Kimura et al . [2014] due to the effect of mass fractionation, and therefore, (2) the interpretations of Kimura et al . [2014] based on the extent of crustal assimilation and the estimated Pb isotopic composition of the crustal component are erroneous. In response to the Comment, we report a flaw in the original paper that the samples from the Aono, Daisen, and Kannabe regions were analyzed using conventional TIMS methods and all other samples were analyzed using thallium-spiked multi-collector inductively-coupled-plasma mass spectrometry (TS-MC-ICP-MS). We have re-analyzed the sample powders from Karasugasen, Daisen, and Aono using TS-MC-ICP-MS. Our new results showed considerable overlap with the data in Pineda-Velasco et al . [2015]. Therefore, the isotopic trends shown by the conventional TIMS in Kimura et al . [2014] were analytical artifacts from mass bias. We conclude that the crustal assimilation proposed by Kimura et al . [2014] was erroneous in terms of Pb isotopes, nevertheless some crustal assimilation in the Karasugasen lava is evident from the chemical zoning of hornblende phenocrysts. Although the original Pb isotope argument for crustal contamination was wrong, the ABS4 modeling is unaffected because of no to subtle changes in estimated mantle source compositions in their ABS4 model. This article is protected by copyright. All rights reserved.

Description: A collection was made in January 2009 of 10 oriented samples from each of 54 sites in lavas on Sao Tome Island (nominal location 0.3º N, 6.5º E). Some sites were affected by lightning leaving a total of 42 sites for analysis of paleosecular variation. Overall magnetic properties were excellent (highly stable magnetizations carried by pseudo-single domain magnetite). After principal component analysis of progressive alternating field demagnetization data for the samples, twenty-two sites had normal polarity magnetizations (D = 0. 6º, I = -8.3º, α 95 = 4.3º, κ = 53.1) and 20 had reverse magnetizations (D = 176.0º, I = 4.2º, α 95 = 7.3º, κ = 20.8); the directions are within 5° of antiparallel, yielding a positive reversal test. The combined data set of 42 site mean virtual geomagnetic poles converted to common (normal) polarity yields a pole position at 86.0°N, 211.5ºE, A 95 =3.1º. Ar/Ar and K/Ar dating reveals that these rocks are Miocene in age (∼5–11 Ma), old enough to allow northerly plate motion to help explain the slightly far-sided pole position. The between-site dispersion in virtual geomagnetic poles was estimated as the angular standard deviation, S b , and equaled 11.4° with 95% confidence interval between 9.9° and 13.4°. This article is protected by copyright. All rights reserved.

Description: Understanding the frequency, magnitude, and nature of explosive volcanic eruptions is essential for hazard planning and risk mitigation. Terrestrial stratigraphic tephra records can be patchy and incomplete due to subsequent erosion and burial processes. In contrast the marine sedimentary record commonly preserves a more complete historical record of volcanic activity as individual events are archived within continually accumulating background sediments. While larger tephra layers are often identifiable by changes in sediment color and/or texture, smaller fallout layers may also be present that are not visible to the naked eye. These cryptotephra are commonly more difficult to identify and often require time-consuming and destructive point counting, petrography, and microscopy work. Here we present several rapid, non-destructive, and quantitative core-scanning methodologies (magnetic susceptibility, visible to shortwave infrared spectroscopy, and XRF core scanning) which, when combined, can be used to identify the presence of increased volcaniclastic components (interpreted to be cryptotephra) in the sedimentary record. We develop a new spectral parameter (BDI1000VIS) that exploits the absorption of the 1 µm near-infrared band in tephra. Using pre-determined mixtures BDI1000VIS can accurately identify tephra layers in concentrations 〉15–20%. When applied to the upper ∼270 kyr record of IODP core U1396C from the Caribbean Sea, and verified by traditional point counting, twenty-nine potential cryptotephra layers were identified as originating from eruptions of the Lesser Antilles Volcanic Arc. Application of these methods in future coring endeavors can be used to minimize the need for physical disaggregation of valuable drill core material and allow for near-real time recognition of tephra units, both visible and cryptotephra. This article is protected by copyright. All rights reserved.

Description: Numerical models show that small-scale convection (SSC) occurring atop a mantle plume is a plausible mechanism to rejuvenate the lithosphere. The triggering of SSC depends on the density contrast and on the rheology of the unstable layer underlying the stagnant upper part of the thermal boundary layer (TBL). Both properties may be changed by partial melting. We analyze, using 2D numerical simulations, how partial melting influences the dynamics of time-dependent SSC instabilities and the resulting thermo-mechanical rejuvenation of an oceanic plate moving atop of a plume. Our simulations show a complex behavior, with acceleration, no change, or delay of the SSC onset, due to competing effects of the latent heat of partial melting, which cools the plume material, and of the buoyancy increase associated with both melt retention and depletion of residue following melt extraction. The melt-induced viscosity reduction is too localized to affect significantly SSC dynamics. Faster SSC triggering is promoted for low melting degrees (low plume temperature anomalies, thick lithosphere, or fast moving plates), which limit both the temperature reduction due to latent heating and the accumulation of depleted buoyant residue to the upper part of the unstable layer. In contrast, high partial melting degrees lead to a strong temperate decrease due to latent heat of melting and development of a thick depleted layer within the sublithospheric convecting layer, which delays the development of gravitational instabilities. Despite differences in SSC dynamics, the thinning of the lithosphere is not significantly enhanced relatively to simulations that neglect partial melting. This article is protected by copyright. All rights reserved.

Description: We use ambient-noise tomography to improve CVM-H11.9, a community velocity model of southern California. Our new 3D shear-velocity model with 0.05 ° x0.05 ° lateral and 1-km vertical blocks reveals new structure beneath the San Andreas Fault (SAF), Peninsular Ranges batholith (PRB), southern Sierra Nevada batholith (SNB) and the Salton Trough (ST). We use four years of data recorded on 849 broadband stations, vastly more than previous studies and including our own broadband Salton Seismic Imaging Project, a 40-station transect across the ST, as well as other campaign stations in both Mexico and the U.S.A. Mean lower-crust and upper-mantle wavespeeds (3.6 km/s @ 20km, 4.2 km/s @ 40km) are low by global standards. Across the SAF, southeast of San Gorgonio Pass, we observe vertical to steeply-dipping lateral velocity contrasts that extend beneath the Moho. Beneath the western PRB and westernmost southern SNB, we observe relatively high shear-velocities (≥ 3.8 km/s) in the lower crust that we interpret as the mafic roots of the overlying arc. Relatively high-velocity upper-mantle (up to ∼4.5 km/s) may be part of the intact arc, or possibly a remnant of the Farallon plate. Beneath the ST, we observe zones of low shear-velocity in the lower crust and upper mantle which permit up to ∼4.5% melt in the lower crust and up to ∼6% melt in the upper mantle, depending on the assumed composition and pore geometry. Our results preclude the existence of older continental crust beneath the ST and support the creation of new crust beneath the ST. This article is protected by copyright. All rights reserved.

Description: This paper develops a Monte Carlo algorithm for extracting three-dimensional lithospheric density models from geophysical data. Empirical scaling relationships between velocity and density create a 3D starting density model, which is then iteratively refined until it reproduces observed gravity and topography. This approach permits deviations from uniform crustal velocity-density scaling, which provide insight into crustal lithology and prevent spurious mapping of crustal anomalies into the mantle. We test this algorithm on the Proterozoic Midcontinent Rift (MCR), north-central U.S. The MCR provides a challenge because it hosts a gravity high overlying low shear-wave velocity crust in a generally flat region. Our initial density estimates are derived from a seismic velocity/crustal thickness model based on joint inversion of surface-wave dispersion and receiver functions. By adjusting these estimates to reproduce gravity and topography, we generate a lithospheric-scale model that reveals dense middle crust and eclogitized lowermost crust within the rift. Mantle lithospheric density beneath the MCR is not anomalous, consistent with geochemical evidence that lithospheric mantle was not the primary source of rift-related magmas and suggesting that extension occurred in response to far-field stress rather than a hot mantle plume. Similarly, the subsequent inversion of normal faults resulted from changing far-field stress that exploited not only warm, recently faulted crust but also a gravitational potential energy low in the MCR. The success of this density modeling algorithm in the face of such apparently contradictory geophysical properties suggests that it may be applicable to a variety of tectonic and geodynamic problems. This article is protected by copyright. All rights reserved.

Description: The volcanic Galápagos Islands are constructed upon a broad platform, with their active westernmost islands marking the current position of the hotspot. Built upon young oceanic lithosphere (〈15 Ma), this platform exhibits unique morphologic features including a system of stepped terraces on the southwestern escarpment with 3 km-relief, contrasting with gentle slopes off the eastern platform toward the Carnegie Ridge. Considering horizontal lithostatic pressure differences associated with this relief, along with high temperatures within the young, hotspot-affected lithosphere, it is likely that lower crustal flow contributes significantly to crustal deformation within the Galápagos platform. Using a 2D, isostatic, thin-sheet approximation for the Stokes flow equation with (Newtonian) space-time dependent viscosity, we suggest that the bathymetric rim along the eastern platform region (where gravimetry indicates Airy isostasy) near Española Island may be the expression of a mature lower crustal flow front developed over the last ∼3 Myr; horizontal mass displacements (∼50 km) associated with this crustal flow episode may have advected mantle plume geochemical signatures towards the southeast, and in directions not necessarily parallel to the hotspot track. Also, the stepped terraces along the southwestern platform may be explained by lower crustal flow-associated backward tilting of the bathymetric surface that, although resulting in small angular changes (∼0.1 deg), effectively hinders the horizontal flow of lava sheets. This backward-tilting process was likely restricted to the last ∼1 Ma or less, and may be a unique event involving extrusion of lavas from within the southwestward-marching lower-crustal flow front. This article is protected by copyright. All rights reserved.

Description: Melt extraction from the Earth's mantle through high-porosity channels is required to explain the composition of the oceanic crust. Feedbacks from reactive melt transport are thought to localize melt into a network of high-porosity channels. Recent studies invoke lithological heterogeneities in the Earth's mantle to seed the localization of partial melts. Therefore, it is necessary to understand the reaction fronts that form as melt flow across the lithological interface between the heterogeneity and the ambient mantle. Here, we present a chromatographic analysis of reactive melt transport across lithological boundaries, using the theory of hyperbolic conservation laws. This is an extension of linear trace element chromatography to the coupling of major elements and energy transport. Our analysis allows the prediction of the non-linear feedbacks that arise in reactive melt transport due to changes in porosity. This study considers the special case of a partially molten porous medium with binary solid solution. As melt traverses a lithological contact, binary solid solution leads to the formation of a reacted zone between an advancing reaction front and the initial contact. The analysis also shows that the behavior of a fertile heterogeneity depends on its absolute concentration, in addition to compositional differences between itself and the refractory background. We present a regime diagram that predicts if melt emanating from a fertile heterogeneity localizes into high porosity channels or develops a zero porosity shell. The theoretical framework presented here provides a useful tool for understanding non-linear feedbacks in reactive melt transport, because it can be extended to more complex and realistic phase behaviors. This article is protected by copyright. All rights reserved.

Description: This study images the upper mantle beneath the D'Entrecasteax Islands, Papua New Guinea, providing insight into mantle deformation beneath a highly rifted continent adjacent to propagating spreading centers. Differential travel times from P - and S -wave teleseisms recorded during the 2010-2011 CDPapua passive seismic experiment are used to invert for separate V P and V S velocity models of the continental rift. A low-velocity structure marks the E-W axis of the rift, correlating with the thinnest crust, high heat flow, and a linear trend of volcanoes. This slow region extends 250 km along strike from the oceanic spreading centers, demonstrating significant mantle extension ahead of seafloor breakup. The rift remains narrow to depth indicating localization of extension, perhaps as a result of mantle hydration. A high- V P structure at depths of 90-120 km beneath the north of the array is more than 6.5% faster than the rift axis and contains well-located intermediate depth earthquakes. These independent observations place firm constraints on the lateral thermal contrast at depth between the rift axis and cold lithosphere to the north that may be related to recent subduction, although the polarity of subduction cannot be resolved. This geometry is gravitationally unstable; downwelling or small-scale convection could have facilitated rifting and rapid lithospheric removal, although this may require a wet mantle to be realistic on the required timescales. The high- V structure agrees with the maximum P,T conditions recorded by young ultra-high pressure rocks exposed on the rift axis and may be implicated in their genesis. This article is protected by copyright. All rights reserved.

Description: The last supereruption from the Yellowstone Plateau formed Yellowstone caldera and ejected the 〉1000 km 3 of rhyolite that composes the Lava Creek Tuff. Tephra from the Lava Creek eruption is a key Quaternary chronostratigraphic marker, in particular for dating the deposition of mid-Pleistocene glacial and pluvial deposits in western North America. To resolve the timing of eruption and crystallization history for the Lava Creek magma, we performed (1) 40 Ar- 39 Ar dating of single sanidine crystals to delimit eruption age and (2) ion microprobe U-Pb and trace-element analyses of the crystal faces and interiors of single zircons to date the interval of zircon crystallization and characterize magmatic evolution. Sanidines from the two informal members composing Lava Creek Tuff yield a preferred 40 Ar/ 39 Ar isochron date of 631.3 ± 4.3 ka. Crystal faces on zircons from both members yield a weighted mean 206 Pb/ 238 U date of 626.5 ± 5.8 ka, and have trace element concentrations that vary with the eruptive stratigraphy. Zircon interiors yield a mean 206 Pb/ 238 U date of 659.8 ± 5.5 ka, and reveal reverse and/or oscillatory zoning of trace element concentrations, with many crystals containing high U concentration cores that likely grew from highly evolved melt. The occurrence of distal Lava Creek tephra in stratigraphic sequences marking the marine isotope stage 16—15 transition supports the apparent eruption age of ca. 631 ka. The combined results reveal that Lava Creek zircons record episodic heating, renewed crystallization, and an overall up-temperature evolution for Yellowstone's subvolcanic reservoir in the 10 3 —10 4 interval before eruption. This article is protected by copyright. All rights reserved.

Description: The implementation of isotopic tracers as constraints on source contributions has become increasingly relevant to understanding Earth surface processes. Interpretation of these isotopic tracers has become more accessible with the development of Bayesian Monte Carlo (BMC) mixing models, which allow uncertainty in mixing end-members and provide methodology for systems with multi-component mixing. This study presents an open source multiple isotope BMC mixing model that is applicable to Earth surface environments with sources exhibiting distinct end-member isotopic signatures. Our model is first applied to new δ 18 O and δD measurements from the Athabasca Glacier, which showed expected seasonal melt evolution trends and vigorously assessed the statistical relevance of the resulting fraction estimations. To highlight the broad applicability of our model to a variety of Earth surface environments and relevant isotopic systems we expand our model to two additional case studies: deriving melt sources from δ 18 O, δD and 222 Rn measurements of Greenland Ice Sheet bulk water samples and assessing nutrient sources from ϵ Nd and 87 Sr/ 86 Sr measurements of Hawaiian soil cores. The model produces results for the Greenland Ice Sheet and Hawaiian soil datasets that are consistent with the originally published fractional contribution estimates. The advantage of this method is that it quantifies the error induced by variability in the end-member compositions, unrealized by the models previously applied to the above case studies. Results from all three case studies demonstrate the broad applicability of this statistical BMC isotopic mixing model for estimating source contribution fractions in a variety of Earth surface systems. This article is protected by copyright. All rights reserved.

Description: Geologically rapid (10s of Myr) partial removal of thick continental lithosphere is evident beneath Precambrian terranes such as North China craton, southern Africa and the North Atlantic Craton, and has been linked with thermo-mechanical erosion by mantle plumes. We performed numerical experiments with realistic viscosities to test this hypothesis and constrain the most important parameters that influence cratonic lithosphere erosion. Our models indicate that the thermo-mechanical erosion by a plume impact on typical Archean lithospheric mantle is unlikely to be more effective than long-term erosion from normal plate-mantle interaction. Therefore, unmodified cratonic roots that have been stable for billions of years will not be significantly disrupted by the erosion of a plume event. However, the buoyancy and strength of highly depleted continental roots can be modified by fluid-melt metasomatism, and our models show this is essential for the thinning of originally stable continental roots. The long-term but punctuated history of metasomatic enrichment beneath ancient continents makes this mode of weakening very likely. The effect of the plume impact is to speed up the erosion significantly and help the removal of the lithospheric root to occur within 10s of Myrs if affected by metasomatic weakening. This article is protected by copyright. All rights reserved.

Description: This study represents the first multi-station investigation of azimuthal anisotropy beneath the interior of north central Africa, including Libya and adjacent regions, using shear wave splitting (SWS) analysis. Data used in the study include recently available broadband seismic data obtained from 15 stations managed by the Libyan Center for Remote Sensing and Space Science, and those from 5 other stations at which data are publicly accessible. A total of 583 pairs of high-quality SWS measurements utilizing the PKS, SKKS, and SKS phases demonstrate primarily N-S fast orientations with an average splitting delay time of approximately 1.2 s. An absence of periodic azimuthal variation of the observed splitting parameters indicates the presence of simple anisotropy, and lack of correlation between surficial features and the splitting parameters suggests that the origin of the observed anisotropy is primarily asthenospheric. This conclusion is enhanced by non-periodic azimuthal variation of the splitting parameters observed at one of the stations located near the boundary of areas with different anisotropic properties. We interpret the observed anisotropy to be the consequence of northward movement of the African plate relative to the asthenosphere toward the Hellenic and Calabrian subduction zones. Local variance in fast orientations may be attributable to flow deflection by the northern edge of the African continental root. The observations provide critical and previously lacking constraints on mantle dynamic models in the vicinity of the convergent boundary between the African and Eurasian plates. This article is protected by copyright. All rights reserved.

Description: A new digital bathymetric model (DBM) for the Northeast Greenland (NEG) continental shelf (74°N to 81°N) is presented. The DBM has a grid cell size of 250 m x 250 m and incorporates bathymetric data from 30 multibeam cruises, more than 20 singlebeam cruises and first reflector depths from industrial seismic lines. The new DBM substantially improves the bathymetry compared to older models. The DBM not only allows a better delineation of previously known seafloor morphology but, in addition, reveals the presence of previously unmapped morphological features including glacially-derived troughs, fjords, grounding-zone wedges and lateral moraines. These submarine landforms are used to infer the past extent and ice-flow dynamics of the Greenland Ice Sheet during the last full-glacial period of the Quaternary and subsequent ice retreat across the continental shelf. The DBM reveals cross-shelf bathymetric troughs that may enable the inflow of warm Atlantic water masses across the shelf, driving enhanced basal melting of the marine-terminating outlet glaciers draining the ice sheet to the coast in Northeast Greenland. Knolls, sinks and hummocky seafloor on the middle shelf are also suggested to be related to salt diapirism. North-south orientated elongate depressions are identified that probably relate to ice-marginal processes in combination with erosion caused by the East Greenland Current. A single guyot-like peak has been discovered and is interpreted to have been produced during a volcanic event approximately 55 Ma ago. This article is protected by copyright. All rights reserved.

Description: Carbonate veins recovered from the mafic/ultramafic footwall of an oceanic detachment fault on the Mid-Atlantic Ridge record multiple episodes of fluid movement through the detachment and secondary faults. High-temperature (∼75-175°C) calcite veins with elevated REE contents and strong positive Eu-anomalies record the mixing of up-welling hydrothermal fluids with infiltrating seawater. Carbonate precipitation is most prominent in olivine-rich troctolite, which also display a much higher degree of greenschist and sub-greenschist alteration relative to gabbro and diabase. Low-temperature calcite and aragonite veins likely precipitated from oxidizing seawater that infiltrated the detachment fault and/or within secondary faults late or post footwall denudation. Oxygen and carbon isotopes lie on a mixing line between seawater and Logatchev-like hydrothermal fluids, but precipitation temperatures are cooler than would be expected for isenthalpic mixing, suggesting conductive cooling during upward flow. There is no depth dependence of vein precipitation temperature, indicating effective cooling of the footwall via seawater infiltration through fault zones. One sample contains textural evidence of low-temperature, seawater-signature veins being cut by high-temperature, hydrothermal-signature veins. This indicates temporal variability in the fluid mixing, possibly caused by deformation-induced porosity changes or dike intrusion. The strong correlation between carbonate precipitation and olivine-rich troctolites suggests that the presence of unaltered olivine is a key requirement for carbonate precipitation from seawater and hydrothermal fluids. Our results also suggest that calcite-talc alteration of troctolites may be a more efficient CO 2 trap than serpentinized peridotite. This article is protected by copyright. All rights reserved.

Description: In nature, subducting slabs and overriding plate segments bordering subduction zones are generally embedded within larger plates. Such large plates can impose far-field boundary conditions that influence the style of subduction and overriding plate deformation. Here we present dynamic laboratory models of progressive subduction in three-dimensional space, in which the far-field boundary conditions at the trailing edges of the subducting plate (SP) and overriding plate (OP) are varied. Four configurations are presented: Free (both plates free), SP-Fixed, OP-Fixed and SP-OP-Fixed. We investigate their impact on the kinematics and dynamics of subduction, particularly focusing on overriding plate deformation. The results indicate that the variation in far-field boundary conditions has an influence on the slab geometry, subduction partitioning and trench migration partitioning. Our models also indicate that in natural (narrow) subduction zones, assuming a homogeneous overriding plate, the formation of backarc basins (e.g., Tyrrhenian Sea, Aegean Sea, Scotia Sea) is generally expected to occur at a comparable location (250-700 km from the trench), irrespective of the boundary condition. In addition, our models indicate that the style of forearc deformation (shortening or extension) is influenced by the mobility of the overriding plate through controlling the force normal to the subduction zone interface (trench suction). Our geodynamic model that uses the SP-OP-Fixed setup is comparable to the Calabria subduction zone with respect to subduction kinematics, slab geometry, trench curvature and accretionary configuration. Furthermore, the model can explain backarc and forearc extension at the Calabria subduction zone since the latest Middle Miocene as a consequence of subduction of the narrow Calabrian slab and the immobility of the subducting African plate and overriding Eurasian plate. This setting induced strong trench suction, driving forearc extension, and forced subduction to be accommodated almost entirely by slab rollback (not trenchward subducting plate motion), while trench retreat was accommodated almost entirely by backarc and forearc extension (not trenchward overriding plate motion), comparable to our SP-OP-Fixed model. This article is protected by copyright. All rights reserved.

Description: The Woodlark seafloor spreading center is propagating westwards into the Australian plate near the D'Entrecasteaux Islands (DI), Papua New Guinea, generating an active transition zone from continental rifting to seafloor spreading. From March 2010 to July 2011, we deployed 31 inland and 8 off-shore broadband seismic stations around the DI region, to explore the dynamic processes of the lithosphere extension and the exhumation of the high-pressure terranes exposed on those islands. We measure the multi-band (10-60 s) Rayleigh-wave phase velocities from both ambient-noise and earthquake signals. These measurements are then inverted for a three-dimensional shear-velocity model for the crust and upper mantle. The results indicate that the lithosphere extension is localized near the rift axis beneath the DI, with a shear-velocity structure in the upper mantle that is similar to mid-ocean ridges. Beneath the Kiribisi Basin west of DI, an ultra-slow shear-velocity anomaly ($\sim$4.0 km/s) is observed at shallow mantle depth (30-60 km), which can be interpreted either by the presence of excess partial melt due to slow melt extraction, or by the existence of felsic crustal material buried to mantle depth and not yet exhumed. This article is protected by copyright. All rights reserved.

Description: The Baltic Sea has experienced three major intervals of bottom water hypoxia following the intrusion of seawater ca. 8 kyrs ago. These intervals occurred during the Holocene Thermal Maximum (HTM), Medieval Climate Anomaly (MCA) and during recent decades. Here, we show that sequestration of both Fe and Mn in Baltic Sea sediments generally increases with water depth, and we attribute this to shelf-to-basin transfer (“shuttling”) of Fe and Mn. Burial of Mn in slope and basin sediments was enhanced following the lake-brackish/marine transition at the beginning of the hypoxic interval during the HTM. During hypoxic intervals, shelf-to-basin transfer of Fe was generally enhanced but that of Mn was reduced. However, intensification of hypoxia within hypoxic intervals led to decreased burial of both Mn and Fe in deep basin sediments. This implies a non-linearity in shelf Fe release upon expanding hypoxia with initial enhanced Fe release relative to oxic conditions followed by increased retention in shelf sediments, likely in the form of iron sulfide minerals. For Mn, extended hypoxia leads to more limited sequestration as Mn carbonate in deep basin sediments, presumably because of more rapid reduction of Mn oxides formed after inflows and subsequent escape of dissolved Mn to the overlying water. Our Fe records suggest that modern Baltic Sea hypoxia is more widespread than in the past. Furthermore, hypoxia-driven variations in shelf-to-basin transfer of Fe may have impacted the dynamics of P and sulfide in the Baltic Sea thus providing potential feedbacks on the further development of hypoxia. This article is protected by copyright. All rights reserved.

Description: The closure of the Central American Seaway is linked with tectonic and magmatic processes that have controlled the evolution of the Isthmus of Panama. We focus on the terminal stages of arc activity in the Central Panama region, and present new geochemical data from ∼9 Ma explosive silicic volcanism preserved in three syngenetic tuff beds from the Gatun Formation. The magmatic evolution of the Gatun Formation is controlled by a series of magma mushes where pyroxene is the dominant early forming mafic mineral, with amphibole appearing only relatively late in the fractionation sequence. Our data shows Gatun lavas exhibit a strong subduction signature, consistent with plate reconstruction models showing arc-normal subduction from Costa Rica to Panama pre-8.5 Ma. However, large ion lithophile elements are depleted in the Gatun Formation in comparison to other regional suites, indicative of a lower flux of subduction fluid to the Gatun Formation mantle source, which is explained by a shift towards magma generation by decompression following the collision of the arc with South America. Oblique subduction commencing ∼8.5 Ma resulted in the shutdown of normal arc activity throughout Panama. We interpret subsequent regional Quaternary adakitic volcanism as a response to this oblique subduction. The now more refractory mantle wedge required greater fluid flux in order to melt. The resultant volatile-rich melts were more prone to deep fractionation of amphibole and garnet cumulates forming adakites. Deep fractionation was potentially enhanced by changing stress regimes on the upper-plate caused by oblique subduction. This article is protected by copyright. All rights reserved.

Description: Multikilometer-sized slivers of high-pressure low-temperature metamorphic oceanic crust and mantle are observed in many mountain belts. These blueschist and eclogite units were detached from the descending plate during subduction. Large-scale thermo-mechanical numerical models based on finite difference marker-in-cell staggered grid technique are implemented to investigate slicing processes that lead to the detachment of oceanic slivers and their exhumation before the onset of the continental collision phase. In particular, we investigate the role of the serpentinized sub-crustal slab mantle in the mechanisms of shallow and deep crustal slicing. Results show that spatially homogeneous serpentinization of the sub-Moho slab mantle leads to complete accretion of oceanic crust within the accretionary wedge. Spatially discontinuous serpentinization of the slab mantle in form of unconnected patches can lead to shallow slicing of the oceanic crust below the accretionary wedge and to its deep slicing at mantle depths depending on the patch length, slab angle, convergence velocity and continental geothermal gradient. P-T paths obtained in this study are compared to natural examples of shallow slicing of the Crescent Terrane below Vancouver Island and deeply sliced crust of the Lago Superiore and Saas-Zermatt units in the Western Alps. This article is protected by copyright. All rights reserved.

Description: The chromium isotope compositions of 27 metamorphic mafic rocks with varying metamorphic degrees from eastern China were systematically measured to investigate the Cr isotope behavior during continental crust subduction. The Cr isotope compositions of all samples studied were Bulk Silicate Earth (BSE)-like, with δ 53 Cr NIST979 of greenschists, amphibolites and eclogites ranging from -0.06‰ to -0.17‰, -0.05‰ to -0.27‰, and -0.01‰ to -0.24‰, respectively. The lack of resolvable isotopic variability among the metamorphic rocks from different metamorphic zones indicated that no systematic Cr isotope fractionation was associated with the degree of metamorphism. However, the Cr isotopic variability among homologous samples may have reflected effects induced by metamorphic dehydration with a change of redox state, rather than protolith heterogeneity (i.e., magma differentiation). In addition, the differences in δ 53 Cr (Δ 53 Cr Cpx-Gt ) between co-existing clinopyroxene (Cpx) and garnet (Gt) from two garnet pyroxenites were 0.06‰ and 0.34‰, indicating that significant inter-mineral Cr isotope disequilibria could occur during metamorphism. To provide a basis for comparison with metamorphic rocks and to provide further constraints on the potential Cr isotope heterogeneity in the mantle and the protolith of some metamorphic rocks, we analyzed mantle-derived chromites and the associated peridotites from Luobusa, and we obtained the following general order: chromite-free peridotites (-0.21‰ to -0.11‰) 〈 chromite-bearing peridotite (-0.07‰) 〈 chromite (-0.06‰). These findings imply potential isotope fractionation between chromite and melt. This article is protected by copyright. All rights reserved.

Description: The three largest Large Igneous Provinces (LIP) of the western Pacific – Ontong Java, Manihiki and Hikurangi plateaus – were emplaced during the Cretaceous Normal Superchron and show strong similarities in their geochemistry and petrology. The plate tectonic relationship between those LIPs, herein referred to as Ontong Java Nui, is uncertain, but a joined emplacement was proposed by Taylor [2006]. Since this hypothesis is still highly debated and struggles to explain features such as the strong differences in crustal thickness between the different plateaus, we revisited the joined emplacement of Ontong Java Nui in light of new data from the Manihiki Plateau. By evaluating seismic refraction/wide-angle reflection data along with seismic reflection records of the margins of the proposed “Super”-LIP, a detailed scenario for the emplacement and the initial phase of break-up has been developed. The LIP is a result of an interaction of the arriving plume head with the Phoenix-Pacific spreading ridge in the early Cretaceous. The break-up of the LIP shows a complicated interplay between multiple micro-plates and tectonic forces such as rifting, shearing and rotation. Our plate kinematic model of the western Pacific incorporates new evidence from the break-up margins of the LIPs, the tectonic fabric of the seafloor as well as previously published tectonic concepts such as the rotation of the LIPs. The updated rotation poles of the western Pacific allow a detailed plate tectonic reconstruction of the region during the Cretaceous Normal Superchron and highlights the important role of LIPs in the plate tectonic framework. This article is protected by copyright. All rights reserved.

Description: We use numerical models to investigate the effects of mineral grain size variation on fluid migration in the mantle wedge at subduction zones and on the location of the volcanic arc. Previous coupled thermal-grain size evolution (T-GSE) models predict small grain size (〈 1 mm) in the corner flow of the mantle wedge, a down-dip grain size increase by ∼two orders of magnitude along the base of the mantle wedge, and finer grain size in the mantle wedge for colder-slab subduction zones. We integrate these T-GSE modeling results with a fluid migration model, in which permeability depends on grain size, and fluid flow through a moving mantle matrix is driven by fluid buoyancy and dynamic pressure gradients induced by mantle flow. Our modeling results indicate that fluids introduced along the base of the mantle wedge beneath the forearc are initially dragged down-dip by corner flow due to the small grain size and low permeability immediately above the slab. As grain size increases with depth, permeability increases, resulting in upward fluid migration. Fluids released beneath the arc and the backarc are also initially dragged down-dip, but typically are not transported as far laterally before they begin to travel upward. As the fluids rise through the backarc mantle wedge, they become deflected towards the trench due to the effect of mantle inflow. The combination of down-dip migration in the forearc and trench-ward migration in the backarc results in pathways that focus fluids to the arc. This article is protected by copyright. All rights reserved.

Description: Tracking marine sediment provenance (e.g., of dust, ash, hydrothermal material, etc.) provides insight into contemporary ocean processes and helps construct paleoceanographic records. In a simple system with only a few end-members that can be easily quantified by a unique chemical or isotopic signal, chemical ratios and normative calculations can help quantify the flux of sediment from the few sources. In a more complex system (e.g., each element comes from multiple sources), more sophisticated mixing models are required. MATLAB codes published in Pisias et al . (2013) solidified the foundation for application of a constrained least square (CLS) multiple linear regression technique that can use many elements and several end-members in a mixing model. However, rigorous sensitivity testing to check the robustness of the CLS model is time- and labor intensive. MATLAB codes provided in this paper reduce the time and labor involved and facilitate finding a robust and stable CLS model. By quickly comparing the goodness of fit between thousands of different end member combinations, users are able to identify trends in the results that reveal the CLS solution uniqueness and the end-member composition precision required for a good fit. Users can also rapidly check that they have the appropriate number and type of end-members in their model. In the end, these codes improve the user's confidence that the final CLS model(s) they select are the most reliable solutions. These advantages are demonstrated by application of the codes in two case studies of well-studied datasets (Nazca Plate, South Pacific Gyre). This article is protected by copyright. All rights reserved.

Description: This paper investigates the causes of a brief, but prominent, cooling episode (1.1–0.8 Ma) that occurred in the equatorial upwelling region of the Atlantic and Pacific during the mid-Pleistocene Transition (MPT) using temporal changes in dust provenance, regional hydrology, and surface productivity recorded in a deep-sea sediment core from the central equatorial Pacific. The 87 Sr/ 86 Sr and ɛ Nd values of the inorganic silicate fraction indicate deposition of dust from Australia and Central/South America before 0.8 Ma, but a gradual increase in Asian dust deposition after 0.8 Ma. The change in dust provenance was accompanied by an increased dust flux and a decrease in surface productivity and salinity. These changes can be explained by the southward movement of the Intertropical Convergence Zone (ITCZ) and North Equatorial Counter Current (NECC) and the direct influence of these features on the site after 0.8 Ma. Our results, together with previously published Atlantic data, suggest the northward position of the ITCZ between 1.1 and 0.9 Ma, and the southward position thereafter. The meridional movement of the ITCZ is in phase with the cooling and warming trend in upwelling regions in the equatorial Pacific and Atlantic, which suggests strengthening of southeast trades relative to its northern counterpart between 1.1 and 0.9 Ma as a plausible cause of this brief cooling event. The southward movement of the ITCZ from 0.9 to 0.8 Ma indicates more significant cooling in the Northern Hemisphere (NH) than in the Southern Hemisphere, which is supportive of the interpretation that the NH ice sheet expanded significantly and stabilized after 0.9 Ma. This article is protected by copyright. All rights reserved.

Description: Oceanic detachment faults are increasingly recognised as playing an integral role in the seafloor spreading process at slow and ultraslow spreading mid-ocean ridges, with significant consequences for the architecture of the oceanic lithosphere. Although melt supply is considered to play a critical control on the formation and evolution of oceanic detachments, much less well understood is how melts and faults interact and influence each other. Few direct constraints on the locus and depth of melt emplacement in the vicinity of detachments are available. Gabbros drilled in ODP Hole 923A near the intersection of the Mid-Atlantic Ridge and the Kane transform fault (23°N; the MARK area) represent magmas emplaced into the footwall of such a detachment fault and unroofed by it. We here present U-Pb zircon dates for these gabbros and associated diorite veins which, when combined with a tectonic reconstruction of the area, allow us to calculate the depths at which the melts crystallized. Th-corrected single zircon U-Pb dates from three samples range from 1.138 ± 0.062 Ma to 1.213 ± 0.021 Ma. We find a crystallization depth of 6.4 +1.7/-1.3 km, and estimate that the melts parental to the gabbros were initially emplaced up to 1.5 km deeper, at 〈8 km below the seafloor. The tectonic reconstruction implies that the detachment fault responsible for the exposure of the sampled sequence likely crossed the ridge axis at depth, suggesting that melt emplacement into the footwall of oceanic detachment faults is an important process. The deep emplacement depth we find associated with ‘detachment mode' spreading at ∼1.2 Ma appears to be significantly greater than the depth of magma reservoirs during the current ‘magmatic mode' of spreading in the area, suggesting that the northern MARK segment preserves a recent switch between two temporally distinct modes of spreading with fundamentally different lithospheric architecture. This article is protected by copyright. All rights reserved.

Description: The processes that are involved in migration and extraction of melt from the mantle are not yet fully understood. Gaining a better understanding of material properties of partially molten rock could help shed light on the behavior of melt on larger scales in the mantle. In this study, we simulate three-dimensional torsional deformation of a partially molten rock that contains a rigid, spherical inclusion. We compare the computed porosity patterns to those found in recent laboratory experiments. The laboratory experiments show emergence of melt-rich bands throughout the rock sample, and pressure shadows around the inclusion. The numerical model displays similar melt-rich bands only for a small bulk-to-shear-viscosity ratio (five or less). The results are consistent with earlier two-dimensional numerical simulations; however, we show that it is easier to form melt-rich bands in three dimensions compared to two. The addition of strain-rate dependence of the viscosity causes a distinct change in the shape of pressure shadows around the inclusion. This change in shape presents an opportunity for experimentalists to identify the strain-rate dependence and therefore the dominant deformation mechanism in torsion experiments with inclusions. This article is protected by copyright. All rights reserved.

Description: ABSTRACT The Semidi segment of the Alaska convergent margin appears capable of generating a giant tsunami like the one produced along the nearby Unimak segment in 1946. Reprocessed legacy seismic reflection data and a compilation of multibeam bathymetric surveys reveal structures that could generate such a tsunami. A 200 km long ridge or escarpment with crests 〉1 km high is the surface expression of an active out-of-sequence fault zone, recently referred to as a splay fault. Such faults are potentially tsunamigenic. This type of fault zone separates the relatively rigid rock of the margin framework from the anelastic accreted sediment prism. Seafloor relief of the ridge exceeds that of similar age accretionary prism ridges indicating preferential slip along the splay fault zone. The greater slip may derive from Quaternary subduction of the Patton Murray hot spot ridge that extends 200 km toward the east across the north Pacific. Estimates of tsunami repeat times from paleotsunami studies indicate that the Semidi segment could be near the end of its current inter-seismic cycle. GPS records from Chirikof Island at the shelf edge indicate 90% locking of plate interface faults. An earthquake in the shallow Semidi subduction zone could generate a tsunami that will inundate the US west coast more than the 1946 and 1964 earthquakes because the Semidi continental slope azimuth directs a tsunami southeastward. This article is protected by copyright. All rights reserved.

Description: Arc volcanism, volatile cycling, mineralisation and continental crust formation are likely regulated by the mantle wedge's flow regime and thermal structure. Wedge flow is often assumed to follow a regular corner-flow pattern. However, studies that incorporate a hydrated rheology and thermal buoyancy predict internal small-scale-convection (SSC). Here, we systematically explore mantle-wedge dynamics in 3-D simulations. We find that longitudinal ‘Richter-rolls’ of SSC (with trench-perpendicular axes) commonly occur if wedge hydration reduces viscosities to $\lesssim1\cdot10^{19}$Pa s, although transient transverse rolls (with trench-parallel axes) can dominate at viscosities of $\sim 5\cdot10^{18} - 1\cdot10^{19}$Pa s. Rolls below the arc and back-arc differ. Sub-arc rolls have similar trench-parallel and trench-perpendicular dimensions of 100–150 km and evolve on a 1–5 Myr time-scale. Sub-back-arc instabilities, on the other hand, coalesce into elongated sheets, usually with a preferential trench-perpendicular alignment, display a wavelength of 150–400 km and vary on a 5–10 Myr time-scale. The modulating influence of sub-back-arc ridges on the sub-arc system increases with stronger wedge hydration, higher subduction velocity and thicker upper plates. We find that trench-parallel averages of wedge velocities and temperature are consistent with those predicted in 2-D models. However, lithospheric thinning through SSC is somewhat enhanced in 3-D, thus expanding hydrous melting regions and shifting dehydration boundaries. Sub-arc Richter-rolls generate time-dependent trench-parallel temperature variations of up to $\sim150$K, which exceed the transient 50–100 K variations predicted in 2–D and may contribute to arc-volcano spacing and the variable seismic velocity structures imaged beneath some arcs. This article is protected by copyright. All rights reserved.