ALBERT

Description: The composite Arctic Alaska–Chukotka terrane plays a central role in tectonic reconstructions of the Arctic. An exotic, non-Laurentian origin of Arctic Alaska–Chukotka has been proposed based on paleobiogeographic faunal affinities and various geochronological constraints from the southwestern portions of the terrane. Here, we report early Paleozoic trilobite and conodont taxa that support a Laurentian origin for the North Slope subterrane of Arctic Alaska, as well as new Neoproterozoic–Cambrian detrital zircon geochronological data, which are both consistent with a Laurentian origin and profoundly different from those derived from similar-aged strata in the southwestern subterranes of Arctic Alaska–Chukotka. The North Slope subterrane is accordingly interpreted as allochthonous with respect to northwestern Laurentia, but it most likely originated farther east along the Canadian Arctic or Atlantic margins. These data demonstrate that construction of the composite Arctic Alaska–Chukotka terrane resulted from juxtaposition of the exotic southwestern fragments of the terrane against the northern margin of Laurentia during protracted Devonian(?)–Carboniferous tectonism.

Description: In the Wet Mountains of central Colorado, we document evidence for increasing metamorphic grade and associated higher amounts of partial melting along a transect from northwest to southeast. Field observations of structural orientation and style, qualitative assessment of strain intensity, analysis of metamorphic mineral assemblages, and macroscopic identification of leucosomes and migmatites are complemented by the use of melt microstructures to carefully document the presence and locations of former partial melt and to identify melt-producing reactions. In the northwest Wet Mountains, migmatitic foliation is moderately well developed, and partial melting occurred via granite wet melting and muscovite-dehydration melting, with rare melt pseudomorphs remaining. At Dawson Mountain in the central part of the range, inferred former melt channels are preserved along grain and subgrain boundaries, deformation appears more intense, and anatexis occurred through biotite-dehydration melting. Farthest to the south, the highest intensity of strain is inferred, with very closely spaced foliations, abundant dynamic recrystallization, and local mylonitization occurring in rocks of granitic composition, and partial melting occurring via granite wet melting. Metapelitic rocks experienced biotite-dehydration melting and contain garnet with Mn-rich rims and Mn-poor cores mantled by plagioclase, decussate biotite, and quartz, textures indicating back-reaction between melt and garnet. These textures indicate there was abundant melt within these highest-grade rocks and extensive melt-rock interaction. Throughout the Wet Mountains, deformation is concentrated in areas where melt-producing reactions occurred, and melt appears to be localized along deformation-related features, suggesting a correlation between partial melting and deformation. The northern Wet Mountains contain few plutons, whereas the central and southern portions of the Wet Mountains contain more pervasive dikes and sills and may contain more former melt as a result of both higher metamorphic grade and widespread thermal insulation. The Wet Mountains represent an exhumed section of formerly molten middle crust located at the transition between upper and lower crust and provide insight into processes ongoing at depth in modern orogenic belts. The microstructures indicative of former partial melt, textures associated with melt-rock interaction, and melting reactions we have identified in the Wet Mountains will greatly facilitate the recognition of other such exhumed sections.

Description: Exposures of continental lower crust provide fundamental constraints on the thermal-mechanical behavior of continental lithosphere during orogeny. The applicability of field-based results, however, requires knowledge of whether these data pertain to deformation during lower-crustal residence or during uplift and exhumation of deep crust. Dating synkinematic monazite-producing reactions provides one way to evaluate deformation styles in the deep crust. We report on the implications of monazite reaction dating for the timing of fabric formation and movement along three crustal-scale shear zones in northern Saskatchewan, western Canadian Shield. The structures accommodated dextral transpressive strain during oblique- and thrust-sense displacement that was coeval with uplift and exhumation of 〉20,000 km 2 of continental lower crust (〉1.0 GPa) to middle-crustal levels (〈0.5 GPa). In situ Th-U–total Pb monazite data reveal that monazite rims in all three shear zones grew synkinematically at 1849 ± 6 Ma (2, mean square of weighted deviates = 0.8). The style of deformation involved localized strain concurrent with segmentation and translation of rheologically strong blocks of deep crust along mutually interacting shear zones during transpression.

Description: The exhumation mechanisms of deep-seated continental crust can be constrained by analyzing the structural and metamorphic imprints left in lithological ensembles. The Santa María de la Alameda dome formed during the collision of Gondwana and Laurussia in late Paleozoic time and is located in the Central Iberian Zone of the Iberian Massif (Spain). Rocks of the dome are part of the autochthonous Gondwanan sections of the Variscan belt, and they occur in the Variscan hinterland. The lithostratigraphy of the dome consists of metasedimentary rocks alternating with orthogneiss massifs showing irregular and sinuous structure. The metamorphic record indicates peak pressures indicative of lower-crust depths followed by isothermal decompression to middle-upper-crust levels. Exhumation resulted in the exposure of different crustal levels (represented by subsolidus vs. supersolidus mineral assemblages). The exhumation was accompanied by initial layer-parallel stretching and subsequent large-scale isoclinal folding developed in a heterogeneous, flat-lying shear zone with top-to-the-SE kinematics. SE-directed shearing and lateral extensional flow occurred in response to thermomechanical disequilibrium of previously thickened orogenic crust, probably assisted by coeval accretion of tectonic slices and lithospheric bending about a vertical axis. Positive feedback among partial melting, exhumation, and crustal attenuation resulted in the formation of a NE-SW–trending, migmatite-cored dome, and in refolding of early isoclinal folds and an associated axial surface regional foliation. The dome formed beneath a set of extensional detachments and was reshaped by WNW-ESE upright folds during later convergent deformation. The latter event brought in further instabilities throughout the belt, triggering in this region the development of a late extensional detachment under low-grade metamorphic conditions (top-to-the-S kinematics). The development of a regional train of flat-lying isoclinal folds is presented here as the macrostructural expression of the combination of vertical and lateral extensional flow, both of which are particularly common in orogens worldwide.

Description: The Colorado Plateau presents a contrast between deep and seemingly recent erosion and apparently only mild late Cenozoic tectonic activity. Researchers have recently proposed multiple sources of epeirogenic uplift and intriguing patterns of differential incision, yet little or no quantitative constraints exist in the heart of the plateau to test these ideas. Here, we use both optically stimulated luminescence (OSL) and uranium-series dating to delimit the record of fluvial strath terraces at Crystal Geyser in southeastern Utah, where the Little Grand Wash fault crosses the Green River in the broad Mancos Shale badlands of the central plateau. Results indicate there has been no deformation of terraces or surface rupture of the fault in the past 100 k.y. The Green River, on the other hand, has incised at a relatively rapid pace of 45 cm/k.y. (450 m/m.y.) over that same time, following a regional pattern of focused incision in the "bull’s-eye" of the central plateau. The Little Grand Wash fault may have initiated during Early Tertiary Laramide tectonism, but it contrasts with related structures of the ancestral Paradox Basin that are presently active due to salt dissolution and focused differential erosion. We also hypothesize there may be a Pliocene component of fault slip in the region linked to broad-wavelength erosional unloading, domal rebound, and extension. An apparent rapid decrease in incision rates just upstream through Desolation Canyon suggests the Green River here may have recently experienced an upstream-migrating wave of incision.

Description: Precise knowledge of the timing of Indo-Eurasian collision is prerequisite for understanding the subsequent evolution of the Himalayan-Tibetan orogenic system, yet the topic remains controversial despite decades of research. We present new data for the Upper Oligocene Basgo Formation of the Indus Basin of NW India that specifically address the proposal that collision initiated no earlier than the Eocene-Oligocene boundary. The Basgo Formation has been cited as the base of the Indus Group because of its previously assumed Maastrichtian age. This age has been revised to Upper Oligocene, but the stratigraphic location has not been re-evaluated. As such, it has been used as evidence of Oligocene-aged collision between India and Eurasia. Based on age constraints in the remainder of the Indus Group, we revise the stratigraphy and place it instead toward the top of the succession. We present evidence that the zircons in the Basgo sandstones originated from the Indian passive margin. Because conglomerate clasts are known to come from the Transhimalayan batholith to the north, our data support mixed provenance and require a minimum late Oligocene age for India-Eurasia collision in the NW Indian Himalaya. (U-Th)/He cooling dates for detrital zircons from the Basgo Formation range from 52.6 to 28.25 Ma, however, implying that their most probable source, the Indian passive margin, was emergent and eroding prior to Oligocene time due to collision. These data alone do not speak to whether the Basgo Formation records pre-Oligocene collision of India and Eurasia or India and the Transhimalayan Ladakh batholith, but as of the date of this publication, there is no evidence for Oligocene collision anywhere else in the Ladakh region. Thus, we interpret our data to demonstrate terminal collision between India and Eurasia prior to Oligocene time.

Description: The eastern escarpment of the Sierra Nevada (USA) forms one of the most prominent topographic and geologic features in the Cordillera, yet the timing and nature of fault displacements along it remain relatively poorly known. The central Sierra Nevada range front is an ideal place to determine the structural evolution of the range front because it has abundant dateable Cenozoic volcanic rocks. The Sonora Pass area of the central Sierra Nevada is particularly good for reconstructing the slip history of range-front faults, because it includes unusually widespread and distinctive high-K volcanic rocks (the ca. 11.5–9 Ma Stanislaus Group) that serve as outstanding strain markers. These include the following, from base to top. (1) The Table Mountain Latite (TML) consists of voluminous trachyandesite, trachybasaltic andesite, and basalt lava flows, erupted from fault-controlled fissures in the Sierra Crest graben-vent system. (2) The Eureka Valley Tuff consists of three trachydacite ignimbrite members erupted from the Little Walker caldera. These ignimbrites are interstratified with lava flows that continued to erupt from the Sierra Crest graben-vent system, and include silicic high-K as well as intermediate to mafic high-K lavas. The graben-vent system consists of a single ~27-km-long, ~8–10-km-wide approximately north-south graben that is along the modern Sierran crest between Sonora Pass and Ebbetts Pass, with a series of approximately north-south half-grabens on its western margin, and an ~24-km-wide northeast transfer zone emanating from the northeast boundary of the graben on the modern range front south of Ebbetts Pass. In this paper we focus on the structural evolution of the Sonora Pass segment of the Sierra Nevada range front, which we do not include in the Sierra Crest graben-vent complex because we have found no vents for high-K lava flows here. However, we show that these faults localized the high-K Little Walker caldera. We demonstrate that the range-front faults at Sonora Pass were active before and during the ca. 11.5–9 Ma high-K volcanism. We show that these faults are dominantly approximately north-south down to the east normal faults, passing northward into a system of approximately northeast-southwest sinistral oblique normal faults that are on the southern end of the ~24-km-wide northeast transfer zone in the Sierra Crest graben-vent complex. At least half the slip on the north-south normal faults on the Sonora Pass range front occurred before and during eruption of the TML, prior to development of the Little Walker caldera. It has previously been suggested that the range-front faults formed a right-stepping transtensional stepover that controlled the siting of the Little Walker caldera; we support that interpretation by showing that synvolcanic throw on the faults increases southward toward the caldera. The Sonora Pass–Little Walker caldera area is shown here to be very similar in structural style and scale to the transtensional stepover at the Quaternary Long Valley field. Furthermore, the broader structural setting of both volcanic fields is similar, because both are associated with a major approximately northeast-southwest sinistral oblique normal fault zone. This structural style is typical of central Walker Lane belt transtension. Previous models have called for westward encroachment of Basin and Range extension into the Sierra Nevada range front after arc volcanism ceased (ca. 6–3.5 Ma); we show instead that Walker Lane transtension is responsible for the formation of the range front, and that it began by ca. 12 Ma. We conclude that Sierra Nevada range-front faulting at Sonora Pass initiated during high-K arc volcanism, under a Walker Lane transtensional strain regime, and that this controlled the siting of the Little Walker caldera.

Description: Normal faults in basalt have distinctive surface-trace morphologies and earthquake evidence that provide information about the slip behavior and earthquake potential. The 47-km-long Hat Creek fault in northern California (USA), a useful case example of this fault style, is a segmented fault system located along the western margin of the Modoc Plateau that is a regional earthquake hazard. In response to interaction with sporadically active volcanic systems, surface ruptures have progressively migrated westward since the late Pleistocene, with older scarps being successively abandoned. The most recent earthquake activity broke the surface through predominantly ca. 24 ka basaltic lavas, forming a scarp with a maximum throw of 56 m. Past work by others identified 7–8 left-stepping scarp segments with a combined length of 23.5 km, but did not explicitly address the throw characteristics, fault evolution, slip history, or earthquake potential. We address these deficiencies in our understanding of the fault system with new field observations and mapping that suggest the active scarp contains 2 additional segments and is at least 6.5 km longer than previously mapped, thus increasing the knowledge of the regional seismic hazard. Our work details scarp geomorphic styles and slip-analysis techniques that can be applied to any normal-faulted basalt environment. Applied to the Hat Creek fault, we estimate that a surface-breaking rupture could produce an earthquake of ~M w (moment magnitude) 6.7 and a recurrence interval of 667 ± 167 yr in response to a rapid slip rate in the range 2.2–3.6 mm/yr, creating a moderate risk given a lack of historical earthquake events.

Description: We present seismic, core, log, and chronologic data on three early to middle Miocene sequences (m5.8, m5.4, and m5.2; ca. 20–14.6 Ma) sampled across a transect of seismic clinothems (prograding sigmoidal sequences) in topset, foreset, and bottomset locations beneath the New Jersey shallow continental shelf (Integrated Ocean Drilling Program Expedition 313, Sites M27–M29). We recognize stratal surfaces and systems tracts by integrating seismic stratigraphy, lithofacies successions, gamma logs, and foraminiferal paleodepth trends. Our interpretations of systems tracts, particularly in the foresets where the sequences are thickest, allow us to test sequence stratigraphic models. Landward of the clinoform rollover, topsets consist of nearshore deposits above merged transgressive surfaces (TS) and sequence boundaries overlain by deepening- and fining-upward transgressive systems tracts (TST) and coarsening- and shallowing-upward highstand systems tracts (HST). Drilling through the foresets yields thin (〈18 m thick) lowstand systems tracts (LST), thin (〈26 m) TST, and thick HST (15–90 m). This contrasts with previously published seismic stratigraphic predictions of thick LST and thin to absent TST. Both HST and LST show regressive patterns in the cores. Falling stage systems tracts (FSST) are tentatively recognized by seismic downstepping, although it is possible that these are truncated HST; in either case, these seismic geometries consist of uniform sands in the cores with a blocky gamma log pattern. Parasequence boundaries (flooding surfaces) are recognized in LST, TST, and HST. TS are recognized as an upsection change from coarsening- to fining-upward successions. We find little evidence for correlative conformities; even in the foresets, where sequences are thickest, there is evidence of erosion and hiatuses associated with sequence boundaries. Sequence m5.8 appears to be a single million-year-scale sequence, but sequence m5.4 is a composite of 3 ~100-k.y.-scale sequences. Sequence m5.2 may also be a composite sequence, although our resolution is insufficient to demonstrate this. We do not resolve the issue of fractal versus hierarchical order, but our data are consistent with arrangement into orders based on Milankovitch forcing on eccentricity (2.4 m.y., 405 and 100 k.y. cycles) and obliquity scales (1.2 m.y. and 41 k.y.).

Description: The temporal and magmatic evolution of central Snake River Plain (SRP; Idaho, USA) olivine tholeiites erupted within the past 4 m.y. is evaluated here. This investigation correlates and merges both geochemical and paleomagnetic measurements to constrain the volcanic history recovered from the 340 m Regional Aquifer Systems Analysis (RASA) test well located near Wendell, Idaho. Only a handful of studies have accomplished this task of shedding light on the chemical stratigraphy of the SRP and the petrogenesis of basalts with depth, and therefore through time. Paleomagnetic relationships suggest that time breaks between individual lava flows represent a few years to decades, time breaks between flow groups represent at least a couple of hundred years or possibly much longer, while significant hiatuses in volcanism, revealed by thick sediment packages or polarity reversals (both are evidenced in this well), are inferred to last thousands to tens of thousands of years. Major element geochemistry from 52 basaltic lava flows demonstrates near primitive compositions (i.e., ~10 wt% MgO) and tholeiitic iron enrichment trends, similar to lavas from the eastern SRP. Trace element concentrations are similar to those of ocean island basalts, with enriched Ba and Pb, and light rare earth element (REE)/heavy REE ratios similar to those of many Neogene volcanics of the western Cordillera. When combined, we identify a total of 11 flow groups, which we also classify as fractionation or recharge on the basis of decreasing or increasing MgO weight percent with depth. Taking into consideration these trends, we review the potential recharge, fractionation, and assimilation processes that characterize much of SRP olivine tholeiite, and conclude that assimilation, in combination with fractional crystallization, is the dominant petrogenesis for the basalts in the central SRP. Although fractionation of Wendell parent magmas was accompanied by assimilation of crustal material, this could not have been assimilation of ancient cratonic crust. The geochemical cycles observed in this well are inferred to represent fractionation and recharge of basaltic magma from a series of sill-like layered mafic intrusions located in the middle crust, similar to what has been proposed for the processes that control the eruptive history of basalts in the eastern SRP.

Description: Palinspastic reconstructions of Phanerozoic geotectonic elements in the Great Basin (Nevada-Utah, USA) to restore the effects of Basin and Range extension, Sevier thrust telescoping, and evolution of the Cordilleran magmatic arc and forearc region to the west provide the following insights: (1) the position of the Neogene Walker Lane incipient transform system was controlled by thermal weakening of the lithosphere beneath the extinct southern segment of the ancestral Cascades arc; (2) Eocene to Miocene magmatism swept across the Great Basin from northeast to southwest, in a direction normal to the continental margin, rather than from north to south parallel to the continental margin; (3) the elevated Paleogene Nevadaplano and Sevier thrust belt between the Sierra Nevada and the Sevier foreland basin was comparable in width to the modern Altiplano and Subandean thrust belt between the Andean volcanic chain and the Brazilian foreland; (4) the late Mesozoic Sierra Nevada and Idaho batholiths formed in the roots of the Cordilleran magmatic arc as segments of a linear batholith belt that lacked curvature; (5) early to middle Mesozoic backarc basins, thrusts, and plutons were coordinated geodynamically with arc accretion and closure of a suture belt in the Sierran foothills, Klamath Mountains, and Blue Mountains of the Cordilleran arc terrane to the west; and (6) Devonian–Mississippian Roberts Mountains and Permian–Triassic Golconda thrusts and allochthons, emplaced successively from the west upon the flank of the Cambrian–Devonian miogeoclinal belt, were formed as linear features subparallel to the Wasatch hinge line marking the western edge of undeformed craton.

Description: Probable fossil ambergris occurs within early Pleistocene shallow-marine clay deposits in western Umbria (central Italy). More than 25 large, permineralized structures are scattered over an area of ~1200 m 2 . These are commonly convex to elongated, helicoidal to concentric, calcium carbonate–rich structures, 30–60 cm high and 60–120 cm wide. Permineralized squid beaks and altered organic matter occur inside these structures. Preliminary chemical data reveal the presence of organic molecules compatible with the degradation of cellular lipids, whose cholic acids indicate the presence of mammalian gastric or intestinal activity; eight free amino acids were also found. The results allow the identification of these structures as intestinal products of sperm whales living ~1.75 m.y. ago. The described fossil structures represent the only known example of Pleistocene sperm whale coprolites.

Description: The depleted nitrogen isotope values observed in Devonian black shales have been ascribed to the dominant role of marine diazotrophic N fixation in providing new reactive N for the productivity that generated the organic matter preserved in these organic-rich facies. However, the emergence of substantial terrestrial ecosystems in the Middle to Late Devonian introduced an additional source of reactive N to shallow epicontinental seas. We examined three sites along a deepening transect of the Late Devonian Appalachian Basin (eastern United States) employing element ratios of C, N, and P, degree of pyritization, and stable isotope analyses of C, N, and S, in order to determine whether basinward changes in N biogeochemistry and 15 N reflect the dominant source of new reactive N. High degrees of pyritization and depleted 34 S indicate a persistently dysoxic to euxinic water column. Sediment N/P rises basinward, indicating that P was relatively more abundant than N in the water column with increasing distance from the shore. Nitrogen isotope values, however, are very consistently ~0 at all three sites despite the evidence of distally increasing N limitation, suggesting that terrestrially derived reactive N was an important source of reactive N and that marine N fixation was not sufficient to address the stoichiometric deficit. Depleted 15 N values in these black shales do not provide a reliable record of diazotrophy; rather, they are a product of the reducing state of the water column and underlying sediments.

Description: A shift from the icehouse climate in which humans evolved to a Late Cretaceous–like greenhouse climate is an often-repeated cautionary prediction of the consequences of continued anthropogenic CO 2 emissions. The corollary, that understanding the past might help predict the future, has justified many Late Cretaceous studies, but important questions remain about climate stability and sensitivity. New 18 O measurements of more than 1000 samples of exceptionally well preserved foraminifera (8 planktic and 11 benthic taxa) from two sites in Tanzania indicate that hot and remarkably stable conditions prevailed in the region during the Turonian, including during a proposed greenhouse glacial event. Planktic taxa have 18 O values largely between –4.0 and –5.0, suggesting surface-water temperatures between 30 and 35 °C. Estimates for seafloor temperatures are between 18 and 25 °C. No parallel shifts in 18 O values are observed among planktic and benthic taxa, contradicting an often-cited line of evidence for greenhouse glaciations and supporting an effectively ice-free Turonian world.

Description: New data for zircon (external morphology, cathodoluminescence zoning, and sensitive high resolution ion microprobe [SHRIMP] U-Pb ages) from the Carvers Gap granulite gneiss of the Mars Hill terrane (Tennessee and North Carolina, United States) require reevaluation of interpretations of the age and origin of this rock. The new results indicate that the zircon is detrital and that the sedimentary protolith of this gneiss (and related Cloudland gneiss) was deposited no earlier than ca. 1.02 Ga and was metamorphosed at ca. 0.98 Ga. Tectonic models that included the gneiss as a piece of 1.8 Ga Amazonian crust (perhaps as part of the hypothetical Columbia supercontinent) are now untenable. The remarkably fast cycle of exhumation, erosion, deposition, and deep burial also is characteristic of other late Grenvillian (post-Ottawan) Mesoproterozoic paragneisses that occur throughout the Appalachians. These rocks provide new evidence for the duration of the formation of the Rodinia supercontinent lasting until at least 0.98 Ma.

Description: Magma unmixing (i.e., separation of a homogeneous silicate melt into two or more liquids) is responsible for sudden changes in the evolution of common melts, element fractionation, and potential formation of orthomagmatic ore deposits. Although immiscible phases are a common phenomenon in the mesostasis of many tholeiitic basalts, evidence of unmixing in intrusive rocks is more difficult to record because of the transient nature of immiscibility during decompression, cooling, and crystallization. In this paper, we document a clear case of liquid immiscibility in an intrusive body of tholeiitic gabbro in the Siberian large igneous province, using textures and compositions of millimeter-sized silicate melt pools in native iron. The native iron crystallized from a metallic iron liquid, which originated as disseminated globules during reduction of the basaltic magma upon interaction with coal-bearing sedimentary rocks in the Siberian craton. The silicate melts entrapped and armored by the native iron are composed of two types of globules that represent the aluminosilicate (60–77 wt% SiO 2 ) and silica-poor, Fe-Ti-Ca-P–rich (in wt%: SiO 2 , 15–46; FeO, 15–22; TiO 2 , 2–7; CaO, 11–27; P 2 O 5 , 5–30) conjugate liquids. Different proportions and the correlated compositions of these globules in individual melt pools suggest a continuously evolving environment of magmatic immiscibility during magma cooling. These natural immiscible melts correspond extremely well to the conjugate liquids experimentally produced in common basaltic compositions at 〈1025 °C. Our results show that immiscibility can occur at large scale in magma chambers and can be instrumental in generating felsic magmas and Fe-Ti-Ca-P–rich melts in the continental igneous provinces.

Description: The Ediacara biota has been long championed as a snapshot of the marine ecosystem on the eve of the Cambrian explosion, providing important insights into the early evolution of animals. Fossiliferous beds in the eponymous Ediacara Member of South Australia have been recently reinterpreted as paleosols and Ediacara fossils as lichens or microbial colonies that lived on terrestrial soils. This reinterpretation, here dubbed the terrestrial Ediacara hypothesis, would fundamentally change our views of biological evolution just prior to the Cambrian explosion. We take a comparative paleobiology approach to test this hypothesis. The Ediacara Member shares a number of forms with assemblages in Ediacaran marine black shales in South China, shales that show no evidence of pedogenesis. Thus, the shared Ediacara fossils, and by extension other co-occurring fossils, are unlikely to have been terrestrial organisms. A terrestrial interpretation is also inconsistent with functional morphological evidence; some of the shared forms are not morphologically adapted to address the most critical challenges for terrestrial life (e.g., mechanical support and desiccation). Thus, the terrestrial Ediacara hypothesis can be falsified on comparative paleobiological and functional morphological grounds, and we urge paleopedologists to critically reevaluate evidence for pedogenesis in the Ediacara Member and other Ediacaran successions.

Description: The 2010 Eyjafjallajökull eruption in Iceland provided a unique opportunity to quantify the evolution of proglacial geomorphology during a series of volcanogenic jökulhlaups (glacial outburst floods) (〉140 events). Time-lapse imagery and repeat terrestrial laser scans before and directly after the eruption show that the jökulhlaup of 14 April 2010 composed 61% of the 57 x 10 6 m 3 total discharge of the combined events, and had the highest peak discharge for the two main flood events, but only deposited 18% of the total volume of sediment in front of Gígjökull glacier. The majority of sediments (67% of a total volume of 17.12 x 10 6 m 3 ) were deposited by the 15 April 2010 jökulhlaup, and this was followed by extensive reworking by a series of smaller jökulhlaups over the following 29 days that deposited 15% of the total sediment. The geomorphological and sedimentary signatures of the two largest jökulhlaups associated with the onset of the eruption have either been reworked by later floods or are buried by later flood deposits. Consequently, the ice-proximal, posteruption landscape cannot be used to reconstruct the characteristics or magnitudes of either of the two largest jökulhlaups. The findings support a complex-response model in which peak discharge and the bulk of the sediment transported is decoupled by changing routing mechanisms and water:sediment ratios during the eruption.

Description: U-Pb zircon data from the uppermost Cottons Breccia, representing the Marinoan glacial-postglacial transition on King Island, Tasmania, provide the first direct age constraint on the Cryogenian-Ediacaran boundary in Australia. Zircons in four samples from the topmost meter of the Cottons Breccia, dated by sensitive high-resolution ion microprobe, exhibit two modes ca. 660 Ma and ca. 635 Ma. The younger component predominates in the uppermost sample, a possibly volcanolithic dolomitic sandstone, apparently lacking glacially transported debris, in the transition to cap carbonate. Chemical abrasion–thermal ionization mass spectrometry (CA-TIMS) U-Pb dating of euhedral zircons from that sample yields a weighted-mean age of 636.41 ± 0.45 Ma. Equivalence to published TIMS ash bed dates from Cryogenian-Ediacaran transitional strata in Namibia (635.51 ± 0.82 Ma, within glacial deposit) and China (635.23 ± 0.84 Ma, 2 m above glacial deposit) supports correlation of those strata to the Australian type sections and globally synchronous deglaciation at the end of the Cryogenian Period.

Description: Age, chemical, and isotopic data from late Cenozoic volcanic rocks in the northern Sierra Nevada, California (USA), from Lake Tahoe north to the southern end of the modern Cascades volcanic arc, were obtained to investigate the evolution of the upper mantle beneath this continental margin during the transition from active subduction to the opening of a slabless window, and to test the possibility that the foundering of mantle lithosphere proposed for the southern Sierra Nevada extended to the northern reaches of the mountain range. Our data are consistent with previous work in the region and illustrate that volcanism shifted from widespread intermediate composition magmatism to small volume, localized trachybasalts to trachyandesites ca. 3 Ma. Similar to southern Cascades volcanism, 87 Sr/ 86 Sr and 206 Pb/ 204 Pb decrease, and Nd increase in the older (older than 3 Ma) volcanic rocks with increasing proportions of a slab component, as measured by increasing (Sr/P) N , where N is primitive-mantle normalized. We interpret these observations as evidence that the older volcanic rocks are subduction related and represent the products of basaltic melts derived from flux melting of mantle wedge that interacted to varying degrees during ascent with lower Nd and higher 87 Sr/ 86 Sr sub–Sierra Nevada continental mantle lithosphere. The younger volcanic rocks lack evidence for the involvement of a slab component in their generation, but have ranges of Nd, Sr, and Pb isotopic compositions similar to those of older volcanic rocks interpreted to have interacted to the greatest extent with the continental mantle lithosphere. However, the younger volcanic rocks have higher high field strength element (HFSE) and higher phosphorus abundances, and higher (La/Yb) N , than their older counterparts, suggesting that they are not simply the products of smaller degrees of partial melting of the same mantle lithosphere involved in the older magmatism. The high HFSE and P contents were more likely controlled by metasomatic accessory carrier phases such as rutile and apatite, the stabilities of which control the abundance of these elements in melts produced from the lithospheric mantle after 3 Ma. One possibility is that the accessory phases were introduced to lithosphere during melt–wall-rock interaction associated with the older magmatic episode. These phases were then purged as a result of conductive heating of the remaining lithospheric mantle triggered by postsubduction upwelling of the sublithospheric mantle. Our data are consistent with lithospheric mantle serving as a melt reactor during the earlier subduction-related magmatism that was baked out during later conductive heating, a process that may be relevant to the production of immediately postsubduction magmatism along other continental margins.

Description: Although Basin and Range–style extension affected large areas of western Mexico after the Late Eocene, most consider that extension in the Gulf of California region began as subduction waned and ended ca. 14–12.5 Ma. A general consensus also exists in considering Early and Middle Miocene volcanism of the Sierra Madre Occidental and Comondú Group as subduction related, whereas volcanism after ca. 12.5 Ma is extension related. Here we present a new regional geologic study of the eastern Gulf of California margin in the states of Nayarit and Sinaloa, Mexico, backed by 43 new Ar-Ar and U-Pb mineral ages, and geochemical data that document an earlier widespread phase of extension. This extension across the southern and central Gulf Extensional Province began between Late Oligocene and Early Miocene time, but was focused in the region of the future Gulf of California in the Middle Miocene. Late Oligocene to Early Miocene rocks across northern Nayarit and southern Sinaloa were affected by major approximately north-south– to north-northwest–striking normal faults prior to ca. 21 Ma. Between ca. 21 and 11 Ma, a system of north-northwest–south-southeast high-angle extensional faults continued extending the southwestern side of the Sierra Madre Occidental. Rhyolitic domes, shallow intrusive bodies, and lesser basalts were emplaced along this extensional belt at 20–17 Ma. Rhyolitic rocks, in particular the domes and lavas, often show strong antecrystic inheritance but only a few Mesozoic or older xenocrysts, suggesting silicic magma generation in the mid-upper crust triggered by an extension-induced basaltic influx. In northern Sinaloa, large grabens were occupied by huge volcanic dome complexes ca. 21–17 Ma and filled by continental sediments with interlayered basalts dated as 15–14 Ma, a stratigraphy and timing very similar to those found in central Sonora (northeastern Gulf of California margin). Early to Middle Miocene volcanism occurred thus in rift basins, and was likely associated with decompression melting of upper mantle (inducing crustal partial melting) rather than with fluxing by fluids from the young and slow subducting microplates. Along the eastern side of the Gulf of California coast, from Farallón de San Ignacio island offshore Los Mochis, Sinaloa, to San Blas, Nayarit, a strike distance of 〉700 km, flat-lying basaltic lavas dated as ca. 11.5–10 Ma are exposed just above the present sea level. Here crustal thickness is almost half that in the unextended core of the adjacent Sierra Madre Occidental, implying significant lithosphere stretching before ca. 11 Ma. This mafic pulse, with subdued Nb-Ta negative spikes, may be related to the detachment of the lower part of the subducted slab, allowing an upward asthenospheric flow into an upper mantle previously modified by fluid fluxes related to past subduction. Widespread eruption of very uniform oceanic island basalt–like lavas occurred by the late Pliocene and Pleistocene, only 20 m.y. after the onset of rifting and ~9 m.y. after the end of subduction, implying that preexisting subduction-modified mantle had now become isolated from melt source regions. Our study shows that rifting across the southern-central Gulf Extensional Province began much earlier than the Late Miocene and provided a fundamental control on the style and composition of volcanism from at least 30 Ma. We envision a sustained period of lithospheric stretching and magmatism during which the pace and breadth of extension changed ca. 20–18 Ma to be narrower, and again after ca. 12.5 Ma, when the kinematics of rifting became more oblique.

Description: The Walker Lane belt of eastern California and western Nevada is the northernmost extension of the Gulf of California transtensional rift, where the process of continental rupture has not yet been completed, and rift initiation can be studied on land. GPS and earthquake focal mechanism studies demonstrate that the Walker Lane belt currently accommodates NW-SE–directed movement between the Sierra Nevada microplate and the North American plate, but the timing and nature of rift initiation remains unclear. I present a model for plate-margin-scale initiation of the Gulf of California and Walker Lane transtensional rifts at ca. 12 Ma; localization of rifting in both was initiated by thermal weakening in the axis of a subduction-related arc undergoing extension due to slab rollback, and thermal weakening in the arc was enhanced by stalling of the trenchward-migrating precursor arc against a thick Cretaceous batholithic lithospheric profile on its western margin. Rifting succeeded very quickly in the Gulf of California, due to stalling of Farallon slabs, but the Walker Lane transtensional rift has been unzipping northward along the axis of the Cascades arc, following the Mendocino triple junction. I infer that plate-margin-scale Walker Lane transtension was signaled by the development of an unusually large and voluminous transtensional arc volcanic center, the ca. 11.5–9 Ma Sierra Crest–Little Walker arc volcanic center. I show that the style of faulting in this large Miocene arc volcanic center closely matches that of Quaternary transtensional structures in the central Walker Lane, where it lies, and that it differs from southern and northern Walker Lane structures. This indicates that the temporal transition from E-W Basin and Range extension to NW-SE Walker Lane transtension occurred earlier than most workers have inferred. I also summarize new data which show that the central Sierra Nevada range front (from Long Valley to the Tahoe Basin) lies squarely within the Walker Lane belt, not to the west of it as previous workers have inferred. The leading tip of Walker Lane transtension is marked by large arc volcanic centers sited in transtensional stepovers; these include the ca. 11.5–9 Ma Sierra Crest–Little Walker volcanic center; north of that, the ca. 6.3–4.8 Ma Ebbetts Pass volcanic center; and north of that, the active Lassen volcanic center. In its wake, large rift volcanic centers are sited on transtensional stepovers or bends; these include the Long Valley and Coso volcanic fields. I predict that any 〈12 Ma large volcanic centers identified by future workers in the Sierra Nevada–Walker Lane region, or in the Gulf of California, will prove to be sited at major releasing bends or stepovers.

Description: Integrated Ocean Drilling Program Expedition 313 continuously cored uppermost Eocene to Miocene sequences on the New Jersey shallow shelf (Sites M27, M28, and M29). Previously, 15 Miocene (ca. 23–13 Ma) seismic sequence boundaries were recognized on several generations of multichannel seismic profiles using criteria of onlap, downlap, erosional truncation, and toplap. We independently recognize sequence boundaries in the cores and logs based on an integrated study of core surfaces, lithostratigraphy and process sedimentology (grain size, mineralogy, facies, and paleoenvironments), facies successions, stacking patterns, benthic foraminiferal water depths, downhole logs, core gamma logs, and chronostratigraphic ages. We use a velocity-depth function to predict the depths of seismic sequence boundaries that were tested by comparison with major core surfaces, downhole and core logs, and synthetic seismograms. Using sonic velocity (core and downhole), core density, and synthetic seismograms, we show that sequence boundaries correspond with acoustic impedance contrasts, although other stratal surfaces (e.g., maximum flooding and transgressive surfaces) also produce reflections. Core data are sufficient to link seismic sequence boundaries to impedance contrasts in 9 of 12 instances at Site M27, 6 of 11 instances at Site M28, and 8 of 14 instances at Site M29. Oligocene sequences have minimal lithologic and seismic expression due to deep-water locations on clinoform bottomsets. Miocene sequences (ca. 23–13 Ma) were sampled across several unconformity clinothems (prograding units) on topset, foreset, and bottomset locations. Excellent recovery allows core-seismic integration that confirms the hypothesis that unconformities are a primary source of impedance contrasts. Our core-seismic-log correlations predict that key seismic surfaces observed in other subsurface investigations without core and/or well logs are stratal surfaces with sequence stratigraphic significance.

Description: The timing and spatial extent of mid-Cenozoic ignimbrite flare-up volcanism of the Sierra Madre Occidental silicic large igneous province of Mexico in relation to crustal extension is relatively unknown. Extension in the Sierra Madre Occidental has been variably interpreted to have preceded, postdated, or begun during Early Oligocene flare-up volcanism of the silicic large igneous province. New geologic mapping, zircon U-Pb laser ablation–inductively coupled plasma–mass spectrometry dating, modal analysis, and geochemical data from the Guazapares Mining District region along the western edge of the northern Sierra Madre Occidental silicic large igneous province have identified three informal synextensional formations. The ca. 27.5 Ma Parajes formation is an ~1-km-thick succession composed primarily of welded to nonwelded silicic outflow ignimbrite sheets erupted from distant sources. The 27–24.5 Ma Témoris formation is interpreted as an andesitic volcanic center composed of locally erupted mafic to intermediate composition lavas and associated intrusions, with interbedded andesite-clast fluvial and debris flow deposits, and an upper section of thin distal silicic outflow ignimbrites. The 24.5–23 Ma Sierra Guazapares formation is composed of silicic vent facies ignimbrites to proximal ignimbrites, lavas, plugs, dome-collapse deposits, and fluvially or debris flow–reworked equivalents. These three formations record (1) the accumulation of outflow ignimbrite sheets, presumably erupted from calderas mapped ~50–100 km east of the study area that were active during the Early Oligocene pulse of the mid-Cenozoic ignimbrite flare-up; (2) development of an andesitic volcanic field in the study area, likely related to rocks of the Southern Cordillera basaltic andesite province that were intermittently erupted across all of the northern Sierra Madre Occidental toward the end of and following the Early Oligocene ignimbrite pulse; and (3) the initiation of explosive and effusive silicic fissure magmatism in the study area during the Early Miocene pulse of the mid-Cenozoic ignimbrite flare-up. The main geologic structures identified in the Guazapares Mining District region are NNW–trending normal faults, with an estimated minimum of 20% total horizontal extension. Normal faults were active during deposition of all three formations (Parajes, Témoris, and Sierra Guazapares), and bound half-graben basins that show evidence of synvolcanic extension (e.g., growth strata) during deposition. Normal faulting began by ca. 27.5 Ma during deposition of the youngest ignimbrites of the Parajes formation, concurrent with the end of the Early Oligocene silicic ignimbrite pulse to the east and before magmatism began in the study area. In addition, preexisting normal faults localized andesitic volcanic vents of the Témoris formation and silicic vents of the Sierra Guazapares formation, and some faults were reactivated during, as well as after, deposition of these formations. We interpret extensional faulting and magmatism in the Guazapares Mining District region to be part of a regional-scale Middle Eocene to Early Miocene southwestward migration of active volcanism and crustal extension in the northern Sierra Madre Occidental. We show that extension accompanied silicic volcanism in the Guazapares region, and overlapped with the peak of mid-Cenozoic ignimbrite flare-up in the Sierra Madre Occidental; this supports the interpretation that there is a relationship between lithospheric extension and silicic large igneous province magmatism.

Description: The stratigraphic utility of carbon-isotope values from terrestrial organic matter is explored for Miocene siliciclastic sediments of the shallow shelf, New Jersey margin, USA (Integrated Ocean Drilling Program [IODP] Expedition 313). These shallow marine strata, rich in terrestrial organic matter, provide a record of deposition equivalent to the Monterey event, a prolonged interval of time characterized by relatively positive carbon-isotope values recorded from foraminiferal carbonate in numerous oceanic settings. Coherent stratigraphic trends and short-term isotopic excursions are observed consistently in palynological preparation residues, concentrated woody phytoclasts, and individually picked woody phytoclasts obtained from the New Jersey sediments. A bulk organic matter curve shows somewhat different stratigraphic trends but, when corrected for mixing of marine-terrestrial components on the basis of measured C/N ratios, a high degree of conformity with the woody phytoclast record is observed. However, assuming that the correlations based on strontium-isotope values and biostratigraphy are correct, the carbon-isotope record from the New Jersey margin contrasts with that previously documented from oceanic settings (i.e., lack of positive excursion of carbon-isotope values in terrestrial organic matter through the Langhian Stage). Factors that may potentially bias local terrestrial carbon-isotope records include reworking from older deposits, degradation and diagenesis, as well as environmental factors affecting vegetation in the sediment source areas. These possible factors are assessed on the basis of pyrolysis data, scanning electron microscope observations, and comparison to palynological indices of environmental change. Some evidence is found for localized degradation and/or reworking of older woody phytoclasts, but where such processes have occurred they do not readily explain the observed carbon-isotope values. It is concluded that the overall carbon-isotope signature for the exchangeable carbon reservoir is distorted, to the extent that the Monterey event excursion is not easily identifiable. The most likely explanation is that phytoclast reworking has indeed occurred in clinoform toe-of-slope facies, but the reason for the resulting relatively heavy carbon-isotope values in the Burdigalian remains obscure.

Description: Recent ocean-bottom geophysical surveys, dredging, and dives, which complement surface data and scientific drilling at the Island of Hawaii, document that evolutionary stages during volcano growth are more diverse than previously described. Based on combining available composition, isotopic age, and geologically constrained volume data for each of the component volcanoes, this overview provides the first integrated models for overall growth of any Hawaiian island. In contrast to prior morphologic models for volcano evolution (preshield, shield, postshield), growth increasingly can be tracked by age and volume (magma supply), defining waxing alkalic, sustained tholeiitic, and waning alkalic stages. Data and estimates for individual volcanoes are used to model changing magma supply during successive compositional stages, to place limits on volcano life spans, and to interpret composite assembly of the island. Volcano volumes vary by an order of magnitude; peak magma supply also varies sizably among edifices but is challenging to quantify because of uncertainty about volcano life spans. Three alternative models are compared: (1) near-constant volcano propagation, (2) near-equal volcano durations, (3) high peak-tholeiite magma supply. These models define inconsistencies with prior geodynamic models, indicate that composite growth at Hawaii peaked ca. 800–400 ka, and demonstrate a lower current rate. Recent age determinations for Kilauea and Kohala define a volcano propagation rate of 8.6 cm/yr that yields plausible inception ages for other volcanoes of the Kea trend. In contrast, a similar propagation rate for the less-constrained Loa trend would require inception of Loihi Seamount in the future and ages that become implausibly large for the older volcanoes. An alternative rate of 10.6 cm/yr for Loa-trend volcanoes is reasonably consistent with ages and volcano spacing, but younger Loa volcanoes are offset from the Kea trend in age-distance plots. Variable magma flux at the Island of Hawaii, and longer-term growth of the Hawaiian chain as discrete islands rather than a continuous ridge, may record pulsed magma flow in the hotspot/plume source.

Description: Integrated Ocean Drilling Program (IODP) Expedition 313 continuously cored Lower to Middle Miocene sequences at three continental shelf sites off New Jersey, USA. The most seaward of these, Site M29, contains a well-preserved Early and Middle Miocene succession of planktonic diatoms that have been independently correlated with the geomagnetic polarity time scale derived in studies from the equatorial and North Pacific. Shallow water diatoms (species of Delphineis , Rhaphoneis , and Sceptroneis ) dominate in onshore sequences in Maryland and Virginia, forming the basis for the East Coast Diatom Zones (ECDZ). Integrated study of both planktonic and shallow water diatoms in Hole M29A as well as in onshore sequences in Maryland (the Baltimore Gas and Electric Company well) and Delaware (the Ocean Drilling Program Bethany Beach corehole) allows the refinement of ECDZ zones into a high-resolution biochronology that can be successfully applied in both onshore and offshore regions of the East Coast of the United States. Strontium isotope stratigraphy supports the diatom biochronology, although for much of the Middle Miocene it suggests ages that are on average 0.4 m.y. older. The ECDZ zonal definitions are updated to include evolutionary events of Delphineis species, and regional occurrences of important planktonic diatom marker taxa are included. Updated taxonomy, reference to published figures, and photographic images are provided that will aid in the application of this diatom biostratigraphy.

Description: Three-dimensional textural observations of inclusion and porosity patterns in a 23-carat carbonado diamond using high-resolution X-ray computed tomography reveal new information bearing on the nature and origin of this enigmatic material. A prominent patinaed surface is texturally linked to a banding and grading of inclusions and pore space beneath, extending several millimeters into the specimen. In situ observation demonstrates that almost all inclusions are polymineralic and show replacement textures, corroborating previous work indicating that the pore network is fully three-dimensionally (3-D) connected, and that virtually all macro-inclusions are secondary. Large metal inclusions are only found immediately adjacent to the margin of the specimen, and are thus also likely to be secondary or even tertiary. However, we also report pseudomorphs after a phase forming pristinely euhedral rhombic dodecahedra, individually and in clusters from 0.3 to 1 mm in diameter; although we could find no evidence of this phase persisting, it nevertheless represents the first "true" macro-inclusion reported in carbonado, which almost certainly formed syngenetically with the diamond material. The pore system is essentially trimodal, consisting of single and clustered pseudomorphs, oblate pores 0.1–0.3 mm in length with a clear preferred orientation, and 20 µm to 〈1 µm pores that form the connected network. Our observations support recent work suggesting that carbonado crystallized from a carbon-supersaturated fluid and suggest that the second stage may correspond with the creation of the pore alignment fabric. We further postulate that, although the present-day macro-inclusions are certainly secondary, the bulk material that comprises them may not be, and may instead be broken-down remains of the original included phase(s). While further verification is needed, a model built around this hypothesis may provide the simplest explanation to many of the unusual features of carbonado.

Description: In the geosciences, fine-scale detail of geomorphic surfaces, commonly parameterized as roughness, is growing in importance as a source of information for modeling natural phenomena and classifying features of interest. Terrestrial light detection and ranging (LiDAR) scanning (TLS), now well known to geologists, is a natural choice for collecting geospatial data. While many recent studies have investigated methodologies for estimating surface roughness from point clouds, research on the influence of instrumental bias on those point clouds and the resulting roughness estimates is scant. A scale-dependent bias in TLS range measurements could affect the outcome of studies relying on high-resolution surface morphology. Growing numbers of research applications in geomorphology, neotectonics, and other disciplines seek to measure the roughness of surfaces with local topographic variations (referred to as asperities) on the order of a few centimeters or less in size. These asperities may manifest as bed forms or pebbles in a streambed, or wavy textures on fault-slip surfaces. In order to assess the feasibility of applying TLS point cloud data sets to the task of measuring centimeter-scale surface roughness, we evaluated the relationship between roughness values of dimensionally controlled test targets measured with TLS scans and numerical simulations. We measured and simulated instrument rangefinder noise to estimate its influence on surface roughness measurements, which was found to decrease with increasing real surface roughness. The size of the area sampled by a single point measurement (effective radius) was also estimated. The ratio of the effective radius to the radius of surface asperities was found to correlate with the disparity between measured and expected roughness. Rangefinder noise was found to overestimate expected roughness by up to ~5%, and the smoothing effect of the measurement size disparity was found to underestimate expected roughness by up to 20%. Based on these results, it is evident that TLS point cloud geometry is correlated with instrument parameters, scan range, and the morphology of the real surface. As different geological applications of TLS may call for relative or absolute measurements of roughness at widely different scales, the presence of these biases imposes constraints on choice of instrument and scan network design. A general solution for such measurement biases lies in the development of calibration processes for TLS roughness measurement strategies, for which the results of this study establish a theoretical basis.

Description: Carbon isotope data of terrestrial organic matter ( 13 C TOM ) obtained in Hokkaido, northern Japan, from the marine Cretaceous Yezo Group along the northwestern Pacific margin elucidated a detailed chemostratigraphy for the Turonian Stage in this region of East Asia. Chemostratigraphic intra-basin correlation reveals three positive 13 C TOM events in the Middle–Upper Turonian of the Yezo Group. 13 C TOM fluctuations in these events show similar patterns in the Yezo Group, indicating that terrestrial organic matter is mixed sufficiently before deposition in the Yezo Basin. These 13 C TOM events are correlated with previously documented 13 C carbonate events in Europe (the Lulworth–Round Down, Glynde–Pewsey, and Late Turonian Events) based on global biostratigraphy. Our chemostratigraphic correlations strengthen the use of these 13 C events for global correlation of the Turonian marine successions. In addition, global correlation of Turonian marine and terrestrial 13 C events identifies changes in isotopic difference between 13 C TOM and 13 C carbonate ( TOM–carbonate ), which are interpreted to reflect changes in atmospheric p CO 2 levels, and climate-driven stresses of humidity and soil processes. In earlier stages of Turonian, TOM–carbonate values are increased. Elevated atmospheric p CO 2 , and increased humidity and soil processes in enhanced greenhouse conditions during mid-Turonian, are interpreted to enlarge TOM–carbonate values. In later stages of Turonian, TOM–carbonate values are at a constant level, and the lowering of atmospheric p CO 2 or decrease of climate stress related to the diverse paleoclimatic cooling is interpreted to have restored the ocean-atmosphere 13 C trends.

Description: Several models have been proposed to explain periodic eruptions of geysers. In essence, the models all use two principally different types of geyser plumbing configurations, dealing with two different physical mechanisms. Here we present data on direct video observations of interior conduit systems for four erupting geysers in Geyser Valley, Kamchatka, Russia. The video footage reveals highly contorted water-filled conduits that periodically discharge voluminous parcels of steam bubbles during eruptions. These observations do not favor the models that use the most popular long vertical conduit type of plumbing, where eruptions are caused by sudden flashing of superheated water into steam. In contrast, our data fit the models using the less-explored type of plumbing, where pressurized steam gradually accumulates in an underground cavity (bubble trap) and periodically erupts through a water-filled, highly contorted conduit with the configuration of an inverted siphon. Hydrodynamic calculations show that such a plumbing configuration produces periodic eruptions when the volume of the bubble trap exceeds the volume of the conduit connecting it to the ground surface. Conduits of the studied geysers were developed from erosion by ascending geothermal water in landslide deposits; chaotic internal structures of the deposits facilitated the formation of conduit systems with highly contorted configurations of the bubble trap type. We suggest that geyser fields are rare on Earth because they require the combination of hydrothermal discharge and geological formations having specific mechanical properties and structures (that facilitate the generation of highly contorted conduits).

Description: While most oceanic volcanism is associated with the passive rise of hot mantle beneath the spreading axes of mid-ocean ridges (MOR), volcanism occurring off-axis reflects intraplate upper-mantle dynamics and composition, yet is poorly understood. Off the south East Pacific Rise (SEPR), volcanism along the Pukapuka, Hotu-Matua, and Sojourn ridges has been attributed to various mechanisms, but none can reconcile its spatial, temporal, and geochemical characteristics. Our three-dimensional numerical models show that asthenospheric shear can excite upwelling and decompression melting at the tip of low-viscosity fingers that are propelled eastward by vigorous sublithospheric flow. This shear-driven upwelling is able to sustain intraplate volcanism that progresses toward the MOR, spreads laterally close to the axis, and weakly continues on the opposite plate. These predictions can explain the anomalously fast eastward progression of volcanism, and its spatial distribution near the SEPR. Moreover, for a heterogeneous mantle source involving a fertile component, the predicted systematics of volcanism can explain the geochemical trend along Pukapuka and the enriched anomaly of SEPR mid-oceanic ridge basalt at 16°–20.5°S. Our study highlights the role of horizontal asthenospheric flow and mantle heterogeneity in producing linear chains of intraplate volcanism independent of a (deep-rooted) buoyancy source.

Description: Large alluvial rivers transport globally significant quantities of water, sediment, and nutrients to the oceans, temporarily storing and cycling this material within the bars, islands, and floodplains that define their morphology. The world’s largest rivers display a remarkable variety of morphologies. However, existing theory and numerical modeling fail to explain this diversity, which remains poorly understood. This study applies a new numerical model of water flow and sediment transport to show how the morphology of large sand-bed rivers is influenced by bed sediment mobility, bank erodibility, and rate of floodplain development. Simulations demonstrate that a wide range of river styles, including meandering, anabranching, and braiding, can occur over a relatively narrow range of environmental conditions. Results highlight the suspension of bed material, which limits the gravitational deflection of sediment in the direction of the local bed slope, as a key control on sediment transport direction and hence river morphology. Moreover, high mobility of bed and bank sediments are hypothesized to favor contrasting river styles, although both may be promoted by increasing stream power. These results explain the inability of existing stream power theory to predict the morphology of the world’s largest rivers, and highlight the potential for investigating river-floodplain co-evolution using physics-based simulation models.

Description: Microtextures of juvenile pyroclasts from Kilauea’s (Hawai‘i) early A.D. 2008 explosive activity record the velocity and depth of convection within the basaltic magma-filled conduit. We use X-ray microtomography (μXRT) to document the spatial distribution of bubbles. We find small bubbles (radii from 5 μm to 70 μm) in a halo surrounding larger millimeter-size bubbles. This suggests that dissolved water was enriched around the larger bubbles—the opposite of what is expected if bubbles grow as water diffuses into the bubble. Such volatile enrichment implies that the volatiles within the large bubbles were redissolving into the melt as they descended into the conduit by the downward motion of convecting magma within the lava lake. The thickness of the small bubble halo is ~100–150 μm, consistent with water diffusing into the melt on time scales on the order of 10 3 s. Eruptions, triggered by rockfall, rapidly exposed this magma to lower pressures, and the haloes of melt with re-dissolved water became sufficiently supersaturated to cause nucleation of the population of smaller bubbles. The required supersaturation pressures are consistent with a depth of a few hundred meters and convection velocities of the order of 0.1 m s –1 , similar to the circulation velocity observed on the surface of the Halema‘uma‘u lava lake.

Description: The empirical probability of submarine mass failure is quantified from a sequence of dated mass-transport deposits. Several different techniques are described to estimate the parameters for a suite of candidate probability models. The techniques, previously developed for analyzing paleoseismic data, include maximum likelihood and Type II (Bayesian) maximum likelihood methods derived from renewal process theory and Monte Carlo methods. The estimated mean return time from these methods, unlike estimates from a simple arithmetic mean of the center age dates and standard likelihood methods, includes the effects of age-dating uncertainty and of open time intervals before the first and after the last event. The likelihood techniques are evaluated using Akaike’s Information Criterion (AIC) and Akaike’s Bayesian Information Criterion (ABIC) to select the optimal model. The techniques are applied to mass transport deposits recorded in two Integrated Ocean Drilling Program (IODP) drill sites located in the Ursa Basin, northern Gulf of Mexico. Dates of the deposits were constrained by regional bio- and magnetostratigraphy from a previous study. Results of the analysis indicate that submarine mass failures in this location occur primarily according to a Poisson process in which failures are independent and return times follow an exponential distribution. However, some of the model results suggest that submarine mass failures may occur quasiperiodically at one of the sites (U1324). The suite of techniques described in this study provides quantitative probability estimates of submarine mass failure occurrence, for any number of deposits and age uncertainty distributions.

Description: Management and restoration of the Mississippi River deltaic plain (southern United States) and associated wetlands require a quantitative understanding of sediment delivery during large flood events, past and present. Here, we investigate the sedimentary fingerprint of the 2011 Mississippi River flood across the Louisiana coast (Atchafalaya Delta, Terrebonne, Barataria, and Mississippi River Delta basins) to assess spatial patterns of sedimentation and to identify key indicators of sediment provenance. The sediment deposited in wetlands during the 2011 flood was distinguished from earlier deposits based on biological characteristics, primarily absence of plant roots and increased presence of centric (planktonic) diatoms indicative of riverine origin. By comparison, the lithological (bulk density, organic matter content, and grain size) and chemical (stable carbon isotopes of bulk organic matter) properties of flood sediments were nearly identical to the underlying deposit. Flood sediment deposition was greatest in wetlands near the Atchafalaya and Mississippi Rivers and accounted for a substantial portion (37% to 85%) of the annual accretion measured at nearby monitoring stations. The amount of sediment delivered to those basins (1.1–1.6 g cm –2 ) was comparable to that reported previously for hurricane sedimentation along the Louisiana coast (0.8–2.1 g cm –2 ). Our findings not only provide insight into how large-scale river floods influence wetland sedimentation, they lay the groundwork for identifying previous flood events in the stratigraphic record.

Description: Although the latest Permian mass extinction and associated 13 C excursion are well documented from the Tethys Ocean, carbonate rocks preserving these events in the eastern Panthalassic Ocean (western Pangea) are unknown. We present 13 C carb from the Gerster and Thaynes (Permian and Triassic) Formations in the western United States and document a negative excursion with no evidence for major breaks in continuity. To further constrain the age of the 13 C carb excursion in the absence of index fossils, we analyzed the same samples for 87 Sr/ 86 Sr. When examining our new carbon and Sr data in the context of biostratigraphy and sequence stratigraphy, we conclude that parts of the western United States may preserve carbonate successions that span the latest Permian extinction.

Description: Lava-fed deltas are extraordinarily useful indicators of fossil water (and ice) levels in glacial, marine, and lacustrine environments. Deltas fed by ‘a‘a lava should be at least as common as those sourced in pahoehoe, yet they have been rarely described. Although facies models for pahoehoe lava-fed deltas are well established, the architecture and lithofacies of ‘a‘a-fed equivalents are substantially different and have thus far largely been unrecognized. This can have profound consequences for paleoenvironmental investigations, particularly those attempting to reconstruct past ice sheets. Essential features of ‘a‘a lava-fed deltas include (1) a subaerial ‘a‘a lava capping unit comprising massive internal sheet lava overlain by clinkers; (2) a crudely developed subaerial to subaqueous transition (passage zone); (3) a chaotic subaqueous association of abundant lava lobes and hyaloclastite with admixed vesicular, often reddened (oxidized) lava clinkers; and (4) rare subaqueous stratification with predominantly lower dips (~10°–20°) than in deltas fed by pahoehoe lava (~25°–40°). We develop a generic facies model and investigate the emplacement conditions of ‘a‘a lava-fed deltas in order to facilitate the recognition and environmental interpretation of these important sequence types in ancient successions.

Description: Magma flow within the subsurface is heavily influenced by the pre-existing structure of the upper crust. During continental rifting, normal faults modify the geometry of igneous networks by providing preferential pathways for the intrusion of magma. However, the way in which magma intrudes into fault planes is poorly understood. Here, we quantitatively document the relationship between fault architecture and intrusion distribution and geometry using three-dimensional seismic reflection data from the Exmouth Sub-basin, offshore northwest Australia. Inclined segments of saucer-shaped sills intrude several faults along convex-into-the-hangingwall fault-plane corrugations. We suggest that stress field perturbations associated with the fault-plane corrugations provide suitable conditions for fault reactivation as magma conduits. Pre-existing faults also modify sill geometries through the offset of stratigraphic horizons that may be preferentially intruded, potentially resulting in the formation of a new sill or the development of minor intrusive steps. This work emphasizes the importance of the pre-existing structural template in controlling the growth and final geometry of intrusive networks.

Description: The Fizzy discovery, a southern North Sea (UK) gas accumulation with ~50% natural CO 2 content, provides an opportunity to study the long-term quantity of CO 2 -related mineral reaction as an analogue for engineered CO 2 storage. The reservoir contains diagenetic dolomite typical of the formation; to identify and quantify any sequestration-related dolomite is challenging. To this end, CO 2 was extracted by stepwise extraction from dolomite from both the Fizzy discovery and equivalent sandstones from a low-CO 2 location. Between 0% and 22% of the dolomite in the Fizzy discovery precipitated due to the high CO 2 concentration. This corresponds to 11% ± 8% of the recent high-CO 2 charge sequestered as dolomite, a relatively low proportion after ~50 m.y. of potential CO 2 -water-rock interaction.

Description: In recent decades, explanations for post-glacial lake acidification have focused on changing climate and biotic factors. Here we present a unique lake sediment data set combining diatom-inferred acidity reconstruction with detailed quantitative assessment of soil base dynamics that challenges this view. We show, at Kråkenes Lake in Norway, that historical development of soil mineral depletion inferred from the lake sediment record is consistent with the extent and timing of early Holocene acidification. The lake-water acidification can be fully accounted for by abiotic soil mineral depletion, suggesting a lesser role for alternative acidifying mechanisms, such as direct climate impacts and successional changes in organic acid production. There are at present few comparable data sets, but those that exist suggest similar rates of soil base depletion. As this acidification is not confined to water, abiotic mineral depletion is likely also to impact terrestrial ecosystems, and dynamic vegetation models that exclude irreversible mineral depletion will fail to capture an important element of global ecology.

Description: Continuous observational monitoring of a study site in eastern Christchurch, New Zealand, following the 2010 M w 7.1 Darfield earthquake has recorded ten distinct liquefaction episodes in the mainshock–aftershock sequence. Three nearby accelerometers allow calibration between the geological expressions of liquefaction and the intensity of earthquake-induced surface ground motion at the site. Sand blow formation was generated by M w 5.2–7.1 earthquakes with M w 7.5–normalized peak ground accelerations (PGA 7.5 ) of ≥ 0.057 g (acceleration due to gravity). Silt drapes between successive sand blow deposits provide markers for delineating distinct liquefaction-inducing earthquakes in the geologic record. However, erosion quickly modifies the surface of sand blows into alluvial and aeolian forms that complicate geologic diagnosis. The two feeder-dike generations identified in subsurface investigations significantly underrepresent the number of liquefaction-inducing earthquakes due to extensive dike reactivation. New constitutive equations enable PGA 7.5 variations to be estimated from the thickness and areal extent of sand blows.

Description: Lithospheres of different thicknesses are often juxtaposed by movement on a continental-transform boundary. Such a boundary with a step change in densities may trigger a gravitational instability as lateral pressure gradients are created where normal mantle lithosphere terminates against less dense asthenospheric mantle. Here we show, for plausible values of the lithospheric viscosity, a mechanism by which the thicker mantle lithosphere will drip off into the lower density asthenosphere. As the mantle deforms it also progressively thickens and then thins the overlying crust, creating a topographic wave that migrates in concert with the removal of mantle lithosphere. Within western North Island, New Zealand, geophysical data define a sharp lithospheric step across the Taranaki-Ruapehu line, and geological observations provide a history of uplift and subsidence that has propagated southward in the past 12 m.y. The rate of observed north to south migration of the wave (~30 mm/yr), its wavelength (~250 km), and amplitude (~±1 km) are compatible with it being caused by progressive removal of mantle lithosphere, if the viscosity of the uppermost lithospheric mantle is ~5 x 10 20 Pa·s, providing one of the clearest examples yet of this fundamental geological process.

Description: Lee’s Ferry (Arizona, United States) lies at an important geologic transition between the Grand Canyon margin and the Canyonlands center of the Colorado Plateau. It marks a knickpoint along the Colorado River at the top of the steep Grand Canyon, and it is central to debate about the patterns of erosion and sources of uplift in this famous landscape. New chronostratigraphic data from the suite of fill terraces here indicate a strong fluvial response to climate drivers superimposed upon an integrated mid-to-late Pleistocene incision rate of ~350 m/m.y. A regional compilation of well-constrained results over the same timescale reveals that this is intermediate between slower rates downstream in Grand Canyon and even faster rates in the central Colorado Plateau, which taper off again farther upstream near the plateau’s eastern edge. This bull’s-eye pattern of rapid incision in the central Colorado Plateau does not match proposed sources of uplift from mantle dynamics at the south and west flank of the plateau, nor patterns of river steepness and energy. Instead we suggest that this incision pattern is primarily driven by transient response to drainage integration and isostatic feedback from the deep exhumation of weak rocks in the central plateau.

Description: The paleoenvironmental evolution of the Black Sea is closely linked to the ingression of Mediterranean seawater over the Bosporus sill after the Last Glacial Maximum. We have reconstructed the temporal and spatial development of the Black Sea suboxic chemocline, which divides oxic surface water from anoxic, sulfidic (euxinic) deep water. By combining high-resolution geochemical records of bulk parameters (carbonate, total organic carbon, sulfur), trace metals (Cu, Mo, V), and an isotopic proxy ( 56 Fe) from seven sediment cores in the Black Sea, we generated a single composite geochemical core log that serves as a reference archive for the entire basin. Our proxy records reflect the changing depositional and redox conditions of the Black Sea and permit us to estimate the inflow budget of Mediterranean seawater throughout the Holocene. Our data indicate a gradual rise of the chemocline until ca. 5.3 ka, when suboxic waters flooded the shelf for the first time. Trace metal and isotopic inventories document one major descent of the chemocline since the onset of brackish/marine conditions before the present stable situation was established.

Description: Banded iron formations (BIFs) are chemical sedimentary rocks comprising alternating layers of iron-rich and silica-rich minerals that have been used to infer the composition of the early Precambrian ocean and ancient microbial processes. However, the identity of the original sediments and their formation is a contentious issue due to postdepositional overprinting and the absence of modern analogues. Petrographic examination of the ca. 2.5 Ga Dales Gorge Member of the Brockman Iron Formation (Hamersley Group), Western Australia, reveals the presence of abundant silt-sized microgranules composed of stilpnomelane. The microgranules are most common in the least-altered BIF where they define sedimentary laminations, implying a depositional origin. We suggest that the precursor mineral was an iron-rich silicate that formed either in the water column or on the seafloor. The microgranular texture may have developed due to clumping of amorphous mud, forming silt-sized floccules. The microgranules were resedimented by dilute density currents and deposited in lamina sets comprising a basal microgranular-rich lamina overlain by amorphous mud with dispersed microgranules. The lamina sets collectively define plane-lamination structure, probably of the lower flow regime. The microgranular textures are preserved only where early diagenetic silica prevented the compaction of lamina sets. Episodic resedimentation of iron silicates alternating with periods of nondeposition and seafloor silicification provides an explanation for some of the characteristic banding in BIF. We propose that for most of the early Precambrian, the persistence of ferruginous oceans with elevated silica concentrations favored the widespread growth of iron silicate minerals, which in environments starved of continental sediments formed extensive deposits of the precursor sediment to iron formation.

Description: High-resolution seismic reflection data from the West Kunlun Range front show that crustal structures beneath the northwestern margin of the Tibetan Plateau are dominated by nappes of upper crustal rocks. Geological cross-section balancing suggests an average of 24.6–54 km (38%–52%) of upper crustal shortening. This horizontal shortening by brittle folding and faulting in the upper crust correlates positively with crustal thickening, an increase in Moho depth by 5–7 km, and the topography. Our work suggests that upper crustal shortening is a chief factor for topographic uplift and crustal thickening at the northwestern margin of the Tibetan Plateau. Such a mechanism is different from that proposed for the plateau south of the Karakax strike-slip fault, where crustal flow may have played a prominent role in plateau uplift.

Description: The Andean broken foreland in west-central Argentina, located 400–800 km from the Peru-Chile Trench, is associated with flat subduction linked to collision of the oceanic Juan Fernandez Ridge. While the conditions associated with flat subduction would be expected to produce increases in rock uplift and exhumation, where thrusting and dynamic forces work together, a prevalence of pre-Cenozoic apatite fission track (AFT) cooling ages suggests that there is no such link. The lack of Cenozoic cooling ages is at odds with structural reconstructions and basin studies along the foreland that show several kilometers of exhumation. This paradox can be reconciled by taking into account the thermal effects of flat subduction that removed the mantle wedge, and a significant source of heat flow into the crust. Reinterpretation of published AFT exhumation data (〉320 ages) using more realistic lower geothermal gradient values allows for substantial exhumation and explains the lack of Cenozoic exhumation ages across the foreland region.

Description: Patterns of origination, evolution, and extinction of early animal life on this planet are largely interpreted from the fossils of the Precambrian soft-bodied Ediacara Biota, spanning nearly 40 m.y. of the terminal Ediacaran period. Localities containing these fossils are loosely considered as part of either the Avalon, White Sea, or Nama Associations. These associations have been interpreted to have temporal, paleobiogeographic, preservational, and/or paleoenvironmental significance. Surprisingly, elements of all three associations occur within the Ediacara Member of the Rawnsley Quartzite of South Australia. An analysis of over 5000 specimens demonstrates that fossil distribution is strongly controlled by facies and taphonomy rather than time or biogeography and that individual taxa vary considerably in their environmental tolerance and taphonomic integrity. The recognition that these taxa represent organisms living in various distinct environments, both juxtaposed and shared, holds strong implications for our interpretation of the record of early animal life on this planet and questions the biostratigraphic utility of the three associations. Furthermore, although in situ soft-bodied preservation provides a unique perspective on composition of benthic fossil assemblages, the record should not be interpreted as a simple "snapshot". Fossil beds represent a range of preservational modifications varying from current winnowed census samples of benthic communities at different depths and ecological maturity, to entirely transported assemblages. Unless the appropriate environments and taphonomic conditions are present for certain taxa, the absence of a particular taxon may or may not indicate its extinction in space or time.

Description: There are two possible origins for ultramafic rocks in subduction-type high-pressure metamorphic terranes: (1) they are derived from the subducted slab or footwall, or (2) they are derived from the mantle wedge in the hanging wall. The origin of ultramafic rock bodies in a classic high-pressure metamorphic belt, the Sambagawa belt of southwest Japan, is assessed based on wide-ranging field studies covering an area of 23 x 30 km 2 and corresponding to metamorphic pressures of 5–25 kbar. Peridotite and/or serpentinite bodies are common in the higher pressure part of the belt, but no occurrence is known in the low-pressure part (the chlorite zone). If the ultramafic rocks originated in the footwall, they should be metamorphosed together with the subducted material and their distribution should not show any correlation with metamorphic pressure. The restricted distribution of mantle rocks to the high-pressure part is strong evidence for a mantle-wedge origin of the ultramafic blocks. The presence of subducted metasediments surrounding the mantle rocks indicates that the subducted slab can tectonically entrain and transport substantial amount of hanging-wall material to the Earth’s surface. The first appearance of ultramafics occurs within the garnet zone, which has a peak metamorphic pressure of 8.0–9.5 kbar, and the corresponding depth (~30–35 km) represents the thickness of the forearc continental crust in the Cretaceous Sambagawa subduction zone.

Description: Diamond is essentially impermeable and unreactive under many conditions, and tiny mineral inclusions within natural diamonds can faithfully preserve information on the chemical and physical conditions during diamond growth. The stable isotope ratios of carbon, nitrogen, oxygen, and sulfur in diamonds and their mineral inclusions have been used to constrain models of diamond formation, but interpretations of the data have differed dramatically. The crux of the controversy lies in the interpretation of the carbon isotope ratios of eclogite-suite diamonds, which range well outside those expected for typical mantle materials such as peridotites, basalts, and carbonatites. Proposed explanations for these anomalous carbon isotope ratios include derivation from primordial mantle inhomogeneities, fractionated mantle fluids, and subducted biogenic carbon. Working with samples from three continents, we have analyzed the carbon isotope compositions of eclogite-suite diamonds and the oxygen isotope composition of their mineral inclusions, primarily by ion microprobe methods. We have discovered a previously unrecognized, remarkably consistent anticorrelation between these two isotopic systems, in that virtually all diamonds with anomalously low carbon isotope ratios have silicate inclusions with anomalously high oxygen isotope ratios. This is a fundamental observation that can only be explained by formation of eclogite-suite diamonds through subduction of seafloor altered basalt, admixed with marine biogenic carbon, into the field of diamond stability.

Description: The Oman ophiolite has an axial volcanic suite and associated sheeted dike complex similar in composition to modern mid-oceanic ridge basalt (MORB). Its internal structure is regarded by many as being directly comparable to ocean lithosphere from the East Pacific Rise. However, there has long been controversy over the geodynamic setting in which the ophiolite formed, and the extent to which the analogy can be drawn, because the MORB-like axial volcanics are overlain by lavas that include depleted arc tholeiites and boninites. To some, this implies that the entire ophiolite formed above a subduction zone; others maintain that it formed at a true open-ocean MOR, and that the water required to generate the arc-like magmas derived from descending near-axis hydrothermal fluids or from ancient subduction. A popular compromise posits that the axial suite formed at a true MOR and the later magmatism documents the initiation of obduction. We test these models by reexamining the "MORB-like" character of the early axial lavas and dikes. We show that fractionation trends require the presence of water at concentrations significantly higher than any open-ocean MORB; instead, trends are identical to those of backarc basin and intraoceanic forearc volcanics. By showing that the entire ophiolite formed in a hydrous system, we rule out all models in which the Oman ophiolite was generated at an open-ocean MOR; instead, it formed at a submarine spreading center above a (probably newly initiated) subduction zone.

Description: We report new measurements of the seismic properties of an upper amphibolite facies metapelite from the Serie dei Laghi basement (southern Alps, northern Italy), presenting evidence for the α-β transition using elastic wave velocity methods and under in-situ conditions. Measurements were made using an internally heated gas apparatus under hydrostatic conditions to 500 MPa and temperatures to 1075 K. P-wave and S-wave velocities were measured parallel and normal to both lineation and foliation at room temperature. An average V p (at 400 MPa) of 6.4 km/s and V p anisotropy of 15% were found, and an average V s1 of 3.6 km/s (8.2% anisotropy) and V s2 of 3.5 km/s (4.1% anisotropy) were also measured. To observe the effects of the α-β quartz transition under hydrostatic conditions, elastic wave velocity was monitored continuously as the sample was heated to 1075 K. At 400 MPa, V p was seen to decrease monotonically with increasing temperature to 950 K; as temperature continued to increase, V p rapidly increased, until 1075 K. The effect was found to be reversible and is interpreted in terms of the α-β transition in quartz, in broad agreement with previous studies. A simple interpretation of the data, taking into account a field case from southern Tuscany as well as natural and elevated geothermal gradient, suggests that thermal and hydraulically driven fracture is a likely cause of "bright spots" seen in these areas. With knowledge of the depth of such features, the transition additionally allows the temperature at depth to be estimated.

Description: Mechanisms that control seismic activity in low strain rate areas such as western Europe remain poorly understood. For example, in spite of low shortening rates of 〈0.5 mm/yr, the Western Alps and the Pyrenees are underlain by moderate but frequent seismicity detectable by instruments. Beneath the elevated part of these mountain ranges, analysis of earthquake focal mechanisms indicates extension, which is commonly interpreted as the result of gravitational collapse. Here we show that erosional processes are the predominant control on present-day deformation and seismicity. We demonstrate, using finite element modeling, that erosion induces extension and rock uplift of the elevated region of mountain ranges accommodating relatively low overall convergence. Our results suggest that an erosion rate of ~1 mm/yr can lead to extension in mountain ranges accommodating significant shortening of 〈3 mm/yr. Based on this study, the seismotectonic framework and seismic hazard assessment for low strain rate areas need to be revisited, because erosion-related earthquakes could increase seismic hazard.

Description: To evaluate sites for long-term geological storage of CO 2 and optimize techniques for monitoring the fate of injected CO 2 , it is crucial to investigate potential CO 2 migration pathways out of a reservoir and surface leakage magnitudes. For the first time, we calculate CO 2 leakage rates and volumes from ancient fault-related travertines and from an abandoned borehole. U-Th–dated travertine along two faults near Green River, Utah (western United States), shows that leakage has occurred in this area for over 400 k.y. and has switched location repeatedly over kilometer-scale distances. One individual travertine was active for at least 11 k.y. Modern leakage is predominantly through the active Crystal Geyser, which erupts from an abandoned exploration well. Using age data and travertine volume, we calculate magnitudes and rates of CO 2 emission. Fault-focused leakage volume is twice as great as diffuse leakage through unconfined aquifers. The leakage rate from a poorly completed borehole is 13 times greater than the long-term time-averaged fault-focused leakage. Although magnitudes and rates of any leakage from future storage sites will be highly dependent on local geology and pressure regime, our results highlight that leakage from abandoned wells is likely to be more significant than through faults.

Description: Oxygen isotope ratios of olivine have become a widely used tool for the study of magmatic systems, especially in the interpretation of source heterogeneities in mantle plume–derived ocean island basalts. The underlying assumption is that fresh minerals provide a better guide to magma 18 O than bulk rock analyses and that olivine is also likely to be a major phenocryst phase in primitive magmas. However, distinctions between source compositions and the effects of subsequent magma evolution have not always been thoroughly scrutinized. For the Azores samples investigated here, we can demonstrate that the 18 O variation (+4.84 to +5.25 Vienna standard mean ocean water) observed in the olivine phenocryst population is closely linked to evolution in the host magmas during ascent to the surface. We observe a linear, positive correlation between forsterite (Fo) content and 18 O in all of the individual island lava suites. This forces us to conclude that the low oxygen isotope ratios result from combined assimilation and fractional crystallization processes, the assimilant being hydrothermally (temperature 〉 250 °C) altered, lower oceanic crust. Linear regression of the measured 18 O olivine values to Fo 89 suggests a homogeneous mantle source with 18 O = +5.2 ± 0.1.

Description: The NASA Mars Science Laboratory (MSL) rover, Curiosity, has safely landed near a 35-km-long dark dune field in Gale Crater on Mars. This dune field crosses the landing site from the northeast to the southwest and lies along Curiosity’s traverse to Aeolis Mons. Here we present the first evidence of recent aeolian activity in the form of ripple and dune migration, and further estimate wind directions within the dune field through analysis of ripple and dune morphologies and the Mars Regional Atmospheric Modeling System (MRAMS). We measured a minimum ripple migration rate of 0.66 m per Earth year, and dune migration rate of 0.4 m per Earth year, in the southwest portion of the field. A strongly bidirectional ripple crestline orientation, nearly orthogonal dune slipfaces, and linear seif or oblique dunes indicate a bidirectional wind regime with winds mainly coming from the ENE and from the northwest; however, MRAMS results indicate primary winds from the ENE. Our constraints on the wind regime provide the unique opportunity to use ground measurements from MSL to test the accuracy of winds predicted from orbital data.

Description: Structural coloration underpins communication strategies in many extant insects but its evolution is poorly understood. This stems, in part, from limited data on how color alters during fossilization. We resolve this by using elevated pressures and temperatures to simulate the effects of burial on structurally colored cuticles of modern beetles. Our experiments show that the color generated by multilayer reflectors changes due to alteration of the refractive index and periodicity of the cuticle layers. Three-dimensional photonic crystals are equally resistant to degradation and thus their absence in fossil insects is not a function of limited preservation potential but implies that these color-producing nanostructures evolved recently. Structural colors alter directly to black above a threshold temperature in experiments, identifying burial temperature as the primary control on their preservation in fossils. Color-producing nanostructures can, however, survive in experimentally treated and fossil cuticles that now are black. An extensive cryptic record is thus available in fossil insects to illuminate the evolution of structural color.

Description: Sulfate reduction has been suggested as a mechanism to induce precipitation of calcium and magnesium carbonates in marine sediments and microbial mats through most of Earth’s history. However, sulfate reduction also causes a drop in pH that favors dissolution rather than precipitation of carbonates. Model results obtained in this study show that in modern seawater, modern hypersaline water, and assumed Precambrian alkaline seawater, sulfate reduction initially lowers the saturation of carbonates due to a rapid decrease in pH. With continuing sulfate reduction, the pH stabilizes between 6.5 and 7, and carbonate saturation slowly increases as a result of increasing dissolved inorganic carbon concentration. However, sulfate reduction in surface microbial mats is not sufficient to cause such an increase in saturation. With increasing salinity, sulfate reduction becomes even less efficient to induce carbonate precipitation. In an alkaline Precambrian ocean, where large amounts of carbonate were formed, induction through sulfate reduction was entirely ineffective. Other metabolic pathways or abiotic factors must be responsible for inducing carbonate formation in microbial mats through Earth’s history.

Description: The emergence of soft-bodied metazoans and the radiation of the earliest skeletal organisms substantially changed the ecological dynamics of Ediacaran environments, leading to the genesis of biogenic hard-part deposits for the first time in Earth’s history. The impact of bioclast origin on sedimentary processes is analyzed herein, focusing on the sedimentology and taphonomy of shell concentrations dominated by the Ediacaran index fossil Cloudina from the Itapucumí Group, Paraguay. Skeletal concentrations include both dense accumulations of parautochthonous, disarticulated specimens ("Type 1 deposits") and in situ specimens preserved as loosely packed assemblages ("Type 2 deposits"). At that time, Cloudina was the critical source of durable biomineralized hard parts in an environment nearly free of other bioclasts. The simple fabric and geometry of these accumulations are typical of Cambrian-style shell beds. Despite their Precambrian age, these deposits indicate that the establishment of the Phanerozoic style of marine substrates and preservation in early shell beds was determined more by the acquisition of hard parts than by environmental changes.

Description: In the year following the end of the 2009 eruption of Redoubt Volcano, Alaska, four significant swarms of low-frequency, low-magnitude (M L 〈 0.1) earthquakes occurred at shallow depths beneath the summit. Because swarms of low-frequency (LF) earthquakes preceded eruptions in 1989 and 2009, the posteruption swarms caused considerable concern and prompted the Alaska Volcano Observatory to raise the monitoring levels on three occasions. None of these swarms led to eruptions, however, and most observers (including us) initially concluded that the swarms had been caused by minor stress adjustments in the new lava dome or in the surrounding summit glaciers. New observations reveal that the shallow LF swarms were accompanied by 2 families of repeating earthquakes at depths between 3 km and 6 km below sea level, where the magma storage region is thought to be. These mid-crustal volcano-tectonic (VT) type earthquakes were identical to earthquakes recorded during the 2009 Redoubt eruption more than 6 months earlier. Focal mechanisms demonstrate that these earthquakes have thrust mechanisms inconsistent with the strike-slip nature of regional faulting. Based on these observations, we conclude that they are generated through processes occurring within the magma storage region. The concurrence of the repeating VT earthquakes with the shallow LF swarms indicates that the shallow LF earthquakes were also magmatically driven. Our results emphasize that even brief episodes of low-amplitude earthquake activity, such as the LF swarms observed at Redoubt following the 2009 eruption, can be indicative of magmatic activity. Perhaps more significant, however, is the demonstration that the conduit system at Redoubt remained active, intact, and capable of transporting heat and fluids to the surface months after the eruption was considered over.

Description: Helium isotope ( 3 He/ 4 He) data from geothermal springs in the Colorado Rocky Mountains (western United States) provide unequivocal evidence for a remarkable mantle-to-groundwater connection, with contributions of up to 27% mantle-derived helium. Hydrochemical modeling of springs shows the mantle helium is associated with high p CO 2 with 76 ± 20% of the CO 2 also derived from endogenic (deep geologic) sources. These springs occur preferentially along faults, have highest 3 He/ 4 He values above domains of low mantle velocity, and demonstrate unexpectedly widespread neotectonic mantle degassing. Total CO 2 flux through these springs is 3 x 10 8 mol/yr, a small but persistent contribution to the CO 2 budget and an important baseline for carbon sequestration/leakage studies.

Description: A fundamental yet unresolved question in fluvial geomorphology is what controls the width of valleys in mountainous terrain. Establishing a predictive relation for valley floor width is critical for realizing links between aquatic ecology and geomorphology because the most productive riverine habitats often occur in low-gradient streams with broad floodplains. Working in the Oregon Coast Range (western United States), we used airborne lidar to explore controls on valley width, and couple these findings with models of salmon habitat potential. We defined how valley floor width varies with drainage area in a catchment that exhibits relatively uniform ridge-and-valley topography sculpted by shallow landslides and debris flows. In drainage areas 〉0.1 km 2 , valley width increases as a power law function of drainage area with an exponent of ~0.6. Consequently, valley width increases more rapidly downstream than channel width (exponent of ~0.4), as derived by local hydraulic geometry. We used this baseline valley width–drainage area function to determine how ancient deep-seated landslides in a nearby catchment influence valley width. Anomalously wide valleys tend to occur upstream of, and adjacent to, large landslides, while downstream valley segments are narrower than predicted from our baseline relation. According to coho salmon habitat-potential models, broad valley segments associated with deep-seated landsliding resulted in a greater proportion of the channel network hosting productive habitat. Because large landslides in this area are structurally controlled, our findings indicate a strong link between geologic properties and aquatic habitat.

Description: Volcanic eruptions are regulated by the rheology of magmas and their ability to degas. Both detail the evolution of stresses within ascending subvolcanic magma. But as magma is forced through the ductile-brittle transition, new pathways emerge as cracks nucleate, propagate, and coalesce, constructing a permeable network. Current analyses of magma dynamics center on models of the glass transition, neglecting important aspects such as incremental strain accommodation and (the key monitoring tool of) seismicity. Here, in a combined-methods study, we report the first high-resolution (20 μm) neutron-computed tomography and microseismic monitoring of magma failure under controlled experimental conditions. The data reconstruction reveals that a competition between extensional and shear fracturing modes controls the total magnitude of strain-to-failure and importantly, the geometry and efficiency of the permeable fracture network that regulates degassing events. Extrapolation of our findings yields magma ascent via strain localization along conduit margins, thereby providing an explanation for gas-and-ash explosions along arcuate fractures at active lava domes. We conclude that a coupled deformation-seismicity analysis holds a derivation of fracture mechanisms and network, and thus holds potential application in forecasting technologies.

Description: The Ediacaran-Cambrian boundary, arguably the most important in the stratigraphic column, is based on the first appearance of the ichnospecies Treptichnus pedum . However, most trace fossils have long temporal ranges and occur in a narrow range of facies, and are typically of little use in biostratigraphic studies. Therefore, understanding the environmental tolerance and range offset of T. pedum is essential to evaluate the reliability of the Ediacaran-Cambrian boundary. Our study in Fortunian units in the Vanrhynsdorp Group of South Africa shows a broad environmental tolerance for the T. pedum producer in shallow-marine clastic settings. This ichnotaxon is not only present in low-energy offshore wave-dominated marine settings, but it also occurs at considerably shallower water in intertidal and shallow-subtidal zones of tide-dominated systems. T. pedum seems to have high values of peak abundance in the upper offshore and lower intertidal sand flats. In many sections, the Ediacaran-Cambrian transition occurs in successions containing a sequence boundary due to incision of valleys that were filled with coarse-grained deposits of fluvial or estuarine origin, both representing facies that are unsuitable for T. pedum . The range offset of this ichnotaxon is typically greater above sequence boundaries and within transgressive systems tracts, providing some constrains on its use. However, the broad environmental tolerance of T. pedum in shallow-marine clastic settings supports evolutionary innovations rather than facies controls as the main mechanism underlying the observed vertical pattern of distribution of this ichnospecies in the relatively continuous succession of the Ediacaran-Cambrian boundary Global Standard Stratotype Section and Point.

Description: There are plenty of good reasons to join your colleagues at any GSA Annual Meeting, but this year affords a special opportunity to reflect on how GSA and its members are building upon the Society's long tradition of achievements and laying the groundwork for future generations of geoscientists.

Description: New geologic and paleomagnetic data from Knight Inlet in the southwestern Coast Mountains Batholith, British Columbia, support significant revision to the paleogeography of the Insular and Intermontane terranes. Recompilation of radiometric ages confirms that after 100 Ma, a magmatic arc migrated northeastward across the Coast Mountains Batholith at ~2 km/m.y. Magmatic age patterns suggest that plutons older than 100 Ma intruded the Intermontane terrane, not the expected Insular terrane. The distribution of brittle faults along Knight Inlet defines a structurally intact central domain, ~45 km wide, flanked to the SW and NE by faulted domains, with no evidence of the widespread Tertiary extension affecting the batholith farther north. Al-in-hornblende geobarometry yields emplacement depths of ~2.5–4 kbar and does not reveal systematic postemplacement tilting. Plutons in the central structural domain yield a consistently oriented paleomagnetic remanence presumably acquired as the Late Cretaceous arc cooled from ca. 110 to 85 Ma. In the absence of recognizable tilting, this result indicates ~1700 km of northward translation since ca. 85 Ma, which is significantly less than predicted for the Insular terrane in the "Baja British Columbia" model but similar to results from the Intermontane terrane. The pluton ages and the paleomagnetic results suggest that the Intermontane terrane, not the Insular terrane, underlies the southwestern flank of the Coast Mountains Batholith. This conclusion is compatible with a paleogeographic model in which the Vancouver Island fragment of Wrangellia was juxtaposed against the Intermontane terrane prior to ca. 120–100 Ma and emplaced in southern British Columbia after ca. 75 Ma.

Description: Stress field perturbations are known to follow earthquakes, but the length scales over which the stress field varies are still poorly understood. Most focal mechanism inversions are assumed to represent the bulk regional stresses, yet fault irregularity and observed small-scale stress changes in boreholes suggest a strong degree of local heterogeneity. We present geologic paleostress inversion results from a segment of the folded West Salton detachment fault located in a left stepover in the San Felipe dextral fault zone of Southern California (United States). Extension and dextral slip in this region were synchronous during latest detachment slip. The local (1–10 km scale) vertical stress field alternated from 1 during extension to 3 during flex-slip folding of the detachment in a restraining bend of the San Felipe fault zone (where 1 , 2 , and 3 are the maximum, intermediate, and minimum principal stress directions, respectively). Regionally (10–100 km scale), we infer that the vertical stress alternates from 1 during extension to 2 during dextral slip. Both are consistent with nearly complete earthquake stress drops. If the fault-slip data set records paleoseismic strains, the magnitudes of differential stress and stress drop must be similar. Both may be relatively small during tectonic transitions.

Description: Low concentrations (tens to hundreds of parts per million) of water in nominally anhydrous minerals (NAMs) of the mantle have been shown to significantly affect the rheology, depth of depressurization melting, and many other properties of the mantle. In this contribution, we evaluate the effect of trace concentrations of water in NAMs on melting of lower continental crust. Water locked in structural sites and in fluid inclusions in nominally anhydrous minerals in lower crustal granitoids may act as a flux for partial melting of these source rocks. Water concentrations of 3000 ppm in minerals that make up large volumes of crustal rocks (K-feldspar, plagioclase, quartz) would lower the dry solidus of granite by 273 °C at 1 GPa. Measurements and maps of water concentration in variably deformed samples of the Stevenson Granite from northern Saskatchewan (Canada) show that, during deformation and recrystallization of K-feldspar megacrysts, water migrated from the interior of megacrysts to finer-grained matrix material. Dark, fine-grained, water-richer matrix material consisting of quartz, plagioclase, alkali feldspar, and fine iron oxides are interpreted to be former melt films that resulted, at least in part, from fluxing by NAM-derived water. Deformation may play a role in moving water from NAMs to phase boundaries where generation of partial melt may lead to further rock weakening and further release of water from NAMs.

Description: Pyroxenites embedded in peridotite are often invoked as a major cause of short-length scale isotopic heterogeneities in the upper mantle, but there has been little direct evidence. We report spatially controlled chemical and Sr-Nd isotopic compositions of pyroxenites and their host peridotites from an ophiolitic mantle sequence in the Northern Apennines, Italy, with depleted mantle compositions, representing a surface exposure of veined upper mantle, a potential source for mid-oceanic-ridge basalts (MORB). Interaction between pyroxenites and adjacent mantle rocks results in centimeter-scale chemical modifications in the host peridotites, systematically lowering their Sm/Nd ratios. Over time, this interaction causes the host peridotite at 〉0.1 m scale to acquire an isotopic heterogeneity larger than the range defined by the peridotite and pyroxenite end-members. Moreover, the 143 Nd/ 144 Nd variation of a single outcrop covers most of the global Nd isotopic variability documented in abyssal peridotites. Such pyroxenite-peridotite veined mantle domains may represent the enriched component rarely found in abyssal peridotites, but often invoked to account for the low end of 143 Nd/ 144 Nd variations in MORB.

Description: Coastal stratigraphy from the Pacific Northwest of the United States contains evidence of sudden subsidence during ruptures of the Cascadia subduction zone. Transfer functions (empirical relationships between assemblages and elevation) can convert microfossil data into coastal subsidence estimates. Coseismic deformation models use the subsidence values to constrain earthquake magnitudes. To test the response of foraminifera, the accuracy of the transfer function method, and the presence of a pre-seismic signal, we simulated a great earthquake near Coos Bay, Oregon, by transplanting a bed of modern high salt-marsh sediment into the tidal flat, an elevation change that mimics a coseismic subsidence of 0.64 m. The transplanted bed was quickly buried by mud; after 12 mo and 5 yr, we sampled it for foraminifera. Reconstruction of the simulated coseismic subsidence using our transfer function was 0.61 m, nearly identical to the actual elevation change. Our transplant experiment, and additional analyses spanning the A.D. 1700 earthquake contact at the nearby Coquille River 15 km to the south, show that sediment mixing may explain assemblage changes previously interpreted as evidence of pre-seismic land-level change in Cascadia and elsewhere.

Description: Gneiss domes in the Pamir (Central Asia) and the Himalaya provide key data on mid- to deep-crustal processes operating during the India-Asia collision. Laser ablation split-stream inductively coupled plasma–mass spectrometry (LASS-ICP-MS) data from monazite in these domes yield a time record from U/Th-Pb dates and a petrologic record from rare earth element (REE) abundances. Seven samples from the Pamir and six samples from the north Himalayan gneiss domes yield almost identical monazite dates of ca. 28–15 Ma. Most monazite has invariant heavy REE (HREE) abundances; two samples, however, have older monazite that records progressive HREE depletion and two samples have younger monazite that records progressive HREE enrichment. These variations in HREE are compatible with increasing garnet abundance—prograde metamorphism—until ca. 20 Ma, and decreasing garnet abundance thereafter. The change from HREE depletion to enrichment may record a transition from crustal thickening and heating to dome exhumation and cooling. This documentation of synchronous Barrovian metamorphism within domes of Indian crust along the margin of the orogen (Himalaya) and within domes of Asian crust within the core of the orogen (Pamir) is best explained by a plate-scale driving force rather than by local events. We propose that widespread, synchronous thickening was initiated by the resumption of Indian subduction following slab breakoff and then terminated by a second slab-tearing event—both plate-scale events inferred from tomography.

Description: Oversampled Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) visible and near-infrared hyperspectral data over Mount Sharp in Gale Crater, Mars, were used to generate spatially sharpened maps of the location of red crystalline hematite within the uppermost stratum of an ~6.5-km-long ridge on the mound’s northern flank. Finely layered strata underlie the ridge to the north and have dips consistent with the nearby Mount Sharp sedimentary sequence. Fe-Mg smectites are exposed in a valley to the south of the ridge. Emplacement of the hematite is hypothesized to result either from exposure of anoxic Fe 2+ -rich groundwater to an oxidizing environment, leading to precipitation of hematite or its precursors, or from in-place weathering of precursor silicate materials under oxidizing conditions. These hypotheses and implications for habitability will be testable with in situ measurements by the Mars rover Curiosity when it reaches Mount Sharp.

Description: Field evidence shows that pyroclastic flows can entrain blocks from underlying substrates formed by earlier geological events, yet, counterintuitively, they are less likely to erode unconsolidated layers of fine particles. Here we report laboratory experiments that reproduce these seemingly contradictory observations and also offer a means to infer pyroclastic flow velocity. Experiments demonstrate that the sliding head of a granular flow generates a dynamic upward pore-pressure gradient at the flow-substrate interface. Associated upward air flux is enough to fluidize a substrate of fines, so that particles are not entrained individually but the substrate instead is subject to small shear instabilities. In contrast, coarse particles forming a non-fluidized substrate are lifted at a critical upward force due to the pore-pressure gradient, according to their individual masses, which provides a basis for a model to calculate the flow velocity. Application to the 18 May 1980 pyroclastic flow deposits at Mount St. Helens (Washington State, USA) gives velocities of ~9–13 m s –1 at ~6–7 km from the vent on gentle slopes (〈4°–6°), in agreement with field observations at this volcano and at others.

Description: Faro are annular reefs, with reef flats near sea level and lagoons of variable depth, characteristic of both the perimeter and lagoons of Maldivian (Indian Ocean) atolls. Their geomorphic development remains largely unknown, but where faro lagoons (termed velu in Maldivian) have infilled and support reef islands, these provide precious habitable land. Understanding the timing and modes of velu infilling is thus directly relevant to questions about reef island development and vulnerability. Here we use a chronostratigraphic data set obtained from a range of atoll-interior faro with partially to fully filled velu (including those with reef islands) from Baa (South Maalhosmadulu) Atoll, Maldives, to determine time scales and modes of velu infilling, and to identify the temporal and spatial thresholds that control reef island formation. Our data suggest a systematic relationship between faro size, velu infilling, and island development. These relationships likely vary between atolls as a function of atoll lagoon depth, but in Baa Atoll, our data set indicates the following faro-size relationships exist: (1) faros 〈~0.5 km 2 have velu that were completely infilled by ca. 3000 calibrated years B.P. (cal yr B.P.) with islands having established on these deposits by ca. 2.5 cal kyr B.P.; (2) faros 〉0.5 km 2 but 〈~1.25 km 2 have velu in late stages of infill, may support unvegetated sand cays and, given sufficient sand supply, may evolve into larger, more permanent islands; and (3) faros 〉~1.25 km 2 have unfilled (deeper) velu which might only infill over long time scales and which are thus unlikely to support new island initiation. These new observations, when combined with previously published data on Maldivian reef island development, suggest that while the velu of the largest faro are unlikely to fill over the next few centuries (at least), other faro with near-infilled velu may provide important foci for future reef-island building, even under present highstand (and slightly rising) sea levels.

Description: The August 2008 avulsion of the Kosi River, northern India, resulted in a maximum eastward shift of 〉100 km and created an avulsion belt of 2722 km 2 . Based on A.D. 2000 Shuttle Radar Topography Mission data and on 2005 Landsat Thematic Mapper satellite image–derived channel network (pre-avulsion), we use a topography-driven connectivity model to simulate the avulsion pathway, which corresponds, to a large extent, to that observed in the post-avulsion period. We then use this model to postulate the avulsive course of the river from another upstream point based on avulsion threshold analysis. Our results demonstrate that this model has the potential for postulating the path of an avulsive channel, and can provide a priori information on the areas likely to be flooded following an embankment breach.

Description: We compare the spatiotemporal progression, geometry, and earthquake source characteristics of a zone of anomalous swarm seismicity between the Maacama and Bartlett Springs faults (California, United States) within the northern San Andreas fault system to both laboratory studies of fracture initiation and structural field observations of fault formation. The similarities between laboratory and field studies of incipient faulting and the earthquake swarms suggest that the seismic lineament on which the swarms occur is a fault in an early stage of development. Kinematic descriptions of faulting and models of fault system development suggest that the ability of existing faults to accommodate deformation across plate boundaries is governed by the length scales over which they accommodate stress. Many of the characteristics of juvenile fault zones, such as segmentation, geometric complexity, and the depth extent of faulting, act to reduce this length scale; this requires the reactivation of existing faults or production of accessory faults to accommodate deformation across the plate boundary.

Description: The Transantarctic Mountains (TAM) are the largest non-compressional mountain belt in the world. Their origin is traditionally related to crustal thickening during the Jurassic Ferrar magmatic event that was followed by episodic uplift in the Early and Late Cretaceous and since the Paleocene. This concept of a long-lived morphological high constitutes a base of virtually all Gondwana reconstructions and global climate models. Here we demonstrate that crossover age relationships between thermochronological (apatite fission track) data and stratigraphic information contradict this established interpretation. Instead these data, together with a wealth of independent thermal indicators and geological evidence require the existence of a vast intra-Gondwana basin between at least Late Triassic and Late Cretaceous times, including during the Ferrar magmatic event. Referred to here as the Mesozoic Victoria Basin (MVB), this basin formed during crustal extension across the paleo-Pacific margin of Antarctica and Australia. Uplift of the TAM with associated basin inversion commenced only with the development of the West Antarctic Rift System in Paleogene times. The recognition of the long-lived MVB has primary consequences for the general understanding of the landscape of Gondwana and the breakup between Antarctica and Australia, West Antarctic rifting and uplift of the TAM, and global long-term climate evolution and faunal radiation.

Description: Exceptional outcrops recently exposed in the Koniambo massif allow the study of the serpentine sole of the peridotite nappe of New Caledonia (southwest Pacific Ocean). Many magnesite veins are observed, with characteristics indicating that they were emplaced during pervasive top-to-the-southwest shear deformation. The oxygen isotope composition of magnesite is homogeneous (27.4 〈 18 O 〈 29.7), while its carbon isotope composition varies widely ( – 16.7 〈 13 C 〈 – 8.5). These new data document an origin of magnesite from meteoric fluids. Laterization on top of the peridotite nappe and carbonation along the sole appear to represent complementary records of meteoric water infiltration. Based on the syn-kinematic character of magnesite veins, we propose that syn-laterization tectonic activity has enhanced water infiltration, favoring the exportation of leached elements like Mg, which has led to widespread carbonation along the serpentine sole. This calls for renewed examination of other magnesite-bearing ophiolites worldwide in order to establish whether active tectonics is commonly a major agent for carbonation.

Description: Shergottites, the most abundant martian meteorites, represent the best source of information about Mars’ mantle and its dissolved water. If the mantle was wet, magmatic degassing could have supplied substantial water to the martian surface early in its history. Researchers have attempted to reconstruct the volatile contents of shergottite parental magmas, with recent analyses confirming that the shergottites contained significant water. However, water is not a passive tracer; it directly affects magma chemistry and physical properties. Deciphering the history of water on Mars requires understanding how that water affected the chemistry of the shergottites and how they fit within Mars’ geologic history. Both topics present difficulties, as no shergottite-like rock has been found in stratigraphic context and there is debate over the timing of eruptions of shergottite-like magmas. Partial melting experiments on terrestrial basalts and new data from orbiters and rovers on Mars provide the information needed to overcome these difficulties and explain the role of water in shergottite magmas. Here we show that shergottite compositions and their martian geologic context can be explained by melting of an originally wet mantle that degassed over time. We also demonstrate that models for the evolution of the martian mantle that do not consider water fail to account for the shergottite compositions, surface distributions, and ages. Finally, we suggest that dehydration of the martian mantle has led to changes in magmatic chemistry over time, with shergottites representing melts of water-bearing mantle and rocks similar to nakhlites representing melts of other mantle sources.

Description: Earthquakes along convergent plate boundaries commonly occur in sequences that are complete within 1 yr, and may include 8–10 events strong enough to generate sand blows. Dune crossbeds within the Jurassic Navajo Sandstone of Utah (western United States) enclose intact and truncated sand blows, and the intrusive structures that fed them. We mapped the distribution of more than 800 soft-sediment dikes and pipes at two small sites. All water-escape structures intersect a single paleo-surface, and are limited to the upper portion of the underlying set of cross-strata and the lower portion of the overlying set. A small portion of one set of crossbeds that represents ~1 yr of dune migration encloses eight generations of eruptive events. We interpret these superimposed depositional and deformational structures as the record of a single shock-aftershock earthquake sequence. The completeness and temporal detail of this paleoseismic record are unique, and were made possible when sand blows repeatedly erupted onto lee slopes of migrating dunes. Similar records should be sought in modern dunefields with shallow water tables.

Description: In nature, barchan dunes typically exist as members of larger fields that display striking, enigmatic structures that cannot be readily explained by examining the dynamics at the scale of single dunes, or by appealing to patterns in external forcing. To explore the possibility that observed structures emerge spontaneously as a collective result of many dunes interacting with each other, we built a numerical model that treats barchans as discrete entities that interact with one another according to simplified rules derived from theoretical and numerical work and from field observations: (1) Dunes exchange sand through the fluxes that leak from the downwind side of each dune and are captured on their upstream sides; (2) when dunes become sufficiently large, small dunes are born on their downwind sides ("calving"); and (3) when dunes collide directly enough, they merge. Results show that these relatively simple interactions provide potential explanations for a range of field-scale phenomena including isolated patches of dunes and heterogeneous arrangements of similarly sized dunes in denser fields. The results also suggest that (1) dune field characteristics depend on the sand flux fed into the upwind boundary, although (2) moving downwind, the system approaches a common attracting state in which the memory of the upwind conditions vanishes. This work supports the hypothesis that calving exerts a first-order control on field-scale phenomena; it prevents individual dunes from growing without bound, as single-dune analyses suggest, and allows the formation of roughly realistic, persistent dune field patterns.

Description: We present a geometric, sediment mass-balance model for the interaction of axial and transverse alluvial systems in a subsiding basin. By comparing the model result with a flume experiment that employed a simplified half-graben tectonic geometry with axial and transverse sediment sources, we quantify rates of axial-transverse erosional sediment mixing. In the experiment, the lateral migration rate of the axial-transverse boundaries due to the sediment mixing scales with sediment supplies delivered by transverse drainages, but not with water (or sediment) discharge from the axial channel or with tectonic tilting rate. Using an empirical lateral erosion rate, the model shows how sediment supply partitioning among the axial, hanging-wall, and footwall drainages controls the width and the location of the axial-channel belt. Comparing the modeling results with field cases demonstrates that transverse sediment fluxes could slow the axial-channel migration or even reverse the movement against the tectonic forcing.

Description: The chemistry of surviving pieces of Eoarchaean mantle together with related crust helps us determine early crust-forming mechanisms. Two lenses of high-Mg, low-Al dunite within a ca. 3720 Ma part of the Isua supracrustal belt in Greenland are interpreted as relicts of Eoarchaean mantle with minimal crustal disturbance. The lenses are within altered, higher Al, Ca ultramafic schists and are intercalated with amphibolitized pillow basalts and gabbros with island arc chemical signatures, all intruded by 3715-3710 Ma tonalites. One variety of dunite is dominated by forsterite (Fo90-92) olivine with accessory chromite and rare clinopyroxene, which does not show high field strength element (HFSE) anomalies. Another variety contains olivine (Fo96-98), some intergrown with Ti-humite group minerals with strong positive HFSE anomalies that are complementary to the negative HFSE anomalies of the adjacent amphibolites. We propose that these dunites are tectonic slivers of ca. 3720 Ma subarc mantle that preserve evidence for varying interaction with mafic magmas in a [≥]850 {degrees}C, 1.7-2.0 GPa subcrustal environment. These are by far the oldest direct geochemical link between coeval mantle and crustal rocks, and are new evidence for subduction zone-like environments on the early Earth.

Description: During the Eocene-Oligocene transition (EOT, ca. 34 Ma), Earth's climate cooled significantly from a greenhouse to an icehouse climate, while the calcite (CaCO3) compensation depth (CCD) in the Pacific Ocean increased rapidly. Fluctuations in the CCD could result from various processes that create an imbalance between calcium (Ca) sources to, and sinks from, the ocean (e.g., weathering and CaCO3 deposition), with different effects on the isotopic composition of dissolved Ca in the oceans due to differences in the Ca isotopic composition of various inputs and outputs. We used Ca isotope ratios ({delta}44/40Ca) of coeval pelagic marine barite and bulk carbonate to evaluate changes in the marine Ca cycle across the EOT. We show that the permanent deepening of the CCD was not accompanied by a pronounced change in seawater {delta}44/40Ca, whereas time intervals in the Neogene with smaller carbonate depositional changes are characterized by seawater {delta}44/40Ca shifts. This suggests that the response of seawater {delta}44/40Ca to changes in weathering fluxes and to imbalances in the oceanic alkalinity budget depends on the chemical composition of seawater. A minor and transient fluctuation in the Ca isotope ratio of bulk carbonate may reflect a change in isotopic fractionation associated with CaCO3 precipitation from seawater due to a combination of factors, including changes in temperature and/or in the assemblages of calcifying organisms.

Description: With the increasing accessibility of terrestrial light detection and ranging scanners (LiDAR), generating tools to elicit meaningful information from high-density point cloud data has become of paramount importance. Surface roughness is one metric that has gained popularity, largely due to the accuracy and density of LiDAR-derived point cloud data. Surface roughness is typically defined as a spread of point distances from a reference datum, the standard deviation of point distances from a model surface being a commonly employed model. Unfortunately, a recent literature review has found that existing surface roughness models are far from standardized and may be prone to error resulting from underlying surface topography. In the research presented here, we develop a surface roughness model that is robust to underlying topographic variability by segmenting the point cloud with a three-dimensional regular grid, establishing local (grid cell) reference planes by orthogonal distance regression, and estimating the surface roughness of each grid cell as the standard deviation of orthogonal point-to-plane distances. This surface roughness model is employed to identify fracture and rubble zone distributions within a terrestrial LiDAR scan from a basalt outcrop in southeast Idaho, and the results are compared to a more common model based on ordinary least-squares plane fitting. Results indicate that the orthogonal regression model is robust to outcrop orientation and that the ordinary least-squares model systematically overestimates surface roughness by contaminating estimates with spatially correlated errors that increase with decreasing grid size.

Description: In Southern California, slip rates derived from geodesy-constrained elastic models are lower than geologic rates along the Mojave and San Bernardino segments of the San Andreas fault and the Garlock fault. In contrast, the summed geodetic rate across the Mojave eastern California shear zone (ECSZ) is significantly higher than the summed geologic rate. We show that geodetic and geologic slip rates in Southern California can be reconciled using a viscoelastic earthquake cycle model that explicitly incorporates time-dependent deformation due to nonsteady interseismic fault creep in the lower crust and viscous flow in the upper mantle. To reconcile geologic and geodetic model rates, our model requires that the southern San Andreas fault and the Garlock fault are in the late stages of the earthquake cycle, resulting in lower current deformation rates than the cycle-averaged rate. Our model implies that the ECSZ and the San Jacinto faults are in the early stages of the earthquake cycle, resulting in high current deformation rates.

Description: (Ultra)high-pressure [(U)HP] rocks form and exhume from deep within subduction channels, but subsequent horizontal transport in the shallower orogenic crust makes it difficult to reconstruct their tectonic histories. We use a conceptual framework and numerical models to show that buoyant exhumation from within a subduction conduit formed during one-sided subduction may lead to emplacement of (U)HP rocks into either the lower plate (prowedge) or upper plate (retrowedge) of an orogen, depending on whether the upper plate crust deforms or acts as a backstop during exhumation. Both modes may operate at different positions or different times within an orogen, leading to emplacement of (U)HP rocks into both plates without changing subduction geometry. We propose that retrotransport during exhumation may explain some (U)HP rocks (e.g., Liverpool Land) situated in the upper plate of the Greenland-Norwegian Caledonides.

Description: The evolution of coastal systems during the Holocene resulted from complex interactions and temporal shifts in the relative contribution of sea-level changes, climate change, and sedimentary processes. Along the Red Sea Coast, a 0.5-2 m highstand of sea level at 5 ka can be directly attributed to far-field effects resulting from the reduction in land ice following the last glacial maximum. At the ancient Egyptian harbor of Mersa/Wadi Gawasis, the site of the world's oldest archaeological evidence of long-distance seafaring, stratigraphic and geomorphologic evidence has been identified for this highstand. Here, wadi sediment input, enhanced by a period of wetter climate of the African Humid Period (early to mid- Holocene), forced the closure of coastal embayments, despite ongoing, relatively rapid sea-level rise. A stable, shallow bay persisted at Mersa/Wadi Gawasis as a result of coincidental aridization and a highstand of sea level during the mid-Holocene. This bay served as the primary harbor for ancient Egyptian trade along the Red Sea coast. During the late Holocene, shoreline progradation was dominated by sea-level fall, driven by isostatic processes. These results demonstrate the interplay of various global (sea level), regional (climate, sea level), and local (sedimentation, bathymetry) controls on the coastal evolution of the Red Sea and how these controls dictated the response of a complex civilization. Furthermore, they highlight the crucial role played by sedimentation in governing coastal response to changing sea levels.

Description: The Plinian columns formed during the magmatic phase of the A.D. 79 eruption of Vesuvius alternated several times between fully stable, buoyantly rising regimes and unstable regimes of partial or total collapse. Six pyroclastic density currents (PDCs) were produced during unstable regimes, and ultimately caused the destruction of Roman towns around the volcano. Through new measurements of juvenile clast density and estimations of ascent parameters, we show that four partial collapses were likely triggered by increases in the abundance of dense juvenile clasts within the eruptive column. In contrast, the total collapse probably occurred in response to an increase in the wall-rock content injected into the plume during a progressive widening of the conduit. A sixth low-energy, small collapse resulted from high abundances in both dense juvenile clasts and wall-rock material. Simulations of eruption column behavior already account for the effects of variations in conduit radius, mass discharge rate, and particle size, but have yet to include variable clast density and wall-rock abundance that cause temporal variations in plume density. Our results suggest that both parameters can exert a significant control on the potential for generation of PDCs.

Description: The National Aeronautics and Space Administration's (NASA) Mars Exploration Program is focused on understanding the past or present habitability of Mars (MEPAG, 2010). Central to this aim is determining the history of liquid water on Mars. Some of the most salient evidence for liquid water is giant outflow channels that debouch into Chryse Planitia, hypothesized to be part of an ancient Martian ocean (Parker et al., 1993; Head et al., 1999; Perron et al., 2007; Di Achille and Hynek, 2010). An ocean should be a fertile nursery for Martian life, based on terrestrial evidence (e.g., Walter et al., 1980; Van Kranendonk et al., 2003). Thus, understanding whether the streamlined forms around Chryse Planitia were created in the outflow channels or within the hypothesized ocean contributes to identifying habitable zones on Mars...

Description: We present the development and application of the novel in situ cosmogenic 14C/10Be chronometer to recently exposed proglacial bedrock of the Rhone Glacier, Switzerland. Results show that during the Holocene, the glacier was smaller than today for 6500 {+/-} 2000 yr and larger than today for 4500 {+/-} 2000 yr. This pattern is consistent with limited data from other techniques for glaciers in the Alps and Scandinavia, but in contrast to glaciers from the Southern Alps of New Zealand, emphasizing the high sensitivity of large mountain glaciers to small climate changes. The 14C/10Be chronometer also shows that abrasion rates beneath the Rhone Glacier increased with ice speed.

Description: To understand probable influences of environmental changes on the decrease and rebound of biodiversity during the Late Ordovician, a detailed analysis of total organic carbon (TOC), pyrite, V/(V + Ni) ratios, and S isotopic compositions of pyrite in the Late Ordovician sediments from Poland was performed. The significant enrichments of TOC and pyrite as well as high V/(V + Ni) ratios (〉0.6) for the Jeleniow Formation of the latest Darriwilian to middle Katian Stages provide evidence for predominant anoxic bottom-water conditions. In contrast, low TOC and pyrite as well as low V/(V + Ni) ratios for the Wolka Formation of late Katian age indicate oxic depositional conditions. The S isotopic data are consistent with the interpretations of oceanic redox conditions based on multiple proxies for paleoredox. The results from the Baltica area reveal a major oceanic redox condition change during the late Katian and suggest that oceanic oxygenation may have contributed to the rebound of marine animal diversity during the Late Ordovician.

Description: Asteroid impacts play an important role in the evolution of planetary surfaces. In the inner solar system, the large majority of impacts occur on bodies (e.g., asteroids, the Moon, Mars) covered by primitive igneous rocks. However, most of the impacts recorded on Earth occur on different rock types and are poor proxies for planetary impacts. The Lonar crater is a 1.88-km-diameter, Quaternary age crater (Fig. 1) located on the ca. 66 Ma Deccan basaltic traps in Maharashtra (India), and is one of the very few craters on Earth emplaced directly on basaltic lava flows. We carried out 12 40Ar/39Ar step-heating experiments on 4 melt rock samples in order to (1) obtain a precise age for the Lonar crater; (2) study the response of isotopic chronometers during impacts on mafic target rocks; and (3) better understand the dating of extraterrestrial impact craters. We obtained 10 plateau and 9 inverse isochron ages on various aliquots. Combination of selected data into a global inverse isochron yielded an age of 570 {+/-} 47 ka (MSWD = 1.1; P = 0.24). In comparison, previous nonisotopic investigations on rocks thought to be affected by secondary processes yielded a range of much younger ages (ca. 12-62 ka). The measured 40Ar/36Ar trapped values offer a direct comparison with the atmospheric benchmark value and allow us to test the inherited 40Ar* degassing capacity of basaltic impact melt rocks. The 40Ar/36Ar ratio of 296.5 {+/-} 1.7 is indistinguishable from the atmospheric composition and suggests that inherited 40Ar* is absent from the melt rock. This result substantiates diffusion models that predict a near-complete degassing of low-viscosity melt (e.g., basalts) during impact, and demonstrates for the first time that inherited 40Ar* is less problematic for 40Ar/39Ar dating of impact events in basaltic igneous rocks compared to Si-rich rocks. These results provide direct evidence that basaltic melt rocks are excellent candidates for recording the timing of planetary impact events and, as far as dating is concerned, should be the preferred targets of sample recovery by future missions.

Description: The catastrophic break-ups of the floating Larsen A and B ice shelves (Antarctica) in 1995 and 2002 and associated acceleration of glaciers that flowed into these ice shelves were among the most dramatic glaciological events observed in historical time. This raises a question about the larger West Antarctic ice shelves. Do these shelves, with their much greater glacial discharge, have a history of collapse? Here we describe features from the seafloor in Pine Island Bay, West Antarctica, which we interpret as having been formed during a massive ice shelf break-up and associated grounding line retreat. This evidence exists in the form of seafloor landforms that we argue were produced daily as a consequence of tidally influenced motion of mega-icebergs maintained upright in an iceberg armada produced from the disintegrating ice shelf and retreating grounding line. The break-up occurred prior to ca. 12 ka and was likely a response to rapid sea-level rise or ocean warming at that time.

Description: Discovery of geomorphological elements such as valleys and channel-like features on the surface of Mars has prompted debate over alternative origins for these morphologies, including erosion by lava, liquid CO2, glaciation, and mass wasting events. Similarities between Martian geomorphological elements and those of certain terrestrial environments suggest that water processes were involved in the formation of some visible Martian landscapes. Recent advances in three-dimensional seismic reflectivity imaging techniques, drawn mainly from oil and gas exploration activities in deep-water regions of the world, have allowed us to describe a variety of internal stratigraphic architectures that resemble some geomorphological features observed in the circum-Chryse Planitia region of Mars. For example, erosional shadow remnants that have been described as components of deep-water mass transport deposits in the eastern offshore margin of Trinidad closely resemble teardrop-shaped islands that have been described at the downstream end of outflow channels within the circum-Chryse Planitia region. These observations suggest that the teardrop-shaped islands might have been formed as a result of catastrophic submarine mass movements similar to those documented within continental margins on Earth.