ALBERT

Description: A rare example is reported in which discrete Ag2L2 ring and (AgL)∞ chain motifs [L = N,N′-bis(3-imidazol-1-yl-propyl)-pyromellitic diimide] co-crystallize in the same crystal lattice with varying ratios and degrees of disorder. Crystal structures obtained from representative crystals reveal compatible packing arrangements of the cyclic and polymeric isomers within the crystal lattice, which enables them to co-exist within a crystalline solid solution. A feasible pathway for transformation between the isomers is suggested via facile rotation of the coordinating imidazolyl groups. This chemical system could provide a chance for direct observation of ring-opening isomerization at the crystal surface. Mass spectrometry and 1H NMR titration show a dynamic equilibrium between cyclic and oligomeric species in solution, and a potential crystallization process is suggested involving alignment of precursors directed by aromatic stacking interactions between pyromellitic diimide units, followed by ring-opening isomerization at the interface between the solid and the solution. Both cyclic and oligomeric species can act as precursors, with interconversion between them being facile due to a low energy barrier for rotation of the imidazole rings. Thermogravimetric analysis and variable-temperature powder X-ray diffraction indicate a transition to a different crystalline phase around 120°C, which is associated with loss of solvent from the crystal lattice.

Description: Closed (endorheic) sedimentary basins are key recorders of the climatic, erosional, and tectonic history of their surrounding topography, playing an active role in its evolution by changing the local geomorphological base level. When these basins become exorheic, the accelerated incision along the new fluvial network can excavate excellent stratigraphic outcrops, but this often removes the uppermost infill, and essential information about the late basin history is lost. Here we propose estimating the opening age and past elevation of captured closed basins by combining the flexural isostatic compensation of the eroded volume with available constraints on sediment age. We use this method to constrain the post-tectonic evolution of the Cenozoic Ebro basin in northeast Iberia. The similar results obtained for 4 dated stratigraphic columns show the robustness of the model and date the basin opening as 12.0–7.5 Ma, with a maximum paleoelevation of the basin of 535–750 m. The isostatic rebound associated with basin erosion, as much as 630 m in the center of the basin, may explain the absence of a canyon excavated by the Ebro River during the Mediterranean sea-level fall associated with the Messinian salinity crisis.

Description: After 137 years without a great earthquake, the M w 8.1 Pisagua event of 1 April 2014 occurred in the central portion of the southern Peru–northern Chile subduction zone. This megathrust earthquake was preceded by more than 2 weeks of foreshock activity migrating ~3.5 km/day toward the mainshock hypocenter. This foreshock sequence was triggered by an M w 6.7 earthquake on a reverse fault in the upper plate that strikes at a high angle to the trench, similar to well-documented reverse faults onshore. These margin-oblique reverse faults accommodate north-south shortening resulting from subduction across a plate boundary that is curved in map view. Reverse slip on the crustal fault unclamped the subduction interface, precipitating the subsequent megathrust foreshock activity that culminated in the great Pisagua earthquake. The combination of crustal reverse faults and a curved subduction margin also occurs in Cascadia and northeastern Japan, indicating that there are two additional localities where great megathrust earthquakes may be triggered by upper plate fault activity.

Description: The spatial variability of Holocene relative sea-level (RSL) change influences the capacities of coastal environments to accommodate a sedimentary record of paleoenvironmental change. In this study we couch a specific investigation in more general terms in order to demonstrate the applicability of the relative sea-level history approach to paleoseismic investigations. Using subsidence stratigraphy, we trace the different modes of coastal sedimentation over the course of time in the eastern Indian Ocean where RSL change evolved from rapidly rising to static from 8000 yr ago to present. Initially, the coastal sites from the Aceh, Sumatra, coastal plain, which are subject to repeated great earthquakes and tsunamis, built up a sedimentary sequence in response to a RSL rise of 1.4 mm/yr. The sequence found at 2 sites 8 km apart contained 3 soils of a mangrove origin ( Rhizophora , Bruguiera / Ceriops , Avicennia pollen, and/or intertidal foraminifera) buried by sudden submergence related to coseismic subsidence and 6 tsunami sands that contain pristine subtidal and planktic foraminifera. After 3800 cal yr B.P. (years before A.D. 1950), sea level stabilized and remained such to the present. The stable relative sea level reduced accommodation space in the late Holocene, suggesting that the continued aggradation of the coastal plain was a consequence of periodic coastal inundation by tsunamis.

Description: Deposits of fluvial systems in highly seasonal tropical climates possess unique architectural and facies characters owing to a flood-prone regime alternating with lengthy periods of ineffective discharge. Distally linked deltaic successions should also feature distinctive attributes, with great potential to preserve the stratigraphic evidence of exceptional discharge events. We describe Late Carboniferous delta-front, valley-confined sandstones from the Pennine Basin (UK), originally deposited at paleoequatorial latitudes during final assembly of the Pangean megacontinent and characterized by giant sedimentary structures with repetitively sigmoidal geometry. Facies traits indicate geologically instantaneous deposition of a large sediment volume from a density current at sustained supercritical-flow conditions, leading to aggradation of cyclic steps, recently identified bedforms developing in high-energy flows and of which this is the first complete outcrop example. The lack of unconformable erosional surfaces and absence of different associated facies point to a single aggradational event during which the structures attained dimensions comparable to those indicated by seismic data sets from which they are remotely detected on modern seafloors. Cyclic-step formation in a deltaic setting suggests that Pangean megamonsoons could have triggered hydrologic events capable of imprinting sedimentologic signatures on shallow-marine deposits.

Description: Interferometric synthetic aperture radar (InSAR) was used to measure ground deformation during explosive eruptions on 27 and 28 May 2010 at Pacaya volcano, Guatemala. Interferograms produced using spaceborne and airborne synthetic aperture radar data reveal ~3 m of along-slope movement of the southwest sector of the edifice during these eruptions. This is the largest measured slope instability witnessed in a single event at a volcano that did not result in a catastrophic landslide. This rapid and extreme movement is particularly concerning given the history of sector collapse and persistent activity at this volcano. These findings emphasize the utility of high-resolution InSAR measurements for monitoring deformation and potential catastrophic slope instability at volcanoes.

Description: Central Turkey represents the only orogenic plateau in the Mediterranean region. Also, the largest closed drainage basin and the largest intracontinental basin of Turkey, the Lake Tuz Basin, is located in this region. Results from a three-dimensional (3-D) computer modeling study of the Lake Tuz Basin indicate a southward-deepening freshwater lake basin with great depth in the Mio–Pliocene, which regressed toward the north during the Plio–Quaternary into the shallow saline lake basin it is today. The spatio-temporal variations of Neogene and Quaternary deposits reflect the main effects of internal forces (isostasy〉volcanism〉faulting) that were caused by lithospheric slab breakoff and subsequent asthenospheric upwelling under central Turkey. Climatic change played a relatively minor role during these periods and was closely associated with the results of internal forces.

Description: The Klamath Mountains province of northwestern California–southwestern Oregon is an anomalous element in the Cascadia margin; these mountains have the highest average topography, the oldest rocks, and the only identified example of late Cenozoic detachment faulting in the coastal mountains of the Cascadia forearc. Low-temperature thermochronology (apatite fission-track, apatite [U-Th]/He) analyses from the central and southern Klamath Mountains province record two distinct exhumation events—a Cretaceous–Paleocene regional cooling and a southward-migrating locus of rapid cooling/exhumation in the middle Tertiary. This younger event is localized within the geographic extent of the La Grange fault. We infer that this pattern reflects two distinct processes of exhumation: regional surface erosion (older) and migrating localized tectonic exhumation (younger). At the southern limit of this region of rapid cooling, slickenside striations on the exposed La Grange fault surface record southward displacement of the upper plate along a shallowly dipping (~20°) detachment surface. Thermochronologic data constrain average dip of the fault to a few degrees, upper-plate thickness to 〈~6–8 km, and fault slip rate to 〈2 mm/yr for a duration of 30 m.y. (ca. 45 Ma to 15 Ma). The fault dip is unusually low compared to that of typical detachment faults; the duration of this extensional event is unusually long compared to other detachment faults; the north-south (margin-parallel) slip direction is roughly perpendicular to that of other Klamath Mountains province faults; and the Eocene to early Miocene timing of extensional faulting does not correlate with recognized tectonic events in northern California. Mid-Tertiary tectonic events in the Oregon Coast Ranges provide a context for understanding the unusual mid-Tertiary tectonism in the Klamath Mountains province. Immediately north of the Klamath Mountains province, early Eocene accretion of a large early Cenozoic igneous province, the Siletz terrane, initiated a westward jump of active subduction. Accretion was followed by late Eocene margin-parallel extension in the Oregon Coast Ranges, recorded by formation of a regional dike swarm. Both the timing of tectonic exhumation and the direction of extension on the La Grange detachment fault suggest that mid-Tertiary tectonism in the southern Klamath Mountains province was likely driven by plate tectonics associated with the accretion of Siletzia and the reestablishment of subduction outboard of the accreted terrane.

Description: The name of one of the authors in the article by Heymann et al. [(2014), IUCrJ, 1, 349–360] is corrected.

Description: Synchrotron radiation has many compelling advantages over conventional radiation sources in the measurement of accurate Bragg diffraction data. The variable photon energy and much higher flux may help to minimize critical systematic effects such as absorption, extinction and anomalous scattering. Based on a survey of selected published results from the last decade, the benefits of using synchrotron radiation in the determination of X-ray electron densities are discussed, and possible future directions of this field are examined.

Description: Crystal structures are presented for two dihydrate polymorphs (DH-I and DH-II) of the non-steroidal anti-inflammatory drug sodium (S)-naproxen. The structure of DH-I is determined from twinned single crystals obtained by solution crystallization. DH-II is obtained by solid-state routes, and its structure is derived using powder X-ray diffraction, solid-state 13C and 23Na MAS NMR, and molecular modelling. The validity of both structures is supported by dispersion-corrected density functional theory (DFT-D) calculations. The structures of DH-I and DH-II, and in particular their relationships to the monohydrate (MH) and anhydrate (AH) structures, provide a basis to rationalize the observed transformation pathways in the sodium (S)-naproxen anhydrate–hydrate system. All structures contain Na+/carboxylate/H2O sections, alternating with sections containing the naproxen molecules. The structure of DH-I is essentially identical to MH in the naproxen region, containing face-to-face arrangements of the naphthalene rings, whereas the structure of DH-II is comparable to AH in the naproxen region, containing edge-to-face arrangements of the naphthalene rings. This structural similarity permits topotactic transformation between AH and DH-II, and between MH and DH-I, but requires re-organization of the naproxen molecules for transformation between any other pair of structures. The topotactic pathways dominate at room temperature or below, while the non-topotactic pathways become active at higher temperatures. Thermochemical data for the dehydration processes are rationalized in the light of this new structural information.

Description: The aim of this article is a general description of the so-called Patterson-function direct methods (PFDM), from their origin to their present state. It covers a 20-year period of methodological contributions to crystal structure solution, most of them published in Acta Crystallographica Section A. The common feature of these variants of direct methods is the introduction of the experimental intensities in the form of the Fourier coefficients of origin-free Patterson-type functions, which allows the active use of both strong and weak reflections. The different optimization algorithms are discussed and their performances compared. This review focuses not only on those PFDM applications related to powder diffraction data but also on some recent results obtained with electron diffraction tomography data.

Description: An emulsion-based serial crystallographic technology has been developed, in which nanolitre-sized droplets of protein solution are encapsulated in oil and stabilized by surfactant. Once the first crystal in a drop is nucleated, the small volume generates a negative feedback mechanism that lowers the supersaturation. This mechanism is exploited to produce one crystal per drop. Diffraction data are measured, one crystal at a time, from a series of room-temperature crystals stored on an X-ray semi-transparent microfluidic chip, and a 93% complete data set is obtained by merging single diffraction frames taken from different unoriented crystals. As proof of concept, the structure of glucose isomerase was solved to 2.1 Å, demonstrating the feasibility of high-throughput serial X-ray crystallography using synchrotron radiation.

Description: A major challenge in paleoclimatology is disagreement between data and models for periods of warm climate. Data generally indicate equable conditions and reduced latitudinal temperature gradients, while models generally produce colder conditions and steeper latitudinal gradients except when using very high CO 2 . Here we show congruence between temperature indicators and climate model output for the cool greenhouse interval of the latest Cretaceous (Maastrichtian) using a global database of terrestrial and marine indicators and fully coupled simulations with the Community Climate System Model version 3. In these simulations we explore potential roles of greenhouse gases and properties of pre-anthropogenic liquid clouds in creating warm conditions. Our model simulations successfully reproduce warm polar temperatures and the latitudinal temperature gradient without overheating the tropics. Best fits for mean annual temperature are simulations that use 6 x preindustrial levels of atmospheric CO 2 , or 2 x preindustrial levels of atmospheric CO 2 and liquid cloud properties that may reflect pre-anthropogenic levels of cloud condensation nuclei. The Siberian interior is problematic, but this may relate to reconstructed elevation and the presence of lakes. Data and models together indicate tropical sea-surface temperatures ~5 °C above modern, an equator-to-pole temperature difference of 25–30 °C, and a mid-latitudinal temperature gradient of ~0.4 °C per 1° latitude, similar to the Eocene. Modified liquid cloud properties allow successful simulation of Maastrichtian climate at the relatively low levels of atmospheric CO 2 indicated by proxies and carbon cycle modeling. This supports the suggestion that altered properties of liquid clouds may be an important mechanism of warming during past greenhouse intervals.

Description: Rejuvenation of previously intruded silicic magma is an important process leading to effusive rhyolite, which is the most common product of volcanism at calderas with protracted histories of eruption and unrest such as Yellowstone caldera (Wyoming), Long Valley caldera (California), and Valles caldera (New Mexico) in the United States. Although orders of magnitude smaller in volume than rare caldera-forming supereruptions, these relatively frequent effusions of rhyolite are comparable to the largest eruptions of the 20 th century, and pose a considerable volcanic hazard. However, the physical pathway from rejuvenation to eruption of silicic magma is unclear, particularly because the time between reheating of a subvolcanic intrusion and eruption is poorly quantified. This study uses nanometer-scale trace element diffusion in sanidine crystals to reveal that rejuvenation of a near-solidus or subsolidus silicic intrusion occurred in ~10 mo or less following a protracted period (220 k.y.) of volcanic repose, and resulted in effusion of ~3 km 3 of high-silica rhyolite lava at the onset of Yellowstone’s last volcanic interval. The future renewal of effusive silicic volcanism at Yellowstone will likely require a comparable energetic intrusion of magma that remelts the shallow subvolcanic reservoir and generates eruptible rhyolite on month to annual time scales.

Description: The details of volcanic plume source conditions or internal structure cannot readily be revealed by simple visual images or other existing remote imaging techniques. For example, one predominant observable quantity, the spreading rate in steady or quasi-steady volcanic plumes, is independent of source buoyancy flux. However, observable morphological features of short-duration unsteady plumes appear to be strongly controlled by volcanic source conditions, as inferred from our recent work. Here we present a new technique for using simple morphological evolution to extract the temporal evolution of source conditions of short-lived unsteady eruptions. In particular, using examples from Stromboli (Italy) and Santiaguito (Guatemala) volcanoes, we illustrate simple morphologic indicators of (1) increasing injection rate during the early phase of an eruption; (2) onset of source injection decline; and (3) the timing of source injection cessation. Combined, these observations indicate changes in eruption discharge rate and injection duration, and may assist in estimating total mass erupted for a given event. In addition, we show how morphology may provide clues about the vertical mass distribution in these plumes, which may be important for predicting ash dispersal patterns.

Description: Resolving the age-distance relation of volcanism along the Walvis Ridge (southern Atlantic Ocean) is essential to understanding relative motion between the African plate and the Tristan-Gough mantle plume since the opening of the South Atlantic. However, tracking the location of the Tristan-Gough plume might not be practicable if most of the complex morphology of the massive Walvis Ridge is related to the proximity of the South Atlantic mid-ocean ridge. Here we use new 40 Ar/ 39 Ar basement ages for the Tristan-Gough hotspot track, together with information about morphology and crustal structure from new swath maps and seismic profiles, to infer that separated age-progressive intraplate segments track the location of the Tristan-Gough mantle plume. The apparent continuity of the inferred age-distance relation between widely separated age-progressive segments implies a connection to a stable or constantly moving source in the mantle.

Description: We assessed tectonic controls on the spatial and temporal distribution of fault zone flow pathways in the Rio Grande rift (New Mexico, USA) by using fault zone calcite cements as a geochemical record of syntectonic fluid flow. Cement 18 O, 13 C, and 87 Sr/ 86 Sr values indicate that older, large-displacement master and basin-margin faults were cemented by more isotopically evolved basinal brines than younger intrabasin faults. These data suggest that diagenetic fluids in basin-bounding faults equilibrated predominantly with downdip Paleozoic carbonates. In contrast, intrabasin faults transmitted fluids from shallow stratigraphic sources. This pattern of flow pathways is linked to the systematic distribution of sediments and faults that record rift evolution, which dictated spatial and temporal variations in fault zone architecture and permeability structure. Our results indicate that the depths from which fluids can be transported in active rift basins ultimately depend on both tectonically mediated variations in the grain size of syntectonic sediments entrained in fault damage zones and fault displacement magnitude.

Description: We study the formation of localized shear zones during the layer-parallel extension of viscous multi-layers using two-dimensional numerical simulations based on the finite-difference method. For power-law viscous layers and a linear viscous embedding medium, the extended multi-layer develops boudins due to necking. For power-law viscous layers embedded in a power-law viscous medium, the extended multi-layer develops first distributed necks, and subsequently a localized shear zone with a vertical offset (with a size of several layer thicknesses) along the multi-layer. During the extension, the deformation style switches from distributed and symmetric necking to localized and asymmetric shearing. A localized shear zone develops in the viscous multi-layer although the rheology is everywhere strain-rate-hardening (power-law stress exponent 〉1) and no material softening and/or energy feedback mechanism (e.g., shear heating combined with a temperature-dependent viscosity) is applied. The shear localization is caused by structural softening because the formation of a localized shear zone decreases the bulk resistance and hence the work required to deform the multi-layer. A localized shear zone forms in the multi-layer when the distance between the stiff layers is approximately equal to or less than the layer thickness. The shear localization was observed in multi-layers with nine and with only three stiff layers.

Description: North America provides an important test for assessing the coupling of large continents with heterogeneous Archean- to Cenozoic-aged lithospheric provinces to the mantle flow. We use the unprecedented spatial coverage of the USArray seismic network to obtain an extensive and consistent data set of shear wave splitting intensity measurements at 1436 stations. Overall, the measurements are consistent with simple shear deformation in the asthenosphere due to viscous coupling to the overriding lithosphere. The fast directions agree with the absolute plate motion direction with a mean difference of 2° with 27° standard deviation. There are, however, deviations from this simple pattern, including a band along the Rocky Mountain front, indicative of flow complication due to gradients in lithospheric thickness, and variations in amplitude through the central United States, which can be explained through varying contributions of lithospheric anisotropy. Thus, seismic anisotropy may be sourced in both the asthenosphere and lithosphere, and variations in splitting intensity are due to lithospheric anisotropy developed during deformation over long time scales.

Description: In this paper, a new approach is applied to test a proposed scenario for the tectonic evolution of the Western Carpathian fold-and-thrust belt–foreland system. A N-S balanced section was constructed across the fold-and-thrust belt, from the Polish foreland to the Slovakia hinterland domain. Its sequential restoration allows us to delineate the tectonic evolution and to predict the cooling history along the section. In addition, the response of low-temperature thermochronometers (apatite fission-track and apatite [U-Th]/He) to the changes in the fold-and-thrust belt geometry produced by fault activity and topography evolution are tested. The effective integration of structural and thermochronometric methods provides, for the first time, a high-resolution thermo-kinematic model of the Western Carpathians from the Early Cretaceous onset of shortening to the present day. The interplay between thick- and thin-skinned thrusting exerts a discernible effect on the distribution of cooling ages along the profile. Our analysis unravels cooling of the Outer Carpathians since ca. 22 Ma. The combination of thrust-related hanging-wall uplift and erosion is interpreted as the dominant exhumation mechanism for the outer portion of the orogen. Younger cooling ages (13–4 Ma) obtained for the Inner Carpathian domain are mainly associated with a later, localized uplift, partly controlled by extensional faulting. These results, which help unravel the response of low-temperature thermochronometers to the sequence of tectonic events and topographic changes, allow us to constrain the tectonic scenario that best honors all available data.

Description: Gas-driven filter pressing is the process of melt expulsion from a volatile-saturated crystal mush, induced by the buildup and subsequent release of gas pressure. Filter pressing is inferred to play a major role in magma fractionation at shallow depths (〈10 km) by moving melt and gas relative to the solid, crystalline framework. However, the magmatic conditions at which this process operates remain poorly constrained. We present novel experimental data that illustrate how the crystal content of the mush affects the ability of gas-driven filter pressing to segregate melt. Hydrous haplogranite (2.1 wt% water in the melt) and dacite (4.2 wt% water in the melt) crystal mushes, with a wide range of crystallinities (34–80 vol% crystals), were investigated using in-situ, high-temperature (500–800 °C) synchrotron X-ray tomographic microscopy with high spatial (3 μm/pixel) and temporal resolution (~8 s per three-dimensional data set). Our experimental results show that gas-driven filter pressing operates only below the maximum packing of bubbles and crystals (~74 vol%). Above this threshold, the mush tends to fracture and gas escapes via fractures. Therefore, the efficiency of gas-driven filter pressing is promoted close to the percolation threshold and in situations where a mush inflates slowly relative to build-up of pressure and expulsion of melt. Such observations offer a likely explanation for the production of eruptible, crystal-poor magmas within Earth’s crust.

Description: The development of semiconductor electronics is reviewed briefly, beginning with the development of germanium devices (band gap Eg = 0.66 eV) after World War II. A tendency towards alternative materials with wider band gaps quickly became apparent, starting with silicon (Eg = 1.12 eV). This improved the signal-to-noise ratio for classical electronic applications. Both semiconductors have a tetrahedral coordination, and by isoelectronic alternative replacement of Ge or Si with carbon or various anions and cations, other semiconductors with wider Eg were obtained. These are transparent to visible light and belong to the group of wide band gap semiconductors. Nowadays, some nitrides, especially GaN and AlN, are the most important materials for optical emission in the ultraviolet and blue regions. Oxide crystals, such as ZnO and β-Ga2O3, offer similarly good electronic properties but still suffer from significant difficulties in obtaining stable and technologically adequate p-type conductivity.

Description: Rv1625c is one of 16 adenylyl cyclases encoded in the genome of Mycobacterium tuberculosis. In solution Rv1625c exists predominantly as a monomer, with a small amount of dimer. It has been shown previously that the monomer is active and the dimeric fraction is inactive. Both fractions of wild-type Rv1625c crystallized as head-to-head inactive domain-swapped dimers as opposed to the head-to-tail dimer seen in other functional adenylyl cyclases. About half of the molecule is involved in extensive domain swapping. The strain created by a serine residue located on a hinge loop and the crystallization condition might have led to this unusual domain swapping. The inactivity of the dimeric form of Rv1625c could be explained by the absence of the required catalytic site in the swapped dimer. A single mutant of the enzyme was also generated by changing a phenylalanine predicted to occur at the functional dimer interface to an arginine. This single mutant exists as a dimer in solution but crystallized as a monomer. Analysis of the structure showed that a salt bridge formed between a glutamate residue in the N-terminal segment and the mutated arginine residue hinders dimer formation by pulling the N-terminal region towards the dimer interface. Both structures reported here show a change in the dimerization-arm region which is involved in formation of the functional dimer. It is concluded that the dimerization arm along with other structural elements such as the N-terminal region and certain loops are vital for determining the oligomeric nature of the enzyme, which in turn dictates its activity.

Description: Hirshfeld atom refinement (HAR) is a method which determines structural parameters from single-crystal X-ray diffraction data by using an aspherical atom partitioning of tailor-made ab initio quantum mechanical molecular electron densities without any further approximation. Here the original HAR method is extended by implementing an iterative procedure of successive cycles of electron density calculations, Hirshfeld atom scattering factor calculations and structural least-squares refinements, repeated until convergence. The importance of this iterative procedure is illustrated via the example of crystalline ammonia. The new HAR method is then applied to X-ray diffraction data of the dipeptide Gly–l-Ala measured at 12, 50, 100, 150, 220 and 295 K, using Hartree–Fock and BLYP density functional theory electron densities and three different basis sets. All positions and anisotropic displacement parameters (ADPs) are freely refined without constraints or restraints – even those for hydrogen atoms. The results are systematically compared with those from neutron diffraction experiments at the temperatures 12, 50, 150 and 295 K. Although non-hydrogen-atom ADPs differ by up to three combined standard uncertainties (csu's), all other structural parameters agree within less than 2 csu's. Using our best calculations (BLYP/cc-pVTZ, recommended for organic molecules), the accuracy of determining bond lengths involving hydrogen atoms from HAR is better than 0.009 Å for temperatures of 150 K or below; for hydrogen-atom ADPs it is better than 0.006 Å2 as judged from the mean absolute X-ray minus neutron differences. These results are among the best ever obtained. Remarkably, the precision of determining bond lengths and ADPs for the hydrogen atoms from the HAR procedure is comparable with that from the neutron measurements – an outcome which is obtained with a routinely achievable resolution of the X-ray data of 0.65 Å.

Description: CTB-MPR is a fusion protein between the B subunit of cholera toxin (CTB) and the membrane-proximal region of gp41 (MPR), the transmembrane envelope protein of Human immunodeficiency virus 1 (HIV-1), and has previously been shown to induce the production of anti-HIV-1 antibodies with antiviral functions. To further improve the design of this candidate vaccine, X-ray crystallography experiments were performed to obtain structural information about this fusion protein. Several variants of CTB-MPR were designed, constructed and recombinantly expressed in Escherichia coli. The first variant contained a flexible GPGP linker between CTB and MPR, and yielded crystals that diffracted to a resolution of 2.3 Å, but only the CTB region was detected in the electron-density map. A second variant, in which the CTB was directly attached to MPR, was shown to destabilize pentamer formation. A third construct containing a polyalanine linker between CTB and MPR proved to stabilize the pentameric form of the protein during purification. The purification procedure was shown to produce a homogeneously pure and monodisperse sample for crystallization. Initial crystallization experiments led to pseudo-crystals which were ordered in only two dimensions and were disordered in the third dimension. Nanocrystals obtained using the same precipitant showed promising X-ray diffraction to 5 Å resolution in femtosecond nanocrystallography experiments at the Linac Coherent Light Source at the SLAC National Accelerator Laboratory. The results demonstrate the utility of femtosecond X-ray crystallography to enable structural analysis based on nano/microcrystals of a protein for which no macroscopic crystals ordered in three dimensions have been observed before.

Description: Lunar Reconnaissance Orbiter Camera images reveal a vast, globally distributed network of over 3200 lobate thrust fault scarps, making them the most common tectonic landform on the Moon. Based on their small scale and crisp appearance, crosscutting relations with small-diameter impact craters, and rates of infilling of associated small, shallow graben, these fault scarps are estimated to be younger than 50 Ma and may be actively forming today. The non-random distribution of the scarp orientations is inconsistent with isotropic stresses from late-stage global contraction as the sole source of stress. We propose that tidal stresses contribute significantly to the current stress state of the lunar crust. Orbital recession stresses superimposed on stresses from global contraction with the addition of diurnal tidal stresses result in non-isotropic compressional stress and thrust faults consistent with lobate scarp orientations. The addition of diurnal tidal stresses at apogee result in peak stresses that may help trigger coseismic slip events on currently active thrust faults on the Moon.

Description: The collision of India with Asia had a profound influence on Cenozoic topography, oceanography, climate, and faunal turnover. However, estimates of the time of the initial collision, when Indian continental crust arrived at the Transhimalayan trench, remain highly controversial. Here we use radiolarian and nannofossil biostratigraphy coupled with detrital zircon geochronology to constrain firmly the time when Asian-derived detritus was first deposited onto India in the classical Sangdanlin section of the central Himalaya, which preserves the best Paleocene stratigraphic record of the distal edge of the Indian continental rise. Deep-sea turbidites of quartzarenite composition and Indian provenance are replaced upsection by turbidites of volcano-plutoniclastic composition and Asian provenance. This sharp transition occurs above abyssal cherts yielding radiolaria of Paleogene radiolarian zones (RP) 4–6 and below abyssal cherts containing radiolaria of zone RP6 and calcareous shales with nannofossils of the Paleocene calcareous nannofossil zone (CNP) 7, constraining the age of collision onset to within the middle Paleocene (Selandian). The youngest U-Pb ages yielded by detrital zircons in the oldest Asia-derived turbidites indicate a maximum depositional age of 58.1 ± 0.9 Ma. Collision onset is thus mutually constrained by biostratigraphy and detrital zircon chronostratigraphy as 59 ± 1 Ma. This age is both more accurate and more precise than those previously obtained from the stratigraphic record of the northwestern Himalaya, and suggests that, within the resolution power of current methods, the India-Asia initial collision took place quasi-synchronously in the western and central Himalaya.

Description: The thick gypsum deposits formed in the Mediterranean Basin during the Messinian salinity crisis incorporate dense mazes of filamentous fossils, which were interpreted as algae or cyanobacteria, thus pointing to a shallow-marine subtidal or intertidal environment. The data presented here reveal that these filaments represent remains of colorless, vacuolated sulfide-oxidizing bacteria. This interpretation is supported by the presence of small crystal aggregates of iron sulfide (pyrite) and associated polysulfide within the filamentous fossils. Pyrite and polysulfide are considered to result from early diagenetic transformation of original zero-valent sulfur globules stored within the cells, which is a clade-diagnostic feature of living and degraded sulfur bacteria. In addition to filamentous fossils, the studied gypsum crystals contain remains of euryhaline and stenohaline diatoms and clay-rich aggregates interpreted as alteration products of marine snow floccules. This peculiar fossil assemblage reflects conditions of increased productivity in the water column, triggered by high fluxes of nutrients into the basin during phases of enhanced riverine runoff and freshwater discharge. This study confirms that gypsum evaporites have great potential to preserve the early stages of the taphonomic alteration of bacterial cells, shedding light on the paleoecology of ancient hypersaline environments.

Description: The common view that frequent overbank flooding leads to gradual aggradation of alluvial strata on floodplains and delta plains has been challenged by a variety of studies that suggest that overbank aggradation occurs in a strongly episodic fashion. However, this remains a largely untested hypothesis due to the difficulty in establishing age models with sufficiently high resolution. Here we use 39 optically stimulated luminescence (OSL) ages from proximal overbank deposits in the Mississippi Delta to demonstrate for the first time that alluvial aggradation over centennial to millennial time scales is predominantly episodic, with aggradation rates of 1–4 cm/yr that can persist for centuries. OSL ages from three separate study areas produce age clusters that are distinctly different yet complement each other. These findings suggest that a substantial portion of the continental stratigraphic record consists of patchworks of relatively discrete, centennial- to millennial-scale sediment bodies assembled by autogenic processes.

Description: Sediment-hosted massive sulfide (SHMS) deposits are an important source of global zinc resources, and the Red Dog Pb-Zn-Ag ± Ba district in Alaska (USA) contains giant deposits of this type. The existing model for ore formation at Red Dog involves early diagenetic replacement of sediment deposited in a restricted basin with stratified suboxic bottom waters. We present new observations of trace fossils Schaubcylindrichnus ichnospecies (isp.) and Chondrites isp. in several Red Dog deposits. The presence of the trace fossils, the size of the largest burrows, and the pervasiveness of the ichnofabric indicate that at least some intervals of the host sediment were deposited in an oxygenated middle to outer shelf environment. The burrow linings and infill are replaced by barite, hydrothermal quartz, and sulfide minerals, and the lack of compaction suggests that mineralization was diagenetically early. To reconcile these data with those from previous regional sedimentological and lithogeochemical studies, we propose a new model whereby the ore-hosting sediment was deposited in a shelfal setting in which redox conditions were affected by a fluctuating oxygen minimum zone. The strong spatial correlation between bioturbation and Red Dog SHMS deposits suggests that the presence of trace fossils may have played an important role in controlling the flow of ore-forming fluids by increasing host sediment permeability.

Description: On Earth, carbon sequestration in geologic units plays an important role in the carbon cycle, scrubbing CO 2 from the atmosphere for long-term storage. While carbonate is identified in low abundances within the dust and soils of Mars, at 〈1 wt% in select meteorites, and in limited outcrops, no massive carbonate rock reservoir on Mars has been identified to date. Here, we investigate the largest exposed carbonate-bearing rock unit, the Nili Fossae plains, combining spectral, thermophysical, and morphological analyses to evaluate the timing and carbon sequestration potential of rocks on Mars. We find that the olivine-enriched (~20%–25%) basalts have been altered, by low-temperature in situ carbonation processes, to at most ~20% Fe-Mg carbonate, thus limiting carbon sequestration in the Nili Fossae region to ~0.25–12 mbar of CO 2 during the late Noachian–early Hesperian, before or concurrent with valley network formation. While this is large compared to modern-day CO 2 reservoirs, the lack of additional, comparably sized post–late Noachian carbonate-bearing deposits on Mars indicates ineffective carbon sequestration in rock units over the past ~3.7 b.y. This implies a thin atmosphere (500 mbar) during valley network formation, extensive post-Noachian atmospheric loss to space, or diffuse, deep sequestration by a yet-to-be understood process. In stark contrast to Earth’s biologically mediated crust:atmosphere carbon reservoir ratio of ~10 4 –10 5 , Mars’ ratio is a mere ~10–10 3 , even if buried pre-Noachian crust holds multiple bars.

Description: In lithospheric-scale strike-slip fault zones, upper crustal strength is well constrained from borehole observations and fault rock deformation experiments, but mantle strength is less well known. Using peridotite xenoliths, we show that the upper mantle below the San Andreas fault system (California, USA) is dry and its maximum resolved shear stress (5–9 MPa) is similar to the shear strength of the upper, seismogenic portion of the fault. These results do not fit with any existing lithospheric strength profile. We propose the "lithospheric feedback" model in which the upper crust and lithospheric mantle act together as an integrated system. Mantle flow controls displacement and loads the upper crust. In contrast, the upper crust controls the stress magnitude in the integrated system. Crustal rupture transiently increases strain rate in the upper mantle below the strike-slip fault, leading to viscous strain localization. The lithospheric feedback model suggests that lithospheric strength is a dynamic property—varying in space and time—in actively deforming regions.

Description: The mid-Pliocene warm period (MPWP, 3.3–2.9 Ma), with reconstructed atmospheric p CO 2 of 350–450 ppm, represents a potential analogue for climate change in the near future. Current highly cited estimates place MPWP maximum global mean sea level (GMSL) at 21 ± 10 m above modern, requiring total loss of the Greenland and marine West Antarctic Ice Sheets and a substantial loss of the East Antarctic Ice Sheet, with only a concurrent 2–3 °C rise in global temperature. Many estimates of Pliocene GMSL are based on the partitioning of oxygen isotope records from benthic foraminifera ( 18 O b ) into changes in deep-sea temperatures and terrestrial ice sheets. These isotopic budgets are underpinned by the assumption that the 18 O of Antarctic ice ( 18 O i ) was the same in the Pliocene as it is today, and while the sensitivity of 18 O b to changing meltwater 18 O has been previously considered, these analyses neglect conservation of 18 O/ 16 O in the ocean-ice system. Using well-calibrated 18 O-temperature relationships for Antarctic precipitation along with estimates of Pliocene Antarctic surface temperatures, we argue that the 18 O i of the Pliocene Antarctic ice sheet was at minimum 1–4 higher than present. Assuming conservation of 18 O/ 16 O in the ocean-ice system, this requires lower Pliocene seawater 18 O without a corresponding change in ice sheet mass. This effect alone accounts for 5%–20% of the 18 O b difference between the MPWP interglacials and the modern. With this amended isotope budget, we present a new Pliocene GMSL estimate of 9–13.5 m above modern, which suggests that the East Antarctic Ice Sheet is less sensitive to radiative forcing than previously inferred from the geologic record.

Description: Natural deformation of rocks is commonly associated with development of mineralogical layering, leading to irreversible transformations of their microstructure. The mechanisms of such chemical differentiation processes during diagenesis, tectonics, metamorphism, or fault differentiation remain poorly understood, as they are difficult to reproduce experimentally due to the very slow kinetics involved. This paper shows that development of differentiated layering, similar to that observed in natural deformation, is stress driven and can be obtained from indenter experiments. Samples of (1) gypsum plaster mixed with clay, and (2) natural diatomite loosely interbedded with volcanic ash, saturated with aqueous solutions in equilibrium, were subjected to loading for several months at 40 °C and 150 °C, respectively. X-ray microtomography and scanning electron microscopy observations show that layering develops by a self-organized pressure solution process. Stress-driven dissolution of the soluble minerals (either gypsum or silica) is initiated in the areas initially richer in insoluble species (clay or volcanic ash), as diffusive mass transfer along the interface between soluble and insoluble minerals is much faster than along the healed boundaries of the soluble minerals. The passive concentration of the insoluble minerals amplifies the dissolution along layers oriented perpendicularly to the maximum compressive stress. Conversely, in areas with an initial low content of insoluble minerals and clustered soluble minerals, dissolution is slower. Consequently, these areas are less deformed; they host the re-deposition of the soluble species and act as rigid objects that concentrate both stress and dissolution near their boundaries, thus amplifying the differentiation and the development of layered microstructures.

Description: Episodic fluid escape from marine sediments results from overpressure development and pressure release, and can occur slowly through geologic time or catastrophically. Morphological features in regions of fluid seepage include doming, mud volcanism, cratering, and pockmark formation. Vertical sediment mobilization and surface erosion are considered the principal mechanisms for these topographic changes. However, the impact of mobilization on the geotechnical properties of sediments has not been explicitly considered. Here we develop a one-dimensional numerical subsidence model that incorporates the well-established behavior of remolded fine-grained cohesive sediments. We use this to show that if subsurface overpressure results in the mobilization of sediments, large settlements (20%–35% reduction in volume) can occur when overpressure dissipates. This presents a novel mechanism to explain changes in seafloor and subsurface topography in areas of fluid escape, while highlighting an important interplay between subsurface fluid flow and the geotechnical properties of fine-grained cohesive sediments.

Description: Terrestrial records from the Canadian Arctic Archipelago indicate that a temperate and moist climate prevailed during the Pliocene, i.e., before the intensification of Northern Hemisphere glaciations, but the controlling factors causing these conditions remains elusive. Here we present marine seismic reflection evidence of contiguous kilometer-scale sedimentary drifts, or contourites, buried below the West Greenland shelf, which relates high-latitude Pliocene warmth to intense geostrophic currents along the eastern Baffin Bay margin. Seismic to borehole correlation indicates that enhanced current-induced deposition began during the middle Miocene, ca. 17–15 Ma, and terminated during the late Pliocene onset of global cooling. The transition from along-slope sediment transport to trough-mouth fan progradation designates the onset of Greenland Ice Sheet glaciers advancing across the continental shelf and coincides with increasing iceberg productivity in Baffin Bay and the Nordic Seas from 3.3 to 2.7 Ma. Our results suggest that the Baffin Bay–West Greenland margin formed a late Neogene passageway for persistent ocean boundary currents, and we infer that the Pliocene high-latitude warmth was to a significant extent caused or enhanced by advection of subtropical heat and moisture to the Arctic regions. Attenuation of the poleward heat flux conveyed by the paleo–current system may have been critical for allowing glacial buildup in the Canadian Arctic and northern Greenland during the late Pliocene. We further surmise that vertical tectonic adjustments of the continental margin played a role in this attenuation, which eventually caused the demise of meridional ocean circulation through Baffin Bay.

Description: Terrestrial impact events have had a profound influence on Earth’s geological, geochemical, and biological evolution. However, the record of Precambrian impacts is poorly constrained due to the dynamic nature of plate tectonics, erosion, and deposition of younger rocks that may destroy or cover the evidence. Here we report the first Precambrian occurrence of the rare mineral reidite (ZrSiO 4 ) within grains of shocked zircon in the ca. 1.18 Ga Stac Fada Member (Stoer Group), northwestern Scotland. The reidite, preserved as 〈2-µm-wide lamellae, is unambiguous evidence of shock pressures in excess of ~30 GPa and confirms the impact origin for the Stac Fada deposit. The reidite lamellae are locally deformed, and sites of deformation record its decomposition to baddeleyite (ZrO 2 ) and amorphous silica, the first natural example of this transformation. The findings demonstrate that reidite and baddeleyite may form and be transported in high-energy ejecta without physical or chemical breakdown and are stable during sedimentary diagenesis and low-grade metamorphism. Thus, reidite may be preserved over time scales exceeding 1 b.y., establishing the use of reidite within detrital shocked zircon from Precambrian strata as a viable and valuable means of recognizing and characterizing ancient terrestrial impact events.

Description: The pyroxenite-peridotite sequence from the External Ligurian (northern Apennines, Italy) ophiolites is evidence of the evolution of recycled crust within the mantle. We present new major and trace element and Nd-Hf isotopic compositions of garnet clinopyroxenites and websterites from this mantle section. The garnet clinopyroxenites display clinopyroxene and bulk-rock rare earth element patterns with distinct positive Eu anomalies, which argue for the involvement of plagioclase-rich precursors in their origin. We propose that the garnet clinopyroxenites formed by crystallization of eclogite-derived melts that underwent negligible interaction with the host peridotites. The garnet websterites are interpreted to have been produced by reactions between the eclogite-derived melts and peridotites, thereby giving rise to hybrid, second-stage pyroxenites with a crustal geochemical fingerprint. In our petrogenetic scenario, a rifting-related event at ca. 220 Ma caused melting of eclogites originating from a mid-oceanic ridge basalt–type gabbroic sequence. These mafic protoliths underwent a long-lived evolution of recycling in the mantle (1.5–1.0 Ga). We show that heterogeneity of crustal protoliths, age of recycling, and interaction with the host peridotites may lead to a significant compositional and isotopic diversity of crust-derived mantle pyroxenites.

Description: The strength, shape, and ultimately seismic behavior of many thin-skinned fold and thrust belts, including marine accretionary wedges, are strongly controlled by large-scale faults that develop from weak, clay-rich sedimentary horizons (décollements). The increase of temperature with depth along clay-rich faults promotes the so-called smectite-illite transition, which may influence the fault strength, fluid distribution, and possibly the onset of seismicity. Here we report on the frictional properties of intact fault rocks retrieved from two large décollements, which were exhumed from depths above and below the smectite-illite transition. We find that all tested rocks are characterized by very low friction (μ = 0.17–0.26), velocity-strengthening behavior, and low rates of frictional healing, suggesting long-term fault weakness. Combining our experimental results with the critical taper theory, we computed the effective friction, F , of megathrusts beneath several accretionary wedges around the world; the result was extremely low (0.03 〈 F 〈 0.14), and in agreement with other independent estimates. Our analysis indicates a long-term weakness that can explain the shape of several tectonic wedges worldwide without invoking diffuse near-lithostatic fluid overpressures.

Description: Global compilations indicate that the geochemistry of arc magmatism is sensitive to Moho depth. Magmatic products are prevalent throughout the history of Cordilleran orogenesis and can be employed to constrain the timing of changes in crustal thickness as well as the magnitude of those changes. We investigate temporal variations in crustal thickness in the United States Cordillera using Sr/Y from intermediate continental arc magmas. Our results suggest that crustal thickening began during the Late Jurassic to Early Cretaceous and culminated with 55–65-km-thick crust at 85–95 Ma. Crustal thicknesses remained elevated until the mid-Eocene to Oligocene, after which time crustal thicknesses decreased to 30–40 km in the Miocene. The results are consistent with independent geologic constraints and suggest that Sr/Y is a viable method for reconstructing crustal thickness through time in convergent orogenic systems.

Description: Earthquakes impart an impressive force on epicentral landscapes, with immediate catastrophic hillslope response. However, their legacy on geomorphic process rates remains poorly constrained. We have determined the evolution of landslide rates in the epicentral areas of four intermediate to large earthquakes (M w , 6.6–7.6). In each area, landsliding correlates with the cumulative precipitation during a given interval. Normalizing for this meteorological forcing, landslide rates have been found to peak after an earthquake and decay to background values in 1–4 yr, with the decay time scale probably proportional to the earthquake magnitude. The transient pulse of landsliding is not related to external forcing such as rainfall or aftershocks, and we tentatively attribute it to the reduction and subsequent recovery of ground strength. Observed geomorphic trends are not linked with groundwater level changes or root system damage, both of which could affect substrate strength. We propose that they are caused by reversible damage of rock mass and/or loosening of regolith. Qualitative accounts of ground cracking due to strong ground motion abound, and our observations are circumstantial evidence of its potential importance in setting landscape sensitivity to meteorological forcing after large earthquakes.

Description: Upwelling hot mantle plumes are thought to disintegrate continental lithosphere and are considered to be drivers of active continental breakup. The formation of the Walvis Ridge during the opening of the South Atlantic is related to a putative plume-induced breakup. We investigated the crustal structure of the Walvis Ridge (southeast Atlantic Ocean) at its intersection with the continental margin and searched for anomalies related to the possible plume head. The overall structure we identify suggests that no broad plume head existed during opening of the South Atlantic and anomalous mantle melting occurred only locally. We therefore question the importance of a plume head as a driver of continental breakup and further speculate that the hotspot was present before the rifting, leaving a track of kimberlites in the African craton.

Description: The evolution of the Antarctic ice sheet during the mid-Pliocene warm period (MPWP) remains uncertain and has important implications for our understanding of ice sheet response to modern global warming. The extent to which marine-based sectors of the East Antarctic Ice Sheet (EAIS) retreated during the MPWP is particularly contentious, with geological observations and geochemical analyses being cited to argue for either a relatively minor or a significant ice sheet retreat in response to mid-Pliocene warming. The stability of marine-based ice sheets is intimately linked to bedrock elevation at their grounding lines, and previous ice sheet modeling assumed that Antarctic bedrock elevation during the MPWP was the same as today with the exception of a correction for the crustal response to ice loading. However, various processes may have perturbed bedrock elevation over the past 3 m.y., most notably vertical deflections of the crust driven by mantle convective flow, or dynamic topography. Here we present simulations of mantle convective flow that are consistent with a wide range of present-day observables and use them to predict changes in dynamic topography and reconstruct bedrock elevations during the MPWP. We incorporate these elevations into a simulation of the Antarctic ice sheet during the MPWP and find that the correction for dynamic topography change has a significant effect on the stability of the EAIS within the marine-based Wilkes Basin, with the ice margin in that sector retreating considerably further inland (200–560 km) relative to simulations that do not include this correction for bedrock elevation.

Description: Volcanic arcs above subduction zones are enriched in volatiles and fluid-mobile elements with respect to mid-oceanic ridge basalts. There is general consensus that this particular subduction zone signature is generated by fluid-induced extraction of these elements from subducted oceanic crust and its sedimentary cover. However, how these fluids are transferred through the mantle wedge to the locus of partial melting and what modification the fluids will experience is unresolved. Here we investigate the interaction of slab fluids with the mantle wedge through a series of high-pressure experiments. We explore two end-member processes of focused and porous reactive flow of hydrous slab melts through the mantle. Transfer by porous flow leads to the formation of hydrous minerals that sequester fluid-mobile elements and residual fluids characterized by trace element patterns inconsistent with typical arc lavas. In contrast, no hydrous minerals are formed in the reaction zone of experiments mimicking focused flow, and the typical trace element signature acquired during fluid extraction from the slab is preserved, indicating that this is an efficient process for element transfer through the mantle wedge.

Description: The formation of platinum-group minerals (PGM) during magma differentiation has been suggested to be an important process in primitive magma evolution, but decisive textural evidence is difficult to obtain because PGM tend to be very small and very rare. We have investigated Cr-spinel phenocrysts from two oxidized magmas (Siberian meimechite and Vanuatu [Ambae Island] arc picrite) and one reduced magma (Uralian [Russia] ankaramite) for PGM inclusions and their platinum-group element (PGE) contents. We observed Os-Ir and Pt-Fe alloys entrapped as inclusions in Cr-spinel in all three suites of lava. The alloys may occur in association with PGE-bearing sulfides and co-trapped silicate melt. Cr-spinel crystals also contain measurable amounts of Os, Ir, Ru, and Rh, which are at concentrations 2 x –100 x higher than mantle values. Thermodynamic models indicate that the arc picrite and ankaramite melts were probably both saturated with the observed PGM phases, whereas the Os-Ir alloy grain observed in the meimechite is not in equilibrium with the "bulk" melt. Our results demonstrate that PGM (alloys and sulfides) occur as liquidus phases in primitive (unfractionated) melts at high temperature and at a variety of redox conditions, and that Cr-spinel is a significant host of PGE, either in the crystal structure or as PGM inclusions.

Description: The evolution of oxygenic photosynthesis was the most important geochemical event in Earth history, causing the Great Oxidation Event (GOE) ~2.4 b.y. ago. However, evidence is mixed as to whether O 2 production occurred locally as much as 2.8 b.y. ago, creating O 2 oases, or initiated just prior to the GOE. The biogeochemical dynamics of possible O 2 oases have been poorly constrained due to the absence of modern analogs. However, cyanobacteria in microbial mats in a perennially anoxic region of Lake Fryxell, Antarctica, create a 1–2 mm O 2 -containing layer in the upper mat during summer, providing the first known modern analog for formation of benthic O 2 oases. In Lake Fryxell, benthic cyanobacteria are present below the oxycline in the lake. Mat photosynthesis rates were slow due to low photon flux rate (1–2 µmol m –2 s –1 ) under thick ice cover, but photosynthetic O 2 production was sufficient to sustain up to 50 µmol O 2 L –1 , sandwiched between anoxic overlying water and anoxic sediments. We hypothesize that Archean cyanobacteria could have similarly created O 2 oases in benthic mats prior to the GOE. Analogous mats may have been at least partly responsible for geological evidence of oxidative weathering prior to the GOE, and habitats such as Lake Fryxell provide natural laboratories where the impact of benthic O 2 oases on biogeochemical signatures can be investigated.

Description: The end-Permian extinction records the greatest ecological catastrophe in Earth history. The vertebrate fossil record in the Karoo Basin, South Africa, has been used for more than a century as the standard for understanding turnover in terrestrial ecosystems, recently claimed to be in synchrony with the marine crisis. Workers assumed that systematic turnover at the Dicynodon assemblage zone boundary, followed by the appearance of new taxa directly above the base of the Lystrosaurus assemblage zone, is the continental expression of the end-Permian event and recovery. To test this hypothesis, we present the first high-precision age on strata close to the inferred Permian-Triassic boundary. A U-Pb isotope dilution–thermal ionization mass spectrometry zircon age of 253.48 ± 0.15 Ma (early Changhsingian) is from a silicified ash layer ~60 m below the current vertebrate-defined boundary at Old Lootsberg Pass (southern South Africa). This section yields newly discovered plants and vertebrates, and is dominated by a normal polarity signature. Our collective data suggest that the Dicynodon - Lystrosaurus assemblage zone boundary is stratigraphically higher than currently reported, and older than the marine extinction event. Therefore, the turnover in vertebrate taxa at this biozone boundary probably does not represent the biological expression of the terrestrial end-Permian mass extinction. The actual Permian-Triassic boundary in the Karoo Basin is either higher in the Katberg Formation or is not preserved. The currently accepted model of the terrestrial ecosystem response to the crisis, both in this basin and its extension globally, requires reevaluation.

Description: Olivine composition and zoning patterns are widely used to reconstruct the evolution of mafic magmas from source to surface and to extract time scales of magmatic processes. Deciphering the olivine zoning code is challenging because the contributions of growth and diffusion may overlap. A detailed multielement (Fe, Mg, P, Al, and Ni) examination of zoning in an exceptional olivine with skeletal morphology allows unequivocal discrimination between these two processes using simple but powerful geometrical arguments. Olivine crystals initially grow rapidly and diagonally from corner locations, whereas diffusion effectively tracks mutually perpendicular crystal lattice orientations. Generating the zoning patterns for our case-study olivine required at least 4–5 months of diffusive reequilibration of Fe-Mg, further demonstrating that crystal morphologies produced by rapid growth can survive at magmatic temperatures for extended periods. No significant major element zoning is preserved after rapid growth, lending further credibility to time scales retrieved via diffusion modeling. Extending multielement approaches to decoding olivine zoning patterns can help determine whether the kinetic relationship between growth- and diffusion-induced zoning recognized herein is widely applicable. Such studies will improve our understanding of time scales of magma storage, solidification, mixing, and/or transit toward the surface.

Description: The structure and mechanical properties of crystalline materials of three boron difluoride dibenzoylmethane (BF2dbm) derivatives were investigated to examine the correlation, if any, among mechanochromic luminescence (ML) behaviour, solid-state structure, and the mechanical behaviour of single crystals. Qualitative mechanical deformation tests show that the crystals of BF2dbm(tBu)2 can be bent permanently, whereas those of BF2dbm(OMe)2 exhibit an inhomogeneous shearing mode of deformation, and finally BF2dbmOMe crystals are brittle. Quantitative mechanical analysis by nanoindentation on the major facets of the crystals shows that BF2dbm(tBu)2 is soft and compliant with low values of elastic modulus, E, and hardness, H, confirming its superior suceptibility for plastic deformation, which is attributed to the presence of a multitude of slip systems in the crystal structure. In contrast, both BF2dbm(OMe)2 and BF2dbmOMe are considerably stiffer and harder with comparable E and H, which are rationalized through analysis of the structural attributes such as the intermolecular interactions, slip systems and their relative orientation with respect to the indentation direction. As expected from the qualitative mechanical behaviour, prominent ML was observed in BF2dbm(tBu)2, whereas BF2dbm(OMe)2 exhibits only a moderate ML and BF2dbmOMe shows no detectable ML, all examined under identical conditions. These results confirm that the extent of ML in crystalline organic solid-state fluorophore materials can be correlated positively with the extent of plasticity (low recovery). In turn, they offer opportunities to design new and improved efficient ML materials using crystal engineering principles.

Description: In the last few decades, supramolecular chemistry has been at the forefront of chemical research, with the aim of understanding chemistry beyond the covalent bond. Since the long-range periodicity in crystals is a product of the directionally specific short-range intermolecular interactions that are responsible for molecular assembly, analysis of crystalline solids provides a primary means to investigate intermolecular interactions and recognition phenomena. This article discusses some areas of contemporary research involving supramolecular interactions in the solid state. The topics covered are: (1) an overview and historical review of halogen bonding; (2) exploring non-ambient conditions to investigate intermolecular interactions in crystals; (3) the role of intermolecular interactions in morphotropy, being the link between isostructurality and polymorphism; (4) strategic realisation of kinetic coordination polymers by exploiting multi-interactive linker molecules. The discussion touches upon many of the prerequisites for controlled preparation and characterization of crystalline materials.

Description: Characterization of transverse coherence is one of the most critical themes for advanced X-ray sources and their applications in many fields of science. However, for hard X-ray free-electron laser (XFEL) sources there is very little knowledge available on their transverse coherence characteristics, despite their extreme importance. This is because the unique characteristics of the sources, such as the ultra-intense nature of XFEL radiation and the shot-by-shot fluctuations in the intensity distribution, make it difficult to apply conventional techniques. Here, an extended Young's interference experiment using a stream of bimodal gold particles is shown to achieve a direct measurement of the modulus of the complex degree of coherence of XFEL pulses. The use of interference patterns from two differently sized particles enables analysis of the transverse coherence on a single-shot basis without a priori knowledge of the instantaneous intensity ratio at the particles. For a focused X-ray spot as small as 1.8 µm (horizontal) × 1.3 µm (vertical) with an ultrahigh intensity that exceeds 1018 W cm−2 from the SPring-8 Ångstrom Compact free-electron LAser (SACLA), the coherence lengths were estimated to be 1.7 ± 0.2 µm (horizontal) and 1.3 ± 0.1 µm (vertical). The ratios between the coherence lengths and the focused beam sizes are almost the same in the horizontal and vertical directions, indicating that the transverse coherence properties of unfocused XFEL pulses are isotropic. The experiment presented here enables measurements free from radiation damage and will be readily applicable to the analysis of the transverse coherence of ultra-intense nanometre-sized focused XFEL beams.

Description: Lakes are major depositional systems for which the related depositional processes have long been considered relatively simple. Breaking this statement, this study presents a detailed analysis of deposits in Lake Saint-Jean, the third largest natural lake in Québec. In addition to postglacial deltaic and coastal depositional systems fringing the lake, current-controlled features such as a large subaqueous prograding wedge and three sediment drifts have been identified in its central portion based on two-dimensional (2-D) acoustic high-resolution subbottom profiles. The large subaqueous prograding wedge is a 4-km-long and up to 15-m-thick heterolithic shelf-like construction in the southeastern part of the lake. The three sediment drifts are 0.1–0.5-km-long and 2–5-m-thick mud mounds distributed on the lake floor in the central portion of the lake. Diatom analyses and radiocarbon dating show that the development of these current-controlled features occurred during the lacustrine phase, after the disconnection with the postglacial marine Laflamme Gulf at 8.5 cal. k.y. B.P. Depositional facies show evidence of recurrent bottom-current activity. Related deposits alternate with pelagic sedimentation stages characterized by the settling of mud and biogenic accumulations. We investigated the origin of bottom currents using a numerical simulation (SYMPHONIE, an oceanographic model), with the aim of modeling wind-induced lake-scale water circulation. Simulations suggest that the subaqueous prograding wedge and the three sediment drifts result from wind-induced bottom currents generated by storm events having wind speed greater than 10 m s –1 . Such strong winds are able to significantly affect sedimentation in the central portion of Lake Saint-Jean. The resulting wind-induced sedimentary features were integrated into a refined lacustrine depositional model that summarizes the evolution of a group of water bodies referred to as "wind-driven water bodies." This study applies a new tool for lake strata characterization and highlights the potential difficulty in differentiating them from marine deposits in the geological record.

Description: In our complementary geochemical study (Part 1), the Malaysian granitoids of the Southeast Asian tin belt were divided into a Middle Permian to Late Triassic I-type–dominated Eastern province (Indochina terrane) and a Triassic to Early Jurassic transitional I/S-type Main Range province (Sibumasu terrane), separated by the Bentong-Raub suture zone which closed in the Late Triassic. Previous geochronology has relied on only a few U-Pb zircon ages together with K-Ar and whole rock Rb-Sr ages that may not accurately record true magmatic ages. We present 39 new high-precision U-Pb zircon ion microprobe ages from granitoids and volcanics across the Malay Peninsula. Our results show that ages from the Eastern province granitoids span 289–220 Ma, with those from the Main Range province granitoids being entirely Late Triassic, spanning 227–201 Ma. A general westerly younging magmatic trend across the Malay Peninsula is considered to reflect steepening and roll-back of the Bentong-Raub subduction zone during progressive closure of Paleo-Tethys. The youngest ages of subduction-related granites in the Eastern province roughly coincide with the youngest ages of marine sedimentary rocks along the Paleo-Tethyan suture zone. Our petrogenetic and U-Pb zircon age data support models that relate the Eastern province granites to pre-collisional Andean-type magmatism and the western Main Range province granites to syn- and post-collisional crustal melting of Sibumasu crust during the Late Triassic. Tin mineralization was mainly associated with the latter phase of magmatism. Two alternative tectonic models are discussed to explain the Triassic evolution of the Malay Peninsula. The first involves a second Late Triassic to Jurassic or Early Cretaceous east-dipping subduction zone west of Sibumasu where subduction-related hornblende and biotite–bearing granites along Sibumasu are paired with Main Range crustal-melt tin-bearing granites, analogous to the Bolivia Cordilleran tin-bearing granite belt. The second model involves westward underthrusting of Indochina beneath the West Malaya Main Range province, resulting in crustal thickening and formation of tin-bearing granites of the Main Ranges. Cretaceous granitoids are also present locally in Singapore (Ubin diorite), on Tioman Island, in the Noring pluton, of the Stong complex (Eastern Province), and along the Sibumasu terrane in southwest Thailand and Burma (Myanmar), reflecting localized crustal melting.

Description: Liquefaction during the 2010 moment magnitude (M w ) 7.1 Darfield earthquake and large aftershocks (known as the Canterbury earthquake sequence) caused severe damage to land and infrastructure in Christchurch, New Zealand. Liquefaction occurred at M w -weighted peak ground accelerations (PGA 7.5 ) as low as 0.06 g at highly susceptible sites. Trenching investigations conducted at two sites in eastern Christchurch enabled documentation of the geologic expressions of recurrent liquefaction and determination of whether evidence of pre–Canterbury earthquake sequence liquefaction is present. Excavation to water table depths (~1–2 m below surface) across sand blow vents and fissures revealed multiple generations of Canterbury earthquake sequence liquefaction "feeder" dikes that crosscut Holocene-to-recent fluvial and anthropogenic stratigraphy. Canterbury earthquake sequence dikes crosscut and intrude oxidized and weathered dikes and sills at both sites that are interpreted as evidence of pre–Canterbury earthquake sequence liquefaction. Crosscutting relationships combined with 14 C dating constrain the timing of the pre–Canterbury earthquake sequence liquefaction to post–A.D. 1660 to pre–ca. A.D. 1905 at one site, and post–A.D. 1415 to pre–ca. A.D. 1910 at another site. The PGA 7.5 of five well-documented historical earthquakes that caused regional damage between 1869 and 1922 are approximated for the study sites using a New Zealand specific ground motion prediction equation. Only the June 1869 M w ~4.8 Christchurch earthquake produces a median modeled PGA 7.5 that exceeds the PGA 7.5 0.06 g threshold for liquefaction. Prehistoric earthquakes sourced from regional faults, including the 1717 Alpine fault M w ~7.9 ± 0.3 and ca. 500–600 yr B.P. M w ≥ 7.1 Porters Pass fault earthquakes, provide additional potential paleoseismic sources for pre–Canterbury earthquake sequence liquefaction. The recognition of pre–Canterbury earthquake sequence liquefaction in late Holocene sediments is consistent with hazard model-based predicted return times of PGAs exceeding the liquefaction triggering threshold in Christchurch. Residential development in eastern Christchurch from ca. 1860 to 2005 occurred in areas where geologic evidence for pre–Canterbury earthquake sequence liquefaction was present, highlighting the potential of paleoliquefaction studies to predict locations of future liquefaction and to contribute to seismic hazard assessments and land-use planning.

Description: The Siberian Traps are one of the largest known continental flood basalt provinces and may be causally related to the end-Permian mass extinction. In some areas, a large fraction of the Siberian Traps volcanic sequence consists of mafic volcaniclastic rocks. Here, we synthesize paleomagnetic, petrographic, and field data to assess the likely origins of these volcaniclastic rocks and their significance for the overall environmental impact of the eruptions. We argue that magma-water interactions, including both lava-water interactions and phreatomagmatic explosions in vents, were important components of Siberian Traps magmatism. Phreatomagmatic episodes may have generated tall water-rich eruption columns, simultaneously promoting removal of highly soluble volcanic gases such as HCl and potentially delivering additional sulfur to the upper atmosphere.

Description: The Central Asian orogenic belt formed by accretion subsequent to the contraction of the paleo–Asian Ocean, and its southeastern segment terminated along the Solonker suture zone, amalgamating the Mongolian arcs and the North China craton by the end of the Early Triassic. Since typical regional collisional features are absent, its tectonic evolution remains speculative. An investigation into the variability of detrital zircon U-Pb ages and Hf ratios in Permian strata along a southeast-northwest transect from the Mongolian arcs to the North China craton reveals distinct differences: Northern basins carry a broad Mesoproterozoic to latest Precambrian age signature, and their sedimentary source terranes are of mixed juvenile to crustal magmatic origin. In contrast, southern basins contain detritus from the North China craton, and their sources are of dominantly crustal contaminated magmatic origin. Provenance analysis suggests that in the early Paleozoic (ca. 429 Ma), the paleo–Asian Ocean was consumed along the Uliastai arc and the North China craton, initiating the formation of the northern and southern accretionary orogens, respectively. By the end of the middle Carboniferous, the Mongolian arcs were consolidated after the accretion of the Uliastai arc. In the late Carboniferous (ca. 314 Ma), the Hegenshan back-arc basin opened, detaching the northern accretionary orogen. While subduction continued there, it may have temporarily ceased in the south after the accretion of a microcontinent (ca. 300 Ma). By the end of the Middle Permian (ca. 269 Ma), back-arc basin closure led to the formation and obduction of the suprasubduction zone Hegenshan ophiolite. Eventually, the paleo–Asian Ocean closed by double-sided subduction. Such accretionary wedge-wedge collision would not involve continental deep subduction, unlike other continental collisions. The early stages of the sequence likely resembled a Pacific-type scenario dominated by the subduction of oceanic lithosphere, including a Japanese Sea–type back-arc basin opening. The late stages involved the accretion of large-scale continental blocks, ultimately leading to final ocean closure, which may be similar to the archipelago-type setting of present-day Southeast Asia.

Description: We report geochemical data from (meta-)sedimentary and igneous rocks that crop out in the Ford Ranges of western Marie Byrd Land and discuss the evolution and reworking of the crust in this region during Paleozoic subduction along the former Gondwanan convergent plate margin. Detrital zircon age spectra from the Swanson Formation, a widespread low-grade metaturbidite sequence, define distinct populations in the late Paleoproterozoic, late Mesoproterozoic, and Neoproterozoic–Cambrian. The late Paleoproterozoic group records magmatism derived from a mixed juvenile and crustal source. By contrast, the late Mesoproterozoic group yields Hf isotope values consistent with derivation from a juvenile Mesoproterozoic source inferred to be an unexposed Grenville-age orogenic belt beneath the East Antarctic ice sheet. For the Neoproterozoic–Cambrian population, Hf isotope values indicate reworking of these older materials during Ross–Delamerian orogenesis. New U-Pb ages from the Devonian–Carboniferous Ford Granodiorite suite across the Ford Ranges reveal an extended period of arc magmatism from 375 to 345 Ma. For four younger samples of Ford Granodiorite, Hf and O isotope values in zircon suggest involvement of a larger (meta-)sedimentary component in the petrogenesis than for two older samples. This contrasts with the secular trend toward more juvenile values documented from Silurian to Permian granite suites in the Tasmanides of eastern Australia and Famennian to Tournasian granite suites in New Zealand, pieces of continental crust that were once contiguous with western Marie Byrd Land along the Gondwana margin. The differences may relate to an along-arc change from the typical extensional accretionary mode in eastern Australia to a neutral or an advancing mode in West Antarctica, and to an across-arc difference in distance from the trench between the New Zealand fragments of Zealandia and western Marie Byrd Land. Upper Devonian anatectic granites in the Ford Ranges most likely record reworking of early Ford Granodiorite suite members during arc magmatism.

Description: Coastal dune accumulations at locations around the world have formed during sea-level highstands, rising sea levels, falling sea levels, and fluctuating sea levels. Initially, episodes of maximum Pleistocene dune activity in Bermuda, which is the type location for eolianite, were correlated with glacio-eustatic regressions. This model had been abandoned by the 1970s in favor of a rising sea-level model for Bermuda’s north shore and autogenic sediment-supply model for the south shore. We report the results of a detailed investigation of the intercalated mid-late Pleistocene beach and coastal dune deposits on the islands of Bermuda, and we test the existing models for these deposits. We contend that the north shore model is invalidated by past misidentification of eolian deposits as transgressive beach deposits. On the south shore, facies analysis of the marine isotope stage 7 (MIS 7) Belmont Formation reveals that coastal deposition was divided into two phases. S1, which includes foreshore and shoreface progradation, is predominantly marine and coincided with rising relative sea level. S2, which commenced with protosol development on top of the marine section prior to burial by advancing dunes, is nonmarine. We conclude that the two successions S1 and S2 resulted from forcing by glacio-hydro-isostatic relative sea-level change spanning a highstand (rising and then falling), rather than from transgressive or autogenic processes. The finding that, in Bermuda, the construction of protective coastal dunes depends on a falling relative sea level has potential implications for many vulnerable shorelines that face rising relative sea levels associated with climate change.

Description: Thin-skinned gravitational gliding and spreading drive deformation on salt-bearing passive margins. Such margins typically have an updip extensional domain kinematically linked to a downdip contractional domain. However, calculating magnitudes of extension and shortening in salt-bearing margins is difficult because the initial widths of diapirs are uncertain. Extension and shortening may be cryptic, being hidden in widening or shortening of diapirs. This uncertainty can lead to controversy in regional analysis. The Santos Basin, offshore Brazil, contains a prime example of this uncertainty in the form of an enigmatic structure known as the "Albian Gap," a zone up to 75 km wide within which the Albian section is missing. The Albian Gap has been variably interpreted as the product of post-Albian extensional faulting (the extension model) or as an Albian salt structure evacuated in response to loading by post-Albian sediments (the expulsion model). We evaluate these two models by: (1) structurally restoring a regional seismic-reflection profile across the Albian Gap using both models; (2) quantitatively analyzing the geometry of the Upper Cretaceous rollover overlying the Albian Gap; and (3) synthesizing and critically evaluating arguments previously advanced in support of extension or expulsion. We propose a revised model for the evolution of the Albian Gap that invokes Albian thin-skinned extension and post-Albian salt expulsion. Our approach shows that critical analysis of geological observations from borehole-constrained seismic-reflection data can be used to assess the relative roles of the key processes in the deformation of salt-bearing passive margins.

Description: Numerous geochronological and geochemical studies of the Late Triassic granite plutons in the Qinling orogen have been conducted over the past few decades. These studies have extensively discussed the genesis and correlations of granite plutons with the collisional processes between the North and South China blocks. However, several contradictory conclusions on the tectonic settings of these plutons (subduction, syncollision, postcollision, or intraplate) have been reached. Moreover, in all these studies, compressional or extensional structures have been always considered to control the magma emplacement, but no direct evidence has been presented so far. In order to clarify the emplacement mechanism of these Late Triassic plutons and avoid the ambiguities from purely geochemical studies, we conducted a multidisciplinary structural study on the Mishuling pluton in West Qinling, one of the biggest Late Triassic plutons in the orogen. New single-grain zircon dating confirmed that the Mishuling pluton was emplaced at 212 ± 2 Ma. Gravity investigations show an overall wedge shape to the pluton: A deep root is located in its western part, the thickness gradually decreases to the east, and it finally pinches out at its eastern extremity. We studied internal structures of the Mishuling pluton through field, microscopic, and AMS (anisotropy of magnetic susceptibility) observations, which show that the pluton did not undergo penetrative secondary deformation, and preserved cryptic internal fabrics were mainly acquired during the final stage of emplacement. In the western part of the pluton, internal fabrics display a concentric pattern. In the eastern part, the lineations mainly plunge west, while the foliations form an analogous W-plunging synform, mainly dipping SW in the northern part and dipping NW in the southern part. The three-dimensional shape and internal structures of the Mishuling pluton are concordant with the wrench tectonics of country rocks that developed during the late stage of the Late Triassic collision between the North and South China blocks. We propose an integrated emplacement model involving both the three-dimensional shape and internal fabrics of the Mishuling pluton, and regional structures of the country rocks. In this model, the magma rose in the western part and then spread eastward parallel to the regional strike-slip movements.

Description: Bleaching of red beds by the migration of reducing pore fluids is a widespread phenomenon, with most previous work on the topic focused on detailed studies in the southwestern United States, and to a lesser extent Germany. Herein, we report on widespread bleaching of Cretaceous red beds in the Neuquén Basin and the relationship of this bleaching to hydrocarbon migration. In the Cerro Granito area of the Huincul High, Neuquén Basin, the basal deposits of the Neuquén Group (Candeleros and Huincul Formations) are bleached. This alteration was apparently controlled by fluid and host-rock composition as well as by the intensity and span of the fluid-rock interaction. Red, fine-grained sandstones and mudstones of the Candeleros Formation were deposited in fluvial and swamp environments and contain authigenic hematite, Na-rich corrensite, micro- and mesoquartz, calcite, and analcite. Coarse-grained sandstones and mudstones of the Huincul Formation, which overlie the Candeleros Formation, were deposited in a braided fluvial system, under more humid and acidic conditions than the Candeleros Formation. Sandstones are similar in composition and texture to the Candeleros sandstones, except they contain less basic volcanic detritus and lack corrensite. Bleached facies in both formations contain bitumen and clusters of calcite + bitumen concretions, and partially dissolved detrital clasts and most cements, including early Fe-oxides, which resulted in an increase in secondary porosity and bleaching of the red beds. Alteration of the Candeleros Formation is confined to medium- to coarse-grained sandstone in which corrensite is partially dissolved and its interlayer charge is satisfied by Ca instead of Na, as in corrensite from the red sandstones. Extensive dissolution occurred in the Huincul Formation, which favored the precipitation of new authigenic minerals such as smectite, mixed-layer chlorite-smectite, and pyrite. Bleaching and associated alteration most likely resulted from interaction of hydrocarbons with the red beds. Hydrocarbon paleomigration in the Cerro Granito area occurred in higher-permeability portions of the Candeleros and Huincul Formations. The seal provided by the Lisandro Formation was breached during uplift driven by Tertiary tectonism, resulting in the exhumation of the Huincul and Candeleros Formations. This study highlights the importance of a variety of alterations, including complex clay mineral paragenesis, contemporaneous with bleaching of the red beds. Such alterations can potentially be used to provide evidence for the passage of chemically reducing fluids associated with hydrocarbons in other regions, including cases where the original red color has been completely removed.

Description: The ~50 m slip of the Tohoku earthquake occurred along a very fine grained red-brown smectitic clay horizon subducting in the Japan Trench. This clay, cored in the plate boundary fault at Integrated Ocean Drilling Program Expedition 345, Site C0019, correlates with similar pelagic clay recovered seaward of the trench at Deep Sea Drilling Project Sites 436 and 1149. Comparable clays occur throughout the northwest Pacific Basin. Backtracking of ocean drilling Sites 436, C0019, and 1149 indicates that they formed during the Early Cretaceous at the Kula-Pacific Ridge. These sites traveled northwestward through the equatorial zone, accumulating siliceous and calcareous oozes until ca. 100–85 Ma. Sites 436, C0019, and 1149 then entered the realm of pelagic clay deposition where they remained until ca. 15 Ma. From ca. 15 Ma to the present, Sites 436, C0019, and 1149 accumulated clays and silty clays with variable amounts of siliceous microfossils and volcanic ash, representing the transition from deep-sea conditions to a continental margin sedimentary environment. The predicted backtracked vertical sequence of sediments fits well with the cores at Sites 436, 1149, and C0019, after accounting for structural complications in the latter. Pelagic clay occurs in numerous boreholes penetrating the relatively smooth ocean floor of the Pacific plate north and northeast of the Tohoku earthquake. Here the widespread pelagic clay apparently fosters tsunami and tsunamigenic earthquakes. Seamounts rising above the normal oceanic crust accumulated sequences of calcareous sediments as their crests remained above the calcite compensation depth for most of their history. A seafloor including pelagic clay and carbonate-covered seamounts occurs south and southeast of the southern extent of the Tohoku earthquake rupture zone. This area has no historic tsunami or tsunamigenic earthquakes along the Japan and Izu-Bonin Trenches with the possible exception of the poorly located Enpo earthquake of A.D. 1677. We believe that the seamounts incoming on the oceanic plate to the south and southeast of the Tohoku rupture zone interfere with long-distance propagation of slip in the pelagic clay, limiting earthquake magnitude, shallow slip, and tsunami generation.

Description: Researchers have long debated whether a transient knickzone, accompanied by a wave of high incision, has migrated through the Grand Canyon in the geologically recent past or if, instead, canyon incision rate has been constant during the past several million years. Resolution of this debate has been hindered until recently by the absence of incision rate data for central Grand Canyon and the short duration (since 385 ka) of the eastern Grand Canyon rate history derived directly from river terraces. Here we constrain incision rate histories since ca. 500 ka at eastern Grand Canyon’s Hermit Creek (river mile 96) and at river mile 159 in central Grand Canyon. At Hermit Creek, U/Th ages of travertine-cemented river terrace fill and other surficial deposits reveal an average incision rate of 519 +55 –58 m/m.y. since 506 ± 33 ka and a maximum rate of 210 +42 –49 m/m.y. since 394 ± 32 ka. These data require an incision rate of 1–4 km/m.y. between ca. 500 and 400 ka followed by at least a fivefold decrease after 400 ka. We attribute this decrease to the migration of a transient knickzone past Hermit Creek between 500 and 400 ka. These same deposits also record a 600–800 m/m.y. retreat rate for the Redwall-Muav escarpment since 500 ka. At river mile 159, we analyze the relative age relationship between intrusion of the basaltic "159-Mile Dikes" and cutting of the Muav Gorge. We conclude that the dikes intruded prior to cutting of the gorge, which requires an average river incision rate of 763 m/m.y. since ca. 520 ka. We interpret this rapid rate (compared to ~100–160 m/m.y. over shorter time scales) as independent evidence of knickzone passage through central and eastern Grand Canyon at ca. 500–400 ka.

Description: We applied multiple geochemical tracers ( 87 Sr/ 86 Sr, [Sr], 13 C, and 18 O) to waters and carbonates of the lower Colorado River system to evaluate its paleohydrology over the past 12 Ma. Modern springs in Grand Canyon reflect mixing of deeply derived (endogenic) fluids with meteoric (epigenic) recharge. Travertine (〈1 Ma) and speleothems (2–4 Ma) yield 87 Sr/ 86 Sr and 13 C and 18 O values that overlap with associated water values, providing justification for use of carbonates as a proxy for the waters from which they were deposited. The Hualapai Limestone (12–6 Ma) and Bouse Formation (5.6–4.8 Ma) record paleohydrology immediately prior to and during integration of the Colorado River. The Hualapai Limestone was deposited from 12 Ma (new ash age) to 6 Ma; carbonates thicken eastward to ~210 m toward the Grand Wash fault, suggesting that deposition was synchronous with fault slip. A fanning-dip geometry is suggested by correlation of ashes between subbasins using tephrochronology. New detrital-zircon ages are consistent with the "Muddy Creek constraint," which posits that Grand Wash Trough was internally drained prior to 6 Ma, with limited or no Colorado Plateau detritus, and that Grand Wash basin was sedimentologically distinct from Gregg and Temple basins until after 6 Ma. New isotopic data from Hualapai Limestone of Grand Wash basin show values and ranges of 87 Sr/ 86 Sr, 13 C, and 18 O that are similar to Grand Canyon springs and travertines, suggesting a long-lived spring-fed lake/marsh system sourced from western Colorado Plateau groundwater. Progressive up-section decrease in 87 Sr/ 86 Sr and 13 C and increase in 18 O in the uppermost 50 m of the Hualapai Limestone indicate an increase in meteoric water relative to endogenic inputs, which we interpret to record progressively increased input of high-elevation Colorado Plateau groundwater from ca. 8 to 6 Ma. Grand Wash, Hualapai, Gregg, and Temple basins, although potentially connected by groundwater, were hydrochemically distinct basins before ca. 6 Ma. The 87 Sr/ 86 Sr, 13 C, and 18 O chemostratigraphic trends are compatible with a model for downward integration of Hualapai basins by groundwater sapping and lake spillover. The Bouse Limestone (5.6–4.8 Ma) was also deposited in several hydrochemically distinct basins separated by bedrock divides. Northern Bouse basins (Cottonwood, Mojave, Havasu) have carbonate chemistry that is nonmarine. The 87 Sr/ 86 Sr data suggest that water in these basins was derived from mixing of high- 87 Sr/ 86 Sr Lake Hualapai waters with lower- 87 Sr/ 86 Sr, first-arriving "Colorado River" waters. Covariation trends of 13 C and 18 O suggest that newly integrated Grand Wash, Gregg, and Temple basin waters were integrated downward to the Cottonwood and Mojave basins at ca. 5–6 Ma. Southern, potentially younger Bouse basins are distinct hydrochemically from each other, which suggests incomplete mixing during continued downward integration of internally drained basins. Bouse carbonates display a southward trend toward less radiogenic 87 Sr/ 86 Sr values, higher [Sr], and heavier 18 O that we attribute to an increased proportion of Colorado River water through time plus increased evaporation from north to south. The 13 C and 18 O trends suggest alternating closed and open systems in progressively lower (southern) basins. We interpret existing data to permit the interpretation that the southernmost Blythe basin may have had intermittent mixing with marine water based on 13 C and 18 O covariation trends, sedimentology, and paleontology. [Sr] versus 87 Sr/ 86 Sr modeling suggests that southern Blythe basin 87 Sr/ 86 Sr values of ~0.710–0.711 could be produced by 25%–75% seawater mixed with river water (depending on [Sr] assumptions) in a delta–marine estuary system. We suggest several refinements to the "lake fill-and-spill" downward integration model for the Colorado River: (1) Lake Hualapai was fed by western Colorado Plateau groundwater from 12 to 8 Ma; (2) high-elevation Colorado Plateau groundwater was progressively introduced to Lake Hualapai from ca. 8 to 6 Ma; (3) Colorado River water arrived at ca. 5–6 Ma; and (4) the combined inputs led to downward integration by a combination of groundwater sapping and sequential lake spillover that first delivered Colorado Plateau water and detritus to the Salton Trough at ca. 5.3 Ma. We propose that the groundwater sapping mechanism strongly influenced lake evolution of the Hualapai and Bouse Limestones and that groundwater flow from the Colorado Plateau to Grand Wash Trough led to Colorado River integration.

Description: Cenozoic strata from forearc basins in southern Alaska record deposition related to two different types of shallow subduction: Paleocene–Eocene spreading-ridge subduction and Oligocene–Recent oceanic plateau subduction. We use detrital zircon geochronology (n = 1368) and clast composition of conglomerate (n = 1068) to reconstruct the upper plate response to these two subduction events as recorded in forearc basin strata and modern river sediment. Following spreading-ridge subduction, the presence of Precambrian and Paleozoic detrital zircon ages in middle Eocene–lower Miocene arc-margin strata and Early Cretaceous ages in lower Miocene accretionary prism–margin strata indicates that sediment was transported to the basin from older terranes in interior Alaska and from the exhumed eastern part of the Cretaceous forearc system, respectively. By middle-late Miocene time, diminished abundances of these populations reflect shallow subduction of an oceanic plateau and associated exhumation that resulted in an overall contraction of the catchment area for the forearc depositional system. In the southern Alaska forearc basin system, upper plate processes associated with subduction of a spreading ridge resulted in an abrupt increase in the diversity of detrital zircon ages that reflect new sediment sources from far inboard regions. The detrital zircon signatures from strata deposited during oceanic plateau subduction record exhumation of the region above the flat slab, with the youngest detrital zircon population reflecting the last period of major arc activity prior to insertion of the flat slab. This study provides a foundation for new tectonic and provenance models of forearc basins that have been modified by shallow subduction processes, and may help to facilitate the use of U-Pb dating of detrital zircons to better understand basins that formed under changing geodynamic plate boundary conditions.

Description: The Mesozoic magmatic history of the North American margin records the evolution from a more segmented assemblage of parautochthonous and allochthonous terranes to the more cohesive northern Cordilleran orogenic belt. We characterize the setting of magmatism, tectonism, and epigenetic mineralization in the western Fortymile mining district, east-central Alaska, where parautochthonous and allochthonous Paleozoic tectonic assemblages are juxtaposed, using sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon geochronology, whole-rock geochemistry, and feldspar Pb isotopes of Mesozoic intrusions and spatially associated mineral prospects. New SHRIMP U-Pb zircon ages and published U-Pb and 40 Ar/ 39 Ar ages indicate four episodes of plutonism in the western Fortymile district: Late Triassic (216–208 Ma), Early Jurassic (199–181 Ma), mid-Cretaceous (112–94 Ma), and Late Cretaceous (70–66 Ma). All age groups have calc-alkalic arc compositions that became more evolved through time. Pb isotope compositions of feldspars from Late Triassic, Early Jurassic, and Late Cretaceous igneous rocks similarly became more radiogenic with time and are consistent with the magmas being mantle derived but extensively contaminated by upper crustal components with evolving Pb isotopic compositions. Feldspar Pb isotopes from mid-Cretaceous rocks have isotopic ratios that indicate magma derivation from upper crustal sources, probably thickened mid-Paleozoic basement. The origin of the mantle component in Late Cretaceous granitoids suggested by Pb isotopic ratios is uncertain, but we propose that it reflects asthenospheric upwelling following slab breakoff and sinking of an inactive inner subduction zone that delivered the previously accreted Wrangellia composite terrane to the North American continental margin, after the outer Farallon subduction zone was established. Epigenetic Pb-Zn-Ag ± Cu prospects in the western Fortymile district are spatially associated with splays of the northeast-trending Kechumstuk sinistral-normal fault zone and with ca. 68–66 Ma felsic intrusions and dikes. The similarity between Pb isotope compositions of feldspars from the Late Cretaceous igneous bodies and sulfides from the epithermal prospects suggests a Late Cretaceous age for most of the mineralization. Fluid flow along the faults undoubtedly played a major role in mineralization. We interpret displacement on the northeast-trending faults to be a far-field effect of dextral translation along Late Cretaceous plate-scale boundaries and faults that were roughly parallel to the subsequently developed Denali and Tintina fault systems, which currently bound the region.

Description: A seismic compressed high-intensity radar pulse (CHIRP) survey of Pyramid Lake, Nevada, defines fault architecture and distribution within a key sector of the northern Walker Lane belt. More than 500 line-kilometers of high-resolution (decimeter) subsurface imagery, together with dated piston and gravity cores, were used to produce the first comprehensive fault map and attendant slip rates beneath the lake. A reversal of fault polarity is observed beneath Pyramid Lake, where down-to-the-east slip on the dextral Pyramid Lake fault to the south switches to down-to-the-west displacement on the Lake Range fault to the north. Extensional deformation within the northern two thirds of the basin is bounded by the Lake Range fault, which exhibits varying degrees of asymmetric tilting and stratal divergence due to along-strike segmentation. This structural configuration likely results from a combination of changes in slip rate along strike and the splaying of fault segments onshore. The potential splaying of fault segments onshore tends to shift the focus of extension away from the lake. The combination of normal- and oblique-slip faults in the northern basin gives Pyramid Lake its distinctive "fanning open to the north" geometry. The oblique-slip faults in the northwestern region of the lake are short and discontinuous in nature, possibly representing a nascent shear zone. In contrast, the Lake Range fault is long and well defined. Vertical slip rates measured across the Lake Range and other faults provide new estimates on extension across the Pyramid Lake basin. A minimum vertical slip rate of ~1.0 mm/yr is estimated along the Lake Range fault. When combined with fault length, slip rates yield a potential earthquake magnitude range between M6.4 and M7.0. Little to no offset on the Lake Range fault is observed in the sediment rapidly emplaced at the end of Tioga glaciation (12.5–9.5 ka). In contrast, since 9.5 ka, CHIRP imagery provides evidence for three or four major earthquakes, assuming a characteristic offset of 2.5 m per event. Regionally, our CHIRP investigation helps to reveal how strain is partitioned along the boundary between the northeastern edge of the Walker Lane and the northwest Basin and Range Province proper.

Description: In light detection and ranging (LiDAR)–integrated outcrop characterizations, coupled utilization of LiDAR-generated virtual outcrop models and the ArcGIS platform has been rarely pursued. As a consequence, there exists a limited appreciation of this coupled approach in stratigraphic analysis. This study presents a novel approach of three-dimensional (3-D) mapping of fluvial channel sand bodies in the Cretaceous Blackhawk Formation outcrops in Utah by exporting quantitative information from a high-resolution (~10 cm) virtual outcrop model into ArcGIS. The adjoining and near-circular character of six contiguous cliff faces in our virtual outcrop model provided both upstream and downstream data sets, allowing us to gather adequate spatial data points ( x , y , and z coordinates for each point) for both basal and top bounding surfaces of individual channel sand bodies. For each sand body, these data points were manipulated in ArcGIS to generate a 3-D geobody, which is a realistic reconstruction of the stratigraphic preservation of that channel sand body in a sedimentary basin. The high resolution of our data set allowed the creation of this 3-D channel body down to individual channel-story level (single-story vs. multilateral). By creating and then populating all channel sand bodies of the entire Blackhawk Formation for our studied outcrop window, this technique generates a robust set of results that is useful for improved understanding of fluvial sand-body organization at a range of spatial scales, over both the short- (single to tens of thousands of years) and long-term (hundreds of thousands to millions of years). Our results are also important for improved reservoir and aquifer exploration and production strategy.

Description: June 2015 GSA Today INTRODUCTION We live on a planet with an active surface that is modified and deformed at multiple temporal and spatial scales owing to diverse processes occurring at plate boundaries and plate interiors. The processes of mid-ocean-ridge spreading, mountain building, subduction of tectonic plates, mantle drag, intra-continental deformation, earthquakes, and volcanism cross traditional disciplinary boundaries (Fig. 1A). Understanding these lithospheric processes is valuable not only for intellectual curiosity and to refine our working knowledge of plate tectonics, but also for understanding threats to life, property, and infrastructure. Computer modeling and simulation are increasingly powerful tools that researchers employ to better understand lithospheric deformation and unravel the complex feedbacks that drive the evolution of Earth’s surface. The field is poised for a significant advance to take advantage of recent expansions in computing power, improved representation of idealized processes, increased data availability, and better communication between software developers and geoscientists.

Description: The hydrologic framework of the Santa Clara Valley in northern California was redefined on the basis of new data and a new hydrologic model. The regional groundwater flow systems can be subdivided into upper-aquifer and lower-aquifer systems that form a convergent flow system within a basin bounded by mountains and hills on three sides and discharge to pumping wells and the southern San Francisco Bay. Faults also control the flow of groundwater within the Santa Clara Valley and subdivide the aquifer system into three subregions. After decades of development and groundwater depletion that resulted in substantial land subsidence, Santa Clara Valley Water District (SCVWD) and the local water purveyors have refilled the basin through conservation and importation of water for direct use and artificial recharge. The natural flow system has been altered by extensive development with flow paths toward major well fields. Climate has not only affected the cycles of sedimentation during the glacial periods over the past million years, but interannual to interdecadal climate cycles also have affected the supply and demand components of the natural and anthropogenic inflows and outflows of water in the valley. Streamflow has been affected by development of the aquifer system and regulated flow from reservoirs, as well as conjunctive use of groundwater and surface water. Interaquifer flow through water-supply wells screened across multiple aquifers is an important component to the flow of groundwater and recapture of artificial recharge in the Santa Clara Valley. Wellbore flow and depth-dependent chemical and isotopic data indicate that flow into wells from multiple aquifers, as well as capture of artificial recharge by pumping of water-supply wells, predominantly is occurring in the upper 500 ft (152 m) of the aquifer system. Artificial recharge represents about one-half of the inflow of water into the valley for the period 1970–1999. Most subsidence is occurring below 250 ft (76 m), and most pumpage occurs within the upper-aquifer system between 300 and 650 ft (between 91 and 198 m) below land surface. Overall, the natural quality of most groundwater in the Santa Clara Valley is good. Isotopic data indicate that artificial recharge is occurring throughout the shallower parts of the upper-aquifer system and that recent recharge (less than 50 yr old) occurs throughout most of the basin in the upper-aquifer system, but many of the wells in the center of the basin with deeper well screens do not contain tritium and recent recharge. Age dates indicate that the groundwater in the upper-aquifer system generally is less than 2000 yr old, and groundwater in the lower-aquifer system generally ranges from 16,700 to 39,900 yr old. Depth-dependent sampling indicates that wellbores are the main path for vertical flow between aquifer layers. Isotopic data indicate as much as 60% of water pumped from production wells originated as artificial recharge. Shallow aquifers not only contain more recent recharge but may be more susceptible to anthropogenic and natural contamination, as evidenced by trace occurrences of iron, nitrate, and volatile organic compounds (VOCs) in selected water-supply wells. Water-resource management issues are centered on sustaining a reliable and good-quality source of water to the residents and industries of the valley. While the basin has been refilled, increased demand owing to growth and droughts could result in renewed storage depletion and the related potential adverse effects of land subsidence and seawater intrusion. The new hydrologic model demonstrates the importance of the aquifer layering, faults, and stream channels in relation to groundwater flow and infiltration of recharge. This model provides a means to analyze water resource issues because it separates the supply and demand components of the inflows and outflows.

Description: The origin and evolution of the central Andes, a noncollisional orogenic system, have been hypothesized to evolve as a result of several dynamic processes, including formation of an eastward-propagating orogenic wedge, segmentation into rhomb-shaped basins as a result of N-S gradients in crustal shortening, reactivation of inherited deep structures, and lithospheric foundering. How these proposed processes dominate the orogen spatially and temporally is uncertain; however, constraining the timing of upper-crustal deformation is critical for investigating these models. We document the formation and deformation of the Pasto Ventura basin (NW Argentina) in the southern Puna Plateau. Through field mapping, deformation analysis, secondary ion mass spectrometry U-Pb dating of zircon from interbedded volcanic ashes, and 40 Ar/ 39 Ar geochronology of volcanics, we show that major basin formation started ca. 11.7–10.5 Ma and continued until at least ca. 7.8 Ma. The basin underwent syndepositional faulting and folding from ca. 10 to 8 Ma. Contractional deformation in the Pasto Ventura basin ended between ca. 7.3 and 4 Ma, based on the onset of regional horizontal extension. Data from the Pasto Ventura region allow us to bridge existing data and complete a regional compilation of upper-crustal deformation for the Puna Plateau. Our analysis shows that late Miocene formation and deformation of the Pasto Ventura basin represent an important out-of-sequence contractional event in the southern Puna Plateau. While a number of geodynamic processes likely shape the evolution of the southern Puna, multidisciplinary data sets, including deformation in the Pasto Ventura basin studied here, highlight the role of the formation and detachment of a late Miocene lithospheric drip in causing the upper-crustal deformation on the southern Puna Plateau since the mid–late Miocene.

Description: Active transpressional fault systems are typically associated with the development of broad zones of deformation and topographic development; however, the complex geometries typically associated with these systems often make it difficult to isolate the important boundary conditions that control transpressional orogenic growth. The Denali fault system is widely recognized as transpressional due to the presence of the Denali fault, a major, active, right-lateral fault, and subparallel zones of thrust faults and fault-related folding along both the north and south flanks of the Alaska Range. Measured Quaternary and Holocene slip rates exist for the Denali fault system and portions of the adjacent thrust system, but the partitioning of fault slip between contractional and translational components of this transpressional system has not been previously studied in detail. Exploiting the relatively simple geometry of the Denali fault, we analyze the style and distribution of active faulting within the Alaska Range to define patterns of strain accommodation and determine how contractional and translational strain is partitioned across the Denali fault system. As the trace of the Denali fault curves by ~70° across central Alaska, the mean strike of the thrust system to the north remains subparallel to the Denali fault, while to the south, the few faults with known or suspected Quaternary offset are oblique to the Denali fault. This relationship suggests that as the Denali fault system accommodates local fault-parallel strike slip, it partitions the residual part of the regional NW-directed plate motion into NW-SE shortening south of the Denali fault and shortening perpendicular to the Denali fault to the north. The degree of slip partitioning is consistent with a balanced slip budget for the two primary faults that contribute displacement to the Denali fault system (the eastern Denali fault and Totschunda fault). The current obliquity of displacement south of the Denali fault is the result of the late Cenozoic development of the Totschunda fault, which provides a more direct connection for the transfer of strain from the Fairweather transform fault to the Denali fault system. The transmitted strain is partitioned into right-lateral slip on the Denali fault and into Denali fault-normal shortening that is accommodated by thrust faulting in the Alaska Range and distributed left-lateral slip faulting within interior Alaska to the north.

Description: The Kikiktat volcanics (new name) of the northeastern Brooks Range of Arctic Alaska are exceptionally well-preserved Neoproterozoic continental tholeiites. This volcanic suite includes high-temperature picritic compositions, making them an excellent probe of mantle composition and temperature underlying the northern margin of Laurentia during the breakup of Rodinia. Detrital zircons from a volcaniclastic sample directly overlying basaltic flows of the Kikiktat volcanics were dated at 719.47 ± 0.29 Ma by U-Pb chemical abrasion–thermal ionization mass spectrometry. This age suggests that the Kikiktat volcanics are an extension of the Franklin large igneous province. Petrogenetic modeling indicates a simple crystallization sequence of olivine -〉 plagioclase -〉 clinopyroxene, recording anhydrous low-pressure fractionation of a picritic parental melt. The composition of this parental liquid requires melting of harzburgite in the spinel stability field, while temperature estimates of the primary melt indicate elevated mantle potential temperatures. In contrast to the ca. 720 Ma Natkusiak basalts of Victoria Island, the Kikiktat volcanics have very low Ti concentrations, consistent with melting of harzburgitic mantle possibly by thermal conduction of an underlying plume. These data are consistent with Neoproterozoic to early Paleozoic tectonic reconstructions that restore the North Slope of Arctic Alaska to the northeastern margin of Laurentia and not directly adjacent to Victoria Island.

Description: In this study, structures in plutons and host rocks are coupled with geochronology to track paleodeformation fields from the late Paleozoic to Late Cretaceous in the central Sierra Nevada. Regional NW-striking host-rock foliation, NE- or SW-vergent thrust faults, and associated folds developed from the early Mesozoic to Early Cretaceous. Dextral transpressional shear zones developed in the Late Cretaceous. Strikes of steep-dipping magmatic foliations in Mesozoic plutons temporally vary from approximately NW (Triassic–Jurassic) to WNW (Late Cretaceous), displaying a progressive counterclockwise rotation. Joint interpretation based on combining host-rock and magmatic structures suggests that intra-arc paleodeformation fields were dominated by coaxial and arc-perpendicular contraction from the early Mesozoic to Early Cretaceous, becoming increasingly dextral transpressive in the Late Cretaceous. The switch from contraction to transpression was likely caused by oblique convergence between the Farallon and North American plates. Based on observations in the study area and other host-rock pendants in the central Sierra Nevada, we propose that the intensity of intra-arc deformation is cyclic. To some extent, it mimics the episodic pattern of arc magmatism: Stronger deformation coincides with magmatic flare-ups. Magmatism promotes intra-arc deformation, which in turn causes crustal thickening during transfer of materials downward to the magma source regions, potentially fertilizing source regions with supracrustal materials and resulting in increased magma generation. Thus, models addressing continental arc tempos should include intra-arc processes. Evolution of continental arcs may be influenced by linked cyclic processes within the arcs accompanied by noncyclic processes driven by events external to the arcs.

Description: Most of the displacement across the North American–Pacific plate boundary in southern California is accommodated by the San Jacinto and the southern San Andreas fault zones. If and how the rate of displacement across these fault zones varies along strike and through time are still being resolved. Here, we present four calculations of late Holocene slip rate and average slip per event from the Claremont fault of the northern San Jacinto fault zone that show variations in strain distribution over the past 2000 yr and illustrate how plate-boundary displacement is distributed between the San Jacinto and southern San Andreas fault zones. We calculate a slip rate of 12.8–18.3 mm/yr and an average slip per event of 2.5 m from two measurements of streams offset by 9–11 earthquakes in the past 1500–2000 yr. Faster slip rates of 21–30 mm/yr and an average slip per event of 2.7–3 m were determined from measurements of a stream and a buried channel that were offset by three earthquakes in the past 400–500 yr. The 2000 yr slip rate is similar to the range in slip rates reported for the adjacent San Bernardino section of the San Andreas fault zone, suggesting that the northern San Jacinto accommodates a similar amount of displacement as the San Andreas fault zone at the same latitude. The rate is also slightly faster (by ~2–3 mm/yr) than reported slip rates from the central San Jacinto fault zone to the southeast. A slip rate of 15 ± 2 mm/yr is within the range of uncertainty for almost all the geologic and geodetic data for the entire length of the San Jacinto fault zone and may be the best approximation for long-term average slip rate of the fault zone. Alternatively, 2–3 mm/yr of slip along the northern San Jacinto fault zone may be accommodated to the south along the lesser-studied Hot Springs, Thomas Mountain, Buck Ridge, and Santa Rosa faults, the lateral slip rates of which are not well known nor included in typical estimates of slip rate along the central San Jacinto fault zone. We infer that the faster slip rate over the past 500 yr is due to a cluster of earthquakes along the Claremont fault between A.D. 1400 and A.D. 1850 and larger-than-average surface displacement of 3 m or more during the third event back. The 3 m or more measurement of displacement in this event corresponds to rupture lengths that are slightly longer than the total length of the Claremont fault, and previously published paleoseismic data indicate that this event occurred coincident in time with an event on the adjacent Clark fault. We propose that this combination of slip per event data and paleoseismic data from adjacent fault strands is strong evidence for rupture through the releasing step over that separates these two segments of the San Jacinto fault zone.

Description: Differences in U-Pb metamorphic monazite ages in the northwestern Thor-Odin culmination of the Monashee complex, southern Canadian Cordillera, are explained in the context of the NNW-trending subvertical transcurrent Paleogene Victor Creek fault. Similar faults are present throughout the Canadian Cordillera. We demonstrate their potential importance in the interpretation of the history of Cordilleran deformation and metamorphism. A pervasive transposition foliation (S T ) is present throughout the Thor-Odin culmination as a result of Cordilleran and possibly earlier deformation. A pre-S T (or early S T ) foliation is preserved as aligned inclusion trails in porphyroblasts such as garnet and kyanite. Monazite U-(Th-)Pb isotope dilution–thermal ionization and secondary ion microprobe mass spectrometry (ID-TIMS, SIMS) ages are used to relate monazite growth to pre- and syn-S T fabrics and associated metamorphism. The ages of both pre- and syn-S T fabrics, and the gap between pre- and syn-S T ages decrease toward the east, in an apparently continuous manner. While monazite west of the Victor Creek fault is latest Cretaceous to earliest Eocene in age, monazite east of the Victor Creek fault is exclusively Eocene. Correlation of rock types across the faults is difficult because the same rock units are repeated many times on either side. However, distinctly different retained ages of metamorphism, and previously recognized differences in structures and detrital zircon signatures across the fault indicate 5–60 km offset along the fault. Similar trends occur across other faults along the western Monashee complex, and faults here and elsewhere in the Canadian Cordillera may have similar geological significance.

Description: Membrane proteins are key players in biological systems, mediating signalling events and the specific transport of e.g. ions and metabolites. Consequently, membrane proteins are targeted by a large number of currently approved drugs. Understanding their functions and molecular mechanisms is greatly dependent on structural information, not least on complexes with functionally or medically important ligands. Structure determination, however, is hampered by the difficulty of obtaining well diffracting, macroscopic crystals. Here, the feasibility of X-ray free-electron-laser-based serial femtosecond crystallography (SFX) for the structure determination of membrane protein–ligand complexes using microcrystals of various native-source and recombinant P-type ATPase complexes is demonstrated. The data reveal the binding sites of a variety of ligands, including lipids and inhibitors such as the hallmark P-type ATPase inhibitor orthovanadate. By analyzing the resolution dependence of ligand densities and overall model qualities, SFX data quality metrics as well as suitable refinement procedures are discussed. Even at relatively low resolution and multiplicity, the identification of ligands can be demonstrated. This makes SFX a useful tool for ligand screening and thus for unravelling the molecular mechanisms of biologically active proteins.

Description: The synchrotron through-the-substrate X-ray microdiffraction technique (tts-μXRD) is extended to the structural study of microvolumes of crystals embedded in polished thin sections of compact materials [Rius, Labrador, Crespi, Frontera, Vallcorba & Melgarejo (2011). J.Synchrotron Rad. 18, 891–898]. The resulting tts-μXRD procedure includes some basic steps: (i) collection of a limited number of consecutive two-dimensional patterns (frames) for each randomly oriented crystal microvolume; (ii) refinement of the metric from the one-dimensional diffraction pattern which results from circularly averaging the sum of collected frames; (iii) determination of the reciprocal lattice orientation of each randomly oriented crystal microvolume which allows assigning the hkl indices to the spots and, consequently, merging the intensities of the different frames into a single-crystal data set (frame merging); and (iv) merging of the individual crystal data sets (multicrystal merging) to produce an extended data set suitable for structure refinement/solution. Its viability for crystal structure solution by Patterson function direct methods (δ recycling) and for accurate single-crystal least-squares refinements is demonstrated with some representative examples from petrology in which different glass substrate thicknesses have been employed. The section of the crystal microvolume must be at least of the same order of magnitude as the focus of the beam (15 × 15 µm in the provided examples). Thanks to its versatility and experimental simplicity, this methodology should be useful for disciplines as disparate as petrology, materials science and cultural heritage.

Description: The crystallization of 28 binary and ternary cocrystals of quercetin with dibasic coformers is analyzed in terms of a combinatorial selection from a solution of preferred molecular conformations and supramolecular synthons. The crystal structures are characterized by distinctive O—H...N and O—H...O based synthons and are classified as nonporous, porous and helical. Variability in molecular conformation and synthon structure led to an increase in the energetic and structural space around the crystallization event. This space is the crystal structure landscape of the compound and is explored by fine-tuning the experimental conditions of crystallization. In the landscape context, we develop a strategy for the isolation of ternary cocrystals with the use of auxiliary template molecules to reduce the molecular and supramolecular `confusion' that is inherent in a molecule like quercetin. The absence of concomitant polymorphism in this study highlights the selectivity in conformation and synthon choice from the virtual combinatorial library in solution.

Description: At the field scale, nearly all fault surfaces contain grooves generated as one side of the fault slips past the other. Grooves are so common that they are one of the key indicators of principal slip surfaces. Here, we show that at sufficiently small scales, grooves do not exist on fault surfaces. A transition to isotropic roughness occurs at 4–500 μm. Although the scale of the transition can vary even between locales on a single fault, the aspect ratio of the roughness at the transition is well defined for a given fault. We interpret the transition between grooved and ungrooved scales as a transition in deformation mode of asperities on the slip surface. Grooves can form when a hard indenter slides past a softer surface. At small scales, the asperities appear to yield plastically and therefore do not generate grooves as hard indenters. The plastic yielding can be a consequence of the high shear strains required to deform the surfaces at small scales where the aspect ratio (roughness) is high. The transition to plastic yielding is predicted to occur at a specific aspect ratio for each fault, as observed. The new observation both shows a limit to one of the most commonly observed features of faults and suggests a change in the mode of failure of faults as a function of scale.