Showing papers in "Geology in 2013"

Potentially induced earthquakes in Oklahoma, USA: Links between wastewater injection and the 2011 Mw 5.7 earthquake sequence

Abstract: Significant earthquakes are increasingly occurring within the continental interior of the United States, including five of moment magnitude (Mw) ≥ 5.0 in 2011 alone. Concurrently, the volume of fluid injected into the subsurface related to the production of unconventional resources continues to rise. Here we identify the largest earthquake potentially related to injection, an Mw 5.7 earthquake in November 2011 in Oklahoma. The earthquake was felt in at least 17 states and caused damage in the epicentral region. It occurred in a sequence, with 2 earthquakes of Mw 5.0 and a prolific sequence of aftershocks. We use the aftershocks to illuminate the faults that ruptured in the sequence, and show that the tip of the initial rupture plane is within ∼200 m of active injection wells and within ∼1 km of the surface; 30% of early aftershocks occur within the sedimentary section. Subsurface data indicate that fluid was injected into effectively sealed compartments, and we interpret that a net fluid volume increase after 18 yr of injection lowered effective stress on reservoir-bounding faults. Significantly, this case indicates that decades-long lags between the commencement of fluid injection and the onset of induced earthquakes are possible, and modifies our common criteria for fluid-induced events. The progressive rupture of three fault planes in this sequence suggests that stress changes from the initial rupture triggered the successive earthquakes, including one larger than the first.

Locating South China in Rodinia and Gondwana: A fragment of greater India lithosphere?

Abstract: From the formation of Rodinia at the end of the Mesoproterozoic to the commencement of Pangea breakup at the end of the Paleozoic, the South China craton fi rst formed and then occupied a position adjacent to Western Australia and northern India. Early Neoproterozoic suprasubduction zone magmatic arc-backarc assemblages in the craton range in age from ca. 1000 Ma to 820 Ma and display a sequential northwest decrease in age. These relations suggest formation and closure of arc systems through southeast-directed subduction, resulting in progressive northwestward accretion onto the periphery of an already assembled Rodinia. Siliciclastic units within an early Paleozoic succession that transgresses across the craton were derived from the southeast and include detritus from beyond the current limits of the craton. Detrital zircon age spectra require an East Gondwana source and are very similar to the Tethyan Himalaya and younger Paleozoic successions from Western Australia, suggesting derivation from a common source and by inference accumulation in linked basins along the northern margin of Gondwana, a situation that continued until rifting and breakup of the craton in the late Paleozoic.

Retrodeforming the Arabia-Eurasia collision zone: Age of collision versus magnitude of continental subduction

Abstract: The Arabia-Eurasia collision has been linked to global cooling, the slowing of Africa, Mediterranean extension, the rifting of the Red Sea, an increase in exhumation and sedimentation on the Eurasian plate, and the slowing and deformation of the Arabian plate. Collision age estimates range from the Late Cretaceous to Pliocene, with most estimates between 35 and 20 Ma. We assess the consequences of these collision ages on the magnitude and location of continental consumption by compiling all documented shortening within the region, and integrating this with plate kinematic reconstructions. Shortening estimates across the orogen allow for ~350 km of Neogene upper crustal contraction, necessitating collision by 20 Ma. A 35 Ma collision requires additional subduction of ~400‐600 km of Arabian continental crust. Using the Oman ophiolite as an analogue, ophiolitic fragments preserved along the Zagros suture zone permit ~180 km of subduction of the Arabian continental margin plus overlying ophiolites. Wholesale subduction of this more dense continental margin plus ophiolites would reconstruct ~400‐500 km of postcollisional Arabia-Eurasia convergence, consistent with a ca. 27 Ma initial collision age. This younger Arabia-Eurasia collision suggests a noncollisional mechanism for the slowing of Africa, and associated extension.

Variation of East Asian monsoon precipitation during the past 21 k.y. and potential CO2 forcing

Abstract: Paleoclimatic research can provide critical insight on causes of change in the East Asian monsoon, which influences the lives of 1.6 billion people today. In this study, we use paleoclimatic indexes from Chinese loess deposits, which have clear climatic implications and are independently dated, to reconstruct the monsoon precipitation since 21 ka. Our results show that monsoon precipitation persistently decreased from 21 ka to ca. 8 ka, and increased after ca. 8 ka, with a precipitation peak at 8–3 ka. These changes in East Asian summer monsoon precipitation are synchronous with changes in high-northern-latitude ice volume/ice cover and atmospheric CO2. These new data suggest that variation of the monsoon precipitation was probably driven by CO2-forced high-northern-latitude temperature changes, shifting the location of the intertropical convergence zone that dominates monsoon precipitation. Our TraCE-21000 modeling experiment supports this interpretation.

Tracking the evolution of large-volume silicic magma reservoirs from assembly to supereruption

Abstract: The most voluminous silicic volcanic eruptions in the geological past were associated with caldera collapses above giant silicic magma reservoirs. The thermal evolution of these subcaldera magma reservoirs controls the volume of eruptible magma and eruptive style. Here we combine high-precision zircon U-Pb geochronology, trace element analyses of the same mineral grains, and mass balance modeling of zircon trace element compositions allowing us to track the thermal and chemical evolution of the Oligocene Fish Canyon Tuff magma reservoir (Colorado, United States) as a function of absolute time. Systematic compositional variations in U-Pb dated zircons record ~440 k.y. of magma evolution. An early phase of volumetric growth was followed by a period of cooling and crystallization, during which the Fish Canyon magma approached complete solidifi cation. Subsequent remelting, due to underplated andesitic recharge magmas, began 219 ± 45 k.y. prior to eruption, and led to the generation of ~5000 km3 of eruptible crystal-rich (~45 vol%) dacite. Age-equivalent, but compositionally different, zircons in an andesite enclave from late-erupted Fish Canyon Tuff tie the growth and thermal evolution of the upper-crustal reservoir to a lower-crustal magma processing zone. Our results demonstrate that the combination of high-precision dating and trace element analyses of accessory zircons can reveal invaluable information about the chemical and thermal histories of silicic magmatic systems and provides critical input parameters for fl uid dynamic modeling.

Volcanic ash reveals time-transgressive abrupt climate change during the Younger Dryas

Abstract: Knowledge of regional variations in response to abrupt climatic transitions is essential to understanding the climate system and anticipating future changes. Global climate models typically assume that major climatic changes occur synchronously over continental to hemispheric distances. The last major reorganization of the ocean-atmosphere system in the North Atlantic realm took place during the Younger Dryas (YD), an ∼1100 yr cold period at the end of the last glaciation. Within this region, several terrestrial records of the YD show at least two phases, an initial cold phase followed by a second phase of climatic amelioration related to a resumption of North Atlantic overturning. We show that the onset of climatic amelioration during the YD cold period was locally abrupt, but time-transgressive across Europe. Atmospheric proxy signals record the resumption of thermohaline circulation midway through the Younger Dryas, occurring 100 yr before deposition of ash from the Icelandic Vedde eruption in a German varve lake record, and 20 yr after the same isochron in western Norway, 1350 km farther north. Synchronization of two high-resolution continental records, using the Vedde Ash layer (12,140 ± 40 varve yr B.P.), allows us to trace the shifting of the polar front as a major control of regional climate amelioration during the YD in the North Atlantic realm. It is critical that future climate models are able to resolve such small spatial and chronological differences in order to properly encapsulate complex regional responses to global climate change.

High-precision temporal calibration of Late Permian vertebrate biostratigraphy: U-Pb zircon constraints from the Karoo Supergroup, South Africa

Abstract: Therapsid and other tetrapod fossils from the South African Karoo Supergroup provide the most detailed and best studied terrestrial vertebrate record of the Middle and Late Permian. The resulting biostratigraphic scheme has global applicability. Establishing a temporal framework for these faunas has proven difficult: magnetostratigraphy has been hampered by a Jurassic overprint, and intercorrelation with Permian marine sequences has been equivocal. Here we report U-Pb zircon isotope dilution–thermal ionization mass spectrometry (ID-TIMS) dates for five volcanic ashes interbedded with fossils from the Pristerognathus , Tropidostoma , and Cistecephalus vertebrate biozones of the Beaufort Group. This temporal framework allows correlation to marine zonations and improves understanding of rates of faunal evolution and patterns of basin evolution. Our results identify no correlative vertebrate extinctions in the Karoo Supergroup to the marine end-Guadalupian mass extinction and raise the question of whether there is any record of a terrestrial extinction related to the Emeishan large igneous province.

On the longevity of large upper crustal silicic magma reservoirs

Abstract: Understanding the processes involved in the formation and maturation of upper crustal magma reservoirs, ultimately sourcing the largest volcanic eruptions on Earth, is one of the most fundamental aspects of volcanology. While such reservoirs are known to assemble incrementally over extended periods of time, debate persists regarding the time scales of melt preservation in the cold upper crust. If rapid cooling individually freezes incoming replenishing intrusions, accumulations of eruptible magma are impossible, precluding the construction of voluminous volcanic reservoirs for all but the highest magma emplacement rates. Recent numerical thermal models have been used to assess the viability of upper crustal silicic magma survival, and have suggested that supervolcanic reservoirs must form on geologically short time scales with anomalously high injection rates, and subsist only ephemerally, making their long-term evolution less predictable. Motivated by geological observations suggesting the contrary, we have improved upon these models by incorporating two fundamental features of natural systems not previously considered: (1) a nonlinear crystallization-temperature relationship adapted for upper crustal silicic magmas and (2) a temperature-dependent thermal conductivity. We demonstrate that the incorporation of both of these properties can allow an upper crustal reservoir to remain above its solidus for hundreds of thousands of years when fed by magma fluxes typical of large magmatic provinces. While the crystallization-temperature path plays the most significant role in maintaining a large pool of eruptible magma, the incorporation of temperature-dependent thermal properties significantly extends the lifetime of such reservoirs. Furthermore, while deeper emplacement levels (e.g., 10 km depth) can both extend magma survival and increase melt availability, we show that a 5 km depth can also provide an adequate magma storage environment. These results provide strong support for long-lived upper crustal mushes as a staging ground for accumulation of highly eruptible, crystal-poor silicic magmas, and further assert the evolutionary link between volcanic and plutonic systems.

“Moist MORB” axial magmatism in the Oman ophiolite: The evidence against a mid-ocean ridge origin

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

Concordant monsoon-driven postglacial hydrological changes in peat and stalagmite records and their impacts on prehistoric cultures in central China

Abstract: Asian monsoon records are widely documented, but specific proxies of monsoonal rainfall are limited. We present here two new independent proxy records from peatland and stalagmite archives that indicate a high degree of concordance between monsoon-driven hydrological changes occurring since the last deglaciation in a broad region of central China. The wet periods elevated the water table in the Dajiuhu peatland, as recorded by reduced mass accumulation rates of hopanoids, biomarkers for aerobic microbes, confirmed by molecular phylogenic analyses. The hopanoid-based reconstruction is supported by the first report of the environmental magnetism parameter ARM/SIRM (anhysteretic remanent magnetization / saturation isothermal remanent magnetization; ratio of fine magnetic particles to total ferrimagnetic particles) in a stalagmite from Heshang Cave in central China. Heavy rainfall resulted in the enhanced transport of coarse particles to the cave and thus low ARM/SIRM values in the stalagmite. The hydrological conditions inferred from the two records reveal three relatively long wet periods in central China: 13–11.5 k.y. ago, 9.5–7.0 k.y. ago, and 3.0–1.5 k.y. ago. Archaeological evidence for the hydrological impacts on regional populations comes from the observation that temporal shifts among six distinctive cultures of the Neolithic Period to the Iron Age in central China occurred during wet periods or flood episodes. Spatiotemporal distributions of >1600 prehistoric settlement sites correlate with the proxy-inferred fluctuating hydrological conditions, with enhanced flooding risk forcing major relocations of human settlements away from riparian zones.

Northeast African vegetation change over 12 m.y.

Abstract: Intense debate surrounds the evolution of grasses using the C 4 (Hatch-Slack) photosynthesis pathway and the emergence of African grasslands, often assumed to be one and the same. Here, we bring new insights with the combination of plant leaf wax carbon isotopic composition (δ 13 C wax ) and pollen data from marine sediments of the Gulf of Aden (northeast Africa), which show that C 4 biomass increases were not necessarily associated with regional grassland expansion. We find broadly opposing trends toward more enriched δ 13 C wax values and decreased grass pollen proportions between 12 and 1.4 Ma. This apparently contradictory evidence can be reconciled if a greater proportion of the Late Miocene northeast African landscape were covered by C 3 grasses than previously thought, such that C 4 grasses and shrubs replaced a C 3 ecosystem including trees and productive grasslands. In addition, δ 13 C wax and pollen both indicate that true rainforests were unlikely to have been extensive in northeast Africa at any time in the last 12 m.y., although seasonally dry forests were a significant component of the regional landscape since the Late Miocene. Here, we extend regionally integrative marine archives of terrestrial vegetation back to 12 Ma, and we evaluate them in the context of an updated compilation of pedogenic carbonate δ 13 C values from East African Rift strata. We identify two distinct phases of increasing C 4 biomass between 11 and 9 Ma (with a reversal by 4.3 Ma) and then a re-expansion between 4.3 and 1.4 Ma; surprisingly, neither was associated with grassland expansion.

Recurrent Early Triassic ocean anoxia

Abstract: The Early Triassic record, from the Smithian stratotype, shows that the organic carbon isotope record from northwest Pangea closely corresponds to major fluctuations in the inorganic carbon records from the Tethys, indicating truly global perturbations of the carbon cycle occurred during this time. Geochemical proxies for anoxia are strongly correlated with carbon isotopes, whereby negative shifts in δ 13 C org are associated with shifts to more anoxic to euxinic conditions, and positive shifts are related to return to more oxic conditions. Rather than by a delayed or prolonged recovery, the Early Triassic is better characterized by a series of aborted biotic recoveries related to shifts back to ocean anoxia, potentially driven by recurrent volcanism.

Geochemical controls on shale microstructure

Abstract: The uptick in hydrocarbon production from shale in the United States has generated interest in metrics of unconventional reservoir quality, like permeability. We use conventional gas sorption to characterize shale microstructure, which provides insight on the features that govern mass transport. The gas sorption data are analyzed to determine the surface area, A S (m2/g), and pore volume, V P (cm3/g) of 30 samples from basins across North America. With this information, we quantify the effect of composition and thermal maturity on shale microstructure. In particular, we find that the specific surface area of the organic component evolves from ∼50 m2/g total organic carbon (TOC) in immature shale to ∼500 m2/g TOC for regions that produce dry gas. The increase in A S is accompanied by an increase in V P and concomitant decline in average pore size (e.g., r H = 4 V P/ A S). We contend that the latter is due to the development of nanometer-sized pores in kerogen as it is converted to mobile hydrocarbon which is ultimately expelled. This hypothesis is supported by similar measurements on companion samples after bitumen extraction or combustion, which underscore the intimate spatial association of petroleum and kerogen. Coupled with information on accessibility to the porosity garnered by varying the particle size, these results establish a clear link between organic matter, thermal maturity, and reservoir quality in unconventional shale systems.

Solidus of alkaline carbonatite in the deep mantle

Abstract: Minor amounts of alkalies (Na and K) can reduce drastically the solidus temperatures of carbonated silicate mantle, by as much as 400–500 °C Low-degree melting of carbonated peridotite and eclogite at pressures of 3–10 GPa produces Na- and K-bearing carbonatite melt Mass-balance calculations of samples obtained below apparent solidi show clear deficits of alkalies, suggesting the presence of minor alkali-rich liquid or solid carbonate phases Here we determine the true solidi in Na- and K-bearing carbonate systems and report the stability of alkaline carbonate phases Melting of subducting alkaline carbonates would likely occur at transition zone depths to produce mobile carbonatite melt diapirs that migrate upward, modifying and oxidizing the upper mantle and initiating volcanism at the surface

Jurassic accretionary complex and ophiolite from northeast Turkey: No evidence for the Cimmerian continental ribbon

Abstract: Permian-Triassic and Late Cretaceous accretionary complexes, ascribed to the consumption of two distinct oceans, the Paleo- and Neo-Tethys, are exposed over extensive areas in the Eastern Mediterranean region. However, a separating continental ribbon, the so-called Cimmeride continent, between the Paleo- and Neo-Tethys during early Mesozoic time cannot be defined. Here we report a previously unknown Early Jurassic metamorphic oceanic accretionary complex and ophiolite from northeast Turkey, bounded by oceanic accretionary complexes of Permian-Triassic and Late Cretaceous age to the north and the south, respectively, without a continental domain in between. This special tectonic position and widespread coexistence of Permian-Triassic and Late Cretaceous accretionary complexes alongside the Izmir-Ankara-Erzincan suture imply that (1) the southern margin of Laurasia in the eastern Mediterranean region grew by episodic accretionary processes from late Paleozoic to end-Mesozoic time without involvement of a Cimmerian continental ribbon, and (2) the Paleo-Tethys and northern branch of the Neo-Tethys were not distinct oceans in the Eastern Mediterranean region.

Fractionation of Nb and Ta by biotite and phengite: Implications for the "missing Nb paradox"

Abstract: The subchondritic Nb/Ta in both the continental crust and the depleted mantle remains enigmatic and is called the "missing Nb paradox." We present partitioning data between biotite and granitic melt for experimental and natural samples that provide evidence that Nb is compatible in biotite and phengite. Nb can thus be enriched in the residue during partial melting of crustal rocks. Additionally, biotite and phengite in equilibrium with granitic melts preferentially incorporate Nb over Ta. Therefore incipient partial melting of biotite-rich crustal rocks produces restites with high Nb/Ta. Progressive melting of such rocks leads to the consumption of biotite and the formation of peritectic rutile or ilmenite, which retain the high-Nb/Ta signature. We suggest that such mid to lower crustal granulites could represent an important Nb-rich reservoir with high Nb/Ta. Similarly, high-Ti phengite that is present in deeply subducted sediments preferentially incorporates Nb over Ta. High-pressure incipient partial melting in the presence of residual phengite thus also produces restites with high Nb/Ta that could be subducted to the deeper mantle.

Recurrent liquefaction in Christchurch, New Zealand, during the Canterbury earthquake sequence

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

Nanograins form carbonate fault mirrors

Abstract: Many faults are characterized by naturally polished, reflective, glossy surfaces, termed fault mirrors (FMs), that form during slip. Recent experiments also find that FMs form during rapid sliding between rock surfaces, and that FM formation coincides with pronounced friction reduction. The structure of FMs and the mechanism of their formation are thus important for understanding the mechanics of frictional sliding, particularly during earthquakes. Here we characterize the small-scale structure of natural carbonate FMs from three different faults along a tectonically active region of the Dead Sea transform. Atomic force microscopy measurements indicate that the FMs have extremely smooth surface topography, accounting for their mirror-like appearance. Electron microscope characterization revealed a thin (

How well do fossil assemblages of the Ediacara Biota tell time

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

Earth is (mostly) flat: Apportionment of the flux of continental sediment over millennial time scales

Abstract: We thank [Warrick et al. (2014)][1] for the Comment on our recent synthesis of 10Be-derived denudation rates ([Willenbring et al., 2013][2]), in which we suggested that gently sloping areas, representing ∼90% of the Earth’s land surface, have sufficiently high rates of denudation to produce a

Mobilization of radiogenic Pb in zircon revealed by ion imaging: Implications for early Earth geochronology

Abstract: Zircon is arguably the most commonly used geochronometer, but the reliability of ages obtained requires a full understanding of processes that might compromise the integrity of its U-Pb systematics. Here we present the results of a multifaceted ion microprobe study of zircon grains from the Napier Complex, East Antarctica, a region affected by pervasive high-temperature metamorphism at 2.5 Ga, and from which previous zircon geochronological interpretations have been problematic. Both U-Pb spot analysis (∼15 μm) and high spatial resolution (∼2 μm) scanning ion imaging of Pb isotopes have been applied in an attempt to quantify the effects of metamorphism. Spot analyses spread along concordia yielding 207 Pb/ 206 Pb ages from 2.5 Ga to 3.9 Ga, with the oldest grains reversely discordant. Ion images of uranogenic Pb reveal a surprising micrometer-scale patchy distribution that is unrelated to crystal morphology or damage. The 207 Pb/ 206 Pb ratios within these subdomains correspond to apparent zircon ages as old as 4.2 Ga. These are interpreted as artifacts of ancient redistribution of radiogenic Pb, a process that can generate meaningless ages, and are not relicts of ancient (including Hadean) zircon. Scanning ion imaging thus facilitates identification of unsupported radiogenic Pb and enables testing of the validity of old ages from zircon known to have a long and complicated history.

Are subduction zones invading the Atlantic? Evidence from the southwest Iberia margin

Abstract: Subduction initiation at passive margins plays a central role in the plate tectonics theory. However, the process by which a passive margin becomes active is not well understood. In this paper we use the southwest Iberia margin (SIM) in the Atlantic Ocean to study the process of passive margin reactivation. Currently there are two tectonic mechanisms operating in the SIM: migration of the Gibraltar Arc and Africa-Eurasia convergence. Based on a new tectonic map, we propose that a new subduction zone is forming at the SIM as a result of both propagation of compressive stresses from the Gibraltar Arc and stresses related to the large-scale Africa-Eurasia convergence. The Gibraltar Arc and the SIM appear to be connected and have the potential to develop into a new eastern Atlantic subduction system. Our work suggests that the formation of new subduction zones in Atlantic-type oceans may not require the spontaneous foundering of its passive margins. Instead, subduction can be seen as an invasive process that propagates from ocean to ocean.

Petrological and Nd-Sr-Os isotopic constraints on the origin of high-Mg adakitic rocks from the North China Craton: Tectonic implications

Abstract: The Mesozoic high-Mg dioritic rocks in the North China Craton have been suggested to be part of adakitic rocks. The origin of the high-Mg diorites has been attributed to equilibration of partial melts from delaminated mafic crust (eclogite) with mantle peridotite. Here we present petrological and Os isotopic data against the delamination model, and propose a process of magma mixing between siliceous crustal melts and basaltic magma from metasomatized mantle in a post-kinematic setting for their origin. The magma mixing process is supported by (1) euhedral overgrowths of high-Ca plagioclase and high-Mg pyroxene over low-Ca plagioclase and low-Mg pyroxene, respectively, and (2) highly radiogenic Os isotopic compositions, and negatively correlated Nd and Sr isotopic ratios. Our proposed model is probably applicable to the general mode of origin and tectonic settings of high-Mg adakitic magmas.

On the source of orogenic gold

Abstract: Gold has historically been a key strategic commodity, and the study of gold deposit formation has long been investigated. Debate on the genesis of so-called “orogenic gold deposits” (>75% of gold recovered through history; [Phillips, 2013][1]), continues largely because it is difficult to

An extensive and dynamic ice sheet on the West Greenland shelf during the last glacial cycle.

Abstract: Considerable uncertainty surrounds the extent and timing of the advance and retreat of the Greenland Ice Sheet (GIS) on the continental shelf bordering Baffin Bay during the last glacial cycle. Here we use marine geophysical and geological data to show that fast-flowing ice sheet outlets, including the ancestral Jakobshavn Isbrae, expanded several hundred kilometers to the shelf edge during the last glaciation ca. 20 ka. Retreat of these outlets was asynchronous. Initial retreat from the shelf edge was underway by 14,880 calibrated (cal) yr B.P. in Uummannaq trough. Radiocarbon dates from the adjacent Disko trough and adjoining trough-mouth fan imply later deglaciation of Jakobshavn Isbrae, and, significantly, an extensive readvance and rapid retreat of this outlet during the Younger Dryas stadial (YD). This is notable because it is the first evidence of a major advance of the GIS during the YD on the West Greenland shelf, although the short duration suggests that it may have been out of phase with YD temperatures.

The thickness of subduction plate boundary faults from the seafloor into the seismogenic zone

Abstract: The thickness of an active plate boundary fault is an important parameter for understanding the strength and spatial heterogeneity of fault behavior. We have compiled direct measurements of the thickness of subduction thrust faults from active and ancient examples observed by ocean drilling and fi eld studies in accretionary wedges. We describe a general geometric model for subduction thrust decollements, which includes multiple simultaneously active, anastomosing fault strands tens of meters thick. The total thickness encompassing all simultaneously active strands increases to ~100–350 m at ~1–2 km below seafl oor, and this thickness is maintained down to a depth of ~15 km. Thin sharp faults representing earthquake slip surfaces or other discrete slip events are found within and along the edges of the tens-ofmeters- thick fault strands. Although fl attening, primary inherited chaotic fabrics, and fault migration through subducting sediments or the frontal prism may build melange sections that are much thicker (to several kilometers), this thickness does not describe the active fault at any depth. These observations suggest that models should treat the subduction thrust plate boundary fault as <1–20 cm thick during earthquakes, with a concentration of postseismic and interseismic creep in single to several strands 5–35 m thick, with lesser distributed interseismic deformation in stratally disrupted rocks surrounding the fault strands.

Pervasive Aeolian Activity Along Rover Curiosity's Traverse in Gale Crater, Mars

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

Mirror-like faults and power dissipation during earthquakes

Abstract: Earthquakes occur along faults in response to plate tectonic movements, but paradoxically, are not widely recognized in the geological record, severely limiting our knowledge of earthquake physics and hampering accurate assessments of seismic hazard. Light-reflective (so-called mirror like) fault surfaces are widely observed geological features, especially in carbonate-bearing rocks of the shallow crust. Here we report on the occurrence of mirror-like fault surfaces cutting dolostone gouges in the Italian Alps. Using friction experiments, we demonstrate that the mirror-like surfaces develop only at seismic slip rates (∼1 m/s) and for applied normal stresses and sliding displacements consistent with those estimated on the natural faults. Under these experimental conditions, the frictional power density dissipated in the samples is comparable to that estimated for natural earthquakes (1–10 MW/m 2 ). Our results indicate that mirror-like surfaces in dolostone gouges are a signature of seismic faulting, and can be used to estimate power dissipation during ancient earthquake ruptures.

Coseismic recrystallization during shallow earthquake slip

Abstract: Solidified frictional melts, or pseudotachylytes, remain the only unambiguous indicator of seismic slip in the geological record. However, pseudotachylytes form at >5 km depth, and there are many rock types in which they do not form at all. We performed low- to high-velocity rock friction experiments designed to impose realistic coseismic slip pulses on calcite fault gouges, and report that localized dynamic recrystallization may be an easy-to-recognize microstructural indicator of seismic slip in shallow, otherwise brittle fault zones. Calcite gouges with starting grain size −1 , and total displacements between 1 and 4 m. At coseismic slip velocities ≥0.1 m s −1 , the gouges were cut by reflective principal slip surfaces lined by polygonal grains 2 + CaO. The recrystallized calcite aggregates resemble those found along the principal slip surface of the Garam thrust, South Korea, exhumed from

A hematite-bearing layer in Gale Crater, Mars: Mapping and implications for past aqueous conditions

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