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The most powerful category of explosive volcanic eruptions, called "ultraplinian," was proposed in 1980 on the basis of the distribution of a single pyroclastic fall product of one phase of the 1800a Taupo eruption in New Zealand. Dispersal data, a measure of the "footprint" of the deposit, were used subsequently to estimate eruption plume heights of 50-51 km, more than 10 km higher than observed or estimated for any historical Plinian eruption. Today, this unit remains the only deposit to have met the rigorous ultraplinian dispersal criteria, and it is an important and widely cited exemplar in physical volcanology. The earlier study was based on total thicknesses for the entire bed and grain-size data across that full thickness. We have now subdivided this bed into 26 subunits and measured their individual thicknesses and selected maximum clast sizes. Our data show that the apparent large footprint of this bed is an artifact of a previously unrecognized shift in the wind field during the eruption, rather than extreme eruptive vigor. Our study demonstrates the dangers of applying full-thickness approaches to even seemingly uniform fall deposits. The results throw into some doubt the need for the term ultraplinian, at least for this deposit. With the revision of plume heights for the Taupo ultraplinian, a height range of 35-40 km may be practical for use as an upper limit for source parameters in models of transport and dispersal of volcanic ash. [ABSTRACT FROM AUTHOR]

How convergent systems distribute strain among frontal thrusts is a major concern regarding seismic hazard assessment. Along the 2500 km Himalayan arc, the seismic behavior of the Bhutan region is unknown, because it corresponds to the only portion of the arc where no evidence of major earthquakes has been reported. This can be due either to the fact that no active tectonic studies have been conducted or to continental shortening being absorbed by the Shillong plateau 150 km farther south. Analyzing offset fluvial terraces in south-central Bhutan shows that two major earthquakes ruptured the Himalayan frontal thrust during the last millennium, and that a comparable rate of Holocene deformation (~20 mm/yr) is accommodated across the Himalaya in Bhutan as in central Nepal. Thus, the propensity for great earthquakes in Bhutan is similar to what is observed in neighboring portions of the Himalaya arc. This in turn suggests that the shortening process beneath the Shillong plateau has little effect on how strain accumulates within the Bhutanese Himalaya. [ABSTRACT FROM AUTHOR]

The geologic record from high-latitude continental margins shows that soft-bedded ice streams are capable of eroding, transporting, and depositing large volumes of sediment. Interpretation of these records relies on correct understanding of how sediments were transferred. We use a three-dimensional numerical ice-flow model, with physically based processes taking place in a Coulomb-plastic basal till layer, to test a new hypothesis related to sedimentary processes occurring beneath ice streams. Based on recent observations of a 15-m-thick debris-bearing basal ice layer (BIL) in Kamb Ice Stream, Antarctica, we propose that sediment entrainment by freeze-on, followed by englacial transport, and eventually meltout, represent efficient mechanisms whereby ice streams erode their bed and redistribute sediments. Our experimental setup produces results where ice stream flow is characterized by oscillations between fast and stagnant modes of flow. We show that there is a strong coupling between the amplitude of the ice stream oscillations and the amount of sediment eroded and transferred out of the modeled system due exclusively to the formation and advection of a BIL. We also show that increased incorporation of water from a basal water system amplifies the oscillations and thus the growth of the BIL. The patterns of ice stream flow, and associated sediment fluxes and transport seen in our model, are consistent with modern Antarctic ice streams as well as the seemingly erratic behavior of paleo--ice streams during the last deglaciation. [ABSTRACT FROM AUTHOR]

Moraines in the Taurus Mountains of south-central Turkey, dated to latest Pleistocene or earliest Holocene, show that glaciers were extraordinarily large, typical of the Last Glacial Maximum (21 ka), and that rates of glacier retreat and temperature rise exceeded those of the past century. Surface exposure ages of 7 moraines in a valley at altitudes between 1100 m and 3100 m above sea level range from 10.2 ± 0.2 ka to 8.6 ± 0.3 ka, computed using our own production rates and spatiotemporal scaling factors. Hitherto unresolved differences in cosmogenic 36Cl production-rate estimates can make these ages significantly older, and therefore the analysis presented here focuses on the rate of change and not on the absolute chronology. During deglaciation, the equilibrium line altitude ascended 1430 m and the air temperature rose by 9 °C. Deglaciation occurred in two phases. During the second, faster phase, which lasted 500 yr, the glacier length decreased at an average rate of 1700 m/100 yr, implying a warming rate of 1.44 °C/100 yr, indicating a rapid climate shift marking the onset of the Holocene in Turkey. [ABSTRACT FROM AUTHOR]

Aeolian dune development is influenced by feedback between surface properties and sediment transport, yet little is known about the larger scale temporal and spatial natures of this relationship. Surface moisture is particularly influential, and is generally recognized in aeolian environments for its ability to increase the critical shear velocity required to entrain sediment in beach settings or, alternatively, to sustain vegetation and stabilize surfaces at a dune-field scale. However, conceptual models and field work have alluded to its importance in protodune initiation, while field observations infer that seasonal moisture input may contribute to residual dune ridge formation at the dune-field scale. This has the potential to reveal geomorphic adaptation to variations in climate, and identify a recognizable signature in the rock record. This article presents a simulation model that produces geomorphological features similar to field observations and is capable of examining the implications of surface and transport feedback at both scales. Results (1) reveal the control of surface moisture at different temporal scales, (2) display complexity in the development of multiple spatial scales within a cellular automaton framework, (3) highlight the importance of transient sand strips and sediment supply frequency in aeolian transport dynamics and protodune development, and (4) explore the relationship and significance of feedback duration, development time, and bedform spatial scale in the development of incipient dunes. This study illustrates the importance of considering geomorphic feedback when assessing the influence of surface moisture in aeolian process--dominated systems. [ABSTRACT FROM AUTHOR]

U-Pb age spectra of detrital zircons in samples from the Paleogene Colton Formation in the Uinta Basin of northeastern Utah and the Late Cretaceous McCoy Mountains Formation of southwestern Arizona (United States) are statistically indistinguishable. This finding refutes previous inferences that arkosic detritus of the Colton was derived from cratonic basement exposed by Laramide tectonism, and instead establishes the Cordilleran magmatic arc (which also provided sediment to the McCoy Mountains Formation) as the primary source. Given the existence of a north-south-trending drainage divide in eastern Nevada and the north-northeast direction of Laramide paleoflow throughout Arizona and southern Utah, we infer that a large river system headed in the arc of the Mojave region fl owed northeast ∼700 km to the Uinta Basin. Named after its source area, this Paleogene California River would have been equal in scale but opposite in direction to the modern Green River-Colorado River system, and the timing and causes of the subsequent drainage reversal are important constraints on the tectonic evolution of the Cordillera and the Colorado Plateau. [ABSTRACT FROM AUTHOR]

A previously unreported easternmost segment of the Nile River deep-sea fan complex has been identified in the deep Levant Basin, offshore Israel, based mainly on a recently released, high-quality three-dimensional seismic data set. This segment is characterized by densely spaced seabed and subseabed turbidite channel complexes, generally fed from the region of the historically active northeastern Damietta and Pelusian branches of the Nile. This pattern suggests a previously unnoticeable symmetry in the modern channel distribution between the eastern and western branches of the deep-sea fan. The base of the Nile's eastern deep-sea fan is determined by a NW-trending system of pre-Nile turbidite slope channels at the top of the Messinian evaporites. The overlying Pliocene-Holocene fan complex is further divided into two distinct stratigraphic units; a lower, stratified, sand-rich unit deposited in a basin-floor setting, and an upper heterogeneous, mud-rich unit consisting of downslope, mass transport, and hemipelagic sediments, with encased slope channel complexes richer in sand. Due to gravity gliding and spreading above the underlying thick, mobile Messinian salt, both stratigraphic units as well as the upper part of the Messinian salt layer are faulted and folded. [ABSTRACT FROM AUTHOR]

Small impact events recorded on the surface of Earth are significantly underrepresented based on expected magnitude-frequency relations. We report the discovery of a 36-m-diameter late Holocene impact crater located in a forested area near the town of Whitecourt, Alberta, Canada. Although undetectable using visible imagery, the presence of the crater is revealed using a bare-Earth digital elevation model obtained through airborne light detection and ranging (LiDAR). The target material comprises deglacial Quaternary sediments, with impact ejecta burying a late Holocene soil dated to ca. 1100 14C yr B.P. Most of the 74 iron meteorites (0.1-1196 g) recovered have an angular exterior morphology. These meteorites were buried at depths <25 cm and are interpreted to result from fragmentation of the original projectile mass, either at low altitude or during the impact event. Impact of the main mass formed the simple bowl-shaped impact structure associated with an ejecta blanket and crater fill. The increasing availability of LiDAR data for many terrestrial surfaces will serve as a useful tool in the discovery of additional small impact features. [ABSTRACT FROM AUTHOR]

We have performed the first known semicontinuous monitoring of particulate organic carbon (POC) fluxes and dissolved Si concentrations delivered to the ocean during a typhoon. Sampling of the Choshui River in Taiwan during Typhoon Mindulle in 2004 revealed a POC flux of 5.00 × 105 t associated with a sediment flux of 61 Mt during a 96 h period. The linkage of high amounts of POC with sediment concentrations capable of generating a hyperpycnal plume upon reaching the ocean provides the first known evidence for the rapid delivery and burial of POC from the terrestrial system. These fluxes, when combined with storm-derived CO2 consumption of 1.65 × 108 mol from silicate weathering, elucidate the important role of these tropical cyclone events on small mountainous rivers as a global sink of CO2. [ABSTRACT FROM AUTHOR]

We model the three-dimensional (3-D) conductive thermal fi eld of the Norwegian margin and evaluate its lithospheric configuration, which is consistent with two independent observables: temperature and gravity. Here, we show that knowledge of the sediment and crustal configuration of the passive continental margin provides constraints for the configuration and thermal structure of the lithosphere. We find that the thickness of the oceanic lithosphere adjacent to the stretched continental margin controls, to a large degree, the shallow conductive thermal fi eld of the entire margin. Our results confirm estimates of lithospheric thickness from seismology but contradict estimates from cooling models, which assume an equilibrium thickness of the oceanic lithosphere of 125 km. We find that the crust is colder in the oceanic than in the continental domain, whereas this trend is reversed downward. We obtain lateral temperature differences of 100 °C at 5 km depth across the margin that increase to 400 °C at 50 km depth. Higher temperatures and heat flows for the oceanic lithospheric mantle compared to the continent indicate that reduced upper-mantle P-wave velocities and densities are thermally induced. [ABSTRACT FROM AUTHOR]

Rock varnish from late to latest Pleistocene geomorphic features in the drylands of the western U.S. provides evidence of nine millennial-scale wet events from 11,500-18,000 calendar yr B.P., represented by regionally replicable and approximately evenly spaced manganeseand barium-rich dark bands in varnish microstratigraphy. Preliminary radiometric age calibration indicates that these events appear to be broadly coeval with millennial-scale cooling events identified in the Greenland Ice Sheet Project 2 (GISP2) ice core record. Six of these wet events are associated with the cold intervals of the Younger Dryas and Heinrich event H1, and the other three with the short-lived cooling phases of the Intra-Allerød Cold Period, the Older Dryas, and the Oldest Dryas. These results, combined with our previous documentation of millennial-scale wet events in the Holocene varnish record for the same region, indicate that such wet oscillations in the western U.S. may be parts of regionally widespread manifestation of well-documented, pervasive millennial-scale cycles of the North Atlantic climate. [ABSTRACT FROM AUTHOR]

Extreme coastal flooding, primarily during hurricane strikes, has deposited sand-rich layers in Laguna Playa Grande, a backbarrier lagoon located on the island of Vieques, Puerto Rico. Siliciclastic grain-size distributions within these overwash deposits fine landward (away from the barrier and toward the mainland). A simple advective-settling model can explain this pattern of lateral sorting and is used to constrain the relative magnitude of past flooding events. A deposit associated with the A.D. 1928 San Felipe hurricane is used as a modern analogue to test the technique, which produces reasonable estimates for wave heights that exceed the barrier during the event. A 5000 yr reconstruction of local flooding intensity is developed that provides a measure of the competence for each overwash event to transport coarser-grained sediment a fixed distance into the lagoon. This reconstruction indicates that although the Laguna Playa Grande record exhibits large-scale changes in hurricane frequency on centennial to millennial time scales, the magnitude of these events has stayed relatively constant. Over the last 5000 yr, no evidence exists for an anomalously large hurricane or tsunami event with a competence for sediment transport greater than historical hurricane events. [ABSTRACT FROM AUTHOR]

The retreat of the southern Greenland Ice Sheet (GIS) during the last deglaciation (Termination I: TI) is poorly dated by conventional means; there is even greater uncertainty about the penultimate deglaciation (Termination II: TII), leading to the assumption that the southern GIS has a signifi cant lag in its response to deglacial warming. Here we use geochemical terrestrial sediment proxies ([Fe] and [Ti]) from a well-studied southern Greenland marine sediment sequence to examine the behavior of the southern GIS during TI and TII. Our records show that during TI and TII the southern GIS response was essentially synchronous with deglacial North Atlantic warming, implying greater climate sensitivity than previously assumed. During TI, elevated ablation lasted 5 k.y., whereas ablation remained elevated for 12 k.y. during TII, suggesting a reduced southern GIS during TII that contributed a signifi cant fraction of the higher sea level during the subsequent interglacial. [ABSTRACT FROM AUTHOR]

Ejecta blankets around impact craters are rarely preserved on Earth. Although impact craters are ubiquitous on solid bodies throughout the solar system, on Earth they are rapidly effaced, and few records exist of the processes that occur during emplacement of ejecta. The Stac Fada Member of the Precambrian Stoer Group in Scotland has previously been described as volcanic in origin. However, shocked quartz and biotite provide evidence for high-pressure shock metamorphism, while chromium isotope values and elevated abundances of platinum group metals and siderophile elements indicate addition of meteoritic material. Thus, the unit is reinterpreted here as having an impact origin. The ejecta blanket reaches >20 m in thickness and contains abundant dark green, vesicular, devitrified glass fragments. Field observations suggest that the deposit was emplaced as a single fluidized flow that formed as a result of an impact into water-saturated sedimentary strata. The continental geological setting and presence of groundwater make this deposit an analogue for Martian fluidized ejecta blankets. [ABSTRACT FROM AUTHOR]

Possible sources of carbon that may have caused global warming at the Paleocene-Eocene boundary are constrained using an intermediate complexity Earth-system model configured with early Eocene paleogeography. We find that 6800 Pg C (δ13C of -22‰) is the smallest pulse modeled here to reasonably reproduce observations of the extent of seafl oor CaCO3 dissolution. This pulse could not have been solely the result of methane hydrate destabilization, suggesting that additional sources of CO2 such as volcanic CO2, the oxidation of sedimentary organic carbon, or thermogenic methane must also have contributed. Observed contrasts in dissolution intensity between Atlantic and Pacific sites are reproduced in the model by reducing bioturbation in the Atlantic during the event, simulating a potential consequence of the spread of low-oxygen bottom waters. [ABSTRACT FROM AUTHOR]

Long Valley caldera, California, formed during the cataclysmic Pleistocene eruption of the Bishop Tuff. Previous stratigraphic and petrologic studies of this eruption deciphered an intriguing pattern of vent migration, thought to mirror the lateral propagation ("unzipping") of magma-tapping ring fractures during caldera collapse. From scaled analog models, we show that this unzipping pattern was intrinsically related to the high plan-view ellipticity of the precollapse magma chamber roof. We also provide a first-order kinematic explanation for the systematic location of initial elliptical roof failure and for the lateral propagation of highly elliptical ring fractures. [ABSTRACT FROM AUTHOR]

The circular pattern of anisotropic fast-axis orientations of split SKS arrivals observed in the western U.S. cannot be attributed reasonably to either preexisting lithospheric fabric or to asthenospheric strain related to global-scale plate motion. A plume origin for this pattern accounts more successfully for the anisotropy field, but little evidence exists for an active plume beneath central Nevada. We suggest that mantle flow around the edge of the sinking Gorda-Juan de Fuca slab is responsible for creating the observed anisotropy. Seismic images and kinematic reconstructions of Gorda-Juan de Fuca plate subduction have the southern edge of this plate extending from the Mendocino triple junction to beneath central Nevada, and flow models of narrow subducted slabs produce a strong toroidal flow field around the edge of the slab, consistent with the observed pattern of anisotropy. This flow may enhance uplift, extension, and magmatism of the northern Basin and Range while inhibiting extension of the southern Basin and Range. [ABSTRACT FROM AUTHOR]

The Eocene-Oligocene (E-O) climate transition (ca. 34 Ma) marks a period of Antarctic ice growth and a major step from early Cenozoic greenhouse conditions toward today's glaciated climate state. The transition is represented by an increase in deep-sea benthic foraminiferal oxygen isotope (δ18O) values occurring in two main steps that reflect the temperature and δ18O of seawater. Existing benthic Mg/Ca paleotemperature records do not display a cooling across the transition, possibly reflecting a saturation state effect on benthic foraminiferal Mg/Ca ratios at deep-water sites. Here we present data from exceptionally well preserved foraminifera deposited well above the calcite compensation depth that provide the first proxy evidence for an ∼2.5 °C ocean cooling associated with the ice growth. This permits interpretation of E-O δ18O records without invoking Northern Hemisphere continental-scale ice. [ABSTRACT FROM AUTHOR]

The West Antarctic rift system differs from other volcanically active rift systems in two unusual respects: (1) the rift floor lies 1000-2000 m lower in elevation than others, and (2) four interior ice-filled troughs extend between 1500 m and 2555 m below sea level. Two troughs are more than twice the depth of Lake Baikal, the world's deepest lake. The Marie Byrd Land dome, by contrast, compares closely with the intra-rift Kenyan and Ethiopian domes in the East African rift. Comparisons between these rift systems suggest (1) that the West Antarctic interior is relatively cool and volcanically inactive, (2) that there is likely to have been an episode of extension during Neogene time in the deep interior basins, and (3) that dome uplift and basin subsidence have greatly changed the West Antarctic landscape over the past 25 m.y. [ABSTRACT FROM AUTHOR]

We have discovered a robust microcrystalline record of the early genesis of North American lithosphere preserved in the U-Pb age and oxygen isotope zoning of zircons from a lower crustal paragneiss in the Neoarchean Superior province. Detrital igneous zircon cores with δ18O values of 5.1‰-7.1‰ record creation of primitive to increasingly evolved crust from 2.85 ± 0.02 Ga to 2.67 ± 0.02 Ga. Sharp chemical unconformity between cores and higher δ18O (8.4‰-10.4‰) metamorphic overgrowths as old as 2.66 ± 0.01 Ga dictates a rapid sequence of arc unroofing, burial of detrital zircons in hydrosphere-altered sediment, and transport to lower crust late in upper plate assembly. The period to 2.58 ± 0.01 Ga included ∼80 m.y. of high-temperature (∼700-650 °C), nearly continuous overgrowth events reflecting stages in maturation of the subjacent mantle root. Huronian continental rifting is recorded by the youngest zircon tip growth at 2512 ± 8 Ma (∼ 600 °C) signaling magma intraplating and the onset of rigid plate behavior. This >150 m.y. microscopic isotope record in single crystals demonstrates the sluggish volume diffusion of U, Pb, and O in zircon throughout protracted regional metamorphism, and the consequent advances now possible in reconstructing planetary dynamics with zircon zoning. [ABSTRACT FROM AUTHOR]

Observations from a number of rifted margins (e.g., South Atlantic salt basin, mid-Norwegian margin, Exmouth Plateau) reveal wide regions of extremely attenuated crust and depositional environments that indicate depth-dependent lithospheric extension. Although the one-dimensional thermal-kinematic consequences of depth-dependent extension are understood, no comprehensive process-based explanation for the complex style of these margins exists. Here, we present self-consistent numerical models of passive-margin formation that explain the depth-dependent extension, the width of the margin, its characteristic tripartite nature, and why such margins are prone to deposition of evaporites under appropriate climatic conditions. Some features that are important to reproducing the observed characteristics include decoupling between upper and lower parts of the lithosphere during stretching, contrasting wide and narrow extensional styles above and below the decoupling level, and progressive focusing of crustal extension toward the rift axis. [ABSTRACT FROM AUTHOR]

Sediment from landscape disturbance often enters temporary storage in valleys and evacuates over longer times, which in steeplands are poorly delimited. We hypothesize that, across process transitions (e.g., debris flow versus fluvial transport), distributions of sediment transit times also change. We use field surveys and extensive radiocarbon dating to assess the distribution of transit (residence) times through the proxy measurement of ages of bank deposits in two mainstem reaches of a 2.23 km² watershed in the Oregon Coast Range. In the downstream reach, debris fans impound fluvial deposits; debris-flow, fine fluvial, and coarse fluvial deposits compose nearly equal parts of the valley fill; and fluvial erosion evacuates deposits . Transit times have a sample mean of 1.22 × 10³ 14C yr and an exponential distribution, indicating uniform probability of evacuation from storage. In the upstream reach, valley-spanning debris jams impound debris-flow deposits composing >95% of the valley fill, which is routinely scoured by debris flows. Transit times have a sample mean of 4.43 × 10² 14C yr and, if >100 14C yr, a power-law distribution, indicating preferential evacuation of younger deposits and retention of older deposits. In both reaches, most sediment has short transit times (<600 14C yr), but significant volumes remain for millennia. Less than 20% of basin-wide denudation passes through these reservoirs, but the latter are still significant buffers between hillslope disturbance and downstream aquatic habitat, especially for coarse sediment. [ABSTRACT FROM AUTHOR]

High-flux pulses of magmatism that make up most of the exposed North American Cordilleran arcs are derived primarily from upper plate lithospheric source materials, and not the mantle wedge as most models would predict, based on a compilation of thousands of previously published Sr, Nd, and O isotopic data. Mass balance calculations show that no more than 50% of that mass can be mantle-derived. Flare-ups must have fundamentally developed simultaneously with crustal/lithospheric thickening, thus implying a connection. Subduction erosion from the trench side, and retroarc shortening from the foreland side are the main tectonic shortening processes that operate in conjunction with high flux magmatism during subduction, and therefore are likely triggers for flare-up events in arc. These arcs represent the sites of crustal differentiation, and thus contribute to net continental growth, only if dense residual lower crust was returned to the convective mantle. Isotopic data shown here suggest that if convective removal of batholithic roots takes place, it must be a consequence and not a cause of episodic flare-ups. The Altiplano-Puna Volcanic Complex in South America may be the most recent continental arc segment in flare-up mode. [ABSTRACT FROM AUTHOR]

The process by which continental crust has formed is not well understood, though such crust mostly forms at convergent plate margins today. It is thus imperative to study modern intra-oceanic arcs, such as those common in the western Pacific Ocean. New seismic studies along the representative Izu-Bonin intra-oceanic arc provide unique along-strike images of arc crust and uppermost mantle to complement earlier, cross-arc lithospheric profiles. These reveal two scales (1000–10 km scale) of variations, one at the scale of the Izu versus Bonin (thick versus thin) arc crust, the other at the intervolcano (∼50 km) scale. These images show that: (1) the bulk composition of the Izu-Bonin arc crust is more mafic than typical continental crust, (2) the middle crust with seismic velocities similar to continental crust is predominantly beneath basaltic arc volcanoes, (3) the bulk composition beneath basaltic volcanoes changes little at thick and thin arc segments, and (4) a process to return lower crustal components to the mantle, such as delamination, is required for an arc crust to evolve into continental crust. Continued thickening of the Izu-Bonin crust, accompanied by delamination of lowermost crust, can yield velocity structure of typical continental crust. [ABSTRACT FROM AUTHOR]

The 231Pa-235U disequilibria provide greater ability to constrain the rate of mantle melting in convergent margin settings than the more often analyzed 238U-230Th-226Ra systems, which are strongly affected by fluid-addition processes from the subducting slab. Here we present new 231Pa-235U data for 12 samples from the Kick'em Jenny (KEJ) submarine volcano in the Southern Lesser Antilles to define the melting rate at a subduction zone with one of the lowest convergence rates. The KEJ samples have the highest average (231Pa)/(235U) yet measured in global arcs, consistent with other studies of the Southern Lesser Antilles lavas. These results reinforce the previously noted negative correlation between average (231Pa)/(235U) and convergence rate in all arc settings globally. We develop a model to explain this negative correlation and to better constrain melting rates at convergent margins. Assuming that the corner flow velocity is coupled to and equal to the subducting slab velocity, the melting rate, directly reflecting the flux of water added, becomes a linear function of subduction rate. This physical model is then coupled to three different melting models previously developed for calculating U-series disequilibria (reactive porous flow, dynamic, and flux melting). All three models reproduce the globally observed negative correlation between subduction rate and 231Pa excess. Although the style of melting cannot be easily discriminated, the good correspondence between models and observation provides an example of how geochemical and geophysical models can be linked to provide a self-consistent model of melt generation in convergent margin settings. [ABSTRACT FROM AUTHOR]

Using triaxial numerical experiments, we investigated the evolution of the state of stress and that of the bulk instantaneous and finite strain during ongoing convergence and subsequent progressive tectonic unloading of a warm and buoyant continental lithosphere. Various unloading histories of the driving tectonic force were considered. As the tectonic force progressively declines, the instantaneous strain evolves from plane strain to horizontal constriction in a direction perpendicular to that of convergence, and finally to horizontal flattening. During the progressive unloading of the tectonic force driving convergence, bulk constrictional strain accommodates the release of accumulated gravitational stress. The decline of the triaxial strain rates to low values reduces the potential for the orogen-parallel linear fabric to be erased by horizontal flattening. This is confirmed by the finite strain ellipsoid that evolves toward plane strain with a long axis parallel to the orogen. In the ca. 2.5 Ga Gawler and Terre Adélie cratons, we have identified a well-preserved and widespread horizontal linear fabric. As suggested by our numerical experiments, we associate the development of this linear fabric with the waning stages of late Archean convergence. [ABSTRACT FROM AUTHOR]

Alluvial fan evolution and morphology are often considered to respond primarily to external forcing (e.g., tectonics, climate, and base-level change). Here we present a numerical model of alluvial fan evolution that shows that dramatic and persistent fan entrenchment may occur in the absence of such forcing. This process is driven by positive autogenic feed-backs between flow width, sediment transport, and rate of fan aggradation. Entrenchment is initiated where sediment accommodation space limits continued fan growth. Our results highlight a need to rethink both the representation of fluvial width adjustment in landscape evolution models and the established framework for the interpretation of fluvial landforms as archives of environmental change. [ABSTRACT FROM AUTHOR]

An array of east-trending ridges 1–2 m high and up to 2 km long occurs on the Tahoe City shelf, a submerged wave-cut bench <15 m deep in the northwest sector of the lake. The shelf is just north of the amphitheater of the giant subaqueous 10 km³ McKinney Bay landslide, which originated on the west wall of Lake Tahoe. Images from a submersible camera show that the ridges are composed of loose piles of boulders and cobbles that lie directly on poorly consolidated, fine-bedded lake beds deposited in an ancestral Lake Tahoe. Dredge hauls from landslide distal blocks, as well as from the walls of the reentrant of the landslide, recovered similar lake sediments. The McKinney Bay landslide generated strong currents, which rearranged previous glacial-derived debris into giant ripples creating the boulder ridges. The uncollapsed part of the sediment bench, including the Tahoe City shelf, poses a hazard because it may fail again, producing a landslide and damaging waves. [ABSTRACT FROM AUTHOR]

Aluminum-26 and beryllium-10 surface exposure dating on cut-and-fill river-terrace surfaces from the lower Sutlej Valley (northwest Himalaya) documents the close link between Indian Summer Monsoon (ISM) oscillations and intervals of enhanced fluvial incision. During the early Holocene ISM optimum, precipitation was enhanced and reached far into the internal parts of the orogen. The amplified sediment flux from these usually dry but glaciated areas caused alluviation of downstream valleys up to 120 m above present grade at ca. 9.9 k.y. B.P. Terrace formation (i.e., incision) in the coarse deposits occurred during century-long weak ISM phases that resulted in reduced moisture availability and most likely in lower sediment flux. Here, we suggest that the lower sediment flux during weak ISM phases allowed rivers to incise episodically into the alluvial fill. [ABSTRACT FROM AUTHOR]

A major earthquake on the Seattle fault, Washington, Ca. A.D. 900-930 was first inferred from uplifted shorelines and tsunami deposits. Despite follow-up geophysical and geological investigations, the rupture parameters of the earthquake and the geometry of the fault are uncertain. Here we estimate the fault geometry, slip direction, and magnitude of the earthquake by modeling shoreline elevation change. The best fitting model geometry is a reverse fault with a shallow roof ramp consisting of at least two back thrusts. The best fitting rupture is a SW-NE oblique reverse slip with horizontal shortening of 15 m, rupture depth of 12.5 km, and magnitude Mw = 7.5. [ABSTRACT FROM AUTHOR]

Seismic refraction and reflection data were acquired in 2000 and 2003 to study the morphology and sedimentary fill of the remote El'gygytgyn crater (Chukotka, northeastern Siberia; diameter 18 km). These data allow a first insight into the deeper structure of this unique impact crater. Wide-angle data from sonobuoys reveal a five-layer model: a water layer, two lacustrine sedimentary units that fill a bowl-shaped apparent crater morphology consisting of an upper layer of fallback breccia with P-wave velocities of ∼3000 m/s, and a lower layer of brecciated bedrock (velocities >3600 m/s). The lowermost layer shows a distinct anticline structure that, by analogy with other terrestrial and lunar craters of similar size, can be interpreted as a central ring structure. The El'gygytgyn crater exhibits a well-expressed morphology that is typical of craters formed in crystalline target rocks. [ABSTRACT FROM AUTHOR]

Magnesium/calcium (Mg/Ca) ratios of foraminiferal shells from a sediment core from the northern South China Sea, a semi-enclosed basin in the western tropical Pacific, document variations in sea-surface temperature (SST) during the past 145 k.y. Glacial SSTs were 4°C colder than interglacial SSTs. During the last deglaciation, most of the warming was accomplished in a single abrupt step after continental ice-sheet decay had already begun, but warming and ice-sheet demise were nearly synchronous during the penultimate deglaciation. Abrupt SST changes of the past 15 k.y. were apparently synchronous with events in East Asian monsoon rainfall, suggesting that variations in monsoon winds and their influence on surface circulation of the western Pacific exerted a strong control on northern South China Sea SSTs. We suggest that this link persisted for the previous 130 k.y., during which time orbital-scale 2-3 °C SST changes and several small (≤2 °C) abrupt SST events occurred in the northern South China Sea. The similar timing of northern South China Sea SST, on a benthic δ18O time scale, to a well-dated speleothem record from eastern China suggests that the demise of ice sheets associated with the penultimate deglaciation did not precede Northern Hemisphere summer insolation increase. Our re suits suggest that surface waters had higher δ18O values during times of strong summer monsoon than during times of weak monsoon, likely reflecting a redistribution of 18O. depleted rainfall from land during times of strong summer monsoons, to the western Pacific during times of weaker summer monsoons. [ABSTRACT FROM AUTHOR]

Life at the Permian-Triassic boundary (ca. 251 Ma) underwent the largest disruption in Earth's history. Paleoclimatic data indicate that Earth was significantly warmer than present and that much of the ocean was anoxic or euxinic for an extended period of time. We present results from the first fully coupled comprehensive climate model using paleogeography for this time period. The coupled climate system model simulates warm high-latitude surface air temperatures related to elevated carbon dioxide levels and a stagnate global ocean circulation in concert with paleodata indicating low oxygen levels at ocean depth. This is the first climate simulation that captures these observed features of this time period. [ABSTRACT FROM AUTHOR]

Ice thickness in West Antarctica at the Last Glacial Maximum (LGM) is poorly known, yet is key information for understanding ice streams and interpreting ice cores. Although trim lines, moraine limits, and exposure-age dating provide geologic constraints on ice thickness near the Transantarctic Mountains and in Marie Byrd Land, lack of exposed bedrock hampers traditional geologic methods in a central, ∼2 × 106 km² region. Here we infer ice-sheet thickness changes in the central Ross Sea Embayment by using a transient ice-flow model to find combinations of accumulation-rate and ice-sheet thickness histories that match the depth-age relationship and the measured layer-thickness pattern in the Siple Dome ice core. After we reject unreasonable accumulation-rate histories, the remaining history pales indicate thinning of 200–400 m since the LGM. Our estimate is lower than previous reconstructions that were constrained by geologic evidence from the Transantarctic Mountains and by marine data from the Ross Sea floor, which indicate that a grounded ice sheet extended to the continental shelf margin during the last glacial period. Low surface elevations in the central Ross Sea Embayment during the LGM do not preclude thicker ice along its boundaries. However, if this grounded ice sheet came over 1000 km from interior West Antarctica, as is usually assumed, then it had very low surface slope, requiring a very slippery bed. Alternatively, the grounded Ross Sea ice might have flowed from the Transantarctic Mountains and western Marie Byrd Land. [ABSTRACT FROM AUTHOR]

Hypoxia on the Louisiana continental shelf is tied to nutrient loading and freshwater stratification from the Mississippi River. Variations in the relative abundance of low-oxygen-tolerant benthic foraminifers in four sediment cores from the Louisiana shelf provide a proxy record of low-oxygen events. Core chronologies are obtained using 210pb dating techniques. The foraminiferal data are consistent with previous studies indicating that the intensity of hypoxic events (oxygen <2 rag/L) has increased over the past 50 yr owing to the higher nutrient loading associated with the use of commercial fertilizer, and also reveal several low-oxygen events between A.D. 1817 and 1910, prior to the widespread use of fertilizer. The pre-1910 low-oxygen events are associated with high Mississippi River discharge rates, indicating that these low-oxygen episodes are related to natural variations in river drainage that enhance transport of nutrients and freshwater to the continental shelf. Our data show that the low-oxygen events of the past few decades were more extreme than any that occurred in the previous ∼180 yr, and support the interpretation that the increased use of fertilizer has amplified an otherwise naturally occurring process. [ABSTRACT FROM AUTHOR]

In contrast to Eocene Himalayan foreland basin sedimentary deposits in India and Pakistan, coeval sedimentary rocks of the Bhainskati Formation in Nepal contain scant petrographic evidence of orogenic input. Such data have been used as evidence to promote models of diachroneity of India-Asia collision. In this paper we document orogenic input into the Eocene foreland basin rocks of Nepal, from fission-truck analyses of detrital zircons. Our data provide evidence that significantly reduces the possible duration of any diachroneity of collision, and brings the interpretation of the sedimentary record into better agreement with ages of early thrusting and metamorphism in the orogen. We also use our detrital fission-track data to bolster previous age determinations of the overlying Dumri Formation, confirming the basin-wide occurrence of a major unconformity. Comparison of our data set with that from coeval along-strike rocks in India suggests that there is no evidence of diachroneity in early stages of the orogen's exhumation. [ABSTRACT FROM AUTHOR]

We studied breccia beds in lacustrine sediments within the active Dead Sea basin. The beds were deformed by M >5.5 earthquakes during the past 60 k.y. Our new analysis considers both the thickness of breccia beds and the lithology of beds directly overlying them in order to identify 11 M >7 earthquakes that originated within the Dead Sea pull-apart between 54 and 16 ka. The resulting time series is a unique long record of earthquakes in a well-constrained segment of a fault system in which the time interval between consecutive earthquakes increased from hundreds of years to a background recurrence interval of ∼11 k.y. since ca. 40 ka. Since this recurrence interval is similar to the M ≥7.2 recurrence interval in the Dead Sea basin, as extrapolated from present seismicity, we suggest that the present seismic regime in the Dead Sea basin, as reflected in its magnitude=-frequency relation as well as in its deficiency in seismic moment, has been stationary for the past ∼40 k.y. Since the increasing interval between consecutive earthquakes in the studied segment of the Dead Sea fault is time-logarithmic, it may be a result of healing of the brittle crust as well as a diminishing strain rate following the first strong earthquake in the sequence. [ABSTRACT FROM AUTHOR]

The Eclogite zone (Tauern Window, Eastern Alps, Austria) represents one of only a few examples of high-pressure units providing both the prograde and the retrograde metamorphic evolution. The eclogites are associated with rocks originating from continental and transitional crust that also have been affected by high-pressure metamorphism. Eclogites that formed along the prograde path indicate metamorphic conditions of 17-20 kbar (1700-2000 MPa) at 550-580 °C; the peak assemblages formed at 21-25 kbar (2100-2500 MPa) at 600-620 °C. Omphacite microstructures, in particular shape fabrics and crystallographic preferred orientations, indicate that the final phases of the prograde evolution were characterized by flattening strain. The evolution at the pressure peak and along the exhumation path shows a constrictional strain geometry. This change is interpreted to have been controlled by the force balance between slab pull (related to an oceanic slab) and the buoyancy of subducted adjacent continental crustal material, including the eclogites. At a certain lithospheric level, where the subducted continental rocks were entirely surrounded by high-density lower-crustal and upper-mantle material, the buoyancy forces exceeded the slab-pull forces. This force change resulted in the buoyancy-driven extrusion of continental material between two lithospheric plates. The part of the subducted slab that remained in buoyant equilibrium was therefore affected by constriction. Moreover, this process may have resulted in breakoff of the subducted oceanic slab. If so, the cessation of slab pull resulted in accelerated buoyancy-driven extrusion of the continental material. Extrusion of these high-pressure sheets would have been accompanied by constriction subparallel to the subduction channel as well. [ABSTRACT FROM AUTHOR]

Among the major ice streams that drain West Antarctica, Kamb Ice Stream (formerly called Ice Stream C) is unique in that it stagnated ∼150 yr ago, but its former fast-flow conditions are virtually unknown. Here we present surface-based radar profiles of the ice stream's undulating internal stratigraphy, which records these conditions. Our analysis of the profiles indicates that prestagnation flow velocities, averaged over a period <740 yr, exceeded 350 m·yr-1 in the trunk of the ice stream. This velocity constraint would be lower if the ice had been thickening (higher if thinning), but suggests mass loss from the ice-stream catchment that is of sufficient magnitude to reverse the gain estimated for today's Siple Coast region. Analysis of other ice streams would allow comparison of velocities over millennial time scales with observations of present-day velocities, useful for evaluating how West Antarctic ice drainage has evolved. [ABSTRACT FROM AUTHOR]

Correlation of continental shelf-slope stratigraphy in Santa Monica Bay (southern California) with Ocean Drilling Program records for nearby slope-basin sites has illuminated the timing and scale of terrigenous sediment dispersal on margin since the Last Glacial Maximum (LGM). Marine flooding surfaces preserved in a transgressive sequence on the Santa Monica Shelf provide a key link between base-level elevation and sediment transport across shelf. Sediment-accumulation rates at slope-basin sites were maximal ca. 15-10 ka, well after the LGM, decreased during the 12-9 ka transition from fluvial-estuarine to fully marine conditions on the shelf, and decelerated throughout the Holocene to 30%-75% of their values at the LGM. The deceleration is interpreted to manifest a landward shift in the margin depocenter with the onset of transgressive sedimentation beginning when sea level surmounted the shelf edge ca. 13 ka, as predicted by sequence-stratigraphic models. However, the records make clear that factors other than base level modulated slope-basin accumulation rates during the deglaciation. [ABSTRACT FROM AUTHOR]

The sediments of Santa Barbara Basin, a valuable source of information about the history of the California Current, show cyclic deposition in three properties here investigated: varve thickness, total organic carbon (TOC) accumulation, and fish-scale abundance. We analyzed the record since A.D. 1350 for varves, A.D. 1120 for TOC, and A.D. 1200 for fish scales. Prominent periods are 169 yr, 93 yr, 51.4 yr, 17.7 yr (varve thickness), 124 yr, 56 yr, 37.4 yr (TOC flux), and 103 yr, 128 yr (fish-scale abundance). A surprising number of the varve and TOC cycles can be interpreted as deriving from the common tidal cycles, i.e., the 4.425 yr perigee period (describing the changing distance between Earth and Moon) and the 18.6 yr lunar nodal period (which pertains to the lineup of Earth, Moon, and Sun). We suggest that the record of the basin is partly a result of pulsed sedimentation aligned with tides. These findings support the hypothesis that a considerable portion of sediment destined for deposition in the center of the basin goes through a phase of shallow storage on the shelf where it can be remobilized through storms and tides. Mechanisms calling on tidally pulsed productivity or tidally influenced climate change are not excluded, but are considered less attractive than remobilization. Fish-scale cycles may reflect the influence of solar forcing. [ABSTRACT FROM AUTHOR]

Presents a study which conducted glacial stratigraphy and geomorphology of the western Lake Superior region, which recorded a complex sequence of late Wisconsinan ice advances and retreats of Laurentide Ice Sheet from accumulation centers in Quebec and Hudson Bay. What geomorphic evidence indicates; Results obtained with the established radiocarbon chronology; Support for the possible linkage between western Lake Superior region ice dynamics and Heinrich events.

Proposes that the hummocky terrain in south-central Alberta, Canada is the product of subglacial erosion, instead of the product of supraglacial letdown during ice disintegration. Forms of hummocks in the area; Significant materials overlying hummocky terrain in the region; Demonstration of the erosional nature of the Travers-McGregor hummocks.

The authors comment on a critic of their study which proposed area-loss feedback (ALF) model for the formation of high-elevation, low-relief landscapes. Topics covered include the model's viability as a primary explanation for the formation of the low-relief surfaces of the southeast Tibetan Plateau, their disagreement that there are four competing models at play on the physics and kinematics of fluvial landscape evolution, and arguments on the diagnostic criteria in landscape evolution.