Search

New geologic mapping and tephrochronologic assessment of strata in extensional basins surrounding Knoll Mountain (Nevada, USA) reveal a geologic history linked to tectonic development of the Yellowstone hotspot and Snake River Plain to the north, and to the Ruby--East Humboldt--Wood Hills metamorphic core complex to the south. Data from these areas are utilized to present a paleogeographic reconstruction of northeastern Nevada--southcentral Idaho depicting the architecture of extensional faulting and basin development during collapse of the Nevadaplano over the past 17 m.y. Knoll Mountain is a northeast-trending horst along the southern margin of the Snake River Plain and track of the Yellowstone hotspot. The horst is bounded on the east by the Thousand Springs fault system and basin, and on the west by the Knoll Mountain fault and basin, where streams currently drain north into the Snake River Plain. The Knoll and Thousand Springs basins form half-grabens that are filled with the ca. 16 Ma to ca. 8-5 Ma Humboldt Formation, which was deposited in alluvial, eolian, and lacustrine environments during slip along range-bounding faults and a series of late-stage synthetic intrabasin faults. Structural, chronologic, and sedimentologic assessment of the Humboldt Formation in the Knoll basin indicates that it records overall southward fluvial drainage with slip along the Knoll Mountain fault beginning ca. 16 Ma and continuing to at least 8 Ma, and that between 8 and ca. 5 Ma, a west-dipping intrabasin fault system had developed. Between ca. 8-5 Ma to ca. 3 Ma, several fundamental changes took place, beginning with the cessation of faulting followed by widespread erosion that in turn was followed by deposition of older alluvium. The reversal of drainage direction from south to north flowing in the Knoll basin also took place during this time period, but its age relative to the widespread erosion or older alluvium is unknown. An integration of our work with previous studies north of Knoll Mountain reveal that the Knoll Mountain and intrabasin faults terminate to the north in the vicinity of the Jurassic Contact pluton, and that this area forms an accommodation zone separating broadly coeval and colinear faults bounding the ca. 10-8 Ma north-trending Rogerson graben, the northern end of which merges with the Snake River Plain. Furthermore, an integration of our work with previous work south of Knoll Mountain reveals that the Knoll Mountain fault formed part of a >190-km long, west-dipping fault zone that included the Ruby--East Humboldt detachment. This fault zone, which we refer to as the Knoll-Ruby fault system, had an extensive hanging-wall basin, the Knoll- Ruby basin. The Knoll-Ruby fault system was a prominent structure facilitating collapse of the Nevadaplano in northeastern Nevada between ca. 16 and ca. 8-5 Ma, and its central part produced partial exhumation of high-grade, mid-crustal metamorphic rocks in the Ruby--East Humboldt--Wood Hills metamorphic core complex. By 8-5 Ma, during the waning stages of extension along the Knoll-Ruby fault system, a series of intrabasin faults developed at about the same time as the integration of streams to form the incipient eastern reaches of the Humboldt River system. Profound changes in tectonics and paleogeography took place between ca. 8-5 Ma and ca. 3 Ma, that included the extinction of the Knoll-Ruby and intrabasin basin fault systems followed by southward migration of significant tectonism away from the Snake River Plain, resulting in development of a set of modern normal faults responsible for uplift of the southern Snake Mountains, Ruby Mountains, East Humboldt Range, and Pequop Mountains. These new faults cut and dismembered the central and southern part of the Knoll-Ruby fault system and basin, effectively ending any fluvial connection between the northern and southern parts of the Knoll-Ruby basin. Since ca. 8-5 Ma to the present, the Knoll Mountain region has remained relatively tectonically quiescent, and continued subsidence in the Snake River Plain to the north induced capture of the drainage system in the Knoll basin and reversed the drainage direction from south to north flowing. Our new findings indicate that (1) the Knoll-Ruby fault system and associated intrabasin faults were active until ca. 8-5 Ma, which is younger than the 12-10 Ma age generally recognized for cessation of major extension elsewhere in the northern Nevada region; (2) although this fault system was responsible for partial exhumation of core-complex metamorphic rocks, it extended well beyond the confines of the core complex proper; and (3) slip along faults in the Knoll Mountain region occurred before, during, and after passage of the hotspot at the longitude of Knoll Mountain. With the exception of significant faulting postdating passage of the hotspot, the timing of faulting in the Knoll Mountain area is consistent in a general way with the space-time pattern of extension recognized elsewhere along the southern margin of the Snake River Plain. However, it is unknown if the rate of fault slip increased during passage of the hotspot as it did in other areas. [ABSTRACT FROM AUTHOR]

Arrival of Laramide uplift sediments to the Texas Gulf Coastal Plain and northwestern Gulf of Mexico during the early Paleogene is recorded in strata of the Wilcox Group as a significant increase in sediment accumulation and with the appearance of 65-52 Ma detrital zircons that correspond with the timing of late Laramide uplift. New U-Pb dating of detrital zircons by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) for samples obtained from the Lower Paleocene Tehuacana Member through the Lower Eocene Queen City Formation in east-central Texas identifies the Hooper Formation of the Wilcox Group as the oldest stratigraphic unit to contain 65-52 Ma ages. Late appearance of 65-52 Ma detrital zircons in the Hooper Formation is correlated with unroofed Laramide magmatic intrusions or nearly syndepositional volcaniclastic sources; whereas older detrital zircons are inferred to be derived primarily from sedimentary cover and basement rocks exposed during uplift of Laramide blocks. Potential source region and Gulf Coastal Plain detrital zircon data support a relatively similar paleodrainage area and sediment sources for east-central Texas Tehuacana Member to Carrizo Formation and central Louisiana Wilcox Group data, and for east-central Texas Queen City Formation and central Louisiana middle-upper Claiborne Group data. South Texas Wilcox Group data contrast with data from these samples and support a different paleodrainage area and sediment sources for the south Texas region. We propose that headwaters sourced from southeastern Wyoming to the southern Rocky Mountain region delivered sediments to east-central Texas and central Louisiana during the Paleocene to Middle Eocene. Pronounced Mesoproterozoic and Neoproterozoic detrital zircons in the lower Claiborne Group of east-central Texas and the middle-upper Claiborne Group of central Louisiana are attributed to new or unroofed recycled sediments with Grenvillian age detrital zircons incorporated from the Ouachita region and other proximal locations in the preexisting paleodrainage area. The inferred paleodrainage area for east-central Texas and central Louisiana includes most of the Rocky Mountain Laramide uplift blocks, has a southern boundary separating it from a south Texas paleodrainage, and an eastern boundary roughly coincident with the Mississippi embayment, which separates it from Appalachian Mountains drainages. [ABSTRACT FROM AUTHOR]

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]

A strike-slip fault is present outboard and subparallel to the Wassuk Range front within the central Walker Lane (Nevada, USA). Recessional shorelines of pluvial Lake Lahontan that reached its highstand ca. 15,475 ± 720 cal. yr B.P. are displaced ~14 m and yield a right-lateral slip-rate estimate approaching 1 mm/yr. The strike-slip fault trace projects southeastward toward the eastern margin of Walker Lake, which is ~15 km to the southeast. The trace is obscured in this region by recessional shorelines features that record the historical dessication of the lake caused by upstream water diversion and consumption. High-resolution seismic CHIRP (compressed high intensity radar pulse) profi les acquired in Walker Lake reveal ~20 k.y. of stratigraphy that is tilted westward ~20- 30 m to the Wassuk Range front, consistent with ~1.0-1.5 mm/yr (20-30 m/20 k.y.) of vertical displacement on the main rangebounding normal fault. Direct evidence of the northwest-trending right-lateral strikeslip fault is not observed, although a set of folds and faults trending N35°E, conjugate to the trend of the strike-slip fault observed to the north, is superimposed on the west-dipping strata. The pattern and trend of folding and faulting beneath the lake are not simply explained; they may record development of Riedel shears in a zone of northwest-directed strike slip. Regardless of their genesis, the faults and folds appear to have been inactive during the past ~10.5 k.y. These observations begin to reconcile what was a mismatch between geodetically predicted deformation rates and geological fault slip rate studies along the Wassuk Range front, and provide another example of strain partitioning between predominantly normal and strikeslip faults that occurs in regions of oblique extension such as the Walker Lane. [ABSTRACT FROM AUTHOR]

In many settings, rivers alternate between carving wide valley bottoms (straths) and cutting narrow gorges over time, thereby creating longitudinally continuous paired bedrock strath terraces along valleys. Strath terraces are used ubiquitously in geomorphology and tectonics; however, how and why they form remain poorly understood. Here, we focus on Arroyo Seco in the central California Coast Ranges, where we test hypotheses for strath planation and subsequent strath terrace formation. Several lines of evidence indicate that strath planation is triggered by braiding in bedrock channels. In particular, hydraulic modeling reveals that the width of Arroyo Seco's most recently formed terrace is comparable to the width of currently braided channel reaches. Additionally, a comparison of currently braided reaches to abandoned bedrock meander cutoffs shows that braided channels have valleys that are several times wider than single-thread meander ing bedrock channel reaches. Lastly, in locations where the modern channel is currently braided, terraces are poorly preserved, suggesting that evidence for past episodes of braiding, in the form of paired strath terraces, is apparently largely destroyed by subsequent episodes of braiding. Field observations combined with mapping of terrace levels using an objective light detection and ranging (LiDAR)-based terrace identification algorithm reveal that temporal variation in tectonic uplift rate, sea level, and/or alluvial cover along the river cannot explain strath planation and subsequent terrace formation in Arroyo Seco. Rather, our results provide evidence that aggra dation and degradation of alluvial sediments downstream of the Reliz Canyon fault result in impulsive base-level forcing of Arroyo Seco's bedrock channel. Strath abandonment and terrace formation apparently occur as incision into downstream alluvial sediments propagates upstream into bedrock. Braiding and planation of straths, in contrast, occur during intervals of low vertical incision rate associated with downstream aggradation or immediately following pulses of vertical lowering triggered by downstream incision of alluvial sediments. [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]

Field analyses of stratigraphy, structure, and tectonic geomorphology of Savu Island define the age and provenance of accreted Australian continental margin sequences and overlying synorogenic cover, and the structure, kinematics, and uplift history of the transition from subduction to collision in the eastern Sunda-Banda arc. The results highlight the dominant influence of lower plate composition and structure in shaping Savu Island and initiating intraforearc shortening. Provenance and biostratigraphic analyses of rocks accreted to the edge of the Sunda-Banda forearc indicate that they mostly consist of Triassic to Cretaceous synrift and postrift successions of the Australian continental margin. These rocks are similar in composition and provenance to Gondwana sequence units found throughout the Banda arc to the east, such as the Triassic Babulu, Jurassic Wai Luli, and Cretaceous Nakfunu Formations of Timor. Previously unrecognized units of pillow basalt are found interlayered with Jurassic beds and incorporated into mélange and mud diapirs. These basalt occurrences have major and trace element compositions similar to those of Indian Ocean mid-oceanic ridge basalt and are likely associated with Jurassic development of the Scott Plateau volcanic margin. South-directed thrusting of these units via a duplex thrust system detached the Middle Triassic section of the underthrust Scott Plateau. The Savu thrust system consists of a series of active north-directed thrust faults found onshore and offshore the north coast of Savu. Thrust faults mapped onshore, penetrated by the Savu #1 well and imaged in vintage seismic reflection profiles, offset the youngest deposits of Savu. The uplift history and deformation pattern associated with the Savu thrust is investigated at a variety of temporal scales. Foraminifera-rich synorogenic deposits indicate low average surface uplift rates until after 1.9 Ma ago, when pelagic chalk deposits were raised from depths of >2500 m to the surface in fewer than 1 Ma. Island emergence is well documented by uplifted coral terraces that encrust the highest ridges to 338 m elevation. U/Th analysis of uplifted coral yields ages of 122 ka near sea level, indicating slow uplift rates of 0.2 mm/a over the past 400 ka. Most synorogenic deposits are stripped from the south coast, exposing parts of the accretionary wedge. The deeply eroded nature of this part of the island, combined with its steep first-order stream gradients, indicates that it underwent rapid rock uplift and exhumation in the past 1-2 Ma. However, the south coast region is now subsiding, as evidenced by drowned streams and southtilted, submerging coral terraces. Streams draining north over the Savu thrust system show convex-upward patterns with gradients commonly associated with intermediate uplift rates. Flights of coral terraces also document growth of the island to the north above thrust-related folds. These results inform us that the transition from subduction to collision involves (1) strain partitioning away from the subduction zone into the upper plate, (2) forearc closure, (3) structure inherited from the lower plate, (4) initiation of a crustal suture zone, and (5) uplift and exhumation of the accretionary wedge. [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]

The poorly known, suspect, Uchee terrane occupies a critical tectonic position with regard to how and when peri-Gondwanan (Carolina) and Gondwanan (Suwannee) terranes were sutured to Laurentia. It lies sandwiched between Laurentian(?) continental basement exposed in the Pine Mountain window and adjacent buried Gondwanan crust of the Suwannee terrane. The Uchee terrane has been proposed as both a septum of Piedmont rocks that once was continuous across the erosionally breached Pine Mountain window or part of the Carolina zone. To help resolve this issue, we conducted U-Pb (SHRIMP-RG) (sensitive high-resolution ion microprobe-reverse geometry) zircon studies and whole-rock isotopic analyses of principal metasedimentary and metaplutonic units. U-Pb ages for zircons from the Phenix City Gneiss suggest igneous crystallization at ca. 620 Ma, inheritance ca. 1000 to ca. 1700 Ma, and a ca. 300 Ma (Alleghanian) overprint recorded by zircon rims. Zircons from the metasedimentary/metavolcaniclastic Moffits Mill Schist yield bimodal dates at ca. 620 and 640 Ma. The 620 to 640 Ma dates make these rocks age-equivalent to the oldest parts of the Carolina slate belt (Virgilina and Savannah River) and strongly suggest a Gondwanan (Pan-African and/or Trans-Brasiliano) origin for the Uchee terrane. Alternatively, the Uchee terrane may be correlative with metamorphic basement of the Suwannee terrane. The ca. 300 Ma overgrowths on zircons are compatible with previously reported 295 to 288 Ma 40Ar/39Ar hornblende dates on Uchee terrane rocks, which were interpreted to indicate deep tectonic burial of the Uchee terrane contemporaneous with the Alleghanian orogeny recorded in the foreland. Temperaturetime paths for the Uchee terrane are similar to that of the Pine Mountain terrane, indicating a minimum age of ca. 295 Ma for docking. In terms of tectono-metamorphic history of the Uchee terrane, it is important to note that no evidence for intermediate "Appalachian" dates (e.g., Acadian or Taconian) has been reported. This younger history, together with the ages of metaigneous rocks and evidence for pre-Grenville basement, suggests the Uchee terrane is likely of Gondwanan origin and may be related to Carolina zone terranes that accreted during the Alleghanian orogeny. [ABSTRACT FROM AUTHOR]

Geophysical and drill-hole data within the Reelfoot rift of Arkansas, Tennessee, Missouri, and Kentucky, USA, were integrated to create a structure contour map and threedimensional computer model of the top of the Precambrian crystalline basement. The basement map and model clearly define the northeast- trending Cambrian Reelfoot rift, which is crosscut by southeast-trending basement faults. The Reelfoot rift consists of two major basins, separated by an intrarift uplift, that are further subdivided into eight subbasins bound by northeast- and southeast-striking rift faults. The rift is bound to the south by the White River fault zone and to the north by the Reelfoot normal fault. The modern Reelfoot thrust fault, responsible for most of the New Madrid seismic zone earthquakes, is interpreted as an inverted basement normal fault. Geologic interpretation of 5077 shallow borings in the central Mississippi River valley enabled the construction of a structure contour map of the Pliocene-Pleistocene unconformity (top of the Eocene-base of Mississippi River alluvium) that overlies most of the Reelfoot rift. This map reveals both river erosion and tectonic deformation. Deformation of the Pliocene-Pleistocene unconformity appears to be controlled by the northeast- and southeast-trending basement faults. The northeast-trending rift faults have undergone and continue to undergo Quaternary dextral transpression. This has resulted in displacement of two major rift blocks and formation of the Lake County uplift, Joiner ridge, and the southern half of Crowley's Ridge as compressional stepover zones that appear to have originated above basement fault intersections. The Lake County uplift has been tectonically active over the past ~2400 yr and corresponds with a major segment of the New Madrid seismic zone. The aseismic Joiner ridge and the southern portion of Crowley's Ridge may reflect earlier uplift, thus indicating Quaternary strain migration within the Reelfoot rift. [ABSTRACT FROM AUTHOR]

Determination of fracture orientation can be an important aspect of structural analysis in reservoir characterization. The availability of ground-based laser scanner systems opens up new possibilities for the determination of fracture surface orientation in rock outcrops. Scanners are available in low-sample-density, low-accuracy, and fast, high-sample-density, high-accuracy models. These automatic laser scanner systems produce enormous volumes or "clouds" of point data at an instrument dependent accuracy and resolution, which can be at the millimeter level. This huge volume of data calls for an automated and objective method of analysis. We have developed a surface classification algorithm based on a multipass partitioning of the point cloud. The method makes use of both spatial proximity and the orientation of an initial coarse-grained model of the point cloud. Unsupervised classification of surface sets is demonstrated herein using the new algorithm. Both previously mentioned types of scanners have been used to map the Jackfork sandstone outcrop at Big Rock Quarry in Little Rock, Arkansas. We apply the surface classification algorithm to these data to extract fracture surface orientations from the point cloud. The effectiveness of these new technologies when applied to fracture analysis is clearly demonstrated in this example. It is also shown that the low-density, low-resolution type of scanner is adequate to define general geomorphology but is inadequate for fracture definition. The surface classification algorithm can be used to reliably extract fracture and bedding strike and dip angles from the three-dimensional point locations acquired using centimeter-accurate, high density laser scanner systems. [ABSTRACT FROM AUTHOR]

Northeastern Nevada is generally interpreted as an area of large-magnitude Eocene extension possibly due to gravitational collapse of crust thickened during the Sevier orogeny. The extensional interpretation is based in part on the presence of widespread Eocene conglomerates and lacustrine basins, as well as on thermochronology-based evidence of major Eocene cooling and uplift of the Ruby Mountains-East Humboldt Range core complex. The distribution of 45-40 Ma ash-flow tuffs and interbedded coarse conglomerates and lacustrine deposits, however, indicates they were predominantly deposited in a system of east-draining paleovalleys incised into a plateau or moderate-relief upland. A large, contiguous sedimentary basin probably was never present. Paleovalleys were as much as 10 km wide and 500 m to possibly as much as 1.6 km deep, based on the thickness of intravalley deposits. Ash-flow tuffs are widely distributed near source calderas but are almost entirely confined to the paleovalleys as little as 20 km from their source. Basal, mostly pre-volcanic conglomerates contain clasts up to 6 m in diameter. The clasts are well rounded, indicating significant fluvial transport, not derivation from nearby fault scarps. Lacustrine deposits also are restricted to paleovalleys and accumulated during two periods that are interpreted to coincide with episodes of minor, northwest-directed extension, one before 41 Ma and possibly as old as 46 Ma, and another between 40 and 38 Ma. Extension formed small displacement, northeast-striking, mostly down-to-thenorthwest faults that temporarily dammed the paleovalleys to form lakes. Lakes probably also formed where volcanic rocks or landslides dammed paleovalleys, a common process both in the Eocene and historically in the western United States. The absence of major Eocene extension suggests that gravitational collapse of overthickened crust, even assisted by thermal weakening of lithosphere by intense magmatism, was not sufficient to generate major extension. Absolute elevation of the high plateau is uncertain, but it was high enough to have paleovalleys as much as 1.6 km deep. Based on published paleoflora data, interfluves could have been at elevations of ~4 km. The Eocene paleovalleys in northeastern Nevada most likely drained eastward to remnants of the Uinta basin. An approximately north-south paleodivide through northeastern Nevada separated these east-draining paleovalleys from paleovalleys that drained westward to the Pacific Ocean. [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]

The Teide and Pico Viejo stratocones and the Northwest and Northeast Rifts are products of the latest eruptive phase of the island of Tenerife, initiated with the lateral collapse of its northern flank that formed the Las Cañadas Caldera and the Icod--La Guancha Valley ca. 200 ka. The eruptive and structural evolution of this volcanic complex has been reconstructed after detailed geological mapping and radioisotopic dating of the significant eruptive events. A set of 54 new 14C and K/Ar ages provides precise age control of the recent eruptive history of Tenerife, particularly Teide Volcano, the third-highest volcanic feature on Earth (3718 m above sea level, >7 km high), and unique in terms of its intraplate setting. The development of the Teide--Pico Viejo Volcanoes may be related to the activity of the Northwest and Northeast Rifts. Volcanic and intrusive activity along both rift zones may have played an important role in activating the gravitational landslide and in the subsequent growth, nested within the collapse embayment, of an increasingly higher central volcano with progressively differentiated magmas. The coeval growth of the central volcano with sustained activity along the rifts led to a clear bimodal distribution in composition of eruptive products, with the basaltic eruptions in the distal part of the rifts and phonolitic and more explosive eruptions in the central area, where the differentiated stratocones developed. Current volcanic hazard in Tenerife is considered to be moderate, because eruptive frequency is low, explosivity is modest, and the eruptive activity of the Teide stratocone seems to have declined over the past 30 k.y., with only one eruption in this period (1150 yr B.P.). [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]

Rock uplift and erosional denudation of orogenic belts have long been the most important geologic processes that serve to shape continental surfaces, but the rate of geomorphic change resulting from these natural phenomena has now been outstripped by human activities associated with agriculture, construction, and mining. Although humans are now the most important geomorphic agent on the planet's surface, natural and anthropogenic processes serve to modify quite different parts of Earth's landscape. In order to better understand the impact of humans on continental erosion, we have examined both long-term and short-term data on rates of sediment transfer in response to glacio-fluvial and anthropogenic processes. Phanerozoic rates of subaerial denudation inferred from preserved volumes of sedimentary rock require a mean continental erosion rate on the order of 16 m per million years (m/m.y.), resulting in the accumulation of ∼5 gigatons of sediment per year (Gt/yr). Erosion irregularly increased over the ∼542 m.y. span of Phanerozoic time to a Pliocene value of 53 m/m.y. (16 Gt/yr). Current estimates of large river sediment loads are similar to this late Neogene value, and require net denudation of ice-free land surfaces at a rate of ∼62 m/m.y. (∼21 Gt/yr). Consideration of the variation in large river sediment loads and the geomorphology of respective river basin catchments suggests that natural erosion is primarily confined to drainage headwaters; ∼83% of the global river sediment flux is derived from the highest 10% of Earth's surface. Subaerial erosion as a result of human activity, primarily through agricultural practices, has resulted in a sharp increase in net rates of continental denudation; although less well constrained than estimates based on surviving rock volumes or current river loads, available data suggest that present farmland denudation is proceeding at a rate of ∼600 m/ m.y. (∼75 Gt/yr), and is largely confined to the lower elevations of Earth's land surface, primarily along passive continental margins; ∼83% of cropland erosion occurs over the lower 65% of Earth∼s surface. The conspicuous disparity between natural sediment fluxes suggested by data on rock volumes and river loads (∼21 Gt/yr) and anthropogenic fluxes inferred from measured and modeled cropland soil losses (75 Gt/yr) is readily resolved by data on thicknesses and ages of alluvial sediment that has been deposited immediately downslope from eroding croplands over the history of human agriculture. Accumulation of postsettlement alluvium on higher-order tributary channels and floodplains (mean rate ∼12,600 m/m.y.) is the most important geomorphic process in terms of the erosion and deposition of sediment that is currently shaping the landscape of Earth. It far exceeds even the impact of Pleistocene continental glaciers or the current impact of alpine erosion by glacial and/ or fluvial processes. Conversely, available data suggest that since 1961, global cropland area has increased by ∼11%, while the global population has approximately doubled. The net effect of both changes is that per capita cropland area has decreased by ∼44% over this same time interval; ∼1% per year. This is ∼25 times the rate of soil area loss anticipated from human denudation of cropland surfaces. In a context of per capita food production, soil loss through cropland erosion is largely insignificant when compared to the impact of population growth. [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]

Southwestern Japan during Paleogene time was affected by subduction of the Kula and Pacific plates beneath the eastern margin of Eurasia, followed by a brief episode of transform faulting to allow the Shikoku Basin to open at 27-15 Ma. Subsequent opening of the Japan Sea back-arc basin in middle Miocene time broke Japan away from Eurasia, and SW Japan rotated clockwise ∼45° as it drifted south (17-15 Ma). Southward drift required subduction of the Philippine Sea plate beneath SW Japan, leading to the formation of a magmatic arc. Subduction of the hot Shikoku Basin lithosphere and spreading ridge resulted in distinctive volcanism in the SW Japan forearc between 17 and 12 Ma. This volcanism included midoceanic-ridge basalt (MORB)-like intrusions and oceanic-island basalt (OIB)-type alkali basalts, felsic plutons in the accretionary prism, and high-magnesium andesites along the Setouchi forearc basin. Mafic magmas in the outermost forearc originated from the subducted Shikoku Basin spreading ridge, whereas Setouchi high-magnesium andesites (HMA) may have resulted from the interaction of melts of the subducted Philippine Sea plate with the overlying mantle. Felsic magmas in the forearc resulted from melting of Shimanto Belt sediments caused by intrusion of HMA magmas. Persistent rear-arc volcanism was caused by upwelling astheno sphere associated with opening of the Sea of Japan. Rear-arc volcanism began ca. 25 Ma as rift-fill low-alkali tholeiite volcanism and was replaced gradually after 12 Ma by alkali basalt volcanism. Upwelling-related alkali volcanism continues up to the present, whereas forearc volcanism ceased at 12 Ma. The volcanic arc narrowed with time as the Philippine Sea slab descended and slowly cooled. Adakitic dacites erupted after 1.7 Ma above the 100-km-depth contour of the subducted Philippine Sea plate, suggesting that melting resulted from interaction of the slab with upwelling asthenosphere. Interactions between upwelled rear-arc asthenosphere and subduction of the hot Philippine Sea slab appear to have been the main controls of magmatism for the Neogene SW Japan arc. [ABSTRACT FROM AUTHOR]

Megathrust earthquakes in western North America may be preceded by a precursor phase several years prior to megathrust, induced earthquakes. For example, on 27 March 1964, a 9.2 magnitude (on the Richter scale) earthquake occurred on the coast of Alaska. Changes in foraminifera and diatom assemblages at Girdwood Flats, Alaska, provide evidence of a precursor to this earthquake, thereby detailing a previously unknown sequence of events. We describe further evidence of precursor phases from marshes in Turnagain Arm, Alaska, United States, and farther south in Netarts Bay, Oregon, United States; this is the first time that two widely spaced locations have been examined for earthquake-related precursor stages. The Alaska earthquake offers the possibility to compare a modern sequence (A.D. 1964) of events with the geologic record. The Netarts Bay marsh has experienced no modern earthquake that could be used for comparison, but the nature of megathrust zones implies that the modern and ancient events should be physically similar. New cores examined from Turnagain Arm include both the 1964 earthquake and an event identified and dated at 1800 yr B.P. Prior to the 1964 event, the foraminifera and thecamoebian assemblages changed from a forest phase to a mildly brackish stage; this sequence was dated as being 15 yr or less in length, using Pb210 and Cs137 dating techniques. The event at 1800 yr B.P. was also associated with a similar precursor stage, indicating a small subsidence prior to the megathrust earthquake-related subsidence event. In Netarts Bay a new core was taken at a previously cored site to make use of carbon-14 dates and the assurance that at least four events were known to have occurred over the past 3000 yr. The new core contains four transitions, each identified by a mineralic deposit, often sharply bounded below and gradationally overlain by marsh peat. Transition in this context refers to the section of core analyzed, ∼15 cm above and below each of the four sand layers. Foraminiferal assemblage analyses indicate that these units represented a high marsh prequake phase followed by a lower marsh precursor stage, the earthquake-related deposition (sand layer), and then a rebound back into postquake marsh deposits. Sand deposits with either no or few foraminifera are inferred as tsunami/ earthquake-related deposition stages. These transitions in two Widely separated geographic areas (Alaska and Oregon) indicate that similar mechanisms operate for large megathrust earthquakes at subduction zones from Alaska to northern California in the Cascadia Subduction Zone, thus implying that precursor events also occur and can be detected by foraminiferal zonation all along this area. In a recent article (Dragert et al., 2001), scientists from the west coast suggested that "slow" or "silent" earthquakes they measured with continuous geographical positioning systems might be indicators of megathrust earthquakes. The transitions we document may be the prehistoric representations of these "silent" quakes. Foraminiferal evidence may help by allowing, more accurate positioning of seismometers along the west coast of North America and thereby lead to more precise and timely earthquake prediction methods. [ABSTRACT FROM AUTHOR]

This study reports U-Pb ages for zircons from the Los Ranchos Formation, which is part of the primitive island arc sequence, the oldest volcanic rocks in the Greater Antilles. Zircons were analyzed from three samples: quartz porphyry (quartz keratophyre) from the lower part of the formation (Quita Suello Member), fragmental quartz porphyry from the upper part of the formation (Pueblo Viejo Member), and the Cotui quartz diorite stock that intrudes the lower part of the formation. The lower part of the Los Ranchos Formation, represented by the Quita Sueño sample, formed at either 113.9 ± 0.8 or 118.6 ± 0.5 Ma depending on interpretation of the data, whereas the upper part, represented by the Pueblo Viejo sample, formed ca. 110.9 ± 0.8 Ma. The Cotui stock (RD-73-601) was emplaced ca. 111.8±0.6 Ma or 112.9± 0.9 Ma depending on whether eight grains reflect subtle inheritance. These results show that the Los Ranchos Formation was emplaced during the Aptian-Albian transition, that the Cotui stock could have supplied magma to the volcanic sequence and fluids to the Pueblo Viejo gold-silver deposit in the upper part of the formation, and that carbonaceous sediments in the upper part of the formation formed at the same time as ocean anoxic event lb (Paquier). Caribbean plate tectonic models involving invasion of anomalous Pacific crust into the Caribbean region appear to agree best with this age because they provide a mechanism for the change from primitive island arc (PIA) to calc-alkaline magmatism and account for the restricted ocean circulation necessary to generate regional anoxic conditions. [ABSTRACT FROM AUTHOR]

The Golden Bear dike in the Sierra Nevada and the Coso dikes in the Coso Range crop out on opposite sides of Owens Valley, California, and strike roughly perpendicular to it. Neither dike reappears along strike across the valley. New data demonstrate that the dike sets are ca. 83 Ma in age, share nearly identical mineralogy and petrography, and intrude similar wall rocks including distinctive 102 Ma leucogranite. Dike bulk-chemical and Sr and Nd isotope compositions are nearly indistinguishable. These data suggest that the dike sets were originally continuous and were offset dextrally by ∼65 km. This displacement estimate is consistent with other recent estimates of total slip across Owens Valley. If faulting began during the Pliocene, the average slip rate was significantly faster than the current rate. Alternatively, motion could have been episodic and have begun as early as the Late Cretaceous. [ABSTRACT FROM AUTHOR]

The NW Zhejiang region of South China occupies a key tectonic position near the suture zone of the Yangtze and Cathaysian blocks and is of critical importance for the assembly of East Asia. Sedimentological and tectonic analyses indicate that the region had a SE-dipping paleoslope in the late Paleozoic to Early Triassic. A transitional sedimentary environment from deep sea to continental molasse is documented in the early Triassic-late Triassic interval. Associated structures are NW-vergent folds and thrusts that root southeastward beneath the high-grade Chencai metamorphic complex. The structural styles of this foreland fold-and-thrust belt are characterized by multifold duplexes and individual folds that are together zoned from SE to NW as follows: (1) core zone characterized by shear folds and ductile thrusts; (2) SE belt with out-of-sequence thrusting of multifold duplexes and an average shortening of 50%; (3) central belt with duplexes, imbricate fans, and an average shortening of 40%; and (4) NW belt with Jura-type folds and a shortening of ∼10%. A tectonic model for the foreland fold-and-thrust belt is discussed in relation to the early Mesozoic archipelago paleogeography of South China. [ABSTRACT FROM AUTHOR]