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Ultrahigh-temperature (UHT) granulites of the central Highland Complex, Sri Lanka, underwent some of the highest known peak temperatures of crustal metamorphism. Zircon and monazite U-Pb systems in granulites near Kandy, the highest grade region (∼1050 °C; 0.9 GPa), preserve both a record of the timing of prograde and retrograde phases of UHT metamorphism and evidence for the ages of older protolith components. Zircon grains from a quartz-saturated granulite containing relics of the peak UHT assemblage have remnant detrital cores with dates of ca. 2.5-0.83 Ga. Date clusters of ca. 1.7 and 1.04-0.83 Ga record episodes of zircon growth in the source region of the protolith sediment. Two generations of over-growths with contrasting Th/U record metamorphic zircon growth at 569 ± 5 and 551 ± 7 Ma, probably in the absence and presence of monazite, respectively. The age of coexisting metamorphic monazite (547 ± 7 Ma) is indistinguishable from that of the younger, low-Th/U zircon overgrowths. Zircon from a quartz-undersaturated monazite-absent UHT granulite with a mainly retrograde assemblage is mostly metamorphic (551 ± 5 Ma). The ca. 570 Ma zircon overgrowths in the quartz-saturated granulite probably record partial melting just before or at the metamorphic peak. The ca. 550 Ma zircon in both rocks, and the ca. 550 Ma monazite in the quartz-saturated sample, record post-peak isothermal decompression. A possible model for this pressure-temperature-time evolution is ultrahot collisional orogeny during the assembly of Gondwana, locally superheated by basaltic underplating, followed by fast extensional exhumation. [ABSTRACT FROM AUTHOR]

A high-resolution grayscale series of the pelagic Fucoid Marls (Piobbico core, central Italy) shows strong, pervasive lithological rhythms throughout the Aptian interval. A hierarchy of centimeter- to meter-scale cycles characterizes the rhythms; when calibrating ∼1 m cycles to Earth's 405 k.y. orbital eccentricity cycle, these rhythms correspond to the periods of the eccentricity, obliquity, and precession index. Tuning to orbital eccentricity cycles provides a high-resolution time scale for the Aptian. Correlation to the Cismon core (northern Italy) extends the tuning to the Aptian-Barremian boundary. The tuning indicates a minimum duration of 13.42 m.y. for the Aptian Stage, where previous estimates range from 6.4 to 13.8 m.y. The combined Aptian-Albian astronomical tuning of the entire 77-m-long Piobbico core (and part of the Cismon core) provides a 25.85-m.y.-long astronomically calibrated time scale for Earth history. [ABSTRACT FROM AUTHOR]

Constraining deep-ocean circulation during past greenhouse climatic periods, such as the Cretaceous, is important for understanding meridional heat transfer processes, controls on ocean anoxia, and the relative roles of climate and tectonics in determining paleocirculation patterns. Ocean circulation models for the Late Cretaceous and early Paleogene suggest that significant deep-water production occurred in the Southern Ocean, but cannot constrain when this process commenced or what the temporal relationship was between opening tectonic gateways and Late Cretaceous climatic cooling. Nd-isotope data obtained from biogenic apatite(fish teeth and bones) are presented from lower bathyal and abyssal sites in the South Atlantic and Indian Oceans. During the mid-Cretaceous, relatively radiogenic Nd-isotope values suggest that deep-water circulation in these basins was sluggish with inputs likely dominated by seawater-particle exchange processes and, possibly, easily weathered volcanic terranes. In the Campanian-Maastrichtian the Nd-isotopic composition of proto-Indian and South Atlantic deep waters became less radiogenic, suggesting the onset of deep-water formation in the Southern Ocean (Southern Component Water, SCW), consistent with Paleogene reconstructions and ocean circulation models. A combination of Southern Hemisphere cooling and the opening of tectonic gateways during the Campanian likely drove the onset of SCW. [ABSTRACT FROM AUTHOR]

We develop a forward model of the generation of faulted seafloor topography (visualization) to estimate the relative roles of tectonic and magmatic extension in the presence of core complexes at slow-spreading ridges. The visualization assumes flexural rotation of 60° normal faults, a constant effective elastic thickness, Te, of young lithosphere, and a continuous infill of the depressed hanging wall by lava flowing from the spreading axis. We obtain a new estimate of Te = 0.5-1 km from the shapes of the toes of 6 well-documented oceanic core complexes. We model an 80-km-long bathymetric profile in the equatorial Atlantic across a core complex and the ridge axis at 13°20'N and estimate the variation in tectonic extension, which yields the variation in the fraction of upper crust extension, M, by magmatic diking at the ridge axis. Core complex formation appears to be stable for all values of M < 0.5. The visualization shows how gabbro emplaced at the base of the lithosphere during extension by magmatic diking is partitioned to each side of the spreading axis, and predicts a high probability of finding gabbros in the domes of core complexes. [ABSTRACT FROM AUTHOR]

The distribution and character of Venus's impact craters led to the widely accepted idea that Venus underwent global catastrophic resurfacing ca. 500 Ma, and thus Venus records only a short history, encompassing surface evolution since postulated catastrophic resurfacing. Ribbon tessera terrain (RTT), a structurally distinctive unit, represents some of Venus's oldest surfaces, and is widely accepted as forming prior to postulated global catastrophic resurfacing. We constructed a global geologic map of RTT unit exposures and structural trends using National Aeronautics and Space Administration (NASA) Magellan data. Map relations illustrate that RTT displays planet-scale patterns that, together with altimetry, record a rich geologic history that predates proposed global catastrophic resurfacing. [ABSTRACT FROM AUTHOR]

The Tanzawa plutonic complex (TPC), central Japan, is a suite of tonalitic-gabbroic plutons exposed in a globally unique arc-arc collision zone, where an active intraoceanic Izu-Bonin-Mariana (IBM) arc is colliding against the Honshu arc. The TPC has been widely accepted as an exposed middle crust section of the IBM arc, chiefly because of geochemical similarities between the TPC and IBM rocks and previously reported precollisional Miocene K-Ar ages. However, new zircon U-Pb ages show that the main pulse of TPC magmatism was syncollisional and that plutons were emplaced rapidly and cooled soon after Pliocene collision. Trace element compositions of TPC zircon show distinctively elevated Th/Nb ratios compared to zircon from other noncollisional IBM silicic plutonic rocks, indicating the involvement of continental sediments from the Honshu arc in their magma genesis. [ABSTRACT FROM AUTHOR]

While the superficial expression of oceanic ridges is generally symmetric, their deeper roots may be asymmetric. Based on a surface wave tomographic three-dimensional model of the Earth's upper 300 km, we construct a global cross section parallel to the equator of the net rotation of the lithosphere, the so-called tectonic equator. Shear wave velocities indicate a difference between the western and eastern flanks of the three major oceanic rift basins (Pacific, Atlantic, and Indian ridges). In general, the western limbs have a faster velocity and thicker lithosphere relative to the eastern or northeastern one, whereas the upper asthenosphere is faster in the eastern limb than in the western limb. We interpret the difference between the two flanks as the combination of mantle depletion along the oceanic rifts and of the westward migration of the ridges and the lithosphere relative to the mantle. The low-velocity layer in the upper asthenosphere at the depth of 120-200 km is assumed to represent the decoupling between the lithosphere and the underlying mantle. It is also well defined by the distribution of radial anisotropy that reaches minimum values close to the rifts, but with an eastward offset. These results could be explained in the frame of the westward drift of the lithosphere relative to the underlying mantle. [ABSTRACT FROM AUTHOR]

Analysis of more than 700 modern continental sedimentary basins that are both endorheic (internally drained) and exorheic (externally drained) and cover a wide range of climatic and tectonic settings shows that sedimentation is dominated by distributive fluvial systems (DFSs). Facies distributions on DFSs are different from those of rivers in degradational settings, yet rivers in non-aggradational settings are commonly used to develop fluvial facies models. DFS rivers typically decrease in size downstream, are not confined to valleys, and form a radial pattern from an apex. Confined rivers are present in specific locations in sedimentary basins, including basin axial positions, areas between adjacent DFSs, and valleys incised into the DFS. DFSs and adjacent axial fluvial systems develop in a predictable manner that allows interpretation and prediction of fluvial architecture at the basin scale. [ABSTRACT FROM AUTHOR]

Drowned carbonate reefs on passive margins are puzzling because of their enormous growth potential compared to typical rates of passive margin subsidence and moderate sea-level fluctuations. A possible solution to this paradox is that slow processes acting over geologic time weaken reefs and contribute to their ultimate demise. The Australian northeastern marginal plateaus, known for their drowned reefs, underwent a period of accelerated tectonic subsidence during the late Miocene to Pliocene that, combined with a sequence of second-order global sea-level rises, outpaced reef growth and drowned the once-thriving Miocene carbonate platforms. However, the mechanism for the observed anomalous subsidence of this relatively mature passive margin 1000 km from the nearest plate boundary is uncertain. We use a coupled plate, kinematic mantle flow model to show that in the late Miocene northeastern Australia overrode subducted slabs from Eocene Melanesian subduction north of Papua New Guinea. We find that the rate of surface subsidence induced by the sinking slabs increases the likelihood that relative sea-level rises outpaced late Miocene reef growth. In addition to the well-known effects of long-term plate processes and short-term global sea-level and climate change, our results demonstrate that deep Earth processes can play a substantial role in driving the evolution of passive margins and coral reefs. [ABSTRACT FROM AUTHOR]

Detrital cooling ages from the pro-foreland and retro-foreland basins of the European Alps record distinctive exhumation trends that correlate with orogenic wedge states inferred from thrust front propagation rates. Periods of rapid hinterland exhumation correlate with relatively slow propagation of deformation toward the foreland and are interpreted to represent subcritical wedge conditions, whereas periods of slow hinterland exhumation correlate with rapid propagation of deformation toward the foreland and indicate supercritical wedge conditions. Similar lag time trends recorded in both the pro-foreland and retro-foreland thus mimic orogenic wedge behavior and suggest that local tectonics and/or climate events do not overprint the regional signal. [ABSTRACT FROM AUTHOR]

The kinematic vorticity number is an important quantity in structural geology and tectonics, giving a nonlinear ratio of simple shear to pure shear deformation. We use natural observations and numerical models to show how rigid clast methods for determining the kinematic vorticity number (Wk) are compromised where strain localization occurs at the matrix-clast interface. Our numerical results show that the critical shape factor cutoff between permanently rotating and stable clasts, used to determine Wk, is highly sensitive to coupling between the clast and the matrix. This fi nding provides an elegant explanation for the fact that rigid clast methods tend to underestimate Wk relative to other methods. We present numerically determined envelopes for clast behavior across a range of kinematic vorticity numbers, clast shape factors, and matrix-clast coupling. Our numerical models show that the shape-preferred orientations of feldspar clasts trend toward the positions of mica fi sh with increasing localization at the clast boundary, suggesting that mica fi sh behave as highly lubricated clasts. Our data and numerical results show that the clast-matrix interface may be several orders of magnitude weaker than the surrounding matrix and that weak interfaces can lead to a marked drop in the bulk shear strength of faults and shear zones. [ABSTRACT FROM AUTHOR]

We report new geochemical data for marine sediments sampled in the frontal prism associated with the Costa Rica subduction zone during Leg 205 of the Ocean Drilling Program (ODP). We describe variation in sediment geochemistry with depth as the décollement zone, the interface between overriding and downgoing tectonic plates, is approached. This variation can be explained by three-component mixing of ash, lower plate sediments (LPS), and frontal prism or upper plate sediments (UPS). We detect in-mixing of LPS in localized sediment intervals, amounting to tens of vertical meters of LPS incorporation; no persuasive evidence of LPS transfer into the prism has been shown until this contribution. This inference of fi ne structure in the prism provides new insight into how tectonic kneading of sediments occurs in décollement zones. [ABSTRACT FROM AUTHOR]

The relative intensity of tectonic and climatic forcing in the western European Alps has been a matter of debate since the recognition of a signifi cant increase in denudation rates over the past few million years. We address this question by quantitatively correlating the spatial pattern of long-term exhumation rates with those of potential short-term tectonic, climatic, and morphologic variables. We find that present-day rock-uplift rates (as measured by geodesy relative to a specific reference point) and mean elevation are correlated with long-term exhumation rates, whereas relief, present-day precipitation, discharge, stream power, and released seismic energy are not, or are only weakly, correlated. We attribute the lack of correlation between long-term exhumation and precipitation to a strong temporal variability in climate and erosional processes during Pliocene-Pleistocene time. The correlations among present-day rock-uplift rates, present-day elevations, and long-term exhumation rates suggest that rockuplift rates have been sustained for millions of years, consistent with rock-uplift rates being the isostatic response to crustal unloading. The lack of a correlation of the released seismic energy with either rock uplift or long-term exhumation denies active tectonics supporting evidence. [ABSTRACT FROM AUTHOR]

We study the recent dynamics of the central Nepal Himalaya, focusing on possible reactivation of the footwall of the Main Central thrust, which is marked by an abrupt topographic transition. Different tectonic mechanisms, such as overthrusting of a major crustal ramp, underplating, or out-of-sequence thrusting, have been suggested to explain the morphology and exhumation patterns in this area. We present 25 new apatite fission-track ages collected along a north-south transect in central Nepal, as well as two age-elevation profiles. Ages are consistently younger than 3 Ma old in the Main Central thrust zone and increase continuously to 4-6 Ma old in the south. No jump in apatite fission-track ages is observed across the topographic transition. Apparent exhumation rates from age-elevation relationships vary from 0.46 +0.13/-0.09 km/Ma in the Palung granite south of Kathmandu to 4.4 +4.8/-1.5 km/Ma in the Main Central thrust zone; the latter rate is probably overestimated by a factor of two due to topographic effects. As shown by a new numerical model, these strongly varying exhumation rates can be explained by overthrusting of a crustal ramp, which exerts a primary control on age patterns, and do not require out-of-sequence reactivation of thrusts in the Main Central thrust zone. [ABSTRACT FROM AUTHOR]

Taiwan is the type example of an arc-continent collision. Numerous tectonic models have been proposed for this orogen, and include both thin-skinned and thick-skinned lithospheric deformation. These models predict very different structures at middle and lower crustal depths, but insufficient geophysical data exist to unequivocally distinguish between them. Long-period magnetotelluric (MT) data were collected in central Taiwan in 2006-2007 to constrain the crustal resistivity structure. A two-dimensional inversion of these MT data revealed a prominent electrical conductor that extends across the décollement predicted by the thin-skinned model. This feature is interpreted to be due to 1%-2% saline fluids, and is inconsistent with the thin-skinned model. In contrast, the thick-skinned model predicts this feature since fluids are generated in the crustal root through metamorphism. Quantitative correlation of the resistivity and seismic velocity models supports small-volume, high-salinity fluids in a thickened crust as the cause of this conductor. [ABSTRACT FROM AUTHOR]

This contribution shows unequivocally that porphyroblasts rotate relative to one another during ductile deformation. The porphyroblasts described here have special significance because they are from the original "millipede" rocks that led to the nonrotation hypothesis. Thus, the debate that has lasted for more than 20 years is settled. Despite this finding, porphyroblast microstructures continue to provide important evidence for deformation and metamorphic histories. Although porphyroblasts clearly rotate relative to one another during ductile deformation, there are several factors that contribute to relatively minor rotation in many instances, including (1) low strain during and after porphyroblast growth in comparison, for example, to mylonitic shear zones; (2) small axial ratios combined with relatively low internal vorticity during growth and post-growth deformation; and (3) strain localization at the porphyroblast-matrix interface. Thus, given the right circumstances, porphyroblasts may preserve the approximate orientations of deformation fabrics present at the time of their growth, but each case must be individually assessed. [ABSTRACT FROM AUTHOR]

The southern highlands of the Moon comprise superposed ejecta layers, individually as thick as a few kilometers, from the major basins. Smaller (1-16-km-diameter) impact craters that penetrate this layered megaregolith and excavate material from depth have radar properties that provide insight into the variability of megaregolith thickness above a postulated basement of large crustal blocks. We observe a signifi cant difference in the population of radar-bright craters, 1-16 km and larger in diameter, between regions of the southeastern nearside highlands north and south of ∼lat 48°S. There are about one-third more radar-bright craters north of this line than to the south, broadly coincident with the mapped boundary between southern deposits mapped as pre-Nectarian age and those of Nectarian-Imbrian age to the north. The radar-bright crater population is consistent with a megaregolith thickness of ∼1.5 km in the north and ∼2.5 km in the south, a difference we attribute to South Pole-Aitken basin ejecta. [ABSTRACT FROM AUTHOR]

We determine the paleoelevation of the northern Sierra Nevada (California) in the Oligocene based on hydrogen stable isotope compositions of meteoric water preserved within volcanic glass from ignimbrites sampled across the range. A 48‰ decrease in the isotopic composition of hydrated glass from ignimbrites located near paleosea level to ignimbrites 100 km to the east reflects the effect of ancient high topography on precipitation. These data show that 31-28 Ma ago, the northern Sierra Nevada had a steep western gradient and elevations similar to the present. This study, placed in the context of other paleoaltimetry studies, suggests that the range was a high topographic feature throughout the Cenozoic and that the majority of uplift occurred in the Late Cretaceous to early Cenozoic, much earlier than some studies have proposed. [ABSTRACT FROM AUTHOR]

Archean cratons are characterized by granite-greenstone belts, by abundant tonalites, trondhjemites, and granodiorites (TTGs), and by the presence of strong and buoyant subcratonic lithospheric mantle (SCLM). Individually, mechanisms for their formation remain controversial, and together they provide no clear picture of the tectonic style prevalent during the late Hadean and Early Archean. Following earlier observation that the dome-and-keel structure of granite-greenstone terranes resembles salt diapirs, we present numerical calculations that show that in pre-Archean to Middle Archean thermal regimes, mafic to ultramafic volcanics overlying a felsic basement will overturn diapirically in as few as 10 Ma, displacing as much as 60% of the volcanics to the lower crust. This suggests that diapirism may have dominated Hadean and Early Archean crustal tectonics. Furthermore, repeated cycles of bimodal volcanism and crustal recycling by diapirism provide a mechanism for making TTGs, leaving a restite that contributes to the SCLM from above. This diapiric tectonism later declined as the Earth cooled and plate tectonics became the dominant paradigm. [ABSTRACT FROM AUTHOR]

Accreted terranes of the Appalachian Iapetan and peri-Gondwanan realms display structural trends that are mainly concordant with promontories and embayments in the Laurentian margin, indicating that during accretion, the shape of the continental margin acted as a template around which accreted terranes were molded. In North Carolina and Newfoundland, post-accretion transcurrent motion appears to be recorded by displaced outboard portions of promontories, no longer concordant with those in Laurentia. A bend in structural trends confined to the peri-Gondwanan realm at the North Carolina-South Carolina state line is interpreted to represent the dextrally displaced outboard portion of the Virginia promontory. In Newfoundland, the Hermitage flexure is interpreted as a dextrally displaced Laurentian promontory that originated along strike to the northeast. In both places, promontories were truncated and dextrally translated for ~220-250 km by a Devonian-Mississippian orogen-parallel transcurrent system, which may well have extended for the length of the eastern Laurentian margin. South of the nearby St. Lawrence promontory, extreme thinning of Appalachian crust beneath the Maritimes Basin is consistent with extension at a releasing stepover. Estimates of extension are consistent with those obtained from promontory offsets. [ABSTRACT FROM AUTHOR]

Shallow subduction of the Farallon plate during the Laramide orogeny (ca. 80-40 Ma) may have resulted in metasomatism of the western North American lithospheric mantle. Olivineand orthopyroxene-hosted melt inclusions from the central Rio Grande Rift are variably enriched in chlorine relative to fluid-immobile elements. Subparallel trends in Cl/K versus Cl/ Nb for alkali basalts and tholeiites can be explained by Cl/K fractionation during low degree partial melting, with DCl ≈DNb < DK. The observed trace element enrichment does not correlate with host Mg# or melt SiO2 wt% as expected for crustal contamination via an assimilation- fractional crystallization (AFC) process. In addition, examples from other volcanic systems suggest that Cl/K decreases with increasing contamination, contrary to observed positive correlations between Cl/K and Ba/Nb and Sr/Nd. The positive correlation of Cl/K and Cl/Nb with typical indices of subduction enrichment (e.g., Ba/Nb and Sr/Nd) supports a model of mantle metasomatism during subduction. [ABSTRACT FROM AUTHOR]

Explosive volcanic eruptions may be triggered by processes in the conduit, including P-T variations, volatile loss, and crystal growth. These unobserved processes may be inferred via textural and chemical examination of eruptive products. Juvenile clasts were examined from vulcanian explosions of the Soufrière Hills volcano. Here we show that secondary ion mass spectrometry (SIMS) depth profiling into plagioclase phenocrysts provides a nearly continuous record of magma ascent from chamber to surface via high-resolution (∼10 nm) chemical profiles of decompression-induced crystal growth. Changes in lithium concentration allow quantification of crystal growth rates, which range from 2 × 10-8 mm/s to 7 × 10-8 mm/s. Anorthite variations suggest conduit temperature increases of 50 to 100 °C, assuming contemporaneous overpressures approaching 20 MPa. These data demonstrate a new means of constraining conduit conditions preceding explosive volcanic eruptions. [ABSTRACT FROM AUTHOR]

Resolving the timing, direction, and magnitude of paleoclimate changes in the southern midlatitudes is a prerequisite for determining the mechanisms underlying abrupt and widespread climate changes between the hemispheres during the Last Glacial-Interglacial transition (LGIT). This issue is still debated, with previous studies producing apparently discordant findings. Here we show evidence for a glacial readvance and a cold episode between ca. 14.8 and 12.6 ka in southwestern Patagonia (50°S), contemporaneous with the Antarctic cold reversal. This was followed by ice recession under cold but relatively milder conditions until ca. 11.5 ka, when paleovegetation records indicate the onset of warm interglacial conditions. These findings differ from those reported in northern Patagonia (∼40°S), where deteriorating conditions before 13.5 ka were followed by the coldest part of the LGIT that lasted until ca. 11.5 ka. We interpret the apparent blend of Greenlandic and Antarctic cold phases as evidence for their co-occurrence in the southern middle latitudes in Patagonia, and hypothesize that the position of the Antarctic Polar Front modulated the strength of these cold events in regions to the north or south of it. [ABSTRACT FROM AUTHOR]

The northern Sierra Nevada and adjacent Basin and Range, western United States, are marked by a widespread regional unconformity at the base of the Eocene-Miocene volcanic and sedimentary section that overlies the Mesozoic batholith and its wall rocks. To help address controversial questions about the origin, uplift, and erosion of the batholith, we compiled a subcrop geologic map of the unconformity prior to Tertiary extension. This simple but underutilized technique reveals the distribution of rock units that could have contributed detritus to the Eocene-Miocene river channels crossing the Sierra Nevada and demonstrates that the Mesozoic Sierra Nevada batholith was continuous to the northeast across the northwestern Basin and Range. More speculatively, the subcrop map implies that Late Cretaceous-Eocene erosional stripping may have been greatest above the axis of the batholith and decreased to the east; thus, drainage in this area may have been eastward and then switched westward in Eocene-Miocene time. [ABSTRACT FROM AUTHOR]

We employ a new thermodynamic method for self-consistent computation of compositional and thermal effects on phase transition depths, density, and seismic velocities. Using these profiles, we compare theoretical and observed differential traveltimes between P410s and P (T410) and between P600s and P410s (T660-410) that are affected only by seismic structure in the upper mantle. The anticorrelation between T410 and T660-410 suggests that variations in T410 and T660-410 of ∼8 s are due to lateral temperature variations in the upper mantle transition zone of ∼400 K. If the mantle is a mechanical mixture of basaltic and harzburgitic components, our traveltime data suggest that the mantle has an average temperature of 1600 ± 50 K, in agreement with temperature estimates from magma compositions of mid-ocean ridge basalts. We infer a 100 K hotter mantle if we assume the mantle to have a homogeneous pyrolitic composition. The transition-zone temperature beneath hotspots and within subduction zones is relatively high and low, respectively. However, the largest variability in T410 and T660-410 is recorded by global stations far from subduction zones and hotspots. This indicates that the 400 K variation in upper mantle temperature is complicated by tilted upwellings, slab flattening and accumulation, ancient subduction, and processes unrelated to present-day subduction and plume ascent. [ABSTRACT FROM AUTHOR]

Stratigraphic, provenance, and subsidence analyses suggest that by the Middle to Late Jurassic a foreland basin system was active in northwestern Montana (United States). U-Pb ages of detrital zircons and detrital modes of sandstones indicate provenance from accreted terranes and deformed miogeoclinal rocks to the west. Subsidence commenced ca. 170 Ma and followed a sigmoidal pattern characteristic of foreland basin systems. Thin Jurassic deposits of the Ellis Group and Morrison Formation accumulated in a backbulge depozone. A regional unconformity and/or paleosol zone separates the Morrison from Early Cretaceous foredeep deposits of the Kootenai Formation. The model presented here is consistent with regional deformation events registered in hinterland regions, and challenges previous interpretations of a strongly diachronous onset of Cordilleran foreland basin deposition from northwestern Montana to southern Canada. [ABSTRACT FROM AUTHOR]

Celestite nodules with subordinate amounts of chalcedony are common in Upper Cretaceous chalk in two cores drilled at Stevns, Denmark. The nodules occur throughout the cores, indicating that the environmental conditions that led to their formation were an integral part of the biogeochemical cycle in the chalk sea. Petrographic evidence indicates that the celestite nodules were formed by replacement of lithistid demosponges in localized reduced environments prior to sediment compaction and chert formation. The 87Sr/86Sr values of the celestite nodules indicate that the Sr is mainly sourced from seawater and the δ34S values indicate activity of sulfate-reducing bacteria. This unusual occurrence of celestite is interpreted as a result of rapid burial of the sponges and following decomposition of the organic tissue by sulfatereducing and sulfide-oxidizing bacteria, forming an anoxic microenvironment. The decomposition produced hydrogen sulfide, which was oxidized to sulfate by sulfide-oxidizing bacteria, resulting in decreasing pH. Strontium precipitated as celestite in dysoxic microenvironments and at the anoxic-dysoxic interface within the sponge. The crystallization of celestite apparently created an Sr gradient that drove Sr into the site of crystallization. This geochemical environment in dead demosponges represents an unknown celestite-forming process. [ABSTRACT FROM AUTHOR]

Skeletal Mg/Ca ratios of well-preserved fossil echinoderms have been used to reconstruct past Mg/Ca ratio in seawater up to the Phanerozoic, taking into account the known temperature effect on this ratio. This study investigates the effects of salinity and growth rate on Mg/Ca and Sr/Ca ratios in starfish calcite skeletons grown in experimental conditions. Both ratios are not related to growth rate: on the contrary, both are positively related to salinity. This effect induces an error on the reconstructed Mg/Ca ratio in seawater that may reach 46%. An intriguing inverse relation between skeletal Sr/Ca ratio and temperature was recorded. The salinity effects are presumably due to physiological regulation processes. [ABSTRACT FROM AUTHOR]

The movements of terminal Pleistocene and Holocene mammoths on Wrangel Island, off northeastern Siberia, were investigated using the isotopic composition of Sr in skeletal remains. A significant shift toward more radiogenic 87Sr/86Sr ratios, from 0.71218 ± 0.00103 to 0.71491 ± 0.00138 in bioapatite, marks the beginning of the Holocene. The higher Sr isotope values in mid-Holocene mammoth remains are interpreted as an influence of radiogenic Sr derived from the Neoproterozoic rocks in the central part of Wrangel Island. The shift implies a change in the ranging and feeding areas of the animals, ultimately reflecting the inundation of the mainland connection and isolation of the population. The shift indicates also that the late Pleistocene animals were not permanent occupants in the territory that makes up presentday Wrangel Island. [ABSTRACT FROM AUTHOR]

The Balcones Escarpment of south-central Texas is the eroded topographic expression of an en echelon fault zone where Cretaceous carbonates have been modified by karst processes influenced by structural and stratigraphic controls. While the modern confined Edwards Aquifer flows through cavernous voids at the base of the escarpment, air-filled caves perched in the escarpment are relicts of paleoaquifer hydrology. The structural geology of the Balcones Escarpment and the phylogeography of its endemic cave spiders provide mutually informative frameworks from which to establish relative dates for the activation of discrete groundwater recharge areas. The mitochondrial genetic variability of troglobitic spiders is correlated with the structural elements of the Balcones fault zone in and around the San Antonio relay ramp. Older (basal) genetic lineages occur in structurally high, mature karst terrains, while the younger (derived) lineages occur in structurally low, emergent karst terrains. Based on mitochondrial deoxyribonucleic acid (mtDNA) data, Cicurina diversity is interpreted as the product of the progressive availability of vadose zone habitat as discrete recharge areas have developed. [ABSTRACT FROM AUTHOR]

Unique wind ripples attaining heights to 2.3 m, wavelengths to 43 m, and a crest maximum grain size of 19 mm occur on the Argentine Puna Plateau at ∼4000 m altitude. These are the largest ripples reported on Earth, comparable only to Mars counterparts. They form in the presence of high proportions of low-density pumice clasts (0.91 g/cm³), although crests are exclusively composed of varnished, normal-density clasts (2.43 g/cm³). Mature ripple profiles are partly excavated on bedrock, so they form by a combination of defiation, winnowing of finer grains, minor wind drift of fine gravel, and lagging of clasts >4 cm. The large ripple size appears to be related to strong winds, dense saltation layers, and a long time for evolution. Ripple sizes are smaller on obstacles, as compared to fiat terrain; there is a lack of correlation between clast size, wavelength, and the extreme ripple size (in spite of the thin atmosphere), all of which suggest that while small-scale gravel ripples may form according to a reptation model, their evolution into large-scale types may relate to aero dynamic instabilities originating at the saltation curtain-air interface. [ABSTRACT FROM AUTHOR]

An Ordovician metamorphic event is recorded by means of conodont paleothermometry in the slope sedimentary sequences of the Western Precordillera. Allochthonous conodonts from reworked deposits of the eastern carbonate platform with CAI 4.5-5 and autochthonous conodonts from the olistostrome matrix with CAI 3 allow for constraining the metamorphic age within the Paltodus deltifer and Lenodus variabilis zones (i.e., ca. 480-465 Ma). The conodont data support an Ordovician instead of an alternative Devonian collision for the Cuyania terrane with the proto-Andean margin of Gondwana, which is consistent with the earlier proposed, but still contentious, microcontinent model. In the latter scenario, the collision of the Cuyania terrane caused unroofing and cannibalism of the leading edge of the carbonate succession and basement rocks. The erosion of these rocks supplied detritus and metamorphosed conodonts to the deeper basinal settings of the Western Precordillera. As the deformation front migrated into the peripheral foreland of the Eastern and Central Precordillera, nonmetamorphosed conodonts were eventually incorporated into the west-prograding synorogenic clastic wedge. The occurrence of allochthonous and autochthonous conodonts in the slope facies of the Western Precordillera provides a time constraint for the early accretion of the Cuyania terrane and its transition to an evolving foreland mountain system in the Ordovician period. [ABSTRACT FROM AUTHOR]

Recent studies suggest that as much as half of the organic carbon (OC) undergoing burial in the sediments of tectonically active continental margins may be the product of fossil shale weathering. These estimates rely on the assumption that vascular plant detritus spends little time sequestered in intermediate reservoirs such as soils, freshwater sediments, and river deltas, and thus only minimally contributes to the extraneously old 14C ages of total organic matter often observed on adjacent shelves. Here we test this paradigm by measuring the Δ14C and δ13C values of individual higher plant wax fatty acids as well as the δ13C values of extractable alkanes isolated from the Eel River margin (California). The isotopic signatures of the long chain fatty acids indicate that vascular plant material has been sequestered for several thousand years before deposition. A coupled molecular isotope mass balance used to reassess the sedimentary carbon budget indicates that the fossil component is less abundant than previously estimated, with pre-aged terrestrial material instead composing a considerable proportion of all organic matter. If these findings are characteristic of other continental margins proximal to small mountainous rivers, then the importance of petrogenic OC burial in marine sediments may need to be reevaluated. [ABSTRACT FROM AUTHOR]

The diffuse deformation zone in the central Indian Ocean is the classical example of distributed deformation of the oceanic lithosphere, where shortening between the Indian and Capricorn plates is manifest as reverse faulting (5-10-km-spaced faults) and long-wavelength (100-300 km) folding. The onset of this deformation is commonly regarded as a key far-field indicator for the start of major uplift of the Himalayas and Tibet, some 4000 km further to the north, due to increased deviatoric stresses within the wider India-Asia area. There has been disagreement concerning the likely timing for the onset of deformation between plate-motion inversions and seismic reflection-based studies. In the present study, fault displacement data from seismic-reflection profiles within the central Indian Ocean demonstrate that compressional activity started much earlier than previously thought, at around 15.4-13.9 Ma. From reconstructions of fault activity histories, we shows that 12% of the total reverse fault population had been activated, and 14% of the total strain accumulated, prior to a sharp increase in the deformation rate at 8.0-7.5 Ma. There is no evidence for any regional unconformity before 8.0-7.5 Ma; early shortening was accommodated by activity on single isolated fault blocks. Total strain estimates derived are more variable and complex than those predicted from plate inversion, and they do not show simple west to east increase. [ABSTRACT FROM AUTHOR]

The Eocene deep-marine siliciclastic fill of the Ainsa basin, Spanish Pyrenees, gives unprecedented temporal resolution of the causes and timing of coarse elastic sediment supply to a deep-marine basin. Early Eocene tectonic subsidence linked to Pyrenean orogenesis created the Ainsa basin, with water depths of ∼400-800 m above a foundered shallow-marine mixed carbonate and clastic shelf. The ∼25 sandbodies or channelized submarine fans in the basin were controlled by the ∼400 k.y. Milankovitch frequency, with modes at ∼100 k.y. and ∼41 k.y. (possibly stacked ∼23 k.y.) influencing bottom-water conditions in the basin, causing periodic stratification in the water column across a submarine sill at the western basin margin (early Boltaña anticline). Intrabasinal tectonics defined and controlled the position of eight sandy systems and their constituent fans, in a process of seesaw tectonics, by (1) westward lateral offset stacking of sandy fans due to growth of the eastern side of the basin, represented today by the Mediano anticline, and (2) eastward (orogenward) back stepping of the depositional axis of each sandy system, due to phases of relative uplift of the Boltafla anticline. During basin infill, uplift of the Boltaña anticline led to increasing basin narrowing and depositional confinement. Unlike the earlier depositional systems, the youngest deep-marine system was fed from a more southern sediment source between the growth anticlines, as was the overlying deltaic system. All the older deep-marine sandy systems were fed from southeast point sources, from canyons and erosional lower-slope channels eroding the growing Mediano anticline. The depositional style outlined in this paper might be common to other active margins where siliciclastic basins evolve between active thrusts. [ABSTRACT FROM AUTHOR]

A log-periodic series of fluid inflow events from the outer rise off the Nicoya Peninsula, Costa Rica, was recorded on a flow meter deployed at the sediment surface. Similar periodic series have been seen in many failure processes in laboratory and field research, including seismic and chemical records preceding earthquakes. Each cycle has the appearance of an impulse and subsequent decay. The hypothesis that the recorded flow pattern is a result of a series of relatively small strain events resulting from aseismic extensional faulting of the outer rise basement is explored, and analytical results supporting this hypothesis are presented. [ABSTRACT FROM AUTHOR]

The Olympus Mons volcano on Mars is notable not only for its immense height and width, but also for substantial asymmetries in its structure. The gently sloped northwest flank extends to a much greater distance from the central caldera complex than the more steeply sloped southeast flank. Furthermore, the northwest flank exhibits lower-flank extensional faults, whereas the southeast shows upper-flank compressional terraces and lower-flank upthrust blocks. However, both the northwest and southeast flanks exhibit characteristic concave-upward profiles and steep bounding scarps, in contrast to other sectors. The NW-SE asymmetries are aligned with the regional slope from the Tharsis rise, but an understanding of the underlying causes has remained elusive. We use particle dynamics models of growing, spreading volcanoes to demonstrate that these flank structures could reflect the properties of the basement materials underlying Olympus Mons. We find that basal slopes alone are insufficient to produce the observed concave-upward slopes and asymmetries in flank extent and deformation style that are observed at Olympus Mons; instead, lateral variations in basal friction are required. These variations are most likely related to the presence of sediments, transported and preferentially accumulated downslope from the Tharsis rise. Such sediments likely correspond to ancient phyllosilicates (clays) recently discovered by the Mars Express mission. [ABSTRACT FROM AUTHOR]

We report here the discovery of a new end-Cryogenian glacial diamictite and an overlying basal Ediacaran cap carbonate within the Tsagaan Oloom Formation of southwestern Mongolia. The identification of the Cryogenian-Ediacaran boundary, coupled with new δ13C chemostratigraphic profiles, facilitates the integration of the Neoproterozoic stratigraphy of Mongolia with records elsewhere. These correlations indicate that a previously unrecognized, -16‰ Cryogenian δ13C anomaly is present in the newly defined Tayshir member (informal) of the Tsagaan Oloom Formation. Furthermore, chemostratigraphic and lithostratigraphic relationships suggest an ~35 m.y. depositional hiatus below the phosphorite-bearing Zunne Arts member (informal) of the upper Tsagaan Oloom Formation and that subsidence renewed in the latest Ediacaran-Early Cambrian. We propose that the lower ~1500 m of the Tsagaan Oloom Formation was deposited on a thermally subsiding passive margin after Rodiniaage rifting, whereas the Zunne Arts member and the overlying ~1600 m of Early Cambrian strata were deposited in a foredeep basin that formed as the southern margin of the Dzabkhan terrane was subducted beneath the Khantayshir-Dariv arc. [ABSTRACT FROM AUTHOR]

Observations related to flood basalts suggest the existence of mantle plumes with large heads and thin trailing tails (cavity plumes). Seismic data suggest the existence of mantle plumes with thick tails (diapir plumes). The conditions required for diapir versus cavity plume generation are different, and in a chemically homogeneous mantle both types are not predicted to coexist. We show, however, that if a variable thickness chemical layer exists at the base of the mantle, consistent with seismic observations, then the coexistence of morphologically distinct plumes types is expected. The chemical layer governs temperature, and thus viscosity variations, in the thermal boundary layer from which mantle plumes rise. A locally thick layer leads to small viscosity variation instabilities and hence to diapir plumes. A locally thin chemical layer allows for large viscosity variations across the active portion of the lower mantle thermal boundary layer and, hence, for cavity plume formation. A chemical layer that can move in response to changing flow patterns allows for the potential that plumes can morphologically transition over their lifetimes. An expectation that the morphology and thermal structure of mantle plumes should vary according to the thickness of a chemical layer is consistent with correlations between seismic observations of chemically distinct material at the core-mantle boundary, the varied morphology of mantle thermal anomalies, and the inferred diversity in hotspot buoyancy fluxes and excess temperatures. [ABSTRACT FROM AUTHOR]

Structural, geomorphic, and interferometric synthetic aperture radar (InSAR) surface displacement data show that uplift of the northernmost onshore Scandinavian passive margin is associated with active normal faulting and anomalous clusters of landslides. Regional-scale normal faults separate alpine footwall ranges with impressive range-front scarps from more gentle hanging-wall topography. In the Lyngen region, rockslides, sackung features (i.e., deep-seated slope gravitational deformation), and active faults cluster inside an ~1000 km2 area of hanging-wall subsidence parallel to an ~30-km-long, faceted range-front scarp. From postglacial to recent time, rockslides have been released along the main range front and other propagating scarps, both in the footwall range and in structurally controlled, glacially oversteepened valleys of the hanging wall. The active rockslides, moving at rates of >10 mm/yr, show a variety of relationships to the controlling structures, reflecting tectonic as well as gravitational faulting. Recent and present-day landscape evolution and large-volume rockslide distribution in Scandinavia should be viewed from the perspective of tectonic processes involving active normal faults. [ABSTRACT FROM AUTHOR]

Stresses released by coseismic faults during subduction toward lawsonite-eclogite facies conditions in the Alpine subduction complex of Corsica can be estimated based on the energy required to form pseudotachylyte fault veins where shear strain can be measured . Congruent peridotite melting at ambient conditions of 1.5 GPa and 470 °C requires a temperature increase of 1280 °C to 1750 °C. We assume that more than 95% of the work is converted to heat during faulting, hence that the stress drop is nearly proportional to the amount of melting and inversely proportional to shear strain. Minimum estimates of released stress are typically greater than 220 MPa and as high as 580 MPa. The abundance of pseudotachylyte on small faults in the studied peridotite suggests that melting is very common on intermediate and deep earthquakes and that shear heating is important for seismic faulting at depth. [ABSTRACT FROM AUTHOR]

Stimulated by the recognition of a rifted continental margin to pre-Grenville Laurentia, we interpret the deformed alkaline igneous rocks and carbonatites (DARCs) adjacent to that margin in the Bancroft domain of Ontario, Canada, to be rocks initially erupted as alkaline rocks and carbonatites (ARCs) in an intra-continental rift. When an ocean began to form, rupturing Laurentia on the site of that rift, the newly formed Laurentian rifted continental margin contained ARCs that had erupted into the intra-continental rift. Later in a Wilson cycle of ocean opening and closing, those ARCs became DARCs during a collision that sutured the Grenville province composite arc belt and Bancroft domain rocks against the margin of Laurentia on the Central Meta-sedimentary Belt boundary thrust zone. Using this interpretation we show that DARCs of the down-dip Bancroft domain suture zone in the mantle at a depth of ∼100 km are likely sources of the Early Cretaceous ARC rocks in the Monteregian province. [ABSTRACT FROM AUTHOR]

Analog and numerical models predict a coupling between climate and tectonics whereby erosion influences the deformation of orogens. A testable prediction from modeling studies is the decrease in width of mountain ranges as a result of increased precipitation. Here we evaluate the effect of climate on a critically tapered orogen, the central Andes, using sequentially restored, balanced cross sections through wet (15°-16°S) and dry (21°S) regions of the orogen. In these regions, tectonics, basin geometry, and style of deformation are similar, allowing us to use variations in propagation (or changes in percent shortening) to evaluate whether alongstrike changes in width and morphology are climate driven in the north. Results indicate similar total percent shortening along the northern (40%) and southern (37%) sections, suggesting that a wetter climate has not limited the width (propagation) in the north. However, comparison of early (45-25 Ma) and recent (ca. 20-0 Ma) shortening indicates that early deformation produced 45% ± 2% shortening of both sections, while recent deformation produced 41% ± 2% (north) versus 32% ± 2% (south) in the actively deforming Subandes. The latter suggests a coupling between climate and tectonics that began between ca. 19 and 8 Ma, and continues to 0 Ma, potentially limiting the width of the northern Subandes by ∼40 km. [ABSTRACT FROM AUTHOR]

In the Miocene, the European Eastern Alps extruded laterally along orogen-scale strikeslip faults due to both extensional gravitational collapse and compressional tectonic forcing. Horizontal extension is most prominently evidenced by detachments east and west of the Tauern Window; it is commonly explained by a retreating slab beneath the Carpathian arc hundreds of kilometers east of the orogen. Horizontal compression is shown by north-south shortening in the Tauern Window and the entire Eastern Alps in response to the convergence of the Adriatic plate with Europe. It is interesting that analogue and numerical models for the Eastern Alps designed to describe the east-directed lateral extrusion have failed to explain the extensional regime in the region of the Tauern Window. Using a numerical model for planview deformation that considers internal faults, we show here that orogen-scale strike-slip faults are mechanically required to cause extension during plate convergence in the Miocene Eastern Alps. We test the idea by coupling this model with a landscape evolution model and by comparing modeled and observed drainage system geometries. [ABSTRACT FROM AUTHOR]

The Middle Stone Age (MSA) of Africa, like the Middle Paleolithic of Europe, is thought to represent a time period wherein toolmakers acquired significant increases in cognitive abilities and physical dexterity. Existing data fail to resolve whether the MSA emerged gradually, abruptly, or discontinuously, and whether this industry reflects the activity of Homo sapiens. Here we present new 40Ar/39Ar geochronological data revealing that advanced MSA archaeology at two sites in the main Ethiopian Rift is older than 276 ka, much older than technologically comparable MSA archaeology from elsewhere. An age of 183 ka for a unit farther upsection, along with the technological stasis observed throughout the section, indicates that similar technology was used here for ∼93 ka. These results suggest that MSA technology evolved asynchronously in different places, and challenge the notion of a distinct time line for either the appearance of the MSA or the disappearance of the earlier Acheulean. These and other recent results indicate that the oldest known MSA consistently predates fossil evidence for the earliest Homo sapiens. [ABSTRACT FROM AUTHOR]

x The manner in which calderas develop is a key question involcanology and has important implications for associated volcanogenic risk and geothermal and ore exploitation. To better understand the structural evolution of calderas, I use a discrete element model of a frictional cover undergoing piston-like subsidence at its base, simulating magma chamber deflation and cover collapse. These novel simulations capture not only the initiation of calderas but also much of the complexity of faulting during their later development. In all models, both normal and reverse faults accommodate deeper subsidence at higher structural levels. Curved, outward-dipping reverse faults are consistently the first structures to develop; subsequent caldera growth is mainly the result of movement on vertical or steeply inward-dipping normal faults. This may be followed by a later phase of lateral collapse on shallower normal faults. The roof aspect ratio (R = cover thickness/piston width) is seen to be an important factor: calderas with low aspect ratios (R≤1) exhibit a coherent central subsiding block, while those with higher aspect ratios have a complexly faulted internal structure. [ABSTRACT FROM AUTHOR]

Shear failure in sediments is universally linked with active boundary conditions, such as those imposed by tectonic stresses. Under conditions of no lateral strain, and in the absence of tectonic stress, soil mechanics theories predict a simple one-dimensional compaction in which sediment particles displace vertically without shear failure during pressure diffusion. Conflicting with this theory, shear failure planes are often found in sediments that formed under near-horizontal burial conditions. We investigated the effect of particle-scale volume contraction as a potential cause of shear failure inuncemented particulate materials and found that loss of particle volume under confined conditions (no external loading) resulted in pronounced lateral stress reduction, often reaching Coulomb frictional failure conditions. Shear strain localization was analytically predicted and modeled numerically, due entirely to volume loss at the grain scale. We define this mode of internally driven shear failure as "contractile" to distinguish it from that caused by external loading, and show that it can explain many natural fracture systems without invoking regional tectonics. [ABSTRACT FROM AUTHOR]

Ongoing oblique collision and flat-slab subduction of the Yakutat terrane has produced Earth's highest coastal mountain range, the Chugach-St. Elias orogen in Southeast Alaska. Massive glaciers and ice fi elds cause extensive erosion, but also limit access to rocks that allow detailed study of the orogen. Fission track and U/Pb dating of detrital zircon from glacial rivers reveal the exhumation history of the Chugach-St. Elias orogen. Orogenic development started ca. 30 Ma and pulses of exhumation occurred 20 ± 2 Ma and 11 ± 2 Ma. Differential exhumation occurred across major fault zones and the locus of exhumation shifted southward. The ca. 5 Ma cooling ages of detrital zircon in the Pliocene-Pleistocene Yakataga Formation reveal that source rock exhumation was likely associated with faulting along the ice-covered Fairweather fault-Contact fault system. [ABSTRACT FROM AUTHOR]

A combination of biostratigraphic markers (ammonites, inoceramid bivalves) and carbon isotope excursions is employed to establish a high-resolution correlation between the middle to late Cenomanian successions of the Western Interior Basin (USA) and the Anglo-Paris Basin (southern UK). Sequences identified from sedimentologic criteria in the Pueblo succession and elsewhere in the Western Interior Basin are shown to coincide precisely with globally recognized sea-level events and were therefore under eustatic control. This evidence refutes arguments that Cenomanian sequences in the Western Interior Basin were formed by local tectonic events. The interaction of longer-term tectonic movements and more rapid eustatic change may have simply enhanced the amount of erosion associated with sequence boundaries. A crossplot of radiometric ages derived from North American bentonites against an orbitally tuned time scale developed in the Anglo-Paris Basin provides support for the argument that the sequences were controlled by the 405-k.y.-long eccentricity cycle. [ABSTRACT FROM AUTHOR]