Showing papers in "Geology in 2002"

Origin of mesozoic adakitic intrusive rocks in the Ningzhen area of east China: Partial melting of delaminated lower continental crust?

Abstract: To the best of our knowledge, modern adakites have not been documented in a nonarc environment. We report geochemical and isotopic data for Early Cretaceous Anjishan adakitic intrusive rocks that are in a continental setting unrelated to subduction. The Anjishan adakitic intrusive rocks, which are exposed in the Ningzhen area of east China, have high Sr/Y and La/Yb ratios coupled with low Yb and Y as well as relatively high MgO contents and Mg numbers (Mg#; 0.4-0.6), similar to products from slab melting. However, low ∈ N d ( t ) values (-6.8 to-9.7) and high ( 8 7 Sr/ 8 6 Sr) i (0.7053-0.7066) are inconsistent with an origin by slab melting. The tectonics and geochemistry lead us to conclude that adakitic magmas were most likely derived from partial melting of mafic material at the base of the continental crust. High Sr/Y and La/Yb ratios of the adakitic intrusive rocks suggest that garnet was stable as a residual phase during partial melting, implying that the crustal thickness exceeded 40 km in the Early Cretaceous. The present thickness of the crust in the Ningzhen area is only 30 km, and therefore the crust appears to have been thinned by at least ∼10 km since the Early Cretaceous. The relatively high MgO contents and Mg# of the Anjishan intrusive rocks suggest that adakitic magmas interacted with mantle rocks, possibly coinciding with lower-crustal delamination, which would also account for the observed thinning.

Spatial distribution of δ18O in meteoric precipitation

Abstract: Proxy data reflecting the oxygen isotope composition of meteoric precipitation (δ18Oppt) are widely used in reconstructions of continental paleoclimate and paleohydrology. However, actual geographic variation in modern water compositions is difficult to estimate from often sparse data. A first step toward understanding the geologic pattern of change in δ18Oppt is to describe the modern distribution in terms of principal geographic parameters. To this end, we empirically model relationships between 18O in modern precipitation and latitude and altitude. We then identify geographic areas where large-scale vapor transport patterns give rise to significant deviations from model δ18Oppt compositions based on latitude and altitude. Model value and residual grids are combined to derive a high-resolution global map of δ18Oppt that can serve as a spatial reference against which proxy data for paleoprecipitation can be compared. Reiteration of the procedure outlined here, for paleo-δ18Oppt data, may illuminate past changes in the climatic and physiographic parameters controlling the distribution of δ18O regimes.

Grenvillian continental collision in South China: new SHRIMP U-Pb zircon results and implications for the configuration of Rodinia

Abstract: The timing of continental collision between the Yangtze and Cathaysia blocks of south China is an issue that bears on the accretion of Asia, as well as on the assembly and configuration of the Neoproterozoic supercontinent Rodinia. We report in this paper SHRIMP (sensitive high-resolution ion microprobe) evidence that suggests a Grenvillian continental collision in south China, including (1) evidence for 1.3–1.0 Ga metamorphism on both sides of the Sibao orogen between the Yangtze and Cathaysia blocks and (2) sedimentary provenance of possible foreland-basin deposits on the Yangtze side of the orogen that were derived from the Cathaysia block and the Sibao orogen during the continental collision. The occurrence of ca. 1430 Ma granodiorites in southern Cathaysia, along with ca. 1800 Ma basement and Archean protoliths in northern Cathaysia, makes Cathaysia a possible western extension of the Mojave province in southwestern Laurentia. Together with regional data, we suggest that the Sibao orogen could be one of the Grenvillian sutures at the center of Rodinia assembly that brought Australia, Yangtze, and Cathaysia-Laurentia together by ca. 1000 Ma.

Deep-sea paleotemperature record of extreme warmth during the Cretaceous

Abstract: Oxygen isotope analyses of well-preserved foraminifera from Blake Nose (30°N paleolatitude, North Atlantic) and globally distributed deep-sea sites provide a long-term paleotemperature record for the late Albian–Maastrichtian interval that is difficult to reconcile with the existence of significant Cretaceous ice sheets. Given reasonable assumptions about the isotopic composition of Cretaceous seawater, our results suggest that middle bathyal water temperatures at Blake Nose increased from ∼12 °C in the late Albian through middle Cenomanian to a maximum of 20 °C during the latest Cenomanian and earliest Turonian. Bottom waters were again ∼12 °C during the middle Campanian and cooled to a minimum of 9 °C during the Maastrichtian. Correlative middle bathyal foraminifera from other ocean basins yield paleotemperature estimates that are very similar to those from Blake Nose. Comparison of global bottom-water temperatures and latitudinal thermal gradients suggests that global climate changed from a warm greenhouse state during the late Albian through late Cenomanian to a hot greenhouse phase during the latest Cenomanian through early Campanian, then to cool greenhouse conditions during the mid-Campanian through Maastrichtian.

Roasting the mantle: Slab melting and the genesis of major Au and Au-rich Cu deposits

Abstract: The generation of large deposits of Au and Cu in suprasubduction-zone settings depends upon a combination of factors, including availability of the chalcophile elements to arc magmas in their mantle source regions, and the operation of suitable hydrothermal systems in the upper crust where the deposits eventually form. The removal of chalcophile elements from the mantle wedge into arc magmas can only occur if sulfide is absent from the melted source rock, requiring oxidation of the mantle wedge to values of log f O 2 > FMQ + 2, where f O 2 is oxygen fugacity and FMQ is the fayalite-magnetite-quartz oxygen buffer. The only agent capable of effecting this change is ferric iron, carried in solution by slab-derived partial melts or supercritical fluids. Arc magmas with high potential to generate Au and Cu deposits will have certain geochemical characteristics; they will have f O 2 more than two log units above FMQ and they will have either adakitic, sodic-alkaline, or potassic-ultrapotassic affinities. Favorable tectonic settings include subduction of very young lithosphere or very slow or oblique convergence, flat subduction, and the cessation of subduction.

On steady states in mountain belts

Abstract: The dynamic system of tectonics and erosion contains important feedback mechanisms such that orogenic systems tend toward a steady state. This concept is often invoked, but the nature of the steady state is commonly not specified. We identify four types of steady state that characterize the orogenic system and illustrate these cases by using numerical-model results and natural examples. These types are (1) flux steady state, (2) topographic steady state, (3) thermal steady state, and (4) exhumational steady state: they refer to the erosional flux, the topography, the subsurface temperature field, and the spatial pattern of cooling ages, respectively. Models suggest that the topography will reach a steady mean form at the scale of an orogenic belt, but perfect topographic steady state is unlikely to be achieved at shorter length scales. Thermal steady state is a precondition for exhumational steady state and in the case of temperature-dependent deformation, topographic steady state. Exhumational steady state is characterized by reset age zones spatially nested according to closure temperature, as illustrated in natural systems from New Zealand, the Cascadia accretionary margin, and Taiwan.

Hot orogens, tectonic switching, and creation of continental crust

Abstract: Many granulite terrains were too hot to have formed during continental collision. Rather, along with many high-grade metamorphic terrains that typify continental crust, most formed in accretionary orogens during tectonic switching, when prolonged lithospheric extension was interrupted by intermittent, transient contraction. Based on modern and ancient examples, tectonic switching occurs when slab retreat induces upper plate extension, causing arc splitting, formation of microcontinent slivers, and backarc basins; then intermittent arrival of buoyant oceanic plateaus induces transient flat subduction (or slab flip) and crustal thickening. During slab retreat, basaltic magmas produced from decompressed asthenosphere advect into the extending orogen, causing granulite facies metamorphism and granite generation, but subsequent thickening during flat subduction cools the region. Thickening is focused in the thermally softened backarc region and, if sediment filled, a hot, short-lived (~10 m.y.), narrow (50– 100 km) orogenic belt forms. Such thickening is often misleadingly ascribed to arc or microcontinent collision. Once slab-retreat mode is reestablished, lithospheric extension recommences and a new arc-backarc system forms, generally outboard. Arrival of another plateau will reverse the procedure, and another short-lived, hot orogen will form within the orogenic system. Cycles of tectonic switching efficiently produce continental crust.

Terrestrial and marine extinction at the Triassic-Jurassic boundary synchronized with major carbon-cycle perturbation: A link to initiation of massive volcanism?

Abstract: Mass extinction at the Triassic-Jurassic (Tr-J) boundary occurred about the same time (200 Ma) as one of the largest volcanic eruptive events known, that which characterized the Central Atlantic magmatic province. Organic carbon isotope data from the UK and Greenland demonstrate that changes in flora and fauna from terrestrial and marine environments occurred synchronously with a light carbon isotope excursion, and that this happened earlier than the Tr-J boundary marked by ammonites in the UK. The results also point toward synchronicity between extinctions and eruption of the first Central Atlantic magmatic province lavas, suggesting a causal link between loss of taxa and the very earliest eruptive phases. The initial isotopic excursion potentially provides a widely correlatable marker for the base of the Jurassic. A temporary return to heavier values followed, but relatively light carbon dominated the shallow oceanic and atmospheric reservoirs for at least 600 k.y.

Could bacteria have formed the Precambrian banded iron formations

Abstract: Banded iron formations (BIFs) are prominent sedimentary deposits of the Precambrian, but despite a century of endeavor, the mechanisms of their deposition are still unresolved. Interactions between microorganisms and dissolved ferrous iron in the ancient oceans offer one plausible means of mineral precipitation, in which bacteria directly generate ferric iron either by chemolithoautotrophic iron oxidation or by photoferrotrophy. On the basis of chemical analyses from BIF units of the 2.5 Ga Hamersley Group, Western Australia, we show here that even during periods of maximum iron precipitation, most, if not all, of the iron in BIFs could be precipitated by iron-oxidizing bacteria in cell densities considerably less than those found in modern Fe-rich aqueous environments. Those ancient microorganisms would also have been easily supported by the concentrations of nutrients (P) and trace metals (V, Mn, Co, Zn, and Mo) found within the same iron-rich bands. These calculations highlight the potential importance of early microbial activity on ancient metal cycling.

Evidence for active subduction beneath Gibraltar

Abstract: We report on a marine seismic survey that images an active accretionary wedge west of Gibraltar. Ramp thrusts offset the seafloor and sole out to an east-dipping decollement, indicating ongoing westward-vergent tectonic shortening. New traveltime tomographic re- sults image a slab of oceanic lithosphere descending from the Atlantic domain of the Gulf of Cadiz, passing through intermediate-depth (60-120 km) seismicity beneath the west- ernmost Alboran Sea, and merging with a region of deep-focus earthquakes 600-660 km below Granada, Spain. Together, these new data provide compelling evidence for an active east-dipping subduction zone.

Secular changes in tonalite-trondhjemite-granodiorite composition as markers of the progressive cooling of Earth

Abstract: Archean tonalite-trondhjemite-granodiorite associations (TTG) are classically thought to generate through partial melting of hydrous metabasalts. However, the chemical composition of the least differentiated TTG parental magmas evolved from 4.0 to 2.5 Ga. During this interval, the Mg# as well as the Ni and Cr contents increased, which is interpreted as reflecting increased interactions between felsic melts generated by metabasalt melting and mantle peridotite. Similarly, (CaO + Na2O) and Sr also increased over time, thus reflecting an increase in the abundance of plagioclase in the melt residue. The presence or absence of residual plagioclase is interpreted in terms of melting depth. The demonstrated interaction between TTG parental magmas and the mantle rules out their genesis by fusion of previously underplated metabasalt and favors the melting of subducted slab material. At 4.0 Ga, Earth's geothermal gradient was sufficiently high to allow slab melting at shallow depths where plagioclase was stable. Consequently, due to the small thickness of the overlying mantle wedge, felsic magmas interacted little with the mantle. At 2.5 Ga, however, owing to lower geothermal gradients, the melting depth was greater and plagioclase became no longer stable in the thick mantle wedge overlying the subducted slab. As a result, felsic magmas reacted strongly with the mantle peridotite. The changes of TTG composition during Archean time can be thus interpreted as reflecting the progressive cooling of Earth.

A cool early Earth

Abstract: No known rocks have survived from the first 500 m.y. of Earth history, but studies of single zircons suggest that some continental crust formed as early as 4.4 Ga, 160 m.y. after accretion of the Earth, and that surface temperatures were low enough for liquid water. Surface temperatures are inferred from high d 18 O values of zircons. The range of d 18 O values is constant throughout the Archean (4.4‐2.6 Ga), suggesting uniformity of processes and conditions. The hypothesis of a cool early Earth suggests long intervals of relatively temperate surface conditions from 4.4 to 4.0 Ga that were conducive to liquidwater oceans and possibly life. Meteorite impacts during this period may have been less frequent than previously thought.

Are Proterozoic cap carbonates and isotopic excursions a record of gas hydrate destabilization following Earth's coldest intervals?

Abstract: Regionally persistent, thin intervals of carbonate rock directly and ubiquitously overlie Proterozoic glacial deposits on almost every continent, and are commonly referred to as cap carbonates. Their unusual facies, stratigraphically abrupt basal and upper contacts, and strongly negative carbon isotopic signature (δ13C values between ∼0‰ and −5‰) suggest a chemical oceanographic origin, the details of which remain unresolved. Here we propose that these enigmatic deposits are related to the destabilization of gas hydrate in terrestrial permafrost following rapid postglacial warming and flooding of widely exposed continental shelves and interior basins. Supporting evidence for this hypothesis includes (1) the common occurrence within the cap carbonates of unusual fabrics, similar to those produced by cold methane seeps; (2) a distinctive time evolution for the carbon isotopic excursions indicative of a pulse addition of isotopically depleted carbon to the ocean- atmosphere system; and (3) agreement between mass-balance estimates of carbon released by hydrate destabilization and carbon buried in the cap carbonate. We infer that during times of low-latitude glaciation, characteristic of the Neoproterozoic, gas hydrates may have been in greater abundance than at any other time in Earth history.

Warming the fuel for the fire: Evidence for the thermal dissociation of methane hydrate during the Paleocene-Eocene thermal maximum

Abstract: Dramatic warming and upheaval of the carbon system at the end of the Paleocene Epoch have been linked to massive dissociation of sedimentary methane hydrate. However, testing the Paleocene-Eocene thermal maximum hydrate dissociation hypothesis has been hindered by the inability of available proxy records to resolve the initial sequence of events. The cause of the Paleocene-Eocene thermal maximum carbon isotope excursion remains speculative, primarily due to uncertainties in the timing and duration of the Paleocene-Eocene thermal maximum. We present new high-resolution stable isotope records based on analyses of single planktonic and benthic foraminiferal shells from Ocean Drilling Program Site 690 (Weddell Sea, Southern Ocean), demonstrating that the initial carbon isotope excursion was geologically instantaneous and was preceded by a brief period of gradual surface-water warming. Both of these findings support the thermal dissociation of methane hydrate as the cause of the Paleocene-Eocene thermal maximum carbon isotope excursion. Furthermore, the data reveal that the methane-derived carbon was mixed from the surface ocean downward, suggesting that a significant fraction of the initial dissociated hydrate methane reached the atmosphere prior to oxidation.

Gneiss domes and orogeny

Abstract: Many gneiss domes record positive feedback between decompression and partial melting of orogenic middle crust. Exhumed orogens are riddled with gneiss domes cored by migmatites that underwent dehydration melting during decompression. The decreasing buoyancy associated with increasing melt fraction drives further decompression at near-isothermal conditions as the partially molten crust rises diapirically. This combination of processes may explain the generation and retention of large volumes of crustally derived melt recorded in many deep-seated migmatite terranes and inferred for active orogens. In exhumed orogens, the signature of the rapid ascent of partially molten crust is a gneiss dome cored by migmatite ± granite. The large volume of material involved in the vertical transfer of partially molten crust indicates that the formation of gneiss domes is an efficient mechanism for heat advection during orogenesis.

Testing the Cretaceous greenhouse hypothesis using glassy foraminiferal calcite from the core of the Turonian tropics on Demerara Rise

Abstract: Glassy Turonian foraminifera preserved in clay-rich sediments from the western tropical Atlantic yield the warmest equivalent ?18O sea-surface temperatures (SSTs) yet reported for the entire Cretaceous-Cenozoic. We estimate Turonian SSTs that were at least as warm as (conservative mean ~30 °C) to significantly warmer (warm mean ~33 °C) than those in the region today. However, if independent evidence for high middle Cretaceous pCO2 is reliable and resulted in greater isotopic fractionation between seawater and calcite because of lower sea-surface pH, our conservative and warm SST estimates would be even higher (32 and 36 °C, respectively). Our new tropical SSTs help reconcile geologic data with the predictions of general circulation models that incorporate high Cretaceous pCO2 and lend support to the hypothesis of a Cretaceous greenhouse. Our data also strengthen the case for a Turonian age for the Cretaceous thermal maximum and highlight a 20–40 m.y. mismatch between peak Cretaceous-Cenozoic global warmth and peak inferred tectonic CO2 production. We infer that this mismatch is either an artifact of a hidden Turonian pulse in global ocean-crust cycling or real evidence of the influence of some other factor on atmospheric CO2 and/or SSTs. A hidden pulse in crust cycling would explain the timing of peak Cretaceous-Cenozoic sea level (also Turonian), but other factors are needed to explain high-frequency (~10–100 k.y.) instability in middle Cretaceous SSTs reported elsewhere.

Late Pliocene age for the Atacama Desert: Implications for the desertification of western South America

Abstract: The Atacama Desert forms one of the major hyperarid deserts of the world. Previous studies suggest that desertification commenced at 14 Ma during global climate desiccation. Sedimentologic data from middle Miocene to upper Pliocene successions in the modern Atacama Desert indicate that a semiarid climate persisted from 8 to 3 Ma, punctuated by a phase of increased aridity at ca. 6 Ma. As such, hyperaridity did not commence until the late Pliocene. Implications are (1) that the rain shadow generated by the Andean Cordillera has had a minor influence on climate change, and (2) that the upwelling, north- flowing, cold Humboldt Current, although important in establishing the generally arid climate of western South America, did not control the shift to hyperaridity. The formation of the hyperarid Atacama Desert in the late Pliocene accompanied the development of the current phase of aridification in the Sahara and Namib Deserts and is attributed to a phase of global climate cooling.

Reconstruction of postglacial to early Holocene vegetation history in terrestrial Central Europe via cuticular lipid biomarkers and pollen records from lake sediments

Abstract: High-resolution environmental reconstruction for Central European terrestrial settings is commonly based on sediment records that provide the pollen spectra used for reconstruction of vegetation development. Alternatively, complementary algal, insect, or zooplankton assemblages and/or carbonate 18 O isotope profiles are investigated. In this study, vegetation development and inferred climate evolution were documented on a decadal to centennial scale by analysis of land-plant cuticular lipids accumulated in lake sediments. Wax lipids derived from land-plant leaves and blades are source diagnostic and highly resistant to degradation. Their aerodynamic properties favor short-distance eolian transport, but they are less affected by possible long-distance transportation that complicates pollen analysis. The late glacial to Holocene sedimentary record of Lake Steisslingen, southwestern Germany, was investigated by palynological and geochemical means. Tundra-type pioneer vegetation consisting predominantly of grasses and herbs is dominated by nC31-alkanes, whereas pioneer birch forest yields preferentially nC27-alkanes. Establishment of a pine assemblage is recognizable by elevated nC29- and nC31-leaf-wax alkanes, and the diversification of deciduous forests in the Holocene is marked by nC29 dominance. Lipid geochemistry and palynology resolve decadal excursions such as the mid‐Younger Dryas event. Centennial climate trends with pronounced and rapid changes at the Oldest Dryas‐Bolling, Allerod‐Younger Dryas, and Younger Dryas‐preBoreal transitions are consistently reflected by molecular and palynological proxy data. The demonstrated suitability of an independent molecular proxy in paleoenvironmental studies provides new perspectives in high-resolution climate reconstruction.

Enhanced aridity and atmospheric high-pressure stability over the western Mediterranean during the North Atlantic cold events of the past 50 k.y.

Abstract: Multiproxy paleoenvironmental records (pollen and planktonic isotope) from Ocean Drilling Program Site 976 (Alboran Sea) document rapid ocean and climate variations during the last glacial that follow the Dansgaard-Oeschger climate oscillations seen in the Greenland ice core records, thus suggesting a close link of the Mediterranean climate swings with North Atlantic climates. Continental conditions rapidly oscillated through cold-arid and warm-wet conditions in the course of stadial-interstadial climate jumps. At the time of Heinrich events, i.e., maximum meltwater flux to the North Atlantic, western Mediterranean marine microflora and microfauna show rapid cooling correlated with increasing continental dryness. Enhanced aridity conceivably points to prolonged wintertime stability of atmospheric high-pressure systems over the southwestern Mediterranean in conjunction with cooling of the North Atlantic.

Geometric and temporal evolution of an extensional detachment fault, Hohhot metamorphic core complex, Inner Mongolia, China

Abstract: The Early Cretaceous Hohhot metamorphic core complex and its master Hohhot detachment fault are ;400 km west of Beijing in the Daqing Shan (Mountains) of Inner Mongolia. The complex developed across the east-trending Yinshan fold-and-thrust belt within ,4 m.y. following cessation of thrusting ca. 125 Ma (see note added in proof in main text). Postcontractional extension was initiated within a mid-crustal zone of mylonitic and ductile shear that was in part controlled by Carboniferous(?) strata sandwiched between its Proterozoic and Archean crystalline basement and an overlying thrust sheet of similar crystalline rocks. The Hohhot detachment fault appears to have rooted into deep, kinematically active levels of the mid-crustal shear zone. Higher, inactive levels of the mylonitic section were transected by the fault and carried upward in its footwall. Geometries of the footwall mylonitic rocks indicate localized ramp-flat geometries of the fault within and across them. The crosscut top of the mylonitic sequence defines a mylonitic front that departs from the gently south dipping detachment fault and dips northward into its footwall. Early Cretaceous extension was widespread elsewhere in northern China, and was particularly pronounced in the Yunmeng Shan core complex north of Beijing. The gravitational collapse of orogenically thickened crust acting in concert with localized centers of deep-seated plutonism appear to have led to the development of isolated metamorphic core complexes within a broad region of more distributed extensional deformation.

Global correlation of Cenomanian (Upper Cretaceous) sequences: Evidence for Milankovitch control on sea level

Abstract: We have investigated the sequence stratigraphy of two widely separated marine Cenomanian successions in southeast India and northwest Europe, and used high-resolution ammonite biostratigraphy to demonstrate that sea-level changes are globally synchronous and therefore must be eustatically controlled. Sequence-scale sea-level changes in the Cenomanian were driven by the long eccentricity cycle (400 k.y.) in the Milankovitch band. We hypothesize that, during pre-Quaternary time, the third-order sequences of Vail and Haq are essentially a sediment response to sea-level changes driven by the 400 k.y. cycle. Construction of a relative sea-level curve for the marginal marine succession in India demonstrates that the short-term sea-level changes are rapid (10-100 m/m.y.) and have a magnitude of 2-20 m. Glacioeustasy is a possible but unproven driving mechanism.

Prehistoric earthquake history revealed by lacustrine slump deposits

Abstract: Five strong paleoseismic events were recorded in the past 15 k.y. in a series of slump deposits in the subsurface of Lake Lucerne, central Switzerland, revealing for the first time the paleoseismic history of one of the most seismically active areas in central Europe. Although many slump deposits in marine and lacustrine environments were previously attributed to historic earthquakes, the lack of detailed three-dimensional stratigraphic correlation in combination with accurate dating hampered the use of multiple slump deposits as paleoseismic indicators. This study investigated the fingerprint of the well-described A.D. 1601 earthquake (I = VII–VIII, Mw ∼ 6.2) in the sediments of Lake Lucerne. The earthquake triggered numerous synchronous slumps and megaturbidites within different subbasins of the lake, producing a characteristic pattern that can be used to assign a seismic triggering mechanism to prehistoric slump events. For each seismic event horizon, the slump synchronicity was established by seismic-stratigraphic correlation between individual slump deposits through a quasi-three-dimensional high-resolution seismic survey grid. Four prehistoric events, dated by accelerator mass spectrometry, 14C measurements, and tephrochronology on a series of long gravity cores, occurred at 2420, 9770, 13,910, and 14,560 calendar yr ago. These recurrence times are essential factors for assessing seismic hazard in the area. The seismic hazard for lakeshore communities is additionally amplified by slump-induced tsunami and seiche waves. Numerical modeling of such tsunami waves revealed wave heights to 3 m, indicating tsunami risk in lacustrine environments.

Effects of orographic precipitation variations on the concavity of steady-state river profiles

Abstract: The concavity, or curvature, of river profiles has long been taken to be a fundamental indicator of the underlying processes governing fluvial erosion, and thereby of landscape evolution. However, erosion laws have generally been derived without accounting for the strong orographically driven gradients in precipitation typically found in mountainous regions. In addition, field measurements have found discrepancies between the form of measured stream profiles and theoretically derived values. Introducing a simple physically based feedback, we find that orographically induced variations in precipitation strongly affect the curvature of steady-state river profiles. This feedback complicates efforts to infer the form of erosion laws from observed profile concavities, but could help explain discrepancies between observations and theory. Our results demonstrate a strong feedback through which climate influences the form of river profiles and show how such climatic effects act to limit the relief of unglaciated mountain ranges.

Boninitic volcanism in the Oman ophiolite: Implications for thermal condition during transition from spreading ridge to arc

Abstract: The discovery of boninite, a typical high-MgO andesite, in the Oman ophiolite is reported. The boninites in the Oman ophiolite occur as lavas and dikes of the Alley volcanic sequence that overlie or crosscut the spreading-ridge-derived lavas (Geotimes volcanic sequence) and sheeted dikes. The phenocryst mineral assemblage and the major and trace element compositions observed for these boninites resemble those of the Izu-Mariana forearc boninites, indicating that the Alley boninites represent primitive melt generated by partial melting of hydrous peridotite. The occurrence of boninite provides strong thermal and chemical constraints on the formation of the Oman ophiolite that require hot, hydrous shallow mantle (>1250 °C at <30 km depth) to have underlain the proto-Oman ophiolite at the time of boninite generation. The initiation of subduction of the young, hot oceanic lithosphere (and obduction of the future Oman ophiolite) near the spreading ridge and the resultant melting of the highly depleted, shallow-mantle wedge metasomatized by slab-derived fluid represent the most favorable mechanism for the genesis of the Alley boninites.

Origin of marine barite deposits: Sr and S isotope characterization

Abstract: Barite can precipitate in microenvironments in the water column (marine barite), from supersaturated pore fluids at the oxic-anoxic boundary within marine sediments and where Ba-rich pore fluids are expelled and come into contact with sulfate-rich seawater (diagenetic barite), or from hydrothermal solutions (hydrothermal barite). Barite is relatively resistant to alteration after burial and has been used in paleoceanographic studies to reconstruct seawater chemistry and productivity through time. For such applications it is very important to determine the origin of the barite used, because both diagenetic and hydrothermal barite deposits may not accurately record the open-ocean contemporaneous seawater chemistry and productivity. We show here that it is possible to distinguish between the different types of barite by using Sr and S isotopes along with crystal morphology and size characteristics.

Direct geological evidence for oceanic detachment faulting: The Mid-Atlantic Ridge, 15°45′N

Abstract: From a detailed survey and sampling study of corrugated massifs north of the Fifteen-Twenty Fracture Zone on the Mid-Atlantic Ridge, we demonstrate that their surfaces are low-angle detachment fault planes, as proposed but not previously verified. Spreading-direction–parallel striations on the massifs occur at wavelengths from kilometers to centimeters. Oriented drill-core samples from the striated surfaces are dominated by fault rocks with low-angle shear planes and highly deformed greenschist facies assemblages that include talc, chlorite, tremolite, and serpentine. Deformation was very localized and occurred in the brittle regime; no evidence is seen for ductile deformation of the footwall. Synkinematic emplacement of diabase dikes into the fault zone from an immediately subjacent gabbro pluton implies that the detachment must have been active as a low-angle fault surface at very shallow levels directly beneath the ridge axis. Strain localization occurred in response to the weakening of a range of hydrous secondary minerals at a very early stage and was highly efficient.

U-Pb zircon dating and Sr isotope systematics of the Vindhyan Supergroup, India

Abstract: The Vindhyan Supergroup of central India, the focus of many paleontological studies, has been reported to contain Cambrian small shelly fossils, Ediacaran fossils, trace fossils, and Proterozoic microfossils and carbonaceous megafossils. New U-Pb zircon and 87 Sr/ 86 Sr isotopic data from the Lower Vindhyan Supergroup require that the rocks are latest Paleoproterozoic to earliest Mesoproterozoic in age. Two rhyolitic volcanic horizons from the Deonar Formation, between the Kajrahat and Rohtasgarh Limestones and below the unit containing trace fossils, yield U-Pb zircon ages of 1631 ± 5 Ma and 1631 ± 1 Ma. The Kajrahat and Rohtasgarh Limestones of the Semri Group that are below and above the reported Mesoproterozoic trace fossils have 87 Sr/ 86 Sr ratios of 0.70460 and 0.70479, respectively. The Bhander Limestone from the Upper Vindhyan Supergroup has an 87 Sr/ 86 Sr ratio of 0.70599, consistent with a Neoproterozoic age for this formation. These results indicate that the Kajrahat Limestone is of latest Paleoproterozoic age and the Rohtasgarh Limestone is of probable Mesoproterozoic age. These findings are in conflict with the report of Cambrian small shelly fossils and fossils of articulate brachiopods in the Rohtasgarh Limestone and argue for a Mesoproterozoic age for the formation that contains the alleged trace fossils. Reports of an Ediacaran fossil Spriggina (?) from the Lower Vindhyan Supergroup from the northern margin of the Vindhyan Basin suggest either incorrect stratigraphic correlation of units or misidentification of this fossil.

Current strain regime in the Western Alps from continuous Global Positioning System measurements, 1996–2001

Abstract: Four to six years of continuous measurements at 10 permanent Global Positioning System sites in the Western Alps show horizontal residual velocities of ,2 mm/yr with respect to stable Europe; uncertainties range from 0.3 to 1.4 mm/yr. These velocities and the associated strain-rate field indicate that the central part of the range is currently dominated by east-west extension, whereas the southern part shows north-south to northwest-southeast compression. The geodetic and seismotectonic data are consistent with a model where strain is essentially controlled by the counterclockwise rotation of the Adriatic microplate with respect to Eurasia. This rotation, together with the arcuate shape of the contact between the Adriatic microplate and the Alps, induces dextral shear kinematic boundary conditions across the Western Alps, with an additional divergence component in their central part and in Switzerland, and a convergence component in their southern part.

Late Pleistocene and Holocene dune activity and wind regimes in the western Sahara Desert of Mauritania

Abstract: The western Sahara Desert in Mauritania is dominated by extensive sand seas consisting largely of linear dunes. Analyses of Landsat images, geomorphic and stratigraphic studies, and optically stimulated luminescence dating of dunes in the Azefal, Agneitir, and Akchar sand seas provide evidence that three main generations of dunes were formed during the periods 25–15 ka (centered around the Last Glacial Maximum), 10–13 ka (spanning the Younger Dryas event), and after 5 ka. The wind regimes that occurred during each of these periods were significantly different, leading to the formation of dunes on three distinct superimposed trends—northeast, north-northeast, and north—and the development of the sand seas as composite geomorphic features.

1.6 Ga U-Pb zircon age for the Chorhat Sandstone, lower Vindhyan, India: Possible implications for early evolution of animals

Abstract: Bedding-plane markings in the Chorhat Sandstone (lower Vindhyan), central India, were recently interpreted as burrows produced by triploblastic animals. Because the rocks were thought to be older than 1000 Ma, these structures were regarded as the oldest fossil evidence for metazoan life. However, the biological origin of the markings has been questioned, as has their age. Current age estimates are based on K-Ar, Rb-Sr, and fission- track dates, though some contentious evidence suggests that the rocks may be only 540 Ma. Here we provide the first robust age data for the lower Vindhyan by using SHRIMP (sensitive, high-resolution ion microprobe) U-Pb zircon geochronology to date silicified tuffs bounding the Chorhat Sandstone. Our results show that the sediments were deposited between 1628 ± 8 Ma and 1599 ± 8 Ma. If the Chorhat markings are burrows left by worm-like animals, then our data suggest that complex metazoans had evolved before 1600 Ma, 1 b.y. before the “Cambrian explosion” when animals rapidly diversified and became ecologically dominant. However, given the doubts expressed about the origin of the bedding-plane structures, as well as the surprisingly “old” age of the host rocks, further studies are urgently required to provide supportive evidence.