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  1. The Great Bank of Guizhou is a 2.5 km thick isolated carbonate platform deposited during the Triassic period. The rocks preserve evidence for multiple episodes of dolomitization, spread across a range of geologic time. Different styles of dolomitization and geochemical evidence support this interpretation. Early dolomitization includes both peritidal cycle cap dolomites and large regions of massively-bedded dolomite in the platform interior, along with isolated dolomitized and partially dolomitized clasts in slope breccias derived from the platform interior. Forms of later stage dolomite include a widespread overprint and modification of massively bedded platform interior dolomites during burial; zones of pervasively dolomitized slope sediments (10s of m thick), some of which are discordant at various scales (0.1 m to 100s of m); partial dolomitization along fractures, bedding planes, and stylolites; alternating stratiform laminae of limestone and dolostone (mm to cm scale) in slope sediments; and matrix-selective dolomitization in some slope breccias. Evidence for early dolomite includes isolated clasts of dolomite in Early Triassic slope breccias surrounded by lime mudstone, pervasive dolomite in platform interior sediments, Sr-isotopes and REE signatures consistent with Early Triassic seawater, and evidence for evaporites and solution collapse breccias in the platform interior. Textures and some geochemical indicators were modified during deep burial. Evidence for later stage dolomite (Late Triassic or later) includes zones of coarse massively dolomitized slope breccias surrounded by selectively dolomitized vertical and bedding plane fractures, stylolites, and alternating stratiform laminae of limestone and dolostone; fluid-inclusions containing brine (12-16 wt. %, NaCl equivalent) with homogenization temperatures of 100°C to 180°C, and some younger (post-burial) U-Pb age dates. Early evaporative-reflux dolomitization in the platform interior likely dominated the dolomite volumetrically before it was overprinted with burial signatures. Pervasively dolomitized slope breccias surrounded by selective dolomitized areas are interpreted to be the result of intrusion of late burial dolomitizing fluids into higher permeability units. 
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  2. Abstract

    Within extreme continental extension areas, ductile middle crust is exhumed at the surface as metamorphic core complexes. Sophisticated quantitative models of extreme extension predicted upward transport of ductile middle-lower crust through time. Here we develop a general model for metamorphic core complexes formation and demonstrate that they result from the collapse of a mountain belt supported by a thickened crustal root. We show that gravitational body forces generated by topography and crustal root cause an upward flow pattern of the ductile lower-middle crust, facilitated by a detachment surface evolving into low-angle normal fault. This detachment surface acquires large amounts of finite strain, consistent with thick mylonite zones found in metamorphic core complexes. Isostatic rebound exposes the detachment in a domed upwarp, while the final Moho discontinuity across the extended region relaxes to a flat geometry. This work suggests that belts of metamorphic core complexes are a fossil signature of collapsed highlands.

     
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  3. Tectonic activity can drive speciation and sedimentation, potentially causing the fossil and rock records to share common patterns through time. The Basin and Range of western North America arose through widespread extension and collapse of topographic highlands in the Miocene, creating numerous basins with rich mammalian fossil records. We analyzed patterns of mammalian species richness from 36 to 0 million years ago in relation to the history of sediment accumulation to test whether intervals of high species richness corresponded with elevated sediment accumulation and fossil burial in response to tectonic deformation. We found that the sedimentary record of the Basin and Range tracks the tectonic evolution of landscapes, whereas species-richness trends reflect actual increased richness in the Miocene rather than increased fossil burial. The sedimentary record of the region broadly determines the preservation of the fossil record but does not drive the Miocene peak in mammalian species richness. 
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  4. Understanding the movement of fluids in the solid Earth system is crucial for answering a wide range of important questions in Earth science. Boron (B) is a perfect tracer for geofluids because of its high solubility and large isotopic fractionation that depends on both temperature and alkalinity. However, the high volatility of boron in acidic solutions at moderate temperatures presents a significant challenge for accurate measurements of the boron concentration and boron isotopic ratios for silicate rock samples. To circumvent this problem, most laboratories use low-temperature dissolution methods that involve concentrated hydrofluoric acid with or without mannitol. However, hydrofluoric acid is highly hazardous and the controlled temperature condition may be difficult to monitor. As a result, relatively few silicate samples have been analyzed for high precision B concentration and isotopic composition measurements, which hinders our understanding of the behavior of B in the solid earth system and the utility of this powerful tracer. Here we report B concentrations and isotopic compositions of the most commonly used geological reference standards dissolved through sodium peroxide sintering and purified using a rapid single-column exchange chromatographic procedure. This streamlined method effectively removes Na and Si from the sample matrix and generates accurate B concentration and isotopic data in as little as a day without the need for expensive lab equipment and reagents. Sintering is already routinely used to dissolve zircon-bearing silicate samples as it ensures complete dissolution. Besides the analysis of boron, other elemental and isotopic analyses can be performed using aliquots of the same dissolution, which greatly speeds up the chemical processing time and reduces uncertainties associated with sample heterogeneity. Using this method, large amounts of material can be processed for ion-exchange chromatography without the need of splitting each sample into separate beakers for dissolution as is often required for the HF + mannitol dissolution method. This new method can rapidly expand the available dataset of the boron concentration and boron isotopes of silicate materials which will certainly advance our understanding of many geologic problems involving fluids. 
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  5. Abstract

    Ice cores and offshore sedimentary records demonstrate enhanced ice loss along Antarctic coastal margins during millennial-scale warm intervals within the last glacial termination. However, the distal location and short temporal coverage of these records leads to uncertainty in both the spatial footprint of ice loss, and whether millennial-scale ice response occurs outside of glacial terminations. Here we present a >100kyr archive of periodic transitions in subglacial precipitate mineralogy that are synchronous with Late Pleistocene millennial-scale climate cycles. Geochemical and geochronologic data provide evidence for opal formation during cold periods via cryoconcentration of subglacial brine, and calcite formation during warm periods through the addition of subglacial meltwater originating from the ice sheet interior. These freeze-flush cycles represent cyclic changes in subglacial hydrologic-connectivity driven by ice sheet velocity fluctuations. Our findings imply that oscillating Southern Ocean temperatures drive a dynamic response in the Antarctic ice sheet on millennial timescales, regardless of the background climate state.

     
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