Shinevar William J.

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Shinevar
First Name
William J.
ORCID
0000-0001-6384-8023

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  • Article
    Compositional dependence of lower crustal viscosity
    (John Wiley & Sons, 2015-10-23) Shinevar, William J. ; Behn, Mark D. ; Hirth, Greg
    We calculate the viscosity structure of the lower continental crust as a function of its bulk composition using multiphase mixing theory. We use the Gibbs free-energy minimization routine Perple_X to calculate mineral assemblages for different crustal compositions under pressure and temperature conditions appropriate for the lower continental crust. The effective aggregate viscosities are then calculated using a rheologic mixing model and flow laws for the major crust-forming minerals. We investigate the viscosity of two lower crustal compositions: (i) basaltic (53 wt % SiO2) and (ii) andesitic (64 wt % SiO2). The andesitic model predicts aggregate viscosities similar to feldspar and approximately 1 order of magnitude greater than that of wet quartz. The viscosity range calculated for the andesitic crustal composition (particularly when hydrous phases are stable) is most similar to independent estimates of lower crust viscosity in actively deforming regions based on postglacial isostatic rebound, postseismic relaxation, and paleolake shoreline deflection.
  • Article
    Causes of oceanic crustal thickness oscillations along a 74-M Mid-Atlantic ridge flow line
    (American Geophysical Union, 2019-11-19) Shinevar, William J. ; Mark, Hannah F. ; Clerc, Fiona ; Codillo, Emmanuel A. ; Gong, Jianhua ; Olive, Jean-Arthur ; Brown, Stephanie M. ; Smalls, Paris T. ; Liao, Yang ; Le Roux, Véronique ; Behn, Mark D.
    Gravity, magnetic, and bathymetry data collected along a continuous 1,400‐km‐long spreading‐parallel flow line across the Mid‐Atlantic Ridge indicate significant tectonic and magmatic fluctuations in the formation of oceanic crust over a range of time scales. The transect spans from 28 Ma on the African Plate to 74 Ma on the North American plate, crossing the Mid‐Atlantic Ridge at 35.8°N. Gravity‐derived crustal thicknesses vary from 3–9 km with a standard deviation of 1.0 km. Spectral analysis of bathymetry and residual mantle Bouguer anomaly show a diffuse power at >1 Myr and concurrent peaks at 390, 550, and 950 kyr. Large‐scale (>10 km) mantle thermal and compositional heterogeneities, variations in upper mantle flow, and detachment faulting likely generate the >1 Myr diffuse power. The 550‐ and 950‐kyr peaks may reflect the presence of magma solitons and/or regularly spaced ~7.7 and 13.3 km short‐wavelength mantle compositional heterogeneities. The 390‐kyr spectral peak corresponds to the characteristic spacing of faults along the flow line. Fault spacing also varies over longer periods (>10 Myr), which we interpret as reflecting long‐lived changes in the fraction of tectonically versus magmatically accommodated extensional strain. A newly discovered off‐axis oceanic core complex (Kafka Dome) found at 8 Ma on the African plate further suggests extended time periods of tectonically‐dominated plate separation. Fault spacing negatively correlates with gravity‐derived crustal thickness, supporting a strong link between magma input and fault style at mid‐ocean ridges.
  • Article
    WISTFUL: whole‐rock interpretative seismic toolbox for ultramafic lithologies
    (American Geophysical Union, 2022-08-11) Shinevar, William J. ; Jagoutz, Oliver ; Behn, Mark D.
    To quantitatively convert upper mantle seismic wave speeds measured into temperature, density, composition, and corresponding and uncertainty, we introduce the Whole-rock Interpretative Seismic Toolbox For Ultramafic Lithologies (WISTFUL). WISTFUL is underpinned by a database of 4,485 ultramafic whole-rock compositions, their calculated mineral modes, elastic moduli, and seismic wave speeds over a range of pressure (P) and temperature (T) (P = 0.5–6 GPa, T = 200–1,600°C) using the Gibbs free energy minimization routine Perple_X. These data are interpreted with a toolbox of MATLAB® functions, scripts, and three general user interfaces: WISTFUL_relations, which plots relationships between calculated parameters and/or composition; WISTFUL_geotherms, which calculates seismic wave speeds along geotherms; and WISTFUL_inversion, which inverts seismic wave speeds for best-fit temperature, composition, and density. To evaluate our methodology and quantify the forward calculation error, we estimate two dominant sources of uncertainty: (a) the predicted mineral modes and compositions, and (b) the elastic properties and mixing equations. To constrain the first source of uncertainty, we compiled 122 well-studied ultramafic xenoliths with known whole-rock compositions, mineral modes, and estimated P-T conditions. We compared the observed mineral modes with modes predicted using five different thermodynamic solid solution models. The Holland et al. (2018, https://doi.org/10.1093/petrology/egy048) solution models best reproduce phase assemblages (∼12 vol. % phase root-mean-square error [RMSE]) and estimated wave speeds. To assess the second source of uncertainty, we compared wave speed measurements of 40 ultramafic rocks with calculated wave speeds, finding excellent agreement (Vp RMSE = 0.11 km/s). WISTFUL easily analyzes seismic datasets, integrates into modeling, and acts as an educational tool.
  • Article
    Mantle thermochemical variations beneath the continental United States through petrologic interpretation of seismic tomography
    (Elsevier, 2023-01-04) Shinevar, William J. ; Golos, Eva M. ; Jagoutz, Oliver ; Behn, Mark D. ; van der Hilst, Robert D.
    The continental lithospheric mantle plays an essential role in stabilizing continents over long geological time scales. Quantifying spatial variations in thermal and compositional properties of the mantle lithosphere is crucial to understanding its formation and its impact on continental stability; however, our understanding of these variations remains limited. Here we apply the Whole-rock Interpretive Seismic Toolbox For Ultramafic Lithologies (WISTFUL) to estimate thermal, compositional, and density variations in the continental mantle beneath the contiguous United States from MITPS_20, a joint body and surface wave tomographic inversion for Vp and Vs with high resolution in the shallow mantle (60–100 km). Our analysis shows lateral variations in temperature beneath the continental United States of up to 800–900°C at 60, 80, and 100 km depth. East of the Rocky Mountains, the mantle lithosphere is generally cold (350–850°C at 60 km), with higher temperatures (up to 1000°C at 60 km) along the Atlantic coastal margin. By contrast, the mantle lithosphere west of the Rocky Mountains is hot (typically >1000°C at 60 km, >1200°C at 80–100 km), with the highest temperatures beneath Holocene volcanoes. In agreement with previous work, we find that the chemical depletion predicted by WISTFUL does not fully offset the density difference due to temperature. Extending our results using Rayleigh-Taylor instability analysis, implies the lithosphere below the United States could be undergoing oscillatory convection, in which cooling, densification, and sinking of a chemically buoyant layer alternates with reheating and rising of that layer.•MITPS_20, a continental US tomographic model, is interpreted in terms of temperature, composition, and density.•Our method predicts temperatures of 260–1430°C, Mg# of 85–92, and density between 3230–3370 kgm−3 between 60–100 km.•Predicted compositional buoyancy of the mantle lithosphere compensates only part (40%) of the negative thermal buoyancy.