Search

Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Tectonics, 38(2), (2019):666-686. doi:10.1029/2018TC005246., A magnetotelluric survey in the Barotse Basin of western Zambia shows clear evidence for thinned lithosphere beneath an orogenic belt. The uppermost asthenosphere, at a depth of 60–70 km, is highly conductive, suggestive of the presence of a small amount of partial melt, despite the fact that there is no surface expression of volcanism in the region. Although the data support the presence of thicker cratonic lithosphere to the southeast of the basin, the lithospheric thickness is not well resolved and models show variations ranging from ~80 to 150 km in this region. Similarly variable is the conductivity of the mantle beneath the basin and immediately beneath the cratonic lithosphere to the southeast, although the conductivity is required to be elevated compared to normal lithospheric mantle. In a general sense, two classes of model are compatible with the magnetotelluric data: one with a moderately conductive mantle and one with more elevated conductivities. This latter class would be consistent with the impingement of a stringer of plume‐fed melt beneath the cratonic lithosphere, with the melt migrating upslope to thermally erode lithosphere beneath the orogenic belt that is overlain by the Barotse Basin. Such processes are potentially important for intraplate volcanism and also for development or propagation of rifting as lithosphere is thinned and weakened by melt. Both models show clear evidence for thinning of the lithosphere beneath the orogenic belt, consistent with elevated heat flow data in the region., Funding for MT acquisition and analysis was provided by the National Science Foundation grant EAR‐1010432 through the Continental Dynamics Program. The data used in this study are available for download at the IRIS Data Management Center through the DOI links cited in Jones et al. (2003–2008; https://doi.org/10.17611/DP/EMTF/SAMTEX) and Evans et al. (2012; https://doi.org/10.17611/DP/EMTF/PRIDE/ZAM). We would like to thank the field crew from the Geological Survey Department, Zambia, for their assistance in collecting data. Matthew Chamberlain, David Margolius, and Colin Skinner, formerly of Northeastern University, are also thanked for their field assistance. Data are available from the corresponding author pending their submission to the IRIS DMC repository at which point they will be publically available. This is Oklahoma State University, Boone Pickens School of Geology contribution number 2019‐99., 2019-07-30

A joint analysis of magnetotelluric and Sp receiver function data, collected along a profile across the central Appalachians, highlights variations in regional lithospheric structure. While the interpretation of each data set by itself is non-unique, we identify three distinct features that are consistent with both the resistivity model and the receiver function image: 1) thin lithosphere beneath the Appalachian Mountains, 2) somewhat thicker lithosphere to the east of the mountains beneath the Coastal Plain, and 3) a lithosphere-asthenosphere boundary that deepens to the west of the mountains. In some regions, the correspondence between seismic velocity discontinuities and resistivity mark the base of the lithosphere, while in other locations we see seismic discontinuities that are contained within the lithosphere. At the western end of our profile a transition from highly resistive lithosphere to more conductive mantle represents the transition across the Grenville front. The thickness of lithosphere beneath the Grenville terrain is ∼140 km. Lithosphere at the eastern end of the profile has a thickness that is not well constrained by our coverage, but is at least 110 km thick. This lithosphere can be associated with a broader region of high resistivity material seen to extend further south. Directly beneath the Appalachian Mountains, lithospheric thickness is inferred to be as thin as ∼80 km, based on observations of elevated mantle conductivities and a westward-dipping seismic converter. Electrical conductivities in the uppermost asthenospheric mantle are sufficiently high (>0.1 S/m) to require the presence of a small volume of partial melt. The location of these elevated conductivities is close (offset ∼50 km to the west) to Eocene volcanic outcrops in and around Harrisonburg, VA. Our observations speak to mechanisms of intraplate volcanism where there is no divergent or convergent plate motion to trigger mantle upwelling or obvious fluid release, either of whic

Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 16 (2015): 4261–4274, doi:10.1002/2015GC006100., We present results of marine MT acquisition in the Alboran sea that also incorporates previously acquired land MT from southern Spain into our analysis. The marine data show complex MT response functions with strong distortion due to seafloor topography and the coastline, but inclusion of high resolution topography and bathymetry and a seismically defined sediment unit into a 3-D inversion model has allowed us to image the structure in the underlying mantle. The resulting resistivity model is broadly consistent with a geodynamic scenario that includes subduction of an eastward trending plate beneath Gibraltar, which plunges nearly vertically beneath the Alboran. Our model contains three primary features of interest: a resistive body beneath the central Alboran, which extends to a depth of ∼150 km. At this depth, the mantle resistivity decreases to values of ∼100 Ohm-m, slightly higher than those seen in typical asthenosphere at the same depth. This transition suggests a change in slab properties with depth, perhaps reflecting a change in the nature of the seafloor subducted in the past. Two conductive features in our model suggest the presence of fluids released by the subducting slab or a small amount of partial melt in the upper mantle (or both). Of these, the one in the center of the Alboran basin, in the uppermost-mantle (20–30 km depth) beneath Neogene volcanics and west of the termination of the Nekkor Fault, is consistent with geochemical models, which infer highly thinned lithosphere and shallow melting in order to explain the petrology of seafloor volcanics., NSF Grant Number: EAR080-9074; Spanish National Projects Grant Number: CTM2009-07039-E/MAR, CTM2011-30400-C02-02, 2016-06-19

We present results of marine MT acquisition in the Alboran sea that also incorporates previously acquired land MT from southern Spain into our analysis. The marine data show complex MT response functions with strong distortion due to seafloor topography and the coastline, but inclusion of high resolution topography and bathymetry and a seismically defined sediment unit into a 3-D inversion model has allowed us to image the structure in the underlying mantle. The resulting resistivity model is broadly consistent with a geodynamic scenario that includes subduction of an eastward trending plate beneath Gibraltar, which plunges nearly vertically beneath the Alboran. Our model contains three primary features of interest: a resistive body beneath the central Alboran, which extends to a depth of ∼150 km. At this depth, the mantle resistivity decreases to values of ∼100 Ohm-m, slightly higher than those seen in typical asthenosphere at the same depth. This transition suggests a change in slab properties with depth, perhaps reflecting a change in the nature of the seafloor subducted in the past. Two conductive features in our model suggest the presence of fluids released by the subducting slab or a small amount of partial melt in the upper mantle (or both). Of these, the one in the center of the Alboran basin, in the uppermost-mantle (20–30 km depth) beneath Neogene volcanics and west of the termination of the Nekkor Fault, is consistent with geochemical models, which infer highly thinned lithosphere and shallow melting in order to explain the petrology of seafloor volcanics.

Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 14 (2013): 1685–1692, doi:10.1002/ggge.20103., Our knowledge of magma dynamics would be improved if geophysical data could be used to infer rheological constraints in melt-bearing zones. Geophysical images of the Earth's interior provide frozen snapshots of a dynamical system. However, knowledge of a rheological parameter such as viscosity would constrain the time-dependent dynamics of melt bearing zones. We propose a model that relates melt viscosity to electrical conductivity for naturally occurring melt compositions (including H2O) and temperature. Based on laboratory measurements of melt conductivity and viscosity, our model provides a rheological dimension to the interpretation of electromagnetic anomalies caused by melt and partially molten rocks (melt fraction ~ >0.7)., We acknowledge partial support under NASA USRA subaward 02153–04, NSF EAR 0739050, and the ASU School of Earth and Space Exploration (SESE) Exploration Postdoctoral Fellowship Program., 2013-12-12

Thesis (S.M.)--Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2007., Includes bibliographical references (p. 43-45)., Grain size is an important material property that has significant effects on the viscosity, dominant deformation mechanism, attenuation, and shear wave velocity of the oceanic upper mantle. Several studies have investigated the kinetics of grain size evolution, but have yet to incorporate these evolution equations into large-scale flow models of the oceanic upper mantle. We construct self-consistent 1.5-D steady-state Couette flow models for the oceanic upper mantle to constrain how grain size evolves with depth assuming a composite diffusion-dislocation creep rheology. We investigate the importance of water content by examining end-member models for a dry, wet, and dehydrated mantle (with dehydration above -60-70 km depth). We find that grain size increases with depth, and varies with both plate age and water content. Specifically, the dehydration model predicts a grain size of -11 mm at a depth of 150 km for 75 Myr-old oceanic mantle. This results in a viscosity of -1019 Pa s, consistent with estimates from geoid and glacial rebound studies. We also find that deformation is dominated by dislocation creep beneath -60-70 km depth, in agreement with observations of seismic anisotropy in the oceanic upper mantle. The calculated grain size profiles are input into a Burger's model system to calculate seismic quality factor (Q) and shear wave velocity (Vs). For ages older than 50 Myrs, we find that Q and Vs predicted by the dehydration case best match seismic reference models for Q and the low seismic shear wave velocity zone (LVZ) observed in the oceanic upper mantle., by James R. Elsenbeck, II., S.M.

Submitted in partial fulfillment of the requirements of the degree Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2007, Grain size is an important material property that has significant effects on the viscosity, dominant deformation mechanism, attenuation, and shear wave velocity of the oceanic upper mantle. Several studies have investigated the kinetics of grain size evolution, but have yet to incorporate these evolution equations into large-scale flow models of the oceanic upper mantle. We construct self-consistent 1.5-D steady-state Couette flow models for the oceanic upper mantle to constrain how grain size evolves with depth assuming a composite diffusion-dislocation creep rheology. We investigate the importance of water content by examining end-member models for a dry, wet, and dehydrated mantle (with dehydration above ~60-70 km depth). We find that grain size increases with depth, and varies with both plate age and water content. Specifically, the dehydration model predicts a grain size of ~11 mm at a depth of 150 km for 75 Myr-old oceanic mantle. This results in a viscosity of ~1019 Pa s, consistent with estimates from geoid and glacial rebound studies. We also find that deformation is dominated by dislocation creep beneath ~60-70 km depth, in agreement with observations of seismic anisotropy in the oceanic upper mantle. The calculated grain size profiles are input into a Burger's model system to calculate seismic quality factor (Q) and shear wave velocity (Vs). For ages older than 50 Myrs, we find that Q and Vs predicted by the dehydration case best match seismic reference models for Q and the low seismic shear wave velocity zone (LVZ) observed in the oceanic upper mantle.