Byrnes Joseph S.

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Joseph S.

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  • Article
    Evaluating models for lithospheric loss and intraplate volcanism beneath the Central Appalachian Mountains
    (American Geophysical Union, 2021-09-16) Long, Maureen D. ; Wagner, Lara S. ; King, Scott D. ; Evans, Rob L. ; Mazza, Sarah E. ; Byrnes, Joseph S. ; Johnson, Elizabeth A. ; Kirby, Eric ; Bezada, Maximiliano J. ; Gazel, Esteban ; Miller, Scott R. ; Aragon, John C. ; Liu, Shangxin
    The eastern margin of North America has been shaped by a series of tectonic events including the Paleozoic Appalachian Orogeny and the breakup of Pangea during the Mesozoic. For the past ∼200 Ma, eastern North America has been a passive continental margin; however, there is evidence in the Central Appalachian Mountains for post-rifting modification of lithospheric structure. This evidence includes two co-located pulses of magmatism that post-date the rifting event (at 152 and 47 Ma) along with low seismic velocities, high seismic attenuation, and high electrical conductivity in the upper mantle. Here, we synthesize and evaluate constraints on the lithospheric evolution of the Central Appalachian Mountains. These include tomographic imaging of seismic velocities, seismic and electrical conductivity imaging along the Mid-Atlantic Geophysical Integrative Collaboration array, gravity and heat flow measurements, geochemical and petrological examination of Jurassic and Eocene magmatic rocks, and estimates of erosion rates from geomorphological data. We discuss and evaluate a set of possible mechanisms for lithospheric loss and intraplate volcanism beneath the region. Taken together, recent observations provide compelling evidence for lithospheric loss beneath the Central Appalachians; while they cannot uniquely identify the processes associated with this loss, they narrow the range of plausible models, with important implications for our understanding of intraplate volcanism and the evolution of continental lithosphere. Our preferred models invoke a combination of (perhaps episodic) lithospheric loss via Rayleigh-Taylor instabilities and subsequent small-scale mantle flow in combination with shear-driven upwelling that maintains the region of thin lithosphere and causes partial melting in the asthenosphere.
  • Article
    Joint analysis of seismic and electrical observables beneath the Central Appalachians requires partial melt in the Upper Mantle
    (American Geophysical Union, 2023-01-25) Mittal, Ved ; Long, Maureen D. ; Evans, Rob L. ; Byrnes, Joseph S. ; Bezada, Maximiliano
    The Central Appalachian Anomaly (CAA) is a region of the upper mantle beneath eastern North America that exhibits pronounced anomalies in its seismic velocity, seismic attenuation, and electrical conductivity structure. The CAA clearly expresses itself in low velocity, high attenuation, and high conductivity values; however, the present‐day composition and state of the asthenospheric upper mantle in the anomalous region remains imperfectly known. The collection of data from densely spaced, co‐located seismic and magnetotelluric arrays during the Mid‐Atlantic Geophysical Integrative Collaboration (MAGIC) experiment affords the opportunity to probe the structure and properties of the upper mantle in the CAA region in detail using multiple types of geophysical observations. Here, we present new observations of P and S wave travel times from teleseismic earthquakes measured at MAGIC stations, including a determination of how travel times deviate from the predictions of a standard 1‐D reference model. These observations constrain the ratio of the P to S wave travel time perturbations associated with the CAA, which in turn allows us to estimate the ratio of P and S wave velocity anomalies. We combine these observations with previously published estimates of seismic attenuation and electrical conductivity in the upper mantle beneath the MAGIC array, and carry out forward modeling to determine reasonable ranges of temperature, partial melt fraction, water content, and composition for the CAA. Our results suggest that 1%–2% partial melt is required to simultaneously explain the velocity, attenuation, and electrical conductivity observations beneath the MAGIC array.Key PointsWe analyze three different types of geophysical observations beneath the Central Appalachian MountainsThe upper mantle exhibits anomalies in its seismic velocity, seismic attenuation, and electrical conductivity structureForward modeling shows that 1%–2% partial melt in the upper mantle can explain the geophysical observations