Evaluating Models for Lithospheric Loss and Intraplate Volcanism Beneath the Central Appalachian Mountains

Abstract

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. Plain Language SummaryFor the past 200 million years, the east coast of North America has been situated in the middle of a tectonic plate. Contrary to the expectations for this setting, a region of the Central Appalachian Mountains centered near the boundary between the U.S. states of Virginia and West Virginia exhibits atypical properties. The unusual observations include volcanic activity in the geologic past far away from a plate boundary, elevated rates of erosion associated with high topography in the Central Appalachians, and anomalous structure in the upper mantle that has been detected using geophysical methods. This article describes, synthesizes, and compares a suite of observations that show that this part of the Central Appalachians is unusual compared to other so-called passive continental margins. We discuss a range of different models that might describe how the lithosphere, or the rigid part of the crust and upper mantle that defines the tectonic plate, has evolved through time beneath our study region. We show that the lithosphere today is thin, and that past episodes of lithospheric loss involving a portion of dense lithosphere "dripping" into the mantle under the force of gravity may provide a goo...

Mantle Structure and Flow Across the Continent‐Ocean Transition of the Eastern North American Margin: Anisotropic S‐Wave Tomography

“…The high velocity lithosphere shallows toward the ocean until it meets the most prominent slow-velocity feature in our model, in Virginia (∼38.5°N, 79°W). This is the previously imaged low-velocity HA (e.g., Long et al, 2021;Savage et al, 2017;Shen & Ritzwoller, 2016;Wagner et al, 2018). This feature dips oceanward from the surface.…”

“…EDC is an attractive hypothesis for explaining a variety of discontinuous, short-wavelength, upper mantle velocity features imaged here and elsewhere without invoking multiple processes (Menke et al, 2016). Another appealing explanation of the HA is shear-driven upwelling (e.g., Long et al, 2021). This is highly localized asthenospheric upwelling caused by lithospheric motion in the case that the LAB has concave-down divots (Figure 7; Conrad et al, 2010).…”

“…Conversely, the slow velocity features may indicate lithospheric delamination and asthenospheric upwelling, which could also account for enigmatic Eocene volcanism (Figure 1; e.g., Biryol et al, 2016;Mazza et al, 2014). Shear-driven upwelling of asthenosphere into a divot in the moving lithosphere has also been suggested to explain mantle anomalies and volcanism (Long et al, 2021). Margin-parallel shear-wave splitting results offshore have been interpreted as reflecting large scale density-driven flow (Lynner & Bodmer, 2017).…”

Uncovering the Controls on Fluvial Bedrock Erodibility and Knickpoint Expression: A High‐Resolution Comparison of Bedrock Properties Between Knickpoints and Non‐Knickpoint Reaches

“…Superimposed on the prevailing lithologic fabric of the Appalachian landscape are smaller geomorphic signals of mid‐Cenozoic topographic rejuvenation, responsible for an estimated 40–400 m of additional relief (Gallen et al., 2013; Rowley et al., 2013). Absent of active tectonic uplift, potential external drivers of this rejuvenation include buoyant mantle upwelling (Gallen et al., 2013; Long et al., 2021; Moucha et al., 2008; Rowley et al., 2013; S. R. Miller et al., 2013), and climatic fluctuations (Harbor et al., 2005; Prince & Spotila, 2013). Compelling autogenic controls have been proposed as well, namely drainage reorganization/stream capture events driven by divide asymmetry (Prince et al., 2010, 2011) or exhumation of heterogeneous bedrock (Gallen, 2018).…”

A Generalized Strategy From S‐Wave Receiver Functions Reveals Distinct Lateral Variations of Lithospheric Thickness in Southeastern Tibet

“…The precise positioning of the LAB provides good constraints on the evolution of cratons and orogens, such as lithospheric thinning owing to extension (e.g., Buck, 1991) or detachment (e.g., Davies & Von Blanckenburg, 1995; Kosarev et al., 1999) and lithospheric thickening owing to collision (e.g., Houseman et al., 1981). A series of methods, such as receiver functions (P‐ and S‐wave receiver function; PRF and SRF, respectively), SS precursors, and body‐wave and surface‐wave tomography, have been developed to detect the lithospheric thickness at both global (Bijwaard & Spakman, 2000; Ritsema et al., 2004; Ritzwoller et al., 2002) and local scales (Long et al., 2021; Mojaver et al., 2021; Rawlinson & Fishwick, 2012; Zhang et al., 2021).…”