Thin lithosphere beneath the central Appalachian Mountains: A combined seismic and magnetotelluric study

Evans, Robert; Benoit, M. H.; Long, Maureen D.; Elsenbeck, J.; Ford, H. A.; Zhu, Jasmine; García, Xavier

doi:10.1016/j.epsl.2019.04.046

Cited by 30 publications

(99 citation statements)

References 44 publications

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“…This mechanism is generally consistent with the inference that there may be partial melt in the uppermost mantle beneath the CAA today, as suggested by Evans et al. (2019) and Byrnes et al. (2019), as ongoing shear‐driven upwelling would result in a continuous process of decompression melting.…”

Section: Which Models Are Most Consistent With the Observations?supporting

confidence: 89%

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

et al. 2021

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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...

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Section: Which Models Are Most Consistent With the Observations?supporting

confidence: 89%

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

et al. 2021

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“…The physical properties (such as age, thickness, composition, temperature, and viscosity) of continental lithosphere contain crucial information about its formation and evolution, and more fundamentally, about Earth's tectonic dynamics through geological time. The thickness of cratonic lithosphere, revealed by fast seismic velocities, is on average ∼200–250 km (e.g., Cammarano & Romanowicz, 2007; Bedle & van der Lee, 2009; Fischer et al., 2020; Hamza & Vieira, 2012; Kind et al., 2020; Schaeffer & Lebedev, 2014), roughly in agreement with estimates from xenolith analysis (O'Reilly & Griffin, 2010; Crépisson et al., 2014), heat flow (Mareschal & Jaupart, 2004), and magnetotelluric data (Adetunji et al., 2014; Evans et al., 2019). It is commonly agreed that cratonic lithosphere is thicker, colder, and more stable than younger continental lithosphere.…”

Section: Introductionsupporting

confidence: 69%

Lithospheric Formation and Evolution of Eastern North American Continent

Gao

2021

Geophysical Research Letters

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Lithospheric layering contains critical information about continental formation and evolution. However, discrepancies on the depth distributions of lithospheric layers have significantly limited our understanding of possible tectonic connections among the layers. Here, we construct a high‐resolution shear velocity model of eastern North America using full‐wave ambient noise simulation and inversion by integrating onshore and offshore seismic datasets. Our new model reveals large lateral variations of lithosphere thickness approximately across the major tectonic boundaries, strong low‐velocity anomalies underlying the thinner lithosphere, and multiple low‐velocity layers within the continental lithosphere. We suggest that the present mantle lithosphere beneath eastern North America was formed and modified through multiple stages of tectonic processes, among which metasomatism may have significantly contributed to the observed intralithospheric low‐velocity layers. The sharp thickness variation of lithosphere promoted edge‐driven mantle convection, which has been consequently modifying the overlying mantle lithosphere and further sharpening the gradient of lithosphere thickness

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“…Those might correspond to plumes formed under the condition of X MTZ ≥ X C2 . We also note that geophysical anomalies in the Appalachian (Evans et al 2019) might represent evidence for a wet plume associated with the breakup of the Pangea. In addition to geophysical observations, geochemical observations suggest the important contributions of MTZ materials to the composition of some volcanic rocks (e.g., Bonatti 1990;Kuritani et al 2013;Li et al 2020;Metrich et al 2014;Nichols et al 2002;Sobolev et al 2019).…”

Section: Evidence Of Wet Plumesmentioning

confidence: 81%

Deep mantle melting, global water circulation and its implications for the stability of the ocean mass

Karato

Karki

Park

2020

Prog Earth Planet Sci

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Oceans on Earth are present as a result of dynamic equilibrium between degassing and regassing through the interaction with Earth’s interior. We review mineral physics, geophysical, and geochemical studies related to the global water circulation and conclude that the water content has a peak in the mantle transition zone (MTZ) with a value of 0.1–1 wt% (with large regional variations). When water-rich MTZ materials are transported out of the MTZ, partial melting occurs. Vertical direction of melt migration is determined by the density contrast between the melts and coexisting minerals. Because a density change associated with a phase transformation occurs sharply for a solid but more gradually for a melt, melts formed above the phase transformation depth are generally heavier than solids, whereas melts formed below the transformation depth are lighter than solids. Consequently, hydrous melts formed either above or below the MTZ return to the MTZ, maintaining its high water content. However, the MTZ water content cannot increase without limit. The melt-solid density contrast above the 410 km depends on the temperature. In cooler regions, melting will occur only in the presence of very water-rich materials. Melts produced in these regions have high water content and hence can be buoyant above the 410 km, removing water from the MTZ. Consequently, cooler regions of melting act as a water valve to maintain the water content of the MTZ near its threshold level (~ 0.1–1.0 wt%). Mass-balance considerations explain the observed near-constant sea-level despite large fluctuations over Earth history. Observations suggesting deep-mantle melting are reviewed including the presence of low-velocity anomalies just above and below the MTZ and geochemical evidence for hydrous melts formed in the MTZ. However, the interpretation of long-term sea-level change and the role of deep mantle melting in the global water circulation are non-unique and alternative models are reviewed. Possible future directions of studies on the global water circulation are proposed including geodynamic modeling, mineral physics and observational studies, and studies integrating results from different disciplines.

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Thin lithosphere beneath the central Appalachian Mountains: A combined seismic and magnetotelluric study

Cited by 30 publications

References 44 publications

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

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

Lithospheric Formation and Evolution of Eastern North American Continent

Deep mantle melting, global water circulation and its implications for the stability of the ocean mass

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