Synthesizing Seemingly Contradictory Seismic and Magnetotelluric Observations in the Southeastern United States to Image Physical Properties of the Lithosphere

Abstract

Although seismic velocity and electrical conductivity are both sensitive to temperature, thermal lithosphere properties are derived almost exclusively from seismic data because conductivity is often too strongly affected by minor highly conductive phases to be a reliable indicator of temperature. However, in certain circumstances, electrical observations can provide strong constraints on mantle temperatures. In the southeastern United States (SEUS), magnetotelluric (MT) data require high resistivity values (>300 Ωm) to at least 200‐km depth. As dry mantle mineral conduction laws provide an upper bound on temperature for an observed resistivity value, the only interpretation is that lithospheric temperatures (<1330 °C) persist to 200 km. However, seismic tomography shows that velocities in this region are generally slightly slow with respect to references models; this observation has led to a view of relatively thin (<150 km), eroded thermal lithosphere beneath the SEUS. We show that MT and seismic (tomography, attenuation, receiver function) results are consistent with thick (~200 km), coherent thermal lithosphere in this region. Reduced seismic velocities (relative to reference models) can be explained by considering the effect of finite grain size (anelasticity). Calculated velocity as a function of temperature is overall slower when including anelastic effects, even at reasonable grain sizes of 1 mm to 1 cm; this permits mantle temperatures that are colder than would typically be inferred. We argue for a geodynamic scenario in which the present thermal lithosphere in the SEUS formed in association with the Central Atlantic Magmatic Province and has subsequently survived intact for ~200 Ma.

Modification of Crust and Mantle Lithosphere Beneath the Southern Part of the Eastern North American Passive Margin

“…Rifting eventually led to the creation of new Atlantic Ocean lithosphere, whose present‐day thickness is estimated to be ∼120–150 km (Savage et al., 2017). In comparison, the continental lithosphere is estimated to be about 200 km thick (e.g., Evans et al., 2019; Murphy & Egbert, 2019; Savage et al., 2017). The thickness difference between the oceanic and continental lithosphere would lead to a lateral variation of temperature and viscosity and consequently trigger an edge‐driven mantle convection and asthenospheric flow beneath the oceanic‐continental margin (Ramsay & Pysklywec, 2011).…”

Insights Into Intraplate Stresses and Geomorphology in the Southeastern United States

“…It is worth noting that the crustal electrical conductors beneath the modern Appalachians that we here map to low viscosity zones are likely the geophysical signature of graphite‐bearing weak zones that formed during subduction‐related and orogenic crustal processes (Murphy & Egbert, ; Wannamaker, ). High electrical resistivities (and consequently high viscosities) beneath the Piedmont and Coastal Plain are likely due to eruption of the Central Atlantic Magmatic Province, which in part comprises a voluminous basaltic dike swarm that is almost entirely confined to those regions (e.g., Ragland et al, ; see also Murphy & Egbert, ). In addition to being highly electrically resistive themselves, the dikes could have eradicated any residual rheologically weak conductive phases during emplacement.…”

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

“…We show an approximate, schematic lithosphere‐asthenosphere boundary (LAB) as a black, dashed line. Different methods suggest inconsistent LAB depths (e.g., Hopper & Fischer, 2018; Murphy & Egbert, 2019; Pasyanos et al., 2014), so we roughly interpreted the LAB depth based on the high velocity material above ∼80–200 km in our model. Cross‐sections run from northwest to southeast (A‐A’ in Figure 3).…”