Electrical conductivity structure of southeastern North America: Implications for lithospheric architecture and Appalachian topographic rejuvenation

Abstract

We present the first three-dimensional view of the lithospheric electrical conductivity structure beneath southeastern North America. By inverting EarthScope long-period magnetotelluric (MT) data, we obtain an electrical conductivity image that provides new insights into both the architecture of the Appalachian Orogen and the cryptic post-rifting geodynamic history of the southeastern United States. Our inverse solutions reveal several elongate electrically conductive features that we interpret as major terrane sutures within the Appalachian Orogen. Most significantly, we resolve a highly electrically resistive layer that extends to mantle depths beneath the modern Piedmont and Coastal Plain physiographic provinces. As high resistivity values in mantle minerals require cold mantle temperatures, the MT data indicate that the sub-Piedmont thermal lithosphere must extend to greater than 200 km depth. This firm bound conflicts with conclusions from seismic results. The boundary between the anomalously thick, resistive sub-Piedmont lithosphere and the relatively thin, moderately conductive sub-Appalachian lithosphere corresponds within resolution to the modern Appalachian topographic

Source biases in midlatitude magnetotelluric transfer functions due to Pc3-4 geomagnetic pulsations

“…Results for a small array located over the highly resistive Piedmont Resistor of Murphy and Egbert (2017), where all six sites show TF biases at ~ 30 s (analyses from two of these sites are shown in Fig. 2), are shown in Fig.…”

“…1) suggests that to first order this problem will be most severe directly over resistive Earth. This pattern is clearest in the central USA, over resistive Archean-to Paleoproterozoicaged cratonic lithosphere that was imaged by Yang et al (2015), and in the southeastern USA, over the highly resistive Piedmont Resistor of Murphy and Egbert (2017). Fig.…”

“…For example, the pervasive presence of biases in the TF estimates from the Piedmont and Coastal Plain regions of the southeastern USA is an additional strong line of evidence for the highly resistive, highly anomalous Piedmont Resistor documented by Murphy and Egbert (2017).…”

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

“…Therefore, in the upper mantle, maximum bulk resistivity cannot exceed the resistivity of dry silicate minerals (Figure 1; see also Murphy & Egbert, 2017). Therefore, in the upper mantle, maximum bulk resistivity cannot exceed the resistivity of dry silicate minerals (Figure 1; see also Murphy & Egbert, 2017).…”

“…Regardless, well-resolved resistivity values are often much lower than would be expected under dry conditions, so trace conductive phases and water appear to be ubiquitous (e.g., Eaton et al, 2009;Jones et al, 2013;Selway, 2014;cf. The southeastern United States (SEUS; Figure 2) is one notable location where well-resolved electrical resistivity values are high enough that electrical conduction mechanisms other than dry temperature-dependent conduction can be ruled out (Murphy & Egbert, 2017). Consequently, regions where MT can provide a firm constraint on temperature are relatively rare.…”