Crustal Composition and Moho Variations of the Central and Eastern United States: Improving Resolution and Geologic Interpretation of EarthScope USArray Seismic Images Using Gravity

Abstract

EarthScope's USArray Transportable Array has shortcomings for the purpose of interpreting geologic features of wavelengths less than the Transportable Array station spacing, but these can be overcome by using higher spatial resolution gravity data. In this study, we exploit USArray receiver functions to reduce nonuniqueness in the interpretation of gravity anomalies. We model gravity anomalies from previously derived density variations of sedimentary basins, crustal Vp/Vs variation, Moho variation, and upper mantle density variation derived from body wave imaging informed by surface wave tomography to estimate Vp/Vs. Although average densities and density contrasts for these seismic variations can be derived, the gravity anomalies modeled from them do not explain the entire observed gravity anomaly field in the United States. We use the unmodeled gravity anomalies (residuals) to reconstruct local variations in densities of the crust associated with geologic sources. The approach uses velocity‐density relationships and differs from density computations that assume isostatic compensation. These intracrustal densities identify geologic sources not sampled by and, in some cases, aliased by the USArray station spacing. We show an example of this improvement in the vicinity of the Bloomfield Pluton, north of the bootheel of Missouri, in the central United States.

Crustal and Upper Mantle Structure Beneath the Southeastern United States From Joint Inversion of Receiver Functions and Rayleigh Wave Dispersion

“…Pn tomography and Rayleigh wave dispersion and RF joint inversion studies (Buehler & Shearer, 2017; Shen & Ritzwoller, 2016) utilizing data from the USArray reveal a southeastward thinning of the crust across the SAM, a feature that has been pervasively observed by numerous passive or active source seismic investigations (see below) and is consistent with the seaward decrease in elevation and increase in Bouguer gravity anomalies (Zhang et al., 2020). Depending on the assumed crustal velocities, the estimated crustal thickness (H) can differ by up to 5 km in the SEUS (Buehler & Shearer, 2017).…”

“…Crustal Vp/Vs () values that are lower than the typical bulk crustal of 1.78 (Christensen, 1996) are observed in the Carolina Terrane and the Inner Piedmont (Hawman et al., 2012; Ma & Lowry, 2017; Parker et al., 2013; Zhang et al., 2020), and a crustal delamination/relamination hypothesis is proposed to explain the low measurements (Parker et al., 2013). Overall, the Appalachian Mountains show some of the highest crustal shear velocities in the continental United States (e.g., Porter et al., 2016; Shen & Ritzwoller, 2016), an observation that is consistent with the hypothesis that crustal densities increase beneath older mountain ranges (Fischer, 2002).…”

Mantle Tomography of Central‐Eastern USA: Influence of Inversion Volume Size

“…The continental lithosphere, that is, the North American Craton (NAC), has remained stable for over 1.3 Ga despite subsequent reworking and deformation with the assembly and breakup of the Rodina and Pangaea supercontinents. In the past decade, seismic velocity structure of the crust and lithospheric mantle beneath the North American continent has been investigated by several techniques, including body wave tomography (Bollmann et al., 2019; Boyce et al., 2019; Chen et al., 2014; Porritt et al., 2014; Savage, 2021; Schmandt & Lin, 2014; Sigloch, 2011; H. Wang et al., 2019), receiver functions (L. Liu & Gao, 2018; Ma & Lowry, 2017; McGlannan & Gilbert, 2016; Yang et al., 2017; H. L. Zhang et al., 2020), full‐waveform inversion (Krischer et al., 2018), ambient noise tomography (Savage et al., 2017; Wu et al., 2020), surface wave tomography (Ekström, 2017; Pollitz & Mooney, 2016; Shen & Ritzwoller, 2016), and simultaneous inversion of receiver function, Rayleigh wave phase velocity and Bouguer gravity (Chai et al., 2022). These studies have provided a wealth of information on structural heterogeneities in the crust and lithosphere.…”

Seismic Evidence for Metamorphic Densification of the Lower Continental Crust in Eastern North America

“…Further, the western and eastern margins of the Grenville Province and the boundary between the United States and Canadian portions of the Grenville Province are characterized by particularly fast and dense lower crust. Several previous studies have also suggested the presence of a relatively high‐velocity layer in the lower crust in parts of eastern North America, including northern New York (Hughes & Luetgert, 1992; Shalev et al., 1991), the southern Appalachians (Wagner et al., 2012), and the central and eastern United States in general (Schulte‐Pelkum et al., 2017; Zhang et al., 2020). Our new findings provide comprehensive maps of thickness, average velocity, and average density of the lowermost crust of eastern North America which, in turn, provide new constraints on its composition and evolution as briefly discussed below.…”