SKS Splitting and Upper Mantle Anisotropy Beneath the Southern New England Appalachians: Constraints From the Dense SEISConn Array

Abstract

Introduction 1.1. Tectonic Background The bedrock geology of southern New England is extraordinary in its complexity, and reflects a range of plate tectonic processes, including subduction and terrane accretion during the Appalachian Orogeny and extension and rifting during the breakup of the Pangea supercontinent. Neoproterozoic rifting of Rodinia created the Laurentian passive margin preserved in western New England (Figure 1). Mesoproterozoic Grenvillian crust is exposed in the Green Mountain and Berkshire massifs in Vermont and Massachusetts (Kara

Systematic Mapping of Upper Mantle Seismic Discontinuities Beneath Northeastern North America

“…For the latter scenario, Lopes et al. (2020) predict roughly N‐S trending axes in the lithosphere to counteract the asthenospheric shearing aligned with APM (roughly ENE‐WSW in this region), which is remarkably consistent with our observations at sites UCCT, M63A, and L64A, down to the depth of about 70 km (Figures 5b and 5c). Given that the crustal thickness here is up to 35 km (Li et al., 2018, 2020), the collective influence of the N‐S trending anisotropy in lithospheric mantle may also be small.…”

“…The consistency between the DF and HF attributes is noted in the Result section at sites in eastern Connecticut and Massachusetts, an area corresponding to the Avalonia terrane (Figure 1b). Here, Lopes et al (2020) report a substantial reduction in the delay times from western Connecticut and suggest two end-member scenarios to explore the possible mechanism that produce the observed lateral variation: a localized transition in the asthenospheric flow, or a cancellation of the anisotropic effect between the lithosphere and the asthenosphere. For the latter scenario, Lopes et al (2020) predict roughly N-S trending axes in the lithosphere to counteract the asthenospheric shearing aligned with APM (roughly ENE-WSW in this region), which is remarkably consistent with our observations at sites UCCT, M63A, and L64A, down to the depth of about 70 km (Figures 5b and 5c).…”

“…Here, Lopes et al (2020) report a substantial reduction in the delay times from western Connecticut and suggest two end-member scenarios to explore the possible mechanism that produce the observed lateral variation: a localized transition in the asthenospheric flow, or a cancellation of the anisotropic effect between the lithosphere and the asthenosphere. For the latter scenario, Lopes et al (2020) predict roughly N-S trending axes in the lithosphere to counteract the asthenospheric shearing aligned with APM (roughly ENE-WSW in this region), which is remarkably consistent with our observations at sites UCCT, M63A, and L64A, down to the depth of about 70 km (Figures 5b and 5c). Given that the crustal thickness here is up to 35 km (Li et al, 2018(Li et al, , 2020, the collective influence of the N-S trending anisotropy in lithospheric mantle may also be small.…”

“…Here, Lopes et al. (2020) report a substantial reduction in the delay times from western Connecticut and suggest two end‐member scenarios to explore the possible mechanism that produce the observed lateral variation: a localized transition in the asthenospheric flow, or a cancellation of the anisotropic effect between the lithosphere and the asthenosphere. For the latter scenario, Lopes et al.…”

“…The depth‐integrated seismic anisotropy has been probed in detail beneath northeastern North America using shear wave splitting (SWS) methodology, exploiting the improved lateral resolution afforded by the Earthscope Transportable Array (e.g., Li et al., 2019; Long et al., 2015; Wagner et al., 2012; Yang et al., 2017). Most studies that include this region report directional variation of the splitting parameters (fast axes orientation and delay time), which indicates multilayered anisotropy within the upper mantle (Levin et al., 1999, 2000), as well as abrupt lateral transitions in splitting patterns (e.g., Levin et al., 2018; Li et al., 2019; Lopes et al., 2020). These findings together suggest complicated anisotropic contributions from both the lithosphere and the asthenosphere.…”