Mantle anisotropy in the lithosphere and asthenosphere beneath the southeastern United States
Abstract:Using teleseismic shear-wave splitting measurements from the SESAME array and EarthScope Transportable Array, we are quantifying sources of seismic anisotropy beneath the southern Appalachians and the adjacent rifted Coastal Plain. SESAME (Southeastern Suture of the Appalachian Margin Experiment; 2010-2014), an EarthScope Flexible Array, consisted of 85 stations that spanned Georgia, northern Florida, and parts of North Carolina and Tennessee. Stations were most densely spaced in southern Georgia across the hypothesized suture between Laurentia and the exotic Suwannee terrane, which is thought to have been part of Gondwana and to have collided with Laurentia in the late Paleozoic. The region experienced rifting in the Mesozoic, leading to the creation of the South Georgia Rift Basin.
Shear-wave splitting in SKS and SKKS phases indicates the presence of significant anisotropy in the mantle. Splitting times range from 0.4 s to 3.2 s, but are typically 1.0-1.5 s. SK(K)S splitting fast directions are dominated by an overall NE-SW trend, but significant fast direction differences occur between sub-regions of the array. At stations within the higher topography of the Blue Ridge terrane, fast directions are predominantly NE-ENE, and show little systematic variation with back-azimuth. At stations further to the southeast in the Inner Piedmont and Carolina terranes, fast directions are ENE-ESE. Closer to the Atlantic coast and in a sub-region that experienced significant rifting in the Mesozoic, a strong variation of fast direction with backazimuth is observed. Spatial overlap of paths with different fast directions indicates that the backazimuthal pattern reflects a rotation of azimuthal anisotropy with depth, rather than simple lateral variations.
The laterally-rapid spatial variations between these groups indicates that the SK(K)S waves are likely sampling differences in lithospheric anisotropy between adjacent terranes, superimposed on any asthenospheric anisotropy. We are in the process of modeling the splitting observed in each group with layered models of anisotropy, including dipping olivine a-axes, with the goal of relating the splitting patterns to past orogenic and rifting processes and present-day asthenospheric flow.