VP/VS VARIATIONS ACROSS TERRANE BOUNDARIES IN THE SOUTHERN APPALACHIANS: IMPLICATIONS FOR ALLEGHANIAN THIN-SKINNED TECTONIC MODELS

The EarthScope SESAME broadband array is designed to investigate crustal evolution in the southeastern United States associated with Paleozoic terrane accretion, Permo-Carboniferous continental collision, and Mesozoic rifting. Previous receiver function results from stations along a NW-trending profile crossing the Carolina terrane, Inner Piedmont, and Blue Ridge provinces of the southern Appalachians show a general trend of increasing crustal thickness and average crustal Vp/Vs towards the high elevations of the Blue Ridge Mountains. Vp/Vs ratios increase from 1.69-1.72 across the Carolina terrane (stations D02-D09) to 1.76-1.77 within the Inner Piedmont (stations D15 and D17), indicating a gradual transition from felsic to intermediate bulk crustal composition to the northwest. Using additional events for H-k analysis, we obtained new constraints on azimuthal variations in crustal structure around stations D13 and D14 within the Inner Piedmont by binning receiver functions by backazimuth. Assuming an average crustal Vp of 6.5 km/s, H-k stacking using events from the northern Pacific yields estimates of 43-44 km for crustal thickness and 1.72-1.74 for average crustal Vp/Vs, whereas analysis of events from the South American trench yields estimates of 39-41 km and 1.74-1.75. The azimuthal variations in crustal thickness are consistent with the trend of increasing thickness to the northwest, and the Vp/Vs ratios from both backazimuths are indicative of a slightly more intermediate composition beneath the Inner Piedmont compared with the Carolina terrane. However, the variation in average crustal Vp/Vs does not correlate with more mafic lithologies exposed across the Carolina arc terrane, suggesting that relatively felsic Grenville-age lower crust strongly controls Vp/Vs measurements. These results are consistent with existing models of Alleghanian thin-skinned tectonics showing the Appalachian detachment extending beneath the Carolina terrane.