INTRA-CRUSTAL DISCONTINUITIES IN THE SOUTHERN APPALACHIANS: EVIDENCE FOR SHEAR LOCALIZATION DURING CONTINENTAL COLLISION

Convergent deformation associated with the Late Paleozoic Alleghanian orogeny in the southern Appalachians is characterized by over-thrusting of an orogenic wedge comprised of accreted terranes over stable Grenville-age continental crust. COCORP and ADCOH seismic reflection profiles across the region suggest that the Blue Ridge-Inner Piedmont (BR/IP) crystalline thrust sheet directly overlies a ~3-km thick Paleozoic passive margin sequence deposited on Grenville basement, but the southeastward extent of the shelf sequence and geometry of the Appalachian detachment beneath the Carolina terrane remains uncertain. We use high-frequency receiver functions from the SESAME broadband array and EarthScope Transportable Array to track the detachment from the southern Appalachian fold-and-thrust belt to the southeast beneath the Carolina terrane. We observe a laterally continuous negative-polarity arrival across the region at 0.8-1.5 seconds (6-13 km) that correlates with the proposed contact between sedimentary rocks and Grenville basement imaged beneath the BR/IP thrust sheet. The negative conversion is interpreted to mark the base of a high shear-wave velocity (Vs=3.8-4.0 km/s) carbonate/sandstone sequence, possibly thickened by duplexing, overlying lower velocity Grenville basement gneisses (3.4-3.6 km/s). Alternatively, the negative conversion may result from the top of a low-velocity (3.2-3.3 km/s), anisotropic shear zone comprised of weak meta-sedimentary strata (e.g. schist, phyllite) that formed during NW-directed shearing between overthrust terranes and Grenville basement. In the Carolina terrane, overthrusting of high-velocity arc rocks (4.0-4.2 km/s) over Grenville basement also provides a simple explanation for the decrease in velocity at depths of 8-13 km. The results are consistent with a low-angle detachment between the Carolina terrane and Grenville lower crust formed within weak passive margin strata, effectively localizing Alleghanian deformation in the upper crustal orogenic wedge.