Constraints on Appalachian Orogenesis and Continental Rifting in the Southeastern United States From Wide-Angle Seismic Data

The Southeastern United States is an ideal location to understand the interactions between mountain building, rifting, and magmatism. Line 2 of the Suwannee suture and Georgia Rift basin refraction seismic experiment in eastern Georgia extends 420 km from the Inner Piedmont to the Georgia coast. We model crustal and upper mantle V_P and upper crustal V_S. The most dramatic model transition occurs at the Higgins-Zietz magnetic boundary, north of which we observe higher upper crustal V_P and V_S and lower V_P/V_S. These observations support the interpretation of the Higgins-Zietz boundary as the Alleghanian suture. North of this boundary, we observe a low-velocity zone less than 2 km thick at ~5-km depth, consistent with a layer of sheared metasedimentary rocks that forms the Appalachian detachment. To the southeast, we interpret synrift sediments and decreasing crustal thickness to represent crustal thinning associated with the South Georgia Rift Basin and subsequent continental breakup. The correspondence of the northern limit of thinning with the interpreted suture location suggests that the orogenic suture zone and/or the Gondwanan crust to the south of the suture helped localize subsequent extension. Lower crustal V_P and V_P/V_S preclude volumetrically significant mafic magmatic addition during rifting or associated with the Central Atlantic Magmatic Province. Structures formed during orogenesis and/or extension appear to influence seismicity in Georgia today; earthquakes localize along a steeply dipping zone that coincides with the northern edge of the South Georgia Basin and the change in upper crustal velocities at the Higgins-Zietz boundary.