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

Abstract

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.

Limited and localized magmatism in the Central Atlantic Magmatic Province

“…We identified refractions through the sedimentary fill, crust, and upper mantle, and reflections off the base of the sedimentary basin (Line 1 only) and the Moho. Travel-time picks of these phases were used to invert for P-wave velocity structure of the sedimentary basins, crust, and upper mantle using the code VMTomo [39][40][41].…”

“…On Line 2 across the eastern South Georgia Rift, the lower crust is almost uniformly <7 km s −1 , implying limited to absent mafic addition to the lower crust. The crust thins abruptly from 38 to 32 km over a distance of 40 km centered at 150 km on the Line 2 transect, which is likely controlled by the Alleghanian suture serving as either a pre-existing weak zone or rheological boundary between Laurentian and Gondwanan crust 39 . In contrast, on Line 1 across the western South Georgia Rift, >7.0 km s −1 lower crustal velocities are observed in the center of the seismic line, but decrease to <7.0 km s −1 to either side ( Fig.…”

“…We infer that thinning of the crust beneath the South Georgia Basin (Fig. 1) was associated with the extension to form this rift basin and that the southeastern portion of Line 2 was likely additionally thinned later during the breakup of Pangea~175-195 Ma [26][27][28]39 . Although the exact timing of continental breakup is uncertain, we would expect this successful rifting event to concentrate crustal stretching and magmatic intrusions towards the margin.…”

“…Formation of the South Georgia Basin was followed by the emplacement of CAMP magmatism~201 Ma 32 , and ultimately the breakup of Pangea~175-195 Ma [26][27][28] . Tectonic sutures that formed during multiple stages of Appalachian orogenesis before rifting 37,38 helped localize extension and the formation of rift basins across eastern North America 33 , including the South Georgia Rift Basin 39 .…”

Seismic imaging of a mid-crustal low-velocity layer beneath the northern coast of the South China Sea and its tectonic implications

“…For example, the well-known mid-crustal LVL in Tibet was proposed to host channel flow, playing an important role in accommodating lithospheric deformations during extension (Nelson et al, 1996;Beaumont et al, 2001;Bao et al, 2015;Sun and Zhao, 2020), however, its geometry, connectivity and therefore importance are still debated (Hetényi et al, 2011). The shallow-angle, long-extent and low-velocity detachments observed in Himalaya (Schulte-Pelkum et al, 2005;Caldwell et al, 2013;Subedi et al, 2018), Central Andes (Yuan et al, 2000) and eastern America (Long et al, 2019;Marzen et al, 2019) indicate a fundamental deformation mode in continental collision zones. In several cases, intra-crustal LVLs result from accretionary complexes that reflect a tectonic transformation from subduction to extension (Smit et al, 2016).…”