Constraints on Appalachian orogenesis and continental rifting in the Southeastern United States from wide-angle seismic data

Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research-Solid Earth 124(7), (2019): 6625-6652, doi: 10.1029/2019JB017611.
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 VP and upper crustal VS. The most dramatic model transition occurs at the Higgins‐Zietz magnetic boundary, north of which we observe higher upper crustal VP and VS and lower VP/VS. 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 VP and VP/VS 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.
The SUGAR experiment would not have been possible without the help of local landowners, county and state officials, the University of Texas El Paso seismic source facility, IRIS PASSCAL instrument center, and the team of students who scouted, deployed, and recovered the geophones. We thank Jim Knapp, Susie Boote, and Ross Cao for helpful discussion and providing the sonic log data from GGS‐3080; Lindsay Worthington for discussion and sharing codes; Bradley Hacker and Mark Behn for sharing their lower crust velocity constraints; Emily Hopper and Karen Fischer for discussions; and Fred Cook for an image of processed COCORP data. We used the PyVM toolbox from Nathan Miller, the VMTomo code from Alistair Harding for tomographic inversions, VMTomo code from Harm van Avendonk for resolution tests, and the Upicker package of MATLAB scripts maintained by W. Wilcock to pick arrivals. Seismic Unix was used for data processing (Cohen & Stockwell, 2002). This project was funded by an NSF GRFP fellowship DGE 16‐44869 and a grant from the National Science Foundation's Division of Earth Sciences (NSF‐EAR) EarthScope program through the collaborative awards EAR‐1144534, EAR‐1144829, and EAR‐1144391. Robert Hawman and two anonymous reviewers provided thorough feedback that improved this manuscript. The refraction seismic data set analyzed in the current study is available on request through the IRIS Data Management Center, report number 14‐023 (http://ds.iris.edu/ds/nodes/dmc/forms/assembled‐data/). The velocity model grid files and arrival picks are available in the supporting information.
2019-12-24