THE WESTERN LIMIT OF MAJOR EXTENSION ASSOCIATED WITH THE IAPETAN RIFTED MARGIN IN THE SOUTHERN AND CENTRAL APPALACHIANS

Specifying the location and extent of rifted crystalline crust (Precambrian basement) associated with the opening of the Iapetus Ocean in the eastern United States is important for seismic hazard evaluation and for models that relate upper mantle structure to tectonic features. As currently mapped by the Central and Eastern U.S. Seismic Source Characterization for Nuclear Facilities (CEUS-SSC) Project, the western limit of major extensional faults and thinned extended crust produced by Iapetan rifting is within the Appalachian Plateau, to the west of the Valley and Ridge province. This limit was defined more than 20 years ago when little information was available about the crustal structure beneath the Appalachians and was based primarily upon the distribution of earthquakes in two intraplate seismic zones, as well as some known basement faults. New estimates, using EarthScope USArray and other data, show that the crustal thickness below the Blue Ridge, Valley and Ridge, and Appalachian Plateau provinces generally exceeds 45 km and is as much as 60 km in places. Such thick crust (equal to that typical of full-thickness continental crust) does not indicate significant extension as in rifted crust. We suggest that the western limit of major extensional faulting along the Iapetan rifted margin is located in the central Piedmont east of the Blue Ridge beneath the Blue Ridge–Piedmont megathrust sheet, and is closely associated with a prominent Bouguer gravity gradient (Appalachian gravity gradient). This location agrees with palinspastic reconstructions of the Iapetan rifted margin. Intracratonic grabens, inboard from the rifted margin, show only minor crustal extension and thinning.

Recognition of thick crust (largely unaffected by Iapetan rifting) below the Blue Ridge and Valley and Ridge provinces will allow more robust geodynamic modeling of the effects of upper mantle structure (such as delamination and abrupt changes in lithospheric thickness) on ongoing earthquake activity and features such as Cenozoic volcanism in the eastern U.S.