TRIASSIC RIFTING IN WESTERN PANGEA

Subcrop maps of the Triassic South Georgia Rift show that contrasting extensional styles occur along strike. Asymmetric grabens predominate to the northeast; whereas southwest of the Jacksonville transfer fault, horsts and grabens are common. A flat Moho underlies both extensional styles in southern Georgia (McBride et al., 1987; Heck, 1989) and indicates distributed deformation. A paleogeographic reconstruction of western Triassic Pangaea also shows that the South Georgia rift, the De Soto basin that lies southwest across the Bahamas transfer fault, the Eagle Mills basin of southern Arkansas, and horsts and grabens off the coast of Yucatan are parts of an originally continuous rift system. The rift system formed in thermally relaxed, thickened crust sub-parallel to, and south of, the Ouachita-Marathon suture. The zone of rifting also abruptly widens in area southwest of the Bahamas transfer fault. Simulations of Van Wijk and Blackman (2005) indicate that northeast narrowing occurred because: 1) rheological differences formed locked zones at each transfer fault; 2) locked zones temporarily stalled rift propagation; and 3) strain became highly distributed to the west-southwest as a result. Horsts and grabens further indicate a boundary control on extension and diachronous distributed deformation on opposite sides of transfer faults. Permo-Triassic, circum-Pacific subduction-related tectonics along the western margin of Pangea induced the boundary control.