RIFT ARCHITECTURE AND TECTONIC EVOLUTION OF THE LATE TRIASSIC TAYLORSVILLE AND RICHMOND RIFTS, VIRGINIA

New models of the structural and stratigraphic architecture of the Late Triassic Taylorsville and Richmond rifts in Virginia and Maryland reveal a complex history including multiple phases of extension and compression that formed and deformed the rifts during the incipient breakup of Pangea and the early phases of passive margin development. Especially intriguing are early episodes of basin inversion under Late Triassic compression. These stratigraphic and tectonic models for the Taylorsville (TVB) and Richmond (RB) basins are well constrained by a large set of subsurface data including over 17 km of cores and well cuttings and about 250 km of seismic profiles. The TVB and RB contain about 4.5 km and 2.2 km of fluvial and lacustrine rocks, respectively, in two unconformity-bounded tectonostratigraphic sequences. A series of distributed, sub-regional half-graben, including the Sequence I strata in the TVB and RB, formed during the early stages of rifting. As extension progressed, some of the sub-regional half-graben normal faults linked to form regional-scale master border fault systems, such as the large (180 km x 50 km) Sequence II half graben of the TVB. The establishment of the regional normal fault created a stress reduction zone, within which the smaller normal faults stopped subsiding. An erosional unconformity was formed between Sequence I and Sequence II during the linkage of the sub-regional-scale normal faults. Late-stage normal faults cut obliquely through basins and form rider blocks on the western margin basins. In the final stage of basin deformation, regional compression created reverse faults and folds at approximately 201 Ma, roughly coincident with the emplacement of diabase dikes. This study shows that the transition from rift (extensional) to drift (compressional) tectonics in the mid-Atlantic area was accomplished by the Early Jurassic and that the transition was asynchronous along the western Atlantic margin.