Middle Jurassic deformation of the Birdsboro cam basin, Mid-Atlantic region, eastern U.S.A.

Rodger T. Faill and Robert C. Smith, II (retired), Pennsylvania Geological Survey, DCNR, 3240 Schoolhouse Road, Middletown, PA 17057-3534

The Newark, Gettysburg, Culpeper, and Barboursville remnants are all that now remain of the early Mesozoic Birdsboro basin. The Birdsboro developed during the late Triassic and early Jurassic (at least to the Sinemurian, ~195Ma) as an elongate, NE-SW-trending sinuous trough in the mid-Atlantic region of eastern U.S. The basin was deformed (monoclinal rotation, faulting, folding, and penetrative flow) after the basin filled with sediment. We propose that basin deformation occurred in the middle Jurassic (Bajocian, ~170Ma), coeval with opening of the central Atlantic Ocean.

A sheet-like mantle plume under the Pangean plate began the process in the middle? Triassic. Erosion of the lower lithosphere through basal shear by the spreading plume head caused thinning and subsidence of the crust across a 300 km-wide zone centered on the Alleghany orogen. Elongate troughs at the surface accumulated sediment shed from the Alleghanides core. Fission-track analyses of zircon and titanite indicate that the plume heated the crust well above the adiabatic cooling from the Alleghany orogeny. By the beginning of the Jurassic, this heating had weakened the crust, making it more ductile, and it culminated in a brief but very widespread intrusion and effusion of tholeiitic magma. Subsequent cooling proceeded linearly throughout the Jurassic, from a level of ~275°C at 199 Ma, to ~200°C by 175 Ma, and ~100°C by 141 Ma. With this cooling, continued crustal extension led to brittle yielding (faulting).

The general N-NE trends of the dikes suggest that the crust continued to be under a general NW-SE extension. This extension probably persisted until the Bajocian, when the continents separated to form the nascent Atlantic ocean. Thus released from the extensile stress, the crust rebounded, abetted by the continued basal shear (now becoming a ridge push), in which the stress state shifted from extension to NW-directed compression. Crustal buckling rotated the Birdsboro basin northwestward, producing the monoclinal structure. As the continents were separating, the northward drift of North America (relative to Africa) created a regional dextral shear along the continental margins, which produced NW-plunging folds. By the beginning of the Cretaceous, Jurassic erosion had largely beveled the deformed Birdsboro basin to the remnants we see today.

Paper presented at Northeastern Section - 38th Annual Meeting (March 27-29, 2003)