A dynamic model of extension in the Baltimore Canyon Trough Region

Abstract

A dynamic model of extension in the Baltimore Canyon trough region of the central North Atlantic indicates that rifting proceeded in two stages and was controlled by laterally offset preexisting weaknesses within the middle crust and upper mantle beneath the Appalachian orogen. The crustal weakness is hypothesized to result from thin‐skinned thrust faulting in the Appalachian foreland, and the upper mantle weakness results from crustal thickening beneath the Appalachian hinterland. During the early stages of rifting (225–185 Ma), the loci of extension in the crust and mantle lie within the respective preweakened regions and are connected by a shear zone in the lower crust in a manner similar to whole lithosphere simple shear. Extension and thinning of the upper crust at this time results in the formation of Late Triassic‐Early Jurassic rift basins in the Appalachian Piedmont and in the development of a horizontal pressure gradient which drives widespread ductile flow in the lower crust. The pressure gradient may also have influenced magma transport, moving melt generated beneath the eastern Baltimore Canyon trough into the more westerly Triassic basin province. During the late stages of rifting (185–175 Ma), extension in the crust shifts to a position overlying the preexisting upper mantle weakness as a result of thermal weakening by the upwelling asthenosphere in this region. The loci of extension in the crust and mantle are then coincident, and the style of rifting becomes similar to whole lithosphere pure shear. The shift in the locus of crustal extension is accompanied by the cessation of subsidence and volcanism in the Triassic basins, the onset of rapid subsidence in the Baltimore Canyon trough, and the formation of a basement hinge zone separating the highly thinned crust beneath the Baltimore Canyon trough from moderately thinned crust beneath the Triassic basin province. Extension becomes increasingly focused beneath the eastern edge of the Baltimore Canyon trough until the onset of seafloor spreading, approximately 50 m.y. after rifting began. The resulting conjugate margins are asymmetric, with most of the region encompassing the Triassic basins stranded on the North American plate.