Early Mesozoic rift basins of eastern North America and their gravity anomalies: The role of detachments during extension

Abstract

Two end‐member models of extension involving detachments have been developed. One model incorporates a fault that soles at mid‐crustal level overlying a broad region of pure shear in the lower crust. The second, referred to as the simple shear model, includes a detachment continuing through the entire crust and terminating in a region of concentrated extension in the lower crust. Both models predict basins with no localized thermal effect. With the inclusion of flexural isostasy, both models predict footwall uplift whose amplitude and wavelength are controlled by the detachment geometry and the lithospheric strength. A gravity anomaly over the hanging wall block distinguishes the simple shear model from the intracrustal detachment model. The early Mesozoic basins of the eastern North America, believed to have formed as the result of the normal‐slip reactivation of a Paleozoic thrust system as the Atlantic opened, are associated with distinctive hanging wall gravity highs. These gravity highs, the basin geometry, the lack of a thermal subsidence phase in the rift basins, and the presence of a highly extended and heated region to the east, suggest that the simple shear model may be applied. The simple shear model fits the outer hanging wall anomaly and permits a region of lower crustal extension to be mapped. These basins contain an abundance of basalt flows and diabase sills despite the lack of evidence for regional heating or thermal subsidence, implying that a source, external to the basin, must exist for this magmatic material. The detachment fault may facilitate the movement of the molten mafic material into the rift basins from an offshore region of greater heating and extension. The addition of 2 km of mafic material along the model detachment accounts for the observed inner gravity high and the lack of a negative gravity anomaly across these basins. The match of this modified simple shear model to the observed gravity suggests that the region of greater extension seaward of the hinge zone is the source for the widespread dikes and sills within the basins and the coastal plain and helps explain the geochemical homogeneity of these intrusives and extrusives along 2000 km of the eastern seaboard.