Abstract

Deep seismic reflection data, allowing crustal and upper mantle reflectors to be delineated, were collected across the sedimentary basins in the Grand Banks region of eastern Canada. Interpretation of the seismic results indicates that the narrow, half-grabens on the Grand Banks formed by extension, which was accommodated on major faults. These bound each of the basins and extend deep into the crust, possibly flattening along the Moho or just above it. Moho reflections show little topography when displayed against two-way travel time, although when converted to depth they will rise slightly beneath the lower velocity sediments of the basins. A reflective lower crustal layer is also observed below the platform regions of the Grand Banks. This layer becomes less pronounced below the basins, and the explanation for this is unknown.

The geometry of the basins, faults and Moho are used to constrain extensional models of basin evolution. Extension along major crustal faults appears to describe upper crustal stretching, whereas the lower crust and subcrustal lithosphere are assumed to undergo penetrative extension. The stretching model based on these assumptions satisfies most observations across the basins, such as the basin shape and the free air gravity anomaly. However, isostatic compensation of the load of sediments and water cannot be modelled by Airy or local isostasy; an elastic plate with finite flexural rigidity was required to support these loads during both the syn-rift and post-rift basin history. Both the seismic data and the modelling results support the idea of extension along major faults in the crust and penetrative extension in the lower crust and subcrustal lithosphere, causing localized deformation along major crustal faults underlain by a broad zone of ductile flow.