Pre-magmatic and magma-assisted rifting of the Eastern North American Margin

Abstract:

Magmatic and tectonic processes contribute to the formation of volcanic continental margins. Such margins are thought to undergo extension across a narrow zone of lithospheric thinning (~100 km). New observations based on both existing and reprocessed data from the Eastern North American Margin contradict the hypothesis tying narrow thinning zones to volcanic margins. Using 64,000 km of 2D seismic reflection data tied to 40 wells and published geological and geophysical data, we quantified along-strike variations in the distribution of rift structures, magmatism, crustal thickness, and early post-rift sediments in an area extending from Cape Hatteras to the Canadian border. This area consists of three segments: the Baltimore Canyon trough (BCT), the Long Island Platform, and the Georges Bank Basin (GBB). Results indicate that BCT is narrow (80-120 km), has few rift basins, and consists of thin crust overlaid by a thick volcanic succession that approximates the hinge line. In contrast, the GBB is wide (~200 km), has many syn-rift structures and contains seaward dipping reflectors located ~ 200 km seaward of the hinge line. Early post-rift subsidence at the GBB coincided with thinner crust suggesting “uniform” thinning of the entire lithosphere under the pre-magmatic rift. Models for the formation of volcanic margins do not explain the wide structure of the GBB. We distinguish between pre-magmatic (before ~195 Ma) and magma-assisted modes of rifting. Variant crustal compositions were determined by the distribution of pre-rift crustal terranes and controlled the pre-magmatic response of the lithosphere to extension. These terranes were accreted to the ENAM during a prolonged Paleozoic convergence to form an asymmetric lithospheric structure with rheological variability. In the BCT, strong rheology resisted necking of the cold lithosphere whereas weaker crustal compositions of the GBB promoted wide pre-magmatic necking. During the magma-assisted phase of the rift, steep geothermal gradients overwhelmed compositional controls on the rheology and facilitated regional strain localization. At the BCT, the localized crustal thinning coupled with intense magmatism acted on a crust unthinned by the pre-magmatic rifting. In contrast, at the GBB, magma-assisted rifting was superimposed on an already thinned crust.