Forward potential-field modeling of the Appalachian orogen in the Gaspe Peninsula (Quebec, Canada): Implications for the extent of rift magmatism and the geometry of the taconian orogenic wedge

Abstract

The nature and extent of potential source bodies associated with prominent, regional-scale gravity and magnetic anomalies in the northern Québec Appalachians are investigated through forward modeling. Potential field anomalies are interpreted to be the geophysical expression of basalt flows extruded during the Neoproterozoic to Cambrian rifting of Laurentia, prior to opening of Iapetus. Geophysical modeling provides substantial new constraints on seismic and structural cross-sections and the map distribution of volcanism. This study indicates that the volcanism was significantly more widespread than previously recognized. Basaltic rocks form a sheet up to 4 kilometers thick, transported on the main Taconic décollement. Most of this sheet is deeply buried and overlain by a thick pile of Paleozoic sediments. Later tectonic modifications by backthrusting and hinterland-directed normal faulting as well as by Acadian strike-slip faulting substantially changed the geometry of the transported volcanic rocks and the structural style both across and along the strike of the orogen. The extension of the gravity anomaly under the St. Lawrence River also suggests that the Ordovician deformation front is 20 to 30 kilometers further to the north than previously interpreted.