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 crosssections 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.

Revisiting the Appalachian structural front and offshore Anticosti Basin (northern Gulf of St. Lawrence, Canada) by integrating old and new geophysical datasets

“…11) that postdate the early Silurian (as sediments younger than the Chicotte Formation are involved) and likely result from the Middle Devonian Acadian deformation event. This interpretation is substantiated by the strong similarities in structural style between the fold and fault belt located in the Honguedo Strait and the Silurian-Devonian Gaspé Belt in northeastern Gaspé Peninsula, the latter being characterized by broad open synclines, narrow anticlines and NW-striking faults with a dextral strike separation of lithologic contacts, axial trace of folds and older east-trending faults (McGerrigle, 1950;Bourque et al, 2001, Pinet et al, 2008. Most of deformation of the Gaspé belt is attributed to the Acadian orogeny.…”

“…Other deformation events do not alter the overall geometry of the Gaspé belt and include early syn-sedimentary normal faulting along the NW-striking faults (Lavoie, 1992;Bourque, 2001) and a late increment of deformation marked by the dextral offset of the Carboniferous Tar Point dyke (Fig. 7, Pinet et al, 2008).…”

“…Offshore of Gaspé Peninsula and Newfoundland, the southern boundary of the basin corresponds to a series of thin-to thick-skinned Appalachian structural slices (Waldron et al, 1993;Stockmal et al, 1998Stockmal et al, , 2004Pinet et al, 2010;Pinet, 2011). Deformation of the frontal Appalachian thrust sheets has been attributed to a long-lived tectonic history including the Taconian (Ordovician), Salinian (Silurian) and Acadian (Devonian) tectonic events (van Staal et al, 2009).…”

“…However, the post-earliest Silurian age of the platform sediments at the contact indicates that the interpretation in which the frontal structure of the Appalachian orogenic wedge is a Middle Ordovician thrust is false. As noted by Pinet et al (2010) alternative hypotheses include: 1) a triangle zone geometry as suggested by vintage seismic data (Fig. 4) and by the interpretation of a wide (90 km NeS) positive (10e32 mGal) gravity anomaly that covers the eastern part of the Gaspé Peninsula and surrounding offshore areas (Pinet et al, 2010); 2) an unconformable relationship between the upper part of the platform succession and the Appalachians and 3) a thick-skinned reactivation of Taconian structures.…”

“…7B) with a magnetic susceptibility of 0.007 cgs (around 88 Â 10 À3 SI). However, geological interpretation of this high magnetic susceptibility body remains hypothetical as it may be related either to the Grenville orogen or to an extensive late Neoproterozoic-early Cambrian magmatic episode (Pinet et al, 2010).…”