Across-strike geometry of the Grand Pabos fault zone: evidence for Devonian dextral transpression in the Quebec Appalachians

Abstract

The principal faults of southeastern Gaspé Peninsula in Quebec consist of a central high-strain zone that is characterized by mainly ductile deformation structures and bordered by low-strain zones each dominated by brittle deformation structures. The overall geometry of shear fractures within the low-strain zones is quite similar to the expected geometry of Riedel shear fractures. The brittle structures overprint the dominant C–S-type fabric of the high-strain zone, which implies that brittle deformation outlasted ductile deformation. The asymmetry of local micro- to meso-scale deformation features along the fault zones reflects the non-coaxiality of the shear. Other features described within the fault zone (stylolitic cleavage, shear bands, and reverse faults) are evidence for a component of shortening perpendicular or oblique to the fault zone. The geometry of the Grand Pabos fault zone (GPFZ), a major fault of southern Gaspé, indicates that deeper seated fault rocks (high-strain zone) have been brought up to higher crustal levels and are presently in contact with brittlely deformed fault rocks (low-strain zone). The proposed model for the evolution of the GPFZ involves Early to Late Devonian, dextral, transcurrent movement accompanied by relatively minor amounts of vertical slip within a dextral transpressive regime. The main pulse of the Acadian orogeny in Gaspé is restricted to the Devonian and therefore occurred later than elsewhere in the Canadian Appalachians.