ACROSS-STRIKE GEOMETRY OF THE GRAND PABOS FAULT ZONE - EVIDENCE FOR DEVONIAN DEXTRAL TRANSPRESSION IN THE QUEBEC APPALACHIANS

The principal faults of southeastern Gaspe 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 dominate d by brittle deformation structures. The overall geometry of shear fra ctures within the low-strain zones is quite similar to the expected ge ometry 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 lo cal micro- to meso-scale deformation features along the fault zones re flects the non-coaxiality of the shear. Other features described withi n 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 Gaspe, indicates that deeper seated fault ro cks (high-strain zone) have been brought up to higher crustal levels a nd are presently in contact with brittlely deformed fault rocks (low-s train 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 transpress ive regime. The main pulse of the Acadian orogeny in Gaspe is restrict ed to the Devonian and therefore occurred later than elsewhere in the Canadian Appalachians.