Brittle faulting in the Canadian Appalachians and the interpretation of reflection seismic data

Abstract

Brittle faults in the Canadian Appalachians, mostly Late Devonian or younger in age, are divided into two groups on the basis of their movement vectors: a strike-slip group and a dip-slip group. The two groups are subdivided on the basis of orientation and sense of movement. They are interpreted in terms of Palaeozoic transpression and Mesozoic extension during Atlantic opening. The Palaeozoic faults comprise strike-slip and reverse faults which were active in Devono-Carboniferous times. Mesozoic strike-slip faults are interpreted as transfer faults and they are coupled with normal faults, many of which developed by reactivation of earlier reverse faults or steeply-dipping surfaces including bedding and earlier foliations. Together these Mesozoic faults comprise an adequate mechanism for crustal extension perpendicular to the orogen. Evidence for listric normal faults is rare in the older rocks of the orogen, and it is suggested that this is due to the adequacy of the deformation mechanisms afforded by the pre-existing planes of weakness. The data presented demonstrate clearly that geological structures are commonly repeated at all scales from outcrop to regional. Following this principle, prominent shallowly-dipping reflectors imaged in the Lithoprobe East seismic profiles are interpreted as large-scale representatives of the Palaeozoic reverse faults, seen in outcrop, that were reactivated as normal faults during Atlantic opening. It is suggested that reverse faulting is to be expected as a deformation mechanism in general in the late stages of a collisional orogen. We draw attention to the importance of the Mesozoic faults in modifying the attitude of earlier surfaces such a bedding, foliations and faults.