PALEOZOIC BASINS AND THE REACTIVATION OF IAPETAN RIFT STRUCTURES IN THE GULF OF ST. LAWRENCE

The Québec reentrant and St. Lawrence promontory are major features of the Appalachian Laurentian margin that were inherited from the Neoproterozoic rifting that led to the opening of the Iapetus Ocean. They represent the largest offset of the Laurentian margin in the Appalachians. Although the rift structures are deeply buried under younger sedimentary rocks of Paleozoic age, they were important in controlling later sedimentary and tectonic features of the orogen.

In the early Paleozoic strata of the Laurentian margin succession, outcrop and industry subsurface data locally show abrupt changes in the thickness of the Neoproterozoic to Early Cambrian Labrador Group that suggest the locations of rift-phase faults. These changes are locally coincident with normal faults that cut the platform succession during the early stages of Taconian deformation, whose orientations are otherwise oblique to the orogen. This suggests that Taconian flexural extension reactivated rift-phase structures. Most of these faults seem to have lain dormant during the subsequent development of a thick post-Taconian foreland basin succession that is imaged in industry seismic profiles. However, deformation associated with the Acadian orogeny reactivated some of these structures again, inverting them locally into thrust faults.

The post-Acadian Maritimes Basin is a major depocenter of Devonian - Pennsylvanian age that is superimposed on the Laurentian margin at the location of the offset between the Québec reentrant and the St. Lawrence Promontory. Abundant evidence for along-margin dextral strike-slip motion suggests that the offset was reactivated in transtension during Devonian-Mississippian strike-slip, and that renewed subsidence and extension created the accommodation space for >12 km of late Paleozoic sediment.