REACTIVATION OF LAURENTIAN MARGIN STRUCTURES IN LATE PALEOZOIC DEFORMATION, AND THE DEVELOPMENT OF THE MARITIMES BASIN, NORTHERN APPALACHIANS

Atlantic Canada contains one of the largest offsets in the trace of the Ouachita-Appalachian-Caledonide orogens, postulated to reflect a transform fault in the early Iapetus. This offset is marks the southern boundary of the St. Lawrence promontory, and the northern boundary of the Quebec embayment, in the Paleozoic margin of Laurentia.

In the Port au Port Peninsula of west Newfoundland, platform strata deposited close to the tip of the St. Lawrence promontory show a complex history of fault reactivation, revealed by petroleum exploration since 1990. A major NE-striking rift-phase fault, oblique to local structural trends, was reactivated during Taconian flexural extension, with ~1 km of stratigraphic throw, and then inverted as a thrust, with >3 km of throw, during mid-Paleozoic (probably Acadian) shortening. In the same region, Tournaisian-Visean extension marked the northern margin of the Maritimes sedimentary basin, which subsided rapidly and accumulated thick deposits of evaporites. In the field, this E-W structure is marked by spectacular sediment-filled fissures, and forms a boundary between thin, predominantly lacustrine Visean successions (to the north) and thick successions, including subsequently remobilized marine evaporites (to the south).

A comparable Mississippian history occurs ~400 km to the SW in Nova Scotia, where seismic profiles reveal that thick Visean evaporites and Namurian clastics are bounded against basement to the south by a large E-W extensional fault subparallel to the Avalon-Meguma terrane boundary. These two extensional structures are linked by NE-SW faults, along which variable dextral strike-slip motion has been postulated.

After Acadian shortening, dextral strike slip along orogen-parallel faults was transferred via a broad zone of transtensional strain and crustal thinning through the developing Maritimes Basin. In Visean time the extensional component of this strain became concentrated at the inherited offset in the orogen at the south margin of the Laurentian promontory and at the Avalon-Meguma boundary, producing a large pull-apart structure bounding the deepest parts of the basin. In this way, structures inherited from Iapetan rifting continued to influence the tectonic style of the orogen well into the Late Paleozoic.