Extension, shortening, and salt tectonics at a Paleozoic transcurrent plate boundary: Cumberland Basin, Nova Scotia

The Late Paleozoic Cumberland Basin lies within the larger Maritimes Basin, north of a major strike-slip boundary between the Meguma and Avalon Terranes of the Northern Appalachians. The basin has been interpreted variously as a rift, a pull-apart related to strike-slip faulting, or as a product of terrane convergence (Bradley, 1982). High-quality seismic lines indicate a complex history. During the Viséan, deposition of as much as 3 km of Windsor Group evaporites and clastics accompanied movement along basement-cutting normal faults. Syndepositional minibasins within the Windsor Group and overlying Mabou Group (Viséan-earliest Namurian) indicate that extension continued throughout the Mississippian and that fault movement nucleated minibasin formation. The contact between these Mississippian units and the overlying Cumberland Group (Pennsylvanian) is marked by a pronounced angular unconformity in the subsurface. The overlying Early to Middle Pennsylvanian coal-bearing succession, the focus of current exploration for coalbed methane, exhibits marked thickness changes in response to contemporaneous salt movement. Folding and faulting of the Cumberland and overlying Pictou Group (Stephanian), together with tilting of basement rocks in the Cobequid Highlands, indicate that reverse faulting accompanied, and possibly triggered, Pennsylvanian halokinesis. This complex history is consistent with an overall environment involving significant dextral strike-slip motion on bounding faults of the Maritimes Basin, alternating between overall transtension (leading to extensional tectonics and minibasin formation) and overall transpression (leading to reverse faulting). Younger parts of the basin-fill were profoundly influenced by salt tectonicism, triggered by these changes in tectonic style.