QUANTIFYING TRANSTENSION: DIFFERENTIAL SUBSIDENCE AND ROTATION OF EXTENSIONAL FAULT BLOCKS IN THE DEVELOPMENT OF PULL-APART BASINS, CANADIAN APPALACHIANS

Late Paleozoic basins in Atlantic Canada subsided within a zone of mainly dextral strike-slip faults, representing an intracontinental transform fault zone developed during the final stages in the assembly of Pangea. The basins contain significant quantities of coal and petroleum.

The Stellarton sub-basin of Nova Scotia is a small (8 x 20 km approx.) but intensively explored basin containing numerous coal seams. The basin is located at a right-handed stepover between major strike-slip faults, and has the geometry of a dextral pull-apart. Borehole records allow analysis of the subsidence history of the basin during the deposition of the most coal-rich successions, and show southward thickening across the basin; coarse clastic facies are concentrated in the thicker, southern side, whereas finer, coal-bearing succession in the north are thinner. This asymmetry persists after decompaction of the logs. Because the coal seams were formed close to the water table, they must represent approximate paleo-horizontal surfaces. The decompacted thicknesses therefore record the asymmetric subsidence history.

Subsequent deformation is documented by observations of faults in large open-pit faces, and in survey plans of historic room-and-pillar mines. This data set provides an unparalleled opportunity to analyze the kinematics of basin deformation. The mine plans show NNW-trending extensional faults with increasing heave toward the north basin margin, allowing the extensional component of strain in the basin floor to be quantified, and demonstrating significant rotation of fault-bounded blocks about vertical axes. Shortening produced ENE-trending folds, that deform both the coal seams and the extensional faults.

These observations indicate that strike-slip and extensional deformations were partitioned during the early subsidence history, but that this regime was replaced by one of distributed transtension soon after deposition. Extension due to faulting slightly exceeds shortening due to folding, suggesting an overall transtensional finite strain. Similar models for deformation of basin floors may be applied to the history of much larger basins in Atlantic Canada and elsewhere.