RELATIVE TIMING OF BASEMENT-INVOLVED FAULTING, DETACHED SHORTENING, AND SALT MOVEMENT DURING RIFTING IN THE SALT-RICH EASTERN ORPHEUS RIFT BASIN, OFFSHORE SOUTHEASTERN CANADA

Many rift basins with salt have undergone both basement-involved and detached deformation during rifting, producing a complex array of rift-related structures. To better understand salt-related deformation in rift basins, we have used a dense-grid of 2D seismic-reflection data to examine the basement-involved and detached deformation within the eastern Orpheus rift basin, offshore southeastern Canada. The Orpheus basin, part of the eastern North American rift system, formed during the Late Triassic to Early Jurassic prior to the breakup of Pangea. A series of south-dipping basement-involved faults with normal separation forms the northern boundary of the eastern Orpheus rift basin. Basement-involved intrabasin faults with normal separation dip mainly toward the south. In the eastern Orpheus rift basin, synrift salt is up to 4.5 km thick and mostly Late Triassic in age because it is intruded by latest Triassic/earliest Jurassic igneous sheets associated with the short-lived Central Atlantic Magmatic Province (CAMP). Salt diapirs, buckle folds, and detached thrust faults, likely related to minibasin formation, are also common throughout the eastern Orpheus rift basin. Growth strata and the truncation of deformed synrift strata and CAMP-related igneous sheets by the Early Jurassic breakup unconformity show that the onset of basement-involved extensional faulting was diachronous. Basement-involved faulting began with the formation of the northern rift-basin border-fault system in the Late Triassic, followed by the development of intrabasin faults in the Early Jurassic. Salt diapirs and shortening-related structures were coeval with minibasins that formed during the later (Early Jurassic) phase of rifting. We propose that increased activity on basement-involved faults in the Early Jurassic was a factor in the development of the detached structures in the eastern Orpheus rift basin. Intensified activity on the northern border faults may have caused the deposition of major alluvial fans locally with higher-density, poorly sorted sediments. As a result, minibasins developed, and their subsidence caused the underlying salt to move laterally to the south, producing shortening-related structures and salt diapirs.