The influence of basin architecture and synrift salt on structural evolution during and after rifting: A case study of the Orpheus rift basin, offshore Nova Scotia and Newfoundland, Canada

The Orpheus rift basin is part of the eastern North American rift system that formed prior to the opening of Atlantic Ocean. Using 2D seismic-reflection and well data and with information from the adjacent Fundy rift basin, I have defined the styles of deformation that formed during the development of the Orpheus rift basin. The basin geometry influenced deformation style by controlling the initial thickness of the massive lower Argo salt. Generally, the lower Argo salt is thin or absent above shallow fault blocks and thick above deep fault blocks. The composition of the upper Argo Formation, which consists of halite and interbedded clastic sedimentary rocks, also influenced the deformation style in the basin. In parts of the basin, the halite of the upper Argo Formation is interbedded with numerous, thick shale beds. In other parts of the basin, however, the upper Argo Formation is predominantly halite with few shale beds, allowing it to behave ductilely like the massive lower Argo salt.;The synrift Argo salt significantly influenced deformation during and after rifting. Growth beds in the upper Argo Formation associated with extensional fault-propagation folds reflect continued activity on basement-involved faults below the salt during its deposition. During the later phases of rifting, paired minibasins and salt walls/columns preferentially formed where the lower Argo salt was thick and/or where the upper Argo Formation had a high proportion of halite. Sediment loading near the northern border faults caused the underlying salt to move laterally, forming the minibasins, salt walls/columns, and possibly detached compressional structures. Immediately after rifting, shortening associated with basin inversion reactivated some basement-involved faults. Detached compressional structures (i.e., salt-cored folds) located to the south and far from minibasins likely resulted from this basement-involved shortening. It is unclear whether the detached compressional structures near the minibasins formed, at least in part, in response to the basement-involved shortening. The nature of the widespread unconformity during the Late Jurassic/Early Cretaceous remains unclear. However, additional postrift deformation during the Oligocene/Miocene again reactivated some basement-involved faults and shortened the buried salt walls/columns, producing domes in the sedimentary cover above them.