UNRAVELING THE COMPLEX INTERPLAY AMONG BASEMENT-INVOLVED FAULTING, SALT DEPOSITION, AND DETACHED DEFORMATION DURING RIFTING: AN EXAMPLE FROM THE SALT-RICH ORPHEUS RIFT BASIN, OFFSHORE SOUTHEASTERN CANADA

Because salt can decouple deep and shallow deformation during rifting, rift basins with viscous salt evolve in a complex manner involving basement-involved and detached deformation. Using well data and a dense grid of 2D seismic-reflection profiles from the salt-rich eastern Orpheus rift basin, offshore southeastern Canada, we have studied how rift-related faulting, synrift salt and clastic deposition, and salt flow during rifting affect deformation patterns in rift basins with salt. Our work shows that a series of E-striking, S-dipping faults with normal separation bound the basin on the north. The deformation within the basin is complex consisting of fault-propagation folds above basement-involved faults, detached folds and thrust faults, and salt diapirs. The synrift salt consists of a lower, older massive evaporite unit that underlies an upper, younger unit with two distinct interfingering facies. Facies A, present near the border-fault system and its relay ramps, consists of salt and interbedded clastic sedimentary rocks (likely shale beds). Facies B, present in the basin center, predominantly consists of massive salt with few clastic interbeds. We propose that, during rifting, relay ramps connected the footwall of the border-fault system with the deep parts of the basin, providing pathways for clastic sediments to enter the salt-rich Orpheus basin. As a result, the depositional patterns of the synrift salt varied from interbedded salt and shales near the relay ramps to predominantly salt in the middle of the basin. The presence of interbedded brittle rocks strongly affected the mechanical stratigraphy of the synrift salt, which controlled the deformation patterns in Orpheus basin during sedimentary loading and/or basement-involved faulting. Fault-propagation folds, detached folds, and thrust faults developed in Facies A, whereas Facies B exhibited a highly ductile behavior, forming massive salt walls and columns. Because all synrift salt units and deformed intrasalt strata are intruded by igneous sheets likely associated with the Central Atlantic Magmatic Province (CAMP), we propose that the salt units in the eastern Orpheus rift basin are mostly Late Triassic in age and that a significant amount of detached deformation began before CAMP igneous activity in latest Triassic/earliest Jurassic time.