DEFORMATION OF SALT LAYERS AND INTERBEDDED IMPURITIES: INSIGHTS FROM THE ORPHEUS RIFT BASIN, OFFSHORE EASTERN CANADA, AND SCALED EXPERIMENTAL MODELS

In many salt-rich basins (e.g., offshore Brazil, onshore Netherlands), rock salt is interbedded with other evaporites, carbonates, shales, or igneous rocks. These impurities can influence salt behavior and produce complex intrasalt structures. For example, our study of the Mesozoic Argo salt in the Orpheus rift basin, offshore eastern Canada, suggests that salt walls and columns formed where Argo salt was composed of mostly halite, whereas buckle folds and thrust faults developed with interbedded salt and shale. Two potential causes of the observed compressional deformation are: 1) basement-involved shortening after rifting (i.e., basin inversion), and 2) lateral salt flow associated with depositional loading (i.e., the formation of minibasins) near the rift-basin border-fault zone. In this study, we used scaled experimental models to better understand the development of the observed compressional deformation and to clarify its origin. In the models, alternating layers of silicone putty and wet clay represented interbedded salt and shale, respectively. To trigger deformation, we placed a narrow wedge of wet clay above the layers to simulate depositional loading near the border-fault zone. In response, a minibasin formed, causing the putty to move laterally away from the depositional load. Beneath the subsiding minibasin, the internal clay layer stretched, producing boudins. Simultaneously, a broad region of shortening developed adjacent to the minibasin. The layers of silicone putty thickened, whereas the internal clay layer underwent shortening with buckle folding and thrust faulting. Near the minibasin, many of the buckle folds had overturned limbs, suggesting that shortening was most intense there. The deformation in our experiments resembles the deformation associated with the interbedded Argo salt of the Orpheus rift basin. Thus, these models suggest that, although postrift basin inversion may have affected the Orpheus rift basin, the formation of minibasins in response to focused sediment loading was sufficient to produce the observed compressional structures during the late stages of rifting.