Controls of listric normal fault styles in the northern Gulf of Mexico: Insights from experimental models

Abstract

Passive margins such as the Gulf of Mexico are characterized by two distinct styles of faulting. Homogenous sand/shale packages in offshore Texas mostly display basinward-dipping listric normal faults with associated rollover structures cut by synthetic and antithetic faults. The fault traces are generally long and show a linear trend. Stratigraphic packages with a ductile substratum (salt) in offshore Louisiana are characterized by basinward and landward-dipping, short arcuate faults detaching within the salt. The structures consist of a series of half-grabens, with the movement of salt from the front to the back of each fault block. Clay experimental models are used to study the controls of fault geometries in the two structural styles and their interaction to form complex transfer zones. The surface of the clay cake is laser-scanned to enable 3D visualization and accurate measurements of structures. The results suggest that within homogeneous sand-shale packages, the dips of the faults and their locations are primarily dependent on the direction of the drop down of the basal detachment along pre-existing discontinuities, with the slope of the basal discontinuity and the direction of extension providing secondary controls. On the other hand, the dips of fault systems in packages underlain by a ductile substratum are primarily controlled by the slope of the basal detachment. Therefore, the more common regional Roho systems typically form above salt sheets with initial basinward slopes, whereas counter-regional fault systems form above salt sheets with initial landward slopes. The direction of extension and the presence of small pre-existing discontinuities impart only secondary controls when ductile basal units are involved. The faults initiate at the head of ductile layer and propagate downslope. Complex transfer zones develop at the boundary of the ductile substratum due to interference between the two fault styles.