Patterns of Quaternary deformation and rupture propagation associated with an active low-angle normal fault, Laguna Salada, Mexico: Evidence of a rolling hinge?

The Laguna Salada rift basin is within the zone of shearing between the Pacific and North American plates and is an asymmetric half-graben controlled on its eastern margin by the Laguna Salada fault and the Canada David detachment. Both faults dip west, have accommodated > 10 km of offset since the middle-late Miocene, and are associated with an extensive late Quaternary fault array. The Laguna Salada fault is a high-angle fault that strikes northwest and has an oblique normal-dextral sense of shear. The Canada David detachment is a low-angle normal fault with a curvilinear trace that extends similar to 55-60 km and contains two prominent megamullion antiform-synform pairs. The late Quaternary scarp array that extends along the entire mountain front shows remarkable variations with antiformal and synformal megamullions. In antiformal domains, the scarp array is generally wider, closer to the mountain front (< 100 m), and contains numerous antithetic scarps. In synformal domains, it is well removed from the mountain front (3.5-10 km) and contains more synthetic scarps. Integrated deformation across the array shows a systematic decrease in the ratio of horizontal: vertical deformation with distance from the Canada David detachment, which reflects the mechanism of accommodation of the horizontal component of slip, and/or is the direct result of a master fault with a near-surface antilistric geometry. Patterns of sedimentation as well as gravity and seismic data are consistent with the latter and strongly suggest that the Canada David detachment takes on a high-angle geometry within 5-10 km of the mountain front. Structural analysis of the scarps and range-bounding fault clearly demonstrates a basin-ward migration of deformation at a variety of scales along the Canada David detachment. The largest steps (3.5-10 km) in this migration are made in synformal megamullion domains, where the strong divergence of the scarp array from the trace of the Canada David detachment results in the abandonment of large segments of the detachment and the transfer of large lozenge-shaped tectonic blocks from the hanging wall to the footwall. The basin-ward migration of deformation and inferred near-surface antilistric geometry are both defining characteristics of the rolling-hinge model of normal faulting. If this model is applicable, our data indicate that the near-surface antilistric bend of the master fault is tighter and more abrupt than previously envisioned.