ORIGIN OF MEGAMULLION CORRUGATIONS AND EVOLUTION OF A ROLLING HINGE: INSIGHTS FROM A MORPHOMETRIC AND STRUCTURAL ANALYSIS OF AN ACTIVE LOW-ANGLE NORMAL FAULT, SIERRA EL MAYOR, BAJA CALIFORNIA

The Canada David detachment (CDD) is a spectacular example of an active LANF that controls the western mountain front of Sierra El Mayor over a strike length of 55 Km. Like most LANFs, the CDD is corrugated and contains two prominent antiform-synform megamullion pairs that strongly control the tectonic geomorphology of the uplifted footwall block and alluvial terraces along the range flank. Quantitative morphometric analysis along the mountain front shows that drainage basins in antiformal domains have systematically higher outlet elevations, higher gradients, greater relief, and much greater hypsometric integrals. Additionally, valley floors in antiformal domains are narrower and dominated by bedrock channels that extend nearly to their outlets, which is reflected in a dramatic decrease in mountain front sinuosity. Detailed mapping and cosmogenic isotopic dating of strath terraces shows that they are much younger and have greater relative heights in antiformal domains. These relations conclusively show that antiformal domains have experienced much higher rates of uplift and erosion of the footwall. The lack of slip gradients on the master fault between synformal and antiformal domains suggests that the megamullions formed by regional buckling perpendicular to the extension direction, and they continue to amplify with progressive slip.

The Quaternary scarp array that extends along the entire mountain front also shows remarkable variations with megamullions. In antiformal domains, the scarp array is 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 dominated by synthetic scarps. Integrated deformation across the array shows a systematic decrease in the ratio of horizontal:vertical deformation with distance from the CDD, which is consistent with an antilistric master-fault geometry. Patterns of sedimentation as well as gravity and seismic data also confirm that the CDD takes on a high-angle geometry within 5-10 km of the mountain front. Our data are consistent with a basinward migration of deformation as envisioned in the “rolling hinge” model of detachment faulting. We conclude that such a migration occurs predominantly in synformal domains as the fault adopts a straighter configuration.