Range-front fault scarps of the Sierra El Mayor, Baja California: Formed above an active low-angle normal fault?

The Canada David detachment, a west-dipping low-angle normal fault, juxtaposes Pliocene and Pleistocene(?) sedimentary units over sheared bedrock in the Sierra El Mayor, Baja California. Along the west-central range front, the detachment is overlapped by Quaternary alluvial fans that are, in turn, cut by fault scarps up to 7 m high. Map relationships suggest that scarp-forming faults may sole into the detachment at very shallow depth (<200-300 m). Palinspastic restoration of a topographic profile across 15 north-striking scarps suggests that the ratio of net heave to net throw along scarp-forming faults is ~2, consistent with them soling into a low-angle (~30°) fault. These 15 scarps may have all formed in one earthquake, with rupture propagation from depth to the near-surface aided by clay gouge along the detachment. The detachment is abandoned in the footwall east of the scarps.