THE NORTHWESTERN SAN MIGUEL-VALLECITOS-CALABASAS FAULT SYSTEM, BAJA CALIFORNIA, MEXICO: TOTAL OFFSET CONSTRAINTS

Baja California, MX is host to several transpeninsular dextral strike slip faults, such as the San Miguel-Vallecitos-Calabasas fault system that directs strain out of the Gulf of California and onto the Continental Borderlands faults, thereby transferring plate-boundary shear around the “Big Bend” of the San Andreas fault. The San Miguel-Vallecitos-Calabasas fault system is comprised of the San Miguel fault to the SE and the Vallecitos and Calabasas faults to the NW. Previous work by Giroux and Hirabayashi on the central and SE section of the San Miguel fault yielded a total offset of 575 m and a slip rate of 0.35 mm/yr, respectively. Mapping of the sub-parallel and overlapping Calabasas and Vallecitos faults for this study has resulted in the documentation of total offset for both faults. Early Cretaceous dikes of the San Marcos dike swarm near Ejido San Marcos are shown to be dextrally offset 340 m. These dikes are steeply dipping and are equally spaced on both sides of the fault, indicating nearly pure strike slip motion along the Calabasas fault. To the south, the Vallecitos fault cuts several northeast-trending drainages, offsetting the channels. The stream in Cañon Bellotas is offset ~660 m and has a natural impoundment behind the deflection, filled with ~3,000,000 m³ of sediment. Three km to the north along the strike of the fault the stream in Cañon El Burro is offset. The active channel here is offset ~400 m, but there is a presumably original, sediment filled drainage that is offset ~650 m from the active channel that we infer to be the total offset. Summing the total offset for the Calabasas and Vallecitos, we arrive at ~1,000 m, nearly twice that reported on the San Miguel fault 50 km to the SE.