EVIDENCE OF LATE PLIOCENE TRANSTENSIONAL TECTONICS, PUNTA EL MANGLE, BAJA CALIFORNIA SUR, MEXICO

The Central Structural Domain of Baja California is a 500-km-long region that extends from Bahía de Los Angeles to Isla San Jose along the Gulf of California. Previous studies within this region have not shown evidence in Pliocene-age rocks of the transtensional tectonic regime active in the gulf for the last 3.5my. Our reconnaissance study of the Pliocene geology on Ensenada El Mangle (25km north of Loreto) reveals a new E-W trending fault that in part defines the northern edge of the bay. "El Coloradito" fault is unusual as it strikes offshore (N55^oW) parallel to the Carmen and Farralon transform faults in the Gulf of California. Landward, this fault dead-ends into a N-S trending fault system in Arroyo El Mangle. The "El Mangle fault system" separates the eastern margin of the Cerro Mencenares volcanic center from an uplifted block of the Miocene Comondú Group. It is worth noting that the Cerro Mencenares volcanic center lies along the strike of El Coloradito, landward of the fault terminus. Reconnaissance of the eastern edge of the 10km² "El Mangle" block also reveals a series of arcuate faults that cross-cut the block from SE to NW. We surmise that these faults branch off El Coloradito fault and merge to the north with El Mangle fault system. As such, they may be expressions of the energy transfer from a transform fault to an older, normal fault along the oblique plate boundary between the Baja California peninsula and the Gulf of California. In addition, a thin limestone bed (S20^oE; 12^oE) with abundant oysters and rare corals truncates the sequence of volcanoclastics, volcanics and marine sediments (N34-40^oE; 25-35^oE) that comprise the south end of El Mangle block. This uplifted limestone unit lithologically and faunally matches upper Pliocene carbonate ramps adjacent to the block. However, the uplifted ramp is structurally oversteepened by approximately 6^o.