STRUCTURE AND HANGING WALL STRATIGRAPHY OF THE LAS CUEVITAS DETACHMENT, CENTRAL SIERRA SAN FELIPE, BAJA CALIFORNIA

The Gulf of California began to open in Late Miocene time. In N Baja California extension occurred mainly along E-dipping normal faults. The E-dipping Las Cuevitas detachment (LCD) is located 25 km W of San Felipe. The fault was active in Mio-Pleistocene time but no longer appears active. The LCD had top-to-the-E slip, has a curviplanar N-S trend and dips 15 to 35˚ E.

Hanging wall strata consist of (1) local basal eolian sandstone and/or metamorphic-clast conglomerate overlain by (2) a thin section of basalt and ignimbrite overlain by (3) fluvial and marine sediments containing locally derived clasts. The contact between marine beds to the north and fluvial beds to the south is most likely a lateral facies change. To the north, mid Miocene fine grained basinal marine sediments record waning volcanism in the lower 95 m (Boehm, 1984). Westward, this grades up into a locally derived marine fault-scarp facies at the LCD. An E-striking fault trends along the north edge of the fluvial sediments and offsets the fluvial-marine contact at least 1 km dextrally, and may cut the detachment. NW-striking dextral faults form SW-facing fault scarps 3-5 m high on colluvial aprons in the base of the fluvial section. These scarps may be shears related to the E-striking dextral fault. Farther east, a steep N-to NNW-striking, E-down normal fault offsets fluvial sediments ~700 m and appears to die out to the north.

The base of the marine section is unexposed, so age of LCD initiation is unknown. At ~6.0 Ma, extension was occurring on the LCD, controlling a marine incursion until ~1.8 Ma when marine deposition was replaced by terrigenous deposition (Boehm, 1984). The LCD is broadly synchronous and en echelon with the Santa Rosa detachment (SRD) mapped by Bryant (1986). Activity on the SRD began between ~12.5 and 8.9±1.2 Ma but constraints on the age of fault cessation are weak (Oskin, 2002). The SRD is inferred to connect with the E-dipping N-NNW-striking steep normal fault SE of the LCD. The detachments are inactive but extension continues to the west on the Sierra San Pedro Mártir fault, and in the east through sea floor spreading in the Delfin Basin since ~2.0 Ma (Stock, 2000).