LATE CENOZOIC EVOLUTION FROM EXTENSION TO TRANSFORM FAULTING IN THE BAHÍA DE GUADALUPE AREA, BAJA CALIFORNIA, MEXICO

Rocks in a transect from southern Bahía de Guadalupe to southern Valle Laguna Seca record initial Cenozoic extension in a rift segment dominated by east tilting before, during(?), and after local subduction-related(?) arc volcanism. Subsequent deformation was restricted to coastal regions and slip on a few valley-forming faults farther inland, recording formation of the adjacent Ballenas transform system.

Plio-Pleistocene marine (and lesser fluvial) mud, sand, coquina, conglomerate and sedimentary breccia near the coast dip west up to ~20° and are intruded by basaltic to dacitic(?) dikes and volcanic feeders, forming pepperite and jasperoid cements. A wave-cut(?) bedrock terrace bevels these deposits at ~100 m elevation. These strata grade westward into subhorizontal mixed fluvial/alluvial and lesser marine beds that are preserved only in present topographic lows and drainages cut in Mesozoic and older crystalline basement; the highest fossiliferous marine horizons are also at ~100 m elevation. Hills and mesas to the west are capped by olivine basalt and local hornblende basaltic andesite flows and dikes that are probably ~15 m.y. old. This sequence is tilted east ~5-10°. It commonly lies on basement rocks but locally is underlain in angular unconformity by a more steeply east-tilted sequence of arkosic and quartz-rich redbeds that largely or entirely predate local volcanism. Tilts in the redbeds average ~20-25° east, but basal dips up to ~70° east are preserved in one small growth fan.

West-down faulting and east tilting of the redbeds and the basaltic sequence are inferred to be related to early extension in a rift segment dominated by top-west slip on an underlying master normal fault (not exposed). West tilting of Plio-Pleistocene beds near the coast and their intrusion by young magma are related to formation of the Gulf transform and spreading center system. The wave-cut(?) bench that bevels them probably records the first step in formation of an unconformity such as those commonly imaged in subsided wrench margins.