Paleomagnetic evidence of localized vertical axis rotation during Neogene extension, Sierra San Fermn, northeastern Baja California, Mexico

Paleomagnetic data from Sierra San Fermfn in the Gulf of California Extensional Province indicate that localized clockwise rotations about vertical axes occurred uring Pliocene through Recent extension and dextral shear. Relative declination discordances in upper Miocene and Pliocene ash flow tuffs indicate a net clockwise rotation of 30 ø _+ 16 ø. Clockwise rotation between 12.5 and 6 Ma is statistically insignificant (11 o _+ 17o). Structural observations and geochronological data suggest that rotations in this area began post-6 Ma, comprising uniformsense block rotations (oblique divergence) associated with extension and dextral slip in the northwest triking boundary between the Pacific and North American plates. Northeast striking sinistral-slip faults and north striking normal faults accommodate distributed extral shear in this • area, allowing fault blocks to rotate in a clockwise sense. A model for oblique divergence predicts -21 km of shear in the direction of relative plate motion and -20% (-7 km) ENE directed extension, perpendicular to the Main Gulf Escarpment. A broad region of northeastern Baja California may have undergone similar distributed shear. Two possible dynamic models may explain this shear. In one model, rotation accumulates above a deep, subhorizontal, basal shear zone. Rotating blocks may extend downward to a detachment beneath the extensional province, either a low-angle astward continuation of the San Pedro M•tir fault or to a basal shear surface on top of a subducted remnant of the Farallon plate. Alternatively, distributed extral shear may be the surface manifestation of a deep vertical shear zone linking transform faults in the northern gulf with dextral transpeninsular faults. In either case, shear may have transferred northward onto faults west of the San Andreas fault, contributing to late Miocene to Recent clockwise rotation of the Western Transverse Ranges. This shear is not accounted for in the 300 km of dextral slip computed from cross-gulf geologic tie points.