PALEOMAGNETIC EVIDENCE OF LOCALIZED VERTICAL AXIS ROTATION DURING NEOGENE EXTENSION, SIERRA SAN-FERMIN, NORTHEASTERN BAJA-CALIFORNIA, MEXICO

Paleomagnetic data from Sierra San Fermin in the Gulf of California Ex tensional Province indicate that localized clockwise rotations about v ertical axes occurred during Pliocene through Recent extension and dex tral shear. Relative declination discordances in upper Miocene and Pli ocene ash flow tuffs indicate a net clockwise rotation of 30 degrees /- 16 degrees. Clockwise rotation between 12.5 and 6 Ma is statistical ly insignificant (11 degrees +/- 17 degrees). Structural observations and geochronological data suggest that rotations in this area began po st-6 Ma, comprising uniform-sense block rotations (oblique divergence) associated with extension and dextral slip in the northwest striking boundary between the Pacific and North American plates. Northeast stri king sinistral-slip faults and north striking normal faults accommodat e distributed dextral shear in this area, allowing fault blocks to rot ate in a clockwise sense. A model for oblique divergence predicts simi lar to 21 lan of shear in the direction of relative plate motion and s imilar to 20% (similar to 7 km) ENE directed extension, perpendicular to the Main Gulf Escarpment. A broad region of northeastern Baja Calif ornia may have undergone similar distributed shear. Two possible dynam ic models may explain this shear. In one model, rotation accumulates a bove a deep, subhorizontal, basal shear zone. Rotating blocks may exte nd downward to a detachment beneath the extensional province, either a low-angle eastward continuation of the San Pedro Martir fault or to a basal shear surface on top of a subducted remnant of the Farallon pla te. Alternatively, distributed dextral shear may be the surface manife station of a deep vertical shear zone linking transform faults in the northern gulf with dextral transpeninsular faults. In either case, she ar may have transferred northward onto faults west of the San Andreas fault, contributing to late Miocene to Recent clockwise rotation of th e Western Transverse Ranges. This shear is not accounted for in the 30 0 km of dextral slip computed from cross-gulf geologic tie points.