LATE MIOCENE TO RECENT TRANSTENSIONAL TECTONICS IN THE SIERRA SAN-FERMIN, NORTHEASTERN BAJA-CALIFORNIA, MEXICO

Basins and ranges within part of the Gulf of California Extensional Pr ovince (Mexico) have experienced complex distributed deformation, incl uding normal and strike-slip faulting and block rotations, linked to d extral shear at the Pacific-North America plate boundary. In the Sierr a San Fermin and southern Sierra San Felipe (northeastern Baja Califor nia), normal faulting began between 12.5 and 6 Ma, although most exten sion occurred between about 6 and 3 Ma, strongly influencing thickness and distribution of ash-flow tuffs and sedimentary deposits. Extensio n is generally < 10% in 6 Ma rocks and somewhat more in 12.5 Ma rocks. Inversion of kinematic data, interpreted together with published pale omagnetic data, suggests that the axis of least principal stress was o riented between W-E and SW-NE in late Miocene time. Our data indicate an important change in the amount of dextral shear, but not necessaril y the least principal stress direction (WSW-ENE), at about 3 Ma. Struc tural constraints limit significant sinistral strike-slip faulting, co njugate to the dextral plate boundary, to the last similar to 3 My. Pr ogressive changes in the geometry of faulting through time are consist ent with regional strain partitioning. within the Pacific-North Americ a plate boundary zone, and are predicted by physical and analytical mo dels of oblique divergence as the orientation of the stretching vector cc changes to lower and lower values. (C) 1998 Elsevier Science Ltd. All rights reserved.