Late Miocene to Recent transtensional tectonics in the Sierra San Fermı́n, northeastern Baja California, Mexico

Basins and ranges within part of the Gulf of California Extensional Province (Mexico) have experienced complex distributed deformation, including normal and strike-slip faulting and block rotations, linked to dextral shear at the Pacific–North America plate boundary. In the Sierra San Fermı́n and southern Sierra San Felipe (northeastern Baja California), normal faulting began between 12.5 and 6 Ma, although most extension occurred between about 6 and 3 Ma, strongly influencing thickness and distribution of ash-flow tuffs and sedimentary deposits. Extension is generally <10% in 6 Ma rocks and somewhat more in 12.5 Ma rocks. Inversion of kinematic data, interpreted together with published paleomagnetic data, suggests that the axis of least principal stress was oriented between W–E and SW–NE in late Miocene time. Our data indicate an important change in the amount of dextral shear, but not necessarily the least principal stress direction (WSW–ENE), at about 3 Ma. Structural constraints limit significant sinistral strike-slip faulting, conjugate to the dextral plate boundary, to the last ∼3 My. Progressive changes in the geometry of faulting through time are consistent with regional strain partitioning within the Pacific–North America plate boundary zone, and are predicted by physical and analytical models of oblique divergence as the orientation of the stretching vector α changes to lower and lower values.