KINEMATICS OF ROTATING PANELS OF E-W FAULTS IN THE SAN ANDREAS SYSTEM: WHAT CAN WE TELL FROM GEODESY?

Panels of E-W-trending sinistral and/or reverse faults occur in various locations within the San Andreas system, and are commonly associated with paleomagnetic evidence for clockwise rotations over geologic time. These panels commonly cut across the trend of active dextral faults, posing questions as to how displacement is transferred across them. Geodetic data indicate that they lie within an overall dextral shear field, and the data are commonly interpreted to indicate little or no slip on the E-W faults.

We model these faults and fault panels as rotating by bookshelf slip in a dextral shear field. This allows prediction of rates of slip, rotation, fault-parallel extension, and fault-normal shortening. We decompose the geodetically determined velocity field into these components, and compare them with our predictions, as illustrated by the following two examples. (1) The velocity field around the central section of the Garlock fault is consistent with a combination of long-term sinistral slip rate of ~7.5 mm/yr on a locked fault, and a counterclockwise rotation rate of 5.2°/m.y. The resultant field comprises a dominant component of dextral shear at a rate of 90 nanostrain/yr, and a residual component of fault-parallel motion which reflects the elastic strain around the locked fault created by shear at depth. (2) The E-W trending faults of the western Transverse Ranges currently lie at ~50° to the Pacific – North America plate motion vector, having been rotated clockwise into this orientation during the since early Miocene time. In this orientation the rate of sinistral slip is small, but they are still rotating at ~2.5°/m.y. Strike-parallel extension at 4 mm/yr is also clearly detectable in the velocity field, as well as strike-normal shortening at up to 6 mm/yr. The rate of strike-normal shortening decreases westward, reflecting transfer of displacement from Inner Borderland dextral faults south of the Transverse Ranges onto faults within the Salinia block.

These two examples demonstrate firstly, that significant sinistral slip on rotating E-W trending faults may only be detectable through second-order elastic effects; and secondly, that previously active sinistral faults may become inactive because of rotation, but may still define a discrete panel deforming by other mechanisms.