TRANSROTATION INDUCED BY CRUSTAL BLOCKS MOVING INTO AND OUT OF RESTRAINING BENDS

Right steps in sinistral systems and left steps in dextral systems result in transpression along high-angle strike-slip faults. Also common is vertical-axis rotation: clockwise within dextral systems and counterclockwise within sinistral systems. Within the San Andreas system of southern California, vertical-axis clockwise rotation is well documented, especially in the Transverse Ranges. These rotations have been attributed to 1. microplate capture, during which North American crust is transferred to the Pacific plate, which imparts basal traction and induces partially constrained northwestward motion of NA crust, and 2. torsional leverage imposed at the ends of rotating panels by friction along margins of shear zones. Both of these mechanisms are useful in explaining overall clockwise rotations during the last 20 Ma in southern California. In detail, however, individual crustal blocks and panels of blocks have rotated in complex ways that demand additional mechanisms. We hypothesize that crustal blocks rotate counterclockwise as they enter dextral restraining bends and clockwise as they leave dextral restraining bends. This hypothesis explains the following features starting at ~6 Ma (after most transrotation due to microplate capture had ceased): 1. rapid 45-degree clockwise rotation of the eastern Transverse Ranges as they exited the southern Big Bend; 2. slow 8-degree counterclockwise rotation of the San Gabriel block as it entered the northern Big Bend; 3. rapid 22-degree clockwise rotation of the Morales Formation in the Cuyama Valley area as it exited the northern Big Bend; and 4. continuing clockwise rotation of the westernmost Transverse Ranges as they exit the northern Big Bend.