EVIDENCE FOR HINGE-PARALLEL EXTENSION IN THE MONTEREY FORMATION: IMPLICATIONS FOR THE KINEMATICS OF DEFORMATION ACROSS THE SAN ANDREAS FAULT SYSTEM

We used deformed sediments in the Coast Ranges to test various transpressional models for deformation in the borderlands between the major fault strands in the San Andreas fault system. Because of its young age, the Monterey Formation is ideal for this study because of the relatively short and simple tectonic history since deposition in the Miocene. Numerous folds in the Monterey Formation are observed adjacent to the Rinconada fault in the central Coast Ranges between Lockwood and Salinas Valleys. Fold orientations are generally N60W, approximately 20° counter-clockwise from the strike of the Rinconada fault.

Several exposures of a syncline of the Monterey Formation were studied in detail along variously oriented outcrops near Williams Hill. The syncline is upright, plunges to the north, and has gentle to moderate limb dips between 10-30°. Faults were only observed along roadcuts that were parallel to the fold-hinge or within ±30° of the fold-hinge, and most faults displayed apparent normal dip separation with offsets ranging from a few centimeters to several meters. The normal faults fall into two distinct populations dipping either moderately to the NW or to the SE. Along a 300 m long continuous section, oriented parallel to the fold hinge, a minimum estimate of 4% extension was calculated based on all observed faults. In the absence of good slip indicators on fault surfaces, we assumed normal motion on all faults.

Total strike-slip partitioned models predict that folds will form parallel to the major faults and remain in the same orientation throughout progressive deformation. Partial strike-slip partitioned models predict that folds will form at an angle to the major faults and rotate towards parallelism with the major faults during progressive deformation. Because of the component of wrench deformation in the latter model, folds will necessarily experience hinge-parallel extension. Therefore, by quantifying both the angle of folds with respect to major faults and how much, if any, hinge-parallel extension has occurred, we can understand the degree of strike-slip partitioning in the system. The angle between the folds and the Rinconada fault together with the observed hinge-parallel extension indicate that a partial strike-slip partitioned model for this area in central California is appropriate.