The Role of the Garlock Fault in the Development of Contractional Structures along the San Andreas Fault North of the Big Bend: An Analogue-Modeling Approach

It has long been noted that the San Andreas fault north of the Big Bend is associated with the development of Late Cenozoic fault-parallel contractional structures in the Californian Coast Ranges. This observation has been interpreted as a result of extremely low friction along the San Andreas fault. This interpretation implies that the general lack of fault-parallel folds south of the Big Bend is due to a significant increase in the fault strength. While the variation of structural styles can be attributed to the fault-strength effect controlled by different types of rocks juxtaposed by the San Andreas fault north (mélange and fore-arc basin strata) and south (Mesozoic arc and cratonal basement) of the Big Bend, we also note that the change in deformation style is spatially correlated with the intersection of the San Andreas fault and the Garlock fault. This relationship raises the possibility that left-slip motion on the Garlock fault may have also played a role in controlling deformation style along the San Andreas fault. To test this possibility, we used simple sandbox models to evaluate the effect of simultaneous motion on the two faults. Our experiments, scaled by fault geometry and relative slip rates, explore two end-member cases: (1) the Big Bend was an original structure and was amplified later by motion on the Garlock fault, and (2) the Big Bend was solely produced by motion on the Garlock fault after the formation of a straight San Andreas fault. Our results suggest that fault-parallel contraction north of the Big Bend can be generated by motion on the coeval Garlock fault, and that the current topographic distribution along the San Andreas fault favors the Big Bend as an original structure.