FOLD DEVELOPMENT AT KETTLEMAN HILLS, CENTRAL CA: IMPLICATIONS FOR OFF-FAULT DEFORMATION IN THE SAN ANDREAS FAULT SYSTEM

We use map- and outcrop-scale features to characterize the development of the Kettleman Hills anticline in central California. This NW-trending fold on the western San Joaquin valley includes three, right-stepping, doubly-plunging anticlines – North, Middle and South Domes – that expose Pliocene and younger sedimentary rocks. Geomorphic indicators, including the degree of incision, drainage orientations, and the relative relief of each dome, suggest that the fold is propagating SE. Thus, each dome captures a different snapshot of fold development.

Both fold-parallel and fold-perpendicular faults develop at all three domes. At the map-scale, these are normal faults that dip ~60˚ in both directions. At the outcrop-scale, they are often expressed as individual deformation bands or deformation band zones, also with ~60˚ dips. The fold-parallel structures tend to dip towards the hinge, and are best developed at North Dome. The fold-perpendicular structures are common in the upright limbs of South and Middle Domes, but are not well developed at North Dome.

Two other sets of faults are also present at all three domes that are oblique to the fold hinges; these faults strike NNE and WNW and dip in both directions. At the map-scale, these faults have ~60˚ dips with oblique-normal slip directions and are best developed near the plunging noses. At the outcrop-scale, the NNE-set tends to have steeper dips and both sets are found at fold noses and along the limbs. At North Dome, deformation bands with NNW- to NS-strikes are also common.

To summarize, the fold-perpendicular faults are important early structures; fold-parallel faults seem to develop once folding is better established. Both are linked to individual dome formation, accommodating hinge-parallel and outer arc extensions. The oblique fault sets develop first near the plunging noses and later become the dominant accommodation structures. Their obliquity suggests strain is three-dimensional at Kettleman Hills, and their orientations may record regional stresses implying a 45˚ to 55˚ angle between S_Hmax and the San Andreas fault. In contrast, earthquake focal mechanisms suggest an 80˚ to 100˚ angle. The discrepancy between these directions suggests deformation is partitioned in time, by depth, or that one or both datasets do not accurately reflect the regional stress field.