WHAT CAN SMALL-SCALE STRUCTURES TELL US ABOUT LARGE-SCALE DEFORMATION?: EXAMPLES FROM DEFORMATION BANDS IN THE KREYENHAGEN HILLS, SAN ANDREAS FAULT BORDERLANDS

The Pliocene Etchegoin formation was deposited in a shallow sea northeast of the San Andreas fault in central California. The unit is folded across the region including in the Kreyenhagen hills, a set of foothills along the west side of the Diablo Range ~20 km from the San Andreas fault. The dip of the Etchegoin formation varies along the 40 km extent of these hills. In the northwest, bedding dips shallowly in gentle folds. Dip values increase towards the southeast as the structure becomes a homocline dipping more than 50˚ towards the northeast.

Deformation bands are well preserved in the Kreyenhagen hills, especially within blue sandstone lenses that are characteristic of the Etchegoin formation. While we have observed deformation bands in other folds northeast of the San Andreas fault, it has been difficult to determine whether deformation bands formed before, during, or after the development of these structures. However, the Kreyenhagen hills are the least deformed of the folded regions and the structural variability along strike allows us to track progressive deformation during the development of the homocline.

In northwestern Kreyenhagen hills where bedding is least tilted, we observe Riedel ladder structures in map view that are traceable for tens of meters. The R- and R’-shears that form these ladders are steeply dipping; those with dextral offsets strike 350˚ whereas those with sinistral offsets strike 060˚. As bedding dips increase to the southeast, these two sets of deformation develop preferential dip directions, rather than having steep dips. This pattern is consistent with the rotation of the early bands during tilting of the homocline. However, based on scatter of the two populations of deformation bands, we believe new bands continue to form during or after folding. When the deformation band data are untilted, we calculate infinitesimal strain axes to infer the local stress directions. We find maximum horizontal stress axes with azimuths between 020˚ and 030˚, which are 60˚ to 70˚ from the strike of the San Andreas fault. These values are consistently lower than those derived from stress measurements at depth, which may have implications for the weak fault interpretation of the San Andreas fault.