PAY DIRT: 3-D EXPOSURES OF REVERSE FAULTS REVEAL MULTIPLE PALEOEARTHQUAKES, SAN ANDREAS FAULT, WRIGHTWOOD, CA

Only 20 meters from the main trace of the San Andreas fault, excavations at the Wrightwood paleoseismic site reveal exquisite examples of coseismic reverse faulting. We show multiple exposures in three areas of a 100-m wide portion of the site and document the interplay of several cycles of coseismic thrust faulting, sedimentation, and erosion. The depositional setting at Wrightwood, continuous peat growth punctuated by instantaneous debris flow deposits, provides an easily resolved and dateable sequence of deposition - deformation cycles.

We commonly observe the following features: in the hanging wall, concentric folds are broken by low-angle reverse faults. Younger deposits thicken across the scarp but are covered by parallel or fanning bedding. We interpret that coseismic deformation compresses existing strata into concentric folds, which are carried up discrete reverse fault planes. Subsequent debris flows fill the depression caused by the reverse fault and create “growth strata.” Depositional contacts between ensuing deposits will be parallel if the depression was completely filled; if not, the deposits may fan into the depression. Repetition of this cycle creates duplexed folds and incorporates older growth strata into the hanging wall; these must be retrodeformed to understand the event sequence.

Basic but fruitful lessons are learned from this study. Interpretation of the paleoearthquake record is significantly enhanced by cutting multiple parallel trenches. In a series of four parallel exposures 1 - 3 meters apart, the evidence improves from three moderate quality events interpreted in the first trench to five higher quality events. Additionally, we observe that in this setting, coseismic deformation is expressed in different areas over time, such that each area records only a few earthquakes. This indicates that poorly expressed geomorphic features may hide older events - rather than smaller deformation from recent events - and should be exploited to lengthen the event record. By studying these small reverse growth structures in 3D in thinly bedded and readily exposed deposits, we gain insights into the kinematics and temporal behavior of larger, less well-exposed reverse faults.