LATE QUATERNARY ACTIVITY OF THE PINTO MOUNTAIN FAULT AT THE OASIS OF MARA: IMPLICATIONS FOR THE EASTERN CALIFORNIA SHEAR ZONE

We excavated a fault perpendicular trench across the central branch of the Pinto Mountain fault approximately 100 m west of the Oasis of Mara in Twentynine Palms, California. Relations between faulted stratigraphy, folding, liquefaction and buttressed deposition indicate the occurrence of five to six surface ruptures on seven fault strands over the last 14 ka. Folded and faulted alluvial gravels in the main fault zone demonstrate rupture of sediment to within 0.5 m of the ground surface. Radiocarbon dating of detrital charcoal recovered from lower deposits in the trench indicate that the earliest event occurred in the late Pleistocene between 11.1-14.1 ka, a probable later event occurred ~9.4 ka, with four events post-9.4 ka. Ongoing OSL analysis will further resolve specific event ages.

Assuming that average displacement during prehistoric earthquakes along the Pinto Mountain fault is similar to the recent 1999 Hector Mine and 1992 Landers surface ruptures (~3 m), five to six events in the last 11.1-14.1 ka yields a slip rate of about 1.1-1.6 mm/yr. Since faulting is thought to have initiated on the Garlock and the Pinto Mountain faults at the same time, the ratio of their slip rates and offsets should be proportional. Using a Quaternary offset of 16±3.6 km on the central segment of the Pinto Mountain fault and a Quaternary slip rate of 5-7 mm/yr with 48-64 km total offset on the Garlock fault, yields a Pinto Mountain slip rate of 1.3-2.3 mm/yr. A low slip rate on the Pinto Mountain fault supports a Mojave block rotation model where the pole of rotation lies south of the Pinto Mountain fault, illustrated by the concave southward curvature of both the Pinto Mountain and Garlock faults.