2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 5
Presentation Time: 1:30 PM-5:30 PM


CADENA, Ana M.1, RUBIN, Charles M.1, ROCKWELL, Thomas K.2, WALLS, Christian3, LINDVALL, Scott4, MADDEN, Chris5, KHATIB, Faten2 and OWEN, Lewis6, (1)Geological Sciences, Central Washington Univ, Geological Sciences MS 7418, 400 E University Way, Ellensburg, WA 98926, (2)Geological Sciences, San Diego State Univ, Geological Sciences MC 1020, 5500 Campanile, San Diego, CA 92182, (3)Unavco, 6350 Nautilus Drive, Boulder, CO 80301, (4)William Lettis & Associates, Inc, 28470 Avenue Stanford, Suite 120, Valencia, CA 91355, (5)Earth Consultants Int'l, 150 El Camino Real, Suite 212, Tustin, CA 92780, (6)Department of Earth Sciences, Univ of California, Riverside, Riverside, CA 92521, ana@geology.cwu.edu

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.