QUANTIFYING ACCOMMODATION OF ACTIVE STRAIN ALONG DISTRIBUTED FAULT ARRAYS IN OWENS VALLEY, CALIFORNIA

Interpretation of differences between geologic slip rates and decadal measures of active strain requires an understanding of the distribution of fault slip in space and time. Such records are often difficult to obtain in regions where strain is accomplished by distributed fault arrays. We assess the role of distributed faulting in active transtension across Owens Valley. Deformation in the southern valley is accommodated by normal slip on the Sierra Nevada fault and by right-lateral slip on the Owens Valley fault. In the northern valley, however, near the junction of the Owens Valley and White Mountain faults, significant strain appears to be accommodated along distributed arrays of normal and oblique-slip faults. We assessed recent slip on these faults using displaced geomorphic markers ranging in age from late Pleistocene to Holocene. On the east side of the valley, in the Waucobi embayment, numerous small faults displace pediment surfaces. Reconstruction of displacement-length profiles suggests Dmax = 0.019L, consistent with global data for normal faults. The age of pediment surfaces is estimated at between 175 – 200 ka, based on depth profiles of cosmogenic ³⁶Cl. Using this age, and summing strain across the Waucobi embayment suggests WSW-directed extension rates ~ 0.2 m/kyr. On the west side of the valley, near the town of Big Pine, numerous north-striking normal faults displace alluvial fans and glacial moraines derived from the Sierra Nevada. At Tinemaha and Birch creeks, Tioga moraines are displaced by ~12 m; cosmogenic ³⁶Cl concentrations in boulders from the moraine crest (Tinemaha) yield ages of 13 - 15 ka. Slip rates on this segment of the Sierra Nevada fault are thus ~1 m/kyr. On the west side of Crater Mountain, basaltic flows are displaced 12-14 m along the Red Mountain fault. Cosmogenic concentrations of samples from this flow suggest an age between 60-80 ka; displacement rates are thus 0.2 – 0.3 m/kyr. Our studies, in conjunction with previous work on the Fish Springs fault, indicate that distributed normal faults throughout the northern Owens Valley accommodate up to 1.2 – 1.5 m/kyr of extension. This rate is significantly higher than those determined along the range front farther south, suggesting that the spatial pattern of extension varies along the Owens Valley – White Mountain fault system.