DYNAMICS AND ACTIVE DEFORMATION IN THE LAKE TAHOE REGION

NW movement of the Sierra Nevada microplate (SNM) has important consequences for active deformation along the Sierra Nevada-Great Basin boundary zone (SNGBBZ), which includes the Lake Tahoe basin (LTB). For example, evidence suggests that the Carson Range, a major spur linked to the SNM, is moving with a northward component, potentially causing local NS shortening. Active faults and the historical record of earthquake activity in the Lake Tahoe region clearly define local tectonic domains. The Lake Tahoe domain, with large, active, ~NS normal fault systems (maximum magnitude ~7) and little historic seismicity, has a low rate of activity indicated by microseismicity studies. Domains of higher seismicity (Truckee and Gardnerville domains) to the north and southeast are characterized by sinistral-oblique-slip faults and local conjugates, and by abundant microseismicity, probably reflecting NS shortening and slip transfer between large normal fault systems. Large normal faulting events in the Lake Tahoe domain dominate the regional geologic strain rate, while in the adjoining domains, the primary mode of faulting is high-angle strike-slip. The difference in the activity rates between the Lake Tahoe domain and the two adjoining domains is a reflection of the recurrence behavior (i.e. b-values) between strike-slip and normal faulting systems. We interpret these data as follows: As the SNM moves NW, the SNGBBZ is characterized by N-S Shmax=s1. This effectively locks the domains of N-S normal faults in an interseismic period. Gradually, weaker transition domains (Truckee and Gardnerville) undergo NS shortening on conjugate strike-slip faults, thus relieving the N-S Shmax. When the magnitude of Shmax is reduced so that s v=s 1, the normal fault domains rupture and undergo EW extension. Then, Shmax builds up again to initiate another cycle. In this way, conjugate high-angle strike-slip faulting and NS normal faulting account for NW translation of the Sierra Nevada microplate.