CUMULATIVE SLIP ESTIMATES ALONG THE EASTERN MARGIN OF THE EASTERN CALIFORNIA SHEAR ZONE, CALIFORNIA

Cumulative slip estimates for faults within the Eastern California shear zone are important for evaluating various kinematic models of the evolution of this wide zone of right lateral shear. Previous estimates of cumulative slip based on offset features and palinspastic reconstructions along the Bristol-Granite Mountains-Soda-Avawatz fault zone (that some consider the eastern margin of the shear zone) range from a few km to as much as 45 km. Based on correlations of offset magnetic anomalies and length of pull-apart basins, the cumulative offsets appear to increase southward from 8-9 km near the Avawatz Mountains to about 16-17 km near the Marble Mountains. South of the Marble Mountains, possible extensions of this fault zone are concealed beneath the sediments of Cadiz Valley. Gravity and seismic-reflection lines indicate that this long, northwest-trending valley is underlain by half grabens filled with basin sediments as much as 1-2 km thick. In the northern Cadiz Valley, seismic and gravity data indicate that the western margin of the basin dips steeply east. The more gently-tilted eastern margin of the basin is marked by reflections that are fairly continuous, precluding significant right slip through this part of the basin. In the middle of the valley, reflectors appear to be locally disrupted, presumably by the continuation of dextral faulting from the north. Correlative magnetic anomalies suggest about 14-15 km of right lateral slip through the area of disrupted reflections. In central Cadiz Valley west of Chubbock, gravity data indicate elevated basement, bounded on the east by the proposed structure responsible for displacing the magnetic anomalies and on the west by a structure that is aligned with sheared Quaternary sediments encountered in a drill hole. Additional gravity data and higher-resolution magnetic information collected closer to the ground surface would help address possible continuation of these structures farther south and test whether they connect to structures in the Colorado River corridor.