AN UPDATED GEOPHYSICAL VIEW OF THE EASTERN CALIFORNIA SHEAR ZONE

Twenty years ago, prompted by the 1999 M7.1 Hector Mine earthquake, analysis of gravity and magnetic data produced estimates of cumulative strike-slip offset along several faults within the Eastern California Shear Zone. Since then, analysis of more than 17,000 gravity measurements and several aeromagnetic surveys has provided additional offset estimates that suggest long-term linkages between adjacent fault zones, such as eastward-decreasing offset along the sinistral Pinto Mountain fault coupled with westward-decreasing offset along the Blue Cut fault in the Eastern Transverse Ranges 25 km to the south. Although the potential-field data indicate that the NW-striking dextral Soda-Avawatz and Bristol-Granite Mountains fault zones are connected, cumulative offset appears to decrease northward over a distance of 100 km, from 16-17 km near the Marble Mountains to 8-9 km north of Baker. Some offset may be transferred westward onto the E-striking Broadwell Mesa and Mesquite Hills faults, each of which has a poorly constrained estimate of 2 km or less total offset. South of the Broadwell Mesa fault, detailed gravity data delineate a possible WNW-striking structural link between the Bristol-Granite Mountains and South Bristol Mountains faults. The latter fault has about 5-6 km of right slip based on offset magnetic patterns. Slip may also be transferred eastward from the Soda-Avawatz fault via a speculative network of dextral and antithetic sinistral faults, such as that documented in Ivanpah Valley, where the dextral Stateline fault appears to terminate in a 3-km deep basin. Here the Stateline fault splays out into gently dipping segments where it intersects the sinistral, steeply dipping Nipton fault that forms the SE basin margin.

Basins highlighted by new gravity data include the N-trending Red Pass Lake basin along the eastern margin of the Northeast Mojave sinistral domain and strike-slip basins formed in the Soda-Avawatz-Bristol-Granite Mountains, Lenwood-Lockhart, and Harper Lake fault zones. The lengths of the basins provide offset estimates for the latter two fault zones of 3 and 4 km. Although the orientations of these basins suggest transtension, the Pinto Mountains and northernmost Soda-Avawatz basins appear to be overprinted by transpression, signaling possible recent changes in stress regime.