CONSTRAINTS ON BASIN GEOMETRY AND FAULT OFFSET NEAR BLYTHE, CALIFORNIA AND IN WESTERN ARIZONA, FROM NEW AEROMAGNETIC AND GRAVITY DATA

Plate boundary reconstructions suggest that 30-35 km of right-lateral slip remains undocumented in the lower Colorado River region. Recently acquired aeromagnetic and gravity data provide constraints on fault continuity and offset and basin geometry where right-lateral faults of the eastern California shear zone project southeast into the Colorado River corridor near Blythe, California, and have documented late Miocene slip. New aeromagnetic data flown at a spacing of 600 m at a nominal height of 305 m above ground were augmented by 712 new gravity measurements in the region. The aeromagnetic data improve resolution of anomalies sourced by Cenozoic volcanic rocks (NW- and N-trending anomalies in the Dome Rock and Trigo mountains) and subtle NW-trending anomalies that bound a structural basin near and NW of Blythe. This structural basin is defined by a NW-trending gravity low with an amplitude of 30-40 mGal, reflecting Cenozoic basin depths of ~2-3 km. Horizontal gravity gradient maxima delineate a ~30 km-long, linear, SW basin margin. At its SE end the prominent gradient changes to a more easterly strike. At its NW end the gradient appears to step right ~3 km (to the NE), where it forms the gradient marking the NE basin margin. The northern and eastern basin margins are more complex. In particular, the eastern margin consists of two parallel, N-trending gradients, one located ~3 km east of the Colorado River and another ~5 km farther east near the base of the Dome Rock mountains. Locally the basin margins defined by gravity correspond with magnetic gradients. Tentative correlation of WNW-trending magnetic anomaly patterns at the NW end of this structural basin, one pair in the Granite and Little Maria mountains and a second pair in the Iron Mountains, suggests apparent right-lateral offsets of 10-15 km on the NW-trending Iron Mountains fault that projects SE into the basin. This new estimate is greater than the ~5.5 km of right-lateral offset estimated on this fault in crystalline rocks in the Iron mountains and may account for 5-10 km of previously undocumented fault offset in the lower Colorado River region. Additional dextral shear, either on other faults that merge into this structural basin, or on other faults across a broader distributed deformation zone outside of this basin, is likely the source of the remaining discrepancy.