LATE MIOCENE DEXTRAL FAULTING IN THE LOWER COLORADO RIVER CORRIDOR NEAR BLYTHE, CALIFORNIA

NW-striking faults with km-scale displacement cut mountain ranges surrounding the Blythe basin in southeastern California, but the geometry, kinematics, and timing of these faults are poorly known. Fault activity is presently only constrained to postdate Late Cretaceous metamorphism and predate Quaternary alluvium that overlaps fault traces. Some authors have hypothesized that these faults are related to dextral transtension associated with the Eastern California shear zone and the Pacific-North America plate boundary starting in the late Miocene. Others have attributed these structures to early–middle Miocene NE-SW extension associated with detachment faulting and core complex exhumation NE of the Blythe basin. We present new gravity, fault kinematic, and calcite U-Pb geochronology data to refine the fault geometry and demonstrate that dextral and oblique-dextral slip on these structures occurred in the late Miocene and influenced the transtensional Blythe basin.

Preliminary interpretation of recently densified gravity measurements suggests a more elongate basin bound by both NW- and N-striking structures. Most NW-striking faults we studied in the bedrock mountains surrounding the basin dip steeply NE to near vertical and record kinematics that range from dip-slip NE-down normal, oblique normal-dextral, dextral strike-slip, and oblique dextral-reverse. The range of slickenline rakes, along with multiple lineation orientations on some of the highest-displacement faults, indicate a polyphase kinematic history that may span the transition from NE-SW extension to distributed dextral shear. Several generations of calcite mineralization are present along faults with trace lengths up to 25 km and as much as 2 km right separation of Cretaceous rocks. Calcite textures such as slickenfiber steps and filled tension fractures indicate precipitation occurred during brittle deformation. New LA-ICP-MS U-Pb dates of this syn-kinematic calcite indicate that NW-striking dextral faults were active ~8–10 Ma, ~3–5 Myr after the end of detachment faulting and contemporaneous with estimates for initiation of dextral shear along ~800 km of the Pacific-North America plate-boundary, from the Eastern California shear zone and Gulf of California shear zone, NW and SE of the Blythe basin, respectively.