SYN- AND POST-6 MA DEFORMATION OF THE BOUSE FORMATION: IMPLICATIONS FOR LOWER COLORADO RIVER TECTONISM AND BOUSE DEPOSITION

The ~6-4.8 Ma Bouse Formation records the first arrival and subsequent integration of the Colorado River to the Gulf of California. Some “non-tectonic” models for the deposition and distribution of Bouse carbonate deposits suggest that modern outcrop elevations reflect original depositional highstand lake levels. Here, we report data in support of tectonism within the corridor. Faults and folds are characterized as N-S extensional, NW-SE dextral-oblique, and E-W contractional, and are observed to deform pre- (~9 Ma), syn- (>6-4.8 Ma), and post-Bouse (<4.8 Ma) strata at numerous locations. Fault slip ranges from minor to significant (cm to 10’s m-scale). Geometric analysis shows prevalence for N-S faults in all basins, while kinematic analysis shows prevalence for N-S dip-slip normal faults and NW-SE dextral-oblique normal faults. E-W contractional structures are few but present in most basins. Bouse deposits commonly exhibit syn-depositional dips up to 30° in draped strata at the 10’s m-scale. In addition, thicker subsurface basin-axis successions and moderate ~5º basinward tilting of strata beneath lower-angle young fanglomerates in some basins produces a vertical distribution of deposits at the 100’s m-scale. These observations have been interpreted to represent progradation of carbonate facies up paleotopography. This is well documented at the 10’s m scale, but the observed 100’s m E-W vertical distribution and tilting suggests a tectonic component. This may reflect tilting due to isostatic response from differential erosion over thin elastic lithosphere. Our reported data also suggests the role of low-strain E-W diffuse extension and/or transtension in the study area that is in agreement with modern geodetics. The observed strain is compatible with inboard stresses set up by dextral shearing on the NW-striking San Andreas plate margin and/or early propagation of the Eastern California shear zone into the lower Colorado River region. Our results have implications for the depositional history of the Bouse Formation (syn-tectonic) and 100-m scale cross-basin uplift/subsidence of Bouse deposits (syn- and post-tectonic). Ongoing studies of basin divides may help explain 100’s m trans-basin elevation changes (e.g., Topock divide) as a result of ongoing regional mantle-driven uplift of the western US.