MULTIPLE LINES OF EVIDENCE SUGGEST 30-35 KM OF OFFSET ON THE BLYTHE BASIN STRIKE-SLIP FAULTS: PART OF THE MISSING DEXTRAL SHEAR ACROSS THE MIOCENE PACIFIC-NORTH AMERICA PLATE BOUNDARY?

Documenting the evolution of Miocene to present Pacific–North America plate boundary strain is a major goal of the tectonics community. Recent regional scale analysis of the Gulf of California to Salton Trough portion of the plate boundary suggests that 30–35 km of plate boundary deformation is unaccounted for in the lower Colorado River region. It is likely that the “missing” strain will be found on dextral strike-slip faults and related zones of transtensional deformation. We hypothesize that a few 10s of km of previously undocumented Neogene dextral shear has occurred on a NW-trending network of faults that cross Blythe Valley CA. A few key observations suggest that this region contains a major portion of the missing strike-slip faulting. First, a recent 1:100,000-scale map compilation suggests that the McCoy Mountains Fm in the McCoy Mountains, CA may correlate to similar exposures located 30-50 km to the southeast in the Dome Rock Mountains, AZ, and allows for ~35-50 km of dextral offset on the southwest side of the Blythe basin. Second, Blythe basin exhibits a gravity low that defines a NW-SE elongate sedimentary basin (~30–35 km long, 6–10 km wide), a typical pull-apart basin shape and orientation for the plate boundary. Based on the mechanics of pull-apart basins, this 30-35 km basin length is likely to be the offset across the dextral faults that bound the SW and NE margins of the basin. Third, known dextral strike-slip faults (Cibola and Big Maria faults) in the ranges surrounding the basin define a right-stepping pull-apart basin when the faults are projected into the basin. Importantly, however, only ~10 km of cumulative offset has been documented on these basin-bounding faults; thus, 20 km or more offset may be unaccounted for. Our hypothesis of 30–35 km dextral offset along the NW striking faults suggests that similar offsets should be found on faults along strike to the SE in SW Arizona and on the Iron Mountain to Bristol and Granite Mountains faults to the NW in SE California; the latter have published offsets of 10-37 km. We will continue to evaluate this 30-35 km offset hypothesis by further study of the McCoy Mountains Fm as a Cretaceous foreland basin, by refining the gravity signature of the Blythe basin, and by compiling all possible fault offsets in the ranges surrounding the basin.