EVIDENCE FOR POST-EARLY MIOCENE INITIATION OF MOVEMENT ON THE GARLOCKFAULT FROM OFFSET OF THE CUDAHY CAMP FORMATION, EAST-CENTRAL CALIFORNIA

The Cudahy Camp Formation, located in the Fl Paso basin, east-central California, consists of a 350-m-thick volcanic and sedimentary sequenc e of early Miocene age. Previous investigators determined from paleocu rrent indicators that the source for these rocks was to the south and southeast, but failed to suggest a specific location for that source. Restoration of 64 km of left slip on the Garlock fault places Cudahy C amp rocks north and northwest of the early Miocene Eagle Crags volcani c field. The unique geochemical nature of basalts found in both locati ons, coincident ages of volcanic rocks, identical clast content of vol canic tuffs, and identical stratigraphic sequence indicate that the so urce for the Cudahy Camp Formation was the Eagle Crags volcanic field. This is the first documented occurrence of Miocene rocks offset by th e Garlock fault. Because the Cudahy Camp rocks and their sources appea r to be offset by the full displacement on the Garlock fault, we concl ude that movement on the Garlock started after earliest middle Miocene time. Further constraining initiation of movement on the Garlock faul t adds significantly to fully understanding the evolution of the weste rn North American plate boundary in the middle to late Cenozoic.