FOSSIL IMPRINT OF THE MENDOCINO FRACTURE ZONE ACROSS THE SAN ANDREAS FAULT

While initial interaction of the Pacific and North American plates was a complex process involving microplate fragmentation, the 4000 km long Mendocino Fracture Zone (MFZ) is known to have intersected the North American margin near the point of initial impact with the Pacific plate. At the time of initial impingement, the MFZ juxtaposed oceanic crust of significantly different age, relief and thermal structure leading previous workers to speculate that the MFZ had a significant tectonic impact upon the over-riding margin. Here we report new evidence that the MFZ triggered extension and deep tectonic denudation of the southern California margin near the time of initial Pacific-North American plate interaction at ca. 28 Ma. The Pelona-Orocopia-Rand schists are the structurally deepest rocks known to underlie Cretaceous batholithic and cratonal crust of the southern California region. These trench-derived rocks were accreted and metamorphosed during Late Cretaceous – early Cenozoic shallow subduction. Previously adjacent schist bodies exposed across the San Andreas fault in Sierra Pelona (north of Los Angeles) and the Orocopia Mountains (east of Salton Sea) were exhumed by mid-Cenozoic detachment faulting. New ⁴⁰Ar/³⁹Ar thermal history data from K-feldspar and other phases reveal that up to 200°C footwall cooling related to detachment faulting occurred from 28-23 Ma in both areas. This includes schist exposed at Mount Pinos that restores to the western end of Sierra Pelona. East of the SAF, the zone of extensional faulting includes the E-W–trending Chocolate Mountains anticlinorium of southeasternmost California and southwesternmost Arizona. This event closely preceded and overlapped with slab gap magmatism (e.g., Telegraph Peak and Mount Barrow) and basaltic volcanism that marked the onset of Pacific-North American plate interaction. We propose that all of the aforementioned deformation was focused upon an originally linear E-trending belt that marked the imprint of the Mendocino Fracture Zone upon the margin. Later SAF-related deformation disrupted the extensional belt by transposing it parallel to the SAF in the Orocopia and Chocolate Mountains area.