DID LATE CRETACEOUS-EARLY CENOZOIC FARALLON-KULA RIDGE SUBDUCTION BREACH THE BATHOLITHIC MARGIN OF SW NORTH AMERICA?

Spatial and temporal patterns of magmatic activity and other geologic relationships indicate that Late Cretaceous-early Cenozoic flat subduction broadly affected western North American during the Late Cretaceous-early Cenozoic. In contrast, deformation along the SW craton margin was sharply focused upon the now highly disrupted Transverse Ranges-Mojave-Salinia arc segment between the comparatively intact Sierra Nevada and Peninsular Ranges batholiths. Subduction of relatively large (up to 500-1000 km) oceanic plateaus (i.e., hypothesized western counterparts of Hess and Shatsky rise in the NW Pacific) have been variously linked to this breached segment of the Cretaceous batholith. However, a growing database of U-Pb detrital zircon and Ar-Ar thermal history constraints upon the timing of schist underplating and removal of mantle lithosphere from southern California and SW Arizona indicate that a topographic feature of far greater extent seems to be required to cause basal erosion of the lower crust/mantle lithosphere for ~35 m.y. (~90-55 Ma). Specifically, given rapid convergence rates over this interval (=>100 km/m.y.), the spatial extent of a topographic feature superposed upon a flat subducting slab that would be required to explain a 35 m.y. duration of subduction erosion is on the order of at least 3500 km. Given the restricted dimensions of the damage zone along the SW North American margin, the most likely geometry of the responsible topographic feature on the sea floor is a ridge segment that intersected a restricted segment of the trench at a steep angle (i.e. analogous to the Nazca Ridge but with 2-3x the convergence rate). Moreover, moderate clockwise rotation relative to North America can explain the southward younging of the deformation we observe. Because oceanic crust is poorly preserved, the Late Cretaceous geometry and position of the Farallon-Kula ridge relative to the North American margin remains ill-constrained and has been widely debated. Nevertheless, we are struck by the restricted nature of the Late Cretaceous-early Cenozoic damage zone along the SW North American margin and believe that serious consideration should be given to the possibility that Farallon-Kula ridge subduction caused the elsewhere continuous and well-preserved Cretaceous batholith to be breached in this locale.