GEOLOGIC DEVELOPMENT OF THE KERN ARCH, SE SAN JOAQUIN BASIN (SJB), CALIFORNIA

We adopt the name Kern arch for the extensively faulted, broad W-plunging basement salient that has grown beneath eastern SJB over the past ~1 m.y., and which has been partly denuded of its Cenozoic cover strata. Subsurface apatite He thermochronometry, vitrinite reflectance, low-grade metamorphic paragenetic and thermal modeling studies show that 1 to 2 km of post 1 m.y. exhumation increased from west to east across the arch, and that ~1500 m of such strata has been exhumed off the adjacent Sierran basement. Seismological and geodynamic relations indicate that this active epeirogenic uplift is driven by the delamination of the underlying mantle lithosphere, and resulting asthenosphere ascent. Rock and surface uplift at mm/yr-scale is further indicated by contemporary vertical velocities measured in GPS monuments across the salient. Such rapid uplift is driving the incision of the lower Kern River gorge as well Poso Creek and the White River. Much of the Kern arch is characterized by widespread erosional lags, which commonly misguide workers into interpreting much of the arch as a series of alluvial fans.

In contrast to the misinterpretation of the Kern arch as a series of alluvial fans, some workers have misinterpreted it is an early Cenozoic uplift. This view may now be more clearly placed into the regional context of profound exhumation of the southern Sierra Nevada batholith and adjacent Great Valley forearc during the Late Cretaceous, which resulted in the rapid removal of ~10-15 km of crust off the area of the modern Kern arch, increasing to ~35 km of crust removed off the area of the Tehachapi Range. This exhumation gradient developed above the northern shoulder of the Shatsky Rise conjugate as it subducted beneath the southernmost Sierra-Mojave region. Complimentary to the southward increasing exhumation gradient, the mantle lithosphere was completely removed from beneath the Tehachapi-Mojave region by shallow flat subduction of the main rise conjugate. The uplift-subsidence patterns of the region were further confounded by the latest Oligocene-early Miocene opening of an underlying slab window, which drove epeirogenic transients and block faulting. Geodynamic relations suggest that the slab window instigated the recent delamination of the residual mantle lithosphere from beneath the Kern arch region.