LATE CRETACEOUS–EARLY TERTIARY EXTENSION IN THE CENTRAL MOJAVE METAMORPHIC CORE COMPLEX: IMPLICATIONS FOR CORE COMPLEX FORMATION AND THE LARAMIDE OROGENY

The central Mojave metamorphic core complex (CMMCC) in southern California has been interpreted as a site of early Miocene large magnitude extension. We present new 40Ar/39Ar thermochronologic and geologic evidence which indicates that the core complex formed through a more protracted and poly-phase evolution than previously recognized. Major extension at the core complex first began during the Late Cretaceous–early Tertiary (ca. 70-60 Ma) based on rapid footwall cooling. Amphibolite-grade extensional mylonites in the footwall support extension during this time. Following a period of tectonic quiescence from ca. 60–21 Ma, the core complex was reactivated during early Miocene extension from ca. 21-17 Ma. The recognition of a poly-phase evolution at the CMMCC has important implications for models of the tectonic development of this core complex. Previous estimates of extension at the CMMCC suggesting 40-70 km of total extension at very high slip rates relied primarily on the offset of Mesozoic markers. The magnitude and rate of Miocene fault slip at the CMMCC may have been substantially less than previously thought because the total extensional strain was distributed over both Laramide and Miocene events. Similarly, assessments of the initial dip of the bounding low-angle detachment fault must consider potential tilting during both Laramide and Miocene extensional phases. Poly-phase extensional histories have been recognized at several other Cordilleran core complexes and may be common. The potential effects of such poly-phase evolutions at other core complexes should be considered. In addition, these results add to growing evidence for significant hinterland extension synchronous with foreland uplift during the Laramide Orogeny. Hinterland extension is seemingly at odds with geodynamic models of flat slab subduction during the Laramide and may indicate that such models need to be reevaluated. These results raise the possibility that hinterland collapse was a significant driving force of foreland uplift and that this deformation was at lease partially decoupled from plate tectonic processes.