EVIDENCE OF EARLY MIOCENE SYNEXTENSIONAL VOLCANISM AND DEPOSITION OF THE JACKHAMMER AND PICKHANDLE FORMATIONS IN THE NORTHERN CALICO MOUNTAINS, CENTRAL MOJAVE METAMORPHIC CORE COMPLEX, SOUTHERN CALIFORNIA

This study of the stratigraphic relationships in the northern Calico Mountains of southern California provides additional age constraints on the relative timing of early Miocene volcanism and synextensional basin development in the central Mojave metamorphic core complex (CMMCC). The Jackhammer Formation is nonconformable with pre-Cenozoic nonmylonitic metaplutonic basement and consists primarily of alluvial deposits and interbedded silicic tuffs, avalanche megabreccia, and mafic lava; in addition, the “Mammut ignimbrite”, a ~130 m-thick crystal-rich welded lapilli tuff, is found only in the easternmost study area and appears to laterally transition into thinner, nonwelded lapilli tuffs to the west. The Pickhandle Formation conformably overlies the Jackhammer Formation and consists of: (1) a lower assemblage of reddish monomict breccias with porphyritic rhyodacitic clasts and silicic block and ash flow deposits of similar composition; and (2) a upper assemblage of polymict (metaplutonic basement and rhyodacite) alluvial deposits, primary to reworked lapilli tuff, and local rhyodacitic lava and block and ash flows. Rhyodacitic lava domes were emplaced during the final stages of deposition, primarily intruded along preexisting normal fault zones. Sedimentary and volcanic lithofacies suggest deposition in a volcanic vent-proximal alluvial fan system that formed within a half graben basin bounded by the SW-dipping “Amphitheatre Fault” on the east. Growth strata, primarily SW-directed paleocurrents, and interbedded debris flow, megabreccia, and lacustrine deposits adjacent to this fault suggest synextensional deposition in an intra-hanging-wall basin the developed in the upper plate of the CMMCC. New ⁴⁰Ar/³⁹Ar ages for six samples from the synextensional Pickhandle and Jackhammer formations have a mean age of 20.10 ± 0.06 Ma, which is 3-4 Myr younger than the maximum age of initial CMMCC extension proposed by previous studies of the Pickhandle formation in adjacent areas of the central Mojave. This suggests that CMMCC extension and coeval volcanism was likely not a synchronous large-magnitude regional event, but was rather more localized and asynchronous, occurring in many smaller basins that eventually culminated in exposure of the CMMCC mylonitic footwall rocks after ca. 20.1 Ma.