SYNEXTENSIONAL DEPOSITION OF THE EARLY MIOCENE JACKHAMMER AND PICKHANDLE FORMATIONS IN THE CALICO MOUNTAINS, SOUTHERN CALIFORNIA: IMPLICATIONS FOR THE TIMING OF INITIAL EXTENSION OF CENTRAL MOJAVE METAMORPHIC CORE COMPLEX

This study in the northern Calico Mountains of southern CA presents new geologic mapping, stratigraphic interpretations, and ⁴⁰Ar/³⁹Ar geochronology of the Jackhammer and Pickhandle formations to provide refined constraints on the relative timing of volcanism and syndepositional extensional basin development in the central Mojave metamorphic core complex (CMMCC) during the Early Miocene. In the study area, the Jackhammer Formation, the oldest Tertiary stratigraphic unit (ca. 20.0 Ma), is nonconformable with pre-Tertiary metasedimentary and plutonic basement rocks and consists of alluvial deposits, primary to reworked tuff-lapilli tuff and local basement-derived avalanche megabreccia, lacustrine limestone, and mafic lava; in addition, the ca. 19.65 Ma “Mammut ignimbrite”, a ~130 m-thick crystal-rich welded lapilli tuff, is exposed in the eastern part of the study area. The ca. 20 to 19 Ma Pickhandle Formation conformably overlies the Jackhammer Formation and consists of: 1) a lower member composed of reddish monomict debris flow breccias with porphyritic rhyodacitic clasts and silicic block and ash flow deposits of similar composition; and 2) a upper member of polymict (metaplutonic basement and rhyodacite) alluvial deposits, primary to reworked lapilli tuff, and local rhyodacitic lava and block and ash flows. Stratigraphic relationships suggest that both formations are synextensional deposits. Reddish porphyritic rhyodacitic lava domes were emplaced synchronously during the final stages of Pickhandle Formation deposition, primarily intruded along preexisting normal fault zones. Previous workers utilized regional stratigraphic interpretations and correlations to constrain the initial timing of CMMCC extension to ca. 23.7 Ma, based in part on the oldest age of Pickhandle-type rocks in the region and the interpretation that they represent supradetachment basin deposits derived from a volcanic center in the Calico Mountains. However, new ⁴⁰Ar/³⁹Ar ages in this study for the Pickhandle and Jackhammer formations are consistently 3-4 Myr younger than this maximum age of initial extension obtained from rocks found >30 km NW of the Calico Mountains and the exposed footwall of the CMMCC, suggesting that the established stratigraphic framework and its relationship to CMMCC extension needs revision.