EARLY MIOCENE SYNEXTENSIONAL VOLCANISM AND DEPOSITION IN THE NORTHERN CALICO MOUNTAINS, CENTRAL MOJAVE METAMORPHIC CORE COMPLEX, SOUTHERN CALIFORNIA

New geologic mapping and ⁴⁰Ar/³⁹Ar geochronology in the northern Calico Mountains provide additional age constraints on the relative timing of volcanism, deposition, and extension in the central Mojave metamorphic core complex (CMMCC) during the Early Miocene. The Jackhammer Formation, the oldest Tertiary stratigraphic unit, is nonconformable with pre-Tertiary nonmylonitic metasedimentary and plutonic basement rocks and consists of alluvial conglomeratic sandstone, tuffaceous sandstone/conglomerate, and nonwelded silicic tuff-lapilli tuff erupted from distal sources. Locally, the Jackhammer Formation is interbedded with basement-derived avalanche megabreccia, lacustrine limestone, and mafic lava; in addition, the “Mammut ignimbrite”, a ~130 m-thick crystal-rich welded lapilli tuff, is only exposed in the eastern part of the study area and appears to transition laterally into thinner, nonwelded lapilli tuff ~6 km to the west.

The Pickhandle Formation conformably overlies the Jackhammer Formation and consists of two members: 1) a lower member of locally erupted dacitic block and ash flow deposits and monomict (dacitic) debris flow breccia of similar composition; and 2) an upper member of alluvial polymict (basement-derived and dacitic) conglomerate and conglomeratic sandstone, primary to reworked lapilli tuff, and local rhyodacitic lava and block and ash flows. Reddish porphyritic dacitic lava domes were emplaced during the final stages of Pickhandle Formation deposition; older upper member rocks are intruded and deformed by the lava domes, and buttress unconformities are observed on the dome’s brecciated margins.

Sedimentary and volcanic lithofacies suggest that the Jackhammer and Pickhandle formations were deposited in a volcanic vent-proximal alluvial system that formed within a half graben basin bounded on the east by the NW-striking “Amphitheatre Fault”. Growth strata (fanning bedding dips and intraformational angular unconformities) within the hanging wall deposits, primarily WSW-directed paleocurrents, and interbedded alluvial debris flow, basement-derived megabreccia, and lacustrine deposits adjacent to this fault suggest a steeply uplifted basement footwall block developed during upper plate extension in the CMMCC.