EVIDENCE OF SYNEXTENSIONAL DEPOSITION OF THE PICKHANDLE AND JACKHAMMER FORMATIONS IN THE NORTHERN CALICO MOUNTAINS, CENTRAL MOJAVE DESERT, CALIFORNIA

The precise timing of extension in the central Mojave metamorphic core complex (CMMCC) is unclear. Previous thermochronology studies suggest that extension occurred between ~21 -17.5 Ma, while stratigraphic studies suggests that extension was active between ~24 - 19 Ma. These previous stratigraphic interpretations imply that the timing of initial extension in the CMMCC is related to the depositional age of the volcanic and coarse-grained volcaniclastic deposits of the early Miocene Pickhandle Formation, inferred to represent synextensional supradetachment basin deposits. However, direct stratigraphic evidence of synextensional deposition has not yet been documented for this formation; therefore, the relation between deposition and inception of extension in the CMMCC warrants further investigation.

The Calico Mountains of the central Mojave Desert, CA are located on the hanging wall block of the Waterman Hills detachment fault in the CMMCC. New geologic mapping in the northern Calico Mountains has found direct evidence of synextensional deposition of the Pickhandle Formation and underlying Jackhammer Formation in an intra-hanging-wall half-graben basin bounded on the east by a high-angle NW-trending, SW-dipping normal fault. The Jackhammer Formation is deposited on nonmylonitic basement composed of Paleozoic metasedimentary and metavolcanic rocks and Mesozoic plutonic rocks. It is composed of fluvially-reworked tuff and lapilli tuff that transitions eastward into a welded ignimbrite, tuffaceous sandstone, and local conglomeratic sandstone, mafic lava, avalanche breccia, and lacustrine limestone. In the Calico Mountains, the Pickhandle Formation is deposited conformably over the Jackhammer Formation. It consists of a lower section of dacitic volcaniclastic breccia with a local dacitic block and ash flow deposits and an upper section of tuffaceous sandstone, conglomeratic sandstone, and fluvially-reworked tuff and lapilli tuff. Evidence of growth strata indicates synextensional deposition of both formations, including sedimentary and volcanic deposits that thicken and coarsen toward the basin-bounding normal fault to the east with some deposits thinning on the half-graben footwall, fanning bedding dips that decrease upsection, and internal angular unconformities.