GEOLOGIC EVOLUTION OF THE SOUTHEASTERN CALICO MOUNTAINS, CENTRAL MOJAVE DESERT, CALIFORNIA

New geologic mapping, structural data, and 40Ar/39Ar geochronology shed light on early Miocene sedimentation and volcanism, and Neogene deformation in the southeastern Calico Mountains, central Mojave Desert, California. Volcaniclastic sedimentation and dacitic volcanism associated with the Pickhandle Formation were restricted to an area north of the Calico fault and occurred primarily between ~19.4 and 19 Ma. Fine-grained lacustrine deposits (here referred to as the Calico Member of the Barstow Formation) are bracketed between ~19 and 17 Ma, and are thus older than the type section of the Barstow Formation in the Mud Hills. Several 17.1 to 16.8 Ma calc-alkaline dacite domes intrude the Calico Member and represent a previously unrecognized volcanic episode in this region.

In the southeastern Calico Mountains, the Calico fault - part of the central Mojave Desert dextral shear system - strikes WNW and forms a transpressional restraining bend with ~3 km of right-lateral slip and ~1 km of reverse (N-side-up) throw distributed on two main fault strands. The Calico fault may have originated as an early Miocene NE- or NNE-dipping normal fault that unroofed metavolcanic basement rocks in the footwall and created a hanging wall basin in which Pickhandle Formation strata accumulated. This extensional slip must have largely ceased prior to deposition of the Calico Member, which unconformably overlies the Pickhandle Formation north of the Calico fault and rests directly on metavolcanic rocks south of the Calico fault. Deposition of the Pickhandle Formation and at least part of the Calico Member were coeval with rapid unroofing and cooling of the central Mojave metamorphic core complex (Gans et al., this vol.), yet extension within the Calico Mountains is minor and is overprinted by strike-slip faulting.

Calico Member beds north of the Calico fault are intensely folded into numerous ~E-W-trending, upright anticlines and synclines that represent 25-30% N-S shortening. Folds are detached from the underlying Pickhandle Formation, which dips homoclinally ~15-30° S and SE. The geometry and distribution of folds are not compatible with gravity driven folding, folding due to dome emplacement, or wrench folding, but are best explained by transpression between the Calico Member and the Pickhandle Formation.