Abstract

New geologic mapping, structural data, and 40Ar/39Ar geochronology document early Miocene sedimentation and volcanism and Neogene deformation in the Calico Mountains, located in a complexly deformed region of California's central Mojave Desert. Across most of the Calico Mountains, volcaniclastic sediments and dacitic rocks of the Pickhandle Formation accumulated rapidly between ca. 19.4 and 19 Ma. Overlying fine-grained lacustrine beds (here referred to as the Calico Member of the Barstow Formation) are bracketed between ca. 19 and 16.9 Ma, and are thus older than the type section of the Barstow Formation in the Mud Hills. Several 17.1–16.8 Ma calc-alkaline dacite domes intrude the Calico Member and represent a previously unrecognized volcanic episode in this region.

In the southern Calico Mountains, the Calico fault (part of the Eastern California shear zone) forms a west-northwest–striking, transpressional restraining bend with ∼3 km of right-lateral slip and perhaps 1 km of reverse (north side up) throw distributed on two main fault strands. Part of the Calico fault appears to have originated as an early Miocene 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 directly overlies metavolcanic rocks south of the Calico fault. Deposition of the Pickhandle Formation and at least part of the Calico Member was coeval with rapid unroofing of the central Mojave metamorphic core complex, yet extension in the Calico Mountains is minor and is overprinted by dextral faulting and transpression.

Calico Member beds north of the Calico fault are intensely folded into numerous east-west–trending, upright anticlines and synclines that represent 25%–33% (up to ∼0.5 km) north-south shortening. Folds are detached along the base of the Calico Member and thrust over the Pickhandle Formation, which dips homoclinally ∼15–30°S to SE. The geometry and distribution of folds are most compatible with localized transpression between the Calico Member and the Pickhandle Formation within a positive flower structure. Transpressional folding and faulting in the Calico Mountains postdate the ca. 17 Ma dacite intrusions and appear to be largely restricted to the area along the Calico fault restraining bend.

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