Structural and stratigraphic evolution of the Calico Mountains: Implications for early Miocene extension and Neogene transpression in the central Mojave Desert, California

Abstract

New geologic mapping, structural data, and 40 Ar/ 39 Ar 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 fi ne-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 hangingwall 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 unroofi ng 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 fl ower 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.

Evidence of early Miocene synextensional volcanism and deposition in the northern Calico Mountains, central Mojave metamorphic core complex, southern California, USA

“…1 and 2;McCulloh, 1952;Dibblee, 1994;Fillmore and Walker, 1996). Previous radiometric dating of the Pickhandle Formation indicates an early Miocene age, bracketed between 23.7 and 18.9 Ma (Burke et al, 1982;Walker et al, 1995;Fillmore and Walker, 1996); additional 40 Ar/ 39 Ar ages by Singleton and Gans (2008) in the southeastern Calico Mountains suggest that the age of volcanic and intrusive rocks of the Pickhandle Formation in this area is 19.3-19.0 Ma. Due to its coarse-grained nature and outcrop proximity to the CMMCC, previous studies suggested that the Pickhandle Formation-and possibly the Jackhammer Formation-represent synextensional supradetachment-basin deposits (Fillmore et al, 1994;Fillmore and Walker, 1996).…”

“…1 and 2; Glazner et al, 2002). East-west-trending folds are also observed within the central Mojave Desert, with crustal shortening attributed to local transpression related to northwest-trending right-lateral faults (e.g., Dibblee, 1980;Singleton and Gans, 2008) or regional north-south compression (Bartley et al, 1990). In the Calico Mountains, the main geologic structure is the northwest-trending Calico fault (Figs.…”

“…1 and 2), which has a maximum right-lateral offset of ~10 km and forms an ~10-km-long restraining bend that resulted in transpressional folding of Barstow Formation lacustrine sediments adjacent to the fault (Fig. 2; Singleton and Gans, 2008).…”

“…Due to its coarse-grained nature and outcrop proximity to the CMMCC, previous studies suggested that the Pickhandle Formation-and possibly the Jackhammer Formation-represent synextensional supradetachment-basin deposits (Fillmore et al, 1994;Fillmore and Walker, 1996). Studies in the southeastern Calico Mountains recognized that extensional faults are not common (possibly due to reactivation as strike-slip faults), yet those present are typically northwest-trending, steeply (≥45°) southwest-dipping normal faults with small offsets (<50 m) resulting in only 5% total magnitude of northeast-southwest extension (Singleton and Gans, 2008). These faults are likely related to CMMCC extension, given that they are present in the Pickhandle Formation but are rare in the overlying stratigraphic units (Singleton and Gans, 2008).…”

The Pickhandle and Jackhammer Formations in the Northern Calico Mountains, Ca: Evidence of Synextensional Deposition and Volcanism in the Central Mojave Metamorphic Core Complex

“…Generalized geologic map of the central Mojave Desert region near the Calico Mountains, showing the location of the Waterman Hills detachment fault (WHDF) and the distribution of early Miocene rocks in the area (modi ed fromSingleton and Gans, 2008…”