STRATIGRAPHY AND STRUCTURE OF THE SOUTHERN MITCHELL RANGE, SAN BERNARDINO COUNTY, CALIFORNIA: IMPLICATIONS FOR THE SOUTHERN MARGIN OF THE CENTRAL MOJAVE METAMORPHIC CORE COMPLEX

The complex array of structures in the southern Mitchell Range is the consequence of a polyphase deformation history involving early NE-SW directed extension associated with early Miocene core complex development, followed by strike-slip faulting and transpressional folding associated with NW-directed dextral shear. New geologic mapping, structural data, and 40Ar/39Ar geochronology of the southern Mitchell Range provides insight into the southern terminus of the Waterman Hills detachment fault, and sheds light on the nature of the structure and stratigraphy related to early Miocene extension. Detailed 1:12,000 mapping in the Lead Mountain and Elephant Mountain areas of the southern Mitchell Range has identified low angle extensional faults in the hanging wall of the Waterman Hills detachment fault. Basement rock and lower Miocene stratigraphy are offset, tilted, and repeated along a set of southeast-dipping low angle normal faults. These faults have apparently been reoriented by clockwise vertical axis rotation, similar to correlative sedimentary and volcanic rocks in the ranges to the south and east, and the original orientation of these faults is interpreted to record top-to-the-northeast extension along the central Mojave metamorphic core complex. Folds in sedimentary units have northeast trending axial traces that are orthogonal to the local extension direction. Folding of lacustrine limestones and sandstones is probably due to transpression along the Harper Lake Fault zone and is influenced by the presence of a rigid intrusive body.

The stratigraphy of the southern Mitchell Range includes intrusive and extrusive volcanic rocks, lacustrine facies limestones and siltstones, and coarse granitic and metavolcanic breccias and conglomerates. Two new 40Ar/39Ar ages constrain the deposition of fine-grained lacustrine sediments between 19.38 ± 0.1 Ma and 17.2 ± 0.1 Ma. These sediments have traditionally been included within the Barstow formation and interpreted as post-extensional, however deposition of these sediments coincides with coarse-grained Pickhandle Formation deposits elsewhere, and with the timing of uplift along the footwall (~20-17.5 Ma, Gans et al., 2005).