Analysis of foreland basement deformation associated with the Clark Mountain thrust complex, southeastern California
Burchfiel, B. C.
Master of Arts
In the Sevier orogenic foreland exposed in the Clark Mountains of southeastern California, results of this study indicate that Precambrian igneous and metamorphic basement, autochthonous relative to eastward vergent Mesozoic thrusting, exhibits two different modes of postPrecambrian foreland deformation in two separate structures. In the Mesquite Mountains antiform, draping of sedimentary cover (which consists of three thrust sheets overlying thin, autochthonous sediments) has resulted from rigid rotation of basement blocks which core the antiform* This antiform, which probably formed after latest Sevier thrusting, is analogous to Larimide basement uplifts and associated drape structures studied by Stearns (197) in the Rocky Mountains. Basement in the complex, overturned Kokoweef svncline probably deformed through a combination of spaced (simple) shear and pervasive (pure) shear in response to folding of sedimentary cover associated with early thrusting in the Sevier orogen. In both structures the unconformable contact between basement and cover is unsheared. Also all foliations and folds in the basement were formed in the Precambrian, and have not been folded during Late MesozoicCenozoic formation of the Mesquite Mountains antiform or the Kokoweef syncline. However basement is more strained on a microscopic scale in the Kokoweef syncline than in the Mesquite Mountains antiform. Basement, during folding of its sedimentary cover, can adjust in three possible ways: 1) by folding, 2) by rigid rotation, or 3) by spaced or pervasive shearing. Detailed mapping of foliations presented in this study shows that the basement was not folded during formation of either structure. The unsheared basement-cover contact in the Mesquite Mountains antiform is planar indicating that rigid rotation of the basement occurred, which requires little or no basement shortening. In the Kokoweef syncline, this same contact is folded, which probably requires extensive basement shortening. Thus basement in the Kokoweef syncline adjusted by shearing along reactivated old foliation planes, although the exact mechanism is unclear and enigmatic. The ultimate cause of foreland basement deformation is dependent on local boundary conditions and not necessarily indicative of regional stresses. Involvement of basement in foreland structures, and the style of this involvement, is controlled by a number of factors including mechanical properties of basement and cover, position and timing of foreland deformation relative to tectonic elements, and scale, all being interrelated. Differences in these factors might explain the lack of pervasively developed basement uplifts in the southern Cordillera relative to the Rocky Mountain foreland.