Description
El Capitan, located 50 km southeast of Yuma, Arizona, consists of complexly deformed and moderately metamorphosed carbonate, clastic, and volcanic strata, granitic and granodioritic plutonic rocks, and mylonitic plutonic gneiss. The metamorphosed carbonate and clastic rocks can be subdivided into formations correlative with the upper Paleozoic-lower Mesozoic cratona1 section of the Colorado Plateau, northwestern Arizona. Metamorphosed lithologic equivalents of the Pennsylvanian-Permian Supai Group, Permian Coconino Sandstone, Permian Toroweap and Kaibab Limestones, Triassic Moenkopi Formation, and Upper Triassic-Lower Jurassic Aztec Sandstone are present. A sequence of metasedimentary rocks overlying the metamorphosed Aztec Sandstone may be correlative with Jurassic strata on the Colorado Plateau; however, the exact correlation is not understood. The metavolcanic rocks at El Capitan are believed to be Jurassic in age based on stratigraphic relationships observed within the study area. A Late Jurassic emplacement age for the granodioritic plutonic rocks is suggested from radiometric studies. All of the rocks at El Capitan were deformed and metamorphosed at upper greenschist-lower amphibolite facies subsequent to the emplacement of the plutonic rocks. Deformation was due to top-to-the northeast simple shear, although a component of pure shear may have been present. Deformation resulted in the formation of thrust faults, mylonitic shear zones, large- and smale-scale folds, and a pervasive axial-planar foliation. The age of this deformational event can be bracketed between Middle Jurassic and earliest Late Cretaceous time. Because the cratonic strata at El Capitan appear to be in place relative to Paleozoic-lower Mesozoic depositional trends, it seems probable that the Mojave-Sonora megashear cannot extend across the eastern Mojave Desert, California, but if it exists, must instead be located west of this area. However, this reconstruction creates distinct problems with the definition of the megashear as separating Precambrian terranes whose boundary is well-documented to be 50 km east of El Capitan. Alternately, the Paleozoic hingeline and miogeoclinal belt may swing around the southwestern United States and northwestern Mexico unbroken by major left-lateral faults. The Mesozoic deformational history recorded in the rocks at El Capitan appears to be remarkably similar to the deformational history of Mesozoic compressional terranes exposed to the north of this area. The similarities in the style and timing of deformation in these areas could be merely coincidental, but more likely, are related by some large-scale tectonic process.
