Geology of a transect of the Northern Argus Range, California

Abstract

Study of a transect of the northern Argus Range reveals Mesozoic structural features, recognized regionally, in complex relationship with local plutons and intrusion-related deformation. These have affected a 2,000-m-thick section of miogeoclinal, predominantly carbonate rock ranging in age from Devonian to Permian. During the Mesozoic the study area was located on the inland margin of a major Andean-type arc, represented by the Sierra Nevada batholith. Lithosphere adjacent to the batholith was greatly weakened by heating and experienced repeated episodes of compressive shortening. What may be the oldest folds in the area, northeast-trending (FA) folds, are located predominantly on Lookout Mountain and may be either rotated younger (FB) folds or related to the early Mesozoic Last Chance thrust system. Northwest-trending FB folds are members of a regionally recognized fold set characterized by a well developed axial surface cleavage and by vergence or overturning to the northeast. The Argus anticline is the main FB fold and is the most prominent structural feature of the study area. In the northern and southern portions of the study area, plutons have intruded and disrupted the hinge area of the Argus anticline. Three plutons of known or probable Middle Jurassic age have chemistry, normative mineralogy and modal mineralogy that are similar to calc-alkaline plutons making up the bulk of the Sierra Nevada batholith. One of these plutons has a unique and complex history of forcible emplacement. At least three fault sets, including a preintrusive, northwest-trending fault, thrust faults and northeast-trending faults are not regionally correlative and may be at least partly resultant of local intrusion-related stresses. The left-normal-slip Osborne Canyon fault significantly offsets the hinge area of the Argus anticline and cuts the Osborne Canyon granite. This fault is probably related to a widespread set of northwest-trending, left-separation faults which, in part, may pre-date Independence dikes of Late Jurassic age. Late Cenozoic Basin and Range normal faulting and basalt volcanism characterize the most recent phase of structural and igneous activity.