Structural geology and deformational history of the southern Inyo Mountains east of Keeler, Inyo County, California

Late Pennsylvanian to Jurassic rocks exposed in a portion of the southern lnyo Mountains were mapped to evaluate the Permian to Recent deformational history of the area. Structures which formed in response to a major Permian deformational event were observed in tile eastern por1ion of the study area, and a belt of Late Jurassic structures dominates the western portion of the study area. Permian deformation occurred in at least three phases: 1) formation of Early Permian, east-northeast-trending folds; 2) initial Early Permian development of the northeast-trending lnyo Crest thrust fault, associated subparallel folds, and the Fishhook thrust, during the development of a submarine ridge which separated Early Permian deposition into the Lone Pine basin to the west and the Santa Rosa Flat basin to the east; and 3) Late Permian to Early Triassic (pre-Anisian) folding of the Lower Permian sedimentary rocks of Santa Rosa Flat and probable continued movement along the lnyo Crest thrust fault. An angular unconformity mapped within the Santa Rosa Flat strata indicates ongoing deformation during Early Permian deposition. Angular discordance of up to 90° between the Santa Rosa Flat strata and the overlying Late Permian to Early Triassic strata observed in the eastern portion of the study area confirms the presence of a previously inferred Triassic overlap sequence in the southern lnyo Mountains and indicates that the bulk of Permian folding occurred in Late Permian time. Late Permian(?), north-vergent, subisoclinal folds, which are similar in style and vergence to folds mapped to the south of the study area near the Talc City thrust fault, were observed west of the Late Jurassic Flagstaff fault. The slip amount for the lnyo Crest thrust fault could not be determined directly in the study area, but its style and age are similar to faults of the Death Valley thrust belt and it may provide a link between the Last Chance thrust fault system exposed to the north of the study area and the Talc City t11rust fault exposed south of the study area. Middle to Late Jurassic deformation is recorded by northeast-vergent folds and thrust faults as well as by tectonic fabrics consisting of pervasive axial plane cleavage, pressure shadows, streaky and stretching lineations, boudins, and tension fractures. Field relations indicate that the folds formed first and were subsequently tightened and cut by thrust faults, and that Permian folds were locally tightened as well. Intermediate to mafic dikes and other intrusions, which may correlate with the Independence dike swarm, intrude these structures but are ductilely deformed, indicating that minor deformation continued after emplacement. The dominant Flagstaff thrust fault emplaced Pennsylvanian to Early Permian rocks eastward over Early Triassic to Jurassic rocks and is inferred to accommodate 3900 m of reverse slip and 3230 m of horizontal shortening. The Flagstaff fault is interpreted to cut obliquely across subparallel Jurassic folds at a low angle, thus providing a mechanism for the emplacement of fault-bounded rock slivers of various ages within the fault zone and allowing space in the footwall of the fault for a stratigraphic pinch out of 1220 m of the Lone Pine Formation, which crops out in the upper plate but not in the lower plate. These Late Jurassic structures are correlated with the East Sierran structural assemblage which formed during compression of heat-weakened rock on the eastern margin of the developing Sierran magmatic arc. Permian and Jurassic structures are overprinted by late Mesozoic to early Cenozoic conjugate strike-slip faults, northeast-trending normal faults, and local northeast-trending warps and folds. Late Cenozoic normal faults associated with Basin and Range extensional deformation are the youngest structures in the study area. These structures initially developed prior to eruption of Late Miocene to Early Pliocene basalt flows and caused local, normal-sense reactivation of Permian and Jurassic thrust faults, including the Flagstaff fault. The total estimated east-west shortening due to folding and faulting across the study area is approximately 70% and may exceed 100% if observed bulk strain is discrete from folding. Total extension across the study area caused by the mapped normal faults is approximately 8%.