NEOGENE TECTONICS AND PALEOGEOGRAPHY OF THE SOUTHWESTERN GREAT BASIN, CALIFORNIA (RANGE, INYO COUNTY, CRUSTAL EXTENSION)

This study characterizes crustal extension in the southwestern Great Basin, California, in terms of the timing of its inception, its evolution, and the areas of present activity through an examination of volcanic activity, timing and orientations of structural features, and sedimentary rock types and source areas. Inception of Basin and Range activity generally proceeded across the study area from east to west. Normal faulting and west-northwest extension may have begun by 20 million years ago in the Death Valley area. About 13 million years ago igneous activity, faulting, and eastward tilting became widespread in areas east of Panamint Valley. West of the Panamint Range detailed study of the Darwin Plateau volcanic field, at the north end of the Argus and Coso Ranges, reveals that igneous activity began about 7.7 million years ago, with eruptions onto a generally flat erosion surface. The peak of volcanism occurred between 5 and 6 million years ago, when the earliest normal faulting also was recorded on the plateau. Faults and dikes indicate that west-northwest extension has been dominant, except during a period of east-northeast extension that lasted at least from 5.7 to 5.3 million years ago. A paleomagnetic study shows that no significant rotation has occurred in the eastern Darwin Plateau. Between three and four million years ago Basin and Range tectonic activity began in the Coso and Saline Valley volcanic fields, south and north of the Darwin Plateau area. At this time the present basins and ranges were developing throughout the southwestern Great Basin. Volcanic activity continued into late Quaternary time in the Coso Range and Owens Valley.; West-northwest extension has been dominant in Neogene time over the entire southwestern Great Basin. Present-day extension over the entire region is also oriented west-northwest, as evidenced by earthquake fault-plane solutions, young faults, and geodetic strain measurements. Right-lateral strike-slip faults trending north-northwest have played a major role in accommodating strain in the southwestern Great Basin. The main pulse of uplift, tilting, and volcanism east of the Panamint Range began when the Mendocino fracture zone was positioned at the latitude of the southern end of Death Valley (13 m.y. ago). A westward shift in volcanic and tectonic activity closely corresponds to a change in relative motion between the Pacific and North American plates (4-6 m.y. ago).