COMPARISON OF DIKE AZIMUTHS IN THE INDEPENDENCE DIKE SWARM, CALIFORNIA, TO PALEOMAGNETIC DATA—IMPLICATIONS FOR LATE CENOZOIC VERTICAL AXIS ROTATIONS

The Late Jurassic Independence Dike Swarm (IDS) extends from the eastern Transverse Ranges northward to the central Sierra Nevada in southeastern California. A comparison of paleomagnetic data suggests that the dike orientations may be a limited guide to late Cenozoic (post-23 Ma) vertical axis rotations. The paleomagnetic declinations determined from previous studies were compared to the azimuths of approximately 3600 dikes from 45 domains throughout the dike swarm. Most IDS azimuths tend to cluster near 312°±10° when corrected for predominantly clockwise rotations, as inferred from paleomagnetic studies of Miocene volcanic rocks. This implies a consistency of pre-rotation strike of the dikes along much of the swarm prior to the vertical axis rotations. However, dike azimuths in a quarter of the domains where paleomagnetic corrections could be applied vary widely from the mean, usually in the clockwise sense. Large deviations from the main trend of the swarm may reflect (1) dike intrusions of other ages, (2) crack filling at angles oblique or perpendicular to the main swarm, (3) pre-Miocene rotations, or (4) unrecognized domain boundaries between sites with paleomagnetic determinations and the dikes. Therefore, we urge caution when inferring rotations from Independence dike trends in the absence of supporting paleomagnetic data.