Simple shear of southern California during Neogene time suggested by paleomagnetic declinations

Abstract

We have studied the paleomagnetism of various Neogene age rocks in southern California as a means of determining the amounts of Oligocene and younger tectonic rotation and translation which has occurred in this region. Our results suggest that fully 25% of this area, in particular the Transverse Ranges, has undergone extreme clockwise rotation. Work in southeastern California implies that 40° of clockwise rotation has occurred here, although paleomagnetic declinations adjacent to a major right lateral fault are apparently rotated over 200°. The crustal block bounded by the San Gabriel and San Andreas fault has undergone a net clockwise rotation of 35° although the data here can be interpreted to show that an original early Miocene rotation of about 50° was followed by a late Miocene or Pliocene counterclockwise rotation of 15°. Paleomagnetic results from the offshore islands suggests that San Clemente, Santa Barbara, and San Nicolas islands have not rotated but Santa Catalina has undergone about 90° of clockwise rotation. All of the northern Channel Islands, including Anacapa, Santa Cruz, Santa Rosa, and San Miguel, are implied to have rotated 70° or 80° clockwise. This result is also found for the Santa Monica Mountains east of Anacapa Island. In the Santa Ynez Range north of the Channel Islands, paleomagnetic study of the Monterey Formation also indicates large clockwise rotations of near 90°. These data also suggest that the Santa Maria Basin is not rotated and that the north boundary of the rotated region is the Santa Ynez River fault. Stratigraphic control on the paleomagnetic data from the Monterey Formation implies that the rotation began about 16 m.y. ago and may be continuing today in the western region. Paleomagnetic inclination data from our study show that the northern Channel Islands, in particular, may have translated 15° northward since middle Miocene time. However, equally valid interpretations of these same data are that the low inclinations are due to the combined effects of erroneous structural corrections, non dipole magnetic field behavior and right offset on the San Andreas fault system. Palinspastic reconstruction of southern California regions for the early Miocene implies that parts of the Transverse Ranges structures were once aligned with north trending extensional structures in the southwestern United States. We propose that Pacific‐American plate interactions both rifted the continental crust to create this pattern and rotated the western‐most structures within a dextral simple shear zone which had a half width of about 400 km.

Tectonics of strike-slip restraining and releasing bends

“…Thus, the progressive evolution of a fault bend may involve local vertical axis rotations within the bend, and vertical axis rotations in the larger region that the bend occurs within (Luyendyk et al 1980;Westaway 1995;Cowgill et al 2004b). This may be indicated by changes in strike trends, and can be proven palaeomagnetically (Luyendyk et al 1985). Progressive vertical-axis rotations within a fault bend will result in changing fault kinematics as the faults rotate relative to the external stress field.…”

Miocene extension and post-Miocene transpression offshore of south-central California. Structure and tectonics of the central offshore Santa Maria and Santa Lucia basins, California: Results from the PG&E/EDGE seismic reflection survey

“…Models indicate that the western Transverse Ranges rifted away from the Peninsular Ranges and rotated about 110° outward about an east hinge (Kamerling and Luyendyk, 1985;Crouch and Suppe, 1993). The resulting large-scale extension was accommodated by detachment faults in these models (Yeats, 1976;Kamerling and Luyendyk, 1985;Legg, 1991;Crouch and Suppe, 1993).…”

“…Rotation models incorporate the geologic tie, indicated by Eocene (Poway) conglomerate clasts, between Santa Rosasouthwestern Santa Cruz Islands and the San Diego area (Abbott andSmith, 1978, 1989;Kamerling and Luyendyk, 1985). Models indicate that the western Transverse Ranges rifted away from the Peninsular Ranges and rotated about 110° outward about an east hinge (Kamerling and Luyendyk, 1985;Crouch and Suppe, 1993).…”

“…2 and Santa Cruz and Santa Rosa Islands (compilation in Weigand and Savage, 1993). Older KAr dates include Oligocene ages on San Miguel Island, while the intrusions which extend from San Luis Obispo to Morro Bay are dated at 22.7*0.9 to 27.2±0.8 Ma (Kamerling and Luyendyk, 1985; others, 1991, recalculated from Turner, 1970). The San Miguel Volcanics mapped on Santa Rosa and San Miguel Islands are stratigraphically above the Rincon Shale .…”

Taking apart the Big Pine fault: Redefining a major structural feature in southern California

“…Hill and Dibblee [1953] believed the fault to be a conjugate shear of the San Andreas fault and partially responsible for the Big Bend. The fault has also been postulated to be the northern boundary of the western Transverse Ranges rotational domain [e.g., Whidden, 1994;Dickinson, 1996], which has experienced approximately 90°of clockwise vertical axis rotation since 18 Ma [e.g., Luyendyk et al, 1985;Hornafius et al, 1986]. Other designations include the current southern limit of the anomalous Salinian terrane [Ross, 1984] and a Quaternary feature accommodating ''tectonic escape'' in the Big Bend of the San Andreas fault [Keller et al, 1997].…”