HR: 1340h
AN: GP43C-1495    [Abstracts]
TI: Clockwise Rotation and Implications for Northward Drift of the Western Transverse Ranges from Paleomagnetism of the Piuma Member, Sespe Formation, near Malibu, California
AU: * Hillhouse, J W
EM: jhillhouse@usgs.gov
AF: USGS, 345 Middlefield Rd., Menlo Park, CA 94025, United States
AB: New paleomagnetic results from mid-Tertiary sedimentary beds in the Santa Monica Mountains of southern California reinforce the evidence for large-scale rotation of the western Transverse Ranges, originally postulated from observations of basement-structure trends and supported by paleodeclination data from Eocene and younger rocks. Previously published paleomagnetic data indicate that post-Oligocene rotation amounts to 70°-110° clockwise, affecting the Channel Islands, Santa Monica Mountains, and Santa Ynez Mountains. The Sespe Formation near Malibu consists of a lower member dominated by nonmarine sandstone and conglomerate and an upper section, the Piuma Member, which consists of gray-red sandstone and mudstone interbedded with minor tuff and limestone beds. The Piuma Member has a paleomagnetic pole at 36.7°N, 326.7°E (A_95min=5.0°, A_95max=9.6°), resulting from thermal demagnetization of 34 oriented cores covering beds from Oligocene to Early Miocene age. The data are consistent with significant clockwise rotation (68°±7°) of the region relative to stable North America. Rotation of the western Transverse Ranges is generally viewed as a consequence of Pacific-North American plate interactions after 28 Ma, when east-west subduction gave way to northwest transform motion in southern California. Inclinations from the Piuma study lead to a paleolatitude anomaly of 11°±7°, or a mean northward drift that exceeds generally accepted San Andreas fault displacement by a factor of three. However, analysis of magnetization-direction distributions from this study combined with J. Liddicoat's work at other Sespe Formation sites (total N=131) reveals that sedimentary inclination error, rather than northward drift, may be the primary contributor to the anomaly. Application of the Tauxe and Kent method, which tests the elliptical shape of a given field-direction dataset against a global geomagnetic field model (TK03.GAD), requires mean inclination flattening of approximately 15° to predict the 1.34:1 azimuthal elongation of the Sespe magnetization directions. Compaction may explain the inclination flattening in these sedimentary rocks, but the process does not adequately explain lower-than-expected inclinations found in previous studies of Miocene volcanic rocks of the western Transverse Ranges.
DE: 1525 Paleomagnetism applied to tectonics: regional, global
DE: 8150 Plate boundary: general (3040)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting