TESTING THE STRUCTURAL ROLE OF THE SANTA MARIA BASIN IN THE ROTATION OF THE WESTERN TRANSVERSE RANGES, CALIFORNIA

The onshore Santa Maria Basin (SMB) of southern California sits between the Southern Coast Ranges (SCR) and the Western Transverse Ranges (WTR). Past paleomagnetic studies conducted in the WTR from early Miocene rocks indicate that area rotated 90° clockwise since early Miocene time (e.g., Kamerling and Luyendyk, 1985; Hornafius, 1985). However, studies conducted in the SCR to the north of the SMB have determined that this area experienced an insignificant amount of rotation (e.g., Onderdonk, 2005) during the same time period. The SMB lies in between these two areas, and has not been previously sampled extensively to determine if it is rotated or not and where the boundaries of the rotated domain are located.

The primary goal of this study is to understand, using paleomagnetism, how differences in rotation across the SMB are distributed. This is significant because it will help define whether the western rotation boundary in the SMB is focused along one fault, or if the boundary exists as a more gradual fault zone across the western SMB. The paleomagnetic data for this study was collected from the middle to upper Miocene Monterey Formation. Because the Monterey Formation was deposited prior to deformation within the SMB it records the rotational history of the basin, and has also been used in previous studies (e.g., Hornafius, 1985; Khan et al. 2001) because the dolomite in the formation typically carries a well-preserved primary remanent magnetization. Samples were obtained from 6 sites in the SMB and were subject to step-wise alternating field and thermal demagnetization analysis. Uncorrected equal area diagrams indicate the samples have a remanence dominated entirely by the present day field, resulting in a failure to answer the main questions of the study. Remagnetization in the Monterey Formation probably occurred due to partial oxidation of the pyrite to magnetite as the rocks were lifted above sea level during the Pleistocene, causing flushing of the formation with meteoric groundwater (e.g., Hornafius, 1984). This process is commonly observed in rocks found in fold and thrust belts (eg., Hornafius, 1984; McCable and Elmore, 1989; McCabe and Channell, 1994; Stamatakos et at., 1996; Enkin et al., 2000).