Paleolatitude of the Butano Sandstone, California, and its implications for the kinematic histories of the Salinian terrane and the San Andreas Fault

The lower to middle Eocene Butano Sandstone of the Santa Cruz Mountains, California, is part of the central portion of the Salinian terrane, a slivered granitic fragment west of the San Andreas fault. The Butano Sandstone has been correlated with the Point of Rocks Sandstone across the San Andreas fault, approximately 315 km to the southeast. The magnitude of this offset is the same as the amount of Neogene San Andreas fault offset documented in other studies. Thus, the correlation of these two Eocene units implies that no pre-San Andreas system motion of the Butano Sandstone has occurred. Paleomagnetic studies indicating large northward translations of Cretaceous and Paleocene rocks of coastal California, however, raise the possibility of continued northward motion of the Salinian terrane during or after deposition of the Butano Sandstone. To better our understanding of the Eocene position of the Salinian terrane, the paleolatitude of the Butano Sandstone was determined by paleomagnetic analysis of azimuthally unoriented cores from exploratory oil wells. Specimens from 30 core intervals from the top to the middle or lower parts of the formation were subjected to alternating field and thermal demagnetization. The resulting paleolatitudes group better when bedding is horizontal than in their drill hole attitudes, providing a positive fold test. Several magnetic polarity zones are present. The mean paleolatitude, 34.6°±2.5°, is significantly lower than that expected if the rocks were deposited in their present-day location on an underformed North American craton. When Neogene slip on the San Andreas and on smaller, inboard faults is restored, however, the measured paleolatitude does not differ significantly from that expected. This result supports the correlation of the Butano and Point of Rocks Sandstones and implies that the Salinian terrane experienced no significant motion between deposition of the Butano Sandstone and establishment of the Neogene San Andreas dextral fault system.