PALEOMAGNETISM OF THE UPPER PALEOCENE LOCATELLI FORMATION, SANTA CRUZ MOUNTAINS, CALIFORNIA: IMPLICATIONS FOR TECTONIC ROTATION AND TRANSLATION OF THE SALINIAN TERRANE

The upper Paleocene Locatelli Formation consists of about 300 m of marine sandstone and siltstone that unconformably overlies Cretaceous granites on the Ben Lomond Block of the northern Salinian terrane in the Santa Cruz Mountains. Magnetic analysis of samples from the Smith Grade section west of Felton showed that the formation is entirely reversed in polarity. Based on the occurrence of late Paleocene planktonic foraminifera (Zone P4-P5) and molluscs of the Turritella infragranulata pachecoensis zone ("Martinez Stage"), this reversed polarity best correlates with Chron C25r (56.3-57.6 Ma). The Locatelli Formation yields a paleolatitude of 22.5 degrees, which is about 15 degrees farther south than its present position, and consistent with previous studies of over 2000 km of northward translation since the late Paleocene. In addition, it is rotated about 136 +/- 9 degrees clockwise, consistent with results reported from Paleocene rocks at Point San Pedro, about 60 km to the northwest on the same block. Since the overlying Eocene-Oligocene rocks are unrotated, such large clockwise rotations on widely separated regions within the same block suggests that either the entire block underwent rotation during the docking of the terrane, or that different portions of the same block underwent the same degree of dextral shear and rotation prior to the early Eocene.