PALEOMAGNETISM OF THE POINT ARENA TERRANE WITH INCLINATION FLATTENING CORRECTION AND IMPLICATIONS FOR TERRANE TRANSPORT

Controversy exists over translation of tectonic terranes along the California coast. Several paleomagnetic studies of sedimentary and batholitic rocks of coastal California all show similar shallow inclinations (Kodama and Davi, 1995), supporting substantial transport of the terranes beyond San Andreas Fault motion. Previous paleomagnetic studies indicate that Eocene turbidites from the Point Arena terrane were deposited at 25°N, indicating ~2000 km of northwestward displacement of the tectonic block (Kanter, 1983). However, critics of large tectonic displacement contend compaction has shallowed the inclination and suggest far less translation history (Kodama and Davi, 1995; Dickenson and Butler, 1998). Dickinson and Butler (1998) place the Point Arena terrane 575 km south of its present position during the Eocene.

A paleomagnetic study of the German Rancho Formation within the Point Arena terrane is in progress to determine the amount of post-Eocene displacement. Preliminary results confirm previous paleomagnetic data reported by Kanter (1983). Both normal and reversed polarity characteristic directions are present and are antipodal. Our results will then be applied to determine the amount of compaction shallowing. Numerous studies have been completed that evaluate the effect of compaction on inclinations (Jackson et al., 1991; Kodama and Davi, 1995; Kodama and Ward, 2001). These methods utilize either anisotropy of magnetic susceptibility (AMS) or anisotropy of anhysteretic remanent magnetization (A-ARM) to determine the compaction-related mineral fabric in sedimentary rocks. Correlations between magnetic fabrics (AMS and A-ARM) and paleomagnetic inclinations are then used to evaluate, and possibly correct for, the occurrence of significant compaction flattening. For these rocks, these data will be used to directly test the hypothesis of Dickinson and Butler (1998), that attributes the shallow remanence inclinations entirely to compaction-flattening processes.