PALEOMAGNETISM OF COASTAL CALIFORNIA AND BAJA-CALIFORNIA - ALTERNATIVE TO LARGE-SCALE NORTHWARD TRANSPORT

Paleomagnetic data from the Santa Lucia-Orocopia (SLOA) and Baja-Borderland (BBA) allochthons of coastal California and Baja California have been interpreted to indicate up to 2500 km of post-mid-Cretaceous northward transport of these regions with respect to interior North America. However, with Neogene strike-slip offsets taken into account, geological interpretations correlate basement rocks of the coastal allochthons with continental basement rocks directly across the San Andreas and related fault systems. We have examined paleomagnetic data from SLOA and BBA and conclude that apparent discordances can be explained without large-scale pre-Neogene tectonic transport. Three major observations are fundamental to this analysis: (1) Paleolatitudes derived from volcanic rocks of the Jurassic Eugenia Formation of BBA and Coast Range ophiolite of SLOA are concordant when compared to revised Jurassic reference paleomagnetic poles from interior North America. (2) Isotopic and paleobarometric data from the Peninsula Ranges batholith in southern California indicate that the batholith has been tilted northeast-side-up by an amount that can account for discordant paleomagnetic directions observed in plutonic rocks of the batholith without large-scale northward transport. (3) Literal interpretation of the paleolatitudes determined from paleomagnetic directions in Upper Cretaceous and Paleogene marine sedimentary rocks of SLOA and BBA requires north-then-south-then-north transport and a complex motion history between the two allochthons. However, concordant paleolatitudes are indicated by some sedimentary rocks while coeval or younger sedimentary rocks of the same allochthon have discordant paleolatitudes. Coupled with recent documentations of compaction shallowing of paleomagnetic inclination in other marine sedimentary rocks, these inconsistencies suggest that paleolatitudes derived from most of the marine sedimentary rocks of SLOA and BBA are biased towards low paleolatitudes by compaction shallowing.