Paleomagnetic and isotope data from southern Mexico and the controversy over the pre-neogene position of Baja California

Abstract

Paleomagnetic and isotope geochronology data for southwestern Mexico, including Baja California (BC), are reviewed to evaluate proposed northward displacements of continental fragments along the western margin of Mexico and North America. The data indicate a relative tectonic stability of southwestern Mexico since mid-Cretaceous. There are no clockwise rotated paleopoles which would point to right lateral shear systems; rather, some counter-clockwise rotated paleopoles indicate the activity of a left lateral shear system. Results from isotope geochronology studies give ages between 120 Ma and 90 Ma for BC, with no distinctive trend along the peninsula. In contrast, there is a clear age trend for undeformed intrusives along the Pacific coast of Mexico, from Puerto Vallarta ("'100 Ma) to Acapulco area ("'55-33 Ma). Ages previously reported for BC vary between 120 Ma and 90 Ma and do not show any trend along the peninsula. Nd-and Srsystematics reveal high crustal contamination for the eastern part of Puerto Vallarta batholith and model age (TDM) higher than 1 Ga. The Los Cabos batholith in BC also shows relatively high crustal contamination and an old model age (TDM = 0.9 Ga), with an intrusion age indistinguishable from the Puerto Vallarta batholith. In combination, these data suggest that the position of BC for the Late Cretaceous - Paleocene was similar to that before the opening of the Gulf of California, with only minor latitudinal displacements.