Correction of inclination shallowing and its tectonic implications: The Cretaceous Perforada Formation, Baja California

Abstract

A paleomagnetic and magnetic fabric study of the Aptian to Albian Perforada Formation, the oldest sedimentary unit of the Cretaceous Valle Group, was conducted to determine whether the Perforada inclination had been shallowed by burial compaction and to correct the inclination for any shallowing. The Perforada Formation is located on the Vizcaino terrane of the Vizcaino Peninsula of west central Baja California. An accurate paleolatitude will indicate whether the Vizcaino terrane is far-travelled. Eighty-eight specimens from 43 independently oriented hand samples and cores were collected from 11 sites. A small (10 m) fold was sampled in order to constrain the age of the magnetization with a fold test. Thermal and alternating field demagnetization was able to isolate characteristic magnetizations for most samples. Maximum unblocking temperatures near 580 °C indicate that magnetite is the magnetic carrier. The characteristic magnetization passes the fold test at the 99% confidence level. Anisotropy of anhysteretic remanence (AAR) and anisotropy of magnetic susceptibility (AMS) measurements show that five sites had well-developed bedding-parallel foliations, while four sites had poorly developed AAR fabrics but foliated AMS fabrics. The remaining two sites had poorly developed foliations for both AMS and AAR. The mean inclination ( I=35.6°) of the five sites with well-developed AAR foliations is 13.7° shallower than the mean inclination ( I=49.3°) of the two sites with poorly developed AAR and AMS fabrics. When the inclinations of the sites with well-developed AAR foliations are corrected using an individual particle anisotropy of 1.5, determined from magnetic extract/epoxy samples dried in 30–50 mT DC magnetic fields, the corrected mean inclination ( I=50.1°) is indistinguishable from the mean inclination of the sites with poorly developed AAR fabrics. A corrected formation mean direction for the Perforada Formation ( D=311.8°, I=47.0°, α 95=9.3°) indicates a 33.5°±11.1° counterclockwise rotation and 5.6°±7.7° of post-Cretaceous northward motion, after 3° of Neogene fault motion has been removed. These results indicate that the Vizcaino terrane has moved little with respect to the Peninsular Ranges terrane and cratonic North America since the Cretaceous.