A PALEOMAGNETIC TRANSECT OF THE PENINSULAR RANGES BATHOLITH NEAR THE 31ST PARALLEL, PART II

Paleodirections in plutons along the western Peninsular Ranges batholith are concordant or slightly clockwise-rotated when compared with the reference direction for North America (D=342°, I=58°), after restoring Baja California to its pre-Neogene position. The San José (SJ) and El Potrero (POT) plutons, on the foothills of Sierra San Pedro Mártir, and essentially emplaced along the Main Mártir Thrust (MMT), exhibit strongly discordant paleodirections. Rotation (tilt) of about 45° about a ~NW horizontal axis brings these directions close to the expected direction and in agreement with directions for the western plutons. In POT, site means are streaked perpendicular to the assumed tilt axis as has been modeled for segmented and deformed plutons. The ChRM of SPM sites indicates a tilt of ~25° down to the west and is also streaked. However, the easternmost  SPM sites appear to carry a higher coercivity and higher unblocking temperature remanence (D=40.7°, I=22.3°; a95=10.0°). Although defined by only 21 samples, this remanence indicates that SPM may have been tilted down-to-the-west by as much as, or even more than, SJ and POT plutons. Systematic rotation respect a NW axis of 30° to 45° may explain a change in emplacement mechanisms, which are brittle in the western plutons and more ductile in eastern plutons, although these may have been emplaced in a pre-heated middle crust. Rotation of plutons such as POT is further supported by magmatic and tectonic foliations in the pluton’s margins and its host rock. Drag during emplacement of SPM, perhaps accommodated by high-angle faults such as the MMT, may be responsible for large-magnitude tilt of SJ and POT. Large-magnitude rotation for SPM is also proposed on the basis on field, geochemical, and geochronological data.