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

The Peninsular Ranges Batholith (PRB) in Baja California shows complex relations between pluton emplacement, arc deformation, and batholith growth.  We carried out paleomagnetic studies in 6 independent plutons or pluton complexes in an E-W transect north of San Quintín. Another complex close at the western end of this transect, San Telmo, had been studied before. Sampling includes close to a 100 sites Aguaje del Burro (the most western), Nueva York, La Zarza, San José, El Potrero, and San Pedro Mártir (SPM) from west to east. Aguaje del Burro outcrops in the low mesas east of the Pacific coastal Baja highway, other plutons outcrop west of and along the foothills of Sierra de San Pedro Mártir, intruding the low-grade metamorphic Alisitos sequence and its suture against high-grade Paleozoic (?) metamorphic rocks. These high-grade rocks are the host of SPM pluton, which is a large “La Posta” type zoned-body. Aguaje del Burro (D=358.5°, I=52.9°; 7 selected sites, k=215.9; a95=4.1°) and NY plutons (D=356.2°, I=54.8°; 8 selected sites, k=114.3; a95=5.2°) yield stable magnetizations residing in magnetite. Tonalitic plutons of Potrero and San José, which lie immediately west of the Main Mártir Thrust, yield largely discordant paleomagnetic directions. The ChRM of Potrero (D=34.6°, I=25.7°; 9 selected sites, k=88.3; a95=5.5°) and San José (D=20.6°, I=29.9°; 15 sites, k=168.9; a95=3.0°) is near univectorial and resides in hematite grains exhibiting high-temperature oxidation textures. SPM yields an overall mean of D=18.9°, I=47.7°; (15 selected sites, k=63.1; a95=5.5°), indicating a tilt of ~25° down to the west. The NRM of SPM is complex, as it includes spurious overprints induced by lightning and a viscous overprint in the direction of the recent dipole field. The ChRM was defined mostly using remagnetization circles and stable-end-points. The ChRM resides primarily in hematite but coarse-grained magnetite is abundant and dominates the rock magnetic signal.