HR: 13:40h
AN: GP12A-01    [PDF]
TI: Magnetic Fabric and Paleomagnetism of the Peninsular Ranges Batholith, Sierra San Pedro M rtir, Baja California.
AU: * Molina-Garza, R S
EM: rmolina@geociencias.unam.mx
AF: Centro de Geociencias, UNAM, Centro de Geociencias, UNAM Campus Juriquilla, km 13 carretera San Luis Potosi, Queretaro, 76230 Mexico
AU: B”hnel, H N
EM: hboehnel@geociencias.unam.mx
AF: Centro de Geociencias, UNAM, Centro de Geociencias, UNAM Campus Juriquilla, km 13 carretera San Luis Potosi, Queretaro, 76230 Mexico
AB: We summarize results of recent paleomagnetic, structural, petrologic and magnetic fabric studies along an east-west (60 km long) transect across the Peninsular Ranges Batholith (PRB) in north-central Baja California. The transect includes both magnetite rich plutons from the western sector of the PRB, and ilmenite rich plutons from the eastern sector, as well as plutons on the eastern and western side of major tectonic discontinuities. We include results for 8 plutons, included well-characterized bodies such as San Pedro M rtir (SP), San Jos‚ (SJ) and La Zarza (LZ), and relatively little known plutons such as Potrero (PO), Aguaje del Burro (AB), El Milagro (MI), and San Telmo (ST). Plutons on the western sector of the PRB yield a paleomagnetic pole at 82$\deg$ N-186.4$\deg$ E (A95=4.8$\deg$). When rotated into a pre- Gulf of California position, the pole (79.2$\deg$ -188.2$\deg$) is statistically undistinguishable from the North American reference pole. In contrast, SP, SJ and PO plutons, on either side of the NW trending Main Martir Thrust yield clearly discordant direction that can only be reconciled with results for the western plutons assuming southwestward tilt of $\sim$ 25$\deg$ for SP and greater than 45$\deg$ for SJ and PO. We find strong evidence in support of tilt of the plutons from thermochronological, structural, and geobarometric data. These data will be discussed elsewhere. Here we focus on magnetic fabric data. AMS for SJ is strongly developed with high values for degree of anisotropy (P= 1.14 a 1.40), but marked east-west asymmetry that contrasts with the general symmetry of the pluton along a north-south axis. Oblate fabrics (T$\sim$ +0.4) with dispersed lineation directions dominate the west side of the pluton and prolate fabrics (T$\sim$ -0.15) with steep to vertical lineations dominate on its eastern side. This fabric is interpreted to result from magma flow. SP, a much larger pluton and sensibly asymmetric, displays high degrees of anisotropy (P$\sim$1.2) on its western side but dominantly oblate (T$\sim$ +0.4) fabric, with foliations parallel to the pluton margins. In contrast, the eastern side of the pluton displays low P values ($\sim$ 1.06-1.10), but markedly oblate fabrics (T$\sim$ +0.6) parallel to the pluton margin. Fabrics in the pluton interior are weakly developed. These data are interpreted to support models of pluton emplacement that involve drag (vertical shear) along the western margin of the pluton along the Main M rtir Thrust during pluton ascent, thus facilitating tilt and deformation of the smaller plutons to the west.
DE: 1518 Magnetic fabrics and anisotropy
DE: 1519 Magnetic mineralogy and petrology
DE: 1525 Paleomagnetism applied to tectonics (regional, global)
DE: 1527 Paleomagnetism applied to geologic processes
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting