HR: 17:35h
AN: GP34A-06    [Abstracts]
TI: Paleomagnetism & Geobarometry of Cretaceous Plutons: Evidence that the Peninsular Ranges Batholith of Baja California is NOT Far-Traveled
AU: * Symons, D T
EM: dsymons@uwindsor.ca
AF: Department of Earth Sciences, University of Windsor, 403 Sunset Ave., Windsor, ON N9B3P4, Canada
AU: Smith, T E
EM: SmtTESMITH@aol.com
AF: Department of Earth Sciences, University of Windsor, 403 Sunset Ave., Windsor, ON N9B3P4, Canada
AU: Blackburn, W H
EM: blackbu@uwindsor.ca
AF: Department of Earth Sciences, University of Windsor, 403 Sunset Ave., Windsor, ON N9B3P4, Canada
AB: Characteristic remanence directions, residing in single and pseudosingle domain magnetite, of D = 1°, I = 51°, A95 = 4° and D = 0°, I = 56°, A95 = 6° have been determined for 18 and 8 sites (311 specimens) in the 108 ± 1 Ma Alpine and 100 ± 1 Ma Ramona tonalite complexes, respectively, in the western zone of the Peninsular Ranges Batholith near San Diego, California. At 16 sites, Al-in-hornblende geothermobarometry gives crystallization depths of 8.8 to 14.1 km that, with previous depths and geologic mapping, indicate orogen-parallel faults with vertical displacements up to several kilometers that bound fault blocks. Paleomagnetic results from the eight available Cretaceous igneous collections, totaling 93 sites, indicate an 11 ± 3° ENE-side-up tilt of the blocks between ~125 and 89 ± 7 Ma. The tilt is attributed to torque from ENE shallow underthrusting of the Farallon Plate and resistive WSW overthrusting of the North American Plate. Also ~440 km of dextral displacement of the western zone of the batholith is predicted on an intra-arc fault. Excluding Neogene-opening of the Gulf of California, the eastern zone has not been displaced relative to North America. This fault block rotation model provides a likely tectonic solution to the Baja California debate between "fixist" geologists and "mobilist" paleomagnetists.
DE: 1525 Paleomagnetism applied to tectonics: regional, global
DE: 8108 Continental tectonics: compressional
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Joint Assembly