Discordant paleomagnetic data for middle-Cretaceous intrusive rocks from northern Baja California:: Latitude displacement, tilt, or vertical axis rotation? -: art. no. 1049

[1] Paleomagnetic results and U/Pb zircon dating from the San Marcos dike swarm and the El Testerazo pluton in the Cretaceous Peninsular Ranges batholith of northern Baja California are used to evaluate alternative pre-Neogene paleogeographic reconstructions of the Baja California peninsula. The San Marcos dike swarm is a dense, northwest striking, regional dike swarm that is exposed over an similar to100 km long segment of the batholith and has yielded a U/Pb zircon crystallization age of 120 +/- 1 Ma. Dike attitudes from the swarm suggest a regionally consistent average similar to320degreesE strike and similar to79degreesNE dip. The El Testerazo pluton is a younger tonalite intrusion that truncates the northern end of the dike swarm. All but one of 36 sites sampled in this study show remanence of normal polarity. Paleopoles for the San Marcos dike swarm and El Testerazo pluton are indistinguishable and were combined into a paleopole at 248.1degreesE, 86.6degreesN, A(95) = 4.8degrees, which is displaced with respect to the 122 Ma reference pole for stable North America at 198.2degreesE, 72.3degreesN, A(95) =3.3degrees. The displacement may be described by an apparent clockwise rotation of 18degrees +/- 6degrees and an apparent northward shift of 8degrees +/- 5degrees. Restoring a northward shift of about 3degrees, related to the separation of Baja California from North America since 10 Ma, only a marginal northward displacement of 5degrees +/- 5degrees is left. The clockwise rotation may be the result of crustal block rotations within the right-lateral shear systems in northern Baja California, although there is no geological evidence that supports this possibility. Alternatively, the difference between paleopole and reference pole may be due to tilting of the study area. Restoring a northeastward tilt of 11degrees, based on the mean dip measured for the San Marcos dike swarm in the study area, yields a paleopole at 187.6degreesE, 70.8degreesN, A(95) = 5.6degrees, which is indistinguishable from the 122 Ma North American reference pole. The tilting hypothesis suggested previously as a potential explanation for low inclination of paleomagnetic data from Baja California therefore seems to be viable for the discordant paleopoles from this segment of the batholith. Data obtained previously about 150 km further south and along the western edge of the batholith defined a concordant paleopole without any tilt correction, indicating that regional tilting may not be valid for all of Baja California.