Paleomagnetic data from the Caborca terrane, Mexico: Implications for Cordilleran tectonics and the Mojave-Sonora megashear hypothesis

Two ancient magnetizations have been isolated in rocks of the Caborca terrane, northwest Mexico. The characteristic magnetizations of Neoproterozoic and Paleozoic miogeoclinal shelf-strata, are-derived Lower Jurassic marine strata, and Jurassic volcanic and volcaniclastic rocks are of dual polarity and east-northeast declination (or south-southwest) and shallow inclination. Magnetizations in Neoproterozoic and Paleozoic miogeoclinal strata are interpreted as secondary (J*) and to be of similar age to those observed in Lower and Middle Jurassic rocks. Remanence acquisition is bracketed between about 190 and 160 Ma. The overall mean (D=15.0 degrees, I=8.5 degrees; n=38 sites; six localities; k=19.1, alpha 95=5.5 degrees) suggests a moderate to large clockwise rotation of 12 to 50 degrees (depending on reference direction assumed) of the Caborca terrane, and rocks of the Sonoran segment of the Cordilleran volcanic are, with respect to the North America craton. When compared with expected inclinations, observed values are not anomalously steep, arguing against statistically significant southward latitudinal displacement of the Caborca block after remanence acquisition. Late Cretaceous intrusions yield primary, dual-polarity steep inclination "K" magnetizations (D=341.4 degrees, I=52.3 degrees; n=10 sites; five localities; k=38.3, alpha 95=7.9 degrees) and have locally remagnetized Neoproterozoic and Jurassic strata. When present, secondary (K*) magnetizations in Neoproterozoic strata are of higher coercivity and higher unblocking temperature than the characteristic (J*) magnetization. Importantly, the regional internal consistency of data for Late Cretaceous intrusions suggests that effects of Tertiary tilt or rotation about a vertical axis over the broad region sampled (similar to 5000 km(2)) are not substantial. Late Cretaceous primary (K) magnetizations and secondary (K*) magnetizations yield a combined mean of D=348.1 degrees, I=50.7 degrees (N=10 localities; 47 sites; k=53.5, alpha 95=6.7 degrees), indicating at most small (<similar to 10 degrees) clockwise rotation of the Caborca region with respect to the craton. Permissible post-late Cretaceous latitudinal displacement is near or below the detection limit of paleomagnetism (<similar to 300 km). Limited data from Lower Cretaceous strata of the Bisbee Group (D=339.9 degrees, I=47.9 degrees; n=4 sites) suggest that the modest clockwise rotations inferred on the basis of J* magnetizations in Jurassic and older strata occurred in Jurassic time. Together, the lack of evidence for southward displacement, yet evidence for statistically significant clockwise rotation, and the overall similarity of Jurassic magnetizations in the Cordilleran are with those of the Caborca block, despite the fact that some of them are clearly secondary, are not consistent with the Mojave-Sonora megashear hypothesis of Late Jurassic left-lateral strike-slip motion of the crust of northern Mexico.