A Comprehensive Paleomagnetic Study on Radiometrically Dated Late Cretaceous Lava Flows from Jalisco Block (Western Mexico)

Abstract:

Western and central Mexico is segmented by several regional structural systems that bound crustal blocs. Paleomagnetic data from the western and eastern Trans Mexican Volcanic Belt as well as from adjacent terrains are still scarce which limits analyses of the both local and regional-scale tectonic evolution. A combined radiometric and paleomagnetic survey performed on late Cretaceous lava flows demonstrate that vertical-axis rotations characterize the paleotectonic evolution of western-central Mexico. The characteristic paleomagnetic directions determined in this study may be considered of primary (thermoremanent) origin. Multicomponent demagnetization plots were observed in some cases.

In general, the polarity obtained for the flows studied is consistent with their stratigraphic position and with the radiometric age determination. The mean inclination is in reasonably good agreement with the expected inclination for the Late Creataceous, as derived from reference poles given by Besse and Courtillot (2002) for the North American craton. The declination, however, is quite different from that expected, which suggests a possible counterclockwise tectonic rotation of at least 12º.

Aceptable palointensity determinations were obtained for only eleven individual samples from two basaltic lava flows. The mean virtual dipole moment (VDM) obtained in this study is 4.2 ± 1.2 _ 1022 A m², which is almost half than the present geomagnetic field strength.