Paleomagnetism of Plio-Quaternary basalts in the Jalisco block, western Mexico

Abstract

The interior of the Jalisco block, western Mexico, is disrupted by several NW-SE to NE-SW oriented grabens and associated recent alkaline volcanics which are bounded by WNW oriented regional lineaments. The geometry of these features suggests deformation from a right-lateral shear couple, consistent with NW motion of the Jalisco block relative to the North American plate. A paleomagnetic study was carried out in the Mascota graben, the Talpa de Allende graben, and the San Sebastian volcanic field. Results from 16 sites (D=354.6°, 1=33.3°, k= 18.1, a95= 8.9°) agree with the expected North America directions. Therefore, no significant recent regional displacement or rotations were found. However, the dispersion of paleomagnetic site means is greater than that expected from secular variation alone, suggesting that differential block rotations may have taken place. Volcanism within the Talpa de Allende graben and the southern part of the Mascota graben is probably older than 0.78 Ma as evidenced by the discovery of four reversely magnetized lava flows.