Earth Planets Space, Vol. 58 (No. 10), pp. 1315-1322, 2006
Roberto S. Molina Garza1 and Luis Alva Valdivia2
1Centro de Geociencias, Universidad Nacional Autónoma de México, Campus Juriquilla, Querétaro, MEXICO 76230
2Instituto de Geofísica, Universidad Nacional Autónoma de México, Coyoacán, MEXICO 04510
(Received January 6, 2006; Revised June 4, 2006; Accepted June 5, 2006; Online published November 8, 2006)
Paleomagnetic data from three plutons of the latest Cretaceous to early Tertiary Mezcala intrusive suite, of the Mezcala mining district in northern Guerrero State, in southern Mexico, are characterized by well-defined near univectorial magnetizations of both normal and reverse polarities. The magnetizations reside primarily in a ferromagnetic cubic phase (magnetite or maghemite) and partly in hematite. Within-site dispersion is relatively small, but between-site dispersion is not. The overall mean for 9 selected sites is of D=318.5° and I=47.1° (k=18.5; α95=12.3°), and is discordant with respect to the reference direction for North America. The discordance may be explained by tilt down-to-the-northeast of the structural block that contains the intrusive suite of about 40° about a NW-SE trending axis, or perhaps more likely and based on additional geological evidence, by a combination of ∼20° of counterclockwise rotation about a vertical axis and a smaller amount of tilt. Vertical-axis-rotation is supported by paleomagnetic data from nearby localities, whereas a small magnitude of tilt is supported by geological field relations. This result is consistent with the hypothesis that Laramide structures of the Guerrero-Morelos platform were reactivated by a younger deformation event, and that there is a deformation event superimposed that involves lateral slip. The contrasting orientation of NW-SE trending structures within the Mezcala mining district affecting the plutons, and N-S trending Laramide structures, suggests that the discordant paleomagnetic directions are best explained by a combination of tilt and rotation.
Key words: Paleomagnetism, Laramide, Guerrero, Mexico.