Paleomagnetic data from the Trans-Mexican Volcanic Belt: implications for tectonics and volcanic stratigraphy

We report a paleomagnetic and rock-magnetic study of Miocene volcanic rocks from the Trans-Mexican Volcanic Belt, A total of 32 sites (238 oriented sa mples) were collected from three localities: Queretaro, Guadalajara and Los Altos dr Jalisco basaltic plateaux, which span from Il to 7.5 Ma. Several rock-magnetic experiments were carried out in order to identify the magneti c carriers and to obtain information about their paleomagnetic stability. M icroscopic observation of polished sections shows that the main magnetic mi neral is Ti-poor titanomagnetite associated with exsolved ilmenite. Continu ous susceptibility measurements with temperature yield in most cases reason ably reversible curves with Curie points close to that of magnetite. Judgin g from the ratios of hysteresis parameters, it seems that all samples fall in the pseudo-single domain (PSD) grain size region, probably indicating a mixture of multidomain (MD) and a significant amount of single domain (SD) grains. Based on our paleomagnetic and available radiometric data, it seems that the volcanic units have been emplaced during a relatively short time span of 1 to 2 My at each locality. The mean paleomagnetic directions obtai ned from each locality differ significantly from that expected for the Midd le Miocene. The mean paleomagnetic direction calculated from 28 sites disca rding those of intermediate polarity is I = 32.46 degrees, D = 341.2 degree s, k = 7.2 and alpha(95) = 11.6 degrees. Comparison with the expected direc tion indicates some 20 degrees anticlockwise tectonic rotations for the stu died area, in accordance with the proposed left-lateral transtensional tect onic regime already proposed for this period.