PALEOMAGNETIC ASSESSMENT OF PSEUDOTACHYLITES IN THE CHIAPAS MASSIF, MEXICO

Crystalline rocks of the Permian age, east-west oriented Chiapas Massif in southeast Mexico are locally host to well-exposed pseudotachylite vein networks. At an exposure along the Tablon River valley near the village of Los Angeles, where the host rock is a medium to coarse-grained equigranular granitoid, individual veins are consistently oriented about 280°, are up to 12 mm wide, and can be traced for tens of meters. A preliminary whole rock 40Ar-39Ar age determination has been obtained for one of the veins, with a climbing Ar release spectrum with minimum apparent ages ca. 90 Ma and maximum apparent ages at ca. 115 Ma, and we interpret to be the time of crystallization of vein material and closure of the Ar system. Sites exposing 10 veins were sampled for paleomagnetic and magnetic fabric studies, with samples collected of vein and host rock up to a distance of about 25 cm. Remanence data show similar NRM intensities for both veins and host rock adjacent to veins. The host rock typically contains a well-defined east-west directed and shallow magnetization, which we interpret as a primary TRM of Late Permian age based on previous studies of the massif. Veins have well-defined magnetizations of north to northwest declination and moderate positive inclination, which resemble the Cretaceous expected field. Rock magnetic parameters suggest that the remanence resides in PSD or perhaps SD magnetite grains. Bulk magnetic susceptibility ranges between ~0.3 and 1.1 * 10-3 SI units, and host rock values do not differ significantly from vein values. There is also good consistency in fabric orientation between vein and host rock. In both, magnetic foliations are near vertical and follow closely the nearly east-west orientation of the veins. Nonetheless, host rock fabrics are consistently prolate, whereas vein fabrics are oblate. This result suggests that the fabric defined by magnetic susceptibility anisotropy was formed under pure shear stress, possibly during the creep phase of fault movement.