PALEOMAGNETIC INVESTIGATION OF THE CHIAPAS MASSIF AND TONALA SHEAR ZONE, WESTERN MAYA BLOCK

The Maya Block in southern Mexico is bordered by the Cocos plate to the west and by the Chortis Block to the south. Major tectonic elements include from west to east the Chiapas Massif (Sierra Soconosco), dominated by Permian crystalline rocks, the Chiapas foldbelt and strike-slip deformation domains, the Tertiary basin province, and the Yucatan Peninsula. Internal deformation of the Maya block may reflect either (1)the diffuse termination of the North American/Caribbean plate boundary (the Motagua-Polochic fault system; MPFS), or (2)Neogene block fragmentation associated with some 20° of CCW rotation of the Chiapas Massif attending deformation in internal domains. Contemporary seismicity defines a steep subduction zone offshore of the Chiapas Massif, although the massif itself is aseismic and amagmatic. Active faulting is associated with the Chiapas foldbelt and strike-slip domains. Along the southwest margin of the massif, late Miocene plutons and Permian host rocks are pervasively sheared along a recently defined zone that is over 120km long and at least 0.5 km wide. This shear zone is very well exposed near Tonala, and probably spans the entire western margin of the Maya Block. To better define the post-Permian tectonics of the Chiapas Massif, paleomagnetic data have been obtained from Miocene plutons,mylonites in the shear zone,Permian host plutonic rocks,and laterally extensive, well-exposed swarms of pseudotachylite veins, likely to be of mid-Cretaceous age, hosted by Permian rocks both south and north of the crystalline massif. Many sites exhibit demagnetization behavior characterized by the well-defined isolation of a north-seeking, shallow positive inclination magnetization, that is typically well-grouped, over a wide range of coercivities and laboratory unblocking temperatures, typically below about 580C. In general,the accepted data are interpreted to suggest that, since the mid-Cretaceous, no large magnitude rotation of the Chiapas massif has taken place, and that the late Miocene intrusions have not been affected by appreciable local tectonism related to the Tonala shear zone. Our interpretations are compromised by the lack of adequate, reliable understanding of the paleohorizontal at the time of remanence acquisition. Low temperature thermochronologic studies are underway to address this important question.