A NEWLY DISCOVERED, RELIC, TRANSCURRENT PLATE BOUNDARY - THE TONALA SHEAR ZONE AND PALEOMAGNETIC EVALUATION OF THE WESTERN MAYA BLOCK, SW MEXICO

The Maya Block of southern Mexico is bound by the Cocos plate to the west and by the Chortis Block to the south. It includes from west to east the Chiapas Massif (Sierra Soconosco), the Chiapas foldbelt and strike-slip deformation domains, the Tertiary basin province, and the Yucatan Peninsula. The internal deformation of the Maya block is thought to 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 a 20° CCW rotation of the Chiapas Massif concomitant with deformation within the internal domains. The latter addresses a modified structural framework of the western Gulf of Mexico and fully incorporates existing paleomagnetic data from the massif. The distribution of deep contemporary seismicity demonstrates that a steep subduction zone lies offshore of the Chiapas Massif while the massif itself is aseismic. Active faulting is associated with the Chiapas foldbelt and strike-slip domains. This study involves additional paleomagnetic data that further address the proposed Neogene fragmentation of the Maya block. A belt of Tertiary plutons (U-Pb, SHRIMP date of 10.3 Ma +/-0.3) emplaced along the western margin of the massif exhibit field relations and textures showing that these intrusions and their host rocks are pervasively sheared along a zone that is at least 120km in length and probably spans the entire western margin of the Maya Block. We name this feature the Tonala Shear Zone; deformation is characterized by a dominantly sub-horizontal lineation along a strikingly linear zone that trends 309°. Mylonitic fabrics are ambiguous between dextral and sinistral shear indicators. Brittle-ductile and brittle structures demonstrate a complex history of sinistral shear that overprints dextral shear. Mafic dikes within the massif define two kinematic events where earlier dikes trend to the ENE and cross-cutting dikes trend to the SE. Given the paucity of seismicity along the Tonala Shear Zone, we interpret this zone as a relic plate boundary that may have originated as a dextral transform fault, which was last reactivated as a sinistral structure when the Chortis block was captured by the Caribbean plate system and translated into its current position and orientation south of the MPFS.