EMPLACEMENT AND DEFORMATION OF PLUTONS ALONG A SHEAR ZONE, THE CASE OF THE MIOCENE SUITE OF COASTAL CHIAPAS

Using high-resolution geochronology, anisotropy of magnetic susceptibility (AMS), paleomagnetic data, structural trends and pluton morphology we studied the emplacement and deformation of Neogene plutons emplaced within a relatively unambiguous tectonic setting, the late Miocene Tonalá shear zone in southern Mexico. The Tonalá mylonite belt in western Chiapas, Mexico, is a crustal scale shear zone extending along the western margin of the Chiapas Massif for about 150 km. 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. Miocene plutons of granitic composition intruded the Tonalá shear-zone at mid to upper crustal levels. Intrusions are elongate, with their long dimension parallel to shear zone, suggesting that the orientation of the shear zone controlled the emplacement of these plutons. Where close to the Tonalá shear zone, plutons were mylonitized by continuous deformation; locally, intricate networks of pseudotachylite veins are exposed. Farther from the the shear zone, plutons preserve magmatic fabrics. Two plutons, near Tonalá and Tres Picos, are markedly asymmetric, with sheared tails that appears to trail behind the intrusion. These are features common to granitoid plutons that have been interpreted to by syntectonically emplaced along shear zones, and their features have been reproduced in analogue models. The greatest areal extent and assumed volume of granitoid plutons is along the northwest section of the Tonala shear zone, and we interpret this to imply emplacement within the extensional tip of the shear zone. Deformation in the shear zone resulted in rotations about vertical axes, in both clockwise and anticlockwise senses, which results from a intrusion within a sinistral transpressive strain field where the vorticity experienced is a function of the initial shape and orientation of the long axis of the evolving magma chambers; possibly this implies the rotation of irregular crustal segments within the shear zone in a fashion similar to, but likely more complicated than, ball-bearings.