CLOCKWISE ROTATION OF CHIAPAS MASSIF WITHIN THE SINISTRAL NORTH AMERICA–CARIBBEAN PLATE BOUNDARY

Synthesising the Cenozoic history of southern Mexico has required integration of multiple efforts, including: definition of relative and absolute plate motions; magmatic arc chronology/migration; detrital zircon geochronology; structural restorations; ZFT, AFT and ZHe thermochronology; paleomagnetism; Cocos slab seismicity / tomography; seismic reflection interpretation; illite geochronology; and geomorphology. After Antilles arc collision and amalgamation of Chortis Block into the Caribbean Plate, Laramide orogenesis in the Oaxaca–Cuicateco transect involved (1) rapid W-ward drift of North America over the mantle and (2) sinistrally transpressive migration of Chortis along S Mexico. Inception of Farallon/Cocos arc magmatism and hanging wall uplift in S Mexico migrated east in the wake of Chortis. Neogene uplift and structural shortening of the Chiapas Massif and its associated Chiapas Foldbelt pertains to the inception of subduction after Chortis passed. The Chiapas Massif has undergone multi-kilometric uplift since the late middle Miocene, evidenced by the provenance of the middle Miocene Nanchital Conglomerate of the Chiapas Foldbelt, and AFT data from the Chiapas Massif. Paleomagnetic data from the Eocene El Bosque Formation, which overlies the massif, indicates a ~20° clockwise (CW) rotation of the massif since the Eocene, presumably during Neogene crustal shortening. The CW block rotation is counter-intuitive to the massif’s position within the sinistral North America–Caribbean plate boundary. We look to the geometry of the Cocos Benioff plane to resolve this unexpected observation and find that the western half of the massif has encountered the eastern flank of the Oaxaca flat-slab portion of the Benioff plane. From this we infer greater uplift and inter-plate coupling at the massif’s western end relative to its eastern end, which, in association with crustal shortening, has caused the CW rotation as a rigid block within the subduction zone hanging wall. These observations carry important implications for the development of the Chiapas Foldbelt (windshield wiper model), and pertain to the Neogene transition of the massif from bounding the former sinistral plate boundary setting (still active to the east) to residing in the subduction zone hanging wall in the wake of Chortis.