MIGRATION OF THE CARIBBEAN-NORTH AMERICA-COCOS TRIPLE JUNCTION: THE ZIPPER MODEL AND THE EVOLUTION OF THE TRANSFORM POLOCHIC-MOTAGUA FAULT SYSTEM (GUATEMALA)

The Polochic-Motagua Fault system (PMFS) is a segment of the sinistral transform plate boundary between the North American and the Caribbean plates. It interacts at its western termination with the Cocos plate subduction to form a diffuse triple plate junction. New structural data, fault kinematics analyses, and geomorphic observations along the PMFS permit to estimate finite and recent deformation rates on the structures that have been involved in the three-plate interaction. They reveal the evolution of this triple junction since Late Miocene-Pliocene. The triple junction is becoming increasingly diffuse and propagates away from the trench within the continental crusts of the North American and Caribbean Plates. Motion along the PMFS is transferred along new transpressional structures in the North American Plate interior and along normal faults in the Caribbean Plate, and takes the form of a horsetail strike-slip fault termination. The Cocos slab is regarded as more rigid that the overriding plates. This generates a deformation field that implies crust shortening and stretching north and south of the transform system, respectively, as a response to the eastward drift of the Caribbean Plate away from the trench. Another tectonic mechanism also accommodates the Caribbean plate departure. A forearc sliver bounded by a trench-parallel fault, conjugate to the transform plate boundary, migrates toward the triple junction, in a sense opposite to the Caribbean Plate motion. Once the departure of the inner part Caribbean plate is achieved, the triple junction migrates eastward, the forearc sliver is left behind while the transform boundary and its conjugate subduction-parallel fault connect and merge. Geological record suggest that this “zipper” process has been operating along the northern boundary of the Caribbean Plate since continental blocks are involved in Caribbean Plate motion, so that a simple triple junction of rigid plates has probably never existed between the North American, Caribbean and pacific realms.