MEXICAN MARGIN OF PANGÆA: LATE PALEOZOIC TECTONIC SWITCHING FOLLOWED BY EARLY MESOZOIC SHIFT FROM ISLAND TO CONTINENTAL ARC DURING PANGÆA BREAKUP

The Acatlán and Ayú complexes (southern Mexico) comprise a lower Paleozoic rifted passive margin overlain by upper Paleozoic-Mesozoic, subduction-related rocks that span the assembly, life and breakup of Pangæa. The lower Paleozoic rocks and Upper Devonian arc-related rocks were removed by subduction-erosion in the latest Devonian, underwent high-pressure metamorphism, and were then extruded into the upper plate during the Mississippian, all synchronous with the deposition of sedimentary rocks intruded by rift tholeiites. Subsequent Pennsylvanian-Middle Permian continental arc magmatism was followed by the development of the Triassic-Lower Jurassic island arc-backarc Ayú Complex comprising rifted passive margin turbidites intruded by tholeiitic dikes. During the Jurassic, the Ayú Complex was thrust beneath the Acatlán Complex, underwent upper amphibolite facies metamorphism and was then exhumed within the Acatlán Complex. The Late Devonian (ca. 385 Ma) initiation of subduction is roughly synchronous with the initial stages of Pangæa amalgamation, the Pennsylvanian-Middle Permian continental arc magmatism is contemporaneous with Pangæa, the Triassic island arc development coincides with the initial rifting of Pangæa, and the Jurassic underthrusting of the inboard part of the backarc basin and subsequent exhumation are synchronous with Pangæa breakup, opening of the Gulf of Mexico and separation of North and South America. The pre-Permian switch from subduction erosion/underthrusting to extrusion/exhumation is not clearly associated with the amalgamation of Pangæa, and is more consistent with tectonic switching whereby the dip of the subduction zone alternates between flat and steep, respectively producing compression and extension. The transition from island to continental arc in the Canadian Cordillera at ca 200 Ma also coincides with Pangæa breakup, suggesting the advance of North America (including Mexico) over the Pacific Ocean following breakup and Jurassic opening of the North Atlantic Ocean. In contrast, continental arc magmatism in the central Andes spans the formation, life and breakup of Pangæa suggesting that advance over the paleo-Pacific prior to amalgamation continued as flat slab subduction before and after the Lower Cretaceous opening of the South Atlantic.