Cenozoic magmatism of the Sierra Madre del Sur and tectonic truncation of the Pacific margin of southern Mexico

The Late Cretaceous-Cenozoic decreasing age trend in the SMS shows three time intervals in which magmatism ceased at different rates. The general southeastward younging trend of magmatism, as well as the left-lateral shear zones of the central and coastal zones of the Sierra Madre del Sur, in combination with constraints imposed by the prevalent model of the Caribbean plate evolution have been invoked to consider the Chortís block as the removed segment during margin truncation. The main drawbacks of this model are the mismatch in the chronology of migmatitic complexes of northern Chortís and the Xolapa terrane in the continental margin of southern Mexico, and the presence of the Late Cretaceous to present unperturbed marine sequence in the Gulf of Tehuantepec in the assumed trajectory of the Chortís block. These considerations and the subduction erosion evidence suggest a more complex scenario than having Chortís juxtaposed immediately adjacent to the present-day margin. There are alternative possibilities to explain removal of the forearc, ranging from a western provenance for the Chortís block, as has been suggested in other models, to intermediate positions for Chortís with a more southern trajectory with respect to the present-day trench. Such a trajectory would leave a broad zone with shear strain structures more susceptible to removal by subduction erosion after the passage of the trench-trench transform triple junction. This would reconcile the evidence of subduction erosion and the indications of the northwestern provenance of the Chortís block.