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

In southern Mexico, the Sierra Madre del Sur (SMS) preserves the record of a long-lived magmatic arc that vanished diachronically during Paleogene-early Miocene time, decreasing in age to the southeast. The magmatic record consists of a coastal plutonic belt dominated by granites and granodiorites with minor mafic and intermediate intrusions and an inland volcanic belt dominated by intermediate and silicic volcanic centers. The juxtaposition of the plutonic belt with the trench indicates the removal of most of the Paleogene forearc during the Oligocene. Abundant evidence indicates that subduction erosion played a significant role in the reconfiguration of the continental margin of southern Mexico following cessation of magmatism. This is evident in the subsidence of continental segments contiguous to the present trench and the loss of lower continental crust as inferred from the present shape of the wedge, as well as in the estimations of the former crustal thickness based on geobarometry of the exposed levels of plutonic complexes. 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 Chortis 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 Chortis 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 Chortis block. These considerations and the subduction erosion evidence suggest a more complex scenario than having Chortis 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 Chortis block, as has been suggested in other models, to intermediate positions for Chortis 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 Chortis block.