Strike-slip faults and K-alkaline volcanism at El Chichon volcano, southeastern Mexico

摘要

El Chichon volcano is located in the northern part of the State of Chiapas in southeastern Mexico. The volcano is situated within the Transcurrent Fault Province characterized by a series of rising and sinking blocks bounded by sinistral strike-slip faults. The basement around El Chichon is a sequence of Jurassic evaporates, Cretaceous limestones, and Tertiary terrigenous rocks folded into open and an echelon structures. The main structures in the area are the La Union and Caimba anticlines, and the Buena Vista syncline. The region is cross-cut by two conjugate fault systems: a dextral strike-slip N-S-trending set, and a sinistral strike-slip E-W set. The most significant fault of the latter system is the San Juan Fault, which controlled the emplacement of a basaltic dike near the village of Chapultenango some 1.1 Ma ago and the activity of El Chichon during the last 0.2 Ma. Additionally, a series of N45°E-trending faults (Chapultengo Fault System) have produced a half-graben geometry of blocks, on top of which El Chichon has been emplaced. Microstructures such as slickensides, tension fractures, layer-parallel slip, stylolites, and macro- and meso-folds of the sedimentary rocks, indicate that, during the late Miocene the El Chichon area was affected by a maximum principal stress (σ_1) oriented N70°E, a minimum principal stress (σ_3) oriented N20°W, and a vertical intermediate principal stress. This stress pattern indicates that the area underwent a strike-slip motion that produced widespread deformation. The occurrence of crustal earthquakes (< 40 km) in the region with sinistral strike-slip focal mechanisms oriented along the major faults suggests that the same tectonic regime has been occurring in southern Mexico from the late Miocene to the Recent, controlling the emplacement and activity of El Chichon since its beginning during the Pliocene to Recent. This tectonic setting has produced K-alkaline trachyandesites (55-59% wt. SiO_2), and K-rich trachybasalts (46-49% wt. SiO_2) in the area represented by the 1.1 Ma fissural Chapultenango basalt, and by mafic enclaves. Despite this alkaline nature, El Chichon magmas display enrichments in K_2O, Rb, and Sr typical of continental arc magmatism.