Geology of part of a long-lived dynamic plate margin: the coastal cordillera of north-central Chile, latitude 30o51'-31oS.

A complex composed of ultrabasic and basaltic lavas, chert, arkose, and conglomerate was assembled in the coast of N central Chile (lat. 30o30'-31oS) prior to 200 Ma. The character of, and relationships between, the rock formations exposed here are consistent with an autochthonous evolution of this part of Chile in the last 200 Ma. Three major episodes of deformation and metamorphism have been observed in this area. The first episode (F₁) produced a compositional layering (S₁) and amphibolite-facies metamorphism coeval with the intrusion of an extensive igneous complex between 220 and 200 Ma. A second episode of deformation (F₂) locally formed reverse faults and tight, recumbent folds in S₁. Units in which F₂ folds are well developed yield K-Ar and Rb-Sr ages between 163 and 140 Ma. At between 140 and 126 Ma, upright, open to tight folds (F₃) formed with sharp hinges and axial planes that strike N and dip steeply E. Structures of similar age, style, and orientation have been reported as far S as Tierra del Fuego. The timing of the F₃ deformation appears to correspond to the opening of the S Atlantic Ocean and accelerated westward motion of South America. -Authors