The continental margin of southern South America facing Panthalassa (or the proto-Pacific) has been characterized by subduction and transcurrent movements at least since mid-Palaeozoic times (Dalziel and Forsythe, 1985; Ramos, 1988a; Breitkreuz et al., 1989). Between the late Palaeozoic and the Triassic, that convergent margin subsided during synchronous stages of Pangaean extension punctuated by diachronous subduction that formed a series of foreland basins by cratonward thrusting of the foldbelt/magmatic arc (Veevers et al., 1994). Those basins were formed by extension (E-I), subsequent foreland shortening (FS), and final extension (E-II). That tectonomagmatic evolution of the continental margin had subtler effects in the interior basins of South America, where sedimentation, although influenced, was not interrupted by major discontinuities. A key element in understanding that evolution is the presence of a rich stratigraphic record, mainly in western Argentina and northern Chile, spanning the Carboniferous– Triassic interval that helps to identify the transition from a compressional to an “extensional” convergent margin. The objective of this chapter is to synthesize our current knowledge of the evolution of the Panthalassan margin of southern South America during the late Palaeozoic and Triassic. For detailed descriptions and discussions, the reader is referred to the numerous publications cited.

On the basis of distinct characteristics to be described later, the convergent margin of South America can be subdivided into three segments (Figure 2.1), namely, (A) a northern segment (north of 20° S), (B) a central segment (20–40° S), and (C) a southern segment (south of 40° S) (López-Gamundí et al., 1994).