Abstract

The oldest Mesozoic record in this region is represented by the Late Triassic deposits of the Potranca Formation in the eastern Chonos metamorphic complex (Patagonian Archipelago). Although there is no subsequent geological information until Middle Jurassic times, Late Jurassic ages for metamorphism and exhumation of the Chonos complex (Thompson & Hervé 2002) suggest that subduction activity was occurring by then. Three stages of Mesozoic and Cenozoic tectonic evolution can be defined for the Patagonian Andes: regional tectonic extension or Rift Phase (Middle to Late Jurassic); thermal subsidence (latest Jurassic to Early Cretaceous); and tectonic inversion with development of an asymmetric foreland basin (Late Cretaceous to Present) (Biddle et al. 1986; Harambour & Soffia 1988; Soffia & Harambour 1989; Skarmeta & Castelli 1997; Mella 2001) (Fig. 3.65). These three stages are broadly correlative with events further north in Chile. The Middle to Late Jurassic regional extension during the first stage of Andean evolution in Patagonia could be interpreted as recording passive-margin conditions and an absence of subduction. However this is not necessarily the case because an extensional setting during active Jurassic subduction existed north of 42°S; this suggests that the general tectonic environment here was probably different. The latest Jurassic-Early Cretaceous thermal subsidence phase in Patagonia coincides chronologically with the second substage of the first Andean stage and shows a similar arc-backarc palaeogeographic organization. In southern Chile low sedimentation rates resulted in a sediment-starved basin, a situation that was to change dramatically with the late mid-Cretaceous onset of tectonic inversion. The initiation of this compressive phase coincides with the beginning of the second stage of Andean evolution north of 42°S (Subhercynian or Peruvian phase), although the palaeogeography south and north of 42°S was considerably different. In southern Chile an eastward migrating foreland basin received huge amounts of westerly derived turbiditic sediments which pass up into shallow marine and continental clastic deposits as the basin filled in Late Cenozoic times. The Patagonian Cordillera can be subdivided into the Palena-Aisén and Magallanes regions, north and south of c. 50°S respectively. The major difference in the evolution of these two regions consists in the development of a basin characterized by extensive bimodal magmatic activity in the south, this being a marginal basin or a branch of the rift system that formed the Atlantic Ocean. Closure (obduction) of this basin occurred at the beginning of tectonic inversion (third stage), with the suture zone forming a zone of weakness that favoured later development of the Patagonian Orocline (see discussion above). The separation zone between these two regions currently coincides with a gap in recent volcanic activity. During the second stage of thermal subsidence in Early Cretaceous times, low sedimentation rates resulted in a depleted (sediment-starved) basin, whereas with the beginning of tectonism (tectonic inversion) and consequent development of the eastward shifting foreland basin, huge amounts of sediments supplied by uplifting and eroding areas to the west determined deposition of thick turbiditic (flysch) successions and later shallow marine and continental clastic deposits that gradually filled the basin in Late Cenozoic times. Filling of the basin with continental deposits also occurred southwards, e.g. in the Última Esperanza section regression occurred in Maastrichtian-Palaeocene times (Cerro Dorotea Formation) whereas in the Riesco section regression occurred in Oligocene times (upper part of the Loreto Formation) (Fig. 3.66).