The Southern Andes Between 39° and 44°S Latitude: The Geological Signature of a Transpressive Tectonic Regime Related to a Magmatic Arc

Abstract

Only the active stratovolcanoes of the main range are more than 2000 m high, in this segment of the Andes where the submerged Central Valley separates the coastal ranges from the main range, south of 41°S lat. A late Palaeozoic accretionary subduction complex, mainly composed of turbiditic sequences with subordinate greenstones and cherts, with low grade metamorphism and deformation increasing to the west, underlies most of the coastal ranges and archipelagos. Isolated occurrences of similar rocks in the main range, which have yielded a Rb-Sr age of ca. 300 Ma, suggest that the same complex forms the basement there. The main range is mostly composed of the North Patagonian Batholith (NPB). Except for minor late Palaeozoic and early Jurassic bodies near its northern end, it has yielded middle Jurassic to Neogene intrusion ages. It is calcalkaline, with a lithologic variation from gabbro to monzogranite, with tonalite and granodiorite as the predominant rock types. The younger plutons appear to be located near the central axis of the batholith, which has a well defined early Cretaceous monzogranitic eastern margin. A backarc basin developed to the east of the batholith during the middle Jurassic to the early Cretaceous, and was filled with marine sediments and volcanic sequences. From the late Cretaceous onwards, only subaerial volcanism is recorded in the backarc position. These rocks show very little deformation. The Liquiñe-Ofqui Fault Zone (LOFZ), a dextral trench-linked strike slip structure runs near the centre of the NPB. It has been active at least since the Eocene, as is indicated in the sedimentary record by pull-apart basins which developed to the west of the fault zone. The LOFZ has controlled the emplacement of syntectonic neogene plutons and the presently active volcanic belt. The described scenario, i.e. the intrusion of the Meso-Cenozoic batholith into a late Palaeozoic accretionary complex, the development of an important transcurrent fault parallel to the continental margin, a lack of migration of the magmatic foci with time and very mild backarc deformation, have resulted in a low mountain range with a relatively thin continental crust. These relations could be useful for interpreting similar situations in the past development of other segments of the Andes.