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The late pre-Andean tectonics of the western margin of the South America plate (SAM) has been active since the late Jurassic to the Paleogene, interacting with the Aluk plate in the Cretaceous, the Aluk- Farallon-SAM triple junction and Farallon plate in the Paleogene. There is agreement on the relationship for subduction with respect to SAM of the Aluk plate in the Cretaceous, and for the Nazca plate in the Neogene, but the fate of the Aluk-Farallon-SAM triple junction in the Paleogene, has been subject to different interpretations that span from a) the uninterrupted subduction, of the Farallon and Aluk plates (Rapela et al. 1987, Pankhurst et al. 1999), to b) the interruption of subduction, with the development of a Farallon-SAM transform margin and the detachment of the Aluk plate (Aragón et al. 2011). The Paleogene magmatic record along the Andes suggests that it is segmented and episodic, with time gaps for magma emplacement, some episodes with a sense of continuous migration of the magmatic axis with time (northern Chile and Peru), others with no sense of migration (northern and southern Patagonian Batholiths), reflecting major changes in subduction processes (Pankhurst et al. 1999). The Paleogene volcanism along the Andes is also segmented and episodic, with unusual events of within-plate-like volcanism such as in northern Chile (Cornejo and Matthews 2000) and Patagonia (Aragón et al. 2011). An ocean-continental plate transform system may be developed as a consequence of the relative translation of plates along the surface of a sphere. This geometry will develop where a segment of a plate boundary is perpendicular to the line from that segment to the pole of relative motion (Fig. 1A). This implies that if the plate boundary is straight and long enough, it will be extensional at one end, continuing as a transform, and convergent at the other end, as the increased convergence angle causes the collapse of the transform into subduction (Fig. 1A). The critical angle in which the transform plate margin collapses into subduction could be >30º as can be observed in the Queen Charlotte transform system of the Pacific- North America plates (Fig 1B), also helped by a change in the coast line. It can also be observed from Figure 1B that in the point of collapse from transform to subduction, the fore-arc is pushed into and beneath the continental plate; the crust margin is duplicated in thickness and deformed into the Alaskan orocline. Furthermore, the complex Pacific-North America (NAM) Plates transform system (Queen Charlotte and San Andres) is a consequence of the Pacific-Farallon-NAM triple junction, and still has remnants of the old Pacific-Farallon active ridge (not yet subducted) that preserve a microplate (Juan da Fuca) and a continuous history of subduction. With this perspective of a complex continent-ocean plate transform system, an analogy can be made with the Aluk-Farallon-SAM triple junction in the Paleogene (Fig. 1C). The Farallon-SAM reconstructions for the 68-28 Ma interval (after Somoza and Ghidella 2005), shows that by that time, the Aluk-Farallon-SAM triple junction had interacted with southern South America and that the Farallon plate had two episodes of moving with respect to SAM much currently to what occurs between the Pacific plate and northern NAM, with an Euler pole geometry as in Figure 1A, and with important strike-slip fault systems in southern Peru-northern Chile and Southern Chile, instead of central Chile (Santiago area), where a thrust and fold system prevails and batholiths of Paleogene age are present. In this scenario, the possibility remains that a few remnants of the Aluk-Farallon ridge could have remained un-subducted, preserving a hypothetical microplate Tupac Amaru, and retaining subduction (through the Cenozoic) in the central Chile segment of the Andes. To the south of the Tupac Amaru microplate, the Patagonia-Farallon transform system was well developed along the Proto-Liquiñe-Ofqui fault (Aragón et al., 2011), having a behaviour equivalent to that of the San Andreas fault, with the transport and docking of peninsular Baja California in the western United States. During this time, magmatic activity shows nearly total quiescence in the Patagonian batholith, with volcanic activity having migrated to the fore- and back-arc (the Eocene convergence change does not seem to interrupt the transform system, Figure 1C). To the north of the Tupac Amaru microplate, a composite transform-obliquely convergent boundary, sliced by a system of strike-slip faults (Atacama and Domeyko fault systems) could have been developed during Paleogene, with an interruption during the Eocene compressive event with subduction. This structurally complex fault system shows uneven strain effects, with vertical and translational shifts of the component blocks and varying along the length of the system and through time; having associated episodic calcalkaline and sporadic within-platelike magmatism. The Peru batholith also shows episodic quiescence of magmatic activity for this time. Finally, as the Farallon-SAM convergence angle increases to the north (away from the Euler pole, Figs. 1A and 1C) the South American margin bends in an Orocline in the Eocene-Oligocene (McQuarrie 2002). The good fit between the age of deformation of the Bolivian Orocline, and the increased convergence angle in the Farallon-SAM complex plate-transform-system at the orocline latitude can sustain an analogy with respect to the genesis of the Alaskan Orocline, implying that the Bolivian Orocline could have developed where the continent-ocean plate transform system collapsed into subduction and the fore-arc was pushed into and beneath the continental plate, duplicating the crustal thickness in that segment of continent margin.

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