The Paleogene Transform Margin of South America

Abstract

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 AlukFarallon-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 PacificNorth 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.