Neogene to Quaternary tectonic evolution of the Patagonian Andes at the latitude of the Chile Triple Junction

Abstract We conducted a preliminary structural and tectonic analysis of the region north and south of the Lake General Carrera–Buenos Aires (LGCBA, Southern Patagonian Andes, Chile and Argentina) in order to study the effects of the subduction of the active South Chile spreading Ridge (SCR) on the development of the tectonic front of the Patagonian Andes at the latitude of the Chile Triple Junction (CTJ). Here, one segment of the SCR entered the trench at 6 Ma and the corresponding slab window is now located below the study area. We emphasize the segmentation of the tectonic front which can be divided into four successive oblique segments (A to D) trending mainly N170 and right laterally offset. This geometrical pattern is consistent with a transpressive stress regime in relation to oblique subduction. The four segments of the tectonic front are separated and locally deformed by N170 oriented, more recent sinistral strike-slip fault zones. Kinematic analysis of minor fault and striaes confirms the abundance of strike-slip faults and suggests rotation of the stress tensor after at least 9 Ma. A consequence of the subduction of the SCR beneath the South American plate at the latitude of the LGCBA is a two-step history: (1) Pre-ridge subduction, oblique to the western margin of the South America plate showing contractional regime with its main axis trending NE–SW responsible for dextral transpressive deformation and (2) Post-ridge subduction more orthogonal convergent regime. Recent uplift occurred later, starting at 3.5 Ma, and could still be active. Neotectonic activity has been evaluated in the region from preliminary analysis of river patterns and fluvial–lacustrine terraces. Anomalies in stream development can be found. Although evidences for recent to active faulting can be shown, effects of syn-glacial and post-glacial-related processes cannot be ruled out. The evolution of this region during the Late Cenozoic can be divided into three distinct periods: (1) 20–15 Ma, deposition of foreland continental detrital sediments (Rio Zeballos Group), followed by a major contractional phase leading to the development of the main thrust front sometimes between 15 and 10–9 Ma. (2) 9–4 Ma, erosion, peneplanation and emplacement of voluminous plateau basalts coeval with fluvial–glacial sedimentation and, (3) 3–0 Ma, renewed orogenic activity, uplift and disruption of former erosional surfaces in relation to activation of strike-slip fault zones and north trending thrusts. Onset of the major contractional event in the region of study can be correlated with the initiation of the subduction of the Chile Ridge at 14–15 Ma in southernmost Patagonia, inducing a different stress regime in the frontal arc region.