Tectonic evolution of the North Patagonian Andes, from the exhumed crystalline rocks to the foreland basin

STUART THOMSON; EZEQUIEL OLAIZOLA; EZEQUIEL GARCÍA MORABITO; VICTOR VALENCIA; STUART THOMSON; EZEQUIEL OLAIZOLA; EZEQUIEL GARCÍA MORABITO; FLORENCIA BECHIS; VICTOR VALENCIA; JULIETA SUTIANO; ALFONSO ENCINAS; SEBASTIAN ORIOLO; FLORENCIA BECHIS; JULIETA SUTIANO; ALFONSO ENCINAS; SEBASTIAN ORIOLO; CAMILA SANTOJA; DANIEL L. YAGUPSKY; JOHN BALLESTEROS; VICTOR A. RAMOS; CAMILA SANTOJA; DANIEL L. YAGUPSKY; JOHN BALLESTEROS; VICTOR A. RAMOS

Quito

Simposio; 8th International Symposium on Andean Geodynamics (ISAG); 2019

Our general goal is to analyze the main tectonic and climatic factors that controlled the late Cenozoicgeodynamic evolution of the North Patagonian Andes at about 41°S. To achieve this objective, weconducted an integrated and multidisciplinary study by analyzing the structural architecture, age, andevolution of deformation and exhumation within the fold and thrust belt. The North Patagonian fold andthrust belt at these latitudes shows a general thick-skinned structural style, with involved basementblocks comprised of highly exhumed Paleozoic to Cenozoic metamorphic and plutonic rocks. Localthin-skinned tectonics is also present near the thrust front, where it deforms synorogenic depositsrepresented by the upper section of the Ñirihuau basin infill. Here we present new thermochronologicand structural data from the exhumed basement blocks, as well as U-Pb dating, structural observations,and sedimentary provenance data from the foreland basin.Zircon and apatite fission track data from Mesozoic to Cenozoic plutonic rocks record a complex andepisodic exhumation history of the basement blocks, showing diverse cooling ages that range from theEarly Cretaceous until the late Miocene. In the central and eastern sectors of the orogen, the youngestpulse of exhumation recorded by these analyses is constrained between ~14 and 8 Ma.A multiphase evolution has been interpreted for the Ñirihuau basin. It originated as an extensional basinduring the Oligocene to early Miocene, registering a later change to a foreland-type basin under acompressional regime. Our structural, geochronologic, paleoenvironmental and provenance data fromthe basin infill constrain the timing of this tectonic switching to between 15 and 13 Ma. Coevalshortening and sedimentation are registered between 13 and 11 Ma, during the foreland basin stage.Our data record a relatively short pulse of shortening in a narrow fold and thrust belt during the middle tolate Miocene (~15 to 8 Ma) in this sector of the North Patagonian Andes. We observe a relatively slowfrontal thrust propagation, with important slip focused along a few reverse faults that uplifted basementrocks, favoring differential exhumation in their hanging walls. In the adjacent foreland basin, highsedimentation rates and little lateral migration of the depositional focus is registered. Thesecharacteristics suggest a fold and thrust belt ? foreland basin system where the rate of surfaceprocesses (erosion and sedimentation) is rapid relative to the rate of deformation (Simpson, 2006),possibly under a steady-state erosional efficiency setting typical of humid climates (Yagupsky et al.,2014).