Apatite (U-Th)/He thermochronometry in the northern Patagonian Andes: new insights into the exhumation history of the thrust beltforeland sector

SAVIGNANO, ELISA; MASSIMILIANO ZATTIN; FRANCHINI MARTA; CÉCILE GAUTHERON; STEFANO MAZZOLI

Nápoles

Congreso; 88th Congress of the Italian Geological Society; 2016

Università degli Studi di Napoli Federico II

The Patagonian Andes represent a unique natural laboratory to study the relationships betweensurface deformation and slab dynamics. In fact, building of the southern Andes has been interpretedto have been controlled by alternating stages of flat- and steep-slab subduction, which producedalternating shortening and upper plate extension episodes1,2,3,4, respectively. This evolution can berecognized in the adjacent thrust belt- foreland system, in which progressive and non-steady-stateprocesses acting in several pulses during Late Cretaceous, Late Eocene and Late Miocene timeshave been detected. Deformation in the Andean retro-wedge sector varied not only in time (i.e. withmajor cycles of mountain building and orogenic collapse, respectively), but also in space, due tothe variable transmission of horizontal compressive stress away from the orogen.The result is a present day complex structural architecture due to heterogeneous deformation.In this study, we integrate field structural observations with apatite (U-Th)/He (AHe)thermochronology in the Esquel-Gastre area (located at latitudes 41.30?43.00°S) in order to analyseand compare the exhumation patterns from the frontal part of the orogen and from the adjacentforeland sector. These data provide new insights into the timing and modes of coupling vs.uncoupling of the deformation between the northern Patagonian fold and thrust belt and its foreland.The collected samples span through different morpho-structural domains, from the variablydeformed foreland to the eastern slope of the Main North Patagonian Andes. Analyzed samplesmainly belong to basement lithologies (see Fig. 1 for the sample location). The resultant AHe agescan be grouped into two major populations: (i) a first group characterized by Miocene?Plioceneages (10.6 to 2.0 Ma) belonging to the Precordillera area, and (ii) a second group of samples withUpper Cretaceous to Eocene ages (26.9 to 123.8 Ma) distributed in the foreland zone. Our studysuggests that configurations of flat-slab vs. steep-slab subduction may exert an important role incontrolling the coupling vs. uncoupling of the deformation between the thrust belt and its forelandin the Patagonian Andes. Late Miocene to Pliocene AHe ages from the frontal part of the northernPatagonian Andes correlate well with a shortening and exhumation stage documented to haveoccurred in the thrust belt during steep subduction characterized by high convergence rates (> 4cm/a). On the other hand, AHe ages obtained for the broken forelandunravelled final exhumationat near-surface conditions during Late Cretaceous to Paleogene times.