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The study of the Cretaceous–Cenozoic evolution of the North Patagonian Andes represents a great opportunity to investigate the effects of coupling between deep lithospheric processes and near-surface deformation. Despite the general along-strike continuity, this mountain belt is characterized by a pronounced internal tectonic segmentation (marked by the variable position of the magmatic arc and of the deformation front to the east). Thus, this plate margin results in a more complex configuration with respect to the simplified notion of “Andean-type” subduction system. Being located in the retro-wedge of the Andes, this sector of the Southern Cordillera experienced a complex evolution characterized by alternating flat- and steep-slab subduction stages, which controlled shortening and extension episodes in the overriding plate. Furthermore, the deformation in this whole retroarc sector varied not only in time (i.e. with major 'cycles' of mountain building and orogenic collapse), but also in space, due to the variable transmission of horizontal compressive stress away from the orogen, that produced an irregular unroofing pattern, recorded by obtained low-temperature (low-T) thermochronometric ages. Indeed, low-T thermochronological systems are ideally suited for detecting events involving rocks in the uppermost part of the crust because they record time and rates of cooling related to exhumation of the top few kilometers of the crust. In this study, apatite (U-Th)/He (AHe) and apatite fission track (AFT) dating are integrated with structural methods in the region located between 40° and 44°S. Two fieldwork periods were carried out throughout the Neuquén, Rio Negro, and Chubut Provinces in order to sample for thermochronological analyses and conduct structural surveys. These methods allowed us comparing the exhumation patterns both between the frontal part of the orogen and its adjacent foreland, and in the same morpho-structural domain at different latitudes. A total of 48 samples have been collected, processed and analyzed. AHe dating was performed at the University of Paris Sud, while AFT dating was done at the University of Padua. Detailed structural surveys were mainly conducted in the less studied zones of the foreland, and in the entire studied area to check major structures. Integrating the different methods allowed us to unravel the complex tectonic scenario characterizing the study area. Three balanced and sequentially restored cross-sections have been integrated with thermochronological information in order to produce a thermo-kinematic model along two different transects located at 40° and 42°S. To do this, the analyzed transects were processed with FetKin, a dedicated software for forward modeling of thermochronometric ages and age prediction along the present-day profile. This in turn, was used to validate the proposed tectonic scenario. New AFT and AHe data obtained in this study highlight two major tectonic events that occurred in North Patagonia: a Late Cretaceous to Paleogene inversion and exhumation stage, that involved the entire fold-and-thrust belt–foreland basin system, and a Miocene-Pliocene inversion stage of focused exhumation in the Andean fold-and-thrust belt. The suggested tectonic scenario was successfully tested with FetKin along the two transects, allowing us to unravel the role and extent of each tectonic stage that occurred since the Mesozoic. The recorded pattern may be interpreted as the result of a variable degree of propagation of the deformation from the Andean chain to the foreland. This appears to be controlled by two main parameters: (i) slab configuration (i.e. steep- vs. flat-subduction, which plays a fundamental role in foreland deformation), and (ii) convergence rate between the Pacific and South-America plates, which rules shortening and exhumation processes in the fold-and-thrust belt.

The Gressoney Shear Zone (GSZ) consists of a ca. 500 m thick, intensely deformed rock panel located at the top of the high-pressure ophiolitic rocks of the Zermatt-Saas Zone in the Western Alps. This greenschist-facies shear zone accommodated multiple non-coaxial deformation events with contrasting kinematics. In this study, detailed field mapping and structural analysis were integrated with the study of crystallographic preferred orientation (CPO) from mylonites associated with the GSZ. Quartz CPO displays a systematic variation across the shear zone: moving from the basal shear zone boundary, the c-axes pattern changes from type II cross-girdle distribution, to an asymmetric pattern characterized by clustering of c-axes on one side of the Z-direction (inclined single girdle), to a central cluster in the Y-direction. The observed CPO patterns are consistent with increasing shear strain toward the basal contact, which probably controls the transition from broad peripheral maxima indicative of basal slip to an inclined single girdle with no maxima, which is indicative of prism slip, and finally an elongate single maximum at the girdle centre produced by a combination of prism and rhomb slip. Our results further indicate that basal slip is dominant in pure quartz domains, whereas with increasing proportion of second phases, prism slip is activated. These features confirm that CPOs obtained from the almost pure quartzites analysed in most published studies, and generally associated with the activation of distinct slip systems controlled by temperature, cannot be straightforwardly applied to the analysis of heterogeneous shear zones and/or polymineralic mylonites. From a regional point of view, the structures observed in the field and the fabric analyses are consistent with top-to-the-SE extension post-dating subductionrelated high-pressure metamorphism and collisional nappe stacking in the studied sector of the Western Alps.