Apatite (U-Th)/He and Fission Track thermochronometry in the Northern Patagonian Andes: New insights into the exhumation history of the thrust belt foreland sector

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 role of slab geometry in the exhumation of cordilleran-type orogens and their forelands: Insights from northern Patagonia

In cordilleran-type orogens, subduction geometry exerts a fundamental control on the tectonic behavior of the overriding plate. An integrated low-temperature, large thermochronological data set is used in this study to investigate the burial and exhumation history of the overriding plate in northern Patagonia (40°–45°S). Thermal inverse modeling allowed us to establish that a ~2.5–4-km-thick section originally overlaid the Jurassic–Lower Cretaceous successions deposited in half-graben systems that are presently exposed in the foreland. Removal of the sedimentary cover started in the late Early Cretaceous. This was coeval with an increase of the convergence rate and a switch to a westward absolute motion of the South American Plate that was accompanied by shallowing of the subducting slab. Unroofing was probably further enhanced by Late Cretaceous to early Paleogene opening of a slab window beneath the overriding plate. Following a tectonically quiescent period, renewed exhumation occurred in the orogen during relatively fast Neogene plate convergence. However, even the highly sensitive apatite (U-Th)/He thermochronometer does not record any coeval cooling in the foreland. The comparison between Late Cretaceous and Neogene exhumation patterns provides clear evidence of the fundamental role played by inter-plate coupling associated with shallow slab configurations in controlling plate-scale deformation. Our results, besides highlighting for the first time how the whole northern Patagonia foreland was affected by an exhumation of several kilometers since the Late Cretaceous, provide unrivalled evidence of the link between deep geodynamic processes affecting the slab and the modes and timing of unroofing of different sectors of the overriding plate.Fil: Genge, Marie C.. Università di Padova; Italia. Centre National de la Recherche Scientifique; Francia. Université de Lille; Francia. Université du Littoral; FranciaFil: Zattin, Massimiliano. Università di Padova; ItaliaFil: Savignano, Elisa. Università di Padova; ItaliaFil: Franchini, Marta Beatriz. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Confluencia; Argentina. Universidad Nacional de Río Negro. Sede Alto Valle. Instituto de Investigaciones en Paleobiología y Geología; Argentina. Universidad Nacional del Comahue; ArgentinaFil: Gautheron, Cécile. Université Paris Sud; Francia. Centre D'etudes de Saclay; Francia. Centre National de la Recherche Scientifique; FranciaFil: Ramos, Victor Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos "Don Pablo Groeber". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Estudios Andinos "Don Pablo Groeber"; ArgentinaFil: Mazzoli, Stefano. Università degli Studi di Camerino; Itali

Apatite (U-Th)/He and Fission Track thermochronometry in the Northern Patagonian Andes: New insights into the exhumation history of the thrust belt foreland sector

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.Lo studio dell’evoluzione Cretacico–Cenozoica delle Ande Nord Patagoniche offre l’opportunità di comprendere le relazioni intercorrenti tra processi profondi riguardanti la dinamica della litosfera e la deformazione più superficiale che coinvolge la crosta. Benché la catena andina sia caratterizzata da un generale andamento lineare nord-sud, nel dettaglio essa è fortemente segmentata da un punto di vista tettonico, come si evince dalla posizione variabile dell’arco magmatico e del fronte di deformazione verso le zone di avampaese. Pertanto, questo margine di placca risulta avere una complessa configurazione, che va oltre la comune nozione di sistema di subduzione di “tipo andino”. Localizzato nella zona di retrocatena, il settore Nord Patagonico della Cordigliera ha subito una complessa evoluzione, caratterizzata dall’alternanza di fasi di subduzione a basso e alto angolo, che ha controllato le fasi rispettivamente di raccorciamento e di estensione in corrispondenza della placca superiore. La variabilità deformativa in questa zona di retroarco è stata sia temporale—dunque legata a fasi di orogenesi e collasso della catena—sia spaziale, data la variabile trasmissione dello stress compressivo verso le zone esterne dell’orogene. La termocronologia di bassa temperatura è in grado di registrare i pattern irregolari di denudamento risultanti, essendo molto sensibile alle variazioni entro i primi chilometri della crosta. Tali sistemi termocronometrici sono ideali per identificare eventi tettonici a livello crostale, poiché sono in grado di registrare il momento e la velocità del raffreddamento legato all’esumazione. In particolare, i metodi (U-Th)/He (AHe) e tracce di fissione (AFT), entrambi su apatite, sono stati integrati in questa tesi con i metodi strutturali. L’area investigata è la regione ubicata tra 40° and 44°S, nel nord della Patagonia argentina. Durante le due campagne, condotte nelle Province di Neuquén, di Rio Negro e del Chubut, sono state campionate le rocce sulle quali compiere le analisi termocronologiche di bassa temperatura e sono stati analizzati dal punto di vista strutturale i lineamenti tettonici più importanti. Questi metodi hanno permesso sia di comparare i pattern di esumazione nella parte frontale della catena e nell’adiacente avampaese sia, al contempo, di comparare le differenze, a diverse latitudini, all’interno di uno stesso dominio morfo-strutturale. Complessivamente, sono stati raccolti, processati e analizzati quarantotto campioni. Le analisi (U-Th)/He sono state eseguite presso l’Università di Parigi Sud mentre quelle di tracce di fissione presso l’Università di Padova. Dal punto di vista geologico-strutturale, le maggiori strutture dell’area di studio sono state rilevate sul terreno e analizzate nel dettaglio, con particolare attenzione nelle zone meno studiate dell’avampaese. Infine, per un’analisi più approfondita di questo complesso scenario i due metodi sono stati integrati. Infatti, sono state costruite tre sezioni bilanciate e retrodeformate integrate con le informazioni termocronologiche al fine di produrre un modello termo-cinematico lungo due diversi transetti posti a 40° e 44°S, mediante l’utilizzo di un software dedicato in grado di modellare le età termocronometriche e fare previsione di queste lungo il profilo topografico odierno, che, a sua volta, è stato usato per validare lo scenario tettonico proposto. I nuovi dati AFT e AHe ottenuti nell’area di studio evidenziano due eventi tettonici principali interessanti l’area settentrionale della Patagonia: una fase d’inversione ed esumazione tra il tardo Cretacico e il Paleogene, che ha coinvolto l’intero sistema dalla catena all’avampaese, e una successiva, Mio-Pliocenica, in cui l’esumazione si è concentrata nella zona di catena. Questo scenario tettonico proposto è stato con successo testato lungo i due transetti analizzati attraverso FetKin, permettendo di chiarire il ruolo e l’entità di ogni fase tettonica avvenuta dal Mesozoico. Il pattern di esumazione così ottenuto è stato interpretato come il risultato del variabile grado di propagazione della deformazione dalla catena andina verso l’avampaese. Quest’ultimo sembra essere controllato principalmente da due parametri: (i) la configurazione della placca in subduzione, in altre parole la sua geometria più o meno orizzontale che gioca un ruolo fondamentale nella deformazione dell’avampaese, e (ii) la velocità di convergenza tra le placche pacifica e sud americana, che governa i processi di raccorciamento ed esumazione nella catena a pieghe e sovrascorrimenti