Tectonic evolution of the North Patagonian Andes (41°–44° S) through recognition of syntectonic strata

Abstract

The North Patagonian fold-thrust belt (41º-44º S) is characterized by a low topography, reduced crustal thickness and a broad lateral development determined by a broken foreland system in the retroarc zone. This particular structural system has not been fully addressed in terms of the age and mechanisms that built this orogenic segment. Here, new field and seismic evidence of syntectonic strata constrain the timing of the main deformational stages, evaluating the prevailing crustal regime for the different mountain domains through time. Growth strata and progressive unconformities, controlled by extensional or compressive structures, were recognized in volcanic and sedimentary rocks from the cordilleran to the extra-Andean domain. These data were used to construct a balanced cross section, whose deep structure was investigated through a thermomechanical model that characterizes the upper plate rheology. Our results indicate two main compressive stages, interrupted by an extensional relaxation period. The first contractional stage in the mid-Cretaceous inverted Jurassic-Lower Cretaceous half graben systems, reactivating the western Cañadón Asfalto rift border ~500 km away from the trench, at a time of arc foreland expansion. For this stage, available thermochronological data reveal forearc cooling episodes, and global tectonic reconstructions indicate mid ocean ridge collisions against the western edge of an upper plate with rapid trenchward displacement. Widespread synextensional volcanism is recognized throughout the Paleogene during plate reorganization; retroarc Paleocene-Eocene flare up activity is interpreted as product of a slab rollback, and fore-to-retroarc Oligocene slab/asthenospheric derived products as an expression of enhanced extension. The second stage of mountain growth occurred in Miocene time associated with Nazca Plate subduction, reaching nearly the same amplitude than the first compressive stage. Extensional weakening of the upper plate predating the described contractional stages appears as a necessary condition for abnormal lateral propagation of deformation.

The Late Paleozoic Tectonometamorphic Evolution of Patagonia Revisited: Insights From the Pressure‐Temperature‐Deformation‐Time (P‐T‐D‐t) Path of the Gondwanide Basement of the North Patagonian Cordillera (Argentina)

“…However, deformation conditions estimated for shear zones (sections and ) indicate that they underwent deformation in a deeper crustal level than Neogene faults, and therefore, a more likely Cretaceous age is inferred for shear zones. This is further supported by middle Cretaceous deformation associated with ENE‐NE shortening directions documented in northwestern Patagonia, where coeval tectonic activity along NNW‐SSE to NNE‐SSW striking thrusts was also reported (Echaurren et al, , ; Gianni et al, ; Orts et al, ). Hence, shear zones might result from an early Andean Cretaceous dextral transpressional event (Figure b), probably associated with coeval magmatism and hydrothermal events recorded by monazite ages, as the latter match the onset of Andean tectonics at circa 100–95 Ma (Somoza & Zafaranna, ).…”

“…In contrast to the Miocene transpressional phase, which is well recorded by structural, kinematic, and geochronologic data of volcano‐sedimentary and granitic rocks (Bechis, Encinas, et al, ; Diraison et al, ; Giacosa et al, , ; Giacosa & Heredia, ; Orts et al, ), the Cretaceous Andean record of the study area is only restricted to few granitoids yielding K‐Ar and Rb‐Sr ages of circa 120–80 Ma (González Díaz, , and references therein). However, coeval thrusting was reported in adjacent regions of northwestern Patagonia (Echaurren et al, , ; Gianni et al, ; Orts et al, ).…”

“…On the other hand, the Jurassic record of the study area is characterized by granitoids of the Cordilleran batholith yielding U‐Pb SHRIMP zircon crystallization ages of circa 176–160 Ma (Castro, Moreno‐Ventas, et al, ) and scarce volcano‐sedimentary sequences (Giacosa et al, ). Based on pluton fabrics, Castro, Moreno‐Ventas, et al () inferred regional sinistral strike‐slip deformation, thus contrasting with the Middle‐Late Jurassic extensional regime recorded by extensional structures of volcanic rock depocenters of the North Patagonian Cordillera located further south (Echaurren et al, , ).…”

Tectonic Regimes of the Central and Southern Andes: Responses to Variations in Plate Coupling During Subduction

“…Second, there remains limited understanding and documentation of contrasting tectonic regimes during the Andean orogeny. As a prime example, the late middle Eocene to earliest Miocene (~40–20 Ma) record of deformation reveals an apparent contradiction in which shortening reported for the thickest part of the central Andes was synchronous with extension in the southern Andes (Echaurren et al, ; Encinas et al, ; Horton, , ; Jordan et al, ; Maksaev & Zentilli, ; McQuarrie et al, ; Oncken et al, ; Orts et al, ). This problem is accentuated at the transition from the central to southern Andes, where diverging interpretations of the Eocene‐Oligocene record invoke either extension or shortening for the magmatic arc, hinterland thrust belt, and retroarc basin system along an east‐west profile at ~30°S (Fosdick et al, ; Litvak et al, ; Winocur et al, ).…”

Constraints on Trenchward Arc Migration and Backarc Magmatism in the North Patagonian Andes in the Context of Nazca Plate Rollback

“…The El Maitén Belt sequences are overlain by the Ñirihuau (~22–11 Ma) and Collón Curá (~16–10 Ma) formations, which represent the main infill of the Ñirihuau basin (Bechis et al, ; Cazau et al, ; Ramos et al, ; Spalletti, ). While extensional conditions controlled the deposition of the basal section of the Ñirihuau Formation, its middle and upper terms and the Collón Curá Formation represent synorogenic deposits, linked to the compressional tectonic phase that affected the North Patagonian Andes at approximately ~18–11 Ma (Bechis et al, , ; Echaurren et al, ; Orts et al, ; Ramos et al, ).…”