Emilio A. Rojas Vera

U–Pb detrital zircon ages of Upper Jurassic continental successions: implications for the provenance and absolute age of the Jurassic–Cretaceous boundary in the Neuquén Basin

Abstract New U–Pb detrital zircon ages are presented for the Tordillo Formation. The ages indicate that the most important source region of sediment supply was the Jurassic Andean arc (peaks at c. 144, 153 and 178 Ma), although two secondary sources were defined at c. 218 and 275 Ma. Temporal variation in the provenance indicates that at the beginning of the sedimentation, Carboniferous to Lower Jurassic magmatic rocks and Lower Palaeozoic metamorphic rocks were the most important sources. Towards the top, the data suggest that the Andean arc becomes the main source region. The comparison between provenance patterns of the Tordillo Formation and of the Avilé Member (Agrio Formation) showed some differences. In the former, the arc region played a considerable role as a source region, but this is not identified in the latter. The results permit a statistically robust estimation of the maximum deposition age for the Tordillo Formation at c. 144 Ma. This younger age represents a discrepancy of at least 7 Ma from the absolute age of the Kimmeridgian and Tithonian boundary (from the chronostratigraphic timescale accepted by the International Commission of Stratigraphy, IUGS), and has strong implications for the absolute age of the Jurassic–Cretaceous boundary. Supplementary material: Sample coordinates, values of the sandstone compositional framework and U–Pb (LAM-MC-ICP-MS) age measurements of zircons grains are available at http://www.geolsoc.org.uk/SUP18718

The Transitional Zone Between the Southern Central and Northern Patagonian Andes (36–39°S)

The transition zone from the Southern Central to the North Patagonian Andes is characterized by a low topography and low shortenings. During its evolution, an extensional event in late Oligocene times affected the western section of the Late Cretaceous to Eocene fold and thrust belt. Late early Miocene contraction then constructed most of the eastern Andean slope as in sequence structures stacked in the frontal sector of the fold and thrust belt. However, out of sequence structures derived mainly from the inversion of the late Oligocene extensional depocenters uplifted the axial Andean zone at these latitudes. Contractional and extensional stages coincide with periods in which the arc expanded and retracted, respectively. Shortening gradients from 30 km in the north to only 11 km in the south and development of synorogenic depocenters are linked to arc dynamics.

Progression of the Deformation in the Southern Central Andes (37°S)

This chapter analyzes the deformational pulses and mechanisms that affected the Southern Central Andes across 37°S. Four zones were analyzed in detail that from west to east are the Cordillera del Viento, the Tromen volcanic plateau, the Sierra de Reyes, and the Chachahuen volcanic complex. Each of these zones shows evidence of one or more deformational stages that affected the Andean margin from the Late Cretaceous onwards. Due to the contrasting structure and geology, different methodologies were applied for their study, (i) balanced structural cross sections to represent structure at depth, (ii) use of potential methods (gravimetry and magnetometry) for a subsurface constraint, (iii) geochronological analyses using U/Pb dating of detrital zircons in synorogenic sequences to determine maximum ages and source areas, (iv) morphometric analyses in the drainage network in order to analyze the activity of neotectonic structures during landscape evolution. As a result, an evolutionary model is presented for the Southern Central Andes in which four pulses of deformation were recognized in agreement with previous proposals, although with a variable distribution reflecting a complex pattern. In particular, the Cordillera del Viento area in the westernmost sector registers an uplift stage that occurred during the Late Cretaceous followed by a late Miocene reactivation. The mechanisms associated with this uplift are related to the selective reactivation of half-grabens and generation of new thrusts cutting through the extensional architecture. To the east, the Tromen volcanic plateau registers a pre-Miocene stage of uplift, later affected by a neotectonic reactivation. Localization of neotectonic activity could be related to the emplacement of asthenospheric material and consequent weakening of the upper crust, as revealed by magnetotelluric studies. In the orogenic front , the Sierra de Reyes was initially uplifted during Eocene times and subsequently suffered a strong reactivation during the Neogene. This last stage produced synorogenic successions in the Sierra de Reyes foredeep describing an unroofing sequence. Finally, the Neogene deformational stage exhumed Lower Cretaceous sequences in the foreland region before 7 Ma expanding considerably the orogenic wedge. Thus, the Chos Malal fold and thrust belt shows a foreland sequence development between the Upper Cretaceous and Eocene, that was followed by a sudden expansion of the orogenic wedge in late Miocene times and reactivation of the western sectors. Finally, Quaternary out-of-sequence thrusts define an active orogenic front at the midsection of the fold and thrust belt.

Cenozoic Deformational Processes in the North Patagonian Andes Between 40° and 43°S

The Cenozoic tectonic evolution of the North Patagonian Andes is analyzed linking geological and geophysical data in order to decipher the deformational processes that acted through time and relate them to basin formation. Field observations and seismic reflection profiles reveal the shallow structure of the retroarc area where contractional structures, associated with Oligocene to early Miocene inverted extensional sections, are partially onlapped by early to late Miocene synorogenic deposits. From the construction of five structural cross sections along the retroarc area between 40° and 43° 30′S, constrained by surface, gravity, and seismic data, a shortening gradient is observed along Andean strike. The highest shortening of 18.7 km (15.34 %) is determined near to 41° 30′S where basement blocks were uplifted in the orogenic front area, and the deepest and broadest synorogenic depocenters were formed toward the foreland. Additionally, eastward shifting of Miocene calc-alkaline arc rocks occurred at these latitudes, which is interpreted as indicative of a significant change in the subduction parameters at this time. Deep crustal retroarc structure is evaluated through inversion of gravity models that made also possible to infer Moho attenuated zones. These coincide with the occurrence of younger than 5 Ma within-plate volcanics as well as with crustal thermal anomalies suggested by shallowing of the Curie isotherm calculated from magnetic data.

Active Deformation, Uplift and Subsidence in Southern South America Throughout the Quaternary: A General Review About Their Development and Mechanisms

A broad range of processes act today and have acted simultaneously during the Quaternary, producing relief from the Chilean coast, where the Pacific Ocean floor is sinking underneath the South American margin, to the Brazilian and Argentine Atlantic Ocean platform area. This picture shows to be complex and responds to a variety of processes which are just started to be considered. These processes involve mountains created in a passive margin setting along vast sections of the Brazilian Atlantic Ocean coast and regions located inland, to “current” orogenic processes along the Andean zone. On one hand, mountains in the passive margin seem to be created in the area where the forearc region eastwardly shifts at a similar rate than the westward advancing continent and, therefore, it can be considered as relatively stationary and dynamically sustained by a perpendicular-to-the-margin asthenospheric flow. On the other hand, the orogenic processes associated with the eastern Andes show to be highly active at two particular areas: the Subandean region, where the trench is stationary and the Pampean flat subduction zone to the south, where a shallower geometry of the Nazca plate creates particular conditions for deformation and rapid propagation of the orogenic front generating a high-amplitude orogen. In the Southern Central and Patagonian Andes, mountain (orogenic) building processes are attenuated, and other mechanisms of regional uplift become dominant, such as the (i) crustal weakening and deformation linked to the impact of mantle plumes originated in the 660 km mantle transition, (ii) the retirement of ice masses from the Andes after the Pleistocene producing an isostatic rebound, (iii) the dynamic topography associated with the opening of asthenospheric windows during the subduction of the Chile ridge and slab tearing processes, (iv) the subduction of oceanic plateaux linked to transform zones and (v) the accretion of oceanic materials beneath the forearc region. Additionally and after recent geodetic studies, (vi) forearc coastal uplift due to co-seismic and post-seismic lithospheric stretching associated with large earthquakes along the subduction zone, also shows to be a factor associated with regional uplift that needs to be further considered as an additional mechanism from the Chilean coast to presumably the arc zone.