The Jáchal river cross-section revisited (Andes of Argentina, 30°S): Constraints from the chronology and geometry of neogene synorogenic deposits

Abstract

In the southern Central Andes, between 27 and 33°S, a flab-slab subduction has been established since the last 10 Ma. This particular subduction geometry has been proposed to control the eastward migration of the deformation, with the Precordillera fold-and-thrust belt development and the uplift of the foreland Sierras Pampeanas basement blocks. In this study, we present a 190 km-long structural section, across the easternmost Cordillera Frontal, the Precordillera and the westernmost sector of the Sierras Pampeanas, and revise both previous kinematic models, and the relationship between the establishment of the flat-slab and deformation in the foreland. For this purpose, we integrate published sedimentological data with a new structural analysis. Our forward kinematic model is fed back by: (i) the geometries of Neogene deformed beds throughout the Precordillera, such as sedimentary wedges and angular unconformities; (ii) flexural subsidence due to the topographic and sedimentary loads for each deformation step, constrained by measured stratigraphic sections; (iii) a revised chronostratigraphy and correlation chart of the Cenozoic sedimentary units; and, (iv) recent constraints on the ages of activity of the faults. This allows us to build a model that considers the timing of deformation for each main structure and its relationship with sedimentation, producing a model with better geometrical, sedimentological and paleogeographical constraints than previous proposals. The results of this modelling are presented in six different evolutionary stages. During the first stage, between 19 and 15 Ma, the eastern Cordillera Frontal is uplifted, associated with subsidence in the Rodeo-Iglesia proximal foreland basin, and low subsidence in the Bermejo distal foreland basin. In the second stage, between 15 and 12 Ma, uplift of the Western Precordillera took place, associated with the creation of a shallow detachment beneath the Rodeo-Iglesia basin. The third stage, between 12 and 8 Ma, was the beginning of the Central Precordillera uplift and the first pulse of uplift in the Valle Fértil range. During the next stage, between 8 and 5 Ma, the Central Precordillera rapidly deformed. Between 5 and 2 Ma, deformation in the Western and Central Precordillera almost ceased, and the focus of contractional deformation shifted to the Eastern Precordillera. The last 2 Ma phase is characterized by strike-slip deformation in the Rodeo-Iglesia basin and contractional deformation in the Eastern Precordillera and the Valle Fértil range. The key results are that the amount of total shortening, 66 km (35%), calculated for the cross-section, is significantly less than previously proposed, and the detachment of the Precordillera fold-and-thrust belt is shallower (∼6.5 km vs 16 km) than previous models. This last point allows us to connect the Cordillera Frontal with the Precordillera, with a ramp-plane geometry of the detachment, as previously suggested, but allowing at the same time the subsidence and the generation of the observed geometries in the Rodeo-Iglesia basin.