Abstract
AbstractThe Andean belt is the only present‐day active case example of a subduction‐type orogeny. However, an existing controversy opposes classical views of Andean growth as an east verging retro wedge, against a recently proposed bivergent model involving a primary west vergent crustal‐scale thrust synthetic to the subduction. We examine these diverging views by quantitatively reevaluating the orogen structural geometry and kinematics at the latitude of 33.5°S. We first provide a 3‐D geological map and build an updated section of the east vergent Aconcagua fold‐and‐thrust belt (Aconcagua FTB), which appears as a critical structural unit in this controversy. We combine these data with geological constraints on nearby structures to derive a complete and larger‐scale section of the Principal Cordillera (PC) within the fore‐arc region. We restore our section and integrate published chronological constraints to build an evolutionary model showing the evolving shortening of this fore‐arc part of the Andes. The proposed kinematics implies uplift of the Frontal Cordillera basement since ~20–25 Ma, supported by westward thrusting over a crustal ramp that transfers shortening further west across the PC. The Aconcagua FTB is evidenced as a secondary east verging roof thrust atop the large‐scale basement antiform culmination of the Frontal Cordillera. We estimate a shortening of ~27–42 km across the PC, of which only ~30% is absorbed by the Aconcagua FTB. Finally, we combine these findings with published geological data on the structure of the eastern back‐arc Andean mountain front and build a crustal‐scale cross section of the entire Andes at 33.5°S. We estimate a total orogenic shortening of ~31–55 km, mainly absorbed by crustal west vergent structures synthetic to the subduction. Our results provide quantitative key geological inferences to revisit this subduction‐type orogeny and compare it to collisional alpine‐type orogenic belts.
Highlights
It is generally admitted that orogeny on Earth results primarily from tectonic shortening and thickening of continental crust associated with continuing plate convergence, most commonly after a protracted period of subduction of oceanic lithosphere under continental lithosphere
We propose a kinematic model of incremental deformation of the Principal Cordillera through 7 temporal snapshots, according to the abovementioned chronological constraints: - Time 1, Late Oligocene: the initial Meso-Cenozoic basin is not yet affected by Andean compressional deformation and is being filled with volcanic and volcano-clastic rocks (Abanico Formation)
The thickness of the Cenozoic to Mesozoic formations located within the eastern West Andean-FTB and the West-Vergent Folds (WVF) imply an initial undeformed ~15 km deep Andean basin
Summary
It is generally admitted that orogeny on Earth results primarily from tectonic shortening and thickening of continental crust associated with continuing plate convergence, most commonly after a protracted period of subduction of oceanic lithosphere under continental lithosphere. Basement culmination using recently published thermochronological data (Hoke et al, 2014) and additional constraints on the Andean eastern front (Garcia et al 2005, Garcia and Casa 2014 and Giambiagi et al 2015) to discriminate between the two existing conceptual models of Andean orogeny These constraints allow us for discussing the mechanics of Andean mountainbuilding and for proposing a crustal-scale section of the Andes at this latitude. To the far east of the Andes, the southern Sierras Pampeanas (Figure 1) are thick-skinned west-vergent structures outcropping pre-Andean basement rocks, with relatively limited amount of cumulative shortening at ~33°S latitude and further south They are further described by Ramos et al (2002) and will not be further considered here
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