Revisiting the Crustal Structure and Kinematics of the Central Andes at 33.5°S: Implications for the Mechanics of Andean Mountain Building

Magali Riesner (1) , Robin Lacassin (2) , Martine Simoes (2) , Daniel Carrizo (3) , Rolando Armijo (1)

1 IPGP - Institut de Physique du Globe de Paris
2 IPG Paris - Institut de Physique du Globe de Paris
3 UCHILE - Universidad de Chile = University of Chile [Santiago]

The 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.

Domaines: Sciences de la Terre

Magali Riesner, Robin Lacassin, Martine Simoes, Daniel Carrizo, Rolando Armijo. Revisiting the Crustal Structure and Kinematics of the Central Andes at 33.5°S: Implications for the Mechanics of Andean Mountain Building. Tectonics, 2018, 37 (5), pp.1347-1375. ⟨10.1002/2017TC004513⟩. ⟨hal-02157984v2⟩