Revisiting the crustal structure and kinematics of the central Andes at 33.5 degrees S: implications for the mechanics of andean mountain building

Abstract

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 degrees 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 similar to 20-25Ma, 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 similar to 27-42km across the PC, of which only similar to 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 degrees S. We estimate a total orogenic shortening of similar to 31-55km, 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.