Surficial architecture of the Principal Cordillera structural lid: an orogenic wedge solution for the Southern Central Andes ( 34°15'S)

The lithospheric scale-configuration, overall structure and vergence, and geodynamics of the Principal Cordillera (PC) at the Southern Central Andes has been subject of ongoing controversy. Our working hypothesis is that the gross structure of the western Andean slope corresponds to the retro-side and apex (structural lid) of a doubly-vergent orogenic wedge, generated by west-directed subduction of the South American lithosphere. The aim of this work is to provide additional structural evidence to test our hypothesis, by means of balanced restored cross sections of the PC at 34°15'S using previous studies in the area and new detailed structural mapping.

Forward kinematic modeling using Move2D software (Midland Valley) was carried out to assess our inferences on the in-depth style of deformation. Overturned west-vergent folds characterize the western Andean deformation front where two high-angle blind reverse faults account for 4.6 km of shortening (38%). Eastward, towards the Andean drainage divide (limit between pro- and retro-sides), adjacent to the Malargüe fold and thrust belt, east-vergent folding accommodated deformation by means of a combination between detachment and fault-propagation folding, rooted into a shallow sub-horizontal décollement. Furthermore, deformation within incompetent sedimentary layers of Neogene series of the PC allows fold hinge collapse, back- and break-through thrusts. The latter suggests multi-detachment asymmetric folding in this area, where shortening reaches a minimum of 4.3 km (47%). Considering the relatively similar shortening estimates along the PC, that major shortening in this part of the Andes is accommodated in the eastern side, and that timing of deformation shows an eastward migration of the front, our results are in general agreement with previous structural interpretations for the PC in between 33°-36°S. This is consistent with an asymmetric doubly-vergent crustal wedge model related to main underthrusting processes involving intracontinental subduction of the South American lithosphere to the west, where the western Andes outcrops belong to a structural lid passively uplifted due to distribution of the incoming material accreted in the Malargüe fold and thrust belt after shortening occurred in the western Andes (mainly before the Late Miocene).