Interplay between constructive deep mechanisms building the Central Andes and the stress field

GIAMBIAGI, L; SPAGNOTTO, S; TASSARA, A.; SURIANO, J; MESCUA, J; LOSSADA, A.; BARRIONUEVO, M.

Hamburg, Germany

Simposio; Latin-American Colloquium of Geosciences; 2019

Both in continental collision and subduction-related orogens, differentkinds of mega-detachments have been proposed to explain the horizontalshortening, crustal thickening and subsequent topographic uplift. Infold-and-thrust belts, these detachments have traditionally been located insidelevels of mechanical anisotropies, such as crystalline basement and sedimentarycover interphase. These orogenic wedges thicken towards the hinterland,reaching depths located in the brittle-ductile transition. At an orogenicscale, these shear zones are controlled mainly by the rheologic structure ofthe crust. Along these areas, the deformation is not controlled by frictionalsliding, but rather by temperature, composition, strain rate, and the stressfield. While the synergies between crustal deformation, exhumation andsedimentation processes are well-known to a first order, it is challenging toevaluate the interplay between constructive deep mechanisms and the in-situstress field.Two outstanding questions in the study of orogenic processes are:?How does shallow structures in the foreland fold-and-thrust belt connect withones in the hinterland under an evolving and changing stress field? ?How longcan a detachment remain active during an orogenic event? To answer these questions,we use the Central Andes as a natural laboratory and choose differentcross-sections to kinematically reconstruct the last episode of crustaldeformation and thickening. Our kinematic-thermomechanical models show that ashallow, sub-horizontal megadetachment located at the shallowestbrittle-ductile transition concentrates most of the horizontal crustalshortening between the fore-arc and the South American craton.We propose that,locally, a threshold in horizontal shortening and crustal thickening isachieved when the buoyancy force equals the horizontal force, and at thispoint, the mega-detachment deactivates, and the crustal root widens eastwards,in concert with ductile deformation in the lower crust and the generation of anew mega-detachment. Our working hypothesis is that, by studying changes in thepaleo-stress fields along the arc region, together with the timing of upliftand exhumation of the morpho-structural units across the transects, we canconstrain the timing of activation/deactivation of the detachments responsiblefor the Andean deformation. We suggest that a change in the stress field fromcompression to strike-slip regime can be used as a proxy for the deactivationof a mega-detachment.