Contemporary stress field, crustal deformation, exhumation and sedimentation during the building of the Central Andes over the last 20 my: Advances in the Central Andean Stress Field Evolution Project.

GIAMBIAGI, LAURA; SPAGNOTTO, SILVANA; SURIANO, JULIETA; LOSSADA, ANA; MESCUA, JOSÉ; QUIROGA, RODRIGO; BARRIONUEVO, MATÍAS

Quito

Congreso; 8 th International Symposium on Andean Geodynamics (ISAG); 2019

Instituto Geofísico, Escuela Politécnica Nacional

The estimation of contemporary stress field, crustal deformation and sedimentation during the evolutionof the southern Central Andes, between 20º and 36ºS, is crucial for the understanding of plate-drivingand gravitational forces, acting during the construction of a subduction-related orogenic system, and themechanical behaviour of the continental plate. While the synergies between crustal deformation,exhumation and sedimentation processes are well-known to first order, it is challenging to evaluate theinterplay between constructive deep mechanisms building the Andes, the stress field, exhumation andbasin development.The goal of our study is to do a systematic analysis of the stress field evolution, in the time intervalbetween 20 Ma to the Quaternary. For this purpose, the temporal and spatial stress field variations as aparameter crucial to understanding the relationship between deep and shallow crustal deformation andassociated exhumation and sedimentation is used.Reduced stress tensors, n=293, from more than 4,000 fault-slip data were obtained. This data set isintegrated with 9,240 published fault-slip data from other authors, into a database to obtain a total of 865location sites with kinematic data or reduced stress tensors. Data from geophysical logs, dyke and sillanalysis, and the compilation of 1,293 earthquake (>2 Mw) focal mechanisms to image the currentstress field in the High Andes and the forelands.The integration of these data suggests that, on the one hand, spatiotemporal changes in the state ofstress in the crust indicate a shift from compression to strike-slip or normal tectonics, associated withσ3/σ2 and σ2/σ1 permutations respectively, in the highest portions of the Andes, due to an increase inthe absolute magnitude of the vertical stress component as the crust thickens and the gravitationalpotential energy increases. This change in local stress regime is closely linked with changes in the styleof basin formation, for example from either foreland or broken foreland to pull-apart settings.On the other hand, the results of paleo and present stress field analysis show a remarkably consistentmaximum horizontal stress direction (either σ1 or σ2) subperpendicular to the plate convergencedirection and/or subperpendicular to spatial variations of the gravitational component of stress. Thissuggests that, during the latest stage of Andean construction of the last 20 my, the SHmax orientation ismainly controlled by plate boundary forces, topography and lithospheric density structure.