Stress modeling in the Andes: Constraints on the South American intraplate stress magnitudes

Abstract

Regions of high topography which exhibit both compressional and extensional deformation provide a natural laboratory to place constraints on horizontal intraplate stress magnitudes. An elastic finite element analysis of the lithospheric stress field in the Cordillera Bianca region of Peru has been used to evaluate the lithospheric stress state and constrain the South American intraplate stress magnitudes. The orthogonal orientation of the Andean high topography to the east–west regional stress field in conjunction with the well‐documented change in deformation style from compression to extension across the Cordillera provides a unique setting for the stress modeling. A two‐dimensional finite element grid consisting of an assembly of isotropic, elastic quadrilateral elements in a state of plane strain was used to represent the lithosphere. An isostatically balanced crustal density model was used to calculate the stresses associated with the lateral density variations between the Andean topography and the Brazilian shield. In some models horizontal stress was concentrated at midcrustal depths by including vertical variation in Young's modulus. The modeling results indicate bounds of 10 and 75 MPa for the magnitude of the average horizontal stress averaged over a 100‐km‐thick lithosphere. A preferred value of 25 MPa is obtained on the basis of fitting all of the data for the rheological models considered and predicting a transition from extension to compression near the 3000‐m elevation contour. Such a far‐field average stress is consistent with a ridge push origin. Because the modeling performed in this study assumes that the topographic load is entirely compensated by the buoyancy of the deflected Moho, these estimates represent upper bounds. The effect of incorporating thermal support for the topographic load of the Cordillera Bianca would be to further reduce our estimate of the intraplate stress magnitudes. Analysis of the distribution of calculated stress with depth indicates that previous assumptions about the vertical plane shear stress within the lithosphere being negligible are acceptable for the geometry of the Andes.