Mechanical Response of the Southern Mexico Lithosphere to Laramide Shortening

We discuss a model for the mechanical response of the southern Mexico lithosphere to Laramide shortening that considers strength differences produced by the geological history. Continental lithospheres are mechanically complex systems because their rheological characteristics can greatly vary in time and space. In such mechanically non-uniform system, strain is accommodated differently in adjacent blocks with contrasting strength. Considering the geological complexity of southern Mexico, previous models proposing a sequential eastward progression of deformation with the development of kinematically compatible structures clearly oversimplify the reality. The available geological data for southwestern Mexico suggest that the mosaic of lithosphere blocks evolved jointly since the early Mesozoic, and Laramide shortening occurred after a period of extension in the continental margin. We discuss the results of scaled physical experiments that simulate the mechanical response of the southern Mexico lithosphere to extension and posterior Laramide shortening, considering a lithosphere system composed of a stable continental margin or cratonic area (Acatlán complex region) flanked to the west by a normal reference crust, in which two built-in lithosphere-scale weak zones represent the Guerrero Morelos Platform and Arcelia Palmar Chico basins. In our models, the deformation is driven by lateral transmission of forces and the continental lithosphere is decoupled from the astenosphere. The results suggest that weak lithospheric zones not only effectively transfer deformation toward the inner parts of the orogen in the early stages of deformation, but can also produce late out-of-sequence structures in the hinterland. Rheological contrasts also exert control on the geometry of structures during moderate oblique convergence and influence the vergence of the orogen producing zones with contrasting transport directions. Finally, our results can help to explain some contrasting features observed in style and amount of deformation in southern Mexico by simply considering shortening of a mechanically heterogeneous lithosphere.