Analogue Models of Polyphase Deformation Around a Built-in Thicker Crustal Block.

Analogue models of polyphase convergence involving crustal differences in strength and density give insights on lateral and vertical strain propagation during the Late Cretaceous - Early Tertiary in southern Mexico. Analogue models reproduced a two-phase deformation characterised by a first stage of east directed compression (11 %\ bulk shortening; b.s.), followed by a second stage of left-lateral transpression directed N 15° E (17 %\ b. s.). Models were designed to simulate two-layer vertical rheology, a brittle upper crust and a lower ductile crust (silicon+sand) with a thicker built-in parallelepiped-shaped block in the upper crust. Maintaining this basic array, we varied the structure of the rigid block using sand or plastic clay, as well as its position (adjacent or separated from a moving wall in the second deformation phase). In all models, similar structures formed around the block: (1) a north-south striking fold-and-thrust belt produced in the first phase, which distance of propagation decreases where the block is present; (2) a main left-lateral transpressive structure in the second phase; (3) counter clockwise rotations and refolding of the first phase structures in the western side of the block; (4) a fold-and-thrust belt paralleling the geometry of the block in front of its north and eastern sides and; (5) vertical upheaval of the block. In the case of the models with the block separated from the moving wall, the transpression was concentrated in the boundary of the block with the adjacent crust and exhumation of the lower crust was observed south of this limit. Models were compared with the deformation observed around the Mixteco-Oaxaca Block (MOB) of southern Mexico. Most structures were reproduced by the models, but structures observed in nature within the MOB were not accurately reproduced. This is likely due to the action of pre-existing structures that influenced the deformation. The similarity of the modeled structures and natural prototype suggest a close analogy between deformation processes and permit the reinterpretation of several key structures for the understanding of the geological evolution of southern Mexico.