Interference between pluton expansion and non-coaxial tectonic deformation: three-dimensional computer model and field implications

Abstract

Pluton emplacement contributes to the complexity of structures in orogenic belts because the geometry of these structures is often controlled by interactions between regional tectonic-related and local pluton-related strain fields. Many questions remain about the genesis of these structures. For example: (1) to what extent does pluton-related strain contribute to the formation of these structures? (2) what is the three-dimensional geometry of these structures? and (3) how does this geometry change with progressive deformation? The present simulation models in three dimensions the position, type and kinematic evolution of structures around plutons that expand in a non-coaxial tectonic environment. The simulation generates patterns of four parameters: strain ellipsoid shapes and magnitudes, and orientation of foliation and mineral stretching lineations. Concave, lens-shaped, regions of high strain form on the sides of the pluton, whereas ellipsoidal regions of low strain form at the ends. In addition, foliation, stretching lineations, and constrictional regions form irregular three-dimensional rings around the pluton. Higher tectonic-related strains and strain rates favor rings that migrate towards the pluton, whereas higher expansion-related strains and strain rates favor ring migration away from the pluton. Furthermore, ring migration rates are much slower than tectonic deformation rates or expansion rates. Although analyses of natural strain fields should always be carried out in conjunction with radiometric age determinations, structural and microstructural analysis and strain measurements, the simulated strain patterns offer practical guidelines for field work. Specifically, these patterns can be used to evaluate: (1) the origin, geometry and evolution of structures around plutons; (2) the three-dimensional sense of movement and orientations of ductile shear zones; and (3) the contribution of pluton-related strains to deformation of country rocks in orogenic belts.