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

Abstract

Structures in orogenic belts are greatly controlled by interactions between pluton-related and tectonic-related strain fields. However, many aspects of the geometry and evolution of these structures are poorly known. For example. (1) What is the geometry of these structures if regional deformation is coaxial? (2) How does this geometry change with progressive deformation? This paper describes a three-dimensional simulation of strain shape and magnitude as well as foliation and lineation patterns formed by interactions between pluton expansion and homogeneous coaxial deformation. This simulation suggests that interactions between pluton expansion and regional uniaxial shortening generate patterns that are distinct from the patterns produced by regional uniaxial extension or regional non-coaxial deformation. For example: shortening produces doughnut-shaped constrictional rings about the pluton, whereas extension produces apple-shaped flattening regions around the pluton. In addition, shortening produces one continuous doughnut-shaped low-strain region around the pluton. On the other hand, extension creates two separate ellipsoidal low-strain regions at the ends of the pluton. Furthermore, shortening forms foliation rings that have triangular two-dimensional sections, and lineation is concentric around the axis of maximum shortening. In contrast, extension produces two separate foliation and lineation cones at the ends of the pluton. All these structural and strain elements migrate away from the pluton with increasing expansion rates and towards the pluton with increasing tectonic deformation rates. Although structural studies around plutons should always integrate microstructural, radiometric and field measurements, simulation-generated patterns can be used to evaluate: (1) origin, geometry and evolution of structures around plutons; (2) mechanisms that explain inclusion trails within porphyroblasts; (3) contribution of pluton-related strains to strains measured in the country rock; and (4) type and three-dimensional orientation of regional kinematic schemes.