Strain fields and foliation trajectories around pre-, syn-, and post-tectonic plutons in coaxially deformed terranes

Abstract

This paper determines, using simple mathematical models, maps of strain fields and foliation trajectories in the neighbourhood of plutonic bodies intruded into a region before, during, and after a lateral compression. The analyses reveal contrasting features for the three cases, which may be useful in distinguishing pre-, syn-, and post-tectonic plutons in coaxially deformed terranes. The theoretically calibrated strain variation curve, along the regional shortening direction, around a pretectonic, rigid plutonic body shows a zone of strain increase (relative to the far-field strain), whereas that around a syntectonic pluton shows a zone of strain decrease along with a zone of strain increase. The foliations deflect outward from a pretectonic pluton, but they bend towards a syntectonic pluton. The analysis also reveals that syntectonic plutons are likely to be elliptical in plan with axial ratio directly dependent on the ratio of the pluton's stress to the tectonic stress. Tectonic foliations deflect away from post-tectonic plutons. However, the foliated area around a post-tectonic pluton is systematically divided into two sets of zones one characterized by foliations showing compressional features, the other showing extensional features.