Is there a relationship between magmatic fabrics and brittle fractures in plutons?: A view based on structural analysis, anisotropy of magnetic susceptibility and thermo-mechanical modelling of the Tanvald pluton (Bohemian Massif)

Abstract

The present study focuses on the thermal and mechanical aspects of cooling of plutonic systems with respect to their internal structure. Analysis of the Tanvald pluton and the nearby Liberec granite (the Krkonoše-Jizera Plutonic Complex, Bohemian Massif) using the anisotropy of magnetic susceptibility (AMS) method indicates the decoupled magnetic fabrics formed by different processes in these two plutons. Two regional sets of vertical fractures cut across the plutons and their host rocks. They are interpreted as having been formed as opening mode fractures (joints) after emplacement and cooling. Our thermo-mechanical finite element modelling indicates that the thermal stresses within the Tanvald pluton were compressive and that fracture criteria were not met during pluton cooling. These results imply that the emplacement depth and confining pressure significantly affect the state of stress within a pluton during cooling and may play a crucial role in the formation of early fractures in plutons. Consequently, the formation of early fractures during pluton cooling is driven by external conditions overriding the role of internal magmatic fabrics. We argue that no genetic, geometric or temporal correlation can exist between magmatic fabrics and fractures in granitoid plutons, and thus that the original concept of Hans Cloos based on fabric/joints relationships should be reconsidered. Instead, an alternative classification of fractures in plutons should include cooling fractures, syntectonic fractures, uplift fractures and post-uplift fractures.