Syntectonic Granites and the Principle of Effective Stress: A General Solution to the Space Problem?

Abstract

Syntectonic granite emplacement in dip slip and strike slip contractional shear zones is now well documented by a number of case histories including the spectacular 1200 km long, 20 km wide Great Tonalite Sill of North America. These examples show fundamentally that magma driving forces can overcome compressional tectonic stresses and suggest that in a contractional orogen it is not in general a necessity to make space for plutons by localised dilation along faults and shear zones. There are a number of other magma driving forces that are available, in addition to buoyancy, which can combine to exceed the tectonic compression, including one which is derived from the compression itself. These extra forces are optimised when ascent and emplacement is achieved along major transcrustal faults and lineaments. The principle of effective stress is applied to the general case of granitic magmas in crust undergoing tectonic horizontal compressive stress and it is argued that the magma pressure is an indistinguishable part of the regional (effective) stress field. This allows a general solution to the space problem in granite emplacement since in the lower crust, where the host rocks are weak, the emplacement strain will also become an indistinguishable part of the regional deformation field. This principle is also likely to underlie space creation mechanisms in the often highly heterogeneous middle crust, where it may be obscured by the observed opportunistic exploitation of dilation along tectonic structures and other weaknesses.