Far- and near- field deformation and granite emplacement

Abstract

The thermal structure of the continental crust controls both the rheological behaviour of the crust and partial melting at depth. As far as material which can be melted, is there, granites are produced and intrude the upper crust. A layered crostai model is assumed with distinct rheologies, quartz or feldspar dependent for the upper and lower crust respectively. The model also assumes heat production in each layer and basal heat flow values. The thermal state of the crust, is examined by varying either the thickness of the uppermost crust, or the basal heat flow. This is equivalent to overthrusting upper crustal slivers as observed during contractional tectonics, or to increase the non-crustal component of heat flow as expected in continental rifts. In both cases (thinning and thickening), crustal deformation, usually brittle at low temperatures, turns to plastic flow with increasing temperature. The mode of emplacement of granitic plutons observed under many tectonic conditions is examined with reference to the proposed models. Granites are emplaced in all tectonic environments and the regional deformation (far-) field controls the bulk morphology of the plutons. The near-field conditions are of major importance in determining the location where granites develop. All plutons are emplaced within locally extensional zones (near-field) because a lower stress level is required to penetrate the brittle crust. The far-field conditions control the geometry, the number of roots and the thickness of the plutons, whereas the near-field conditions control the geometry of their roots.