Fabric development in syn-tectonic intrusive sheets as a consequence of melt-dominated flow and thermal softening of the crust

The temperature dependence of ductile deformational processes suggests that the thermal anomaly associated with a cooling, syn-tectonic intrusion will produce a crustal low-viscosity horizon that will concentrate deformation in the vicinity of the pluton during cooling. This effect should be most prominent when magma is present but should also occur after solidification of the magma when flow is by solid-state processes. This paper evaluates this hypothesis for sheet-like intrusions using one-dimensional, time-dependent thermal models and accompanying predictions of viscosity vs. time history based on experimental flow laws. These models predict that for systems with an initial “normal” geothermal gradient (e.g., extensional systems, strike-slip systems, or thrust systems with low displacement rates), the base of a large plutonic sheet cools more slowly than the top and fabric development should be most pronounced on the floor of the pluton. In contrast, in megathrust systems where displacements are sufficiently rapid to produce temperature inversions, cooling is also “upside-down” and fabric development should preferentially occur along the top of plutonic sheets. Moreover, when a pluton is emplaced within a zone of inverted isotherms, the heat may be trapped within the inversion. A natural system characterized by this history should show a sharp thermal front coincident with the top of a paleo-temperature inversion. If a pluton is weaker than its country rock it will form a weak horizon in the crust throughout its cooling history and the plutonic sheet should take up the bulk of the deformation throughout its cooling history. If a pluton is stronger than its country rock under conditions of solid-state flow, however, models predict a two-phase deformational history: prior to solidification the deformation should be concentrated in the pluton with the magma representing a weak horizon in the crust, but upon crystallization the pluton should become relatively rigid with respect to its country rock and deformation should be concentrated in the wall rocks. These two cases predict markedly different structural histories, and the history could easily be misinterpreted. In the first case static mineral growth might be prominent in the wall rocks and these textures could be misinterpreted as evidence for post-tectonic emplacement. In the second case, the extensive high-T deformation of the wall rocks could be readily ascribed to non-tectonic, emplacement-related deformation rather than a consequence of syn-tectonic emplacement. The model predictions are tested with three examples of well-exposed plutonic sheets: a Neogene extensional system in Death Valley, California, and two megathrust systems in southern Alaska (Border Ranges fault system and Mclaren metamorphic belt). Structural observations from these systems are broadly consistent with the model predictions and lend support to first-order characteristics of the models. Nonetheless, more sophisticated 2-D models of the coupled thermal-mechanical system are needed to thoroughly test the hypothesis.