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Institute für Geologie, Universität Bern, CH-3012 Bern, Switzerland
A two-dimensional heat flow model designed to predict the transient thermal consequences of mass movement in tectonically active regions is presented. Although the techniques developed have applicability to numerous tectonic and magmatic scenarios, the MELONPIT model presented here is set up to explore the thermal evolution of destructive plate margins. The solution to the time-dependent heat flow equation that allows for heat conduction, advection, and source/sinks is approximated using the finite element method (FEM). The FEM code uses an adaptive gridding approach that allows for deformation and differential movements of subgrids to simulate tectonic mass flow. This technique requires modification of the standard FEM to allow for special hybrid, triangular, four-node "slip elements" to be used along the lines of movement and standard three-node elements to be used elsewhere. The geometry and velocities of plate convergence, large-scale strain partitioning (subduction channel, slab break-off, tectonic exhumation), and erosion are described explicitly as functions of time. At present, the kinematic model involves a down-going slab, tectonic fragments that are first subducted then obducted, and a convecting (subcontinental) mantle. The thermal anomaly developing in the subduction-collision cycle, its spatial-temporal evolution during and following the tectonic exhumation, and its decay during a backthrusting stage (which involves rapid uplift and erosion) can be modeled. The accuracy of the technique has been validated by comparison of MELONPIT model results with published analytical and numerical solutions for steady-state pressure-temperature conditions along the subduction shear zone.
A brief application of the model is presented that explores the thermochronologic effects of several subduction-related processes, using the Central Alps as a guide in the model setup. Results of these simulations indicate that the shape of paths linking the highest pressure stage with the thermal maximum stage is typically not as linear as interpretations of thermobarometric datasets commonly suggest. In fact, such "interpolated" paths may lead to false kinematic implications. In our models, the precise path followed in any location depends critically on the balance between opposing effects such as conductive cooling upon ascent and shear heating along major contacts, as well as the accretion of upper crustal material within the subduction channel.
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