Thermo-mechanical Models of Variscan Orogenic Root Evolution - Constraints for Tectonic Hypotheses J. Arnold & W. R. Jacoby Institut für Geowissenschaften, Johannes Gutenberg-Universität Mainz, 55099 Mainz The European Variscides have collapsed due to post-collisional high-thermal overprint combined with uplift, tectonic denudation, crustal melting and underplating. Seismic profiles across the variscan belt show uniform crustal thickness of 30 km beneath the variscan belt. Lithospheric mantle had eventually dropped down, allowing asthenosphere to reach a higher level in the upper mantle and the buoyancy forces at the moho to degrade the crustal root and thin the crust. Uplift and extension could be accompanied by magmatic activity, intrusions taking advantage of pre-existing shear zones to infuse into the lower crust. These processes would all tend to stabalize the Moho at a uniform level. Geodynamic modeling collision stage of the Variscan belt should cover the following aspects: (1) delamination of lithospheric mantle creating a large scale buoancy instability (Schott & Schmeling 1998) (2) rapid exhumation of the metamorphic core complexes with uplift rates of up to 10 mm/a (Masonne 1999, Willner et al. 2000) (3) fast melt emplacement within 10-20 million years after maximum thickening (O'Brien 1999). The question arises wether melting in the orogenic crustal root may be due to heating from within (1) by radiogenic heating of stacked crust (Gerdes et al 1997) (2) by viscous dissipation within the crust (Schott et al. 2000) or additionally heating from below (3) asthenospheric rise to the base of the thickened crust after delamination (Schott & Schmeling 1998) (4) subduction of young oceanic lithosphere ("ridge subduction"). Numerical calculations have been carried out with the finte differences convection code FDCON by H. Schmeling. Beside the finite differences formulation. Flow paths of markers, which carry chemical information, are calculated with a Lagrangian formulation for two purposes: (1) to carry information about physical properties of the markers along flow paths, and (2) to estimate synthetic PTt-paths and to reconstruct the deformation history of units of geological interest. To take mechanical interaction between crust and mantle into account, the models cover convection in the upper mantle. The rheology is assumed to be temperature- and pressure-dependent. Parameters are chosen for upper and lower crust according to Wilks & Carter 1990 and for the mantle according to Bai et al. 1991. Thermal boundary conditions are 0° C at the surface and a constant heat flux from the lower mantle of 20 mW/m2. The adiabatic temperature gradient is about 0.32 K/km. The potential mantle temperature is 1200°C. Crustal radioactive heating is described by exponential decay with depth with a characteristic lengthscale of 30 km. Compressional forces are imposed by 2 cm/a convergence on the model lithosphere on the left-hand side using an initially enforced convection cell. In the models the crustal root reaches a thickness of >100 km with a `tail' of lower crustal material dragged down to >200 km depth between the two `plates'(mechanically decoupled by an assumed weakzone within the lithosphere). Near the `upper plate', lower crust and some mantle are carried to shallow levels. The double vortex is driven by both the stiff upper crust and the descending mantle plate. Crustal material is piled up, the root deepens and broadens, the structures become more complex, upper crustal material is pushed deeper and `slivers' of mantle are carried higher. The models show, that crustal root formation strongly depends on the choice of rheology and the existence of weak zones in the lithosphere. Melting only due to radioactive heatproduction within stacked crust takes too much time to count for the Variscan orogenic evolution. Additional heatsources are certainly needed to produce melting temperatures within geological short timescales (less than 20 Ma). Additional heat can be provided by delamination of lithospheric mantle (Schott and Schmeling 1998), followed by rapid exhumation of metamorphic core complexes (Willner et. al. 2000) and asthenospheric rise to the base of the crust producing melts within tens of million years (H. Schmeling). Open Questions: (1) Could subduction of young oceanic lithosphere ("ridge subduction") support enough heat to produce sufficient magmatic activity within short timescales required for variscan orogeny? (2) How "realistic" are kinematic boundary conditions (timescale, dynamic evolution)? (3) How "realistic" are weakzones (not created by thermo-mechanical coupling)? References: Bai, Q., Mackwell, S.J., Kohlstedt, D. L. 1991. 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