Numerical modelling of dynamic continental collision: quantifying the slab rollback orogeny model for the Central Alps
Agtmaal, L.J.H. van
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The Central Alps are one of the continental collision zones that cannot be explained by conventional collision tectonics. The striking isostatic disequilibrium has inspired the formulation of the slab rollback orogeny (SRO) model. This concept revolves around the notion that vertical buoyancy forces dominate over horizontal forces. Recently it has been applied to the Central Alps in a mostly kinematic framework, but lacks supporting dynamic evidence. This research uses quantitative (seismo-)thermomechanical models of continental collision with a fixed lower plate to evaluate the SRO model, both in general terms and in application to the Central Alps. Specifically, the most important vertical and horizontal forces (i.e. slab pull and mantle tractions below the overriding plate) are monitored to explain the observed evolution of the collisional systems. The role of the lower continental crust in both continents is assessed as well in order to explore circumstances for lower crustal indentation as interpreted in the Central Alps. Several models produce orogenic and sub-lithospheric architectures that resemble the Central Alps surprisingly well. The modelling results and dynamic analysis show that the SRO model underestimates the importance of mantle drag in driving motions of tectonic plates. Rollback after slab detachment does not occur, due to detachment-induced rebound and unbending of the slab which invariably generates mantle drag away from the trench. Extensive parameter studies revealed that ocean length and frictional strength ratios between various layers have a large influence on the modelling outcome. The lower crust of the overriding plate is found to be much more important than that of the down-going plate in controlling the geometry of the orogen. Indentation is favoured when the lithospheric mantle is somewhat weaker and the upper crust of the overriding plate decouples from its lower crust. These results demonstrate the effectiveness of thermo-mechanical models in producing structures similar to those observed in nature. Furthermore, they provide important insights into the parameters that control their development and they can be used to quantify the forces that effectuate the structures.