Convergence and extension driven by lithospheric gravitational instability: Evolution of the Alpine-Carpathian-Pannonian system

Abstract

Lithospheric gravitational instability may play a significant role in the Neogene Alpine-Carpathian-Pannonian system, where the extensional Pannonian Basin formed over a ∼10 Myr period simultaneously with contraction in the surrounding Alpine and Carpathian orogens. We investigate the idea that gravitational collapse of low-density crust, previously thickened in the collision of the European plate with Adria, has caused the simultaneous development of depth-variable lithospheric thinning beneath the Pannonian Basin and contractional deformation in the Carpathian arc. Implicit in this model is asthenospheric upwelling beneath the basin and lithospheric downwelling beneath the surrounding mountains. We investigate the interplay between convergence and extension in this type of continental orogen using 2-D axisymmetric finite element models. We show how key model parameters, such as crust to lithosphere thickness and viscosity ratios, affect the evolution of the system: an initially thicker lithosphere enhances mantle instability and leads to a greater degree of thinning of the crust and lithospheric mantle, and stronger downwelling in the mantle. Increasing the viscosity of the crust relative to the mantle slows the growth of the instability and reduces crustal deformation. Relatively low crustal viscosity, however, implies too rapid a collapse of initial topography and rapid re-equilibration after deformation induced by mantle flow. Non-Newtonian viscosity localizes the deformation in a narrower spatial region and a shorter time interval, following a relatively slower phase of initial growth. In the Alpine-Carpathian-Pannonian system, development of the instability requires lithospheric thickness prior to extension on the order of 150 km, and lithospheric mantle viscosity on the order of 2 × 1020 Pa s, with crustal strength similar to that of the mantle. © 2007 The Authors Journal compilation © 2007 RAS.