FROM OROGENIC HINTERLANDS TO MEDITERRANEAN-STYLE BACK-ARC BASINS: A COMPARATIVE ANALYSIS

Abstract

Hinterland plateaux and Mediterranean-style back-arc basins both form behind active subduction zones or collisional megathrusts, and share many characteristics: (1) early crustal thickening to about twice normal continental thickness; (2) thin lithospheric mantle; (3) mixed magmatism including asthenospheric, lithospheric, and crustal melts; (4) late-stage horizontal extension accompanied by vertical shortening. Horizontal extension and vertical shortening are driven by high gravitational potential energy (GPE) contrasts between the hinterland and surrounding lithosphere, which may reach 7 3 10 12 N m À1 , equivalent to about 2.5 times the ridge-push force. If extension is rapid relative to the rate of lithospheric cooling, GPE may remain positive even as extension continues, declining to the mid-ocean-ridge value as crustal thickness approaches zero. This suggests that hinterland plateaux could ultimately evolve into oceanic back-arc basins. The rate, direction, and amount of extension, and the rate of vertical shortening, depend on the plate boundary conditions and the GPE of the surrounding lithosphere. Vertical shortening in Tibet is limited by work required to deform the surrounding Asian lithosphere, whereas Mediterranean back-arc basins can extend at the expense of regions of thin continental or oceanic crust. Continental collision zones are typically marked by a major thrust belt that accommodates a large proportion of the total convergence, behind which lies a hinterland plateau characterized by significant crustal thickening but relatively low strain rates. This plateau may in turn be flanked by thin-or thick-skinned thrust belts on its rear margins. The type example of this configuration is the Tibetan plateau, bounded by the Himalayan thrust belt to the south, and by convergent or transpressive zones to the north and east.