HINTERLAND PLATEAUX AND MEDITERRANEAN-STYLE BACK-ARC BASINS AS AN EVOLUTIONARY SEQUENCE

Hinterland plateaux and Mediterranean-style back-arc basins both form behind active subduction zones or collisional megathrusts, and share many characteristics: (i) Early crustal thickening to as much as twice normal continental thickness; (ii) late-stage whole-lithosphere thinning; (iii) thin or absent lithospheric mantle; (iv) mixed magmatism including asthenospheric, lithospheric, and crustal melts; (v) core complexes exhuming mid to lower crustal rocks below extensional detachments. Thinning and lateral “escape” are driven by high gravitational potential energy (GPE) associated with shallow asthenosphere and/or thick crust. GPE may reach 10¹³ Nm^-1 relative to mature oceanic lithosphere – roughly 2.5 times the ridge-push force. GPE remains positive as extension continues, declining to the mid-ocean-ridge value when the crust ruptures. This suggests that hinterland plateaux may ultimately evolve into oceanic back-arc basins. The rate and amount of thinning depend on the boundary conditions. E-W extension in Tibet is compensated by N-S India-Eurasia shortening, producing almost plane strain with very minor thinning. Where extension is accommodated by an adjacent subduction zone, as in the pre-middle-Miocene Basin & Range Province, rates of thinning are much larger. Several Mediterranean back-arc basins are adjacent to active subduction zones, and thinning is driven by the GPE contrast of about 4 x 10¹² Nm^-1 between the back-arc basin and the subducting oceanic lithosphere. Originally thickened orogenic crust beneath these basins has been thinned by a factor of up to ten prior to rupture, locally exhuming the base of the crust and the underlying mantle.