From orogenic hinterlands to Mediterranean-style back-arc basins: a comparative analysis

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 N m , 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 thinor 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 (Fig. 1). Similar configurations have developed along the southern margin of Asia further west, in the Iranian and Anatolian plateaux, and analogous structures may be developing in the incipient collision zone at the western end of the Sunda arc and in New Guinea. More generally, very similar structures have developed in the Andean and Cordilleran orogens on the western margin of the Americas, where large plateaux have formed through intracontinental shortening behind the major subduction zones along the Pacific rim. Several of these plateaux are currently undergoing some degree of normal faulting at high elevations, and in the US Cordillera this process has led to about 50% vertical shortening in the Basin and Range Province, reducing the crust to about half of its post-orogenic thickness (Fig. 2). A similar process has proceeded much further in the complex orogens of the Mediterranean region, where tightly arcuate thrust belts surround extended hinterlands, several of which have subsided below sea level, and some of which have rifted to form young ocean basins (Fig. 3). Some of these Mediterranean back-arc basins are clearly related to active subduction zones, and may therefore be directly analogous to the back-arc basins of the western Pacific; others, such as the Pannonian basin, the Alboran Sea and the northern Tyrrhenian Sea, have formed in essentially intracontinental settings, and the role of subduction is less clear. The purpose of this paper is to make the case that collisional plateaux, extending Cordilleran hinterlands, and Mediterranean back-arc basins represent a spectrum of hinterland behaviour with many geological, tectonic, and mechanical features in common. I focus on three examples: the Tibetan plateau, the Basin and Range Province, and the Alboran Domain of the western Mediterranean (Fig. 4). These, and many other examples, share the following characteristics, documented in more detail below: (1) an initial stage of crustal thickening, to as much as twice normal continental thickness; (2) removal of much or all of the lithospheric mantle, either during or after crustal thickening; (3) mixed mode magmatism including asthenospheric, lithospheric, and crustal melts; (4) late-stage vertical shortening and horizontal extension (which may be coeval with continuing plate convergence), reflected by normal faults and conjugate sets of strike-slip faults in the upper crust; (5) core complexes exhuming midto lower crustal rocks below extensional detachments. There are many marked differences between these examples, reflecting their differing geological settings, and differences in the relative importance of the tectonic processes involved. As is commonly the case in the Earth Sciences, identification of fundamental underlying processes requires us to sift through the intricate details of the geology to identify common themes.