Thermally softened continental extensional zones (arcs and rifts) as precursors to thickened orogenic belts

Intra-continental deformation of soft zones during continental collision re quires weak continental lithosphere which is able to be shortened across co nsiderable width during later convergence. This enables significant thicken ing with formation of an orogenic root. WI: have examined models with a his tory of lithospheric thinning by pure shear during an earlier phase of intr acontinental extension with associated heating. Geologically this situation is appropriate to intra-continental rifts and back-are basins. If thinning of elevated thermal structure is decoupled from the thinning of lithology then a weak (soft) lower crust and subarc/rift mantle result. Th is weak structure has a favoured theology for subsequent convergent thicken ing while the lithosphere is still hot. These regions are associated with f ormation of granulites and metamorphic assemblages typical of high-temperat ure/low-pressure (HT/LP). If convergence starts while the heat input is still active then the failed rifts and arcs are shut off by lateral wedging of the hard lower crust and upper mantle of shoulder regions into the softened arc/rift domain. Such si tes are ideal for the formation and for the exhumation of metamorphic core complexes. Subsequent thickening during convergence leads to HT eclogites w hen the previous arc/rift was hot and to medium-T eclogites for a thickened "standard" geotherm, These P-T paths are counter clockwise and their shape s are strongly dependent on the amount of previous thinning and type of ini tial geotherm. If the compression starts long after cessation of the extensional event and associated thermal anomaly, then the geotherm of the extended area relaxes and the whole region hardens. In this case, no homogeneous thickening occu rs and deep continental roots cannot form. (C) 2001 Elsevier Science B.V. A ll rights reserved.