THERMALLY SOFTENED CONTINENTAL EXTENSIONAL ZONES (ARCS AND RIFTS) AS PRECURSORS TO THICKENED OROGENIC BELTS

Abstract

Intra-continental deformation of soft zones during continental collision requires weak continental lithosphere which is able to be shortened across considerable width during later convergence. This enables significant thickening with formation of an orogenic root. We have examined models with a history of lithospheric thinning by pure shear during an earlier phase of intra-continental extension with associated heating. Geologically this situation is appropriate to intra-continental rifts and back-arc basins.If thinning of elevated thermal structure is decoupled from the thinning of lithology then a weak (soft) lower crust and sub-arc/rift mantle result. This weak structure has a favoured rheology for subsequent convergent thickening while the lithosphere is still hot. These regions are associated with formation of granulites and metamorphic assemblages typical of high-temperature/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 sites are ideal for the formation and for the exhumation of metamorphic core complexes. Subsequent thickening during convergence leads to HT eclogites when the previous arc/rift was hot and to medium-T eclogites for a thickened ''standard'' geotherm. These P-T paths are counterclockwise and their shapes are strongly dependent on the amount of previous thinning and type of initial 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 occurs and deep continental roots cannot form.