CRUSTAL DEFORMATION, DEEP CRUSTAL FLOW AND PTτ EVOLUTION DURING EXTENSIONAL TECTONICS IN HOT CONTINENTS

Observations of mountain belts and orogenic plateaux indicate that hot crustal rocks flow at depth and ascend to very shallow crustal levels by various mechanisms. In Archean cratons, deep crustal rocks fill anorogenic gneiss domes surrounded by greenstone belts. In all cases, exhumation of the deep crust requires that space be made via lateral displacement or extension in upper crust, downward return flow of upper crust (sagduction), localized erosion, or a combination of these mechanisms. A characterization of strain, temperature and pressure evolutions of hot middle to lower crustal rocks can help the inference of geologic settings from PTt data.

As deep rocks approach their solidus, their viscosity decreases and they become sensitive to lateral pressure gradients. Normal to oblique extension of the upper crust leads to lateral pressure gradients that drive horizontal and upward flow of low-viscosity crust into the extending upper crust. Although deep crustal flow is dynamically linked to upper crust extension, the upper crust and deeper crust may display apparently opposite tectonic regimes and contrasting PT-strain evolutions, as semi-rigid divergent motion in the upper crust may trigger convergent flow in the deep crust. This process explains common features of orogens where lower crust contractional structures (upright folds, steeply dipping foliation and high-strain zones) are coeval with upper crust extension and basin development in and around gneiss domes. This result poses an interesting challenge; since tectonic regimes can be strongly partitioned vertically and laterally: can we infer motion of tectonic boundaries from the observations of finite strain in the ductile crust? High-resolution geochronologic-thermochronologic work, added to a good understanding of the metamorphic path of deep rocks in relation to melting and crystallization, can help demonstrate the coupling between deep crustal flow and upper crustal deformation.