THE ROLE OF EXTENSION IN DEVELOPMENT OF OROGENIC GEOTHERMS: HOT OROGENS MAY NOT BE SO HOT (Invited Presentation)

Hot orogens are characterized by large tracts of migmatite and associated high-grade rocks that typically record mid-crustal conditions. Some migmatites may form at the low- to moderate P and high T recorded for their crystallization, but evidence from rare high-P relics and predictions from numerical modeling of extension of thick (60 km) orogenic crust indicate that migmatite terranes may derive from greater depth, with transport of partially molten crust to shallower levels driven by extension or transtension. In these cases, which we propose are common, the rapid, near-isothermal ascent of hot, deep crust to mid crustal levels – and the associated attenuation of mantling rocks – accounts for the high transient thermal gradients observed. The geotherm corresponding to conditions prior to extension is determined from the T at the maximum depth (P), and that depth may be tens of km greater than the depth of emplacement recorded by migmatites. For example, in migmatite of the Montagne Noire (French Massif Central), part of the Variscan hot orogen, P-T conditions of orogenic eclogite (in which HP zircon ages are coeval with the oldest ages of migmatite crystallization) correspond to a thermal gradient of 15 °C/km. Calculation of the transient geotherm for the emplacement of migmatite at ~10 km is >70 °C/km, and measurements across the gneiss dome-schist carapace contact locally indicate >> 100 °C/km, but these high numbers do not represent the orogenic geotherm. Rapid ascent of deeply-sourced migmatite may be accompanied by intrusion of mafic magma generated by extension-driven upwelling of the lithospheric mantle, but additional heat sources such as this are not required to account for the P-T history of migmatite and associated rocks. Development of regional-scale migmatite complexes therefore does not necessarily indicate an unusually hot orogen, and may indicate crustal thickness >50 km and development of a weak (typically partially molten) deep crust that can flow laterally and vertically, forming the large tracts of migmatites in the core of orogens.