The role of melt in the uplift and exhumation of orogenic belts

Abstract

In some orogenic belts, rapid pulses of uplift occur when melt is present. Detailed work on the relation of melt to structural and metamorphic features in several localities shows that hot metamorphic rocks, and especially anatectic migmatite, can be physically transported from the lower to middle crust on decollements. The decollements form in the lower crust because, even though the lower crust is structurally weak throughout due to thermally activated crystal plastic deformation mechanisms, the crust is substantially weakened where melt forms or is intruded; melt in the lower crust is present long enough to accomodate strain, whereas in the upper crust it crystallizes too fast to be a factor in strain localization. Following crustal loading due to thrusting, exhumation results in the formation of additional melt in the migmatites due to decompression melting. Support of this sequence of events includes: (1) evidence for near isothermal decompression of high temperature metamorphic rocks, with early high pressure minerals having formed pre or synchronous with penetrative deformation and with lower pressure minerals having formed after the deformation, (2) the occurrence of magma sheets that occur along decollements between high temperature metamorphic rocks and lower temperature metamorphic rocks (in either thrust or normal configurations), (3) nearly concordant KAr cooling dates for biotite and hornblende suggesting rapid cooling rates, and (4) “post-tectonic” intrusions of leucogranite or leucotonalite that may represent the products of decompression melting. The crystalline slabs of the high Himalaya, the compressional surges of the Coast orogen of British Columbia and Alaska, the Cordillera of southern British Columbia, and the southern belt of the Hercynian chain are examples where melt assisted rapid pulses of uplift appear to have occurred. During the terminal phases of uplift, and following uplift, these orogenic belts were exhumed during extension while melt was still present.