GNEISS DOMES AND OROGENY

Many high-grade metamorphic terranes, including gneiss domes, record clockwise P-T-t paths, with segments of near-isothermal decompression indicating that rocks ascended from >25-30 km to shallow crustal levels (10 km) without significant cooling. These P-T paths cross dehydration melting solidi, producing melt once decompression has initiated. The correspondence of the near-isothermal decompression segment of the path with mica dehydration melting suggests a dynamic relationship between decompression and partial melting. In exhumed orogens, the signature of the rapid ascent of partially molten crust is a gneiss dome cored by migmatite and/or granitoids. Gneiss domes represent partially molten diapirs emplaced sufficiently high in the crust that they freeze and retain their domal shape.

Decompression, partial melting, and generation of gneiss domes may be driven by erosion if the erosion is extremely rapid and localized, such as in orogenic syntaxes (Nanga Parbat, Namche Barwa) or narrow, wrench-dominated orogens (Red River shear zone). On a larger scale (Karakorum, Zanskar, northern N. America Cordillera), decompression may be initiated by thinning of thickened crust and/or by solid-state diapirism, and is sustained at high temperature by the relationship between decompression and partial melting. The most efficient mechanism for deep orogenic crust to ascend without cooling is by buoyant rise of partially molten crust, with buoyancy-driven flow coupled with crustal thinning, especially if the upper crust is removed by extension and/or erosion. It is not clear whether the rising migmatite diapir localizes upper crustal extension or whether the removal of upper crust drives the buoyant rise of partially molten crust. It is likely that these processes are coupled because they involve positive feedback between partial melting and decompression.

The large volume of material involved in the vertical transfer of partially molten crust indicates that the formation of gneiss domes is an efficient mechanism for heat advection and intracrustal differentiation. Given the ubiquity of gneiss domes in orogens of various ages and tectonic settings, the diapiric rise of deep orogenic crust is a fundamental process in the thermal, mechanical, and chemical evolution of continents.