Gneiss domes and crustal flow

Gneiss domes are ubiquitous structures in all exhumed orogens, and their formation represents a first order thermal-tectonic process that has operated from the Archean to the present. The vertical flow of crust to create domal structures is a significant factor in the redistribution of heat and material in orogens and therefore in the evolution of continents. Worldwide, gneiss domes display many similarities in geometry (aspect ratio), petrology, and structure, and these similarities transcend differences in tectonic setting. Gneiss domes are cored by high-grade metamorphic rocks (including migmatite) ± granitoids, and the core rocks commonly record a component of isothermal decompression, in contrast to mantling schists, and may exhibit a late, low-pressure-high-temperature metamorphic assemblage. Rapid cooling typically follows isothermal decompression, as hot rocks are rapidly emplaced at higher structural levels. Most gneiss domes are elongate parallel to the strike of the orogen. Domes with long dimension ≤90 km have a ratio of long to short axes of ∼2:1-3:1. The elliptical shape of gneiss domes worldwide suggests that their morphology, and therefore genesis, is controlled by crustal flow dynamics, including the magnitude of vertical versus lateral crustal flow. The conditions and mechanisms involved in dome formation inform the relative rates of vertical and lateral crustal flow during orogeny.