MAGMATIC UNDERPLATING IN CONTINENTAL LITHOSPHERE - NOT ONLY IN EXTENSIONAL SETTINGS

Underplating was originally proposed as the process of magma ponding at the base of the crust and was inferred from petrologic consideration, often thought to be associated with extensional regions. The process may have main importance for the formation of continental crust with its characteristic low average density, and may have been mainly active during the early evolution of the Earth. However, despite the assumed importance of underplating processes and associated fractionation, the available geophysical images of underplated material remain relatively sparse and confined to specific tectonic environments. Direct ponding of magma at the Moho is only observed in very few locations, probably because the magma interacts with the surrounding crustal rocks. There is no direct discriminator between traditionally underplated material and lower crust magmatic intrusions in the form of batholiths and sill-like features, and in the current review of new results on underplating we include both these phenomena. Underplating in this sense is observed in a variety of tectonic settings, including island arcs, wide extensional continental areas, rift zones, and continental margins. Recent new data show that magmatic processes around Moho level may take the form of sill intrusions in the lower crust (at continental rift zones and slopes) as well as in the vicinity of major crustal intrusions, and that underplating may even take the form of massive magmatic intrusion into the crust in giant magma chambers. We review the structural styles observed by geophysical imaging and discuss these first order observations in relation to the Moho.