Abstract
Summary Continental lower crust comes in a limited number of types that are best distinguished tectonically. Lower crust is formed primarily in oceanic or continental zones of lithospheric plate convergence and secondarily in regions of intracontinental rifting. Formation processes involve magmatic intrusion of mantle-derived basalt and structural burial of pre-existing upper crust by thrust or reverse faults. In both cases, metamorphism, crustal melting and ductile deformation result from the accompanying heating and release of volatiles. A compositional conundrum exists for both arcs and continental rifts since the new continental crust has a basaltic rather than an andesitic (mean crustal) composition. Possible solutions to this compositional disparity include formation of more silicic lower crust in the Archaean, and lower crustal delamination which could preferentially remove tectonically immature basaltic crust.
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