Comparative planetology and the origin of continental crust

Abstract

Abstract This paper summarizes crustal evolution in silicate planets (the Moon, Mercury, Mars, Venus and meteorite parent bodies), and outlines a general theory for the origin of continental crust based on comparative planetology and terrestrial evidence. Silicate planets and some meteorite parent bodies have undergone early global differentiation (nonplate-tectonic), followed by a late basin-forming bombardment and a long basaltic second differentiation. Evidence for analogous stages (except a late bombardment) can be found in terrestrial geology, modified by conditions of hydrous magma generation and persistent tectonism. Continental crust is inferred to have largely been formed in the early Archean by andesitic volcanism accompanying catastrophic early degassing. This global crust was disrupted by basin-forming impacts that initiated mantle upwelling and sea-floor spreading. Basaltic overplating and underplating, from mantle plumes, continued on the continents producing greenstone belts, mafic dike swarms, and, by partial melting of underplated ‘basalt’ and sialic crust the granitoids of Archean granite-greenstone terrane. This process produced most of the present continental crust by ∼2.5 Ga ago, when the global tectonic style evolved into the present one of tectonism marginal to and within continents. Subsequent crustal evolution has largely been deformation, metamorphism, anatexis and rearrangement of previously formed crustal segments, not separation of new sialic crust from the mantle. The crustal growth that has occurred since the Archean has primarily been crustal thickening resulting directly or indirectly from generation of basalt in the subcontinental tectosphere. Viewed in the light of evidence from comparative planetology, the continents are interpreted as the greatly altered remnants of an originally global primordial crust, the result of an evolutionary sequence shared by all the silicate planets to some degree depending on their mass, volatile content and location in the solar system. This theory can be tested by deep drilling in Archean terranes to locate remnants of the original crust, by determination of the composition of the highland crust of Mars, and by investigations of the crust of Venus.