Continental Crust, Crustal Evolution, and the Caribbean

Attention is focused on the genesis and tectonic behavior of the crust, especially the continental crust. A distinction is made between the rigid upper mantle, or peridosphere, and the crust which overlies it. Crust and peridosphere together make up the lithosphere. Five major types of crust are recognized. Continental crust is distinguished especially by its great thickness and by the wide-spread distribution of Precambrian gneisses. Oceanic crust, overlying the Moho in the deep ocean basins, is thinner, younger, and more widespread over the Earth's surface. Undeformed crustal masses which contain great thicknesses of basaltic rocks overlying otherwise normal oceanic crust are called platillo crust. The undeformed crust of smaller ocean basins, being thicker than normal oceanic crust, is believed to represent a great thickness of sedimentary layers overlying normal oceanic crust. Such crust has been called transitional crust (Menard, 1967). Where Phanerozoic crustal rocks have accumulated into masses of tectonites of continental thickness, the crust is called tectonitic. A map of crustal types of the Caribbean area is presented as an illustration of classification. Basalt is assumed to be the primary ultimate source of the continental crustal materials. Many processes, especially tectonic ones, have built up the continental crust to a relatively uniform equilibrium thickness. That thickness is determined by the interplay of tectonics, erosion, and isostasy. Lateral additions probably dominate the process of continental growth. Sedimentary, igneous, and tectonic processes at the continental margins are important contributors. Away from the continental margins, some additions to the continental mass are possible by sedimentation on the surface, by limited intrusion near the surface, and perhaps by sill-like intrusions along the Moho. Intrusion of basic magma at the base of the continent is a mechanism which can explain epeirogenic uplift, the Conrad discontinuity, and the slight increase of density with depth in the continents. The disappearance of widespread granulite metamorphic facies terrains at the end of the Precambrian is interpreted to be the result of cooling in the crust. It is suggested that the 550° C isotherm descended to levels below the continental crust near the end of Precambrian time. This would simultaneously permit the formation of serpentinite along the Moho and increase the rigidity of the crust. The possibility of slip, or free sliding, of the continents along the Moho is thus increased in Phanerozoic time. Continental drift is seen to involve three possible processes: passive rafting, overriding, and free sliding. All three may operate simultaneously to produce the net effect known as drift.