Metamorphism associated with lithospheric extension

Abstract

The Earth’s oceanic crust has formed by extension from oceanic ridges. In oceans where subduction zones are not yet extensively developed, such as the Atlantic, these ridges are close to the centre of the ocean basin. In oceans such as the Pacific, which are surrounded by active subduction zones, the ridges are not central. Oceanic crust and the upper mantle, together constituting the oceanic lithosphere, are constructed by extension at the ridges. Geophysical studies indicate that oceanic ridge extension creates new crust symmetrically on either side of relatively narrow extensional rift regions, often marked by topographical graben structures. The oceanic crust so formed is mostly composed of basic igneous rocks (basalts when erupted on the ocean floor, dolerites and gabbros when intruded into previously constructed oceanic crust), and the oceanic mantle of ultrabasic rocks.