Ophiolites

Ophiolites are sections of oceanic crust and upper mantle that have been emplaced onto continental crust, typically through obduction, a tectonic process in which slices of ocean lithosphere are thrust onto land rather than subducted back into the mantle. They provide direct exposure to systems that formed at oceanic spreading ridges and in subduction zones, allowing study of mantle rocks, crustal rocks, and hydrothermal alteration that are rarely preserved in modern oceans.

The classic ophiolite sequence, though variable, is commonly described from base to top as: mantle rocks such as peridotite or dunite and harzburgite representing the upper mantle; layered gabbro representing lower crust; a sheeted dike complex recording intrusion of dikes into the crust; and overlying pillow lavas and related basaltic rocks representing the upper oceanic crust, often capped by pelagic sediments such as radiolarian cherts. Serpentinized mantle rocks and, in some occurrences, chromitite deposits can occur near the base. Not all ophiolites preserve every element of this sequence, but the overall assemblage is diagnostic of oceanic lithosphere.

Formation and significance: Ophiolites are remnants of ancient oceanic lithosphere that have been tectonically transported onto continental margins, typically in association with convergent plate boundaries and collisional orogenies. The study of ophiolites has been crucial for understanding the creation and evolution of oceanic crust, mantle composition, and the interactions between tectonics, magmatism, and hydrothermal systems. Prominent and well-studied examples include the Semail ophiolite of Oman and numerous belts across the Mediterranean, the Alps, and other orogenic regions.