Two contrasting field relationships may reflect different tectonic settings of subduction initiation preserved in orogenic belts. “Hot” subduction initiation assemblages include a large ophiolite unit (up to kms thick, extending tens to hundreds of km along strike) with supra subduction zone (SSZ) geochemical affinity that structurally overlies a thin (<500 m thick) sheet of high-pressure (HP), high-temperature (HT), primarily metamafic rocks called a metamorphic sole. The ophiolite generally lacks burial metamorphism and includes variably serpentinized peridotite at its base. The sole structurally overlies subduction complex rocks made up of oceanic materials (igneous part of oceanic crust and overlying pelagic sedimentary rocks, and clastic sedimentary rocks of trench fill affinity) and/or passive margin assemblages; some of the subduction complex may be metamorphosed under HP-low temperature (LT) conditions (such as blueschist facies). The field relationships suggest initiation of subduction within young (<15 My) and “hot” oceanic lithosphere and that the sole represents the first slice(s) of material transferred from the subducting to upper plate. Examples include the Neotethyan and northern Appalachian ophiolites and units beneath them, and the Coast Range ophiolite and subjacent Franciscan subduction complex of California.

“Cold” subduction initiation assemblages lack SSZ ophiolite and island arc components and a metamorphic sole. Instead, the upper plate above the subduction complex is made up of continental lithosphere that last experienced significant heating during a passive-margin forming rift event. The protoliths of the rocks subducted were >70 My in age at the time of subduction initiation. The HP-LT subduction complex is composed of slices of continental crust and oceanic crust representing parts of a hyperextended continental margin. These field relationships suggest initiation of subduction along a continental margin within old (“cold”) hyperextended continental lithosphere. Examples include the Apennine subduction zone, exposed in Calabria, Italy, and the Alpine orogenic belt, both remnants of the Alpine Tethys.