HOT AND COLD SUBDUCTION INITIATION

We propose that two contrasting field relationships reflect different types and settings of subduction initiation in orogenic belts. "Hot" subduction initiation assemblages include a large ophiolite unit (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-temperature high-pressure (HP), high-temperature (HT) metamorphic 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 (including HP-LT rocks) and/or passive margin assemblages. The sole comprises metamafic rocks with subordinate metachert and metaclastics. Sole protoliths were <10 Ma when subducted. The age of SSZ ophiolite formation and metamorphism in the sole differ by <10 Ma. These relationships suggest initiation of subduction within young (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.

The tectonic stack associated with "cold" subduction initiation lacks an SSZ ophiolite and metamorphic sole. Instead, the upper plate above the subduction complex is made up of continental lithosphere which last experienced significant heating during a passive-margin forming rift event. The structurally highest subduction-related metamorphic rocks underlie and are interleaved with serpentinite, are HP-LT and include thin slices of continental crust as well as metamafic and metapelagic oceanic rocks. The protoliths of the oldest HP-LT rocks were old (tens of Ma and older) at the time of subduction. The HP-LT rocks are part of a subduction complex, commonly in fault contact with subjacent LP continental margin slivers and oceanic crust representing the distal part 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.