A
terrane in
geology is a fragment of crustal material formed on, or broken off from, one tectonic plate and accreted---"sutured"---to crust lying on another plate. The crustal block or fragment preserves its own distinctive geologic history, which is different from the surrounding areas (thus the occasional term "exotic" terrane). The suture zone between a terrane and the crust it attaches to is usually represented by a
fault.
In the lithospheric scheme of plate tectonics, a terrane is not an independent microplate, but a piece of crust which lies atop a larger plate which includes other, usually oceanic, crustal material. It can accrete to another piece of crust on another plate when the plate of which it is a part collides with the plate associated with the other crust. Typically, accreting terranes are portions of continental crust which have rifted off from another plate by a tectonic change, or they may represent old island arcs formed at some distant subduction zone. In some instances, this occurs with the interposition of a third plate, usually oceanic, which carries the terrane off some distance to be attached to another plate.
The transfer of a terrane from one plate to another typically occurs as the plate that carries it is subducted under the plate to which the terrane attaches. The terrane detaches from the denser subducting plate while being driven into the crust which lies on the other plate. This process may occur with such force that the terrane is driven on to the other plate, sometimes overriding crustal material already carried by the plate, and sometimes by compressing it. If the receiving plate is oceanic, these processes occur under the sea and involve the thinner oceanic crust. If the receiving plate is continental, these processes will occur on land, producing very complex topography. Typically, the receiving plate is part oceanic and part continental.
It is not yet clear whether or not terranes can actually change plate boundaries themselves. Since terranes are not microplates, they have no "plate of their own". Yet if a plate boundary changes as a result of terrane accretion, this implies that some terranes do in fact include a plate fragment as well, or else that a hypothesized process known as "plating" creates new plate material beneath accreted terranes, or that the plate which delivered the terrane starts a new subduction zone behind the terrane while the old subduction zone is closed off below the place where the terrane sutured on to the receiving plate. In this latter instance which seems to be favored, not only does the terrane suture on, but so does the "stranded" fragment of the plate which delivered it and remains beneath it. Research is ongoing.
