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Ingeology, aterrane (/təˈreɪn,ˈtɛreɪn/;[1][2] in full, atectonostratigraphic terrane) is acrust fragment formed on atectonic plate (or broken off from it) andaccreted or "sutured" to crust lying on another plate. The crustal block or fragment preserves its distinctive geologic history, which is different from the surrounding areas—hence the term "exotic" terrane. The suture zone between a terrane and the crust it attaches to is usually identifiable as afault. Asedimentary deposit that buries the contact of the terrane with adjacent rock is called anoverlap formation. Anigneous intrusion that has intruded and obscured the contact of a terrane with adjacent rock is called astitching pluton.
There is also an older usage of the termterrane, which described a series of relatedrock formations or an area with a preponderance of a particular rock or rock group.
A tectonostratigraphic terrane did not necessarily originate as an independentmicroplate, since it may not contain the full thickness of thelithosphere. It is a piece ofcrust that has been transported laterally, usually as part of a larger plate, and is relatively buoyant due to thickness or low density. When the plate of which it was a partsubducted under another plate, the terrane failed to subduct, detached from its transporting plate, and accreted onto the overriding plate. Therefore, the terrane transferred from one plate to the other. Typically, accreting terranes are portions ofcontinental crust which haverifted off another continental mass and been transported surrounded by oceanic crust, or they are oldisland arcs formed at some distant subduction zones.
A tectonostratigraphic terrane is a fault-bounded package of rocks of at least regional extent characterized by a geologic history that differs from that of neighboring terranes. The essential characteristic of these terranes is that the present spatial relations are incompatible with the inferred geologic histories. Where terranes that lie next to each other possess strata of the same age, they are considered separate terranes only if it can be demonstrated that the geologic evolutions are different and incompatible. There must be an absence of intermediatelithofacies that could link the strata.
The concept oftectonostratigraphic terrane developed from studies in the 1970s of the complicatedPacific Cordilleranorogenic margin ofNorth America, a complex and diverse geological potpourri that was difficult to explain until the new science of plate tectonics illuminated the ability of crustal fragments to "drift" thousands of miles from their origin and attach themselves, crumpled, to an exotic shore. Such terranes were dubbed "accreted terranes" bygeologists. Geologist J. N. Carney writes:
It was soon determined that these exotic crustal slices had in fact originated as "suspect terranes" in regions at some considerable remove, frequently thousands of kilometers, from theorogenic belt where they had eventually ended up. It followed that the present orogenic belt was itself an accretionary collage, composed of numerous terranes derived from around the circum-Pacific region and now sutured together along major faults. These concepts were soon applied to other, older orogenic belts, e.g. theAppalachian belt of North America.... Support for the new hypothesis came not only from structural and lithological studies, but also from studies of faunalbiodiversity andpalaeomagnetism.[3]
When terranes are composed of repeated accretionary events, and hence are composed of subunits with distinct history and structure, they may be calledsuperterranes.[4]
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