A tectonostratigraphic terrane did not necessarily originate as an independent
microplate, since it may not contain the full thickness of the
lithosphere. It is a piece of
crust 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 part
subducted 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 of
continental crust which have
rifted off another continental mass and been transported surrounded by oceanic crust, or they are old
island 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 intermediate
lithofacies that could link the strata. The concept of
tectonostratigraphic terrane developed from studies in the 1970s of the complicated
Pacific Cordilleran
orogenic margin of
North 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" by
geologists. Geologist J. N. Carney writes: When terranes are composed of repeated accretionary events, and hence are composed of subunits with distinct history and structure, they may be called
superterranes. ==List of tectonostratigraphic terranes==