Terranes

Terranes are crustal blocks with distinct geological histories that have become part of a larger tectonic plate through accretion during the growth of continents. They are typically bounded by faults or sutures that mark the boundaries with neighboring crustal blocks. Terranes can comprise continental crust, island arcs, oceanic crust fragments, or combinations thereof, and they may exhibit internal stratigraphy, metamorphic histories, and paleogeographic origins that differ from surrounding rocks.

Recognition relies on differences in lithology, fossil assemblages, isotopic signatures (such as strontium, neodymium, and lead), and paleomagnetic data, as well as structural and metamorphic contrasts. When terranes weld to continents, their boundaries may be reconfigured by later deformation, but sutures preserve evidence of former ocean basins and collisions. Terranes form and grow when plate tectonics detaches pieces of crust, which are transported on subducting slabs or along transform boundaries and then accrete to a continental margin.

Formation processes often involve island arcs, fragments of another continent, or oceanic plate pieces that collide with a larger plate. Accretion typically occurs during mountain-building (orogenic) cycles, producing a mosaic of crustal blocks with complex tectonostratigraphic histories that record long belts of plate interaction.

Examples and significance: In western North America, recognized terranes include the Alexander, Wrangellia, Yukon-Tanana, Stikinia, and Cache Creek terranes, among others, reflecting multiple episodes of accretion along the Cordilleran margin. Similar terrane concepts apply to other convergent margins worldwide. Studying terranes helps reconstruct past continental configurations, track crustal growth, and understand the kinematic history of plate tectonics, as well as guide exploration for mineral resources concentrated at sutures and accreted blocks.