A "cordilleran-type" orogen has previously been defined as contractional deformation of a continental margin due to subcontinental subduction. The plate tectonic setting of cordilleran-type orogenesis is fundamentally different from that of "collisional" orogenesis. Intracontinental orogens such as the Himalayas and Alps form by collision between a continental margin and a continental margin subduction zone. Orogens at continental margins may form due to subcontinental subduction (cordilleran-type orogeny), or due to collision of a continental margin with an intra-oceanic subduction zone which is typically followed by a reversal of subduction polarity. In the latter case, collisional orogenesis may proceed along with the reversed, subcontinental subduction. Modern arcs are mostly extensional or neutral, so subduction hangingwall contraction is not a necessary consequence of subduction. Crustal contraction in known collisional orogens lasts for tens of millions of years, so collisional deformation at a continental margin may last for tens of millions of years after arc-continent collision and subduction reversal. Collisional orogenesis is a predictable consequence of subduction, and collisional deformation is kinematically linked to the kinematics of subduction. Noncollisional cordilleran-type orogenesis is not predicted by plate theory. A geologic history that would demonstrate noncollisional orogenesis is a magmatic arc built into a passive margin sequence with strong deformation but without an earlier arc or ophiolite accretion event; this history has not been recognized in the geologic record. The classic examples of cordilleran-type orogens, including the Cretaceous-Cenozoic of the Andes and the Jurassic-early Tertiary of the western United States, may be interpreted as arc-continent collisions. However, the youngest increments of deformation in these two orogens occurred 100 million years after the youngest collisions and are thus difficult to ascribe to collisional processes. Notwithstanding the long time scales, these deformations occurred at the end of space-time continua of probable collisional orogenies. Alternatively, these deformations may represent lithosphere-scale fault-bend folding, duplex folding, backthrusting, and/or out-of-sequence forethrusting during shallow subduction.
