Alpine Tectonics and the Western US Cordillera

The western margin of the US Cordillera exhibits a complex tectonic assemblage of ophiolitic and other terranes whose histories extend back into the late Precambrian. Application of concepts derived from the Alps assist in gaining insight into the US Pacific margin. In particular, the recognition of nappe structures and sutures, careful structural reconstruction, understanding the tectonic importance of ophiolite emplacement, and the correspondence between thrust-belt polarity and subduction direction, subduction zone collision and polarity flip -- concepts derived from the Alps—all usefully provide insight into Cordilleran tectonic events. Ophiolites are tectonic sheets emplaced along mantle-rooted thrusts. These thrusts are fossil subduction zones, either incipient or mature. Modern thrust belts dominantly are synthetic to subuction polarity. Similarly, models for the tectonic evolution of the Alps, involving collision, subduction polarity flip, development of new subduction zones provide key insight into Cordilleran tectonic development. Models of evolution of mountain systems based upon lithology, petrology, and geochemistry need to be augmented with structural geology and comparison with Alpine evolutionary models. In particular, many ophiolitic, island arc rocks, and subduction complexes are folded thrust sheets, reminiscent of Alpine nappes. Recognition of this fact allows for previously unattainable insight into tectonic evolution. As many as four sutures with resultant overthrust (nappe) sequences may be involved in the Mesozoic tectonic assembly of the US Pacific margin.