Along Pacific convergent margins of the Americas, high-standing relief on the subducting oceanic plate “collides” with continental slopes and subducts. Features common to many collisions are uplift of the continental margin, accelerated seafloor erosion, accelerated basal subduction erosion, a flat slab, and a lack of active volcanism. Each collision along America’s margins has exceptions to a single explanation. Subduction of an ~600 km segment of the Yakutat terrane is associated with >5000-m-high coastal mountains. The terrane may currently be adding its unsubducted mass to the continent by a seaward jump of the deformation front and could be a model for docking of terranes in the past. Cocos Ridge subduction is associated with >3000-m-high mountains, but its shallow subduction zone is not followed by a flat slab. The entry point of the Nazca and Juan Fernandez Ridges into the subduction zone has migrated southward along the South American margin and the adjacent coast without unusually high mountains. The Nazca Ridge and Juan Fernandez Ridges are not actively spreading but the Chile Rise collision is a triple junction. These collisions form barriers to trench sediment transport and separate accreting from eroding segments of the frontal prism. They also occur at the separation of a flat slab from a steeply dipping one. At a smaller scale, the subduction of seamounts and lesser ridges causes temporary surface uplift as long as they remain attached to the subducting plate. Off Costa Rica, these features remain attached beneath the continental shelf. They illustrate, at a small scale, the processes of collision.

Suprasubduction-zone ophiolites have been recognized in the geologic record for over thirty years. These ophiolites are essentially intact structurally and stratigraphically, show evidence for synmagmatic extension, and contain lavas with geochemical characteristics of arc-volcanic rocks. They are now inferred to have formed by hinge retreat in the forearc of nascent or reconfigured island arcs. Emplacement of these forearc assemblages onto the leading edge of partially subducted continental margins is a normal part of their evolution. A recent paper has challenged this interpretation. The authors assert that the “ophiolite conundrum” (seafloor spreading shown by dike complexes versus arc geochemistry) can be resolved by a model called “historical contingency,” which holds that most ophiolites form at mid-ocean ridges that tap upper-mantle sources previously modified by subduction. They support this model with examples of modern mid-ocean ridges where suprasubduction zone–like compositions have been detected (e.g., ridge-trench triple junctions). The historical contingency model is flawed for several reasons: (1) the major- and trace-element compositions of magmatic rocks in suprasubduction-zone ophiolites strongly resemble rocks formed in primitive island-arc settings and exhibit distinct differences from rocks formed at mid-ocean-ridge spreading centers; (2) slab-influenced compositions reported from modern ridge-trench triple junctions and subduction reversals are subtle and/or do not compare favorably with either modern subduction zones or suprasubduction-zone ophiolites; (3) crystallization sequences, hydrous minerals, miarolitic cavities, and reaction textures in suprasubduction-zone ophiolites imply crystallization from magmas with high water activities, rather than mid-ocean-ridge systems; (4) models of whole Earth convection, subduction recycling, and ocean-island basalt isotopic compositions ignore the fact that these components represent the residue of slab melting, not the low field strength element–enriched component found in active arc-volcanic suites and suprasubduction-zone ophiolites; and (5) isotopic components indicative of mantle heterogeneities (related to subduction recycling) are observed in modern mid-ocean-ridge basalts (MORB), but, in contrast to the prediction of the historical contingency model, these basalts do not exhibit suprasubduction zone–like geochemistry. The formation of suprasubduction-zone ophiolites in the upper plate of subduction zones favors intact preservation either by obduction onto a passive continental margin, or by accretionary uplift above a subduction zone. Ophiolites characterized by lavas with MORB geochemistry are typically disrupted and found as fragments in accretionary complexes (e.g., Franciscan), in contrast to suprasubduction-zone ophiolites. This must result from the fact that oceanic crust is unlikely to be obducted for mechanical reasons, but it may be preserved where it is scraped off of the subducting slab.