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.

Offshore of the Pacific side of Costa Rica, the Caribbean plate converges with the subducting Cocos plate along the Middle America Trench. The tectonics of both plates, from the Cocos Ridge to the Nicoya Peninsula, were studied with swathmapping, magnetic anomalies, and samples. Three morphological domains on the Cocos plate were defined by mapping. The broadly arched Cocos Ridge forms the southeastern domain. Adjacent to the northwest flank of Cocos Ridge is a domain where seamounts and their aprons cover about 40% of the ocean floor. Farther northwest, a sharp juncture in the oceanic crust separates the seamount domain from a deep sea plain. These three contrasting oceanic seafloor morphologies are mimicked in the morphology of the Pacific continental margin of Costa Rica. Opposite the subducting Cocos Ridge are a broad continental shelf and Osa Peninsula, which are attributed to large-scale domal uplift. Where the seamount domain has been subducted, a rugged continental slope has developed, including 55-km-long furrows trending parallel to the Cocos-Caribbean interplate convergence direction. We propose that the furrows represent paths of disruption produced by subducting seamounts. Where the smooth deep sea plain has been subducted, a well-organized accretionary prism covered by slope deposits forms a relatively smooth morphology. The Costa Rican margin illustrates the effects of subducting seafloor morphology on the continental margin structure and morphology.

Abstract After the 1964 Alaska earthquake, seismic records were collected across the eastern Aleutian Trench to study subduction-zone tectonics because geologic and seismologic studies indicated a subduction-related cause. Single-channel seismic-reflection records collected during the 1960s penetrated strata in the trench and about 1,000 m (3,281 ft) of rocks under the slope; only rarely was the top of the igneous oceanic crust detected for more than several kilometers beneath the front of the subduction zone (von Huene, 1972). In the first published multichannel seismicreflection records across the eastern Aleutian Trench (Seely, 1977), the reflection from the oceanic crust can be traced about 40 km (25 mi) landward of the trench, and the deformed stratification above this crust was shown in sufficient detail that a general structural style could be outlined. Structural style was also apparent in a series of records collected subsequently (von Huene, 1979; von Huene et al, 1979), but none of these multichannel records are migrated. The record presented here, from the Aleutian subduction zone off southern Kodiak Island (Figure 1), is migrated. A comparison of the unmigrated and migrated sections shows the improved resolution of structural detail that results from the migration process. This record and those from previous publications show a remarkable diversity of structural style in the subduction zone near Kodiak Island. In the record displayed here, the foot of the landward slope of the trench has a relatively simple structure in comparison with that evident in adjacent records from a seismic network off southern Kodiak Island. This record exemplifies one structural style of the subduction zone off southern Kodiak Island and shows the local formation of thrust packets within an accretionary complex.