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.

The geological development of Panama’s isthmus resulted from intermittent magmatism and oceanic plate interactions over approximately the past 100 m.y. Geochemical data from ~300 volcanic and intrusive rocks sampled along the Cordillera de Panama document this evolution and are used to place it in a tectonic framework. Three distinct trace-element signatures are recognized in the oldest basement rocks: (1) oceanic basement of the Caribbean large igneous province (CLIP basement) displays flat trace-element patterns, (2) CLIP terranes show enriched ocean-island basalt (OIB) signatures, and (3) CLIP rocks exhibit arc signatures. The Chagres igneous complex represents the oldest evidence of arc magmatism in Panama. These rocks are tholeiitic, and they have enriched but highly variable fluid-mobile element (Cs, Ba, Rb, K, Sr) abundances. Ratios of these large ion lithophile elements LILEs) to immobile trace elements (e.g., Nb, Ta, middle and heavy rare earth elements) have a typical, but variably depleted, arc-type character that was produced by subduction below the CLIP oceanic plateau. These early arc rocks likely comprise much of the upper crust of the Cordillera de Panama and indicate that by 66 Ma, the mantle wedge beneath Panama was chemically distinct (i.e., more depleted) and highly variable in composition compared to the Galapagos mantle material, from which earlier CLIP magmas were derived. Younger Miocene andesites were erupted across the Cordillera de Panama from 20 to 5 Ma, and these display relatively uniform trace-element patterns. High field strength elements (HFSEs) increase from tholeiitic to medium-K arc compositions. The change in mantle sources from CLIP basement to arc magmas indicates that enriched sub-CLIP (i.e., plume) mantle material was no longer present in the mantle wedge by the time that subduction magmatism commenced in the area. Instead, a large spectrum of mantle compositions was present at the onset of arc magmatism, onto which the arc fluid signature was imprinted. Arc maturation led to a more homogeneous mantle wedge, which became progressively less depleted due to mixing or entrainment of less-depleted backarc mantle through time. Normal arc magmatism in the Cordillera de Panama terminated around 5 Ma due to the collision of a series of aseismic ridges with the developing and emergent Panama landmass. Younger heavy rare earth element–depleted magmas (younger than 2 Ma), which still carry a strong arc geochemical signature, were probably produced by ocean-ridge melting after their collision.

We present on- and offshore structural data from the Nazca-Panama plate boundary zone in the Gulf of Chiriquí and surrounding onshore areas of southwest Panama. Major offshore structures interpreted on multichannel seismic profiles from the Gulf of Chiriquí include Cébaco basin complex, a series of northeast-striking, Plio-Pleistocene half-grabens, and Montuosa basin, an asymmetric Plio-Pleistocene sag basin associated with a major strike-slip fault. We interpret Cébaco basin complex as a pull-apart basin between two major, left-lateral strike-slip faults that accommodate oblique motion between the Nazca plate and the mainland of southwestern Panama. Interpretation of regional seismic stratigraphic data indicates that the Plio-Pleistocene extensional phase that produced the Cébaco basin complex extended the area by about 7%. We studied outcrop-scale, conjugate strike-slip fault systems exposed on landmasses surrounding the Gulf of Chiriquí in order to place kinematic and age constraints on large-scale faults mapped on seismic profiles. Fault systems deforming Eocene to Lower Miocene sedimentary rocks on Coiba Island and the Azuero and Soná Peninsulas suggest an approximately northwest-southeast orientation of maximum extensional strain in an area that encompasses the offshore Cébaco basin complex. We propose three possible models to explain the observed pattern of strike-slip deformation observed in the Gulf of Chiriquí: (1) Neogene oblique subduction of the Nazca plate beneath Panama produces left-lateral strike-slip faulting and related northwest-oriented extension within the forearc (Gulf of Chiriquí) (2) Plio-Pleistocene shallow subduction/collision between the Cocos ridge and Costa Rica produces southwestward motion or “escape” of a Gulf of Chiriquí block that is detached from the rest of Panama by left-lateral strike-slip faults, and (3) Neogene bending of the Panama island arc following collision with the South American continent is accommodated in part by strike-slip motion and underthrusting along the southwest margin of Panama. Observed deformation may be a composite effect of more than one of these tectonic mechanisms.

The western part of the North Panama deformed belt (NPDB) undergoes a number of changes in the orientation of the frontal thrusts, in marked contrast to the eastern part of the NPDB, which shows very little change over distances of 150 km. In the western part of the belt, vergence and structural style of the frontal thrusts vary rapidly along the belt, again in contrast to the eastern part of the belt. The northeast-trending part of the western NPDB appears to be constrained in orientation by the slope of the Panama arc, which lies parallel to the thrust front. Farther west the belt trends northwest, and the thrust front migrates across the slope to the shelf area, where it has been identified by studies of the April 22, 1991, earthquake. Seismic data offshore from the surface location of this earthquake show no signs of crustal deformation, further constraining the location of the thrust front. The northwest orientation of the thrust belt on- and offshore of Costa Rica is consistent with a kinematic mechanism related to collision of Costa Rica by the Cocos Ridge. A proposal to explain the fact that the thrust front is located on the shelf, rather than at the base of the slope in its westernmost position, is impedance of thrusting as a result of the heavy load of sediment deposited by the Costa Rica fan. Thus several kinematic mechanisms are operating around the NPDB, producing very different directions of thrust belt orientation. In addition, several more mechanisms, such as slope stress, sediment loading, and possibly the structure of the lower plate, are operating to modify the orientation of the frontal thrusts.

Shallow subduction of the Cocos Ridge beneath the Costa Rican island arc results in six major tectonic effects. These effects include a volcanic gap in the Costa Rican volcanic arc chain, a shallowing of the dip of the subducted Cocos plate beneath Costa Rica, forearc indentation of the Pacific margin of Costa Rica, structural inversion of forearc (Terraba) and backarc (Limon) basins, arching of on- and offshore acoustic basement in a direction parallel to plate convergence between Costa Rica and the Cocos plate, and a radial stress pattern around the underthrust area of the Cocos Ridge as inferred from earthquake and geologic indicators. Structures formed in forearc basin sedimentary and volcanic rocks of the Térraba belt above the subducted Cocos Ridge include major reverse faults that consistently place older lithologic units over younger lithologic units. One of these faults, the Ballena-Celmira fault zone, forms a prominent linear contact between Quaternary alluvium of the Pacific coastal plain and the Térraba belt. Bedding plane and fault data in the Térraba belt constrain a maximum shortening direction of N30-34°E for the central and eastern Térraba belt. This direction of maximum shortening corresponds closely to the N35°E direction of maximum shortening of Corrigan et al. (1990) from Plio-Pleistocene rocks of the outer forearc in the Burica/Osa area to the south and southeast of the Térraba belt. Assuming that the predicted plate convergence direction (N32°E) and the direction of maximum shortening in the forearc subparallel, thrusting and tilting in the forearc of westernmost Panama and eastern and central Costa Rica is interpreted as the result of regional northeast-southwest-oriented maximum compressive stresses exerted by post-Miocene shallow subduction of the Cocos Ridge.