[1] The submarine Cocos ridge in the northwestern Panamá basin, a bathymetric feature more than 1000-km long and 250-500 km broad, is about 2 km shallower than the adjacent basin. It is generally interpreted as the trace of the Galápagos hot spot. Two 127-and 260-km long seismic wide-angle sections were recorded along and across this ridge, offshore the Osa peninsula, Costa Rica. Crustal thickening is seen everywhere along the sections. On the northwestern outer ridge flank, increased thickness is exclusively attributed to the upper crust and expressed by 2-km thick flow basalts. The Quepos plateau caps the upper crust in this area. Toward the center of the Cocos ridge, the Moho deepens from 11-12 to 21 km depth and crustal thickening is almost entirely attributed to the lower crust which makes up 80% of the crust and is three times the thickness of normal oceanic lower crust. It is homogeneously structured and the velocities which range from 6.5 km/s at the top to 7.35 km/s at the base are comparable to normal lower crust under these depth conditions and suggest no differences to a gabbroic rock composition. Similarities to the crustal velocity structure of Iceland, central Kerguelen plateau, and Broken ridge are consistent with a formation of this 13-15 Ma old Cocos ridge segment by excessive magmatism in a nearplate boundary setting.

Seismic wide‐angle measurements across the Pacific margin of Nicaragua were carried out using ocean bottom hydrophones and land stations recording marine airgun shots. The structure and the P‐wave velocity of the subducting Cocos and overriding Caribbean Plates were determined by modelling wide‐angle data and further constrained by coincident seismic reflection, borehole and gravity data. The oceanic crust of the Cocos Plate is 5.5 km thick, with a thin pelagic sediment cover. The plate boundary can be traced to 40 km depth and is generally similar to configurations derived earlier from the local seismic network. A major feature of the upper plate is an 80‐km‐wide high‐velocity, high‐density rock unit reaching from the front of the margin to about the middle of the shelf. This wedge‐shaped unit is 15 km thick beneath the shelf edge and is composed of a 5‐km‐thick upper part with velocities increasing from 3.5 km s−1 near the trench to 5.2 km s−1 at the shelf break overlying a 10‐km‐thick lower part with velocities of 6.0 km s−1. Analysis of the gravity field requires densities of 2.6–2.7 g cm−3 for the upper part and 2.9 g cm−3 for the lower part of this unit. These velocities and densities suggest that the margin wedge is composed of ophiolitic rock similar to those sampled offshore Guatemala and exposed on the Nicoya Peninsula, Costa Rica. The velocity structure of this ophiolitic unit ends about 50 km offshore. Landward, the basement underneath the Sandino Basin is formed by older igneous rock drilled beneath upper Cretaceous sedimentary rock onshore Nicaragua. Beneath the ophiolitic basement we find a sliver with velocities typical of mantle material that begins at depths of 12–18 km and coincides with the down‐dip limit of the seismogenic zone. Mantle densities are required for the sliver to match the gravity data. In a tectonic reconstruction, the suture of an oceanic plateau on the Farallon Plate against the Chortis Block in upper Cretaceous time is suggested. Suturing left the former trench and margin in deep water, consistent with the late Cretaceous to Palaeocene deep‐water sediment of the Rivas Formation at the base of the Sandino Basin. Suture of the Farallon Plate and Chortis Block might have initiated the strike‐slip movement along the Motagua–Polochic Fault System. The development of a new subduction zone in Eocene–Oligocene times left the ophiolitic basement and a sliver of oceanic mantle attached to the Chortis Block and shifted the volcanic arc about 70 km southwestwards, close to its present position.

[1] A seismic wide-angle section offshore Costa Rica is presented across the boundary between oceanic crust generated at the East Pacific rise (EPR) and at the Galápagos spreading center (GSC) as indicated by magnetic anomalies. This suture, where the Farallon plate broke up $23 Ma ago, is marked by pronounced velocity variations throughout the crust including a low-velocity body in the lower crust. This body is well constrained by refracted waves above the inversion zone and by strong P m P reflections from its lower boundary. The distinctness of this body and the local gravity field point to an igneous intrusion rather than serpentinized rock. Typical oceanic crust is found adjacent to the suture zone.

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