In southern Panama, a change from convergent to transform plate motion at the continental margin influenced the history of the associated continental shelf. Normal subduction of the Farallon and Cocos plates near Coiba Island (82° W, 7.5°N) off southern Panama ceased ∼3–5 m.y. ago when left-lateral transform motion was introduced due to the passage of the Nazca-Cocos-Caribbean triple junction. Subduction of the Farallon and Cocos plates during the Tertiary Period was accompanied by the landward development of a volcanic basement with a thick overlying sedimentary sequence. Mid-Tertiary uplift of the basement resulted in an unconformable surface which since subsided, was covered with shelf deposits, and was structurally faulted and folded.
An industry 24-fold multichannel seismic profile which crosses the continental shelf on the Gulf of Chiriqui, near Coiba Island, is presented. The outer continental shelf here is topographically downbowed by a sedimentary basin. The mid-Tertiary acoustic basement is about 1 km beneath the basin and rises to the ocean floor between the basin and the shelf break. It probably represents a mid-Miocene erosional surface of the volcanic basement. Disruption of the acoustic basement and the overlying sedimentary reflectors indicates Quaternary episodes of normal faulting associated with the transform boundary.
The Gulf of Osa, Costa Rica, is situated northwest of the triple junction and is presently undergoing subduction. Comparison of a seismic profile across the Gulf of Osa continental shelf with the Panama seismic profile reveals an acoustic basement with similar characteristics, but it is not cut by the high-angle faults which are present in the Gulf of Chiriqui. These faults may have been introduced in Panama by the conversion from a convergent to a transform plate boundary. Several of these faults may be projected across the continental shelf under the Gulf of Chiriqui for more than 100 km.