Tectonics, crustal structure and evolution of the Galapagos triple junction

Abstract

Deep seismic-reflection data were used to study the area of the triple junction of the Pacific, Cocos, and Nazca lithospheric plates. Clearly identifiable are: (1) oceanic lithosphere created by the spreading of the East Pacific Rise, with up to five seismic horizons identified in it, of which the 3rd horizon corresponds to the Mohorovičič discontinuity and the 4th and 5th to the uppermost mantle layers; and (2) lithosphere formed in the Cocos-Nazca spreading center (Galapagos rift), into which the above seismic horizons cannot be traced, whilst deeper-lying upper-mantle layers presumably approach closer to the surface. On the western continuation of the Galapagos rift an anomalous crustal zone has been identified, 100 km long and 20–40 km wide, stretching across the East Pacific Rise crest. The 3rd seismic horizon, i.e., the Mohorovičič discontinuity, is observed here to lie 2–3 km higher than in the adjacent areas. This zone of uplifted mantle is not accompanied by gravity anomalies and this points to a low-density mantle. The formation of the anomalous zone is interpreted as the result of the fan-like accretion of a triangular crustal area in the regions of triple junction of the ridge-ridge-ridge type.