GARIMAG - Magnatism at the fossil spreading axis of the Galapagos Rise, SE Pacific, Cruise Report SO-160, Guayaquil - Antofagasta, 18.9.2001 - 8.10.2001 (English)

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The cruise SO 160 mapped and sampled a region of the seafloor of the Galapagos Rise, a fossil spreading centre in the SE Pacific Ocean. The objective of the cruise was to determine the exact location of the fossil axis as well as its structure, age, and composition. Previous work in the 1970ies had suggested the fossil axis further in the east of the structure that we studied. However, satellite altimetric data clearly indicated the exact position of the fossil spreading centre. This feature was mapped in detail and we also conducted gravimetry and magnetic measurements during the mapping. The mapping revealed an e10ngated structure which is bounded by a large fracture zone in the north and which can be divided into three segments, each of about 50 km length and a NNE strike. The northernmost segment consists of a deep rift which shows clear similarities to a slow-spreading centre, e.g. the Mid-Atlantic Ridge. The southern two segments are volcanic ridges with numerous volcanic flank cones which reach water depths up to 490 m. We interpret this volcanic ridge as a continuation of the fossil spreading axis, which is magmatically starved in the north but apparently erupted large volumes of magmas after cessation of spreading in the south. The rock samples we recovered are mainly plagioc1ase-phyric lavas with abundant glass rims and manganese crusts on the rift flanks in the north. On the two volcanic ridge segments in the south we recovered plagioclase- and olivine-phyric lavas which are highly vesicular in contrast to the lavas from the northern rift zone. The apparent difference in volatile content between the northern rift and the southern two ridge segments indicated a significant compositional variation. The southern lavas were classified as alkaline basalts to trachytic lavas while the northern samples resembled typical tholeiitic Mid-ocean Ridge Basalts. The trachytic samples were recovered at the peak of the volcanic ridge where round domes with heights of several hundred metres occurred.