The Galapagos Spreading Centre: A Near-Bottom Geophysical Survey

Abstract

A near-bottom geophysical survey of the Galapagos spreading centre at 86"W shows the highly lineated nature of oceanic crust generated at a fastspreading ridge. The bathymetric relief is dominated by small blocks which tilt slightly away from the spreading centre and which are bounded by inward-facing scarps. This relief is similar to that found on slow-spreading ridges, but on a smaller scale, and when covered with a thin layer of sediments, it looks like typical abyssal hill terrain. The magnetization distribution, sediment pattern and the fine-scale bathymetric structure are used to locate the centre of spreading and the region in which the upper portion of the crust is being formed. Recent volcanic activity is primarily occurring within 2 km of the centre, a region which is nearly sediment-free and which contains a highly magnetized ridge.Possible models of ridge crest mechanics are constrained by these observations over a ridge spreading with a half-rate of 35 mm/yr. There appears to be a small median valley which has a width of 16 km and a depth of 150 m. Most of the near-surface volcanism appears to cease when the rigid blocks are initially faulted apart from the central crustal generation zone at about 2 to 4 km from the centre. The central block is fairly level with a relief which appears to have been formed by constructional volcanism while the rigid blocks on either side of this central one already are tilted slightly outwards and are bounded by inward facing scarps.

Geochemistry and Origin of Eocene-Oligocene Metalliferous Sediments from the Central Equatorial Pacific: Deep Sea Drilling Project Sites 573 and 574

“…Lenses of nontronite are interbedded with pelagic sediments beneath hydrothermal mounds and ridges 25 to 50 m across, which form positive topographic features on the seafloor (Klitgord and Mudie, 1974;Lonsdale, 1977). It has been suggested (Corliss et al, 1978;Hekinian et al, 1978) that nontronite forms from reducing, metal-rich hydrothermal fluids leached from basalt, which replace pelagic sediments during early diagenesis (Honnorez, Von Herzen et al, 1981Moorby, 1983).…”

Contribution of volcanism and tectonism to axial and flank morphology of the southern East Pacific Rise, 17°10′–17°40′S, from a study of layer 2A geometry

“…The volcanic draping of outward facing slopes on the EPR reported by Macdonald et al [ 1996] will produce slopes with a volcanic surficial morphology, although the underlying architecture is fault-controlled. In comparison, on the intermediate spreading ridges abyssal hills appear to be backtilted blocks bounded by faults on their axis-facing sides with gently dipping, volcanic, outward facing slopes [Klitgord and Mudie, 1974;Kappel and Ryan, 1986]. Kappel and Ryan [1986] propose that crustal accretion on the Juan de Fuca Ridge is episodic with periods of magmatic activity building crestal ridges which are split and rafted off axis, creating abyssal hills alternating with predominantly tectonic phases resulting in abyssal valleys.…”

Site 508

“…The existence of low heat-flow values in this area had been determined from a detailed survey south of the spreading center by Green et al (1981). In addition, deep-tow surveys (Klitgord and Mudie, 1974;Lonsdale, 1977) show the site to be free of any mounds, such as are found to the north. Morphologically, the site is near the middle of a gently northward tilted block several kilometers wide and extending in an east-west direction.…”