Abstract

Research : Solid earth sciences

The Atlantis Bank Gabbro Massif, Southwest Indian Ridge

Henry J.B. Dick, Astri J.S. Kvassnes, Paul T. Robinson, Christopher J. MacLeod, H Kinoshita

This paper presents the first detailed geologic map of in situ lower ocean crust; the product of six surveys of Atlantis Bank on the SW Indian Ridge. This combined with major and trace element compositions of primary magmatic phases in 99 seafloor gabbros shows there are both significant vertical and ridge-parallel variations in crustal composition and thickness, but a continuity of the basic stratigraphy parallel to spreading. This stratigraphy is not that of magmatic sedimentation in a large crustal magma chamber. Instead, it is the product of dynamic accretion where the lower crust formed by episodic intrusion, large-scale upward migration of interstitial melt due to crystal mush compaction, and continuous tectonic extension accompanied by hyper- and sub-solidus, crystal-plastic deformation.

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East-west cross-section of Atlantis Bank through Hole 735B based on the seafloor geology and ODP and IODP drill cores. Subsurface geology at depth including dunite melt transport zone and basal cumulates, and the location of crust-mantle boundary is somewhat arbitrary as the seismic data are not definitive, and we only know that a considerable volume of primitive cumulates must lie below the hole. Heavy black lines show the Moho reflectors (Minshull et al. 1998; Muller et al. 2000; Muller et al. 1997). The gaps are attributed to the geometry of ocean bottom seismometer emplacement and ray tracing. Best guess model for Moho and crust-mantle boundary are shown—but only drilling will tell. Lithologic units in the gabbro massif are identified by color in the inset dive profiles.

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