Site U1473: Expedition 360

Abstract

International Ocean Discovery (IODP) Expedition 360 constituted Phase I Leg 1 of the SloMo (shorthand for “The nature of the lower crust and Moho at slower spreading ridges”) Project, a multiphase drilling program that aims ultimately to drill through the Mohorovičić seismic discontinuity (Moho) at a slow-spreading mid-ocean ridge. The overall goal of SloMo is to test the hypothesis that the Moho may represent an alteration horizon such as a serpentinization front rather than the igneous crust/mantle boundary. If correct, the igneous crust/mantle boundary could lie at any depth above the seismic boundary; furthermore, because serpentinization is a methanogenic process, it raises the possibility that a significant biosphere exists below the crust at slow-spreading ridges. The site chosen for this endeavor is Atlantis Bank, an oceanic core complex on the Southwest Indian Ridge (SWIR) proven from previous drilling to expose a substantial section of gabbro and local areas of serpentinite, beneath which a Moho reflector is present at ~5.5 km below the seafloor. Phase I of SloMo is to drill 3 km through lower crustal gabbro and to penetrate the inferred crust/mantle boundary; Phase II proposes using the Japanese riser D/V Chikyu to drill to 6 km through the Moho itself and into the mantle. The principal aim of Expedition 360 was to establish a legacy hole at Atlantis Bank suitable for deep penetration (now Hole U1473A) and then drill as deep as possible into the lower crustal gabbro layer. By doing so, Expedition 360 could in its own right address a number of first-order scientific questions about the mechanisms of accretion of the igneous lower ocean crust and exhumation by oceanic detachment faults. By comparing the results of Expedition 360 drilling with those from prior Ocean Drilling Program (ODP) drilling at Atlantis Bank (Holes 735B and 1105A), we had the opportunity to examine for the first time the lateral continuity of igneous, metamorphic, and structural stratigraphy on a kilometer scale, and hence to assess the spatial and temporal scales of magmatic accretion of the plutonic lower crust and the mechanisms by which it is exhumed. A further aim was to determine the nature of the source rock in the lower crust responsible for magnetic anomalies by drilling through a magnetic reversal boundary that dips southward beneath Atlantis Bank. In addition, by employing modern protocols for obtaining samples free from contamination, we aimed to determine the microbiology of the nonvolcanic ocean crust and hence explore the extent to which life exists in the subseafloor.