Site 1070 lies on the crest of a north-south basement high 20 km west of the peridotite ridge and 30 km east of the crest of magnetic anomaly J, which is a definitive indicator of oceanic crust. The magnetic anomaly modeling near the site suggests that it is underlain by oceanic crust (Whitmarsh and Miles, 1995), as does recent seismic velocity modeling ~40 km further south (Dean et al., 2000), although a less well-resolved seismic velocity structure over the site, which was published earlier, is less convincingly oceanic (Whitmarsh et al., 1990); the presence of elongate, isochron-parallel basement highs and lows is consistent with the presence of oceanic crust. The spreading rate used in the magnetic anomaly models also implies that the crust formed within ~2 m.y. of the onset of continuous seafloor spreading. Basement cores consist of matrix-supported serpentinized peridotite breccias separated by a tectonic contact from an underlying pegmatitic gabbro. The original igneous mineralogy suggests that the gabbro crystallized from a differentiated mafic magma. The pegmatite is intruded into weakly deformed serpentinized peridotite, as are gabbroic veinlets. The peridotite protolith, which was highly heterogeneous, included lherzolites, harzburgites, and dunites and graded locally into pyroxenite (Shipboard Scientific Party, 1998d).
Remarkably, no rocks from the upper oceanic crust (neither basalts nor sheeted dikes) were encountered at Site 1070. The oldest sediments are upper Lower Cretaceous (upper Aptian) claystones that were deposited near or below the calcium carbonate compensation depth and contain abyssal microfossils, traces of manganese and hematite, and grains of volcaniclastic sediment.
Thus, the cores suggest the site is underlain by serpentinized upper mantle with minor intrusive bodies of gabbro and lacks extrusive material. Such a structure, close to that described at some slow-spreading ridges (Cannat, 1993), implies a lack of melt generation in the upwelling asthenosphere, possibly because of vertical and lateral loss of heat (perhaps accentuated by hydrothermal circulation) at the onset of seafloor spreading. After its intrusion, the gabbro was tectonically exposed at the seafloor. Such tectonism might also explain the lack of lavas at Site 1070. Given the geophysical interpretation that this site lies over or very close to crust with oceanic affinities, we cannot be certain that cores from this single site are representative of the surrounding crust.