Site 1161 is located in western Zone A, ~78 km east of the ~127°E fracture zone and 43 km south of Site 1158. The seafloor magnetic age is ~19 Ma. The site was located during the Leg 187 survey transit between Sites 1158 and 1159. The site is in a small, sediment-filled basin in rugged terrain that we interpret as the transferred lithosphere zone of an extinct westward-directed propagating rift system, identified from aeromagnetic data by Vogt et al. (1984). Site 1162 is ~45 km farther south, in a valley formed by this propagating rift. This site is part of the north-south transect in western Zone A; it was intended to check for Indian-type mantle between the previously identified Pacific-type Sites 1158 and 1159. Together with Site 1162, Site 1161 could, if it is of Indian type, test whether rift propagation reintroduced Pacific-type mantle beneath the region.
Hole 1161A was spudded in ~5020 m water depth and was washed through ~116 m of sediment, recovering a single wash barrel containing a slurry of drilling-induced clay pellets with pieces of basalt in the bottom of the core catcher. Rotary drilling continued 29.3 m into volcanic basement, recovering 4.4 m (~15%) of basaltic rubble with fragments of aphyric to plagioclase-olivine phyric basaltic breccia. Between the large clasts is a finer grained breccia with clasts of altered basalt and palagonitized glass in a clay-rich sediment matrix. Hole 1161A was abandoned because of drilling problems.
Hole 1161B was spudded 200 m north of Hole 1161B and was washed through ~158 m of sediment with no recovery. Rotary drilling continued only 8.5 m into volcanic basement, recovering 0.9 m (~10%) of aphyric to highly plagioclase-olivine phyric basalt and basaltic breccia similar to that recovered from Hole 1161A. Basalts from both holes are moderately to strongly altered with Fe oxyhydroxide and clay replacing olivine phenocrysts and basaltic groundmass.
A basalt glass from Hole 1161B was analyzed for major and trace elements by shipboard inductively coupled plasma-atomic emission spectrometry (ICP-AES), and a whole rock from Hole 1161A was analyzed by X-ray fluorescence (XRF). The Ba and Zr contents of the glass clearly indicate that it is derived from Indian-type mantle. This occurrence, separated by <1 Ma from the Pacific-type mantle beneath Site 1158, documents a second incursion of Indian mantle into Zone A (Site 1157 documents the first).
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