The Sedimentary Record of an Intraoceanic Magmatic Arc, from Inception through Maturation to Abandonment: IODP Expedition 351, Site U1438
Friday, 19 December 2014
International Ocean Discovery Program (IODP) Expedition 351 recovered an unprecedented ~1.4-km thick volcaniclastic sedimentary record documenting the initiation and subsequent evolution of the Izu-Bonin-Mariana (IBM) intra-oceanic arc-basin system. The oldest sedimentary rocks in the 50 m above igneous basement may correspond to the time of subduction initiation to the east. They are lithologically complex, consisting of a mix of variably tuffaceous mudstone, sandstone and breccia-conglomerate with some basaltic andesite passing upsection into 50 m of reddish-brown, radiolarian-bearing hemipelagic mudstone with thin volcaniclastic siltstone to sandstone beds. Coarser volcaniclastic rocks (andesitic average whole rock composition) then dominate the overlying section from 1360 to 160 mbsf. Variably graded tuffaceous sandstone to conglomerate and breccia beds range from a few cm to 9 m in thickness and exhibit a wide range of bedding contact relationships and sedimentary structures, including partial to complete Bouma sequences. Gravel to silt-sized volcaniclastic sediment was likely delivered to the site by gravity flows ranging from low- to high-concentration turbidity currents to debris flows. The depositional setting was ~ 50 km from the main arc front (Palau-Kyushu Ridge, PKR), so these may represent very distal apron to fan deposits. The sharp decline in coarse volcanic sediment supply and switch to hemipelagic mud accumulation at ~160mbsf is coincident with the C. 25 Ma transformation of the KPR to a remnant arc by backarc spreading in the adjacent Shikoku Basin and the eastward migration of magmatic arc activity to the IBM.