IODP

doi:10.14379/iodp.pr.351.2015

Abstract

The intraoceanic Izu-Bonin-Mariana (IBM) arc in the western Pacific has been intensively examined over the past few decades, and the outlines of its overall tectonic and magmatic history have been revealed. Arc inception occurred at ~52 Ma, concurrent with a major change in the motion of the Pacific plate. Rifting of the active volcanic axis took place at ~25 Ma, with accompanying seafloor spreading and eastward migration of the active volcanic front forming a volcanically inactive remnant arc (Kyushu-Palau Ridge; KPR). The Amami Sankaku Basin (ASB) flanks the northern KPR; the ASB seafloor has a simple structure comprising ~1.5 km of sediment overlying igneous oceanic crust. International Ocean Discovery Program Expedition 351 targeted the ASB anticipating recovery of the sedimentary record of the earliest stages of arc inception and evolution of the northern IBM arc. Igneous basement samples would permit determination of the petrological, geochemical, and age characteristics of the pre-KPR crust in the region, from which the geochemical composition of the mantle prior to IBM arc inception and growth could be inferred.

The expedition successfully accomplished its primary and most of its secondary objectives at Site U1438 in 4700 m water depth. Drilling penetrated 1461 m of sediment and 150 m of variably altered and veined aphyric to sparsely phyric tholeiitic basalt lava flows, which form the uppermost igneous oceanic basement (lithostratigraphic Unit 1). Four sedimentary units have been established in the sedimentary column above basement, described here from oldest to youngest. Lithostratigraphic Unit IV (99.7 m) consists of early Eocene mudstone, tuffaceous siltstone, breccia-conglomerate, sandstone, and radiolarian-bearing mudstone. The oldest fossil (radiolarian) age is 50–53 Ma (Core 351-U1438E-63R-1; 40 m above basement). Lithostratigraphic Unit III (1046 m) consists of Eocene–Oligocene tuffaceous mudstone, tuffaceous sandstone, sandstone with gravel, and breccia-conglomerate with pebble/cobble-sized volcanic and sedimentary rock clasts. Lithostratigraphic Unit II (139.4 m) is Oligocene tuffaceous mudstone, siltstone, and fine sandstone with localized slumping. Lithostratigraphic Unit I (160.3 m) is latest Oligocene to recent mud and ooze of terrigenous and biogenic origin, with interspersed discrete tephra layers.

In addition to the fossil age constraint, in situ downhole temperature measurements and thermal conductivity measurements on core material from Unit I give a calculated heat flow of 73.7 mW/m2, implying a thermal age for the underlying lithosphere of 40–60 Ma. The recovery at Site U1438 of an extensive sediment sequence of early Eocene age in Unit IV, coeval with the putative initiation of the IBM arc at ~52 Ma determined by radiometric dating of fore-arc igneous samples, will allow comprehensive analysis of the provenance, geochemical and petrological characteristics, and style of earliest arc magmatic activity in the KPR. The geochemical and petrological equivalence of the Unit 1 lava flows with the “fore-arc basalts” of the IBM fore arc has critical implications regarding the style of magmatism accompanying arc initiation and the lateral (across-strike) extent of this type of igneous activity. The apparent absence of boninite at Site U1438, however, may indicate petrologic provinciality was established within the first few million years of IBM magmatism.

The voluminous volcaniclastic rocks recovered from the Eocene through Oligocene sequence of Units II, III, and IV contain sufficiently fresh glass (at least in the shallower section) and igneous minerals and clasts to allow comprehensive petrological and geochemical description of the compositional evolution during the Paleogene IBM arc. A noteworthy feature of the mineral assemblage is the ubiquitous occurrence of amphibole. This phase is rare in the tephra recovered from previously drilled sites in the IBM fore arc and may indicate that significant across-strike variability of magma compositions was established in the Paleogene.

Formation MicroScanner images show bedding and other features that may help characterize the large-scale tectonic development of the ASB; structural orientation may characterize the geographic provenance of the coarse-grained sediments. Sonic and density data from logs and cores will give a seismic traveltime-depth relationship for the site, providing characterization of seismic boundaries and accurate tie-points between core/log data and seismic data. The seismic velocity structure of the ASB specifically and the adjacent KPR more generally will become better constrained.

The quality of the cores obtained in the uppermost Unit I provide an excellent paleoceanographic sedimentary record, including the Oligocene–Miocene transition, the mid-Miocene climatic optimum, and the Pliocene–Pleistocene onset of northern hemisphere glaciation and mid-Pliocene transition. Numerous layers of volcanic ash were recovered in the recent to Miocene sediments of Unit I, comprising vitric pumice and shards, as well as phenocrysts and isolated volcanogenic crystals of pyroxene, plagioclase, biotite, quartz, and opaque minerals. Comparison of glass and mineral compositions with Ryukyu-Kyushu, Honshu, and Neogene IBM tephras will permit identification of the sources of volcanic ash; pending further age constraints, new data from Site U1438 will permit refined modeling of volumes, explosivity, and geochemical properties of the explosive output of the respective arcs surrounding the ASB.

The fundamentally important discoveries of the age (~50–55 Ma) and composition of the oceanic basement at Site U1438 provide critical constraints on the inception of the intraoceanic IBM arc. It appears a major change in Pacific plate motion following subduction of the Izanagi-Pacific Ridge beneath East Asia led to reorganization of equatorially located networks of island arc systems in the region between the Australian and Asian plates. The Philippine Sea plate developed in this region and experienced trench roll-back at one or more of its bounding plate margins. Rotation of the Philippine Sea plate led to propagation of transpressional forces on its northeastern margin, accompanied by rifting and seafloor spreading invading the former arc edifices forming much of the oldest part of the plate. Localization of a defined volcanic front in the IBM arc developed as the Philippine Sea plate migrated northward, and the transpressional boundary rotated clockwise to orthogonality with the westward-subducting Pacific plate. The oldest magmatic basement of the IBM arc developed in a latitudinally and longitudinally extensive seafloor spreading regime and is dominated by tholeiitic magmas sourced from highly depleted upper mantle.