Brandl, Philipp A.

Hamada, Morihisa

Ishizuka, Osamu

Kanayama, Kyoko

Kusano, Yuki

OceanRep

Deap sea drilling at the Amami Sankaku Basin revealed wide distribution of fore-arcbasalts across the Izu-Bonin-Mariana Arc: Evidence for spontaneous subduction initiation52 million years ago *Morihisa Hamada1, Philipp A. Brandl2, Kyoko Kanayama3, Yuki Kusano4, Osamu Ishizuka4, IODP Exp.351Scientists 1.Department of Solid Earth Geochemistry, Japan Agency for Marine-Earth Science and Technology,2.GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany, 3.San'in Kaigan Global GeoparkPromotion Office, Tottori Prefectural Government, 4.Research Institute of Earthquake and VolcanoGeology, Geological Survey of Japan, AIST In 2014, a triplet of IODP expeditions (Exp. 350-352) focused on the Izu-Bonin-Mariana (IBM) arcwere conducted with the aim of comprehensively understanding the evolution of oceanic island arcand the origin of the continental crust. Exp.351 drilled at Site U1438 in the Amami Sankaku Basin(ASB), about 100 km west of the Kyushu-Palau Ridge, a remnant of the IBM arc. A Cretaceous age (ca. 120 Ma) island arc system now preserved around the ASB (Amami Plateau, DaitoRidge and Oki-Daito Ridge) suggests that the oceanic crust beneath the ASB had existed beforesubduction initiation ca. 52 Ma. The primary objectives of Exp.351 were to constrain thegeochemical nature of the mantle when the oceanic crust was formed, to identify and model theprocesses of subduction initiation, and to reconstruct the evolution of the IBM arc during thePaleogene. Exp.351 cored a 1611-m-long cores composed of 1461-m of sediments and 150-m of igneous basementrocks from the seafloor at a depth of 4711 m. The age for the bottom of the sedimentary layers isestimated to be ca. 50 Ma, based on biostratigraphic and paleomagnetic studies; therefore, igneousbasement below the sedimentary layers should be equivalent or only slightly older than 50 Ma. Themeasured heat flow at Site U1438 is 73.7 mW/m2, implying that the thermal age for the underlyinglithosphere is 40-60 Ma. This age range is consistent with that constrained based onbiostratigraphic and paleomagnetic studies (50 Ma or older), which is much younger than theCretaceous age (ca.120 Ma), as estimated before the expedition. We then analyzed the chemical composition of the uppermost igneous basement rocks on board. Wefound that they are not mid-ocean ridge basalts, but are very similar to fore-arc basalts (FABs)commonly found in the adjacent IBM fore-arc region that erupted ca. 52 Ma when subduction of thePacific plate beneath the Philippine Sea plate was initiated. The igneous basement beneath the ASBshould be FAB itself formed at the onset of subduction ca. 52 Ma. FABs are distributed not only inthe IBM fore-arc region but also in the ASB, which is at the rear-arc side of the IBM arc. Thisunexpected widespread distribution of FABs suggests that the subduction zone was under anextensional environment across the arc, and that the igneous basement of the IBM arc was formedover a much wider area during the subduction inception in a mode consistent with “spontaneous”subduction. The results of our analysis will provide significant insights into the process ofsubduction initiation in this area, as well as into the subsequent evolution of the IBM arc. After Exp.351, we analyzed major and volatile elements (S and Cl) of melt inclusions collected fromthe top to the bottom of sedimentary Unit III (30-40 Ma based on onboard biostratigraphy) usingelectron probe micro-analyzers. Most of the host minerals of the melt inclusions are clinopyroxeneand plagioclase. The compositions of the melt inclusions are diverse, ranging from basalt throughrhyolite, and also ranging from low-K through medium-K series. In terms of major elements, low-Kmelt inclusions are consistent with the melt compositions reported from the volcanic front of theIBM arc. Major element composition of medium-K melt inclusions overlaps with the melt compositionMIS16-14 Japan Geoscience Union Meeting 2016©2016. Japan Geoscience Union. All Right Reserved. - MIS16-14 -Keywords:reported from the IBM rear-arc, such as volcanoes on the KPR and/or near the ASB. Theseobservations suggest that the turbidites accumulated at the ASB originate not only from the IBMrear arc, but also from the IBM frontal arc. We will further investigate the temporal evolution ofarc volcanism by analyzing the trace elements and isotopes in the melt inclusions. International Ocean Discovery Program, Izu-Bonin-Mariana arc, Amami Sankaku Basin MIS16-14 Japan Geoscience Union Meeting 2016©2016. Japan Geoscience Union. All Right Reserved. - MIS16-14 -

2.	Deep sea drilling at the Amami Sankaku Basin revealed wide distribution of fore-arc basalts across the Izu-Bonin-Mariana Arc: Evidence for spontaneous subduction initiation 52 million years ago

2. Deep sea drilling at the Amami Sankaku Basin revealed wide distribution of fore-arc basalts across the Izu-Bonin-Mariana Arc: Evidence for spontaneous subduction initiation 52 million years ago

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