Geochemical characteristics of back-arc basin lower crust and upper mantle at final spreading stage of Shikoku Basin: an example of Mado Megamullion

This paper explores the evolutional process of back-arc basin (BAB) magma system at final spreading stage of extinct BAB, Shikoku Basin (Philippine Sea) and assesses its tectonic evolution using a newly discovered oceanic core complex, the Mado Megamullion. Bulk and in-situ chemical compositions together with in-situ Pb isotope composition of dolerite, oxide gabbro, gabbro, olivine gabbro, dunite, and peridotite are presented. Compositional ranges and trends of the igneous and peridotitic rocks from the Mado Megamullion are similar to those from the slow- to ultraslow-spreading mid-ocean ridges (MOR). Since the timing of the Mado Megamullion exhumation corresponds to the very end of the Shikoku Basin opening, the magma supply was subdued and highly episodic, leading to extreme magma differentiation to form ferrobasaltic, hydrous magmas. In-situ Pb isotope composition of magmatic brown amphibole in the oxide gabbro is identical to that of depleted source mantle for mid-ocean ridge basalt (MORB). In the context of hydrous BAB magma genesis, the magmatic water was derived solely from the MORB source mantle. The distance from the back-arc spreading center to the arc front increased away through maturing of the Shikoku Basin to cause MORB-like magmatism. After the exhumation of Mado Megamullion along detachment faults, dolerite dikes intruded as a post-spreading magmatism. The final magmatism along with post-spreading Kinan Seamount Chain volcanism were introduced around the extinct back-arc spreading center after the opening of Shikoku Basin by residual mantle upwelling.

Seafloor Pressure Change Excited at the Northwest Corner of the Shikoku Basin by the Formation of the Kuroshio Large-Meander in September 2017

The Kuroshio takes a greatly southward displaced path called a large-meander (LM) path off the southern coast of Japan on interannual to decadal time scales. The transition of the current path from a non-large-meander path to an LM path is the most salient ocean current variation south of Japan. The change in pressure on the seafloor due to the formation of the LM path in September 2017 is of critical importance to understand the dynamics of the LM path and to distinguish the change due to the Kuroshio path variation from changes due to crustal deformation. Hence, we examined the seafloor pressure across the continental slope off the eastern coast of Kyushu for the period March 2014 to April 2019. The pressure and its cross-slope gradient over the continental slope shallower than 3,000 m beneath near the Kuroshio are invariable. As a mesoscale current path disturbance, called a small meander, passed over the observation stations, the pressure decreased by approximately 0.1 dbar on the continental slope deeper than 3000 m and was kept low until the end of the observation period (April 2019). The pressure decrease is consistent with the changes in sea surface height and subsurface water density and is caused by the baroclinic enhancement of the Shikoku Basin local recirculation. This seafloor pressure change implies a strengthening of the deep southwestward current, possibly as a part of a deep cyclonic circulation in the Shikoku Basin. The present study demonstrated that, in addition to altimetric sea surface height data, hydrographic data are useful to distinguish the ocean variation in seafloor pressure from the variation due to crustal deformation, and vice versa.

Geochemical Variation of Miocene Basalts within Shikoku Basin: Magma Source Compositions and Geodynamic Implications

Shikoku Basin is unique as being located within a trench-ridge-trench triple junction. Here, we report mineral compositions, major, trace-element, and Sr-Nd-Pb isotopic compositions of bulk-rocks from Sites C0012 (>18.9 Ma) and 1173 (13–15 Ma) of the Shikoku Basin. Samples from Sites C0012 and 1173 are tholeiitic in composition and display relative depletion in light rare earth elements (REEs) and enrichment in heavy REEs, generally similar to normal mid-ocean ridge basalts (N-MORB). Specifically, Site C0012 samples display more pronounced positive anomalies in Rb, Ba, K, Pb and Sr, and negative anomalies in Th, U, Nb, and Ta, as well as negative Nb relative to La and Th. Site 1173 basalts have relatively uniform Sr-Nd-Pb isotopic compositions, close to the end member of depleted mantle, while Site C0012 samples show slightly enriched Sr-Nd-Pb isotopic signature, indicating a possible involvement of enriched mantle 1 (EM1) and EM2 sources, which could be attributed to the metasomatism of the fluids released from the dehydrated subduction slab, but with the little involvement of subducted slab-derived sedimentary component. Additionally, the Shikoku Basin record the formation of the back-arc basin was a mantle conversion process from an island arc to a typical MORB. The formation of the Shikoku Basin is different from that of the adjacent Japan Sea and Parece Vela Basin, mainly in terms of the metasomatized subduction-related components, the nature of mantle source, and partial melting processes.

Draft Genome Sequence of Salinispora sp. Strain H7-4, Isolated from Deep-Sea Sediments of the Shikoku Basin

ABSTRACT Salinispora spp. are obligate marine actinomycetes reported mainly from shallow tropical and subtropical marine habitats. In this announcement, we present the isolation and draft genome sequence of Salinispora sp. strain H7-4 of a new 16S rDNA phylotype from deep-sea sediments of the Shikoku Basin.

Characterization of porosity from Cation Exchange Capacity and resistivity data at IODP Expedition 358 Site C0024: insights on compaction state, stress and deformation history at the frontal thrust of the Nankai margin

<p>In subduction zones, megathrust seismicity depends on the hydrogeological, thermal, physical and mechanical properties of the sediments before they enter the subduction zone and how these properties evolve through the subduction process. In particular, fluids are progressively released by compaction and/or mineral dehydration reactions as burial increases, resulting in the build-up of pore fluid pressure in low-permeability sediments that strongly affects fault behavior through its control on effective normal stress.</p><p>We use porosity, logging and chemical data to characterize compaction state and bound water content of sediments at Site C0024. This site was drilled in March 2019 during International Ocean Discovery Program Expedition 358 in the anticline overlying the frontal thrust of the Nankai margin, a few kilometers landward of Sites C0006 and C0007 that were previously drilled during the NanTroSEIZE project. Sites C0024, C0006 and C0007 transect the décollement and overlying accreted Upper Shikoku Basin and wedge slope deposits. At Site C0024, the main frontal thrust at ~820 mbsf, the top of its footwall (up to ~870 mbsf) and its hanging-wall were logged. Two intervals were cored in the hanging-wall (~0-320 mbsf and ~510-652 mbsf). Four sedimentary facies were identified : (1) the slope apron (~0-4 mbsf) composed of silty clay to clayey silt hemipelagites, (2) accreted trench wedge sediments (~4–519 mbsf) composed of hemipelagites with volcanic ash layers, silt and sand turbidites, (3) outer trench-wedge sediments (~519-555 mbsf) mainly composed of hemipelagites with rare silt turbidites and volcanic ash layers and (4) accreted Upper Shikoku Basin (>555mbsf) composed of hemipelagites and volcanic ash layers. The Pliocene to Miocene accreted Upper Shikoku Basin deposits at the frontal thrust are correlated with undeformed Upper Shikoku Basin deposits at reference Sites C0011 and C0012 seaward in the incoming Shikoku Basin.</p><p>Following previous studies, we use Cation Exchange Capacity (CEC) to determine bound water content and interstitial porosity in the cored interval. Unlike total porosity commonly measured onboard, interstitial porosity is representative of the compaction state of sediments. We use interstitial porosity data to calibrate resistivity-derived porosity through the hanging-wall, the décollement and below. Resistivity-derived porosity is obtained with a resistivity model accounting for the high surface conductivity of clays based on CEC, exchangeable cation composition, LWD resistivity and gamma ray logs. We also document the evolution of the structure of micro- to macropores with depth using low pressure nitrogen adsorption/desorption, mercury injection capillary pressure and nuclear magnetic resonance.  Finally, we compare the porosity dataset at Site C0024 with that of Sites C0006 and C0007 in the frontal thrust and reference Sites C0011 and C0012 in the entering Shikoku Basin to characterize the evolution of the compaction state of sediments during accretion.</p>