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

AbstractThis 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

Depth dependence of the frictional behavior of montmorillonite fault gouge: Implications for seismicity along a décollement zone

To understand the seismogenic potential of shallow plate‐boundary thrust faults (décollements) in relatively warm subduction zones, water‐saturated Na‐montmorillonite gouges were sheared at a pore fluid pressure of 10 MPa, effective normal stresses (σneff) of 10–70 MPa, temperatures (T) of 25–150°C, and axial displacement rates of 0.03–3 μm/s. The Na‐montmorillonite gouges were frictionally very weak at all conditions tested (steady state friction coefficient μss = 0.05–0.09). At T ≤ 60°C, Na‐montmorillonite showed a transition from velocity‐strengthening to velocity‐weakening behavior with increasing σneff, whereas at T ≥ 90°C it was largely velocity neutral or velocity strengthening, irrespective of σneff. The rates of frictional healing (β) showed extremely low values (mostly <0.001) at all temperatures. Our results suggest that the existence of Na‐montmorillonite in the décollement zone at Costa Rica and Nankai promotes aseismic slip, particularly at shallow depths, forming weakly coupled regions

Low silica activity for hydrogen generation during serpentinization: An example of natural serpentinites in the Mineoka ophiolite complex, central Japan

The textural evolution in the serpentinite of the Mineoka ophiolite complex has been investigated to constrain the natural environment for hydrogen production in the serpentinite-hosted hydrothermal vent systems. Textural relations of the serpentinites from the Mineoka ophiolite indicate at least two stages in the process of serpentinization, with the replacement of olivine by a mesh texture of serpentine and brucite, followed by the development of magnetite-bearing or -free serpentine veins. The generation of hydrogen during serpentinization, which accompanies the formation of magnetite, involves a silica-depletion reaction, as evidenced by the low abundance of serpentine in the magnetite-bearing veins and the absence of magnetite in pseudomorphs of orthopyroxene. Direct evidence for the production of hydrogen and strongly reducing conditions is provided by CH4 and H-2-bearing inclusions in relic olivine crystals; the production of methane and hydrogen may have provided a suitable environment for microbial activity in hydrothermal vent systems along the seafloor. Our results indicate that low silica activity plays a key role in the generation of hydrogen during serpentinization, and that low silica activity environments are possible in olivine-rich rocks such as dunite, or during local disequilibrium in other silica-poor rocks in the mantle lithosphere

Data repository of "Effects of rheological stratification and elasticity of lithosphere on subduction initiation"

This dataset contains the calculation results of plate elasticity, flow stress, strength envelope, and numerical simulation

The role of metasomatic alteration on frictional properties of subduction thrusts : An example from a serpentinite body in the Franciscan Complex, California

Fluid–rock interaction within accretionary prisms drastically changes the frictional strength and slip stability of the fault zone. In order to understand the effect of ultramafic components on the degree of interplate coupling, we present data on frictional properties of a reaction zone between serpentinite and argillite in a tectonic mélange of the Franciscan Complex, central California. Field and petrographic observations indicate that the argillite and serpentinite along the lithological boundary are metasomatized to tremolite and talc schists, respectively, forming a ductile shear zone. Simulated gouges made from fault and wall rock samples collected are sheared at effective normal stresses (σneff) of 60–180 MPa and temperatures (T) of 20–400C∘, using a hydrothermal ring shear machine. The serpentinite is frictionally strong (steady-state friction coefficient μss=0.5–0.6), and exhibits both velocity-weakening and velocity-strengthening behavior. The μss of the argillite and tremolite schist increases up to 0.7 and 0.6, respectively, with increasing σneff and T, while (a−b) transitions from positive to negative with increasing T. At a given experimental condition, μ and (a−b) values are lower and higher for the tremolite schist than for the argillite. In contrast, the talc schist exhibits very low friction (μss=0.1–0.2) and velocity-strengthening behavior at all experimental conditions tested. Our results demonstrate that Si-metasomatism along the argillite–serpentinite contact results in reduced friction and stabilized slip, forming frictionally weak, velocity-strengthening regions in the megathrust zone

The role of metasomatic alteration on frictional properties of subduction thrusts:An example from a serpentinite body in the Franciscan Complex, California

Fluid–rock interaction within accretionary prisms drastically changes the frictional strength and slip stability of the fault zone. In order to understand the effect of ultramafic components on the degree of interplate coupling, we present data on frictional properties of a reaction zone between serpentinite and argillite in a tectonic mélange of the Franciscan Complex, central California. Field and petrographic observations indicate that the argillite and serpentinite along the lithological boundary are metasomatized to tremolite and talc schists, respectively, forming a ductile shear zone. Simulated gouges made from fault and wall rock samples collected are sheared at effective normal stresses (σneff) of 60–180 MPa and temperatures (T) of 20–400C∘, using a hydrothermal ring shear machine. The serpentinite is frictionally strong (steady-state friction coefficient μss=0.5–0.6), and exhibits both velocity-weakening and velocity-strengthening behavior. The μss of the argillite and tremolite schist increases up to 0.7 and 0.6, respectively, with increasing σneff and T, while (a−b) transitions from positive to negative with increasing T. At a given experimental condition, μ and (a−b) values are lower and higher for the tremolite schist than for the argillite. In contrast, the talc schist exhibits very low friction (μss=0.1–0.2) and velocity-strengthening behavior at all experimental conditions tested. Our results demonstrate that Si-metasomatism along the argillite–serpentinite contact results in reduced friction and stabilized slip, forming frictionally weak, velocity-strengthening regions in the megathrust zone