“…3 a), which may also be caused by persistent brittle faulting. The RMS amplitude averaged over a region of 65–84 km distance, which is characterized by thin sediment on the igneous basement, is approximately 23% lower than that of the weak reflectivity zone (40–46 km distance), probably because of considerable seismic attenuation by petit-spot magmatism originated from partial melts along lithospheric fractures in response to plate flexure during subduction 22 . Figure 3 Seismic reflection data on lines A4 (Miyagi transect) and 1 (Iwate transect).…”
Section: Resultsmentioning
confidence: 87%
“…Gravity coring sites of PC1–PC9 are shown. Two petit-spot volcanoes 21 , 22 marked by red triangles, which are the nearest from lines A4 and 1, are projected to each line. ( a ) RTM depth section with automatic gain control (AGC) on line A4.…”
Section: Resultsmentioning
confidence: 99%
“…Two yellow squares (MC3 and MC4) indicate the bottom seawater sampling sites. Two blue triangles mark petit-spot volcanoes 21 , 22 nearest from lines A4 and 1, respectively. …”
Plate bending-related normal faults (i.e. bend-faults) develop at the outer trench-slope of the oceanic plate incoming into the subduction zone. Numerous geophysical studies and numerical simulations suggest that bend-faults play a key role by providing pathways for seawater to flow into the oceanic crust and the upper mantle, thereby promoting hydration of the oceanic plate. However, deep penetration of seawater along bend-faults remains controversial because fluids that have percolated down into the mantle are difficult to detect. This report presents anomalously high helium isotope (3He/4He) ratios in sediment pore water and seismic reflection data which suggest fluid infiltration into the upper mantle and subsequent outflow through bend-faults across the outer slope of the Japan trench. The 3He/4He and 4He/20Ne ratios at sites near-trench bend-faults, which are close to the isotopic ratios of bottom seawater, are almost constant with depth, supporting local seawater inflow. Our findings provide the first reported evidence for a potentially large-scale active hydrothermal circulation system through bend-faults across the Moho (crust-mantle boundary) in and out of the oceanic lithospheric mantle.
“…3 a), which may also be caused by persistent brittle faulting. The RMS amplitude averaged over a region of 65–84 km distance, which is characterized by thin sediment on the igneous basement, is approximately 23% lower than that of the weak reflectivity zone (40–46 km distance), probably because of considerable seismic attenuation by petit-spot magmatism originated from partial melts along lithospheric fractures in response to plate flexure during subduction 22 . Figure 3 Seismic reflection data on lines A4 (Miyagi transect) and 1 (Iwate transect).…”
Section: Resultsmentioning
confidence: 87%
“…Gravity coring sites of PC1–PC9 are shown. Two petit-spot volcanoes 21 , 22 marked by red triangles, which are the nearest from lines A4 and 1, are projected to each line. ( a ) RTM depth section with automatic gain control (AGC) on line A4.…”
Section: Resultsmentioning
confidence: 99%
“…Two yellow squares (MC3 and MC4) indicate the bottom seawater sampling sites. Two blue triangles mark petit-spot volcanoes 21 , 22 nearest from lines A4 and 1, respectively. …”
Plate bending-related normal faults (i.e. bend-faults) develop at the outer trench-slope of the oceanic plate incoming into the subduction zone. Numerous geophysical studies and numerical simulations suggest that bend-faults play a key role by providing pathways for seawater to flow into the oceanic crust and the upper mantle, thereby promoting hydration of the oceanic plate. However, deep penetration of seawater along bend-faults remains controversial because fluids that have percolated down into the mantle are difficult to detect. This report presents anomalously high helium isotope (3He/4He) ratios in sediment pore water and seismic reflection data which suggest fluid infiltration into the upper mantle and subsequent outflow through bend-faults across the outer slope of the Japan trench. The 3He/4He and 4He/20Ne ratios at sites near-trench bend-faults, which are close to the isotopic ratios of bottom seawater, are almost constant with depth, supporting local seawater inflow. Our findings provide the first reported evidence for a potentially large-scale active hydrothermal circulation system through bend-faults across the Moho (crust-mantle boundary) in and out of the oceanic lithospheric mantle.
“…Young (~8.5 Ma) intraplate volcanism, called petit-spot volcanism, has been newly discovered in the outer-rise region of the Japan Trench (e.g., Hirano, 2011;Hirano et al, 2006Hirano et al, , 2008. Petit-spot volcanoes are considered to be formed by magma ascending from the asthenosphere in response to plate flexure during subduction (e.g., Hirano et al, 2006Hirano et al, , 2008Machida et al, 2015;Pilet et al, 2016;Sato et al, 2017), and petit-spot volcanoes have been reported not only in the Japan Trench but also in the Tonga (Hirano et al, 2008), Chile (Hirano et al, 2013), and Sunda trenches (Taneja et al, 2015). Therefore, we infer that in subduction zones around the globe, in addition to the effects of bending-related faults, petit-spot volcanism, if present, may also play a role in governing structural changes of the incoming oceanic plate.…”
The structure of an incoming oceanic plate in a subduction zone systematically changes toward the trench because of the flexure of the plate. Changes in seismic velocity from the outer rise toward the trench are widely considered to be caused by the development of bending-related faults. However, few studies have focused on the effects of igneous activities preceding the subduction, which lead to structural changes of the incoming plate. Here we present active-source seismic data acquired around petit-spot volcanoes situated in the outer-rise region of the Japan Trench. The resultant velocity model showed low seismic velocities (V p < 4.0 km/s) at the top of the oceanic crust under the petit-spot volcanoes. Moreover, the velocity reduction was significantly larger than that caused only by bending-related faults. Our results suggest that petit-spot volcanism is a contributing factor affecting structural variation and modification of the incoming oceanic plate in this subduction zone.
Plain Language SummaryIn subduction zones, the oceanic plate is bent in trench-outer-rise region before its subduction. By using marine seismic methods with airgun seismic sources, many studies have shown that the seismic velocity of the incoming oceanic plate reduces in the trench-outer-rise region due to bending-related faults. Here we present the results of marine seismic experiments in trench-outer-rise region of the Japan Trench, which showed a large velocity reduction (V p < 4.0 km/s) under the petit-spot volcanoes that cannot be explained only by the effects of bending-related faults. This implies that petit-spot volcanism is one of the contributing factors leading to structural variation and modification of the incoming oceanic plate before its subduction.
“…Young basalts have been discovered beside the outer rise on the northwestern Pacific Plate: so-called petit-spot volcanoes (Hirano et al, 2006). They erupted at the oceanward base of the outer rise of the descending Pacific Plate where flexure of the subducting plate causes fractures in the lithosphere (Hirano et al, 2001(Hirano et al, , 2004(Hirano et al, , 2006(Hirano et al, , 2008Machida et al, 2015;Sato et al, 2018). Based on their geochemical signatures, these lavas presumably originated from partial melts in the asthenosphere, but the original depth of the petit-spot magma remains uncertain.…”
We measured noble gas isotopic compositions of quenched lavas sampled from seamounts, so-called petit-spot volcanoes, on the 160-million-year-old northwestern Pacific Plate. The samples 3 He/ 4 He and 40 Ar/ 36 Ar ratios were, respectively, 2.5-8.3 Ra and up to 1735, where Ra stands for atmospheric 3 He/ 4 He, which are analogous to or lower than those of MORB. Considering narrow sampling regions, a secondary effect might be responsible for variation of the data. During ascent and subsequent cooling of magma in the oceanic lithosphere, chemical components in the magma will be assimilated with those in the lithosphere. Correlation between CO 2 / 3 He ratios and carbon isotopic ratios suggests that carbon was affected by the incorporation of seafloor carbonate. The same would be true of noble gases. The mixing of noble gases among a mantle source, an atmospheric component dissolved in seawater and a radiogenic component can explain the data distribution. No 3 He/ 4 He ratio exceeds the MORB-like value. The mantle source of the petit-spot magma was likely to have had a MORB-like 3 He/ 4 He ratio originally. The eruption of petit-spot magma shows a close relation with the bending of subducting oceanic plates. The MORB-like 3 He/ 4 He ratio supports the hypothesis that the petit-spot magma is derived from the lithosphere-asthenosphere boundary.
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