The role of mud volcanism and deep‐seated dewatering processes in the Nankai Trough accretionary prism and Kumano Basin, Japan

Abstract

Circulation of water at moderate depths in subduction zones is dominantly driven by clay mineral dehydration over distinct pressure and temperature gradients. The signature of these dehydration reactions is found in mud volcano pore waters, however, it is largely unknown, how much of the deep‐seated fluids are emitted at mud volcanoes. To unravel this relation for the region off the Kii Peninsula, Japan, we calculated the water volume that is subducted in the Nankai Trough using input data from IODP holes C0011 and C0012 and the correspondent water volume released from the subducted plate under the Kumano Basin, in an area where 13 mud volcanoes are located. According to our model, water released at depth in the mud volcano area is derived almost entirely from basaltic saponite and sedimentary smectite transformation [up to 96%]. Nonetheless, the mud volcanoes themselves expel ≪1% of the total volume. To test the contribution of the accreted strata and the Kumano Basin fill to the water budget, we run a second model. Water loss due to compaction of sediments and smectite‐illite transition below the basin floor have been calculated. The results were compared with salinity measurements on background cores scattered in the study area to extrapolate the volume of water loss at depth. The comparison of the two methods yielded similar results and led us to conclude that the bulk part of the deep‐seated fluid reenters the hydrosphere via the basin floor, a mechanism rarely taken into account in fluid budgets in the literature.

Clay Mineral Suites in Submarine Mud Volcanoes in the Kumano Forearc Basin, Nankai Trough: Constraints on the Origin of Mud Volcano Sediments

“…Nishio et al [16] inferred that Li isotope ratios in porewater of the drilled core from KMV#5 indicate temperatures as high as 310 • C, which is significantly higher than the S-I conversion temperature range of 60-150 • C. They suggested that a portion of fluid was derived from the periodic injection of deep-seated fluid accumulated in the corner of the serpentinized forearc mantle wedge, transported upward along a thrust fault [16]. Modeling of the water circulation in mud volcanoes of the Kumano basin has suggested that the fluids in the mud volcano sediment are derived by S-I transformation in the accretionary prism and decomposition of basaltic saponite in the subducting slab (Figure 8) [13]. In sum, the fluid in the mud volcanoes of the Kumano basin appears to originate in or beneath the accretionary prism underlying the Kumano forearc basin sediment.…”

“…Because our porewater samples from KMV#10 show the same trends in δ 18 O and δD values with respect to Cl -concentrations, KMV#10 also was affected by water derived from clay mineral dehydration. Indeed, this process has previously been reported for other mud volcanoes in the Kumano basin (KMV#2-11) [13].…”

Deep-biosphere methane production stimulated by geofluids in the Nankai accretionary complex

“…In the Kumano Basin, 13 mud volcanoes have been observed at anticlines along the landward megasplay faults ( Fig. 1 , B and C) ( 17 – 20 ). Chemosynthetic macrofaunal communities occur at seep environments on the summits of these features ( 19 ).…”

Discharge of deeply rooted fluids from submarine mud volcanism in the Taiwan accretionary prism

“…Sediment compaction and mineral dehydration are the most important mechanisms for fluid generation and circulation in an accretionary prism 6,7 . In essence, both mechanisms are driven by the increasing pressure and temperature associated with sediment burial and deformation.…”