Studies on formation mechanism and source depth of mud volcanoes by using of drilling cores in the Kumano forearc basin, SW Japan
Abstract
Submarine mud volcanoes are formed as conical mounds composed of erupted unconsolidated or partially consolidated sediments from mud diapirs which are induced by high pore-fluid pressure and buoyancy developed in the deep underground. Most of them were discovered around subduction zones. Mud diapir that brings deep underground materials to seafloor has an important role for material circulations in subduction zones. Moreover, methane seepages at mound summits are suggested by existences of chemosynthetic biological communities, and accumulation of methane hydrate is expected from core samples and seismic reflection studies. Therefore, mud volcano is also significant in terms of global warming and energy resource. In order to understand material circulations by mud volcanoes, information about formation mechanism, source layer and its depth is important. In addition, despite mud diapir is generally regarded as rising phenomenon by buoyancy and abnormal high pore pressure, those physical properties are not well investigated. In this study, we discuss the formation mechanism and source depth of mud diapir by using of samples derived form mud volcanoes. We obtained drilling samples from two sites at the summit of the mud volcano in the Kumano Trough, off Kii Peninsula, SW Japan, during CK09-01 using Deep-Sea Drilling Vessel CHIKYU, in March, 2009. Those sites are near the central part of the vent of the mud volcano. To understand formation process of mud volcano, anisotropy of magnetic susceptibility, vitrinite reflectance, density, geological description of breccia are conducted. Anisotropy of magnetic susceptibility shows particle arrangement within samples to understand sedimentation and deformation fabrics. While muddy sediments usually exhibit the ellipsoidal body characterized by oblate shape, the samples from the mud volcano show prolate shape rather than oblate shape. Moreover, long axis of the ellipsoidal body shows mostly vertical direction. Therefore, we expected that the drilling site is influenced by vertical material flow. Porosity of the matrix from the mud volcano is almost constant around 50%. In contrast, the porosity from deposits of the normal basin sediment decreases with the depth and show larger values than those of the mud volcano within 20 m below seafloor. Constant value of porosity of mud volcanoes indicates recent eruption without gravitational compaction. On the other hand, the porosity of breccias shows 20-40 %. These values are smaller than those of the surface basin sediment and the matrix of the mud volcano suggesting their derivations from consolidated host formations. We measured vitrinite reflectances of breccias and estimated absolute maximum temperature of breccias using nanno fossil age by a previous study. The source depth of a breccia is calculated to be 2000 meters using the temperature from vitrinite reflectance and the geothermal gradient of this area. Therefore, it can be said that the source depth of the mud volcano is more than 2000 meters.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFMOS11A1178M
- Keywords:
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- 3045 MARINE GEOLOGY AND GEOPHYSICS / Seafloor morphology;
- geology;
- and geophysics;
- 8003 STRUCTURAL GEOLOGY / Diapir and diapirism