1:45 PM - 3:15 PM
[SSS06-P01] The structures of low-angle thrust penetrating accretionary prisms and igneous rock, and the traces of high-temperature-pressure fluid passage along steep joints on the thrust, in Hyuga City, Miyazaki Prefecture, Japan
Keywords:Thrust, duplex, anastomosing structure, hydraulic fracturing, pinch-and-swell structure, spherical block
1. Preface
The thrust (approximately 5 km traceable) that strikes east-west and dips about 15N shears Upper Shimanto Super-Group (Oligocene to early Miocene ; Hyuga Group and Kadogawa Group) and Osuzuyama volcanic plutonic complex at 14-15 Ma in Hyuga City. And the various structures that are considered to have occurred during fault sliding have been observed along the thrust.
2. Observations
1) Osuzuyama volcanic plutonic complex
The thrust that strikes east-west and dips 10N with developed large-scale anastomosing structure of wavelength of 30 m to 50 m and duplex structure (4 m high and 14 m wide) is observed in the welded dacitic tuff in Takeshima. In addition, white minerals (several millimeters thick) are precipitated in the steep joints near the thrust.
2) Shimanto Super Group (Hyuga Group/Kadogawa Group)
The thrust fault with strike of N80E to N80W and dip of about 16N penetrates in the shale sandstone that has contact thermal metamorphism due to the heat of the Osuzuyama volcanic plutonic complex, and sometimes bifurcates. Structural lower thrust become white clay layer with a thickness of more than 1 m due to weathering of fault gouges. On the other hand, the upper thrust has anastomosing structures with a wavelength of several meters, mine tunnel traces, and constant spring water. And the characteristic steep joints of conjugated systems perpendicular to the thrust fault are observed between the above two thrusts. Around the steep joint, there are S-C (pinch-and-swell) structures, hydraulic fracturing, thermal metamorphism, and quartz veins, rotation of the fluid itself (10mm to 20mm in diameter) and other structures are observed, and disappear as the distance from the steep joint increases. In addition, there is the spherical sandstone tectonic block with a diameter of 10 m near the steep joints, and small-scale duplex structures continues along the boundary of spherical sandstone-block and surrounding shale, and the fluidized quartz grains can be seen under the microscope at the edge of the spherical block.
3. Summary and discussion
The thrust, which strikes east-west and dips gently to the north, develops anastomosing structures and duplexes, but the fault plane is unclear. Therefore it is considered that the thrust has finished its activity in early stage of sliding. And various structures showing high temperature-pressure stress along the steep joints of conjugated systems on the thrust indicate that high temperature-pressure fluids flowed along the steep joints. This high-temperature-pressure fluid is thought to have been generated by generated frictional heat and kinetic energy with sliding of the thrust propagating to the pore-water in the thrust zone.
The thrust (approximately 5 km traceable) that strikes east-west and dips about 15N shears Upper Shimanto Super-Group (Oligocene to early Miocene ; Hyuga Group and Kadogawa Group) and Osuzuyama volcanic plutonic complex at 14-15 Ma in Hyuga City. And the various structures that are considered to have occurred during fault sliding have been observed along the thrust.
2. Observations
1) Osuzuyama volcanic plutonic complex
The thrust that strikes east-west and dips 10N with developed large-scale anastomosing structure of wavelength of 30 m to 50 m and duplex structure (4 m high and 14 m wide) is observed in the welded dacitic tuff in Takeshima. In addition, white minerals (several millimeters thick) are precipitated in the steep joints near the thrust.
2) Shimanto Super Group (Hyuga Group/Kadogawa Group)
The thrust fault with strike of N80E to N80W and dip of about 16N penetrates in the shale sandstone that has contact thermal metamorphism due to the heat of the Osuzuyama volcanic plutonic complex, and sometimes bifurcates. Structural lower thrust become white clay layer with a thickness of more than 1 m due to weathering of fault gouges. On the other hand, the upper thrust has anastomosing structures with a wavelength of several meters, mine tunnel traces, and constant spring water. And the characteristic steep joints of conjugated systems perpendicular to the thrust fault are observed between the above two thrusts. Around the steep joint, there are S-C (pinch-and-swell) structures, hydraulic fracturing, thermal metamorphism, and quartz veins, rotation of the fluid itself (10mm to 20mm in diameter) and other structures are observed, and disappear as the distance from the steep joint increases. In addition, there is the spherical sandstone tectonic block with a diameter of 10 m near the steep joints, and small-scale duplex structures continues along the boundary of spherical sandstone-block and surrounding shale, and the fluidized quartz grains can be seen under the microscope at the edge of the spherical block.
3. Summary and discussion
The thrust, which strikes east-west and dips gently to the north, develops anastomosing structures and duplexes, but the fault plane is unclear. Therefore it is considered that the thrust has finished its activity in early stage of sliding. And various structures showing high temperature-pressure stress along the steep joints of conjugated systems on the thrust indicate that high temperature-pressure fluids flowed along the steep joints. This high-temperature-pressure fluid is thought to have been generated by generated frictional heat and kinetic energy with sliding of the thrust propagating to the pore-water in the thrust zone.