[S-SS10] Fault Rheology and Earthquake Physics

[SSS10-22] Fault zone architecture, creep-cavitation and ductile fractures in crustal-scale faults at the base of the seismogenic zone

★Invited Papers

*Norio Shigematsu¹, Thomas Yeo^1,2,3, Simon Richard Wallis², Kiyokazu Oohashi¹, Takuma Katori⁴, Keita Kobayashi⁵, Tatsuya Sumita⁶, Ayumu Miyakawa⁶, Kohtaro Ujiie³, Chunjie Zhang⁷ (1.Research Institute of Earthquake and Volcano Geology, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, 2.School of Science, University of Tokyo, 3.School of Life and Environmental Sciences, Tsukuba University, 4.Fossa Magna Museum, 5.Research Institute for Material and Chemical Measurement, National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology, 6.Research Institute of Geology and Geoinformation, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, 7.Earthquake Research Institute, University of Tokyo)

Keywords:Median Tectonic Line, Ductile fracture, Ductile deformation, Creep cavitation

The fault zone architecture, creep cavities, and ductile fractures were examined to understand the processes at the base of the seismogenic zone in an exhumed fault zone, the Median Tectonic Line (MTL) in Mie Prefecture, SW Japan, which is the largest on-land fault (>1000 km along strike) of Japan. In Mie Prefecture, the MTL trends almost E-W and dips to the north. The fault zone along the MTL can be divided into structures falling under two main types: those formed by older sinistral faulting at temperatures higher than ~300^oC, and those resulting from younger faulting at temperatures lower than 250^oC. Younger brittle structures are exposed near the lithological boundary, while older structures are exposed distant from the lithological boundary and are isolated from younger brittle overprinting. In older structures, ultramylonites deformed at temperatures of ~350^oC commonly contain fractures parallel to the mylonitic foliation.
Using a heterogeneously deformed ultramylonite, we establish a correlation between cavity density and the quartz recrystallized fraction, which served as a general indicator for strain. The cavity density increases with increasing deformation. Ductile failure occurs at a critical cavity density of ~ 7.5%. This trend indicates that ductile fracture results from the evolution of creep cavities.
Ultramylonites visually inspected at outcrops are found to associate with a high quartz recrystallized fraction and cavitation density larger than 7.5% implying the occurrence of ductile fractures. We also observed an outcrop where ultramylonites are associated with fractures with a continuous exposure of 30 meters. The distribution of mylonite can be classified into four zones. Zone A is situated in the north exhibits heterogeneous deformation and is characterized by numerous localized shear zones. Zone B, situated in the second north, is characterized by an approximately 20 m thick strongly deformed mylonite zone. This zone is interpreted as the main shear zone within the outcrop. Zone C, situated in the second south, exhibits shear zone structures featuring numerous localized shear zones. Zone D, situated in the south, is characterized by weakly deformed rocks. The interpolation of exposures of mylonites indicates that zone B is traced over a length of > 1.2 km.
Creep cavitation induces a transition from a steady state creep into transient creep and ultimately lead to ductile fracture in metals and alloys. The volume where creep cavitation and ductile fracturing occur extends for > 1.2 km at the base of the seismogenic zone. Several previous studies suggest that the base of the seismogenic zone is of great importance for the behaviors of crustal-scale faults. Therefore, the evolution of creep cavities to ductile fractures possibly controls the macroscopic behavior of crustal-scale faults.