10:45 AM - 12:15 PM
[SCG45-P16] Dislocation creep in cataclasite with seismic slip zone in an exhumed accretionary complex
Keywords:dislocation creep, fossil seismogenic fault, accretionary complex, slow earthquake
Faults slip at various speeds from slow to fast as observed in geophysical observations. The spatial relationship between slow and fast slips will provide a clue to understanding the interactions between the slips with different speeds. Geological constraints on the slip rate along faults are characterized by deformation mechanisms such as frictional melting for fast slip and dislocation creeps for slow deformations. However, the examination for slip rate has not been conducted in a single fault zone. In the Cretaceous Shimanto Belt, an exhumed accretionary complex, fossil seismogenic faults have been identified as that including pseudotachylyte, which are always accompanied by cataclasite. The dynamically recrystallized quartz grains were observed within the cataclasite. In this study, we conducted microscopic observations on the cataclasite to examine the dislocation creeps by EBSD and crystal axis orientations. Finally, we discuss about the slip sense and slip rate for the slow deformations in the cataclasite including a seismogenic fault.
The study area is the Yokonami mélange, the Cretaceous Shimanto Belt. A fossil seismogenic fault zone is located at the northern end of the Yokonami mélange. The cataclasitic shear zones are about 20 cm in thickness. Thin (less than 1mm) of discrete slip zone is developed in the cataclasitc shear zone. The paleo-maximum temperature is about 250 degC for host rocks based on vitrinite reflectance.
Microstructural observations indicate that the cataclasite comprises sandstone blocks including quartz and calcite grains surrounded by shale matrix. The shale matrix depicts a flow texture around sandstone blocks, representing asymmetric shapes. Quartz grains characterize undulate extinction and serrated grain boundaries.
We conducted EBSD analysis on the quartz grains to examine the orientations of the crystal axes. EBSD maps were collected with 15kV acceleration voltage, 17mA of current and 0.3 µm of step size. EBSD data was processed by MTEX toolbox. To pick recrystallized grains, grain orientation spread (GOS) was examined. The grains with GOS smaller than the GOS threshold and with grain size smaller than 10 µm were selected as recrystallized grains.
The pole figures show crystallographic preferred orientations (CPO) for [c] axes of quartz indicating asymmetric peripheral distribution and for [a] axes with a girdle distribution perpendicular to the [c] axes in some analyzed areas. The existence of CPO indicates that the dislocation creep occurred in quartz grains. The paleo-maximum temperature in the host rocks is about 250 degC, while the temperature required for dislocation creep of quartz is estimated to be at least 300 degC (Passchier and Trouw, 2005). This indicates that the dynamically recrystallized quartz in the cataclasite suggests a local heating due to frictional heating or hot fluid migration. The idea of local heating in cataclasite is also supported by chemical analyses and paleo-magnetic analyses estimating 300-360 degC. The strain rate for the dislocation creep was constrained as 10-13–10-14/s using flow law with 300 degC in temperature, lithostatic pore fluid pressure and 155MPa in differential stress. Taking into account 20 cm of the average thickness of the cataclasite, the strain rate corresponds to 10-7m/year of displacement rate for the dislocation creep, which is much smaller than even plate convergent rate. To compensate the lack of displacement rate, strain of shale matrix may need to be expected. The asymmetric peripheral distribution of [c] axes in the pole figure suggests that the shear sense of the dislocation creep is consistent with the asymmetric shape of the sandstone blocks. The consistency suggests that the ductile deformations in shale matrix and dislocation creep in quartz in the cataclasite occurred simultaneously. The local exothermic event is required for the dislocation creep. Such slow and heat-generating events could correspond to slow earthquakes.
The study area is the Yokonami mélange, the Cretaceous Shimanto Belt. A fossil seismogenic fault zone is located at the northern end of the Yokonami mélange. The cataclasitic shear zones are about 20 cm in thickness. Thin (less than 1mm) of discrete slip zone is developed in the cataclasitc shear zone. The paleo-maximum temperature is about 250 degC for host rocks based on vitrinite reflectance.
Microstructural observations indicate that the cataclasite comprises sandstone blocks including quartz and calcite grains surrounded by shale matrix. The shale matrix depicts a flow texture around sandstone blocks, representing asymmetric shapes. Quartz grains characterize undulate extinction and serrated grain boundaries.
We conducted EBSD analysis on the quartz grains to examine the orientations of the crystal axes. EBSD maps were collected with 15kV acceleration voltage, 17mA of current and 0.3 µm of step size. EBSD data was processed by MTEX toolbox. To pick recrystallized grains, grain orientation spread (GOS) was examined. The grains with GOS smaller than the GOS threshold and with grain size smaller than 10 µm were selected as recrystallized grains.
The pole figures show crystallographic preferred orientations (CPO) for [c] axes of quartz indicating asymmetric peripheral distribution and for [a] axes with a girdle distribution perpendicular to the [c] axes in some analyzed areas. The existence of CPO indicates that the dislocation creep occurred in quartz grains. The paleo-maximum temperature in the host rocks is about 250 degC, while the temperature required for dislocation creep of quartz is estimated to be at least 300 degC (Passchier and Trouw, 2005). This indicates that the dynamically recrystallized quartz in the cataclasite suggests a local heating due to frictional heating or hot fluid migration. The idea of local heating in cataclasite is also supported by chemical analyses and paleo-magnetic analyses estimating 300-360 degC. The strain rate for the dislocation creep was constrained as 10-13–10-14/s using flow law with 300 degC in temperature, lithostatic pore fluid pressure and 155MPa in differential stress. Taking into account 20 cm of the average thickness of the cataclasite, the strain rate corresponds to 10-7m/year of displacement rate for the dislocation creep, which is much smaller than even plate convergent rate. To compensate the lack of displacement rate, strain of shale matrix may need to be expected. The asymmetric peripheral distribution of [c] axes in the pole figure suggests that the shear sense of the dislocation creep is consistent with the asymmetric shape of the sandstone blocks. The consistency suggests that the ductile deformations in shale matrix and dislocation creep in quartz in the cataclasite occurred simultaneously. The local exothermic event is required for the dislocation creep. Such slow and heat-generating events could correspond to slow earthquakes.