[S-SS08] 地震発生の物理・断層のレオロジー

[SSS08-16] Development of crystallographic preferred orientation during normal faulting in the Pleistocene Ryukyu Limestone

*井上 郁¹、氏家 恒太郎²、永冶 方敬³ (1.筑波大学大学院生命環境研究科、2.筑波大学生命環境系、3.東京大学大学院理学系研究科)

キーワード：琉球石灰岩、結晶方位定向配列、正断層運動

Limestone is one of the major shallow crustal sedimentary rocks in seismically active area. The northwest-southeast striking Yonabaru Fault in Miyako Island, southern Ryukyu Arc has been thought to be an active fault because it displaced the Pleistocene Ryukyu Limestone derived from coral reef. We examined the fault rocks of the Yonabaru Fault at the southernmost part of Miyako Island, based on fieldwork and microstructural analysis using optical microscope and electron backscatter diffraction. The fault zone is ~50 m-thick and is composed of subvertical principal slip zones that sharply cut subhorizontal Ryukyu Limestone. The kinematic data obtained from the polished and lineated slip surfaces indicate the normal faulting in a northeast-southwest extension field. The principal slip zones are defined by the slip surfaces flanked by ~1-3 mm-thick zones composed of limestone fragments in the very fine-grained calcite matrix. The matrix calcite has monoclinic crystallographic preferred orientation(CPO) and shape preferred orientation(SPO). The EBSD map with grain boundaries defined by misorientation angles larger than 10° shows fine calcite grains and similar CPO patterns characterized by c-axes inclined against the shear direction, regardless of the degree of the misorientation angle within the grain. SPO is also marked by the alignment of inclined long axes at 60-70° to the shear direction. These CPO and SPO resemble those formed by dynamic recrystallization during the friction experiments on powdered calcite at seismic slip rates and the torsion experiments on Carrara marble at temperatures of 500-600 ℃. However, similar CPO was also obtained from dislocation glide in nano-crystalline gouge sheared at subseismic slip rates, which was enhanced at temperatures higher than 80 ℃. Our study demonstrates that intracrystalline deformation of calcite can occur during normal faulting at very shallow depths. Considering that crystal plasticity in calcite is strongly thermally activated, intracrystalline deformation of calcite observed in the principal slip zones may represent the past heat during normal faulting at very shallow depths and thus potentially an indicator of past seismic slip in the Ryukyu Limestone.