[S-CG45] Science of slow-to-fast earthquakes

[SCG45-P12] Deformation mechanisms and rheology of mélange shear zones associated with seamount subduction

*Madison Frank¹、Kohtaro Ujiie²、Ginta Motohashi¹、Takayoshi Nagaya³ (1.Graduate School of Life and Environmental Sciences, University of Tsukuba、2.Faculty of Life and Environmental Sciences, University of Tsukuba、3.Graduate School of Science, University of Tokyo)

キーワード：slow earthquakes, seamount subduction, rheology, deformation mechanisms, Amami-Oshima Island

Exhumed subduction mélanges are expected to provide critical information on the lithology, fluid conditions, and deformation mechanisms related to slow slip events or aseismic creep along a megathrust. Slow earthquakes are frequently observed where seamounts are subducting, but mélanges formed in association with seamount subduction have rarely been examined. The Chichibu accretionary complex in Amami-Oshima, Ryukyu Arc, contains shear zones composed of mudstone-dominated mélange (MDM) and basalt-limestone mélange (BLM), showing a shear sense consistent with megathrust shear. MDM is characterized by lenses of sandstone, siliceous mudstone, and rare massive basalt in an illitic matrix. BLM is marked by lenses of micritic limestone and basalt blocks in a chloritic matrix, which originated from the mixing of limestone and basalt at the foot of seamount. Peak temperature of MDM from Raman carbonaceous material geothermometers is ~285 °C. Deformation microstructures indicate that shear deformation in MDM was accommodated by dislocation creep of quartz and combined quartz pressure solution and frictional sliding of illite, whereas dominant deformation mechanisms in BLM indicate diffusion creep of very fine-grained calcite and frictional sliding of chlorite. To estimate the range of strain rates responsible for megathrust shear, we constructed rheological models of mélange shear zones taking deformation mechanisms of the MDM and BLM and experimentally derived constitutive laws for quartz, combined quartz + illite, calcite, and chlorite into consideration. For the MDM, our model predicts that dislocation creep of quartz and combined quartz pressure solution and frictional sliding of illite can occur at strain rates between ~10^-15 to 10^-12 s^-1 over fluid pressures ranging from hydrostatic to near-lithostatic. For the BLM, frictional sliding of chlorite and diffusion creep of calcite can occur at strain rates of ~10^-13 to 10^-10 s^-1 between hydrostatic to near-lithostatic fluid pressure, two orders of magnitude faster than the MDM. Our models indicate that the BLM recorded faster slip events with respect to the MDM. If the mélange shear zones are 100 m thick, BLM could accommodate slow slip rates while MDM could accommodate aseismic creep rates.