11:30 AM - 11:45 AM
[SCG39-04] Lithological heterogeneity and fluid flow related to seamount subduction: an exhumed example from Amami-Oshima Island
Keywords:seamount, subduction, slow earthquakes, mélange, lithological heterogeneity, Amami-Oshima Island
Recent geophysical surveys and deep ocean drilling revealed that shallow slow earthquakes have been spatially and temporally correlated to lithological heterogeneity associated with the subduction of seamounts. Furthermore, it has been proposed that the rise in fluid pressure and fluid flow on the leading edge of the seamount during subduction results in tectonic tremor. However, geological perspective of shallow slow earthquakes related to seamount subduction remains elusive. Our preliminary geologic investigations conducted on an exhumed accretionary complex exposed in Amami-Oshima Island, Ryukyu Arc reveal a section containing mélange sequences related to subduction of a seamount. Outcrops include altered basalt, a sequence of variable limestone and low grade basaltic sediments, and a mudstone dominated sequence showing a block-in-matrix fabric. A complex quartz vein network is present in the mudstone dominated mélange where extensional fractures have formed in competent blocks. Viscous shear was accommodated along the mixed rocks composed of limestone and basaltic materials, which is consistent with lithological heterogeneity inferred for the shallow slow slip source region. The quartz-filled fracture network in the mudstone dominated mélange may represent tremorgenic fluid flow along brittle fractures that occurred near the subducting seamount. We suggest that the exhumed section at Amami-Oshima Island is representative of the source conditions under which shallow slow earthquakes occur during seamount subduction.