Rapid exhumation of subducted sediments along an out-of-sequence thrust in the modern Eastern Nankai accretionary prism

Abstract

A foliated mudstone with illite crystallization was recovered at an outcrop during dive surveys of manned submersible SHINKAI 6500 to the bottom of the Tenryu Submarine Canyon near the Tokai Thrust, which is one of the largest out-of-sequence thrusts within the Nankai accretionary prism (NAP) off Southwest Japan. The mudstone contains a strong preferred orientation of illite flakes that define two foliations (S1 and S2): both are schistosity-like texture defined by shape and crystallographic preferred orientation. The S2 foliation is subparallel to a later tension fracture (S3). The illite flakes within the foliated mudstone clearly formed from diagenetic recrystallization rather than being detrital in origin. The peak metamorphic temperature for the mudstone samples was estimated to be ~200–230°C on the basis of illite crystallinity. Based on the representative thermal gradients within the NAP at present (~50°C/km), the rock was exhumed from 4 to 5 km in depth, corresponding to the depth of the seismogenic zone. Nannofossils in the foliated mudstone indicate a depositional age older than 3.8. Ma. Radiolarians inserted within the tension fractures of the sample indicate <0.42. Ma, suggesting the sediments subducted and then returned to the surface <0.42. Ma ago. The present sample site is located ca. 30 km from the leading edge of the trough. Present subduction rate of the Philippine Sea plate (4.0–7.0 cm/year) gives estimates for the rate of lateral transportation of sediments. These results indicate that the mudstone along the out-of-sequence thrust were rapidly exhumed from the seismogenic zone.