Mud diapirs observed in two piston cores from the landward slope of the northern Ryukyu Trench, northwestern Pacific Ocean

Two piston cores were collected from the cones of two mud volcanoes identified by a side-scan sonar survey east of Tanega-shima Island on the landward slope of the northern Ryukyu Trench to clarify the internal nature of the mud diapir. The upper part (Unit I) of both cores consists of normal hemipelagic sediments, whereas the lower portions (Unit II) of the cores are composed of mud-supported breccia. Distinct decrease in porosity from Unit II to Unit I indicates that the pore fluid pressure for mud diapirism was changed after expulsion on the sea floor due to the extruding conditions associated with dewatering and degassing. The Late Eocene to Pleistocene planktonic foraminifera found in the clasts, and calcareous nannofossils from muddy matrix in Unit II suggest that the various-aged surrounding rocks of the diapiric vents were torn off and taken into the mud diapir via hydraulic fracturing under intrusive conditions. Based on the planktonic oxygen isotope stratigraphy, AMS C-14 measurements and tephrochronology, Unit I represents continuous deposition of hemipelagic mud after similar to 38 and similar to 17 ka to the two cores, respectively, and marks the time cessation of diapiric activity in these mud volcanoes. The findings of this study reveal the detailed behavior of mud diapirs from first stage (intruding) to final stage (extruding) in diapirism. Although these mud volcanoes are located near sites, the activities show a wide age spectrum. Therefore, it is possible that there is a characteristic relationship between the mud diapir development and the tectonic setting of this region. (C) 2000 Elsevier Science B.V. All rights reserved.