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 identif ied 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 hemipe lagic sediments, whereas the lower portions (Unit II) of the cores are comp osed of mud-supported breccia. Distinct decrease in porosity from Unit II t o Unit I indicates that the pore fluid pressure for mud diapirism was chang ed after expulsion on the sea floor due to the extruding conditions associa ted with dewatering and degassing. The Late Eocene to Pleistocene planktoni c foraminifera found in the clasts, and calcareous nannofossils from muddy matrix in Unit II suggest that the various-aged surrounding rocks of the di apiric vents were torn off and taken into the mud diapir via hydraulic frac turing under intrusive conditions. Based on the planktonic oxygen isotope s tratigraphy, 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 diap iric activity in these mud volcanoes. The findings of this study reveal the detailed behavior of mud diapirs from first stage (intruding) to final sta ge (extruding) in diapirism. Although these mud volcanoes are located near sites, the activities show a wide age spectrum. Therefore, it is possible t hat there is a characteristic relationship between the mud diapir developme nt and the tectonic setting of this region. (C) 2000 Elsevier Science B.V. All rights reserved.