Parametric and non-parametric statistical approaches to the determination of paleostress from dilatant fractures: Application to an Early Miocene dike swarm in central Japan

Several methods have been proposed for determining paleostress states from orientations of dilatant fractures such as dikes and veins. Recently a stochastic inversion method was invented to objectively estimate the principal stress axes and the stress ratio. Whether a fracture is dilated or not is controlled by the balance of the fluid pressure and the normal stress acting on it. The magnitude of normal stress depends on the fracture orientation, which causes anisotropic orientation distribution of dilatant fractures. The inversion method assumes that the orientation distribution of fractures can be approximated by a Bingham distribution, an exponential probability distribution on the unit sphere, of which symmetric axes are interpreted as the principal stress axes. However, it is unknown if the exponential type of distribution function is suitable or not. Here, we examine the distribution functions and propose two improved methods. One method uses the shifted power-law function as the shape of probability distribution, which is more flexible than the Bingham distribution and is applicable to various shapes of orientation distributions. Furthermore, an index of the driving fluid pressure can be estimated with a confidence interval. The other is a non-parametric (distribution-free) method, which can avoid the a priori assumption on the shape of distribution function without significantly losing accuracy or precision. The new methods were applied to an Early Miocene dike swarm formed during the back-arc opening of the Japan Sea. A normal-faulting stress regime with the minimum principal stress axis trending roughly perpendicular to the arc was obtained from the dikes. A moderately high stress ratio and a high fluid pressure were also estimated

Hot fluids, burial metamorphism and thermal histories in the underthrust sediments at IODP 370 site C0023, Nankai Accretionary Complex

This research used samples and data provided by the International Ocean Discovery Program (IODP). The authors are grateful to the IODP and the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT). We thank crew, drilling team, geologists and lab technicians on D/V Chikyu and the staff of the Kochi Institute for Core Sample Research for supporting operations. This work was supported by the ECORD research grant [2017 to MYT]; and the NERC grant [NE/P015182/1 2017 to SAB]. ZW acknowledges technical support provided by Colin Taylor at the University of Aberdeen. Petromod 2017 was provided by Schlumberger. VBH and KUH acknowledge funding from the Deutsche Forschungsgemeinschaft through the Cluster of Excellence, The Ocean Floor – Earth’s Uncharted Interface“ and Project Grant HE8034/1-1 2019. This is a contribution to the Deep Carbon Observatory.Peer reviewedPublisher PD

A New Method for Quality Control of Geological Cores by X-Ray Computed Tomography: Application in IODP Expedition 370

ACKNOWLEDGMENTS This research used data provided by the International Ocean Discovery Program (IODP). We are grateful to the IODP and thank crew, drilling team, geologists and lab technicians on Chikyu and the staff of the Kochi Institute for Core Sample Research for supporting IODP 370-operations. We would like to thank Lucia Mancini for handling the editorial process and the three reviewers for submitting their helpful comments and improving the manuscript.Peer reviewedPublisher PD

In-situ mechanical weakness of subducting sediments beneath a plate boundary décollement in the Nankai Trough

© 2018, The Author(s). The study investigates the in-situ strength of sediments across a plate boundary décollement using drilling parameters recorded when a 1180-m-deep borehole was established during International Ocean Discovery Program (IODP) Expedition 370, Temperature-Limit of the Deep Biosphere off Muroto (T-Limit). Information of the in-situ strength of the shallow portion in/around a plate boundary fault zone is critical for understanding the development of accretionary prisms and of the décollement itself. Studies using seismic reflection surveys and scientific ocean drillings have recently revealed the existence of high pore pressure zones around frontal accretionary prisms, which may reduce the effective strength of the sediments. A direct measurement of in-situ strength by experiments, however, has not been executed due to the difficulty in estimating in-situ stress conditions. In this study, we derived a depth profile for the in-situ strength of a frontal accretionary prism across a décollement from drilling parameters using the recently established equivalent strength (EST) method. At site C0023, the toe of the accretionary prism area off Cape Muroto, Japan, the EST gradually increases with depth but undergoes a sudden change at ~ 800 mbsf, corresponding to the top of the subducting sediment. At this depth, directly below the décollement zone, the EST decreases from ~ 10 to 2 MPa, with a change in the baseline. This mechanically weak zone in the subducting sediments extends over 250 m (~ 800–1050 mbsf), corresponding to the zone where the fluid influx was discovered, and high-fluid pressure was suggested by previous seismic imaging observations. Although the origin of the fluids or absolute values of the strength remain unclear, our investigations support previous studies suggesting that elevated pore pressure beneath the décollement weakens the subducting sediments. [Figure not available: see fulltext.]

Stochastic modeling for the stress inversion of vein orientations: Paleostress analysis of Pliocene epithermal veins in southwestern Kyushu, Japan

Plio-Pleistocene epithermal quartz veins in southern Kyushu, Japan, include gold deposits. The coherent trend of the ore veins suggests tectonic control for their formation. However, the stress regime during the formation has been controversial. To solve this problem, we improved existing methods for inferring paleostresses from vein orientations. It was assumed that veins making a cluster were formed intermittently from thermal fluids with various pressures. The present method determines stress ratio and stress axes with 95% confidence regions. The method was applied to mid Pliocene quartz veins cropping out at Hashima, southwestern Kyushu. We obtained a normal faulting regime of stress with the trend of sigma3 at 167◦ ± 10◦ and the stress ratio at 0:20 +0:13=−0:09. The low stress ratio and the lack of slickenlines and slickenfibers on vein walls suggest that the host rock was subject to a small differential stress, i.e., a weak tectonic stress, when the veins were formed