Abstract

Deformation characteristics of sedimentary rocks are anisotropic for their sedimentary structures. Such anisotropy further evolves with changes of the structure due to stress conditions. The same phenomena can be observed to sedimentary rocks sampled from near subduction zone because the structure of such rock changes by tectonic movements. To detect the anisotropic properties of such rock is important for predicting the movement of oceanic plate. In this study, to evaluate the deformation anisotropy of subduction zone rocks, consolidated-drained triaxial compression tests were conducted on sedimentary rocks sampled from deep seabed, 1023 m below the seafloor, in the top basement of the Philippine sea plate. The results during isotropic consolidation demonstrated anisotropic deformations because different values of the normal strains were measured in axial and circumferential directions. The orientations of maximum principal strain tend to move toward the normal orientation of Philippine sea plate. The orientations of the plane including intermediate and minimum principal strains almost agree with the dip (subduction) orientation of the plate. Values of these intermediate and minimum strains were similar. The anisotropic behaviors are the results of in-situ complex stresses owing to the tectonic movement of Philippine plate.

1. Introduction

Deformation characteristics of sedimentary rocks are anisotropic for their sedimentary structures (Amadei 1996). For example, Japanese tuff at Neogene period shows the variation of Young's moduli in bedding plane and perpendicular orientation by a factor of 1.5 -2.7 (Togashi et al. 2018b). Japanese mudstone of Kazusa layer also show the similar tendency by a factor of 0.6−2.8 (Togashi et al. 2018a). Such anisotropy further evolves with changes of the structure due to stress conditions. In a geologically active area like Japan, crustal movements as folding and faulting make the in-situ stress histories rather complicated. Especially, stress states near the subduction zone must be complicated, and the structure of sedimentary rock must show complex deformation anisotropy. To understand the tectonic movement of oceanic plate, to evaluate the deformation anisotropy of the sedimentary rock sampled in plate boundary is also important.

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