[S-CG52] Dynamics in mobile belts

[SCG52-10] Experimental verification of densification without shear-induced fabric along plate boundary décollement

*Yuzuru Yamamoto¹, Yudai Suzuki², Yuhei Kurimoto³, Feng Zhang², Hide Sakaguchi⁴ (1.Kobe University, 2.Nagoya Institute of Tecnology, 3.Shimizu Corporation, 4.OPRI, Sasagawa Peace Foundation)

Keywords:Plate boundary-mega thrust, Consolidation test, Ocean drilling

The initiation of décollement at plate boundaries is a fundamental geological process that makes mountain belts, and controls subduction zone geometries. ODP (Ocean Drilling Program) and IODP (Integrated Ocean Drilling Program 2003-2013; International Ocean Discovery Program after 2013) expeditions have penetrated the shallow portions of décollements at plate boundaries and identified unique fabrics within. That is, the shallow parts of décollement zones at Nankai (off Muroto Peninsula, Japan) and Costa Rica (off Nicoya Peninsula) are composed of fragmented mudstone which possess a higher bulk density. Although the surfaces of fragments are represented by slickenlines and narrow zones of clay-minerals with preferred orientations, the core of the fragments is characterized by random fabrics. The identification of these characteristics is quite notable, if contrary, because it requires densification to occur without the formation of shear-induced fabrics within highly sheared fault zones. Similar fabrics are also reported from ancient shear zones at plate boundaries such as the Northern Apennines, Italy and Boso Peninsula, Japan. Our hypothesis is that weakly cemented intervals developed at our reference sites, and these intervals were sensitive to the fluctuations of effective stress that occurred due to earthquakes or fluid flow along plate boundaries. In fact, at some of the aforementioned reference sites décollement-zones have equivalent horizons that are characterized by weak inter-granular cementation.
To verify the hypothesis, we established a new-type of experimental apparatus which achieves fluctuations in consolidation stress of up to 5 Hz and overburden pressures up to 80 MPa. Then we conducted classical consolidation and dynamic consolidation tests and examined the fabric changes before and after the experiments using Anisotropy of Magnetic Susceptibility (AMS). The former test applied up to 80 MPa (0.01%/min strain rate) and obtained consolidation yielding stress (Pc). The latter applied 0.94 Pc ± 0.68 Pc (0.5Hz), with the number of fluctuations applied being 1000, 5000, and 10000. Specimens were taken from just above the décollement zone (IODP Hole C0023A) and its equivalent interval at a the the reference site (ODP Hole U1173A).
After the classical consolidation tests, specimens exhibited obvious flattening fabrics due to consolidation under uniaxial conditions. By comparison, after the dynamic tests, specimens retained their original fabric despite the accumulated strain being greater than 10%. More interestingly, overconsolidation states are apparent under the original confining pressure and are observed after the experiments. Therefore, we suggest that densification with a random fabric is a characteristic of the shallow part of décollement zones, and is produced by fluctuation of effective stress during the primary accretion processes.