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Ultra) high-pressure (HP) rocks can be exhumed rapidly by subduction reversal or divergent plate motion. Recent studies show that subduction reversal can in particular occur in a divergent double subduction zone when the slab pull of one slab exceeds that of the other, shorter one, which then experiences a net upward pull. This recent hypothesis, first proposed for Triassic HP-rocks exposed in the central Qiangtang mélange belt in central Tibet, can explain the exhumation of (ultra) HP rocks through upward slab movement. However, this model lacks the support of kinematic evidence. In this study, based on the recognition of multiple deformational phases, we analyze the kinematics of the HP-bearing mélange in central Qiangtang. Based on new 40 Ar-39 Ar geochronology data and those collected from the literature, we present a temporal framework for the new observations. We recognize a switch in sense of shear between the prograde (D1) and exhumation (D2-3) paths. The change of shear sense reflects the reversal from downward to upward movement of the oceanic slab below. Early D2 represents the early exhumation stage that caused retrograde metamorphism from eclogite to blueschist facies. No magmatism occurred during this period. Continued exhumation from blueschist facies to greenschist facies resulted in D2-D3 structures. Voluminous igneous activity occurred during this stage. We suggest that subduction reversal in a divergent double subduction zone can best explain the kinematic evolution and temporal framework above. This exhumation model may provide a new perspective on the exhumation mechanism for other HP rocks around the world.

It has been well recognized that unsaturated natural loess shows significant volume contraction upon wetting due to its metastable internal structure. But the structural effect on stress–strain relationship of saturated natural (undisturbed) loess is much less explored. Few attempts have been made in proposing a constitutive model for saturated natural loess. This study presents both laboratory tests and constitutive modeling of a saturated natural loess, with special focus on the structural effect and evolution of structure damage during loading. Oedometer and drained triaxial compression tests have been carried out on undisturbed and remolded saturated loess samples. It is found that the natural soil structure has dramatic influence on mechanical behavior of loess, including the compressibility, dilatancy, and shear strength. Destructuration, which is the damage of soil structure with deformation, is observed in both oedometer and triaxial tests. A constitutive model is proposed for saturated loess based on the experimental observations. The model is established within the theoretical framework of subloading and superloading surface concepts. Destructuration of loess is assumed to be affected by both plastic volumetric and shear strain. A new method for determining the initial degree of structure is proposed. The model can reasonably predict the compression and shear behavior of both undisturbed and remolded saturated loess.

For shale gas reservoirs with the characteristics of bedding planes developed, hydrated easily and strong anisotropy of the mechanical properties, this paper couples conventional logging, image logging and real-time drilling data with laboratory rock mechanical and in-site stress tests, and establishes a wellbore stability analysis model for shale reservoirs. This analysis covers weak bedding planes, mechanical anisotropy and time effects. Besides, the effect of these above factors on the wellbore stability for horizontal wells in shale reservoirs is studied, and wellbore collapse mechanism for the shale reservoirs in Sichuan Basin is also analyzed. Based on the established wellbore stability analysis model, this paper analyses the well W-H1, the analysis results are in good agreement with real drilling data. Study and field application results indicate that weak bedding plane effects mainly affect wellbore collapse of horizontal wells for shale reservoirs, which differs from conventional sand reservoirs and carbonate reservoirs, even for the vertical wells in the shale reservoirs. The study provides a theoretical basis for W-H1 well drilling and completion design, such as drilling fluid design, wellbore structure optimization and completion method optimization, etc.

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The inherent anisotropy of soft sedimentary rock is a very important factor that influences the mechanical behavior of the rock. The confining-stress dependency of the shear stress ratio at the critical state, briefly referred to as the confining-stress dependency, is another important factor that should be taken into consideration when discussing the mechanical behavior of the rock. In this paper, based on an elasto-viscoplastic constitutive model for soft sedimentary rock (Zhang et al., 2005), a new model capable of describing both the inherent anisotropy and the confining-stress dependency of soft sedimentary rock is proposed in the framework of generalized stress space, called the t ij concept (Nakai and Mihara, 1984), and the subloading yield surface (Hashiguchi and Ueno, 1977;Hashiguchi, 1989). In order to describe the confining-stress dependency, an evolution equation for the shear stress ratio at the critical state M n is introduced. A transformed stress, proposed by Boehler and Sawczuk (1977) and first introduced into a constitutive model by Oka et al. (2002), is also adopted into the model. In order to examine the performance of the newly proposed model, triaxial compression tests for soft sedimentary rock under different loading conditions were simulated and the results have been compared with the corresponding test results obtained from the authors and other researches available in the literature. It is found that the model can describe both the inherent anisotropy and the confining-stress dependency of soft sedimentary rock using a set of parameters with a fixed value for a given material.