Depth–Strength In-Situ Stress Model for Granite in Southeastern Tibetan Plateau and Its Implications for Stress Estimation in the Upper Crust

Abstract

Extracting the effective information of a tectonic stress field from in-situ stress data is significantly important for engineering projects and solid Earth sciences but extremely difficult to perform because of the scattered distribution of stress data and the complexity of rock mechanical properties. A depth–strength in-situ stress model is proposed in this study by deriving the local maximum horizontal strain (ε_H) via an analysis of the depth trend of the maximum horizontal principal stress (S_H) for granite in the southeastern Tibetan Plateau (SETP). In this model, the depth trend of S_H is divided into three segments in accordance with the values of S_H and the variations in rock mechanical properties. The first segment (0–15 MPa) shows the complexity of S_H affected by multiple factors of local settings on the near-surface. The second segment (15–45 MPa) exhibits the slope of S_H against depth mainly caused by elastic variations against the decrease in porosity (or fracture closure) and increase in pressure. The third segment (> 45 MPa) explains the stable depth trend of S_H in relation to the increase in pressure only. The ε_H can be extracted using the second segment, and it is in the range of 4 × 10⁻⁴–6 × 10⁻⁴ in the granite area of the SETP. The depth trend of stress in the shallow layer of the crust calculated based on the extracted ε_H is consistent with that from in-situ stress measurements. Moreover, the stress distribution along the Xianshuihe fault calculated based on the extracted ε_H suggests that the values of differential stress range within 200–250 MPa at a depth range of 10–16 km, a result that agrees well with local focal depths. The proposed model connects the in-situ stress in the shallow layers with the tectonic stress environment in the deep crust.