Near-surface rock stress orientations in alpine topography derived from exfoliation fracture surface markings and 3D numerical modelling

• We investigated near-surface principal stress orientations in Alpine valleys.
• Elastic numerical modelling revealed remarkable stress rotations in Alpine valleys.
• Stress rotations are supported by exfoliation (sheeting) joint fractographic data.
• Variability in stress directions is governed by topographic stress perturbation.
• Sheeting joint plume axes are valuable proxies for principal stress directions.

The fractographic analysis of plumose axes of exfoliation fracture surfaces from a preceding study in the Grimsel region of the Swiss Alps suggested complex directional trends of near-surface (i.e., within ~100 m below ground surface) maximum principal stress (σ1) trajectories within the investigated inner-trough valleys (i.e., U-shaped valleys). The stress trajectories describe a pattern governed by local topographic variations. In situ stress measurements from the region are scarce, locally scattered, based on different methods, and, thus, difficult to interpret at regional scale. In this study, we inferred that plumose structure axes form parallel to compressive σ1, and improve our knowledge of the near-surface three-dimensional stress field in alpine settings with complex topography. We investigated near-surface stress tensors utilising three-dimensional, elastic numerical models. Our models account for morphological details of the Grimsel area at the decametre scale. We used two models with vertical boundaries aligned N–S/W–E (0°-model) and NW–SE/SW–NE (45°-model). These models allowed investigating gravitational stresses and superimposed isotropic and anisotropic compressive stresses arising from (active) tectonic shortening and/or stresses induced by exhumation (remnant stresses). Our model results illustrate that the superposition of gravitational stresses and realistic horizontal strains reveals complex near-surface stress trajectories that widely follow the patterns of exfoliation fracture plumose axes. Our models demonstrate large variations of stress orientations within the shallow subsurface, including depth levels where exfoliation fractures formed. These variations cannot be captured by a small number of local stress measurements. Our study reveals that directional data from exfoliation fracture plumose axes of Middle to Upper Pleistocene ages can be used to constrain geologically recent and current maximum principal stress directions of the shallow subsurface of up to a few hundred metres below ground.

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