Near-surface principal stress trajectories in Alpine valleys (Grimsel region, Switzerland) as inferred from exfoliation fracture plumose structure axes and numerical modeling

In this study we will show how fractographic (i.e., fracture surface morphology) data of exfoliation fractures (i.e., fractures subparallel to landscape surfaces limited to near-surface rock masses) and three-dimensional numerical modeling can be used to infer orientations of principal rock mass stresses in topographically complex Alpine areas. Analysis of exfoliation fracture plumose axes, i.e., fractographic features that indicate main fracture propagation directions, and supposed local maximum compressive principal stress (σ1) orientations at the time of fracture formation, suggests complex directional trends of near-surface σ1 within trough valleys of the Grimsel region (central Swiss Alps). We investigated near-surface stress tensors with a threedimensional, elastic numerical model to 1. deduce evidence that plumose axes form parallel to σ1 in an overall compressive (farfield) stress field, and to 2. increase our knowledge of near-surface stress orientations in Alpine settings. Model results illustrate that superposition of topographic stresses with realistic horizontal strains reveals complex near-surface σ1 trajectories that widely follow the patterns of plumose axes. The model results demonstrate large variations of stress orientations, which cannot be captured by small numbers of classical stress measurements. In-situ stress measurements support exfoliation fracture formation under compression and principal stress directions as inferred from our numerical model.

, 2014