Growth of exfoliation joints and near-surface stress orientations inferred from fractographic markings observed in the upper Aar valley (Swiss Alps)

Granitic rock mass of the upper Aar valley (Grimsel area, Switzerland) contains distinct generations of exfoliation joints, which formed during different stages of the Pleistocene, subparallel to distinct glacial valley palaeotopography. The bulk of exfoliation joints shows prominent, common fractographic features: (1) radial plumose structures with distinct plume axes; (2) arrest marks superimposed by plumose striations; and (3) gradually-developing en échelon fringe cracks.Multiple arrestmarks reveal that exfoliation joints formed incrementally and, togetherwith the absence of hackle fringes, suggest stable, i.e., subcritical fracturing conditions. Smooth transitions fromplumose structures on the parent plane to en échelon fringe cracks, combinedwith nonsystematic stepping senses of fringe cracks, suggest local (vs. temporal) stress field variations. Assuming that plume axes formed parallel to the maximum principal compressive stress (σ₁) enables us to infer near-surface palaeostress orientations and compare them with classical borehole-based in-situ stress data. The majority of plume axes suggest (1) persistently subhorizontal to slightly inclined σ₁ orientations at trough valley slopes and (2) near-surface variability of σ₁ orientations originating from topographic perturbation caused by glacial valley erosion superimposed on the regional stress field. Our investigations of fracture surface morphologies yield unique insights into exfoliation fracture formation, such as directional trends of fracture propagation and associated palaeostress orientations within Alpine valley slopes.