Near-surface extensional fracturing (exfoliation jointing) in granitic rocks of the central Swiss Alps
Exfoliation joints are well-known natural fractures limited to near the ground surface. Relatively few details, however, are known about their distribution and age in the Swiss Alps. Exfoliation joints follow the landscape surface at the time of their formation; the age of the associated landscape feature then provides a maximum age of exfoliation joints. While landscape forms can change through time, exfoliation joints preserve elements of former landscape morphologies by their undisturbed orientations. The Grimsel region of the Central Alps in Switzerland is well-suited for analysing the impact of erosional episodes, and accompanying stress changes, on exfoliation joint formation in granitic rocks. Mapping above and below ground revealed that exfoliation joints are widespread and occur between valley bottoms and mountain crests within glacial (inner and hanging U-shaped trough valleys, glacial cirques, and steep mountain crests) and predominantly fluvial landforms (gently inclined linear slopes above the inner trough valleys, narrow inner-valley gorges, and steep V-shaped side gullies). Relating the mapped distribution and characteristics of exfoliation joints to identified erosional episodes and landscape features of known and inferred ages, respectively, enables us to distinguish four exfoliation joint generations in the Grimsel area, ranging from the lower Pleistocene (~1.5–1 Ma) to the Late Glacial/Holocene (<0.02 Ma). The most prominent and deepest exfoliation joint generation is associated with erosion of the inner glacial troughs of the upper Aar valley, which likely occurred during the mid-Pleistocene Revolution. The bulk of exfoliation joints shows prominent, common fracture surface markings (i.e., 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 arrest marks reveal that exfoliation joints formed incrementally and, together with the absence of hackle fringes, suggest stable, i.e., subcritical fracturing conditions. Assuming that plume axes formed parallel to the maximum principal compressive stress (σ1), active at the time of fracture formation, one can infer near-surface palaeostress orientations and compare them with classical borehole-based in-situ stress data. The results indicate complex directional trends of near-surface σ1 within the trough valleys of the Grimsel region: the majority of plume axes suggests (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. Three-dimensional, elastic numerical modelling was used to investigate stress tensors at the near surface of the topographically complex Grimsel area. The model results support our hypothesis that plumose axes form parallel to σ1 in an overall compressive (far-field) stress field and further increase our knowledge of near-surface stress orientations in Alpine settings. The model findings illustrate that superposition of topographic stresses with realistic horizontal strains reveals complex near-surface σ1-trajectories that widely follow the patterns of exfoliation fracture plumose axes. Thus, also the model results demonstrate large variations of stress orientations, which cannot be captured by small numbers of classical stress measurements. In summary, this study shows - how exfoliation joint episodes can be dated, and, conversely, - that better knowledge of the distribution of exfoliation joint sets can reveal unique insights into the morphological evolution of an Alpine valley, - that exfoliation fracture surface morphologies can be used to, e.g., infer directional trends of fracture propagation and associated palaeostress orientations within Alpine valley slopes, - that simplified numerical models support exfoliation fracture formation under compression and principal stress directions as inferred from the majority of in-situ stress measurements, and - that exfoliation fracture plumose axes can be used as a proxy for near-surface principal stress orientations Show more
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Journal / seriesFrontiers in Earth Sciences
PublisherMunich University, Geophysics, Department of Earth and Environmental Sciences
Organisational unit03465 - Löw, Simon
NotesLecture at Munich University on 12 December 2014.
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