When do background fractures form?

Predicting the geometry of natural fracture networks in the subsurface is a challenging endeavor. In this study, we present a model in which burial-related stress curves are combined with the theory of sub-critical crack growth to investigate the timing of propagation of fractures that constitute the background network, i.e. not genetically related to faults and/or folds.

To predict principal stress orientations and magnitudes as a function of crustal depth, we relate the vertical stress to the density of the overburden, and the horizontal stresses are estimated by calculating the Poisson effect caused by this overburden. In addition, we assume a tectonic stress in the direction of the maximum horizontal stress. We correct the resulting values for the effects of pore fluid pressure.

Based on the orientation and magnitude of the principal stresses at various depths, we calculate the normal and shear stresses on planes ideally oriented for opening and shear fractures (perpendicular to σ₃ and at 30 degrees with σ₁, parallel to σ₂ respectively). Following linear elastic fracture mechanics, the normal and shear stresses are used to compute the stress intensity at fracture tips and estimate the related fracture propagation rate adopting sub-critical crack growth theory.

This simplified model gives insight into the relationship between depth and i) magnitudes of horizontal and vertical stresses, ii) permutations of principal stresses and associated changes of stress regimes and iii) the magnitude of fracture stresses driving fracturing.

We test our model in the Lower Cretaceous limestones of the Geneva Basin, a naturally fractured formation targeted for geothermal exploitation. The burial curve of this formation is marked by two distinct burial phases. The first is in the Late Cretaceous with maximum burial depths of +-500 m. After this, the carbonate rocks have been exhumed to the surface in the Paleogene, followed by deep burial (up to 4000 m) in the foreland of the emerging Alpes in the Miocene. Our model predicts that sub-critical fracture growth only occurred during the latest burial phase, in a reverse and strike-slip regime.  

The results are compared with analogue outcrops of the Lower Cretaceous carbonate rocks. Multiple generations of calcite veins from different mountain ranges surrounding the Geneva Basin (Jura, Vuache, Bornes Massif) are sampled for absolute dating with the U/Pb geochronology. The obtained ages confirm a change in stress regime from reverse to strike-slip in the Oligocene to Miocene times