Fracture pattern analysis as a tool for constraining the interaction between regional and diapir-related stress fields: Poza de la Sal Diapir (Basque Pyrenees, Spain)

During diapir evolution the local stress field results from the interaction between diapirrelated and remote ‘regional’ stress fields. The fracture pattern hosted in the overburden can register this evolving interaction, providing significant insights for the unravelling of the diapir kinematics. In this work we present the example of the Poza de la Sal Diapir, which is a salt diapir that pierces the synorogenic deposits of the Basque Pyrenees. The integrated analysis of mesostructural data, which includes field-collected faults, joints and fracture traces digitalized on high-resolution orthophotos, allowed us to establish an evolutionary model for the stress field around diapirs. The presented data support the idea that, during diapir evolution, stress fields evolve from mostly regional-related to mostly diapir-related. Contrasting with other geological data (seismic data, geological map and cross-section), we observe that different methods provide complementary information, coherent with the proposed evolutionary model of the Poza de la Sal Diapir. The study of fracture development associated with the growth of salt diapirs is of interest for both academic and industrial purposes. In fact, fractures play an important role in fluid flow and its accumulation in salt diapir-related reservoirs (Johnson & Bredeson 1971; Koestler & Ehrmann 1987; Davison et al. 2000a, b). On the other hand, the analysis of fracture patterns is useful to constrain the host structure evolution (Thorbjornsen & Dunne 1997; Tavani et al. 2008) and, accordingly, can provide useful information for understanding diapir kinematics (Alsop 1996; Stewart 2006; Yin & Groshong 2007, Yin et al. 2009). The fracture pattern associated with salt diapirs (and in general with doming structures) is characterized by two classes of elements: radial and concentric (Fig. 1a, b; e.g. Parker & McDowell 1955; Withjack 1979; Squyres et al. 1992; Branney 1995; Malthe-Sorenssen et al. 1999; Rowan et al. 1999, 2003; Davison et al. 2000a, b; Freed et al. 2001; Stewart 2006). This pattern is documented in simple structures where the remote stress field is negligible in relation to the diapir-related stress fields. When such a condition is not verified, the interaction of these two stress fields leads to the development of more complex patterns (Ode 1957; Withjack & Scheiner 1982). Studies of interacting regional and punctual stress fields (e.g. Ode 1957) indicate that the influence of diapir-induced stress progressively reduces away from the dome (i.e. the punctual structure). As a consequence, far from the diapir the fracture pattern responds to the remote stress field and, approaching the diapir, the fracture pattern tends to be radial and concentric (Fig. 1c). Although these patterns have been observed by several authors in different structures (Withjack & Scheiner 1982; Mege & Masson 1996; Ernst et al. 2003), there are no detailed field studies focusing on this topic. In this work we present a simple 2D theoretical stress field model, where the stress tensor resulting from the interaction between remote and diapirrelated stress fields is assumed to be, for a fixed distance from the diapir centre, a 2×2 matrix provided by the sum of the horizontal components of the two stress tensors. The eigenvectors associated with this matrix are assumed to be the principal axes of the local stress field. The working hypothesis is that if the stress field changes, the azimuth of fractures (joints) developed under these stress fields also change. By understanding interactions between diapir-related and regional stress fields, we can predict the azimuth of the developed fractures. To test the validity of this theoretical model, we have applied it to a well-exposed salt diapir located at the Western Pyrenees (Poza de la Sal Diapir). This is a near-circular diapir with a mainly carbonate overburden affected by a complex pattern of fractures. Results of this comparison indicate that, despite its simplicity, the provided stress model is a useful tool for understanding the kinematics of salt diapirs. Accordingly, in this paper we provide From: Alsop, G. I., Archer, S. G., Hartley, A. J., Grant, N. T. & Hodgkinson, R. (eds) 2012. Salt Tectonics, Sediments and Prospectivity. Geological Society, London, Special Publications, 363, 521–532. http://dx.doi.org/10.1144/SP363.25 # The Geological Society of London 2012. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics guidelines to obtain a fracture pattern using the theoretical model presented to complement other techniques of investigation such as field work or 3D seismic interpretations.