Fault-related dolomitization in the Vajont Limestone (Southern Alps, Italy): photogrammetric 3D outcrop reconstruction, visualization with textured surfaces, and structural analysis

摘要

The Vajont Gorge (Dolomiti Bellunesi, Italy) provides spectacular outcrops of Jurassic limestones (Vajont Limestone Formation) in which Mesozoic and Alpine faults and fracture corridors are continuously exposed. Some of these faults acted as conduits for fluids, resulting in structurally-controlled dolomitization of the Vajont Limestone, associated with significant porosity increase. We carried out a 3D surface characterization of the outcrops, combining high resolution topography and imaging to provide a quantitative framework for structural analysis and 3D characterisation of the dolostone geobodies, enabling interdisciplinary reconstruction of coupled brittle deformation and fluid flow processes. 3D imaging of outcrop surfaces has been carried out by means of photogrammetric techniques. This methodology has advantages with respect to Lidar-based projects in terms of arbitrary spatial resolution, quality of imagery, acquisition and processing timing and cost. The survey resulted in a “virtual outcrop” dataset (700 m x 350 m x 300 m) consisting of continuous triangulated surfaces representing the outcrop surfaces textured with high resolution images (c.f. Bistacchi et al., 2011). Interpretation and modelling work performed on this dataset include: (1) georeferencing of structural measurements and sampling stations; (2) tracing of stratigraphic boundaries, structural surfaces, and dolomitization fronts (partly performed in the field for direct comparison with outcrops); (3) correlation and extrapolation of realistic 3D surfaces from their traces; and (4) development of a 3D geological model at the scale of the Vajont Gorge, including stratigraphy, faults, dolomitization fronts, and volumetric meshes suitable for the statistical analysis of structural, diagenetic and geochemical parameters. The model allows reconstruction of subtle offsets (10-30 m range) across faults which provided the main pathways for Mg-rich fluids, but which are now heavily masked by the dolomitization process. This was particularly important in the development of a tectonic and geochemical scenario to explain dolomitization. As a further development, we plan to populate the model (e.g. as in Smith et al., 2013) in terms of: morphological features of individual structure sets (orientation statistics, mean length of fractures, displacement/length ratios for faults, etc.), and aggregated parameters of the fracture/fault network (clustering/anticlustering, terminations, interconnection, fracture density in 1D or 2D, etc.). These parameters will be used in subsequent modelling steps, including a DFN approach.ReferencesBistacchi A., Massironi M., Menegon L., 2010. Three-dimensional characterization of a crustal-scale fault zone: The Pusteria and Sprechenstein fault system (Eastern Alps). Journal of Structural Geology, 32 (12), 2022-2041, doi: 10.1016/j.jsg.2010.06.003.Bistacchi A., Griffith W.A., Smith S.A., Di Toro G., Jones R., and Nielsen S., 2011. Fault Roughness at Seismogenic Depths from LIDAR and Photogrammetric Analysis. Pure and Applied Geophysics, 168 (12), 2345-2363. doi: 10.1007/s00024-011-0301-7.Smith S.A.F., Bistacchi A., Mitchell T.M., Mittempergher S., Di Toro G., 2013. The Structure of an Exhumed intraplate Seismogenic Fault in Crystalline Basement. Tectonophysics, in press.