Structurally Controlled Silica Precipitation Within Multi-Stage Fault Damage Zones During the Rifting of the Pannonian Basin

Abstract

Development of brittle fracture zones as passages for fluid migration within the shallow crust results in substantial petrophysical and rheological changes that strongly influence deformation localisation, promoting reactivation at evolved inhomogeneities in the host rock. A natural example of multi-stage fault zone evolution with different generations of deformation elements and mode of silica cementation was investigated using combined structural, burial, micropetrographic, geochemical and geochronological analyses in a sandstone predating the main rifting phase. Deformation mechanisms progressively evolved from proto-cataclasis, through advanced cataclasis connected with inhomogeneous silica cementation, to siliceous fluid-enhanced slip along discrete fault planes or vein formation; all of these processes are well correlated with burial and volcanic phases. The established relationships allowed reconstruction of the evolutionary steps within the fault zones as the initially porous sediment was structurally and diagenetically hardened and then softened, and the geometry of the fault system changed during rifting. The age of silica-associated fracture systems (syn-tectonic silica cementation) is constrained by early type deformation bands (having the same pattern as silica-associated fractures) occurring in the ~15.3 Ma pyroclastic rocks bordering the sandstone. Silica precipitation can be related primarily to structurally controlled fluid pulses and rapid cooling as fluids pass through the propagating syn-rift fractures in an initially good siliciclastic aquifer. Such large-scale hydrothermal fluid migration, resulting in tens of km² siliceous cementation, was facilitated by the onset of volcanic activity. The accompanying general increase in fluid pressure may have led to the permutation of the maximum and the intermediate principal stress axes. As a result, the early syn-rift extension switched to a transtension during the main syn-rift phase. Meanwhile, vertical axis rotations also contributed to the change in the apparent stress field, resulting in the development of a fault pattern analogous to an oblique rift. The developed fault sets, with three characteristic orientations and frequent reactivation, may have formed in relation to an inherited structural weakness zone.