Fault zone architecture and deformation processes within evaporitic rocks in the upper crust

Recently, in the Northern Apennines, geophysical data have identified the Triassic Evaporites (TE, anhydrites and dolomites) as the source region of the major extensional earthquakes of the area (M-6). In order to characterize fault zone architecture and deformation processes within the TE, we have studied exhumed evaporite-bearing normal faults within the upper crust. The structure of large displacement (>100 m) normal faults is given by 1) a zoned fault core with a wider portion of fault-parallel foliated Ca-sulphates (ductile deformation), overprinted by an inner fault core (IFC) of localized brittle deformation, and 2) wide (dolostones) to absent (Ca-sulphates) damage zones of fault fracture patterns. Fault rock assemblage within the IFC is characterized by fault breccia, gouge, and cataclasites of different grain size. Most of the deformation within the IFC is localized along thin and fault parallel principal slip surfaces (PSS) made of dolomite-rich fine-grained cataclasite. SEM analyses show an evolution from Ca- to St- to gypsum-rich mineralization, due to episodic fluid flow events channeled along the fault zones during different stages of fault exhumation. The development of the observed fault geometry can be explained by a mechanical fault evolution model where initial faulting occurs along broad and ductile shear zones within the anhydrites and causes fracturing within the dolostones. Progressive deformation within the fault core leads to the development of fault parallel dolomite-rich cataclastic layers. Their reactivation coupled with transient fluid overpressures can produce embrittlement and localization of brittle deformation within the IFC. Copyright 2008 by the American Geophysical Union