Mechanical Stratigraphy Controls Width of Damage Zone in Normal Faults: an Example from Carbonates of the Island of Gozo (Malta)

Fault damage zones (DZ) are fractured volumes of rock that surround the fault core(s), and their structure can have an important impact both on fault mechanics and hydraulic properties (with impact on groundwater flow, ore-deposits, hydrocarbon reservoirs, nuclear waste disposal, etc.). It is generally accepted that DZ width is related with fault displacement. However, published data on DZ thickness in faulted carbonates show a two-order-of-magnitude scatter, suggesting that this parameter is controlled also by other factors. Here we present results obtained on normal faults in platform carbonates of the Island of Gozo (Malta). Two stratigraphic units with different facies and mechanical properties - grain-dominated vs. micrite-dominated - are characterized by completely different DZ widths along the same fault (80-100 m displacement; Martinelli et al., 2017, AAPG conference, London). More competent grain-dominated carbonates show DZ thickness of several hundreds of meters, while fracturing in the less competent micrite-dominated rocks is developed only very close to the fault core, with a DZ width of a few tens of meters. In order to explain this counterintuitive facies-controlled behavior, we performed petrophysical (porosity, density, permeability) and geotechnical (uniaxial, Brazilian, triaxial tests) analyses to characterize the mechanical stratigraphy and be able to develop a numerical modelling study. This highlights the heterogeneous stress distribution in a multilayer with variable elastic and plastic parameters subject to horizontal extension. Stress is concentrated in the stiffer layers (stress channeling), and these can yield before the more compliant ones even if they are stronger. Also the width of the yielding zone is increased in the stiffer layers, leading to a wider DZ.