Extensional faults in fine grained carbonates – analysis of fault core lithology and thickness–displacement relationships

A study of 103 extensional faults hosted by fine grained carbonates in western Sinai, Svalbard and Oman reveals that faults vary geometrically between simple cores and cores comprising fault splays, lenses, segment linkages and overlap structures. Fault core rocks are typically carbonate breccias, carbonate and shale gouge, shale smear, secondary calcite cement and veins, and host rock lenses.There is a significant scatter in the core thickness for any given displacement, but the overall pattern is that the thickness increases with displacement. This increase best fits a power law function (0.29D0.56) that describes a gradual decrease in the thickness/displacement relationship for increasing slip along faults. In more detail, the general function can be seen as the sum of two (power law) trend lines; the first representing thin localized fault cores with generally simple and planar geometry, the second representing thicker fault cores with complex geometry of lenses and overlap structures and with fault rock membranes.The studied faults show a significant change in composition and geometry from small (0–1 m), to moderate (1–10 m) and to large offset faults (10–400 m). The overall pattern is that fault initiates as fractures filled with calcite veins and thin shear fractures that hosts gouge membranes. With increased fault offset, complexity increases with breakdown of veins, more extensive fault rock membranes, and a trend towards development of lenses. When offset exceeds 100 m, cores become complex, with multiple slip zones, cemented breccia and shale smear membranes, and various types of lenses. We envision that the fault development as reflected by offset is dominated by forces (extension, compression) acting in the fault, mechanical heterogeneity of wall rocks, the core lithologies and their developing rheology, and especially geometric effects arising from fault irregularities.

Research highlights► Thickness and compositional data were collected from 103 fault cores in carbonate rocks. Data indicate that small carbonate faults are dominated by calcite cement, while breccias and shale smear dominate larger faults. Displacement-thickness relations vary over three orders of magnitude. General thickness-displacement ratio decreases for larger faults, corresponding to a power law trend with exponent of 0,6.