There are numerous examples of fault-controlled, so-called hydrothermal dolomite (HTD), many of which host economic mineral deposits or hydrocarbons, but there remains a lack of consensus as to how they form. In particular, multiple phases of diagenetic overprinting can obscure geochemical fingerprints. Study of a Cenozoic succession with a relatively simple burial history here provides new insights into the development of differentially dolomitized beds. The Hammam Faraun fault (HFF) block within the Suez Rift, Egypt, hosts both massive and stratabound dolostone bodies. Non-fabric-selective massive dolostone is limited to the damage zone of the fault, while fabric-selective stratabound dolostone bodies penetrate nearly 2 km into the footwall. Oligo-Miocene seawater is interpreted to have been drawn down discrete faults into a deep aquifer and convected upwards along the HFF. Escape of fluids from the incipient HFF into the lower Thebes Formation led to differential, stratabound dolomitization. Once the HFF breached the surface, fluid circulation focused along the fault plane to form younger, massive dolostone bodies. This study provides a snapshot of dolomitization during the earliest phases of extension, unobscured by subsequent recrystallization and geochemical modification. Contrary to many models, stratabound dolomitization preceded non-stratabound dolomitization. Fluids were hydrothermal, but with little evidence for rapid cooling and brecciation common to many HTD bodies. These results suggest that many of the features used to interpret and predict the geometry of HTD in the subsurface form during later phases of structural deformation, perhaps overprinting less structurally complex dolomite bodies.

Post-Caledonian extension during orogenic collapse and Mesozoic rifting in the West Norway-northern North Sea region was accommodated by the formation and repeated reactivation of ductile shear zones and brittle faults. Offshore, the Late Palaeozoic-Mesozoic rift history is relatively well known; extension occurred mainly during two rift phases in the Permo-Triassic (Phase 1) and Mid-Late Jurassic (Phase 2). Normal faults in the northern North Sea, e.g., on the Horda Platform, in the East Shetland Basin and in the Viking Graben, were initiated or reactivated during both rift phases. Onshore, on the other hand, information on periods of tectonic activity is sparse as faults in crystalline basement rocks are difficult to date. KAr dating of illite that grows synkinematically in fine-grained fault rocks (gouge) can greatly help to determine the time of fault activity, and we apply the method to nine faults from the Bergen area. The K-Ar ages are complemented with X-ray diffraction analyses to determine the mineralogy, illite crystallinity and polytype composition of the samples. Based on these new data, four periods of onshore fault activity could be defined: (1) the earliest growth of fault-related illite in the Late Devonian-Early Carboniferous (>340 Ma) marks the waning stages of orogenic collapse; (2) widespread latest Carboniferous-Mid Permian (305-270 Ma) fault activity is interpreted as the onset of Phase 1 rifting, contemporaneous with rift-related volcanism in the central North Sea and Oslo Rift; (3) a Late Triassic-Early Jurassic (215-180 Ma) period of onshore fault activity postdates Phase 1 rifting and predates Phase 2 rifting and is currently poorly documented in offshore areas; and (4) Early Cretaceous (120-110 Ma) fault reactivation can be linked either to late Phase 2 North Sea rifting or to North Atlantic rifting.

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There are numerous examples of fault-controlled, so-called hydrothermal dolomite (HTD), many of which host economic mineral deposits or hydrocarbons, but there remains a lack of consensus as to how they form. In particular, multiple phases of diagenetic overprinting can obscure geochemical fingerprints. Study of a Cenozoic succession with a relatively simple burial history here provides new insights into the development of differentially dolomitized beds. The Hammam Faraun fault (HFF) block within the Suez Rift, Egypt, hosts both massive and stratabound dolostone bodies. Non-fabric-selective massive dolostone is limited to the damage zone of the fault, while fabric-selective stratabound dolostone bodies penetrate nearly 2 km into the footwall. Oligo-Miocene seawater is interpreted to have been drawn down discrete faults into a deep aquifer and convected upwards along the HFF. Escape of fluids from the incipient HFF into the lower Thebes Formation led to differential, stratabound dolomitization. Once the HFF breached the surface, fluid circulation focused along the fault plane to form younger, massive dolostone bodies. This study provides a snapshot of dolomitization during the earliest phases of extension, unobscured by subsequent recrystallization and geochemical modification. Contrary to many models, stratabound dolomitization preceded non-stratabound dolomitization. Fluids were hydrothermal, but with little evidence for rapid cooling and brecciation common to many HTD bodies. These results suggest that many of the features used to interpret and predict the geometry of HTD in the subsurface form during later phases of structural deformation, perhaps overprinting less structurally complex dolomite bodies.

A field study focusing on fracture systems in a fault linkage zone from the Suez Rift, Egypt, is presented to elucidate the role of fault linkage zones in the permeability structure of segmented normal faults in tight carbonate rocks. Fracture systems in the linking damage zone show significantly increased structural complexity compared to that typical of isolated faults. The linkage zone is characterized by high fracture frequencies and multiple fracture sets of different orientations. Notably, pervasive fracture corridors strike at high angles to the fault trend and are interpreted to have formed during the latest evolutionary stages of what is interpreted as a breached relay. The structural observations indicate that along segmented normal faults in carbonate rocks, fault linkage zones represents locations of progressively increased cross- and along-fault permeability through the stages of relay growth and breaching. Our findings, in combination with previously published work, indicate that fault linkage zones represent localized conduits not only for increased fluid flow across faults, but also (vertically) within fault zones. Appreciating this has wide-ranging implications for understanding fluid transport in carbonate rocks and other naturally fractured lithologies.

Fault-controlled dolostone bodies have been described as potential hydrocarbonbearing reservoirs. Numerous case studies have described the shape and size of these often non fabric selective dolostone bodies within the vicinity of crustal-scale lineaments, usually from Palaeozoic or Mesozoic carbonate platforms, which have undergone one or more phases of burial and exhumation. There has been little attention paid, however, to fault-strike variability in dolostone distribution or the preferential localization of these bodies on particular faults. This study focuses on dolostone bodies adjacent to the Hammam Faraun Fault (HFF), Gulf of Suez. This crustalscale normal fault was activated in the Late Oligocene, coincident with the onset of extension within the Suez Rift. Dolomitization in the prerift Eocene Thebes Formation occurred in the immediate footwall of the HFF forming two massive, non facies selective dolostone bodies, ca. 500 m wide. Facies-controlled tongues of dolostone on the margins of the massive dolostone bodies extend for up to 100 m. The geochemical signature of the dolostone bodies is consistent with replacement by Miocene seawater, contemporaneous with the rift climax and localization of strain along the HFF. A conceptual model of dolomitization from seawater that circulated within the HFF during the rift climax is presented. Seawater was either directly drawn down the HFF or circulated from the hanging wall basin via a permeable aquifer towards the HFF. The lateral extent of the massive dolostone bodies was controlled by pre-existing HFF-parallel fracture corridors on the outer margins of the damage zone of the fault. The behaviour of these fracture corridors alternated between acting as barriers to fluid flow before rupture and acting as flow conduits during or after rupture. Multiple phases of dolomitization and recrystallization during the ca.10 Ma period in which dolomitization occurred led to mottled petrographical textures and wide-ranging isotopic signatures. The localization of dolomitization on the HFF is interpreted to reflect its proximity to a rift accommodation zone which facilitated vertical fluid flow due to perturbed and enhanced stresses during fault interaction. It is possible that the presence of jogs along the strike of the fault further focused fluid flux. As such, it is suggested that the massive dolostones described in --

We investigate the permeability and flow effects of deformation bands in porous granular carbonate rocks in Malta and use results from flow simulations to discuss the practical implications of deformation bands in carbonate and siliciclastic reservoirs rocks in general. Image-and laboratory-based analyses of deformation bands show permeabilities that are 1 -2 orders of magnitude lower than the adjacent host rocks. Small-scale outcrop-based flow models (1 × 1 m) focus on the effect of deformation band on flow at the scale of individual bands. Two-phase flow simulations (water displacing oil) show that at the local scale a decrease in deformation band permeability led to increasing flow complexity, reduced and irregular waterfront propagation and reduction in sweep efficiency. A reduction in host rock permeability is associated with increased sensitivity to deformation bands. In low-permeable host rocks, a single magnitude-order reduction of deformation band permeability significantly delays flow, whereas in higher-permeable host rocks the effect is less pronounced. Hence, in some cases, deformation bands may represent a significant impediment to flow already when they are only 1 -2 orders of magnitude less permeable than host rock. Consequently, deformation bands may have greater practical implications than previously thought, particularly in reservoir rocks with moderate to low host rock permeability.