Abstract: 

Fault zones are known to play a key role in controlling fluid migration, including hydrocarbons, at local and regional scales. In this contribution physicochemical data from veins are used to document the extent and maintenance of fluid conductivity associated with fault-damage zones in an anticlinal structure occurring in the frontal part of a foreland fold and thrust belt. Migration of fluids during major deformational events has been recorded by several generations of calcite and localized dolomite cementation in veins in marine Cretaceous carbonates of the Jabal Qusaybah Anticline (North Oman). Applying detailed petrographic and paragenetic relations, geochemical analysis (stable carbon and oxygen isotopes, strontium isotopes, and trace elements), and microthermometry, coupled with structural studies of fault zones and associated calcite veins, made it possible to link fluid circulation to the structural evolution of the Jabal Qusaybah Anticline. Based on the structural framework and fault systems, three groups of veins were distinguished. Group 1 veins are interpreted to be linked to burial and early-tectonic deformation in association with E–W extensional fault zones. Calcite cementation in this group occurred in a closed to semiclosed fluid system, which originated as a result of intraformational fluid movement along a low-connectivity fracture network. The δ13C values of these veins range from 3‰ to 3.5‰, and δ18O values vary from –10.3 to –8.1‰ V-PDB. Group 2 veins are related to syntectonic deformation along NE–SW strike–slip and incipient N–S extensional faults linked to culmination of the Jabal Qusaybah Anticline. From this stage onward, fluids correspond to progressive fault connectivity and dilation related to interaction of fault–folding systems. The cements precipitated during this stage have more variable δ13C values ranging from 0.9 to 3.2‰ and possess a wide range of δ18O values varying between –11.2 and 0.3‰ V-PDB. The cements in Group 2 veins originated from fluids characterized by a relatively broad range of precipitation temperature (75–107°C) and salinities evolving from seawater to moderately saline fluids (3.4–12.8 eq. wt% NaCl), with H2O–NaCl composition. The source of salinity is inferred to be evaporitic brines that originated from the Ara Group, and the large range of salinity is interpreted as the result of mixing between those brines and modified seawater. Group 3 veins are considered to be related to late–tectonic deformation corresponding to late fold amplification of the anticline hinge, transition from NE–SW left-lateral strike–slip to N–S extensional faulting, progressive regional and local uplift, and exhumation. Stable-isotope data support the occurrence of bacterial methanogenesis or CO2 degassing at the end of this stage. Their associated cements are characterized by higher δ13C and δ18O values compared to other groups of veins, ranging from 0.6 to 8.0‰ and –3.6 to 1.5‰ V-PDB, respectively. Fluids responsible for calcite precipitation in this group evolved from a moderate saline to a more saline (8.5–16 eq. wt. % NaCl) H2O–NaCl–CaCl2 brine. The progressive 18O enrichment relative to seawater and preceding cements and its covariation with fluid salinity in the later stage suggest the higher contribution of the evaporitic brines. This hypothesis is further supported by the increase in their 87Sr/86Sr ratios compared to both middle to Upper Cretaceous marine carbonates and other groups of veins. Although during this episode calcite cementation occurred at temperatures below 50°C, their geochemical signatures reveal that the faults were sufficiently conductive to transfer extraformational fluids late in the deformation history of this structure. The interference between strike-slip and extensional fault systems and their interaction with fold amplification is invoked to be responsible for enhanced conductivity in fault-damage zones.

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