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This study investigates hydrothermal silicification along rift border faults in the Cretaceous Rio do Peixe Basin, Brazil, through field geology, petrography, mineralogy, and fluid inclusion microthermometry. We distinguish three main structural-diagenetic domains: (1) a non-silicified fault damage zone, (2) a silicified damage zone composed of deformation bands and centimeter-scale quartz veins in the hanging wall and footwall, respectively, and (3) a silicified fault core with meter-scale wide quartz veins and mosaic breccia textures. We recognize three main events of silicification in the fault zone. The first silicification event occurs after the widespread development of deformation bands in the hanging wall damage zone that may reach 270 m in width. During subsequent deformation events, multiple fracturing episodes were accompanied with precipitation of the second quartz generation in hydraulic mosaic breccias and the third quartz cement in vein stockworks. Shallow burial conditions of faulting (<2 km), combined with fluid inclusion homogenization temperatures of the second quartz generation, indicate that silicification occurred in a hydrothermal system characterized by fluid temperatures in the range of 71–391 °C (mean value of 141 °C). We conclude that hydrothermal silicification was imprinted during the late synrift stage, when the rift border faults were already established. This contribution has implications for understanding how structural processes may control the geometrical, petrographic and petrophysical attributes of hydrothermal products. • Syn-Kinematic silicification occurs along rift border faults. • Three generations of hydrothermal silicification were recognized. • The second quartz generation confirms hydrothermal fluids having an average temperature of 141 °C. • Silicification occurred during late rifting and subsequent fault reactivation. [ABSTRACT FROM AUTHOR]

Abstract In this contribution we show the variation of the uniaxial compressive strength (UCS) and petrophysical properties of different architectural elements of fault zones affecting poorly lithified coarse sandstone and conglomerates of the Rio do Peixe basin, NE Brazil. We worked on three distinct outcrops; (1) non-deformed (protolith); (2) single fault zone, presenting moderate strain; and (3) complex fault zone, presenting a high strain intensity, where multiple and well developed fault cores occurs. To characterize the structural domains, we performed scanlines and uniaxial geomechanical survey across deformation bands and clustering zones. We combined geomechanical data with porosity data acquired thru image analysis and gas expansion analysis. We acquired data on deformation band cores and zones between deformation bands (i.e. , deformed zones that do not present deformation bands). Our results show that the complex fault zone exhibits a strong increase on the rock resistance, thus presenting UCS values three times higher than those obtained on the single fault zone outcrop and four times higher when compared with the non-deformed outcrop. It suggests that there is a direct relation between strain and rock resistance. The porosity is also strongly affected by the deformation, exhibiting an inverse relation with the strain intensity. Complex fault zone presents up to 75% of porosity reduction, while single fault zone, only about 10% of reduction. The petrophysical properties were calculated in the complex fault zone, where the resistance of the DBs was as high as twice that of the zone without bands, which had effects on the Young's and the incompressibility's moduli. The results suggest that even though DBs generate up to a 40% increase in the UCS and a 75% decrease in the porosity, the structural domains where the rock is located strongly controls the geomechanical and petrophysical properties, thus, directly affecting the anisotropy of a reservoir. [ABSTRACT FROM AUTHOR]

Abstract The objective of this study is to analyze the influence of brittle reactivations of continental-scale Precambrian ductile shear zones on the evolution of deformation band damage zones. Our results indicate that the Rio do Peixe Basin was formed by the Cretaceous reactivation of its basement EW- and NE-SW-striking major fault segments. These fault segments are composed of a footwall with Precambrian crystalline mylonites and a hanging wall with Cretaceous fine to coarse sandstones. In the hanging wall, deformation bands occur in poorly sorted, fine to very coarse sandstones. These deformation bands occur as single bands or clusters mostly within damage zones ~84 m from the basement faults and are widespread in structural highs. Our data plot slightly above the medium fault displacement vs. the damage zone width solution but falls within the point cloud of global patterns. The slip surfaces strike EW and NE-SW and are parallel or slightly oblique to the basement rift faults. The kinematics of slip surfaces are consistent with those of the reactivated basement faults. In addition, a logarithmic decrease in deformation band frequency occurs away from the basement slip surface. The deformation band frequencies peak close to the fault core with values as high as 22–48 bands/m. The lithology grain size also influences deformation as clusters of deformation bands and slip surfaces are only observed in coarse sandstones. This new case of deformation bands in damage zones of basement faults is characterized by a strong attitude and geometric and kinematic inheritance but follows the deformation band frequency of global patterns. Highlights • We analyze the evolution and the behavior of deformation bands (DB) in damage zones. • Precambrian ductile shear zones reactivated as rift faults in the early Cretaceous. • DBs occur as single or clusters close to master rift faults. • DB frequencies peak close to the fault core to as much as 22–48 bands/m. • DB are widespread in a structural high bounded by breached ramps. [ABSTRACT FROM AUTHOR]

Hydrothermal silicification and deformation bands influence the physical properties of porous siliciclastic rocks. However, the impacts generated by the coexistence of these two processes on fault zone flow properties, such as porosity and permeability reductions, are still debatable. We integrated structural, geomechanical, and petrophysical data to investigate the influence of hydrothermal silicification on the physical properties of a fault zone. The fault affects Precambrian crystalline rocks and porous sandstones in the Rio do Peixe Basin, northeastern Brazil. The results indicate that quartz cementation is confined to a halo along the fault. Silicification decreases away from the fault zone toward the basin, which we subdivided into three main zones: (1) nonsilicified sandstone – Zone I, (2) moderately silicified sandstone – Zone II, and (3) intensely silicified sandstone – Zone III. The elongated geometry, the thickness of the silicified body, the proximity to the fault, and the high silica cement concentration indicate an external silica source. Nevertheless, we also propose internal sources related to feldspar dissolution. The primary porosity is rarely preserved in Zone III, while the secondary porosity is filled with silica cement. The primary sandstone porosity is well-preserved in Zone II, while many dissolved feldspar grains are not filled with cement. The petrographic and petrophysical analysis (cataclastic matrix amount) indicates that cement precipitation also occurred inside the deformation bands. Silicification increased the uniaxial compressive strength (UCS) by one order of magnitude in the fault damage zone relative to the protolith. On the other hand, the porosity and permeability in the silicified fault zone exhibit a reduction of two and four orders of magnitude, respectively, relative to the undeformed and nonsilicified protolith. This study indicates that silicification along a fault zone has crucial implications for the reservoir properties of siliciclastic rocks, strongly reducing the fluid flow properties and increasing the UCS values. [Display omitted] • We recorded hydrothermal silicification in a deformation band fault zone at the Rio do Peixe Basin, Brazil • We assessed how hydrothermal silicification affects the physical properties of arkosic sandstone in a fault zone • Hydrothermal silicification splits the fault zone into non-silicified, moderately silicified, and intensely silicified zones • Percolation of hydrothermal fluid and precipitation of silica cement occur within deformation bands and adjacent host rocks • Silicification increases UCS by 65% and decreases porosity-permeability by up to 2-4 orders of magnitude in fault zones [ABSTRACT FROM AUTHOR]