ALBERT

Description: 〈span〉Reconstructing the paleofluid evolution in mature fault zones, which typically have complex structural architectures, is a challenging task because reactivation of pre-existing deformation structures and dissolution-reprecipitation processes are very abundant. Understanding why specific structural elements are preferentially mineralized and what are the factors leading to rapid fluid migration and accumulation, bears geological and economic implications, especially in seismically active fault zones. We studied the Compione Fault on the Tyrrhenian Sea side of the Northern Apennines orogenic wedge, Italy, which is a segment of the 30-km-long Northern Lunigiana high-angle extensional fault system still active today. The Compione Fault propagated from the metamorphic basement and accumulated about 1.5 km of displacement. We used structural, petrographic, isotopic, microthermometric, compositional, and organic matter analyses to constrain fluid and host rock properties during fault zone evolution. This approach allowed us to quantify the thermal anomaly in the fault zone and to infer the processes responsible for such a disequilibrium. Specifically, we show that in the fault process zone ahead of the upper fault tip, which is twice as wide as the damage zone, seismic pumping caused suprahydrostatic fluid pressures and that local dilation promoted the nucleation of a highly permeable mesh of conjugate extensional shear fractures hosting calc-silicate mineralization. The thermal difference between hydrothermal minerals in the conjugate fracture mesh and the host rock is 60−90 °C. The mineralizing fluids were deeply sourced from metamorphic reactions. Propagation of the upper fault tip caused process zone folding and incorporation into the fault damage zones. As the upper fault tip breached through shallower structural levels, it favored mixing between deep and meteoric fluids.〈/span〉

Description: Classical frictional fault reactivation models indicate that slip along misoriented fault planes is not possible under most conditions. Nevertheless, active or exhumed low-angle normal faults have been described in many settings worldwide. This discrepancy is addressed by contrasting models: (1) those proposing that low-angle normal faults result from postkinematic passive rotation of former high-angle extensional faults; and (2) those proposing that specific conditions can promote slip along misoriented fault planes. This paper describes the Tellaro detachment, a mid–late Miocene low-angle normal fault that was responsible for ~500 m of tectonic vertical thinning in the carbonate-dominated Triassic to Lower Miocene succession of the Northern Apennines, Italy. By integrating structural, petrographic, isotopic, and fluid inclusion data, we show that: (1) the main kinematic activity of the Tellaro detachment occurred between ~8 and 4 km depths and peak temperature ~190 °C; (2) dilational breccias, tens of cubic meters in volume, are frequently associated with major low-angle fault segments; (3) slip along misoriented planes was favored by elevated fluid pressures and low differential stress; and (4) the fault system was characterized by transient permeability pulses and overpressure buildups, associated with multiple fracturing and cementation events that caused the downward migration of master slip surfaces. Results presented in this study show that: (1) in a fluid-active regime, continental crustal thinning can occur for shallow values of fault dip; (2) low-angle normal faults have a great influence on fluid circulation within the upper crust; and (3) episodic permeability enhancement and destruction in detachment faults can promote overpressure buildups, triggering deformation episodes.

Description: Different remote sensing technologies, including photogrammetry and LIDAR (light detection and ranging), allow collecting three-dimensional (3D) data sets that can be used to create 3D digital representations of outcrop surfaces, called digital outcrop models (DOM). The main advantages of photogrammetry over LIDAR are represented by the very simple and lightweight field equipment (a digital camera), and by the arbitrary spatial resolution, that can be increased simply getting closer to the outcrop or by using a different lens. The quality of photogrammetric data sets obtained with structure from motion (SFM) techniques has shown a tremendous improvement over the past few years, and this is becoming one of the more effective ways to collect DOM data sets. The Vajont Gorge (Belluno Dolomites, Italy) provides spectacular outcrops of jurassic limestones (Vajont Limestone Formation) in which mesozoic faults and fracture corridors are continuously exposed. Some of these faults acted as conduits for fluids, resulting in structurally controlled dolomitization. A 3D DOM study, based on a photogrammetric SFM data set, was carried out, aimed at enabling interdisciplinary characterization and reconstruction of coupled brittle deformation and fluid flow processes. For this study we used a DOM (730 m x 360 m x 270 m) consisting of continuous triangulated surfaces representing the outcrop, textured with high-resolution images. Interpretation and modeling performed on this data set include (1) georeferencing of structural measurements and sampling stations; (2) tracing of stratigraphic boundaries, structural surfaces, and dolomitization fronts (ground-truthed); (3) correlation and extrapolation of realistic 3D surfaces from these traces; and (4) development of a 3D geological model at the scale of the Vajont Gorge, including stratigraphy, faults, dolomitization fronts, and volumetric meshes suitable for the statistical analysis of structural, diagenetic, and geochemical parameters. The DOM study highlighted the close relationship between faults and dolostone geobodies, demonstrating that dolomitization was guided by fluid infiltration along Mesozoic normal faults. In order to explore the uncertainty associated with the 3D model of irregularly shaped dolostone bodies, three different 3D dolostone geobody realizations have been modeled, providing a minimum, intermediate, and maximum estimate of the dolostone/limestone volumetric facies ratio, while honoring the field constraints.

Description: We studied a structurally oversimplified, extensional fault zone developed in poorly lithified, quartz-rich, high-porosity sandy sediments of the seismically active Crotone Basin (southern Italy). The fault zone consists of a cm-thick, discrete fault core embedded in virtually undeformed wall sediments. By combining grain size, shape, and microstructural analyses with mineralogical analyses and permeability measurements, we investigated the influence of initial sedimentological characteristics of sands on the final faulted granular products and related hydrologic properties. Faulting produces a general grain-size and porosity reduction by changing both the grain-size and shape distributions. We document a combination of intragranular fracturing, spalling, and flaking of grain edges in the fault core, which do not depend on grain mineralogy. The dominance of cataclasis, also confirmed by fractal dimensions 〉2.6, is generally not expected at a deformation depth

Description: 〈span〉〈div〉Abstract〈/div〉Reconstructing the paleofluid evolution in mature fault zones, which typically have complex structural architectures, is a challenging task because reactivation of pre-existing deformation structures and dissolution-reprecipitation processes are very abundant. Understanding why specific structural elements are preferentially mineralized and what are the factors leading to rapid fluid migration and accumulation, bears geological and economic implications, especially in seismically active fault zones. We studied the Compione Fault on the Tyrrhenian Sea side of the Northern Apennines orogenic wedge, Italy, which is a segment of the 30-km-long Northern Lunigiana high-angle extensional fault system still active today. The Compione Fault propagated from the metamorphic basement and accumulated about 1.5 km of displacement. We used structural, petrographic, isotopic, microthermometric, compositional, and organic matter analyses to constrain fluid and host rock properties during fault zone evolution. This approach allowed us to quantify the thermal anomaly in the fault zone and to infer the processes responsible for such a disequilibrium. Specifically, we show that in the fault process zone ahead of the upper fault tip, which is twice as wide as the damage zone, seismic pumping caused suprahydrostatic fluid pressures and that local dilation promoted the nucleation of a highly permeable mesh of conjugate extensional shear fractures hosting calc-silicate mineralization. The thermal difference between hydrothermal minerals in the conjugate fracture mesh and the host rock is 60–90 °C. The mineralizing fluids were deeply sourced from metamorphic reactions. Propagation of the upper fault tip caused process zone folding and incorporation into the fault damage zones. As the upper fault tip breached through shallower structural levels, it favored mixing between deep and meteoric fluids.〈/span〉

Description: Seismic slip episodically occurring along shallow creeping faults in poorly lithified sediments represents an unsolved paradox, largely due to our poor understanding of the mechanics governing creeping faults and the lack of documented geological evidence showing how coseismic rupturing overprints creep in near-surface conditions. Here we describe the signature of seismic ruptures propagating along shallow creeping faults affecting unconsolidated forearc sediments. Field observations of deformation band–dominated fault zones show widespread foliated cataclasites in fault cores, locally overprinted by sharp slip surfaces decorated by thin (0.5–1.5 cm) black gouge layers (herein, black gouge). Compared to foliated cataclasites, black gouges have much lower grain size, porosity, and permeability. Moreover, they are characterized by distinct mineralogical assemblages compatible with high temperatures (180–200 °C) due to frictional heating during seismic slip. Foliated cataclasites were also produced by laboratory experiments performed on host sediments at subseismic slip rates (≤0.1 m/s), displaying high residual friction (µ f = 0.65) and strain-hardening behavior. Black gouges were produced during experiments performed at seismic (1 m/s) slip rates, displaying low residual friction (µ f = 0.3) due to dynamic weakening. Our results show that black gouges represent a potential diagnostic marker for seismic faulting in shallow creeping faults. These findings can help understanding the time-space partitioning between aseismic and seismic behavior of faults at shallow crustal levels.

Description: The E-W–trending Jabal Qusaybah anticline, at the western termination of the Salakh arch, Oman Mountains, is characterized by a complex fault network that developed in layered Cretaceous carbonates. This network includes NE-SW left-lateral, N-S extensional, and subordinate E-W extensional fault zones. The N-S–striking extensional faults zones are roughly perpendicular to the fold axis and are best developed in the longitudinally bulged central sector of the anticlinal crest. They are likely due to along-strike outer-arc extension associated with positive fault inversion and salt migration. These extensional fault zones are confined within, and locally abut, major NE-SW left-lateral strike-slip fault zones. Extensional fault displacements range between a few decimeters and ~60 m, whereas the maximum exposed trace lengths range between a few meters and ~800 m. Narrow (~1–15-cm-thick) cataclastic fault cores are surrounded by vein-dominated damage zones as thick as tens of meters. Moreover, fault zones show widespread evidence for substantial dilation in the form of (1) dilation breccias, (2) infilling by large columnar calcite crystals and aggregates, and (3) centimeter- to meter-thick veins. Dilation breccias and calcite infillings are primarily localized at fault tips, fault overlaps, and interaction zones between strike-slip and extensional fault segments. Displacement profiles along the N-S–striking extensional fault zones indicate that they are one order of magnitude shorter than values predicted by most published displacement-length scaling laws. By analyzing fault abutting geometries, detailed vein relative chronology, 13 C and 18 O signatures, and fluid inclusion data from calcite veins and calcite fault infillings, we propose a model whereby a deep-seated, regionally sized, left-lateral strike-slip fault system that was active during anticline growth inhibited the lateral propagation of late-stage transversal extensional fault zones. Our findings show that, in this geological setting, the structural position, rather than fault displacement, is the parameter controlling the location of the more dilatants (and permeable) fault segments. Results of the present work suggest that fault intersections may be more useful than fault throw for predicting zones of enhanced vertical fluid flow in structurally complex carbonate reservoirs.

Description: Iron-oxide coloration and deposits in sandstone are significant indicators of the mobility of solutes (Fe 2+ and O 2 ) in groundwater, mainly controlled by host-rock porosity and permeability. We describe the occurrence and geometry of different types of iron-oxide deposits developed within the vadose zone along faults affecting poorly lithified, quartz-dominated, heterolithic sands in the Paraíba Basin, NE Brazil. The development of highly permeable damage zones (10 0 –10 2 Darcy) and low-permeability fault-core–mixed zones (10 –3 –10 1 Darcy) promotes the physical mixing of Fe 2+ -rich waters and oxygenated groundwater. This arrangement favors iron-oxide precipitation as meter-scale sand impregnations, centimeter- to decimeter-scale concretions, and well-cemented decimeter- to meter-thick mineral masses. The formation of hydraulically isolated compartments along hard-linked strike-slip faults promotes: (1) the development of Liesegang bands in a reaction zone dominated by pore-water molecular diffusion of O 2 into Fe 2+ -rich stagnant water, and (2) the precipitation of iron-oxide impregnations and concretions in the fault-core–mixed zone boundaries, likely by O 2 diffusion in flowing Fe 2+ -rich waters. Late-stage fault reactivation provides preferential pathways for the circulation of gravity-driven reducing fluids, resulting in localized dissolution of iron and bleaching along fractures and iron remobilization. These relationships reveal the roles of tectonic activity and near-surface sandstone diagenesis in determining preferential hydraulic pathways for the physicochemical interaction between oxygenated groundwater and iron-rich fluids. Structural setting, fault-zone architecture, and related grain-size–permeability structures determine the dominant mode of solution interaction, leading to the formation of iron-oxide Liesegang bands where O 2 diffuses into stagnant Fe 2+ -rich water, and concretions when diffusion is complemented by Fe 2+ advective flow.

Description: Natural fracture networks exert a first-order control on the exploitation of resources such as aquifers, hydrocarbons, and geothermal reservoirs, and on environmental issues like underground gas storage and waste disposal. Fractures and the mechanical stratigraphy of layered sequences have been intensively studied to unravel the relationships between bed thickness and fracture spacing, but less attention has been paid to intrabed fracturing patterns due to the intrinsic local variability of sedimentary processes and products. Among sedimentary rocks, turbidites show great lateral and vertical variability of textural characteristics and depositional facies, which are expected to strongly influence the location and density of fractures. To better understand the contribution of stratigraphic, sedimentologic, and petrophysical properties on fracture patterns, we performed a high-resolution study on a selected stratigraphic interval of jointed foredeep turbidites in the Miocene Marnoso-Arenacea Formation (Northern Apennines, Italy). Cumulative statistical relations of field and laboratory structural, sedimentologic, and petrophysical data significantly improved when analyzed at the sedimentary facies scale. In particular, for facies recording different cross-flow (i.e., longitudinal to the paleocurrents) depositional conditions within the parent turbidity currents, we observed three-dimensional anisotropies of rock hardness (i.e., uniaxial compression) that were positively correlated with normalized fracture intensities, indicating a primary sedimentary control on fracture distribution. This type of intrabed joint distribution has crucial practical implications for the lateral prediction and evaluation of mesoscale fracture patterns in turbidite sequences.

Description: 〈span〉In this work, we report the results of a multidisciplinary study describing the structural architecture and diagenetic evolution of the Rocca di Neto extensional fault zone developed in poorly lithified sandstones of the Crotone Basin, Southern Italy. The studied fault zone has an estimated displacement of ∼90 m and consists of: (1) a low-deformation zone with subsidiary faults and widely spaced deformation bands; (2) an ∼10-m-wide damage zone, characterized by a dense network of conjugate deformation bands; (3) an ∼3-m-wide mixed zone produced by tectonic mixing of sediments with different grain size; (4) an ∼1-m-wide fault core with bedding transposed into foliation and ultra-comminute black gouge layers. Microstructural investigations indicate that particulate flow was the dominant early-stage deformation mechanism, while cataclasis became predominant after porosity loss, shallow burial, and selective calcite cementation. The combination of tectonic compaction and preferential cementation led to a strain-hardening behavior inducing the formation of “inclined conjugate deformation band sets” inside the damage zone, caused by the kinematic stress field associated with fault activity. Conversely, conjugate deformation band sets with a vertical bisector formed outside the damage zone in response to the regional extensional stress field. Stable isotope analysis helped in constraining the diagenetic environment of deformation, which is characterized by mixed marine-meteoric signature for cements hosted inside the damage zone, while it progressively becomes more meteoric moving outside the fault zone. This evidence supports the outward propagation of fault-related deformation structures in the footwall damage zone.〈/span〉