Ioiô Cave is a 4.7 km long maze cave in the southern tip of the Irecê Basin (Bahìa, Brazil), and although still actively forming today, it hosts signs of a long speleogenetic history. Deep rising hydrothermal fluids weathered the carbonates, creating dark ghost-rocks and quartz and dolomite veins, mainly in the anticlinal hinges and below the siliciclastic seals. This silicification, although not directly dated, is probably associated with the end of the Brasiliano-age tectono-thermal activity (Lower Cambrian) based on isotopic and trace element data and regional tectonic correlations. Since the Plio-Quaternary, the progressive exhumation of the carbonate reservoir increasingly favored the introduction of meteoric oxygen-rich water from the surface, causing sulfide oxidation at shallow aquifer depth. The CO₂ produced by Sulfuric Acid Speleogenesis (SAS) rose along fractures and degassed at shallow depth, producing carbonic speleogenesis close to the water table. This carbonic speleogenesis, probably still active, produced a maze network, by horizontal diffusion of aggressive fluids from the feeders. Surface breaching increased air flow activating degassing and supersaturation of the basins, with deposition of subaqueous calcite shelves, carved with bubble trails resulting from CO₂ degassing related to still-ongoing pyrite oxidation (localized SAS).

In this study, we present a method that uses associations of discontinuity sets to demonstrate similarities between the outcrop and the subsurface. A discontinuity association comprises up to four discontinuity sets (fractures and stylolites) that can form coeval in a single stress field, a well-known concept that is rarely applied for subsurface characterization of discontinuities. We use this concept to improve the interpretation of borehole image logs of naturally fractured geothermal reservoirs in the Geneva Basin, Switzerland. Here, the naturally fractured Lower Cretaceous pre-foredeep carbonate rocks are targeted for geothermal exploitation, and exposures of this formation are found in three mountain ranges that surround the basin. In these outcrops, the orientations of the discontinuity associations are used as paleostress indicators in order to map out principal stress trajectories of regional discontinuity-forming events that created the background discontinuity network. We document two multiscale discontinuity-forming events that formed prior to Alpine fold-and-thrusting and thus constitute the regional-scale background network. Given the regional character of these events, we predict that the target reservoir is impacted by them as well. This prediction is subsequently used to isolate the background-related discontinuities on image logs from two boreholes that penetrate the target reservoir in the Geneva Basin. This analysis reveals that ∼ 45 % of the observed discontinuities can be understood in the framework of the regional-scale background. In this way, we demonstrate that defining discontinuity associations in outcrops is a powerful tool to predict the geometry of natural discontinuity networks in the subsurface and subsequently can be used to develop geothermal exploitation strategies in naturally fractured reservoirs.

Sandstones from the Main Buntsandstein Subgroup represent a promising deep geothermal target in the subsurface of the Netherlands considering their widespread distribution and temperatures locally reaching 140-150 °C at depths of ~ 3 to 5 km. The Main Buntsandstein Subgroup is a sand-prone interval, but the reservoir quality of these sandstones is known to be heterogeneous as result of an interplay between depositional and diagenetic processes. This makes the Buntsandstein sediments an uncertain and risky geothermal play.

In this project, we assess the syn- and post-depositional history of these sediments. The aim is to define structural, sedimentary, and diagenetic heterogeneities within the Main Buntsandstein sediments and assess their impact on reservoir quality. This will help reduce uncertainties for geothermal operations in the Triassic in the southern Netherlands and beyond.

The structural analysis of the study area using seismic and well data reveals that the Main Buntsandstein sediments represent an early syn-rift sequence and that their present-day distribution is strongly controlled by faulting. In parallel, the study of the sedimentology and stratigraphy conducted on core and wireline data indicates that the depositional environment evolves through the Buntsandstein stratigraphy, resulting in the development of different reservoir architectures. Diagenesis has largely altered the primary relationship between sedimentary facies and porosity and permeability. Overall cementation seems to have a larger impact on reducing reservoir quality than compaction, with quartz, dolomite, and illite representing the most abundant types of cement. The analysis of fractures using core and image logs suggests that the fracture density is driven by the lithological variability within the Main Buntsandstein and that fracture joints and stylolites locally may contribute to enhancing the system permeability.

The integrated assessment of the results allows the development of prospect play maps for the Buntsandstein in the southern Netherlands, addressing uncertainties and providing future recommendations for further exploration and optimizing geothermal operations in the Triassic.

Aiming to contribute to the energy transition, this study provides an integrated picture of the geothermal system hosted in the West Netherlands Basin and shows how the reconstruction of the basin's geological history can contribute to the correct exploration and exploitation of its geothermal resources. In the West Netherlands Basin, the main geothermal targets are found in the Cretaceous and Jurassic strata that were deposited during the rifting and post-rifting stages and were deformed during the subsequent basin inversion. Despite multiple studies on the tectonic setting, the timing and tectono-stratigraphic architecture of the rift system and its overall control on the development and evolution of geothermal systems are still to be fully deciphered. In this study, a detailed seismo-stratigraphic interpretation of the syn- and post-rift intervals in the West Netherlands Basin will be given within the framework of geothermal exploration. A recently released and reprocessed 3D seismic cube is used, covering a large portion of the onshore section of the basin. We identified two major Jurassic rifting episodes and a Late Cretaceous inversion event. During the Jurassic rifting phases, the compartmentalization of the basin and the creation of accommodation space led to the deposition of the Late Jurassic Nieuwerkerk Formation, which is the main regional geothermal producing target. Within this formation, we individuate growth synclines located in the central portions of the Jurassic half-grabens as sites that show good potential for geothermal exploration.

Fracture networks play a critical role in fluid flow within reservoirs, and it is therefore important to understand the interactions and influences of these networks. Our study focuses on the Southern Chotts–Jeffara Basin, which hosts reservoirs within Triassic, Permian and Ordovician units containing significant hydrocarbon accumulations. Recent developments on the structural understanding of the basin have proved that a regional shortening phase occurred between the Permian and Jurassic, forming open folds and a distributed fracture network. Analysis of late Paleozoic and Mesozoic outcrops within the basin has identified several sets of fractures (with dip directions and dip angles of 150/80 and 212/86) and compressional structural features that support this shortening hypothesis. We have integrated fracture data from surface analogues and subsurface analysis of advanced seismic attributes and well data through structural linking to form a 2D hybrid fracture model of the reservoirs in the region. Through analytical aperture modelling and numerical simulation, we found that the fractures orientated 212° in combination with large-scale fractures contribute significantly to the fluid-flow orientation and potential reservoir permeability. Our presented fracture workflow and framework provide an insight into network characterization within naturally fractured reservoirs of Tunisia, and how certain structures form fluid pathways that influence flow and production.

In this study, a re-evaluation is performed of the well data of the NLW-GT-01 and VAL-01 wells in the Lower Triassic sandstones in the West Netherlands Basin. Core, geophysical and image logs, are compared to document the characteristics of natural fractures distribution, and investigate the geological parameters influencing their development. The main control on the distribution is identified. Natural fractures are concentrated in heterolithic lithological intervals of the VAL-01 and NLW-GT-01 wells. The identification of more fractures in VAL-01 compared to NLW-GT-01 can be explained by the difference in basin location. VAL-01 was located in the centre of the basin where distal playa environments produced fine-grained material alternating with coarser sands. The more proximal NLW-GT-01 was dominated by fluvial sands. The lithological variability produces Young’s modulus variability that seems to be driving increased fracture density rather than that the absolute value of the Young’s modulus. These fluctuations could be the result of concentrated compressional stress, which is supported by the existence of stylolites, indicative of high compressional stresses in the same intervals. The identification of controls on fracture distribution in the targeted stratigraphic interval can be used to optimize the planning of future geothermal doublets and to de-risk upcoming operations.

Southern Tunisia is known to be less deformed and simpler than its neighboring Atlassic domain to the north. This area is complex and basin evolution in the Southern Chotts-Jeffara (SCJ) basin is debated. In this paper we combined surface and subsurface data with low temperature thermochronology (LTT) to reinvestigate the tectono-sedimentary evolution of the SCJ basin from Permian to Jurassic. We reconstruct the present-day architecture of the SCJ basin along two regional sections. In these sections, we focused mainly on regional thickness variations and on internal reflections interpreted from seismic data. We observe three structural elements: (a) A Paleozoic culmination, oriented E-W, capped by Mid-Upper Triassic deposits; (b) the Tebaga of Medenine (ToM), a culmination also oriented E-W but located ∼50 km north of the Paleozoic culmination; and (c) A Triassic culmination in the eastern part of the area, oriented NW-SE. We note the absence of major normal faults along the sections. The LTT data we present are the first published in this area and allow to reconstruct the timing and magnitude of vertical movements. These data prove: (a) exhumation at ∼230 Ma of the Permian and Lower Triassic units associated with the onset of the ToM removing locally about 900 m of pre-Cretaceous sediments; and (b) the development of the Triassic culmination ∼180 Ma removing 2000 m of pre-Cretaceous sediments in the Jebel Rehach. This study demonstrates that vertical movements in the SCJ basin are controlled by long-wavelength processes developed essentially in shortening regimes.

Fractured and karstified carbonate units are key exploration targets for the hydrocarbon industry as they represent important reservoirs. Furthermore, large water reserves and geothermal systems are hosted in carbonate aquifers. This paper documents the relationships between stratigraphy, structural patterns, silicification, and the spatial-morphological organization of a 3D multistorey cave system developed in a Neoproterozoic mixed carbonate-siliciclastic sequence. We found that the combination of lithology, silicification, fracture patterns (controlled by lithostratigraphic variability), and petrophysical properties control the formation of high or low permeability zones; their distribution was fundamental for the spatial organization of dissolution and the compartmentalization of the resulting conduit system in different speleogenetic storeys. We propose a deep-seated hydrothermal origin for the fluids involved in the main phases of karst formation. Warm and alkaline hydrothermal fluids caused silica dissolution, followed by chalcedony and quartz reprecipitation in pore space and fractures. Rising fluids concentrated along through-going vertical fracture zones in the lower storey, whereas sub-horizontal bedding-parallel fluid flow was focused on sedimentary packages containing highly silicified dolostones (SiO₂>80 wt%) characterized by high permeability. The Calixto Cave is an enlightening example for the complex speleogenetic history affecting a mixed carbonate-siliciclastic succession where the combined effect of silicification and hydrothermal karst dissolution can potentially generate high-quality reservoirs.

Fracture networks are abundant in subsurface applications (e.g., geothermal energy production, CO₂ sequestration). Fractured reservoirs often have a very complex structure, making modeling flow and transport in such networks slow and unstable. Consequently, this limits our ability to perform uncertainty quantification and increases development costs and environmental risks. This study provides an advanced methodology for simulation based on Discrete Fracture Model approach. The preprocessing framework results in a fully conformal, uniformly distributed grid for realistic 2D fracture networks at a required level of precision. The simplified geometry and topology of the resulting network are compared with input (i.e., unchanged) data to evaluate the preprocessing influence. The resulting mesh-related parameters, such as volume distributions and orthogonality of control volume connections, are analyzed. Furthermore, changes in fluid-flow response related to preprocessing are evaluated using a high-enthalpy two-phase flow geothermal simulator. The simplified topology directly improves meshing results and, consequently, the accuracy and efficiency of numerical simulation. The main novelty of this work is the introduction of an automatic preprocessing framework allowing us to simplify the fracture network down to required level of complexity and addition of a fracture aperture correction capable of handling heterogeneous aperture distributions, low connectivity fracture networks, and sealing fractures. The graph-based framework is fully open-source and explicitly resolves small-angle intersections within the fracture network. A rigorous analysis of changes in the static and dynamic impact of the preprocessing algorithm demonstrates that explicit fracture representation can be computationally efficient, enabling their use in large-scale uncertainty quantification studies.

The Songwe geothermal prospect is situated in western Tanzania in the Rukwa Rift of the western branch of the East African Rift System. Thermal springs discharge along NW–SE oriented fracture zones in two separate areas: in the main Songwe graben (Iyola, Main springs, Rambo and Kaguri) and eastern Songwe graben (Ikumbi). Lithologies forming and filling the Songwe graben are metamorphic gneiss and shist as basement rocks, overlain by the Karoo sandstones, and Red sandstones, both silt- and sandstones with a carbonatic matrix. In some areas of the graben, volcanic rocks intruded these formations forming basalt outflows. The discharge temperatures of springs are between 37 and 85 °C with Na-HCO₃ type fluids. Carbonate deposits surround most of the springs. Using previous geophysical, geological studies and historical fluid geochemical data and mineral data, the Songwe geothermal system interpretation was updated, including new reservoir fluid temperature, fluid flow pathway and water–rock interaction models. The classical geothermometers of K-Mg and Na-K-Ca _{(Mg correction)} were used to predict the reservoir fluid temperature and show that fluid emerging in the Songwe area reaches temperatures between 125 and 148 °C. Reservoir fluid characteristics are reconstructed based on the geothermometer calculation and a PHREEQC model in which the deep fluid reacts with certain lithologies. Minerals precipitating at the surface and reservoir depth were used to calibrate the models. The models run at surface temperature were calibrated with minerals precipitating around the springs and suggest that Songwe thermal fluids interact with Red sandstone only, while Ikumbi spring water is the only spring that interacts with all lithologies (simplified referred to as: metamorphic rocks, Karoo and Red sandstone). The model run at reservoir temperature indicates that rising water is also in contact with Karoo sandstones and Ikumbi spring water composition is again influenced by the contact with all lithologies in the graben. Our conceptual model summarizes all data showing the meteoric origin of the fluids, the travel through the basement, rising along the Mbeya fault and the main reaction with sandstones through a lateral travel towards the hot springs. The proposed models reinforce the idea that carbonate dissolution from the sandstone layers is the most common water–rock interaction. Our model is supported by carbonate deposition observed in all springs, dominated by HCO₃ and Na.

The southern Chotts basin (SCB), Central Tunisia, has shown hydrocarbon potential since the end of the 1980s. This basin records a complex structural history which appears decoupled at the Hercynian or Variscan unconformity. The Paleozoic series is deformed by short to medium wavelength folds (kilometres-multi kilometres scale) and by steep normal faults. The Mesozoic series is largely less deformed. The evolution of the basin through time is still a matter of debate as the preserved Paleozoic series is fragmented (e.g. affected by erosions). In this paper, we proposed a reconstruction of the vertical movements affecting the basin and an evaluation of their magnitude. Using basin modelling techniques, we provided new insights on the possible thermal evolution of the basin that might be used in the future exploration phases. This study was completed by structural restorations allowing the reconstruction of the paleogeography of the basin at the time of deposition of principal reservoir formations.

The present study used a multitool approach to characterize fractures of several orders of magnitude in large fracture corridors, caves, and canyons to investigate their impact on fluid flow in carbonate units. The study area is the Brejões carbonate karst system that is located in the Neoproterozoic Salitre Formation in the Irecê Basin, São Francisco Craton, Brazil. The approach included satellite imagery, used for interpreting the regional structural context, Unmanned Aerial Vehicle (UAV) and ground-based Light Detection And Ranging (LiDAR) imagery, used for detailed structural interpretation. Regional interpretation revealed that fracture corridors, caves and canyons occur along a N–S-oriented anticline hinge. An advanced stage of karstification caused fracture enlargement and intrabed dissolution, and the formation of caves and canyons. A river captured by the highly fractured zone along the anticline hinge played an important role as an erosive agent. Detailed characterization of fracture corridors comprised structural analysis, topological studies, persistence estimations, power-law fitting of fracture trace length distributions, and identification of network backbones. Our results indicate that fracture corridors comprise four subvertical fracture sets: N–S and E-W and a conjugate pair, NNE-SSW and NW-SE. Fractures observed in the caves show the same dominant directions. Fracture directions are consistent with a common origin associated with the anticline folding. Fracture traces range from 1.0 m to 300 m, comprising both subseismic (<50 m) and seismic scale fractures (>50 m). Networks have dominance of node terminations Y and X (notably Y), C_B values higher than 1.8, high P₂₀ and P₂₁ persistence values, and highly interconnected backbones. Fracture network connectivity is associated with power-law exponents greater than 2.5 for the fracture trace distributions, indicating large influence of subseismic-scale fractures on fluid flow. As the final result of folding and karstification, large volumes of secondary macroporosity were created, particularly in the zone of maximum fracture intensity around the hinge zone of the anticline. This scenario can be used to understand better oil reservoirs formed in similar structural controls in near-surface conditions.

Evaporite mobilisation in evaporite-cored anticlines leads to topographic growth that can alter sedimentary routing in shallow marine environments. This paper analyses two evaporite-cored anticlines perpendicular to the NW Africa coast to understand how their tectonic evolution influenced sediment pathways during the Early to Middle Jurassic and Early Cretaceous exhumation of the Mesozoic margin hinterland. The Essaouira-Agadir Basin in Morocco underwent evaporite deposition during the Atlasic and Atlantic rifting. Subsequent loading and tectonics resulted in re-mobilisation and generation of a variety of evaporite structures. Structural data obtained from analysis of Google Earth images processed using Move 2D allowed derivation of the thickness of sedimentary units on both flanks of the structures. Integrated with sedimentary logs from nine locations around evaporite-cored structures and two onshore wells, this data constrains the depositional record and allow an assessment of siliciclastic flux in the Essaouira-Agadir Basin. The results show the importance of syn-sedimentary evaporite tectonics on basin morphology, which influences the distribution of clastics delivered by fluvial systems and deepwater processes into the basin. This paper constrains the evolution of the Amsittène and Imouzzer anticlines in the Early to Middle Jurassic and support that their growth is associated with halokinesis. Mesozoic syn-sedimentary fold growth would have provided an important control on sediment discharge pathways and on the location of sediment entry points on the shelf margin, that ultimately fed deep water fan systems. This knowledge is key to predict the reservoir presence on the eastern margin of the Central Atlantic Ocean and to some extent, its conjugate margin in Nova Scotia.

Rock fractures organize as networks, exhibiting natural variation in their spatial arrangements. Therefore, identifying, quantifying, and comparing variations in spatial arrangements within network geometries are of interest when explicit fracture representations or discrete fracture network models are chosen to capture the influence of fractures on bulk rock behaviour. Treating fracture networks as spatial graphs, we introduce a novel approach to quantify spatial variation. The method combines graph similarity measures with hierarchical clustering and is applied to investigate the spatial variation within large-scale 2-D fracture networks digitized from the well-known Lilstock limestone pavements, Bristol Channel, UK. We consider three large, fractured regions, comprising nearly 300 000 fractures spread over 14 200 m2 from the Lilstock pavements. Using a moving-window sampling approach, we first subsample the large networks into subgraphs. Four graph similarity measures – fingerprint distance, D-measure, Network Laplacian spectral descriptor (NetLSD), and portrait divergence – that encapsulate topological relationships and geometry of fracture networks are then used to compute pair-wise subgraph distances serving as input for the statistical hierarchical clustering technique. In the form of hierarchical dendrograms and derived spatial variation maps, the results indicate spatial autocorrelation with localized spatial clusters that gradually vary over distances of tens of metres with visually discernable and quantifiable boundaries. Fractures within the identified clusters exhibit differences in fracture orientations and topology. The comparison of graph similarity-derived clusters with fracture persistence measures indicates an intra-network spatial variation that is not immediately obvious from the ubiquitous fracture intensity and density maps. The proposed method provides a quantitative way to identify spatial variations in fracture networks, guiding stochastic and geostatistical approaches to fracture network modelling.

Multiphase mass and heat transfer are ubiquitous in the subsurface within manifold applications. The presence of fractures over several scales and complex geometry magnifies the uncertainty of the heat transfer phenomena, which will significantly impact, or even dominate, the dynamic transport process. Capturing the details of fluid and heat transport within the fractured system is beneficial to the subsurface operations. However, accurate modeling methodologies for thermal high-enthalpy multiphase flow within fractured reservoirs are quite limited. In this work, multiphase flow in fractured geothermal reservoirs is numerically investigated. A discrete-fracture model is utilized to describe the fractured system. To characterize the thermal transport process accurately and efficiently, the resolution of discretization is necessarily optimized. A synthetic fracture model is firstly selected to run on different levels of discretization with different initial thermodynamic conditions. A comprehensive analysis is conducted to compare the convergence and computational efficiency of simulations. The numerical scheme is implemented within the Delft Advanced Research Terra Simulator (DARTS), which can provide fast and robust simulation to energy applications in the subsurface. Based on the converged numerical solutions, a thermal Péclet number is defined to characterize the interplay between thermal convection and conduction, which are the two governing mechanisms in geothermal development. Different heat transfer stages are recognized on the Péclet curve in conjunction with production regimes of the synthetic fractured reservoir. A fracture network, sketched and scaled up from a digital map of a realistic outcrop, is then utilized to perform a sensitivity analysis of the key parameters influencing the heat and mass transfer. Thermal propagation and Péclet number are found to be sensitive to flow rate and thermal parameters (e.g., rock heat conductivity and heat capacity). This paper presents a numerical simulation framework for fractured geothermal reservoirs, which provides the necessary procedures for practical investigations regarding geothermal developments with uncertainties.

Caves developed in carbonate units have a significant role in fluid flow, but most of these subsurface voids are below seismic resolution. We concentrated our study on four caves to determine the roles of fractures and folds in the development of karst conduits that may form flow pathways in carbonate reservoirs. We performed structural field investigations, petrographic analyses, and geometric characterization using Light Detection and Ranging (LIDAR) for caves in Neoproterozoic carbonates of the Salitre Formation, central part of the São Francisco Craton, Brazil. We found that the conduit shape, usually with an ellipsoidal cross-section, reflects the tectonic features and textural variations. Carbonate layers containing pyrite and low detritic mineral contents are generally karstified and appear to act as favorable flow pathways. Our results indicate that the development of the karst system is related to fracture corridors formed along parallel and orthogonal sets of fold hinges, which provide preferential pathways for fluid flow and contribute to the development of super-K zones. This study provides insights into the prediction of subseismic-scale voids in carbonate reservoirs, with direct application for the hydrocarbon and hydrogeology flow and storage.

The positive impact that natural fractures can have on geothermal heat production from low-permeability reservoirs has become increasingly recognised and proven by subsurface case studies. In this study, we assess the potential impact of natural fractures on heat extraction from the tight Lower Buntsandstein Subgroup targeted by the recently drilled NLW-GT-01 well (West Netherlands Basin (WNB)). We integrate: (1) reservoir property characterisation using petrophysical analysis and geostatistical inversion, (2) image-log and core interpretation, (3) large-scale seismic fault extraction and characterisation, (4) Discrete Fracture Network (DFN) modelling and permeability upscaling, and (5) fluid-flow and temperature modelling. First, the results of the petrophysical analysis and geostatistical inversion indicate that the Volpriehausen has almost no intrinsic porosity or permeability in the rock volume surrounding the NLW-GT-01 well. The Detfurth and Hardegsen sandstones show better reservoir properties. Second, the image-log interpretation shows predominately NW-SE-orientated fractures, which are hydraulically conductive and show log-normal and negative-power-law behaviour for their length and aperture, respectively. Third, the faults extracted from the seismic data have four different orientations: NW-SE, N-S, NE-SW and E-W, with faults in proximity to the NLW-GT-01 having a similar strike to the observed fractures. Fourth, inspection of the reservoir-scale 2D DFNs, upscaled permeability models and fluid-flow/temperature simulations indicates that these potentially open natural fractures significantly enhance the effective permeability and heat production of the normally tight reservoir volume. However, our modelling results also show that when the natural fractures are closed, production values are negligible. Furthermore, because active well tests were not performed prior to the abandonment of the Triassic formations targeted by the NLW-GT-01, no conclusive data exist on whether the observed natural fractures are connected and hydraulically conductive under subsurface conditions. Therefore, based on the presented findings and remaining uncertainties, we propose that measures which can test the potential of fracture-enhanced permeability under subsurface conditions should become standard procedure in projects targeting deep and potentially fractured geothermal reservoirs.

This study combines multiscale analyses of geological, fault, fracture, and stable isotope data to investigate strike-slip deformation and channeling of hydrothermal fluids along the Cafarnaum fault and calcite veins at different distances from the fault, which is a structure in the São Francisco Craton, northeastern Brazil. Meteoric fluids with δD values near −45‰ and δ¹⁸O values near −6.5‰ and temperatures at 40–70 °C precipitated as calcite veins in the host carbonate units. The Cafarnaum fault, a N-S-striking vertical, ~170 km long fault zone, juxtaposes Neoproterozoic carbonate rocks in the western block and Mesoproterozoic siliciclastic rocks in the eastern block. A zone of restraining bends occurs at the central part of the fault, whereas termination zones of horsetail geometry occur at both ends of the Cafarnaum fault. These zones are marked by NW-SE-striking extensional faults that are oblique to the main N-S-striking fault zone, where hydrothermal deposits occur. The zone of influence of the Cafarnaum fault is ~ 20 km wide around the main fault. The fault formed during the Brasiliano orogeny (740–560 Ma) after Neoproterozoic carbonate platform deposition. In contrast with the host units, fluids along the fault zone originated in deeper levels of the crust and show much lower δ¹⁸O values, indicating higher crystallization temperatures. These fluids caused brecciation in the Neoproterozoic carbonate host rocks, whereas a subsequent decrease in fluid pressure and cooling near the surface resulted in the precipitation of a hydrothermal paragenesis in veins, also affecting the host rock.

Diagnostic morphological features (e.g., rectilinear seafloor scarps) and lateral offsets of the Upper Quaternary deposits are used to infer active faults in offshore areas. Although they deform a significant seafloor region, the active faults are not necessarily capable of producing large earthquakes as they correspond to shallow structures formed in response to local stresses. We present a multiscale approach to reconstruct the structural pattern in offshore areas and distinguish between shallow, non-seismogenic, active faults, and deep blind faults, potentially associated with large seismic moment release. The approach is based on the interpretation of marine seismic reflection data and quantitative morphometric analysis of multibeam bathymetry, and tested on the Sant’Eufemia Gulf (southeastern Tyrrhenian Sea). Data highlights the occurrence of three major tectonic events since the Late Miocene. The first extensional or transtensional phase occurred during the Late Miocene. Since the Early Pliocene, a right-lateral transpressional tectonic event caused the positive inversion of deep (>3 km) tectonic features, and the formation of NE-SW faults in the central sector of the gulf. Also, NNE-SSW to NE-SW trending anticlines (e.g., Maida Ridge) developed in the eastern part of the area. Since the Early Pleistocene (Calabrian), shallow (<1.5 km) NNE-SSW oriented structures formed in a left-lateral transtensional regime. The new results integrated with previous literature indicates that the Late Miocene to Recent transpressional/transtensional structures developed in an ∼E-W oriented main displacement zone that extends from the Sant’Eufemia Gulf to the Squillace Basin (Ionian offshore), and likely represents the upper plate response to a tear fault of the lower plate. The quantitative morphometric analysis of the study area and the bathymetric analysis of the Angitola Canyon indicate that NNE-SSW to NE-SW trending anticlines were negatively reactivated during the last tectonic phase. We also suggest that the deep structure below the Maida Ridge may correspond to the seismogenic source of the large magnitude earthquake that struck the western Calabrian region in 1905. The multiscale approach contributes to understanding the tectonic imprint of active faults from different hierarchical orders and the geometry of seismogenic faults developed in a lithospheric strike-slip zone orthogonal to the Calabrian Arc.

The Lilstock outcrop in the southern Bristol Channel provides exceptional exposures of several limestone beds displaying stratabound fracture networks, providing the opportunity to create a very large, complete, and ground-truthed fracture model. Here we present the result of automated fracture extraction of high-resolution photogrammetric images (0.9 cm/pixel) of the full outcrop, obtained using an unmanned aerial vehicle, to obtain a spatially extensive, full-resolution map of the complete fracture network with nearly 350,000 ground-truthed fractures. We developed graph-based functions to resolve some common issues that arise in automatic fracture tracing such as incomplete traces, incorrect topology, artificial fragmentation, and linking of fracture segments to generate geologically significant trace interpretations. The fracture networks corresponding to different regions within the outcrop are compared using several network metrics and the results indicate both inter- and intra-network (layer to layer) structural variabilities. The dataset is a valuable benchmark in the study of large-scale natural fracture networks and its extension to stochastic network generation in geomodelling. The dataset also highlights the intrinsic spatial variation in natural fracture networks that can occur even in weakly-deformed rocks over relatively short length scales of tens of metres.