Extensional Faults Research Articles

The implications of structural control on the miocene carbonate reservoirs of Bakr-Al Hamd oil fields, Gulf of Suez

The present work aims to provide a tectonostratigraphic model of the Miocene carbonate reservoirs accumulated in Bakr-Al-Hamd ridge to help unlock an estimated statistical yet-to-find over 10 MMbbl of oil. The structural ridge is located in the western central Gulf of Suez and the hydrocarbon exploration within this NE-dipping structural high began in 1951. The model integrated several interpreted 3-D seismic volumes and their attributes, a complete set of well-log data, borehole images, and the resultant extensional structures from the natural and physical models. The structural model proposed the following: (1) a major gulf-parallel curved and faulted detachment between the two differentially strained sections of Miocene and Pre-Miocene sediments which were distinguished by seismic attributes and confirmed by borehole images, (2) the synthetic Gulf-parallel faults which represent footwall collapsing structural style of extensional faults, delineated Al Hamd Miocene Nullipore carbonate reservoir, (3) south-westward dislocation of the western gulf-parallel boundary fault of Al Hamd Nullipore facies and its allocation at the present-day shoreline, (4) three classification of the Miocene carbonate reef were interpreted; fringe reef in Bakr ridge, barrier reef in Al-Hamd, and patch reef in the intra-field. The achievements of the present study prompted exploration activity and two discoveries were announced in 2021 and 2022 in the vicinity of Bakr and Al-Hamd intra-fields. The recent discoveries penetrated more than 200 m of Miocene carbonate reef and dolomitic reservoirs accumulated on the detachment surface. The present study workflow could be used in similar petroliferous rift basins to maximize hydrocarbon resources, enhance production performance, and revive brownfields.

Behavior of Amagmatic Orogenic Geothermal Systems: Insights From the Agua Blanca Fault, Baja California, Mexico

AbstractAmagmatic geothermal systems within regional‐scale orogenic faults are promising renewable resources for heat and possibly electricity production. However, their behavior needs to be better understood to improve their exploration and assessment of energy potential. To provide more insight, we report geochemical, geological, and geophysical studies from seven hot spring sites strung along a 90 km segment of the Agua Blanca Fault, which traverses a mountainous region of northern Baja California, Mexico. Our results show that topographic heads drive infiltration of meteoric water deep into basement rocks, where it is heated according to the local geothermal gradients. Long paths lead to long water residence times and high 3He/Hetotal fractions. The hot water ascends along preferentially permeable zones within the ABF, discharging at temperatures from 37°C in inland springs to 102°C on the Pacific coast. Higher discharge temperatures correlate positively with the degree of extensional fault displacement (a proxy for fault permeability). Correlations between hydraulic head gradients, residence times, and 3He/Hetotal of the thermal waters show that the hydraulic head gradient controls the length and depth of the flow paths, whereas the magnitudes and locations of the discharge sites are controlled by fault permeability. Optimal conditions at the coast allow the 120°C temperature threshold for electricity production to be reached at relatively shallow depths (<2 km), demonstrating the potential of orogenic geothermal systems not only for exploitation of hot discharging water but also for EGS exploitation of the hot rocks that surround the water upflow zones.

New model of linkage evolution for the transtensional fault systems in the Nanpu Sag of Bohai Bay Basin: Insight from seismic interpretation and analogue modelling

The evolution of faults within the same stress field is frequently influenced by numerous factors, involving the reactivation of pre-existing structures, stress transmission through ductile detachment layers, and the growth, interaction, as well as linkage of new fault segments. This study analyses a complex multi-phase oblique extension fault system in the Nanpu Sag (NPS) of the Bohai Bay Basin (BBB), China. High-resolution three-dimensional (3D) seismic data and analogue modelling indicate that the oblique extensional reactivation of pre-existing structures governs the sequential arrangement of fault segments in the caprock, and they dip synthetically to the reactivated fault at depth. During the NW–SE extension in the Eocene, the predominant movement of the pre-existing fault is strike-slip. Subsequently, during the N–S extension since the Oligocene, inclined at 20° to the pre-existing fault, forming splay fault segments and ultimately creating large en-échelon arcuate faults linked by relay ramps. Using fault throw-distance (T-D) and laser scanning, we reconstructed the fault evolution model of oblique extension reactivation in the presence of a ductile detachment basement. Our study illustrates that the arcuate faults can be categorized into linear master fault segments controlled by pre-existing structures, bending splay faults in the termination zone, and normal fault segments responding to the regional stress field. The interaction between faults occurs among normal faults and strike-slip faults, and the kinematic unification of the two fault systems is accomplished in the intersection zone. As the faults continue to evolve, the new fault segments tend to relinquish the control of pre-existing structures and concentrate more on the development of planar and continuous major faults. The ductile detachment layer significantly contributes to the uniform distribution of strain, resulting in narrow shear zones and discontinuous normal faults in its absence.

Seeps and Tectonic Structure of the Hydrothermal System of the Panarea Volcanic Complex (Aeolian Islands, Tyrrhenian Sea)

High-definition bathymetry mapping, combined with the measurement of dissolved benthic fluxes and water column biogeochemical properties, allows for a description of new biogeochemical processes around the Panarea Volcanic island. Investigations focused on the CO2 releases from the bottom sea on the east of the Panarea volcanic complex provided insights into the geological setup of the marine area east and south of the Panarea Island. Between the Panarea Island and the Basiluzzo Islet lies a SW-NE-stretching graben structure where a central depression, the Smoking Land Valley, is bounded by extensional faults. Abundant acidic fluids rich in dissolved inorganic Carbon are released on the edges of the graben, along the extensional faults, either diffusely from the seafloor, from hydrothermal chimneys, or at the center of craters of different sizes. The precipitation of iron dissolved in the acidic fluids forms Fe-oxyhydroxides bottom sea crusts that act as a plug, thus preventing the release of the underlying gases until their mounting pressure generates a bursting release. This process is cyclic and results in intermittent gas release from the bottom, leaving extinct craters and quiescent chimneys. The measurement of dissolved benthic fluxes allowed us to estimate the volcanic DIC venting at 15 Mt of CO2 over the past 10,000 years. The fluxes are not distributed homogeneously but rather concentrate along fractures and fault planes, which facilitate their rise to the seafloor. The acidic fluids released affect the chemical properties and structure of the water column through the formation of layers with a lower pH under the pycnocline, which can limit volcanic CO2 release to the atmosphere. Further and continuous monitoring and investigation of the area are needed in order to complete a thorough picture of the variations in fluid releases through time and space. The importance of such monitoring lies in the development of a new method for detecting and quantifying the diffusive dissolved benthic fluxes on a volcanic sea bottom affected by hydrothermal seeps.

Structural architecture and maturity of Val d’Agri faults, Italy: Inferences from natural and induced seismicity

The Val d’Agri Basin is a Quaternary sedimentary basin topping multiple tectonic units of the southern Apennines fold-and-thrust belt and a giant oilfield within deeper Apulian Platform carbonates. This basin is bounded by the seismically active East Agri (EAFS) and Monti della Maddalena (MMFS) extensional fault systems. The reservoir rocks are sealed and separated from shallower thrust sheets by a clay-rich and overpressured mélange. The role of this mélange during fault evolution at shallow crustal levels is widely debated and perhaps underestimated. Here, through multi-scale structural analyses and U–Pb dating of syn-tectonic calcite mineralizations, we gain new insights into the Val d’Agri fault system architecture, their structural maturity, and their relations with both natural and induced seismicity. Consistent with present-day NE-SW crustal stretching, the macro-scale structural architecture of both EAFS and MMFS is controlled by NW-SE and NE-SW fault sets, which displaced and in part re-sheared inherited pre- and syn-orogenic structures. The lack of evident clustering of meso-scale faults and the radial pattern of related slickenlines suggest that polygonal-like faulting occurred, particularly along the EAFS, due to lateral spreading of the Irpinia mélange in the subsurface. Structural data show that the MMFS is characterized by a higher structural maturity (slip longevity), with calcite U–Pb ages indicating the onset of long-lasting extensional tectonics in Early-Middle Miocene time. The original results are discussed in terms of seismotectonic setting of the study area, emphasizing the role played by both the thickness and spatial distribution of plastic mélange in modulating fluid pressure and seismic faulting.

Fold localization at pre-existing normal faults: field observations and analogue modelling of the Achental structure, Northern Calcareous Alps, Austria

Abstract. Within the Northern Calcareous Alps (NCA) fold-and-thrust belt of the Eastern Alps, multiple pre-shortening deformation phases have contributed to the structural grain that controlled localization of deformation at later stages. In particular, Jurassic rifting and opening of the Alpine Tethys led to the formation of extensional basins at the northern margin of the Apulian plate. Subsequent Cretaceous shortening within the Northern Calcareous Alps produced the enigmatic Achental structure, which forms a sigmoidal transition zone between two E–W-striking major synclines. One of the major complexities of the Achental structure is that all structural elements are oblique to the Cretaceous direction of shortening. Its sigmoidal form was, therefore, proposed to be a result of forced folding at the boundaries of the Jurassic Achental basin. This study analyses the structural evolution of the Achental structure through integrating field observations with crustal-scale physical analogue modelling to elucidate the influence of pre-existing crustal heterogeneities on oblique basin inversion. From brittle–ductile models that include a weak basal décollement, we infer that oblique shortening of pre-existing extensional faults can lead to the localization of deformation at the pre-existing structure and predicts thrust and fold structures that are consistent with field observations. Consequently, the Achental low-angle thrust and sigmoidal fold train was able to localize at the former Jurassic basin margin, with a vergence opposite to the controlling normal fault, creating the characteristic sigmoidal morphology during a single phase of NW-directed shortening.

Active Shortening Simultaneous to Normal Faulting Based on GNSS, Geophysical, and Geological Data: The Seismogenic Ventas de Zafarraya Fault (Betic Cordillera, Southern Spain)

AbstractThe central Betic Cordillera, southern Spain, is affected by an uplift related to the NNW–SSE Eurasia‐Nubia convergence and shallow ENE–WSW orthogonal extension accommodated by the extensional system of the Granada Basin. The combination of geophysical, geodetic, and geological data reveals that the southwestern boundary of this extensional system is a seismically active compressional front extending from the W to the SW of the Granada Basin. The near‐field Global Navigation Satellite System data determine NNE–SSW shortening of up to 2 mm/yr of the compressional front in the Zafarraya Polje. In this setting, the normal Ventas de Zafarraya Fault developed as a result of the bending‐moment extension of the Sierra de Alhama antiform and was last reactivated during the 1884 Andalusian earthquake (Mw 6.5). The uplift in the central Betic Cordillera together with the subsidence in the Western Alborán Basin may facilitate a westward to southwestward gravitational collapse through the extensional detachment of the Granada Basin. The heterogeneous crust of the Betic Cordillera would generate the compressional front, which is divided into two sectors: thrusting to the west, and folding associated with buttressing to the south. Our results evidence that basal detachments, linking extensional fault activity with compressional fronts, may determine the activity of local surface structures and the geological hazard in densely populated regions.

Application of wide-field electromagnetic method for favorable target optimization in the Heishan granite geothermal area of Yunnan Province

The Heishan geothermal area is positioned above the sole Yunnan–Tibet high-temperature geothermal belt, where huge geothermal energy resources are available. Utilizing the characteristics of large exploration depth, extensive coverage, and high precision of the wide-field electromagnetic method, four survey lines were deployed, totaling 29.8 km, to enable a comprehensive analysis of the granite structure and fault distribution. The results indicate that the rocks within the area can be vertically divided into a granite basement and a fractured layer. Moreover, three different zones of resistivity were identified: the granite basement zone has a resistivity range of 2500–20000 Ω m, whereas the compressional shear zone and the secondary fault zone have a resistivity range of 750–2500 Ω m, and the extensional fault zone and the main fault zone have resistivity values below 750 Ω m. The Heishan-Hejian fault and the fault zone formed during its right-lateral strike-slip process, as well as the controlled area of the Qianmaihe fault, are all favorable targets. Of these, the Qianmahe fault possesses a larger-scale heat-conducting and water-controlling structure. In addition, in the secondary fault-controlled area extending to the west, the zone between F2’ and F4 exhibits renewed tectonic activity, suggesting a greater potential for geothermal resources.

Structural inheritance through 3D reservoir modeling and ant-tracking attribute implications for structural configuration of Upper Cretaceous Pab Sandstone, Kirthar Fold Belt, Pakistan

This research presents a comprehensive investigation to enhance understanding of subsurface structural complexities within the Upper Cretaceous Pab Sandstone reservoir level in a gas-producing field within the Kirthar Fold Belt of the Southern Indus Basin, Pakistan. The seismic interpretation for accurate structural delineation and characterization of intricate fault systems is often challenging in Fold and Thrust Belt settings. The 3D structural maps of Upper Cretaceous Pab Sandstone at the reservoir level and the application of the ant-tracking attribute for fault extraction improve the structural understanding of the gas-producing field in the Kirthar Fold Belt in the Southern Indus Basin, Pakistan. Attribute-assisted 3D seismic interpretation has revealed several subsurface structure elements, including a large thrusted anticline extended towards the north-south, the pattern of north-south oblique ramp thrusts on the southeastern flank, and a combination of easterly vergent thrusts with a counter-back thrust creating a local pop-up structure in the area. A 3D structural model at the reservoir level was generated using horizon and fault framework volume-based modeling approach, serving as a structural model for Pab sandstone. The structural smoothing, variance and ant-tracking attribute were applied iteratively to get the best results and correlated with manual interpretation. 3D seismic interpretation and application of ant-tracking for fault extraction identifies discrete structural styles and revealed that all thrusting occurred due to compressional tectonics of the Plio-Pleistocene period. The apparent different styles result from the reactivation of earlier extensional fault systems and probably during phased periods of compression. The resulting 3D reservoir model can be used as an input to populate petrophysical properties and results can be extended for future field development plans. Integration of well and 3D seismic data interpretation and the application of automatic fault extraction techniques are highly recommended in structurally complex areas and have an equal implication for worldwide basins with similar conditions for reliable structural interpretation in Fold and Thrust Belts with structural complexity.

Assessing the Activity of Eastern Himalayan Extensional Structures: Evidence from Low-Temperature Thermochronology of Granitic Rocks from Yadong

The east–west-trending South Tibetan Detachment System (STDS) and north–south-trending rifts (NSTRs) are the two main types of extensional structures that have developed within the Tibetan Plateau during continent–continent collision since the early Cenozoic. They have played significant roles in the evolution of the plateau, but it is unclear how they are related genetically. In the Yadong area of the eastern Himalaya, the NSTRs cross-cut the STDS. Apatite and zircon fission track ages of a leucogranite pluton in the footwall of the two extensional faults can be used to reconstruct the cooling and exhumation history and thereby constrain the activity of extensional structures. The new AFT ages range from 10.96 ± 0.70 to 5.68 ± 0.37 Ma, and the ZFT age is 13.57 ± 0.61 Ma. Track length distributions are unimodal, albeit negatively skewed, with standard deviations between 1.4 and 2.1 µm and mean track lengths between 11.6 and 13.4 µm. In conjunction with previously published datasets, the thermal history of the region is best explained by three distinct pulses of exhumation in the last 16 Ma. The first pulse (16–12 Ma) records a brittle slip on the STDS. The two subsequent pulses are attributed to the movement on the Yadong normal fault. The normal fault initiated at ~12 Ma and experienced a pulse of accelerated exhumation between 6.2 and 4.7 Ma, probably reflecting the occurrence of two distinct phases of fault activity within the NSTRs, which were primarily instigated by slab tear of the subducting Indian plate.

Magma‐Assisted Continental Rifting: The Broadly Rifted Zone in SW Ethiopia, East Africa

AbstractThe Gofa Province and Chew Bahir Basin in the Broadly Rifted Zone (BRZ) between the southern Main Ethiopian Rift (sMER) and the northern Kenya Rift (nKR) record early volcanism and associated faulting in East Africa; however, the spatiotemporal relationships between volcanism and faulting remain poorly constrained. We applied apatite (U‐Th)/He (AHe) and zircon (U‐Th)/He (ZHe) thermochronometry to Neoproterozoic basement rocks from exhumed footwall blocks of the extensional Gofa Province and Chew Bahir Basin, and analyzed our result in the context of well‐dated regional volcanic units in the BRZ to unravel the interplay between tectonic exhumation, faulting and volcanism. Single‐grain AHe ages ranging from 1.0 to 136.8 Ma were recorded in 32 samples, and single‐grain ZHe ages from three samples range between 142.2 and 335.6 Ma. The youngest AHe ages were obtained from the Chew Bahir Basin and the narrow deformation zone in the Gofa Province. Our thermal modeling results reflect little or no significant regional crustal cooling prior to extensive volcanism, which started at about 45 Ma. Conversely, new and previously published thermal history models suggest that widespread crustal cooling related to regional extension occurred between ∼27 and 20 Ma. Thermal modeling results from subsets of samples indicate that following this initial diffuse extensional deformation, renewed exhumation occurred along a narrow zone within the Gofa Province and the Chew Bahir Basin during the middle to late Miocene (15‐6 Ma) and Pliocene (<5 Ma), respectively. The crustal cooling phases follow a regional trend in volcanic episodes. For example, initial cooling between 27 and 20 Ma corresponds with the end of widespread flood‐basalt volcanism (45–28 Ma), suggesting that spatially diffuse normal faulting may have initiated shortly after the emplacement of voluminous and areally extensive flood basalts. The Miocene and Pliocene shifts in deformation along the Mali‐Dancha and Bala‐Kela basins in the Gofa Province and the Chew Bahir Basin, respectively, may indicate strain localization during the late stage of rifting and ongoing tectonic interaction between the sMER and the nKR. Our results support the notion of crustal weakening by massive volcanism as a precursor to widespread extensional faulting, and thus offer further insights into magma‐assisted deformation processes in the East African Rift System.

Inherited extensional faults and bed-parallel slip in Mae Moh Basin, Thailand with implication for basement fabrics

The 3D seismic analysis with structural reconstruction and outcrop study in Mae Moh mine, located in the Central Sub-basin of the Mae Moh Basin, unveil multiple extensional phases and the controls of pre-existing basement fabrics on extensional faults, bed-parallel slip (BPS) and post-rift folding. The coexistence of orthogonal and oblique extensional fault systems characterizes the mine area as a result of regional E-W extension in the Middle Miocene. The dominant NNE-striking normal faults comprise easterly dipping domino-style faults with minor western dipping antithetic faults. Left-stepping en-echelon arrays of NNE-to NNW-striking normal faults at coal levels in the 3D survey follow the deeper early-rift fault that bends along strike from NNE to NW direction. Locations of intra-basin fault bends and offset of basin-bounding faults reflect E-W to ENE-WSW striking basement fabrics that potentially activate during rifting as a transfer zone across the Mae Moh Basin. The focus of rifting migrates eastward with up to 56% extension in the mine. The initial depocenter of early rift phase and the deepest economical coal seams at the center of the post-rift syncline align above the transfer zone. The presence of BPS is observed at outcrop scale, characterized by the offsetting of normal faults or fault zones with antithetic movement. BPS develops during the late rift phase under a NW-SE extension preceding the Late Miocene-Pliocene inversion.

Structural evolution of the North Himalaya domes as revealed by crustal-scale seismic-reflection surveying

As the typical products of collisional orogeny, gneiss domes are important geological units with which to decipher the crustal deformation and evolutionary history of continental collision. However, their formation mechanisms remain poorly understood. This issue is well illustrated by the debate surrounding the origin of the North Himalaya gneiss dome zone, which has been attributed to middle-crustal channel flow, thrust-duplex development, extensional detachment faulting, or diapiric flow related to partial crustal melting. These models predict different internal structures within individual domes that can be tested by high-resolution seismic imaging. Here, we present newly acquired seismic-reflection data collected along an ∼120-km-long north-south traverse across the central segment of the North Himalaya gneiss dome zone. Analysis and interpretation of the seismic data constrained by surface geology observations imply that (1) the subducting Indian lower crust is decoupled from the deformed middle and upper crust in the North Himalaya, (2) a crustal-scale stack of antiformal duplexes with a structural thickness of ∼35 km defines the cores of the gneiss domes imaged by the seismic survey, and (3) highly reflective, sheetlike bodies imaged in our seismic profile are best interpreted as leucocratic intrusions developed synchronously during gneiss dome development. As a whole, our work suggests that the North Himalaya gneiss dome zone was created by coeval crustal shortening and partial melting of orogenic crust.

Mode of extension and final architecture of the south Angolan margins controlled by Precambrian lithospheric heterogeneity

We use offshore 2-3D seismic reflection and potential field data coupled with field work to investigate the control of lithospheric heterogeneity and shear zone rejuvenation on the mode of extension and the architectural segmentation of the southern Angola margins. The pre-rift basement is inherited from several Precambrian deformation events ended by the Pan-African/Brasiliano orogen that accreted four different age-dependent lithospheric domains with contrasted thickness, fabrics, thermal structure, and chemical composition. From north to south, the continental crust is composed of Neoproterozoic metapelites, Paleoproterozoic gneisses and migmatites and Archean greenstone belts pervasively intruded by TTG granitoids. The latter are compartmentalized by lithospheric-scale Archean – Paleoproterozoic shear zones with a general sub-vertical fabric. Upon extension, these pre-existing shear zones nucleated extensional deformation, focalized magmatism and accommodated different rates of extension and width of offshore rift domains. In the north, younger Neoproterozoic lithospheric domain deformed with a ductile behavior and multiple crustal detachments promoting lithospheric decoupling and a distributed extension, with late continental break-up. It favored the formation of a wide magma-poor margin where extension was accommodated by low-angle, seaward-dipping extensional faults and mantle-exhumation. To the south, Archean and Paleoproterozoic lithospheric domain with stronger and brittle strength profile promoted a more localized extensional deformation and favored an early lithospheric coupling and continental break-up. This ultimately led to the formation of a short margin with increased magmatic budget, where extension was accommodated by high-angle, landward-dipping extensional faults and mantle-exhumation. We demonstrate that the pre-rift heterogeneity of the lithosphere controlled the mode of extension during the rift and the along-strike variation of margin geometry. Moreover, the rejuvenation of Precambrian shear zones as extensional transverse fault zones also promoted the activation of lithospheric-scale thermal drains facilitating the emplacement of mantle-related melts into the upper crust.

Mass-Transfer and Fluid Flow along Extensional Detachment Faults in Hyperextended Rift Systems: The Examples of Tasna in the Alps, Mauléon in the Pyrenees, and Hobby High Offshore Iberia

Hyperextended rift systems are characterized by extreme crustal thinning and mantle exhumation associated with extensional detachment faults. These faults cut through thinned continental crust, reaching the underlying mantle and allowing for seawater to infiltrate and react with the crustal and mantle rocks. Hydrothermal fluid systems linked to detachment faults result in fluid–rock reactions occurring along the detachments, resulting in the breakdown and alteration of minerals, loss of elements and strain weakening in both mantle and crustal rocks. We present new geological observations and geochemical data from the modern Iberia and fossil Alpine Tethys Ocean Continent Transition and the West Pyrenean Mauléon hyperextended rift basin. We show evidence for a km-scale fluid flow along detachment faults and discuss the conditions under which fluid flow and mass transfer occurred. Convective fluid systems are of major importance for mass transfer between the mantle, crustal and marine reservoirs. We identified gains in Si, Mg, Fe, Mn, Ca, Ni, Cr and V along extensional detachment faults that we relate to channelized, hydrothermal crust- and mantle-reacted fluid systems migrating along detachments in the hyperextended continental crust. The observation that fault rocks of extensional detachment and syn-extensional sedimentary rocks are enriched in mantle-derived elements such as Cr, Ni and V enables us to define the pathways of fluids, as well as to estimate their age relative to detachment faulting and sedimentation. Because all three examples show a similar mass transport of elements along detachment systems at km-scale, we conclude that these examples are linked to convective fluid systems that may affect the thermal state of the lithosphere, as well as the rheology and chemistry of rocks in hyperextended systems, and may have implications for ore mineral exploration in hyperextended rift systems.

Shale 3D Flow and Interaction With Basement Faults in the Niger Delta Deep‐Water Fold and Thrust Belt

AbstractBased on a large 3D seismic data set in the deep‐water domain of the Niger Delta, this study challenges previous interpretations involving the occurrence of multiple detachments and extensive thrust flats, illustrating timing and mode of shales flow at the toe of the gravity system. Five units of syn‐kinematic sediments, reaching a maximum thickness of ∼800 m, accumulated in the tectonically subsiding synclines during fold amplification between ∼9.5 and ∼1.4 Ma. The volumes of syn‐kinematic units roughly balance those of the shales accumulated in the thickened cores of WNW trending anticlines. This feature is consistent with folding resulting from buckling controlled by the competence contrast between isopach Cenozoic units and underlying overpressured shales of the Akata Formation. A dense network of NE‐SW striking oblique extensional faults offsets a prominent anticline characterized by a NE‐SW trend (which is almost perpendicular to the regional fold trend). These faults form a narrow, continuous deformation zone extending for tens of kilometers along and beyond the length of the anticline. The faults, rooting within the shales of the Akata Formation, formed since ∼5 Ma and deform the seabed. Displacement distribution suggests mechanical interaction between isolated fault segments within the deformation zone. The latter is interpreted as the shallow expression of a deep‐seated fault zone inherited from the segmented passive margin and marked by gravity and magnetic data. Our results, providing a comprehensive picture of active deformation features and their relationships with deep‐seated faults, shed new light into the modes of interaction between gravity systems and underlying basement structures.

Local Seismicity and Sediment Deformation in the West Svalbard Margin: Implications of Neotectonics for Seafloor Seepage

AbstractIn the Fram Strait, mid‐ocean ridge spreading is represented by the ultra‐slow system of the Molloy Ridge, the Molloy Transform Fault and the Knipovich Ridge. Sediments on oceanic and continental crust are gas charged and there are several locations with documented seafloor seepage. Sedimentary faulting shows recent stress release in the sub‐surface, but the drivers of stress change and its influence on fluid flow are not entirely understood. We present here the results of an 11‐month‐long ocean bottom seismometer survey conducted over the highly faulted sediment drift northwards from the Knipovich Ridge to monitor seismicity and infer the regional state of stress. We obtain a detailed earthquake catalog that improves the spatial resolution of mid‐ocean ridge seismicity compared with published data. Seismicity at the Molloy Transform Fault is occurring southwards from the bathymetric imprint of the fault, as supported by a seismic profile. Earthquakes in the northern termination of the Knipovich Ridge extend eastwards from the ridge valley, which together with syn‐rift faulting identified in seismic reflection data, suggests that a portion of the currently active spreading center is buried under sediments away from the bathymetric expression of the rift valley. This hints at the direct link between crustal rifting processes and faulting in shallow sediments. Two earthquakes occur close to the seepage system of the Vestnesa Ridge further north from the network. We suggest that deeper rift structures, reactivated by gravity and/or post‐glacial subsidence, may lead to accommodation of stress through shallow extensional faults, therefore impacting seepage dynamics.

Tectonism and fracture-related dissolution induced the formation of the large-scale metamorphic granite reservoir: Implications from Bozhong 26–6 Precambrian bedrock trap, offshore Bohai Bay basin, Northern China

Crystalline rocks in basins are unconventional sites for hydrocarbon exploration. Owing to the lack of primary pores in the bedrock, it remains unclear how later tectonism and dissolution alter crystalline rock within large-scale reservoirs. Notably, the recent discovery of a 200 million-ton oil field of Precambrian metamorphic granite in the Bonan buried hill, Bohai Bay Basin, China provides an important opportunity for exploring this mechanism. Herein, the relationship between fractures and the regional tectonic evolution of this buried metamorphic granite mass, as well as the accompanying phenomena of dissolution and vein filling (which record related fluid interactions contributing to second pore formation from tight crystalline rocks) were investigated. The results indicated that the reservoir space was primarily composed of tectonic fractures and associated dissolution pores (maximum porosity ≤18%). The densely interlaced fractures with dominant NWW-trending flat surfaces indicated that the majority of Precambrian bedrock fractures were induced by Indosinian tectonic compression. The Cenozoic north–south tectonic extensional fault obliquely intersects these NWW-trending fractures, accounting for 73% of the previous dense shearing fractures transformed into open fractures. The dissolution pores were primarily distributed along the fractures and increased the porosity of these fracture-related reservoirs by ≤ 12%. The carbon and oxygen isotopes of the calcite fillings within the fractures suggested that atmospheric freshwater was the primary dissolution fluid, whereas some pores were formed by the dissolution action of deep hydrothermal and organic acid fluids. Thus, regional compressive stress is a necessary tectonic mechanism for the formation of densely interlaced fractures in tight crystalline rocks. The later extensional setting contributing to fracture release and fracture dissolution in the epi-diagenesis stage are two immediate factors in the formation of permeability and storage capacity within buried bedrock traps.

Continental rift asymmetry and segmentation – contributions from the African plate

Prior to the mid-1980s, continental rifts were generally depicted as roughly symmetrical graben, with little complexity along the strike of the basin. In reality, most continental rifts originate as asymmetric, large-scale half graben that are strongly segmented along-strike. This basic architecture does not usually persist unaltered to the rift-to-drift transition but can commonly be observed in both failed continental rift systems and along the passive margins of oceanic plates where rifting succeeded. These geometric aspects of continental rifting have fundamental implications for both hydrocarbon exploration and understanding the causes and processes of continental rupture. Contributions to our understanding of rift asymmetry and segmentation have spanned the past seven decades and come from many settings around the world. However, Africa has played a key role in this undertaking, due to both the presence of well-exposed, active extensional basins in the Afro-Arabian rift system and numerous buried Mesozoic and early Cenozoic rift systems.The first well-documented examples of both rift asymmetry and along-strike segmentation occurred in the Gulf of Suez, northern Egypt, in the 1970s. These early ideas were greatly enhanced and expanded by later studies in both the Western and Eastern branches of the East Africa Rift, including field and refraction/reflection seismic work in Burundi, the Democratic Republic of the Congo, Ethiopia, Kenya, Malawi, Tanzania, and Uganda during the 1980s and 1990s. Asymmetry and segmentation were recognized to be fundamental characteristics of both magmatic and amagmatic rifts, although important differences regarding scale and temporal evolution are evident. At smaller scales, hard-linkage of growing and interacting extensional faults was another key component of subsurface interpretation in the Gulf of Suez by the mid-1970s. Later, outcrop studies in East Africa, supported by subsurface seismic and well data, led to greatly improved models of syn-tectonic sedimentation in step with the advances in understanding of structural geometries. Most recently, concepts derived from the continental margins of other plate settings have been incorporated with the earlier African observations to produce new models of the tectonic evolution of the Red Sea and Gulf of Aden, which in turn has then influenced our overall understanding of the progression from continental to oceanic rifting. This is an ongoing topic of research and discussion that will certainly proceed for many years and require the collaboration of both the international scientific community and local African earth science expertise.

Salt tectonics along the High Zagros Fault in Iran, faulting through welded salt walls

In salt-bearing foreland fold-thrust belts, precursor salt structures affect deformation and accommodate fault zones during orogenic shortening. The complex structure and the halokinetic history of these belts, including the spatiotemporal relationship between diapirism and thrust faulting, have yet to be fully understood. The well-exposed Ediacaran–Early Cambrian Hormuz salt diapirs and the Cambrian–Pliocene overburden stratigraphic sequences along the High Zagros Fault (HZF) in Iran are interesting examples in this regard. The detailed field investigation of the diapirs, the halokinetic deformations, and the thickness variations of the sedimentary units toward the flanks of eight salt structures along the fault show that the first movement of the Hormuz salt could be as old as the Middle Cambrian, occurring long before the initiation of regional folding in the early Miocene. This marks the first field-based and the oldest proposed time for the onset of Hormuz salt diapirism in the Zagros fold-thrust belt. Our field and remote-sensing mapping show the presence of precursor linear salt structures like salt walls. The Hormuz salt diapirs likely began to rise as elongated salt walls above extensional basement faults coeval with the Neo-Tethyan rifting in Early Mesozoic. The squeezing of the salt walls during Zagros shortening from Miocene resulted in the partial accommodation of the HZF as thrust welds. This spatiotemporal evolution, showing the interplay between salt diapirism and late thrusting, can be considered as an analog for other fold-and-thrust belts with pre-orogenic diapiric systems.