Fault-bend Folds Research Articles

Structural style & kinematic analysis of deformation in the northern Dezful Embayment, Zagros Fold-Thrust Belt, SW Iran

A comprehensive understanding of structure and kinematic characteristics of fold and thrust belts provides significant information for hydrocarbon exploration and production. The kinematic evolution and structural style of three subsurface oilfields, Zeloi, Lali and Karun, in the northern part of the Dezful Embayment, a significant petroleum province of the Zagros Fold-Thrust Belt in SW Iran, were investigated using 2D seismic data. Interpretation of the 2D seismic profiles with up to 5 km depth consisting of upper Jurassic to Pleistocene sediments and the balanced cross-sections constructed revealed diverse geometries and kinematics along the oilfields. These results demonstrate that the structural style of the oilfields was controlled by the Gachsaran and Dashtak formations as upper and middle detachment levels, respectively. Tear faults affected the along-strike variations in structural style of the oilfields. Based on the analysis of growth strata, folding evolved as limbs rotated and hinges migrated, beginning in the mid-Miocene. During the later stages of deformation, the initial detachment folds transformed into fault-bend folds.

Unraveling the tectonic evolution of the Andean hinterland (Argentina and Chile, 30°S) using multi-sample thermal history models

The Andean hinterland of Chile and Argentina defines a region of voluminous magmatism, polyphase deformation, and high topography above a long-lived subduction zone. Construction of this high-elevation domain has been variably attributed to isostatic uplift during crustal thickening induced by internal hinterland shortening, underthrusting during growth of the external thrust belt, or lower crustal flow. Alternatively, uplift may be related to dynamic processes associated with lithospheric foundering or flat-slab subduction. This study integrates geo/thermochronological, structural, magmatic, and sedimentological datasets to reconstruct hinterland evolution and evaluate potential drivers of exhumation. Our thermal history modeling approach implements a new time-depth extension to HeFTy 2 software, which enables simultaneous inversion of multiple samples along a structural or topographic profile. This extension addresses transient effects such as isotherm deflection and the transition from geothermal to atmospheric gradients and permits changes in the relative position among samples (folding, tilting) within user-defined constraints. Single- and multi-sample modeling results based on published apatite (U-Th)/He and fission track data (AHe, AFT) along the western Andean hinterland confirm late Eocene (∼30–40 Ma) cooling below at least ∼120°C, whereas thermal histories derived from new AHe, AFT, and zircon He analyses from the eastern hinterland resolve Mesozoic cooling below ∼80–160°C, followed by protracted residence at ∼60–80°C and rapid exhumational cooling in the early Miocene. Multi-sample models further require ∼10° eastward tilting of eastern sample locations, compatible with hinterland uplift via underthrusting and development of a crustal-scale fault-bend fold that was geometrically and kinematically linked to shortening in the external (eastern) thrust belt. Results underscore the enhanced resolution of multi-sample thermal history models in testing structural hypotheses and deciphering the timing, magnitude, and mechanisms of exhumational cooling during changing geodynamic conditions.

Paipote fold-and-thrust belt, a key element in understanding the upper crustal shortening mechanisms of the Central Andean forearc

The crustal shortening of the Andean forearc in northern Chile was accommodated by a combination of both thin- and thick-sinned structural styles. However, fold-related inversion of normal faults appears to be the most important structures in the area. To understand the upper crustal shortening mechanisms that acted during the tectonic uplift of this region, an original structural investigation was carried out, integrated with outcrop and regional-scale observations, balanced cross-sections, and pre-shortening restorations of structures exposed along the Paipote fold-and-thrust belt. On this basis, we presented the first balanced cross-section of this region extending for nearly 27 km is presented. The structural styles consisted of east-directed asymmetrical folds involving Paleozoic to Cenozoic strata. The folds were kinematically related to inverted normal faults and thrust ramps that penetrated downward into the basement. The inverted structures resulted from the reverse reactivation of preexisting Upper Paleozoic to Jurassic west-dipping, basement-rooted normal faults that accommodated the tectonic extension that preceeding the Andean orogenesis. The reverse-reactivation of these extensional structures controlled the development of east-verging anticlines, along which the Mesozoic syn-rift strata were elevated above their regional elevation. Other folds exhibit the typical geometry of fold-related thrust ramps (fault–bend folds and fault–propagation folds). These are proposed to result from the development of low-angle thrusts propagating across precursor normal faults with shortcut trajectories, that detach along Jurassic shales, thus forming complex thin-skinned structures in shallow structural levels. The latter is responsible for accommodating a major crustal shortening (nearly 5 km). The east-directed tectonic transport direction was influenced by the original attitude of precursor extensional faults.

A Regional Shallow Décollement at the Front of the Mexican Fold and Thrust Belt: Microstructure of the Santiago Shale

Abstract This contribution analyses the role played by the mechanical properties of a decollément shale layer in the evolution of the Mexican Fold and Thrust Belt (MFTB). The mobility of overpressured shales can accommodate large strains by grain-scale plastic mechanisms, and affect the folding and thrusting styles of the overburden. Research on shale deformation mechanisms is necessary to improve the knowledge of these processes and their influence on the structural style of fold and thrust belts. The ductile behavior of rocks involving grain-scale plasticity was documented in the Jurassic Santiago shale sequence using geological mapping, microstructural observations on thin-oriented sections, and scanning electron microscopy (SEM) imaging. Structural styles such as detachment folding, fault-bend folding, and shale-cored fold-thrusts were observed at the regional scale. At the outcrop scale, the shale developed strong foliation and pencil cleavage, with immersed packstone boudins. Observed structures include thrusting, soft and open folds, and buckle folding. In thin section, the ductile textures include a strong penetrative foliation with lenticular and wavy-parallel laminae composed of carbonates, ribbons of reoriented clays and organic matter (clay+OM), s-c structures, porphyroblasts microtextures, development of oblique cleavage concerning folded foliation (crenulation cleavage), and carbonates dissolution. The Santiago shale shows also evidence of brittle deformation including calcite-filled fractures and cataclastic gouges. X-ray diffraction (XRD) analysis of the clay size fraction suggests that the authigenic calcareous shale was deformed in conditions of the deep diagenetic zone (between 100 and 200°C) and fluid overpressure (>70 MPa). The results help to improve the understanding of ductile microstructure and its role in shale deformation cretaceous cover, promoting the formation of localized fault propagation folds in the overburden. This study aims to open new perspectives in the kinematics and rheology interpretations for this sector of the MFTB, highlighting the role of the décollement layers during the progression of the orogen.

An improved incremental shortening calculation method of the shear fault-bend fold

Shear fault-bend folds, a common feature of fault bend folds in foreland fold-and-thrust belts, originate in the early deformation stage. Building upon Suppe's geometric model of 'shear fault-bend folding,' we present refined calculation formulas for total shortening, incremental shortening, and the shear coefficient specific to these folds, as well as ideas to verify model parameters. The practicality and versatility of our refined approach are confirmed through case studies of the Pakuashan anticline in Taiwan and the Tugulu anticline in North Tian Shan. This improved incremental shortening calculation formula, conserves the bed length and eliminates errors of the widely used shortening calculation formula, is crucial for understanding deformation processes and assessing seismic risks shear fault bend folding. To obtain more precise incremental shortening calculations, it should further refine the geometric and kinematic models of shear fault bend folding in the future.

Seismic data, cross-section balancing, and hydrocarbon prospectivity of the western Sulaiman fold belt, northwest Pakistan

Over the past few decades, hydrocarbon exploration has considerably improved our understanding of foreland deformation. This study used the interpretation of seismic reflection profiles and cross-section balancing techniques to understand the passive-roof duplex geometry of foreland structures in the Western Sulaiman Fold Belt (SFB), Pakistan. Seismic reflection profiles were interpreted to show the presence of hinterland-vergent fault-propagation and fault-bend folds as prospects over a detachment located at a depth of ∼6 km in the pelitic Triassic succession. Cross-section balancing showed intraplate deformation in the hanging-wall flat of the frontal duplex. Clear seismic reflectors above 3 s two-way travel time (TWT), estimates of stratigraphic and tectonic thicknesses with field geology and isopach mapping were used to locate the décollement in the pelitic Permo-Triassic strata at a depth of ∼11 km and interpret stacked duplexes at the mountain front. The foreland deformation of the western SFB was compared with that of the central/eastern SFB. The comparison shows the presence of discovered gas fields, with intraplate deformation of the duplex in the western, and pop-ups in the roof-sequence in eastern SFB depicting the complexity of deformation for the assessment of hydrocarbon prospectivity in the fold belt. The 33 km deformed balanced cross-section was restored to ∼71 km, with 38 km (54%) shortening in the foreland. Approximately 12 km (17%) of shortening was passively accommodated by the hinterland-vergent out-of-sequence passive backthrust in the frontal horse. This description of foreland deformation using the example of the SFB adds to our understanding of the structures of the mountain front that are recognized as zones of prolific hydrocarbon prospects.

Elucidating fault-related fold mechanics: a 2D finite element analysis of bending, slip, and buckling mechanisms

Fault-related folds are present in most tectonic settings and may serve as structural traps for hydrocarbons. Due to their economic importance, many kinematic models present for them. Unfortunately, most of them have predominantly concentrated on the sliding mechanism parallel to the layering and often ignore the integral role of buckling in folding processes. This study is at the forefront of exploring the interplay among, sliding, buckling, and bending in the formation of the three fundamental types of fault-related folds: detachment, fault-propagation, and fault-bend folds. To this end, we developed five sets of two-dimensional (2D) finite element models, embodying both elastic and elastic-plastic behaviors. Our results indicate that sliding parallel to layering and faults, in conjunction with buckling, are the predominant mechanisms in fault-related folding. The strain ellipse patterns in our models are consistent with those observed in buckling models, thus affirming the significance of buckling in these geological structures. Furthermore, our models demonstrate that fault slip diminishes from the periphery towards the center in all three types of fault-related folds, in contrast to interlayer slip, which intensifies from the edge towards the center. In essence, a diminution in fault slip at the center is balanced by an augmentation in interlayer slip, leading to thickening and buckling. The genesis of all three fault-related fold types is attributed to the reduction in fault slip, with their distinctiveness defined by the location of this reduction: at the detachment fault tip for detachment folds, at the ramp tip for fault-propagation folds, and at the upper flat for fault-bend folds.

Structural geometry and geomorphic evolution of the Trans Indus Manzai Ranges, Western Himalayas, Pakistan

At the leading edge of fold-thrust belts, structural deformation can either be accommodated by thick-skinned faulting, and development of structural wedges near the mountain fronts, or by foreland propagation into the basins along the detachment, forming thin-skinned structures. Manzai Ranges at the western extremity of the Trans Indus Ranges were chosen as a case study to describe the structural deformation accommodated in the foreland basin and to study the folded mountain front. These ranges also provide an opportunity to compare along-strike structural variations of the Himalayan frontal thrust system. Both surface (field structural measurements and digital elevation model) and subsurface (2D seismic reflection and well) data were integrated to document different structural styles and their corresponding geomorphic expressions. Based on seismic reflection profiles, the results of structural interpretations constrain westward dipping basement (2.5o) at a depth of ∼10 km in the foreland, which dips deeper in the hinterland. In addition, our interpretation shows a basin-wide Eocene-Miocene unconformity that marks the Himalayan orogenic events. The structural geometry of the frontal Surqamar anticline is quantitatively characterized to be detachment fold, while Jandola, Kundi and Manzai anticlines demonstrate staircase geometry and characterized as fault-bend folds. The kinematics of development varies along-strike in the Manzai Ranges as displacement transfers along the detachment from typical fault-bend folding in the Manzai anticline to detachment folding in the Surqamar anticline, forming thin-skinned structures. The results of this study highlight the structural styles and their evolution in the context of active Himalayan tectonics.

Discovery of the large-scale thrust nappe and its geological significance in the southwestern Santanghu fold–thrust belt, NW China

Studying the structural evolution of the piedmont fold and thrust belt is one of the most important methods to interpret the mechanism of intracontinental collision orogeny. In this study, we have discovered a long-distance large-scale thrust nappe with a width of approximately 20 km in the southwestern margin of the Santanghu Basin, which provides a good evidence for studying the Mesozoic–Cenozoic tectonic evolution mechanism of Moqinwula Mountain. Using the electromagnetic and high-resolution seismic profiles, we have determined that the hanging wall of the nappe is composed of the pre-growth strata of the Carboniferous–Middle Jurassic period and the syntectonic strata of the Cretaceous–Quaternary period. The nappe is subjected to two branch faults of the Baiyishan thrust, forming a breakthrough fault-propagation fold and a fault-bend fold along the detachment layer of the Haerjiawu Formation, and a large monoclinic is formed by the basement structural wedge near the mountain root. The growth strata and unconformity structure record that the fold–thrust belt has experienced five episodes of thrusting from the Late Triassic to Quaternary period. Based on sequential restoration and forward modeling, we propose that the southwestern margin of the Santanghu Basin has been shortened by at least 55 km, especially in the Late Jurassic and Late Cretaceous. Our results provide an excellent example for studying the mechanism of the transition from Yanshanian transpression to Himalayan thrust compression in the Eastern Tianshan region.

A mechanical basis of fault-bend folding

A mechanical basis of fault-bend folding

Tectonics of the Western Internal Jura fold-and-thrust belt: 2D kinematic forward modelling

The balancing technique, called 2D kinematic forward modelling, is a powerful tool to understand the kinematic evolution of fold-and-thrust belts. This study presents a new 2D kinematic forward model for the westernmost Internal Jura fold-and-thrust belt (FTB), situated immediately adjacent to the Geneva Basin. The technique used not only provides a new valid balanced cross-section but also offers new insights regarding the kinematic evolution of the Western Internal Jura FTB. Our model proposes a pure thin-skinned style dominated by forward stepping deformation accompanied by minor back-stepping thrust sequences. A first deformation step is attributed to the thrusting of the Crêt de la Neige Anticline, followed by the Crêt Chalam Thrust and its imbrications. This is followed by thrusting along the Tacon and the Bienne thrusts. Imbricate fault-bend folding explains the steep southern limb of the Crêt de la Neige and the Bellecombe anticlines. 2D kinematic forward modelling yields a total amount of shortening by 23.6 km for the Western Internal Jura FTB. In addition to the primary décollement located at the base of the Keuper Group evaporites, three other décollements are found within the marly layers of the Aalenian “faciès de transition” units, the Oxfordian “Couches d’Effingen-Geissberg” members and the Berriasian Goldberg formation. The multiple thrust horizon approach is supported by new precise seismic interpretations. Our model provides a valid alternative to previous models that either propose local thickening of the Triassic evaporites or inversion of normal faults in the basement. This fully explains the elevated position of the Mesozoic cover in the Jura FTB.

Structural Origin of Sinjar Anticline, NW Iraq

The Sinjar anticline is a double plunging, trending almost E-W in the northwestern part of Iraq. It extends in Syria for about 42 km, whereas in Iraq, its length is about 91 km, and the width is about 31 km. The northern limb (45° - 80°) is steeper than the southern limb (15° - 25°), with average plunges dip of 35° and axial plane dipping of 47.5° southwards. The exposed rocks in the anticline range in age from Upper Cretaceous, represented by the Shiranish Formation, to Upper Miocene, represented by the Injana Formation. Google Earth image was used to calculate structural data, which were used to indicate the structural origin of the Sinjar anticline. This was achieved by calculating the Aspect Ratio (AR), Fold Symmetry Index (IFS or IFS), and length of the mountain front (FS). Accordingly, it was found that the structural origin of the Sinjar anticline is a fault-bend fold.

Three-Dimensional Fault-Fold Growth Deciphered from Combined Seismic and Geological Data: A Case Study from the Xiongpo Anticline, Longmen Shan Piedmont

The Xiongpo fault-fold belt shows prominent NE, ENE- and ~N–S-trending relief, which resulted from multi-stage upper crustal shortening in the Longmen Shan piedmont during the eastward growth of the eastern Tibetan Plateau. Previous studies have determined its 2D structural configurations from seismic profiles and field-based geological cross-sections. Here, we extend this analysis into the entire belt to explore the 3D structural evolution of this complex fault-fold belt and have built a 3D regional fault model. The results reveal along-strike variation of subsurface structural architecture of the Xiongpo fault-fold belt, which is characterized by transformation from a complex superimposition of a deep fault-bend fold beneath a shallow structural wedge in the center segment to a simple shallow fault-bend fold on both ends of the structure, and then to a trishear fault propagation fold on the plunging edges. This structural transformation determines the contrast between the NE-striking relief of the central segment, and the ENE- and ~N-S-striking relief in the two plunging zones. We combine our results with published low-temperature thermochronology and growth strata results to propose a three-stage evolution for the Xiongpo fault-fold belt that closely relates with regional stress field changes, including a NE-striking fault under the NW–SE compression between 40–25 Ma and 15–10 Ma, lateral propagation of the NE-striking fault and initiation of ENE-striking fault by WNW–ESE compression from ~5–2 Ma, ~N–S fault under ~E–W compression until the present. This work enhances our understanding of the stress field changes of eastern Tibet since the Late Eocene. It also can serve as a typical case study deciphering 3D fault-fold growth using seismic and geological imaging, which is helpful to understand 3D structural and landscape evolutions of other complex fault-fold belts worldwide.

Recent paleoseismic investigations at the hidden Thakhek fault in Thailand

• The branches of the Thakhek Fault zone were first studied in Northeast Thailand. • A single paleoearthquake in the trench (M 7.1 moment magnitude) was examined. • This fault is active based on the inferred age of the displacement events. We conducted a preliminary paleoseismological study at the branches of the Thakhek fault zone in Northeast Thailand. Locating the fault in Thailand is challenging as there are no evident morphotectonic landforms indicative of an active fault. We utilized remote sensing to delineate fault traces, supplemented by geological and geophysical ground-based surveys. We identified a suitable site for trenching through a 2D electrical resistivity tomography (ERT) survey. Trenches were excavated to determine the succession of sedimentary units. The trench-log stratigraphy was compiled and sedimentary samples were dated by optically-stimulated luminescence (OSL) for determining the age sedimentary layers, thereby inferring the putative seismic events. Analysis of the digital elevation model revealed three minor faults extending from Laos into Northeast Thailand with lengths of 138, 159, and 35 km. The ERT survey indicated a clear resistivity contrast, which was interpreted as mudstone with low resistivity juxtaposing sediment layers with higher resistivity. Subsequently, two trenches were excavated. Displacement resulting from reverse faulting appeared to have moved older mudstone over the Quaternary sediment layers with vertical offsets of >3.5 m in the first trench; thrust faulting resulted in a transition fold to fault-bend fold with mudstone ramping over the sediment layers in the second trench. The sediment units cut by the fault plane were dated by the OSL method between 18,120 ± 1920 and 36,960 ± 2780 years in the Pleistocene. Results indicated a single paleoearthquake in the trench with greater than M 7.1 moment magnitude and 0.009 mm/a average slip rate. This fault is potentially active based on the inferred age of the displacement events.

How do differences in interpreting seismic images affect estimates of geological slip rates?

Abstract. Uncertainties of geological structural geometry constructed based on seismic reflections can stem from data acquisition, processing, analysis, or interpretation. Uncertainties arising from structural interpretations and subsequent estimates of geological slip have been particularly less quantified and discussed. To illustrate the implications of interpretation uncertainties for seismic potential and structural evolution, I use an example of a shear fault-bend fold in the central Himalaya. I apply a simple solution from the kinematic model of shear fault-bend folding to resolve the geological input slip of given structure and then compare the result with a previous study to show how differences in structural interpretations could impact dependent conclusions. The findings show that only a little variance in interpretations owing to subjectivity or an unclear seismic image could yield geological slip rates differing by up to ∼ 10 mm yr−1, resulting in significantly different scenarios of seismic potential. To reduce unavoidable subjectivity, this study also suggests that the epistemic uncertainty in raw data should be included in interpretations and conclusions.

Cenozoic double-layered structure in the western Qaidam Basin, northern Tibetan Plateau, China

• Cenozoic double-layered structure was identified in the western Qaidam Basin. • The Qaidam Basin suffered much stronger shortening deformation in west than east. • The double-layered structure in the western Qaidam Basin was initiated at Pliocene . • Double-layered structure was due to far-field effect of the India-Asia collision. The Qaidam Basin in the northern Tibetan Plateau lies above over 3000 m elevation above sea level, being the inland terrestrial basin with the highest altitude in the world. This basin is surrounded by the Eastern Kunlun, Altyn and Qilian Mountains, and represents a key area to decipher the tectonic and kinematic evolution of the northern Tibetan Plateau during Cenozoic times. The Cenozoic deformation of the Qaidam Basin is synchronous with the uplift of the northern Tibetan Plateau. The structural coupling between Cenozoic shallow and deep deformation systems in the Qaidam Basin is complicated and poorly understood, because of the lack of studies of the deformation at the depth. Based on the detailed structural analysis of the Honggouzi and Nanyishan anticlines in the western Qaidam Basin, we found that the fault-propagation fold was usually developed in the shallow part and “Y” shaped thrust faults occur at both sides of the anticlinal core, while fault-bend folds developed above the bottom detachment fault in the deep, which together constitute the Cenozoic double-layered structure (DLS). The DLS is relatively well developed in the northwest and gradually disappears to the southeast in the Qaidam Basin, indicating that the deformation and shortening in the western Qaidam Basin is much stronger than that in the eastern Qaidam Basin. The initial formation time of the DLS started from Pliocene (Shizigou Fm.) and continue to present, which supports that the Qaidam Basin was a “plate-shape-like” compressive flexing basin without obvious deformation during Eocene to Pliocene. The formation of the DLS in the western Qaidam Basin was due to the continuing N/NNE-directed compression in the southwest and the large-scale sinistral strike-slip displacement along the Altyn Tagh fault in the northwest related to the far-field stress effect of the India-Asia collision.

New insights into the structural development and shortening of the southern Jasmund Glacitectonic Complex (Rügen, Germany) based on balanced cross sections

Late Pleistocene glacitectonism at the southern Scandinavian Ice Sheet margin caused folding and thrusting of Upper Cretaceous chalk layers and Pleistocene glacial deposits in parts of the southwestern Baltic Sea area in Europe. Beside Møns Klint (SE Denmark), the Jasmund Glacitectonic Complex (JGC) on Rügen Island (NE Germany) is a similar striking example of glacitectonic deformation creating large composite ridges. In spite of a long research history and new results from modern datasets, the structural development of the JGC is still poorly understood, especially the detailed evolution of the southern JGC and its relationship to the northern JGC remain enigmatic. In this contribution, we demonstrate how the understanding of the JGC benefits from the application of established structural geological methods comprehending the formation of fold-and-thrust belts. The methods include cross-section balancing of the eastern coast (southern JGC) and quantification of the amount of folding and faulting. The proposed geometric model shows the current fold-and-thrust stack of glacially deformed sedimentary strata ca. 5720 m in length evolved by shortening from the original length (11,230 m) by 5510 m (49.1%). We present a spatial and temporal development of fault-related folding with a transition from detachment folds through fault-propagation folds to fault-bend folds. Together with morphological information from a digital elevation model, the thrust faults mapped in the cliff section are mainly inclined towards the S to SW and imply that a local glacier push occurred from the south. These results highlight the complexity and individual architecture of the JGC when compared to other Pleistocene and modern glacitectonic complexes. Resolving its structural development provides new insight into the deformation history and shortening of this spectacular glacitectonic complex lying in the southwestern Baltic Sea region.

Structural characteristics and implications on oil/gas accumulation in north segment of the Longmenshan piedmont, northwestern Sichuan Basin, SW China

By integrating surface geology, seismic data, resistivity sections, and drilling data, the structural deformation characteristics of the frontier fault of thrust nappes were delineated in detail. The frontier fault of thrust nappes in northwest Scihuan Basin is a buried thrust fault with partial exposure in the Xiangshuichang—Jiangyou area, forming fault propagation folds in the hanging-wall and without presenting large-scale basin-ward displacement along the gypsum-salt layer of the Triassic Jialingjiang Formation to the Triassic Leikoupo Formation. The southwestern portion of the frontier fault of thrust nappes (southwest of Houba) forms fault bend folds with multiple ramps and flats, giving rise to the Zhongba anticline due to hanging-wall slip along the upper flat of the Jialingjiang Formation. In contrast, the northeastern portion of the frontier fault of thrust nappes (northeast of Houba) presents upward steepening geometry, leading to surface exposure of Cambrian in its hanging-wall. With the frontier fault of thrust nappes as the boundary between the Longmenshan Mountain and the Sichuan Basin, the imbricated structural belt in the hanging-wall thrusted strongly in the Indosinian orogeny and was reactivated in the Himalayan orogeny, while the piedmont buried structural belt in the footwall was formed in the Himalayan orogeny. In the footwall of the frontier fault of thrust nappes, the piedmont buried structural belt has good configuration of source rocks, reservoir rocks and cap rocks, presenting good potential to form large gas reservoirs. In comparison, the hanging-wall of the frontier fault of thrust nappes north of Chonghua has poor condition of oil/gas preservation due to the surface exposure of Triassic and deeper strata, while the fault blocks in the hanging-wall from Chonghua to Wudu, with Jurassic cover and thicker gypsum-salt layer of the Jialingjiang formation, has relative better oil/gas preservation conditions and thus potential of oil/gas accumulation. The frontier fault of thrust nappes is not only the boundary between the Longmenshan Mountain and the Sichuan Basin, but also the boundary of the oil/gas accumulation system in northwestern Sichuan Basin.

Controls of fault-bend fold on natural fractures: Insight from discrete element simulation and outcrops in the southern margin of the Junggar Basin, Western China

The fault-bend fold, a typical structure in foreland thrust belts, is suitable for hydrocarbon accumulation. Natural fractures are ubiquitously developed and usually interlaced to form a complex network in fault-bend folds, which can play a fundamental role in hydrocarbon migration and enrichment. This study utilized outcrops and the discrete element method (DEM) to simulate and analyze the characteristics, distribution patterns, and development mechanisms of natural fractures in the fault-bend fold. The study anticline, located in the southern margin of the Junggar Basin, is a well exposed fault-bend fold. Statistical results of dips and dip direction of fracture in different structural positions show that the fold-related fractures strike NWW-SEE parallel to the axis of the study fold. The fractures include layer-parallel shortening related fractures (LPSF), interlayer slipping related fractures (ILSF), active hinge parallel shearing related fractures (AHSF), and curvature-related fractures (CF). The LPSF and ILSF are strata-bound fractures, whereas the CF and AHSF are not strictly confined in layers. Furthermore, results of the DEM and outcrop measurements demonstrate that LPSF are formed at the embryonic stage. ILSF and AHSF are formed in the early stage, and the mid-late stage is the main formation period of CF. By integrating strata deformation history with fracture types and development characteristics, a development pattern of fractures within the fault-bend fold has been established, and five deformation panels are developed (panels I–V). From panels I to V, the fracture area density increases consecutively, and panel I (unfolded) is limited to regional fractures. Next, LPSF and ILSF are more highly developed in panel V (forelimb) than in panels IV (strongly folded layers of the backlimb), III (core), and II (weakly folded layers of the backlimb). Moreover, CF are predominantly found in panel III, and AHSF are mostly developed in panel IV. Overall, this study investigated the characteristics of fractures in deformation panels of the fault-bend fold and thus provides guidance for locating favorable reservoirs in foreland thrust belts.

Structural geometry and evolution of the Rukwa Rift Basin, Tanzania: Implications for helium potential

AbstractThe Rukwa Rift Basin, Tanzania is regarded as a modern example of a cratonic rift zone despite complex polyphase extensional and episodic inversion structures. We interpret 2D seismic reflection data tied to wells to identify and describe structures controlling stratigraphic sequences (Late Carboniferous to Pleistocene) in two main segmented Rukwa Rift domains, A and B, which are controlled by the Chisi and Saza shear zones. Fault geometry and stratal patterns are illustrated in relation to their kinematic interaction with folds. Fold structures reflect both extensional and compressional deformation and were mapped with a particular interest for their helium potential. We illustrate fault bend folds, fault propagation folds and fault propagation monoclines that are related to extension events. Folds related to compression exhibit various structural styles reflecting at least two phases of episodic and widespread inversion. First, Early Jurassic inversion phase which involved multi‐faulted anticlines in the Karoo strata. Second, a mild and widespread inversion structures during the Pleistocene which are characterised by both symmetrical and asymmetrical anticlines styles. Taken together, the extensional and compressional fold structures, stratal juxtapositions and unconformities define stratigraphic packages that are widely distributed in the Rukwa Rift Basin, and form potential subsurface traps for helium‐nitrogen–rich gases, from which some seep to the surface, evidently documented in thermal springs across the region.