Negative Flower Structures Research Articles

Identification and Verification of the Movement of the Hidden Active Fault Using Electrical Resistivity Tomography and Excavation

Identifying the movement of the branches of the hidden Thakhek fault in Thailand is challenging due to the absence of evident landforms indicating an active fault. In this study, we analyzed a digital elevation model (DEM) to identify potential landforms. A 2D Electrical Resistivity Tomography (ERT) survey was conducted to locate the hidden Thakhek fault. The results reveal vivid images of resistivity contrast, interpreted as two reverse faults, with mudstone exhibiting low resistivity in the middle, flanked by thick sediment layers with higher resistivity. Three trenches were excavated perpendicular to the two interpreted reverse faults. The displacement of reverse faulting appears to have shifted mudstone over Quaternary sediments, with vertical offsets revealed in trenches NWY-1, NWY-2, and NWY-3. This movement could be identified as a positive flower structure. Additionally, lakes are identified as a negative flower structure along the traces. These features result from strike-slip strains under a locally appropriate compressional and extensional environment in a shearing strike-slip fault.

Characteristics and genesis of the Zhongnan Fault Zone in the South China Sea oceanic basin: Insights from an integrated analysis of multibeam bathymetric and 2D multichannel seismic data

The Zhongnan Fault Zone (ZFZ) is a large-scale tectonic belt in the South China Sea (SCS) oceanic basin and plays an important role in the formation and evolution of the basin. Despite numerous studies over the past few decades, debates persist regarding its location, orientation, nature, time of activity, and genesis. In this study, we evaluate our findings on the characteristics and origin of the fault zone through an integrated analysis of multibeam bathymetric and 2D multichannel seismic data. Our results reveal the ZFZ as a fault zone approximately 400 km long and 50–90 km wide, situated between the east subbasin (ESB) and southwest subbasin (SWSB) of the SCS. The fault zone is oriented N8°W, roughly perpendicular to and laterally displacing the relict spreading center and related spreading lineaments. Bounded by discontinuous linear seamounts, the fault zone comprises two V-shaped subparallel pull-apart basins and a separating basement high. Steeply dipping (>60°) normal basement-involved faults bound these pull-apart basins, forming typical negative flower structures. Numerous northeast-oriented en-echelon linear bathymetric highs within the fault zone are identified as secondary antithetic shears (P′ shears). These shears are characterized by their orientation relative to the principal displacement zones defined along the pull-apart basins. The ZFZ exhibits differences from the adjacent subbasins in water depth, basement burial, stratal thickness, and seismic stratigraphic characteristics. Five seismic sequences (S1–S5 upward) in the ZFZ and nearby ESB and SWSB are defined, dating to the Early Miocene synspreading (S1) and the Middle Miocene to Recent postspreading (S2–S5) stages, respectively. The difficulty in correlating seismic facies in sequences S1–S3, compared with the comparable seismic facies in sequences S4–S5 between the ZFZ and adjacent subbasins, suggests a horizontal displacement during the synspreading and early postspreading stages. We propose that the ZFZ functioned as a right-lateral transform fault zone during the synspreading period (the Early Miocene, approximately 24–16 Ma) of the SWSB and transitioned into a left-lateral strike-slip fault zone during the successive early postspreading period (the Middle-Late Miocene, approximately 16–5.3 Ma).

Variable behaviour of Abu Jir fault zone, western Iraq

This study sheds light on the dynamic nature of the Abu Jir fault zone, uncovering intriguing contrasts along its path. It was conducted to know the variation through the zone, western Iraq, that starts at Razzazah Lake’s west and continues to Anah city. It is divided into two parts, southern and northern, utilized seismic sections for the southern part whereas fieldwork for the northern part. Seismic sections indicate that the southern part of the fault zone is negative flower structure which develops depressions, while field studies reveal that the northern part is positive behaviour forms some structures such as ridges, relay ramps and controlled the distribution of valleys within its zone. The Abu Jir fault zone not continuous fault but consists of many segments that subject to differential forces of strike-slip movements during Early-middle Miocene. The variation of behaviours is attributed to the difference of the direction between the northern and southern parts of the fault with sense of movement.

Fault structure, seismicity, and magmatism of the Littoral Fault zone, northern South China sea: Insights from high-resolution seismic reflection data

Four destructive M7+ earthquakes and 18 M6+ earthquakes demonstrate that the Littoral Fault Zone (LFZ) is one of the most seismically active regions in the offshore area of South China. However, the location, geometry, and structural characteristics of the LFZ, which spans over 1000 km, have not been properly examined. In this study, we investigate the precise position of the LFZ offshore East Guangdong (EGD) and map the fault structure using recently acquired high-resolution multi-channel seismic (MCS) reflection data. The LFZ consists of an array of normal faults that collectively form a terraced sidewall fault zone linked to the continental rifting of the South China Sea (SCS). This fault zone is approximately 35–75 km wide and runs parallel to the shoreline. The primary faults of the LFZ EGD segment correlate to a high-gravity gradient zone and serve as the northern boundary of the Zhu-I Depression. During the syn-rift phase, the LFZ experienced significant intra-plate deformation, resulting in fault throws of up to 1000 m. This deformation was locally influenced by pre-existing structures, giving rise to a listric geometry at depth. In the post-rift period, the LFZ exhibited inherited activity along its rift-related faults. The presence of growth fault structures suggests that the LFZ remained active throughout the Cenozoic, with numerous active faults extending practically to the seafloor. The negative flower structure confirms the strike-slip character of the LFZ, which is influenced by the modern tectonic stress field. Intense multi-phase Cenozoic magmatism, including igneous intrusion, volcanism, and lava flows, has been detected in the offshore area of Nan'ao Island. The frequent earthquakes in the study area are primarily attributed to far-field stress release resulting from the subduction of the Philippine Sea Plate beneath the Eurasian Plate. Additionally, the intense magmatism weakens crustal structures, further facilitating stress release and the frequent occurrence of earthquakes.

Surface rupturing earthquakes along the eastern Rhine Graben Boundary Fault near Ettlingen-Oberweier (Germany)

The eastern Rhine Graben Boundary Fault (eastern RGBF) forms the eastern margin of the Upper Rhine Graben (URG), the most seismically active area in the plate interiors of Europe. Despite seismic activity posing a significant threat to the densely populated URG and critical facilities therein, only a few studies have documented the paleoearthquake history and associated seismic hazard, focusing mainly on the western margin. We present the results of the first paleoseismological trenching ever conducted on the central section of the eastern RGBF. Following high-resolution near-surface geophysical studies, we excavated six trenches near Ettlingen-Oberweier (south of Karlsruhe, Germany) on one of its secondary fault strands. The nearly-vertical fault is transtensional left-lateral and splits into several NNW-SSE en échelon branches, forming a negative flower structure. Stratigraphic and structural relationships along with radiocarbon and Optically Stimulated Luminescence dating reveal a minimum of three surface-rupturing paleoearthquakes with a Mw of potentially 6.5 ± 0.5, occurring from old to young, >56 ka (EX), between 54 and 19 ka (EY), and between 15 and 1 ka (EZ). The events are poorly constrained in age due to erosional unconformities. Based on the cumulative vertical separation of 1.2 ± 0.3 m, we calculate an average vertical slip rate of 0.02 ± 0.005 mm/yr. From a horizontally offset alluvial channel, we infer a cumulative left-lateral slip of 5.9 ± 0.7 m and derive an average horizontal slip rate of 0.1 ± 0.01 mm/yr. The average net slip rate is 0.1 ± 0.02 mm/yr for the past 59.5 ± 3.8 ka. This value represents a minimum slip rate for the eastern RGBF, considering slip distribution within the different fault strands of the fault system. Our findings highlight the seismic potential of the eastern RGBF, providing new evidence of the Late Pleistocene and Holocene tectonic activity of its central section.

The controls of strike-slip faults on fracture systems: Insights from 3D seismic data in the central Tarim Basin, northwest China

The central Tarim Basin has gained wide attention for its petroleum reserves, especially the recent commercial discovery of carbonate reservoirs of the Ordovician age in the Shunbei oil and gas field. A systematic analysis is conducted based on 3D seismic interpretation in the central Tarim Basin. The results indicate the presence of several major strike-slip faults and associated fracture systems. The characteristics of major strike-slip faults indicate a lower positive or negative flower structure in the Lower-Middle Ordovician and faults with a normal sense of movement in the Upper Ordovician due to the regional stress field. The major strike-slip faults commonly cause fractures in multiple scales. The fracture systems in different segments of major strike-slip faults contain significant differences in characteristics. In addition, two models are established with regard to strike-slip fault-associated fracture systems in a compressional setting and an extensional setting. The development of fracture systems varies to a large extent in relation to the scales of a strike-slip fractured zone. The fracture systems in a small-scale shear zone or strike-slip fractured zone usually develop along the fracture plane. In contrast, the fracture systems in a large-scale strike-slip fractured zone commonly develop along the fault zone on both sides.

Reactivated fracture-controlled uranium mineralization: An example from NNE-SSW Kamaguttapalle–Kammapalle tract, Kadapa district, Andhra Pradesh, India

The Cuddapah Basin (CB) and its basement can be considered as an appropriate uranium metallogenic province. The basement complex of CB in the Eastern Dharwar Craton (EDC), India, has been the target for structurally controlled uranium exploration since the early 1980s. Most of the brittle-ductile deformation induced reactivated narrow linear fracture zones often exhibit significant surface and sub-surface uranium occurrence. There are about 10 fracture zones associated with uranium mineralization. Amongst them, the Kamaguttapalle–Kammapalle fracture zone (KKF) in the south of the CB is quite interesting from an exploration viewpoint with a suitable geologic setting in space and time frame. Geomorphic, structural and petro-mineralogic exploration guides imply the possibility of economic uranium concentration along the NNE-SSW trend where tectonic reactivation took place over pre-existing ductile shear zone. Progressive exhumation and transition from ductile to brittle deformation regime led to the development of dominant strike slip tectonics with an associated riedel fracture system, which are subsidiary coeval shear fractures used in depicting sense of movement. Intense illitization and hematitization are often observed. Quartz reefs and veins along the NNE-SSW trend cut the preexisting mylonites at places. About 9 stages of fracturing is observed out of which 3 stages are related to uranium mineralization. All the 9 deformations are related to strike slip tectonics which in turn is part of Grenville orogeny. Paragenetic sequence and mode of occurrences of uranium phases deciphers about the importance of low to moderate level epi‑meso type hydrothermal (100°–250 °C) fluids as carrier of U. Reactivated fractures and quartz veination also act as controlling factors of mineralization. Strain analysis points to positive and negative flower structures at different segments of the fracture zone. Based on the litho-structural attributes it is established that maximum compression (σ1) acted along the NNW-SSE direction. Fractured quartz veins provide well defined permeability and trapping facility by restricting transverse fluid movement as a mechanical barrier. The presented model gives an idea on role of strike-slip tectonics in uranium mineralization for narrowing down the target zone for sub-surface exploration.

Structural and Tectonic Evolution of the Porgera Gold Mine; Highlands of Papua New Guinea

The Porgera Transfer Zone (PTZ) is a major crustal and probably lithospheric structure across Papua New Guinea recording >50 km offset of ophiolites and very different patterns of geology and topography on either side. In the Late Jurassic, the PTZ probably separated oceanic crust and thick Jurassic Om shales to the west from a continental promontory to the east. During the Late Miocene to Recent orogenesis, the differential compression of these features is interpreted to have created a dextral strike slip fault across the fold belt with pull-apart basins at sites of fault relays. This facilitated the ascent of intrusions and mineralization at Porgera. The acquisition of high-resolution LIDAR data semi-regionally around the Porgera Gold Mine greatly improved interpretation of the regional geology and particularly the recognition of normal faults. By correlating with sparse dip data and paly-dated samples, it was possible to create stratigraphic sections and interpret structural cross-sections using the LIDAR data. As the area involved strike–slip offsets, it was important to construct sections in multiple orientations in order to interpret the 3D geology. Both dips and fault orientation could be directly inferred from the LIDAR data such that sections could be constructed orthogonally to them. A balanced, restored and forward-modelled cross-section illustrates the interaction between thrust faults and normal faults during compression and that it was synchronous with the development of a pull-apart basin. A semi-regional 3D geological model, which was developed mainly from the LIDAR data, supports the hypothesis of inversion of the thick Om beds to the west before or during compression of the continental promontory to the east resulting in dextral strike–slip offsets across the PTZ. A jog or relay in the faults occurred and caused a pull-apart collapse basin to develop in the area of the Porgera mine. Similar pull-apart graben, or negative flower structures, were detected nearby and may be areas for future exploration.

Differential deformation of a strike-slip fault in the Paleozoic carbonate reservoirs of the Tarim Basin, China

Ultra-deep carbonate reservoirs buried over 7000 m are distributed around strike-slip faults, which control hydrocarbon accumulation and enrichment in the Tarim Basin, China. This study proposes a fracture development index (FDI) method for analyzing the width of the strike-slip fault zone based on 3D seismic data. Based on the FDI method and geological and geophysical data, we analyze the structural characteristics of the SB5 strike-slip fault and the scaling of fault zone width and throw.In map view, at depth the strike-slip fault is characterized by single fault segments, extensional overlaps, and contractional overlaps, at intermediate depths by left-stepping en echelon normal faults, and in shallow layers by right-stepping en echelon normal faults. In cross section, we find composite flower structures and positive flower structures and medium-shallow layer negative flower structures or deep layer negative flower structures and medium-shallow layer negative flower structures. The formation time of deep, middle, and shallow layer faults is the middle Caledonian, Late Caledonian, and Late Hercynian. The heterogeneity of the strike-slip fault zone varies from strong to weak from contractional overlaps, to extensional overlaps, and single fault segments. Strike-slip fault-related fractures are dominated by three sets intersecting with strike-slip faults at small angles. Where throw is less than 100 m, the fault zone width and throw have a good power law relationship. Where the throw is over 100 m, the width tends to diminish with throw.

Afterslip on Conjugate Faults of the 2020 Mw 6.3 Nima Earthquake in the Central Tibetan Plateau: Evidence from InSAR Measurements

ABSTRACT Afterslip could help to reveal seismogenic fault structure. The 2020 Mw 6.3 Nima earthquake happened in a pull-apart basin within the Qiangtang block, central Tibetan plateau. Previous studies have explained the coseismic and early (<6 mo) postseismic deformation by rupture and afterslip on a normal fault bounding the western side of the basin. Here, we resolved the 19-month Interferometric Synthetic Aperture Radar-measured sequences of postseismic displacements that revealed a second postseismic displacement center ~12 km to the east of the main fault. Fitting the postseismic displacement requires afterslip on both the main fault and an antithetic fault that probably forms a y-shaped pair of conjugate faults in a negative flower structure. Stress-driven afterslip models suggest that the required afterslip on the antithetic fault could be triggered by coseismic rupture of the main fault or by a simultaneous rupture on the antithetic fault. The afterslip on both faults occurred mainly up-dip to the coseismic slip and has released moment ~15%–19% of that by the coseismic rupture. These results provide insights into active extension in the central Tibetan plateau and highlight the complex nature of fault rupture and afterslip.

Geometry and Kinematics of strike-slip faults in the Dibrugarh field of the Upper Assam foreland basin, NE India

Sedimentary basins within fold-and-thrust belts preserve the structural record of complex deformations and are essential to many orogenic systems worldwide. One of the component of such deformations is strike-slip tectonics. This research uses high-quality, three-dimensional seismic reflection data to investigate the geometry and kinematics of strike-slip faults developed within a geologically complex region in the Upper Assam foreland basin, NE India. Faults in the basin have a dominant NE-SW trend, with most faults in the SE and SW parts exhibiting sinistral strike-slip. These lateral movements are evidenced by extensional horsetail splay faults, negative flower structures, and minor transfer faults. The displacement profiles of the strike-slip faults also reveal complex segmentation, linkage, and mechanical interactions at different structural levels, indicating their prolonged histories and development. Tectonic tilts of the basin, along with sediment loading from the bordering fold-and-thrust belts over the geologic time, structurally changed the basin configuration giving rise to strike-slip deformations in the study region. These findings contribute to our knowledge of sedimentary basins within fold-and-thrust belts and their structural records of complex deformations.

An integrated study for seismic structural interpretation and reservoir estimation of Sawan gas field, Lower Indus Basin, Pakistan

The information about the subsurface structure, type of fluids present in the reservoir, and physical properties of the rocks is essential for identifying potential leads. The integrated approach of petrophysical analysis, seismic data interpretation, seismic attributes analysis, lithology, mineralogy identification, and Gassmann fluid substitution were used for this purpose. The structural interpretation with the help of seismic data indicated the extensional regime with horst and graben structures in the study area. The two negative flower structures are cutting the entire Cretaceous deposits. The depth contour map also indicate favorable structures for hydrocarbon accumulation. The four possible reservoir zones in the Sawan-01 well and two zones in the Judge-01 well at B sand and C sand levels are identified based on well data interpretation. The main lithology of the Lower Goru Formation is sandstone with thin beds of shale. The clay types confirm the marine depositional environment for Lower Goru Formation. The water substitution in the reservoir at B sand and C sand levels indicated increased P-wave velocity and density. The water substitution affected the shear wave velocity varies slightly due to density changes. The cross plots of P-impedance versus Vp/Vs ratio differentiate the sandstone with low P-impedance and low Vp/Vs ratio from shaly sandstone with high values in the reservoir area. The P-impedance and S-impedance cross plot indicate increasing gas saturation with a decrease in impedance values. The low values of Lambda-Rho and Mu-Rho indicated the gas sandstone in the cross plot.

Structures evolution along strike-slip fault zones: The role of rheology revealed by PIV analysis of analog modeling

This study investigates how rheology can influence the nucleation and evolution of strike-slip structures and, consequently, the structural architecture. To that end, we carried out a series of analog experiments using materials with different rheological behavior. The materials' properties, such as the angle of internal friction, cohesion, and density, were measured. The Particle Image Velocimetry (PIV) technique was applied to investigate the strain pattern on the surface, the amplitude of damage zones and fault core, the activeness/inactiveness of the structures over the evolution of strike-slip fault, and the earliest nucleation stage of fractures. Two sets of experiments were performed: i) single-layer experiments constituted by rheologically homogeneous material (quartz sand, dry clay, plaster powder, and a mixture of quartz sand and plaster powder), and ii) multilayer experiment using a heterogeneous stratified sequence of materials with different rheology. Except for the plaster powder model, during the first increments of deformation, the strain is widely distributed to give rise to a deforming zone with varying widths, where subsidiary structures nucleate. As deformation evolves, the deformation zone progressively narrows as the maximum strain concentrates along the main fault. Nucleation, development, and relative chronology of structures, such as R, R', P, Y-fractures, pull-apart basins, and positive and negative flower structures, varied according to the material used. The structural architecture of the dry clay and plaster powder models are more similar, with a greater quantity of smaller fractures than the other models. P-shears and pull-apart basins are also present. The quartz sand and mixture models developed a more complex damage zone and a higher topographic relief. The results emphasize that rheology is a controlling factor in the deformation zone architecture. Rheology controls fracture and fault arrangement, structure relative chronology, and the localization of contractional and extensional domains over all the stages of development of strike-slip tectonics.

Analyzing the seismic attributes, structural and petrophysical analyses of the Lower Goru Formation: A case study from Middle Indus Basin Pakistan

The purpose of this research is to delineate the structures of the Lower Goru Formation, investigate fluid properties, and clarify the hydrocarbon-prone areas through seismic attributes analysis. First, the acquired data was matched by the interpretation datum. Structural analysis was done by performing horizon interpretation, fault interpretation, and contour mapping on the C-Interval of the Lower Goru Formation. Hydrocarbon zones were marked with the help of attribute analysis on seismic sections and were justified by petrophysical analysis. An integrated approach such as seismic structural interpretation, seismic attribute, spectral decomposition, and petrophysical analyses was used in current research to better understand geological structure and features. This research showed that normal faults are present in the area showing negative flower structure, horst and graben, and faults oriented north-west to south-east. The contour map shows structural inclination and faults bound closure near well locations. Variance attribute and spectral decomposition attribute were used to verify horizon lineation and fault behavior. Instantaneous amplitude and instantaneous phase attributes justify hydrocarbon bearing zones, and bright spots are present on seismic sections at C–Interval of Lower Goru Formation. Petrophysical analysis of the available wells showed a number of significant hydrocarbon zones having more than 55% of hydrocarbon saturation at the C-Interval of the Lower Goru Formation. The four possible reservoir zones in Sawan-02 well, two zones in Sawan-07 well, and three zones in Sawan-09 well are identified based on well data interpretation. Based on these analyses, the area of interest has a very good reservoir potential, structural closure, and visible bright spots. The current finding of this research will be helpful for future exploration and development of the Sawan area.

Fault characteristics and their control on oil and gas accumulation in the southwestern Ordos Basin

3D seismic data recently acquired from the Ordos Basin shows three sets of regularly distributed fault systems, which overrides previous understanding that no faults were developed in this basin. Seismic interpretation suggests that the faults in the southwestern Ordos Basin have three basic characteristics, namely extreme micro-scale, distinct vertical stratification, and regularity of planar distribution. These NS-, NW-, and NE-trending fault systems developed in the Meso-Neoproterozoic – Lower Ordovician strata. Of these, the NS-trending fault system mainly consists of consequent and antithetic faults which show clear syndepositional deformation. The fault systems in the Carboniferous – Middle-Lower Triassic strata are not clear on seismic reflection profiles. The NW- and NE-trending fault systems are developed in the Upper Triassic – Middle Jurassic strata. Of these, the NW-trending fault system appears as a negative flower structure in sectional view and in an en echelon pattern in plan-view; they show transtensional deformation. A NE-trending fault system that developed in the Lower Cretaceous – Cenozoic strata shows a Y-shaped structural style and tension-shear properties. A comprehensive analysis of the regional stress fields at different geologic times is essential to determine the development, distribution direction, and intensity of the activity of fault systems in the Ordos Basin. Current exploration suggests three aspects in which the faults within the Ordos Basin are crucial to oil and gas accumulation. Firstly, these faults serve as vertical barriers that cause the formation of two sets of relatively independent petroleum systems in the Paleozoic and Mesozoic strata respectively; this is the basis for the ‘upper oil and lower gas’ distribution pattern. Secondly, the vertical communication of these faults is favorable for oil and gas migration, thus contributing to the typical characteristics of multiple oil and gas fields within the basin, i.e. oil and gas reservoirs with multiple superimposed strata. Finally, these faults and their associated fractures improve the permeability of Mesozoic tight reservoirs, providing favorable conditions for oil enrichment in areas around the fault systems.

Determination of the coal-bearing zones and the alteration zones containing uranium ore by using two dimensional (2D) seismic reflection method in Thrace Basin

It is aimed to reveal the basement topography, the coal-bearing levels, the alteration zones containing uranium ore, and the tectonic structure prevailing by using 2D seismic reflection method in the Thrace Paleogene-Neogene Basin. In this context, seismic data collected on the six profiles were interpreted by correlating with the data of 97 wells. In the seismic lines, respectively, the Metamorphic Basement-Eocene boundary, the top of the coal-bearing zone and the boundary of the Danişmen-Ergene formations were confirmed by using the borehole data. By evaluating seismic data, the coal propagation is modeled with 3D figures. Moreover, coal accumulation starts from the southwest of the field and continues towards the northeast, and it is supported by the results obtained from the borehole data. The presence of uranium ore in some of the alteration zones and borehole data indicated that all alteration zones determined should be inspected for uranium ore. In addition to normal and reverse faults, positive and negative flower structures formed in the strike-slip fault zones were determined, and lignite deposits were cut in the flank of these structures. Finally, it is recommended to carry out seismic studies before drilling, to investigate potential coal and uranium areas and to plan more seismic lines.

A back‐arc transtensional origin for the Nanpanjiang basin in the pre‐Norian Triassic, with implications for the broader intracontinental development of South China

AbstractThe generally accepted foreland interpretation of the Nanpanjiang basin has been regarded as providing support for the hypothesized Indosinian collision between the South China and Indochina blocks, and it has led to the long‐standing view that crustal shortening dominated the Triassic intracontinental development of South China. Here, we examine the stratigraphic architecture, magmatism and gross structural features of the Nanpanjiang basin. Outcrop studies indicate that a north‐stepping axial turbidite system was confined by intrabasinal faults, the active development of which is indicated by mass‐transport complexes. Field mapping combined with U–Pb dating of baddeleyite and zircon reveals that magmatism in the southern part of the basin coincides with a distinct episode of bimodal volcanism from ca. 244 to 242 Ma, and is unrelated to the middle Permian Emeishan plume. Differences in tectonic subsidence between hanging‐wall and footwall blocks, as that has been quantified by the backstripping of composite sections, show that synsedimentary dip slip along intrabasinal faults was of normal sense. Meanwhile, a role for strike‐slip deformation is registered by the left‐lateral offset of middle Permian and Middle Triassic piercing points and the exceptional preservation in outcrop of syndepositional negative flower structures. For these reasons, therefore, the pre‐Norian Triassic Nanpanjiang basin is thought to represent a transtensional back‐arc setting, a result that leads to an unresolved paradox: in adjacent parts of South China to the east, coeval development involves crustal thickening, nonmarine sedimentation and granitic plutonism. A new lateral decoupling model consistent with palaeomagnetic evidence for large‐scale block rotation is tentatively proposed here as a working hypothesis to explain the observed intracontinental evolution of South China during the Triassic assembly of eastern Asia. We propose that such decoupling may relate in part to the configuration of inherited crustal weaknesses.

The Tucano Basin (Cretaceous, Brazil) – a world-class example of an aborted rift system with anomalous depth

The structural and stratigraphic characteristics of the central portion of the Tucano Basin, a Cretaceous continental rift basin in northeast Brazil, was investigated, using post-stack 2-D reflection seismic, gravity and well data. Methodology was steered by the principles of seismic stratigraphy, targeting at an adequate identification of stratal terminations and on the mapping of structural features, where a negative flower structure was interpreted. The identification of erosional truncations led to a stratigraphic framework of seven intra-rift unconformities, forming eight third-order rift sequences, while the literature on the Tucano basin recognizes only two depositional sequences forming the rift sedimentary infill. In order to study the abnormal depth of Central Tucano depocenter, gravity data were processed and a spectral analysis was performed, which located the depocenter at approximately 16.8 km, a rare configuration for a rift basin. Gravity models were created in order to verify the seismic interpretation and the depth found by the spectral analysis; and to provide a view of the basin in a regional scale, analysing its crustal architecture. The modelling confirmed the abnormal depth and also indicated a severe crustal thinning under Central Tucano Basin, where the Moho was interpreted around 24 km and a major crustal necking was observed. The models introduced the geometry of the lower continental crust and indicated the presence of a high-density layer (3000 kg/m3) at its base, which this work suggests to be a result of magmatic underplating. The paper closes discussing the role of tectonics and basement inheritance over the Central Tucano Basin structural framework and commenting on the mode of extension for the Recôncavo-Tucano-Jatobá rift system.

Dynamic processes of upper plate deformation at the New Guinea Trench

The interactions between arc-continent collision and subduction zones are still poorly understood. Here, we use 2D seismic data to document upper plate deformational features and identify different structural styles along the New Guinea Trench, western Pacific, and discuss their broader significance. In the western section, normal faults and negative flower structures occur within the strata of the upper plate. An unconformity represents the signal of tectonically driven subsidence. In the eastern section, both thick-skinned and thin-skinned thrusting are found within the frontal upper plate. In the central section, rare extensional or shortening deformation is identified within the forearc basement and overlying strata. Horizontal, thick sedimentary sequences within the Caroline subbasin have been subducted beneath the upper plate. This suggests that Miocene subduction in the eastern section led to the formation of a thrust-deformed ophiolitic basement in the upper plate. Since the Miocene, strike-slip tectonic processes dominated in the upper plate, causing compressional deformation in the strata in the eastern section and extensional to transtensional deformation in the strata in the western section. The strike-slip tectonic processes might have been weakened in the upper plate in central section due to the incoming Eauripik Rise. Over geologic time, arc-continent collision processes and related changing of regional tectonic forces can have a long-lived influence on shaping and deforming subduction systems.

Seismic interpretation and hydrocarbon assessment of the post-rift Cenomanian Bahariya reservoir, Beni Suef Basin, Egypt

The geophysical interpretations for a collection of high-quality 2D seismic data plus well logging data for six drilled wells exposed the structural controls and the hydrocarbon potentiality of the Beni Suef Basin, north Western Desert, Egypt. The created structural map on the top of the Early Cenomanian Bahariya Formation (the significant oil reserve in north Western Desert), exhibits a series of NW–SE normal faults as well as a master ENE strike-slip major fault. The extensional faults formed horst structures while the strike-slip fault created a negative flower structure in the central part of the study area. These structural features represent the main petroleum traps within Beni Suef Basin. The reservoir quality was assessed through the petrophysical analysis of the Bahariya Formation in wells; Azhar A2, Azhar E-2X, Azhar E3, Yusif-4X, Fayoum-2X and Beni Suef W-1X. This analysis proved that the Lower Bahariya unit has better reservoir quality than the Upper Bahariya reservoir, with shale volume less than 10%; effective porosity between 10 and 20%, water saturation between 25 and 75% and total pay thickness between 11 and 88 ft. The obtained results encourage drilling more exploratory wells in Beni Suef Basin to enhance the productivity from the sandstones of the Lower Bahariya reservoir.