Extensional Faults Research Articles

Petroleum geological features and hydrocarbon enrichment of Linhe Depression in Hetao Basin, NW China

Based on paleogeomorphology, drilling and seismic data, this paper systematically studies the structural and sedimentary evolution, source rock characteristics, reservoir characteristics and formation mechanism, hydrocarbon accumulation model and enrichment law in the Linhe Depression of the Hetao Basin, NW China. The Hetao Basin mainly experienced three stages of evolution, namely, weak extensional fault depression, strong extensional fault depression and strike-slip transformation, giving rise to four positive structural belts (Jilantai, Shabu, Nalinhu and Xinglong), which are favorable areas for oil and gas accumulation. The two main saline lacustrine source rocks, Lower Cretaceous Guyang Formation and Oligocene Linhe Formation, are characterized by high sulfur content, rich algae, early maturity, early expulsion, and wide oil generation window. The large structural transition belt in the intermountain area around the Hetao Basin controls the formation of large-scale braided river delta deposits, which are characterized by high quartz content (50%–76%), long-term shallow burial and weak compaction, low cement content, and good reservoir properties in delta front sandbody. The burial depth of the effective Paleogene reservoirs is predicted to reach 8000 m. Three hydrocarbon accumulation models, nose-uplift near sag, buried hill surrounding sag, fault nose near source rock, are constructed. The law of hydrocarbon accumulation in the Linhe Depression is finally clarified as follows: near-source around the depression is the foundation, high-quality thick reservoir is the premise, good tectonic setting and trap conditions are the key.

Structure and dynamics of the Ecuador Fracture Zone, Panama Basin

SUMMARYIn this study, multiple geophysical data types are used to investigate the structure and dynamics of the Ecuador Fracture Zone—a complex multistranded strike-slip fault system located in the Panama Basin. Gravity modelling reveals a 25–30-km-wide region of ∼3-km-thick, low-density crust beneath this system and an anomalously low-density region in the uppermost mantle. Along both edges, the transition to the ‘normal’ structure and thickness oceanic crust formed at both the Ecuador and Costa Rica Rifts is abrupt. Within the Ecuador Fracture Zone itself, normal faults bound the median ridges. These faults traverse the entire thickness of accumulated sediment and offset the seabed, while sediment layer geometries document multiple phases of relative uplift, with the most recent phase still ongoing. Active extensional faulting, with an approximately spreading ridge-parallel strike, is also observed in 6–7 Ma Costa Rica Rift crust. The median ridges and the transverse ridge at the eastern edge of the Ecuador Fracture Zone also have contrasting crustal density structures. Both median ridges have a lower density crust than between the intervening valleys, while the transverse ridge crust has an equivalent thickness and density structure to that formed at the Costa Rica Rift. The active median valley basement-cutting normal faults allow seawater ingress and alternation of the crustal footwall, and also flow to mantle depth where, based on gravity modelling, 30–50 per cent serpentinization of mantle peridotite occurs. The resulting serpentinite-driven buoyancy acts as the primary control on the observed median ridge relative vertical tectonism. In contrast, the relative uplift of the transverse ridge results from lithospheric flexure in response to a change in spreading direction between the Ecuador and Costa Rica Rifts. Contrary to the widely accepted assumption that fracture zones are tectonically inactive systems, the Ecuador Fracture Zone provides evidence of extension, serpentinization due to ongoing hydrothermal circulation and relative uplift.

The effects of subsalt relief on gravity-driven salt tectonics: Results from analogue modelling

Gravity-driven salt-related deformation on passive margins is commonly interpreted as kinematically-linked domains of updip extension and downdip contraction with an intermediate, undeformed zone of translation. Our study uses analogue models to show how salt flow is affected by the subsalt geometry resulting in complex deformation and the alternation of extensional and contractional domains in space and time in translational salt provinces. In our work, the analogue models incorporate subsalt relief associated with half-graben and graben that are realistic geometries along passive margins and combine the effects of gravity spreading in which deformation is driven by differential sedimentary loading, and an early-stage of gravity gliding induced by basin tilt. This approach allows for analysis of how salt flow and overburden deformation are affected by: (1) smoothly dipping subsalt relief; (2) subsalt relief characterized by half-graben with a gently landward dipping extensional fault; (3) subsalt half-graben connected laterally with a distal symmetric graben. For each of these models, we evaluate: 1) the interplay between slipping deformation corresponding to gravity gliding, gravity spreading mechanisms, and subsalt geometry; 2) the spatial and temporal distribution of salt-related structural domains; and 3) the contrasting styles of salt tectonics for different subsalt geometries. In addition, some of the experiments have been repeated considering thicker salt and pre-kinematic overburden and different sedimentation rates. We show that the structures of the subsalt relief variably resist salt flow producing flow mismatch. This variable resistance controls the orientation of the extension and contraction supra-salt structures. Salt surplus and contraction occur over landward dipping base-salt ramps, and a zone of salt depletion and subsidence limited by extensional and contractional hinges occurs over the basinward dipping base-salt ramp. Variations in width and the dip of these base-salt ramps result in stronger flow mismatches and deformation complexity. We also discuss variations in salt flow profile in response to the gravitational forces that drive deformation, allowing a good first-order approximation of viscous salt flow at a regional scale that can be directly compared to examples from various salt-bearing continental margins, particularly the relatively less understood but extremely economically important distal domains of Brazil's Campos and Santos basins.

Seismicity at the intersection of the Coast Shear Zone and Anahim Volcanic Belt near Bella Coola, British Columbia, Canada

In the Coast Mountains of western British Columbia, an anomalous seismicity concentration exists near the intersection of the Coast Shear Zone, a major northwest–southeast trending Eocene-age shear zone that accommodated deformation between the Pacific and North America plates, with the Anahim Volcanic Belt, an east-northeast–west-northwest trending zone of volcanic features that decrease in age to the east. To better characterize seismicity in the Coast Mountains, we augment the existing Natural Resources Canada seismicity catalogue by applying an automatic detection and location algorithm to both permanent Canadian National Seismic Network stations and temporary stations from the 2005–2006 BATHOLITHS deployment, resulting in 837 relocated events with at least three paired P- and S-phase picks. Double-difference relocation reveals several small-scale linear strands subparallel to the Coast Shear Zone and within the Anahim Volcanic Belt and three clusters of events striking at a high angle to the Coast Shear Zone that occurred as swarms in 2015 and 2017. First-motion focal mechanisms exhibit extensional and strike-slip faulting. Our observations indicate that most of these events are not associated with surficial processes such as landslides, but rather, we hypothesize that the interaction of the Anahim Volcanic Belt and Coast Shear Zone has weakened the lithosphere in this region, leading to current-day strain localization and high heat flow that manifest seismicity, including swarm-like activity.

Structural and tectonic assessment of the western Huincul High, Neuquén Basin (Argentina)—The role of structural inheritance and mechanical stratigraphy in inversion systems

AbstractThe Neuquén Basin is a major Mesozoic sedimentary depocentre located in the retroarc foreland of the Argentinian Andes. The basin hosts world renowned inversion systems that have been the target of georesource exploration for the last three decades. The Huincul High is a structurally and economically prominent ca. 270 km long, E–W trending feature that formed by the accretion of exotic Palaeozoic terranes, influencing subsequent Mesozoic deformation in the basin. Exploration in the Huincul High has been mainly focused on the shallow part of the inversion structures leaving a limited understanding of the deep structural architecture and early tectonic evolution, particularly in the western reaches of the high. This research reveals that Late Triassic extensional faulting was followed by widespread thermal subsidence in the Early Jurassic, as shown by the occurrence of an extensive ca. 60 km long, ca. 20 km wide, NE–SW‐trending, central depocentre. In the Early Jurassic, as contraction ensued across this regional sag basin, atypical inversion geometries were developed. These exhibited prominent thickening in the hanging‐wall and, strikingly, in the footwall of reactivated faults. The style of inversion was also markedly influenced by the mechanically weak stratigraphy of the thick, Lower Jurassic, Los Molles formation that promoted broad inversion folding, inhibited shortcut fault creation, and decoupled post inversion deformation from earlier faulting. Quantitative fault analysis suggests that the reactivated faults originated during the Late Triassic extensional phase as separate ca. 10 km long fault segments. The analysis also indicates that segmentation of extensional faults, as well as their orientation to the later contractional vector (SH), spatially dictated style and magnitude of inversion. This research highlights the critical role played by structural inheritance and mechanical stratigraphy in the development of inversion in the Neuquén Basin, which might be of relevance for characterising inversion systems elsewhere. This research also proposes an evolutionary model for the western reaches of the Huincul High that suggests crustal weakening and thermal sag in the Early Jurassic. Moreover, the model highlights a previously unknown late Early Cretaceous transtensional phase that overprints the main Early Jurassic–Early Cretaceous inversion.

Characteristics and Formation Mechanism of Water-Eroded Ground Fissures in the Weihe Basin of China

The Weihe Basin is a crucial part of the Fenwei Graben System and is a typical example of an extensional fault basin. Over time, the region has been developed around 212 ground fissures, with the majority of them being associated with heavy rainfall. These water-eroded ground fissures have caused serious damage to farmland, houses, and roads, with the Liangjiacun ground fissures, Meiyuancun ground fissures, and Dizhentai ground fissures being the most known. To understand the developmental characteristics and formation mechanisms of water-eroded ground fissures in the Weihe Basin, we utilized a variety of methods, including ground surveys, mapping, trenching, drilling, and geophysical exploration. These three ground fissures are distributed along corresponding fault zones, with their orientations being closely linked to local topographic changes. Additionally, the trenching profiles showed that all three typical ground fissure zones have distinct hidden holes above the paleosol. The formation of these ground fissures is closely associated with tectonic activity and heavy rainfall, with soil ruptures resulting from tectonic activity becoming the dominant channels for surface water infiltration. Furthermore, the erosion caused by heavy rainfall accelerates the rupture and expansion of ground fissures in the shallow subsurface. This paper is a valuable resource for disaster prevention and mitigation strategies aimed at mitigating the damage caused by ground fissures in the Weihe Basin.

Growth and transtensional reactivation of a basin-bounding fault: Chengbei Sag, Bohai Bay Basin, East China

Models of extensional fault systems are predominantly based on examples where extension is approximately orthogonal to rift boundaries and deformation can be regarded as plane strain. In contrast, extensional faults in transtensional or strike-slip settings present complex geometric and kinematic features due to rotational deformation, and distinct rift and post-rift phases may be difficult to separate. Well-constrained 3D seismic data from the Bohai Bay Basin, East China reveal the growth history and transtensional reactivation of a basin-bounding fault (Chengbei Fault). During the syn-rift phase, the Chengbei Fault reached its near-final length within ∼24 Ma via the propagation and linkage of six initial fault segments, accompanied by the accumulation of 26% of its total displacement; in the following ∼18.7 Ma of the Paleogene, it accumulated the remaining 74% of displacement without an obvious increase in fault length. Following a phase of relative quiescence in the early Neogene, the Chengbei Fault was reactivated at ∼12 Ma, associated with the enhanced activity of a continental-scale strike-slip fault. At least 17 fault segments developed quickly and linked vertically to the lower fault system of the Chengbei Fault. The plan-view en-echelon and horse-tail configuration, and the flower structure seen in section views of the fault system, indicate that the post-rift activity of Chengbei Fault has had both dip-slip and strike-slip components. A calculation of the rotational deformation using fault heaves and orientations indicates that the tectonic reactivation occurred in a strike-slip-dominated transtensional setting accompanied by an increasing extension component with time. In basins affected by strike slip, kinematic analyses that consider rotational deformation can provide more detailed and convincing information on the tectonic setting.

Basin-scale interaction between post-LGM faulting and morpho-sedimentary processes in the S. Eufemia Gulf (Southern Tyrrhenian Sea)

The integrated interpretation of high-resolution multibeam bathymetry, seismic profiles and backscatter data in the S. Eufemia Gulf (SEG; Calabro-Tyrrhenian continental margin, south-eastern Tyrrhenian Sea) documents the relationship between postglacial fault activity and morpho-sedimentary processes. Three systems of active normal faults that affect the seafloor or the shallow subsurface, have been identified: 1) the S. Eufemia fault system located on the continental shelf with fault planes mainly oriented N26E-N40E; 2) the offshore fault system that lies on the continental slope off Capo Suvero with fault planes mainly oriented N28E-N60E; 3) the Angitola Canyon fault system located on the seafloor adjacent to the canyon having fault planes oriented N60E-N85E. The faults produce a belt of linear escarpments with vertical displacement varying from a few decimeters to about 12 m. One of the most prominent active structures is the fault F1 with the highest fault length (about 9.5 km). Two main segments of this fault are identified: a segment characterised by seafloor deformation with metric slip affecting Holocene deposits; a segment characterised by folding of the seafloor. A combined tectono-stratigraphic model of an extensional fault propagation fold is proposed here to explain such different deformation.In addition to the seabed escarpments produced by fault deformation, in the SEG, a strong control of fault activity on recent sedimentary processes is clearly observed. For example, canyons and channels frequently change their course in response to their interaction with main tectonic structures. Moreover, the upper branch of the Angitola Canyon shows straight flanks determined by fault scarps. Tectonics also determined different sediment accumulation rates and types of sedimentation (e.g., the accumulation of hanging wall turbidite deposits and the development of contourite deposits around the Maida Ridge). Furthermore, the distribution of landslides is often connected to main fault scarps and fluids are locally confined in the hanging wall side of faults and can escape at the seabed, generating pockmarks aligned along their footwall.

Timing, Thrusting Mode, and Negative Inversion Along the Circeo Thrust, Apennines, Italy: How the Accretion‐To‐Extension Transition Operated During Slab Rollback

AbstractThe evolution of the Apennine wedge has seen the time‐space migration of the forebulge, foredeep, thrust wedge, and back‐arc extension phases in the wake of the Eastward rollback of the subducting Adria slab. In this framework, thrusting and post‐orogenic extensional faulting have occurred in two parallel forelandward‐migrating ribbons, with extensional deformation overprinting or partly exploiting anisotropies of the inherited thrust system. Here, we explore the tectonic framework and the timing of thrusting and subsequent negative inversion of the Circeo thrust, one of the major thrusts in the inner portion of the central Apennines, with the main aim to constrain the timing and mode of the compression to extension switch. Structural analysis, carbonate C and O and clumped isotopes analysis, X‐ray diffraction of clay minerals, and U‐Pb dating of calcite slickenfibers have been integrated with seismic interpretation, cross‐section balancing, and 1D burial and thermal modeling. We show that the Circeo thrust developed during Langhian‐Serravallian time. Its extensional reactivation is dated at the Serravallian, during the stacking of an underlying thrust slice, before the onset of Pliocene back‐arc extension in the area. Combination of our data with the age of thrusts, extensional basins, and base of the foredeep infill of the central Apennines, demonstrates that forelandward migration of the foredeep‐thrust system occurred at variable velocities. Accelerations and decelerations are synchronous, respectively, with the opening of the Liguro‐Provençal and Tyrrhenian back‐arc basins and with the interluding quiescent period.

Alpine transpression in the Passo Rolle area (Dolomites, Italy): new structural and paleostress constraints

The Dolomites geologically belong to the south-verging eastern Southern Alps of Italy. They record the effects of multiple deformation episodes from the Permian onward and thus offer the possibility to study the role of earlier inherited structures upon the Cretaceous-Neogene Alpine deformation history. In the Passo Rolle area, Permian extension was followed by Late Triassic-Early Jurassic extensional tectonics during which the NNW-SSE striking Passo Rolle Fault (PRF) contributed to the development of horst and graben structures. The Alpine orogenesis in the Passo Rolle area was responsible for the build-up of the Pale di San Martino mountain range during ~ NNW- SSE compression. During this stage, the NNW-verging Cimon della Pala backthrust formed concomitant with the activation of minor thrusts and strike-slip faults, and the reactivation of Late Triassic-Early Jurassic extensional faults. The genetic relationships between the broad suite of tectonic structures in the Passo Rolle area have never been investigated in detail. To this end we performed mesostructural analysis of key representative exposures to add geometric and kinematic constraints on the tectonic structures associated with the different tectonic phases registered in the Passo Rolle area. We focused on collecting fault-slip data that we used for paleostress analysis aiming at reconstructing the local paleostress evolution through time. We show that (i) the PRF accommodated significant Alpine transpression and (ii) the Permian-to-Jurassic tectono-stratigraphic framework promoted the localization of Alpine deformation to the east of the PRF, where a thick carbonate-siliciclastic multilayer deformed through first-order folds and thrusts. To the west of the PRF, where the Permian ignimbrite formed instead a relatively rigid and thus less deformable block, only minor thrusts and strike-slip faults formed, commonly exploiting suitably oriented inherited structures.

Structural control of inherited salt structures during inversion of a domino basement-fault system from an analogue modelling approach

Abstract. The geometries of inverted rift systems are different depending on a large variety of factors that include, among others, the presence of decoupling layers, the thickness of the pre- and syn-extension successions, or structural inheritances. Our study focuses on the inversion of an extensional domino-style basement-fault system with a pre-extension salt layer using analogue models to understand the role of pre-existing structural features during inversion. Models investigate how different overburden and salt thicknesses, inherited extensional structures, and salt distributions condition the evolution during inversion. The experimental results show that models with thick salt can partially or totally preserve the extensional ramp–syncline basin geometry independently of the overburden thickness. In contrast, models with a thin salt layer result in a total inversion of the ramp–syncline basins with the development of crestal collapse grabens and extensional faults affecting the overburden. Inversion also triggered the growth or reactivation of salt-related structures such as primary weld reopening and/or obliteration, diapir rejuvenation, salt thickening, or thrust emplacement. The use of analogue modelling allowed us to address the processes that controlled the growth and evolution of these structural elements during the inversion. Experimental results also provide a template of different structural styles resulting from the positive inversion of basins with a pre-extensional salt layer that can help subsurface interpretation in areas with poor seismic imaging.

Using seismic and well data to constrain hydrocarbon trap potential of offshore Lamu Basin, Kenya

Structural analysis of seismic data, combined with well data, provides information for hydrocarbon exploration in a prospective basin. These techniques have been employed in the offshore Lamu Basin to characterize structural and stratigraphic setting of carbonate reefs and assess structural deformation and its effect on trapping mechanisms in the offshore basin. Structural interpretation was based on seismic and well data. A combination of microfossil, nannofossil and palynomorph data provided broad age-control for the stratigraphic interpretations. Seismic amplitude brightening attribute was used for tracing structural continuity. Faulted anticlinal traps were formed in the Upper Cretaceous and Tertiary periods. There are inversion features with recent extensional faulting. Favourable structural traps are in the near coastal region. The carbonate reefs in the offshore basin are interpreted from the Early Paleocene to Miocene periods. These reefs formed in the near coastal region, are likely to be quality traps/reservoirs for hydrocarbon accumulation. Five main structures are identified from the depth seismic maps for the Early Paleocene, Late Paleocene and Early Miocene. The reef front may have subsided early, the overlying shales offering the sealing mechanisms. The Davie-Walu transformation was likely to affect the trapping mechanism in the central and southern regions of offshore Lamu Basin. The study provides better understanding of the carbonate reefs’ hydrocarbon potential and trapping mechanisms in the offshore basin.

Sentinel-1 Interferometry and UAV Aerial Survey for Mapping Coseismic Ruptures: Mts. Sibillini vs. Mt. Etna Volcano

The survey and structural analysis of surface coseismic ruptures are essential tools for characterizing seismogenic structures. In this work, a procedure to survey coseismic ruptures using satellite interferometric synthetic aperture radar (InSAR) data, directing the survey using Unmanned Aerial Vehicles (UAV), is proposed together with a field validation of the results. The Sentinel-1 A/B Interferometric Wide (IW) Swath TOPSAR mode offers the possibility of acquiring images with a short revisit time. This huge amount of open data is extremely useful for geohazards monitoring, such as for earthquakes. Interferograms show the deformation field associated with earthquakes. Phase discontinuities appearing on wrapped interferograms or loss-of-coherence areas could represent small ground displacements associated with the fault’s ruptures. Low-altitude flight platforms such as UAV permit the acquisition of high resolution images and generate 3D spatial geolocalized clouds of data with centimeter-level accuracy. The generated topography maps and orthomosaic images are the direct products of this technology, allowing the possibility of analyzing geological structures from many viewpoints. We present two case studies. The first one is relative to the 2016 central Italian earthquakes, astride which the InSAR outcomes highlighted quite accurately the field displacement of extensional faults in the Mt. Vettore–M. Bove area. Here, the geological effect of the earthquake is represented by more than 35 km of ground ruptures with a complex pattern composed by subparallel and overlapping synthetic and antithetic fault splays. The second case is relative to the Mt. Etna earthquake of 26 December 2018, following which several ground ruptures were detected. The analysis of the unwrapped phase and the application of edge detector filtering and other discontinuity enhancers allowed the identification of a complex pattern of ground ruptures. In the Pennisi and Fiandaca areas different generation of ruptures can be distinguished, while previously unknown ruptures pertaining to the Acireale and Ragalna faults can be identify and analyzed.

Extensional tectonics, structural architecture modeling and geodynamic evolution in the Cretaceous Tinghir-Errachidia-Boudenib basin (Pre-African Trough, Morocco)

Structural evolution of the Cretaceous Tinghir-Errachidia-Boudenib foreland basin in the south front of the Moroccan High Atlas belt is documented here. Fieldwork, lithostratigraphic and new structural analysis coupled with borehole and geoelectrical data identify two ENE-to NE-trending sub-basins in the large Cretaceous Tinghir-Errachidia-Boudenib (TEB) basin. One named Errachidia sub-basin is situated in the west and the other Boudenib sub-basin is located in the east. They are separated by a major NE-SW trending anticlinal, inherited from a paleohigh structure, where the thickening of the Cretaceous series increases towards the center of the sub-basins and in their northern borders with the central-eastern High Atlas. The Cretaceous series clearly show a considerable thickness variation, angular unconformities and lateral facies change that was related to the syn-sedimentary normal faulting activity. The Albian-Turonian period is characterized by the development of ENE- and NW-trending fault systems. These latter are linked to an extensional tectonics event responsible for half-graben basin geometry associated with significant sedimentary rates. This considerable thickness depocenters geometry is related to normal fault coupled with the intra-basin faults. The intensity of subsidence increases from south to north and from west to east, with a high rate in the Boudenib sub-Basin. The compressive Cenozoic events caused the inversion of these extensional faulting and the folding of the High Atlas area with ENE-trending structures.

Deciphering the Structural History of Ulysses Fossae, Mars, Using Fault Pattern Analysis

AbstractUlysses Fossae is a faulted region surrounded by lava flows that sits between the major volcanoes of the Tharsis Rise volcanic province on Mars. This area is unique, as it is the only exposure of extensional faulting which can be related to the Olympus Mons volcano and one of the only a few faulted areas relating to the Tharsis Montes. In order to determine the area's structural evolution through time, we mapped all identifiable faults, divided them into 10 fault groups determined by their orientation and morphology, and performed detailed crater size‐frequency analysis of the geological units. We divide the fault groups into two overall types based on their genesis: local diking and regional extension. The complex structural evolution recorded in the extensional faults in Ulysses Fossae is dominated by local dike‐related activity (8/10 fault groups), along with deformation from large, regional scale extensional processes related to the development of the Tharsis province as a whole (2/10 fault groups). Strain values measured across the 1 fault groups vary between 0.4%–2.2%, with the grabens related to regional extension accommodating larger extensional strains than the local dike‐related grabens. The crater size‐frequency distribution analysis of three distinct areas in Ulysses Fossae (UF Dome, UF North, and UF South) revealed that the majority of the faulting in Ulysses Fossae was active during the Early Amazonian.

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.

Structural characteristics and genetic mechanism of transfer zones in an extensional rift zone: An example from the Xihu Sag, East China Sea Basin

The Xihu rift zone within the East China Sea Shelf Basin (ECSSB) is governed by NE-striking segmented extensional faults, which were formed by Cenozoic orthogonal NW-SE extension. However, the segmentation of the NE-striking faults has up to now received only limited attention. Here, we employ seismic data, information about pre-existing faults, the Cenozoic fault system, and fault activity rate to analyze the structural characteristics and genetic mechanism of the transfer zones in the Xihu sag. Our results demonstrate that: (1) The Cenozoic structures and evolution of the Xihu sag are fundamentally controlled by the NE-striking extensional faults, while showing influence from the pre-existing NW-striking basement structures. (2) The transfer zones are preliminarily recognized on the fault system map along the bottom of the Pinghu Formation (T40 surface, ∼43 Ma), which is segmented by the NE-striking faults. The cross faults are identified in the transfer zones, which are commonly truncated by the T30 (∼33.9 Ma) surface and above the T100 (∼66 Ma) surface. These faults are generally composed of strike-slip structures. In addition, the transfer zones are primarily developed above the pre-existing NW-striking basement faults. (3) A hybrid genetic model is proposed to interpret the formation of the NW-striking strike-slip transfer zones in the Xihu sag, including the influence of the pre-existing NW-striking basement faults, and the different extensions along the rifts. Finally, six genetic-mechanism-structural models are established for general cases, based on whether the pre-existing faults are developed and the different extensions existed or not. This study may have significant implication for future study on the transfer zones in this area.

Cenozoic Fault Growth Mechanisms in the Outer Apulian Platform

This work focuses on a ca. 55 km-long extensional fault zone buried underneath the foredeep deposits of the southern Apennines, Italy, with the goal of deciphering the Cenozoic fault growth mechanisms in the Outer Apulian Platform. By considering public 2D seismic reflection profiles, well logs, and isochron maps data, the study normal fault zone is interpreted as made up of four individual fault segments crosscutting Top Cretaceous, Top Eocene, Top Miocene, and Top Pliocene chrono-stratigraphic surfaces. The computed cumulative throw profiles form either bell-shaped or flat-shaped geometries along portions of the single fault segments. The computed incremental throw profiles also show an initial fault segmentation not corresponding with the present-day structural configuration. Data are consistent with the initial, post-Cretaceous fault segments coalescing together during Miocene–Pliocene deformation and with fault linkage processes localizing at the stepover/relay zones. Pleistocene faulting determined the evolution of a coherent fault system. The computed n-values obtained for the single time intervals by considering the maximum fault throw–fault length relations indicate that the fault segments formed scale-dependent geometries. Variations of these computed values are interpreted as due to the higher degree of maturity reached by the entire fault system during Miocene to Pleistocene deformation.

Basement structures exert a crucial influence on the structural configuration of later rift development: Insights from the Shanan and Chengbei sag, Bohai Bay Basin

The Early Cretaceous and Paleogene rift structures of the Shanan and Chengbei sags were inherited from basement tectonic deformation of Indosinian and Yanshanian tectonism. However, the architecture of the basement structures and its influence on the structural configuration of later rift development are poorly understood, which impeded hydrocarbon exploration efforts in the study area. Based on 2D high-resolution seismic reflection data in the study area and onshore field data in the Luxi area, we recognize the Indosinian NW-striking low-angle fold-thrust belts and Yanshanian NE-striking high-angle thrusts in basement of the study area. The pre-rift NW- and NE-striking basement structures exert a crucial influence on the two-phase rift architecture and structural evolution of the study area: (1) Low-angle NW-striking Indosinian thrusts are dominantly reactivated as dip-slip major normal faults during Early Cretaceous, such as the Shanan fault and Chengbei faults. The high-angle Yanshanian NE-striking thrusts are primarily reactivated as left-lateral strike-slip faults during this stage, such as the Shaxi fault. (2) During the Paleogene, oblique extension took place across the pre-existing NW-striking reactivated normal faults. A set of half-grabens formed along the major NW-striking oblique-slip faults and large amount of ENE-striking secondary normal faults developed in the NW-striking sags. The pre-existing left-lateral NNE-striking faults shifted to become right-lateral and extensional faults. The Early Cretaceous rift was primarily controlled by NE-SW-directed extension, which was derived from the large-scale left-lateral strike-slip movement of the Tan-Lu Fault Zone. The Paleogene oblique rift was induced by NNW-SSE-directed extension related to the upwelling water-rich mantle wedge of the Pacific subduction zone.

Synsedimentary slump folding: Examples and consequences of an under-recognized process in epicratonic basins

Small-scale synsedimentary folds in the Eagle Ford Formation in southwest Texas formed during deposition in a shallow epicratonic basin on the North American craton. These folds formed by slumping (i.e., gravity-driven deformation) and are characterized by (i) plastic deformation of limestone beds, (ii) discontinuous bedding and thickness changes of limestone beds, (iii) horizontal compactional fabric at high angle to folded bedding, (iv) low-angle or recumbent axial surfaces, and (v) erosion of slumped intervals and/or onlapping by resumed sedimentation. Exposures of the Eagle Ford near the northwest margin of the Maverick Basin reveal slump folds in 1–2 m thick zones with observed lateral extents of 10's to 100's of meters to >1 km – actual extents are likely much larger. Curvilinear fold hinges changing trend by >90° over short distances, indicating markedly non-cylindrical fold geometries. Bed thickness changes reflect flowage of unlithified sediment during slumping, and subsequent compaction. Previous descriptions of slump folds in the Eagle Ford Formation have focused on examples from the hydrodynamic facies of the basal portion, but slump folds are also present in pelagic facies of the upper Eagle Ford. Slump folding locally represents significant shortening (e.g., 50%) and thickening (e.g., 100%) of slumped intervals, with up-dip absence of the same interval represented by missing section, bed terminations, boudinage, and/or extensional faulting. Repeated slumping can result in significant cumulative updip thinning and downdip thickening along depositional slopes. Our experience working unconventional reservoirs in a variety of North American epicratonic basins indicates that slump folding is common and underrepresented in the literature. Criteria for recognizing slumping in the subsurface include a combination of discontinuous and contorted beds, thickening into fold hinges, compactional fabric postdating folds, and inverted or repeated section. Natural fracture networks may be less regular than in similar but flat-lying beds. Occurrence of slump folded intervals adds complexity and unpredictability to unconventional reservoirs, with implications for reservoir quality, drilling, and optimal stimulation during completion.