Structural Wedges Research Articles

Seismicity in the Weiyuan-Rongxian area, Sichuan basin, SW China

The Weiyuan-Rongxian shale gas field in the Sichuan basin, China, has experienced numerous earthquakes in recent years, causing significant casualties and property loss. To investigate the spatial–temporal evolution of seismicity and seismic fault structures, in this study, we first utilize a deep-learning-based picking algorithm –(PhaseNet) to identify seismic phases from raw seismic data between March 2019 and October 2020, and then employ a Rapid Earthquake Association and Location technique (REAL) to associate these seismic picks. Finally, the VELEST and GrowClust methods are performed to build a high-precision earthquake catalog containing 18,912 earthquakes. Our results reveal that the spatial–temporal distribution of earthquake swarms indicates a complex fault system. In the northern part of Weidong block (R1), the earthquakes illuminate a shovel-shaped structure that the dip angle of the fault plane gradually decreases with depth. In the south of Weidong block (R2), the natural fractures with dip angles of about 80° control the distribution pattern of earthquakes. In the Weixi block (R3), the Molin fault and basement fault form a typical structural wedge that bounds a triangular or wedge-shaped fault block. The majority of earthquakes located below the buried depth of the shale gas and spatio-temporal correlations between seismicity and industry operations both suggest that they are probably induced by hydraulic fracturing. The migration front of the earthquakes can be modeled by fluid diffusion with the diffusivity D between 0.2 m2/s and 3.5 m2/s. The Ms 5.2 earthquake occurring on 18 December 2019 is located in a positive ∼10 kpa Coulomb failure stress area generated by the 8 September 2019, Ms 5.4 earthquake, reaching the typical static triggering threshold. Therefore, in addition to fluid diffusion, the stress perturbation caused by previous seismic ruptures may be considered as another factor of earthquakes occurring in the Weiyuan-Rongxian. Our study provides evidences for better understanding the seismicity characteristics, seismogenic mechanism, and fault structure in Weiyuan-Rongxian shale gas field.

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.

Interpretation and analysis of seismic and analog modeling data of triangle zone: a case study from the frontal part of western Kura foreland fold-and-thrust belt, Georgia

2D seismic reflection profiles revealed the presence of a triangle zone at the frontal part of the western Kura foreland fold-and-thrust belt of the pro-wedge of the Greater Caucasus. To understand the triangle zone geometry, seismic interpretations should be substantiated by forward kinematic modeling, supported by analog experiments. This study presents a new structural model for the region by integrating field observations, well data, and seismic reflection data. East-West directed along-strike structural variation of the frontal thrust is observed on the interpreted seismic profiles which affected the fold geometry. The Bitsmendi breakthrough fault-propagation fold gradually transits into a wedge structure in the W-E direction and is represented by the triangle zone. The seismic profiles interpretation results completely match with analog models of similar triangle zones. The analysis of the experimental results helps us to further understand the kinematic evolution of natural systems and improve seismic interpretation. The triangle zone developed in the western part of the Bitsmendi breakthrough fault-propagation fold is related to double fault-bend fold structural wedges and is characterized by the presence of passive, and active wedges, and passive-backthrust and passive-forethrust.

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.

რიონის ფორლანდის ნაოჭა-შეცოცებითი სარტყლის აგებულება, მიმოხილვა

The article presents an overview of the structural structure of the Rioni Forland Fold-and-thrust Belt. The structures are mainly represented by folds and duplexes associated with south-vergent faults. The synclines are overlaid by Middle Miocene-Pleistocene syntectonic sediments and are represented by piggy-back basins. Faultrelated folds are mainly represented by growing fault-propogation folds. The kinematic evolution of the Rioni Forland Basin in the Late Alpine period is related to the structural wedge (or duplexes) of the Caucasus foundation moving southwards, and its modern structure is represented by a thin-skinned fold-and-thrust belt.

Shallow Faults Reactivated by Hydraulic Fracturing: The 2019 Weiyuan Earthquake Sequences in Sichuan, China

AbstractHuman activity-induced earthquakes are emerging as a global issue, and revealing its underlying mechanisms is essential for earthquake hazard mitigation and energy development. We investigated the relationship between the seismotectonic model and seismic sequences from moderate Mw 4.3 and Mw 5.2 earthquakes that occurred in February and September 2019, respectively, in the Weiyuan anticline of Sichuan basin, China. We found that the Mw 5.2 earthquake ruptured a back thrust of structural wedges and released most aftershocks near the wedge tip. However, the two foreshocks of the Mw 4.3 earthquake sequence occurred in hydrofractured Silurian shale at depth of 2.5–3 km, and the mainshock ruptured the overlying oblique tear fault at a depth of ∼1 km. Hydraulic fracturing in the sedimentary cover of this block may induce earthquakes through fluid pressure diffusion in the Silurian shale and through poroelastic effects on back thrusts within structural wedges, respectively. We assessed the hazard potential of four seismic sources in the Weiyuan block and suggest it is critical to conduct a coupled flow-geomechanics assessment and management on induced seismicity and related cascading effects in the densely inhabited and seismically active Sichuan basin.

Development and growth of basement-involved structural wedges in the northwestern Qaidam Basin, China

Structural wedges contain two connected fault segments, a fore thrust and back thrust, that bound a triangular- or wedge-shaped fault block. Coeval displacements on both faults drive the wedge into surrounding rock, causing distinctive patterns of uplift and folding. We identified a series of thick-skinned structural wedges in northwestern Qaidam Basin, China, where syntectonic strata record deformation timing and kinematics. The Qaidam Basin is located at the northern margin of the Tibetan Plateau and contains more than 100 large anticlinal structures, many of which exhibit characteristics of structural wedges. We interpreted and modeled these structures using seismic reflection data, well logs, surface geological exposures, remote sensing images, and digital elevation data. These wedge structures are distinguished by having elevated and deformed strata in the footwalls of the back thrusts and synclinal folds that extend upward from the wedge tips. With the constraints of fold shape, growth strata, structural relief, and shallow fault geometry, we developed a series of forward models that describe the geometry and kinematic evolution of these wedge structures. The analysis suggests that some of the wedge structures reactivate preexisting normal faults. Our results provide a better understanding of how to identify thick-skinned wedge systems and model their geometry and kinematic evolution, which has implications for studying the tectonic history of the basin. Understanding the geometry and kinematics of the wedge structures is also crucial for petroleum exploration because both hanging-wall and footwall traps constitute an important component of current exploration targets.

Image analysis pipeline for segmentation of a biological porosity network, the lacuno-canalicular system in stingray tesserae

A prerequisite for many analysis tasks in modern comparative biology is the segmentation of 3-dimensional (3D) images of the specimens being investigated (e.g. from microCT data). Depending on the specific imaging technique that was used to acquire the images and on the image resolution, different segmentation tools are required. While some standard tools exist that can often be applied for specific subtasks, building whole processing pipelines solely from standard tools is often difficult. Some tasks may even necessitate the implementation of manual interaction tools to achieve a quality that is sufficient for subsequent analysis. In this work, we present a pipeline of segmentation tools that can be used for the semiautomatic segmentation and quantitative analysis of voids in tissue (i.e. internal structural porosity). We use this pipeline to analyze lacuno-canalicular networks in stingray tesserae from 3D images acquired with synchrotron microCT.•The first step of this pipeline, the segmentation of the tesserae, was performed using standard marker-based watershed segmentation.•The efficient processing of the next two steps, that is, the segmentation of all lacunae spaces belonging to a specific tessera and the separation of these spaces into individual lacunae required recently developed, novel tools.•For error correction, we developed an interactive method that allowed us to quickly split lacunae that were accidentally merged, and to merge lacunae that were wrongly split.•Finally, the tesserae and their corresponding lacunae were subdivided into structural wedges (i.e. specific anatomical regions) using a semi-manual approach.With this processing pipeline, analysis of a variety of interconnected structural networks (e.g. vascular or lacuno-canalicular networks) can be achieved in a comparatively high-throughput fashion. In our study system, we were able to efficiently segment more than 12,000 lacunae in high-resolution scans of nine tesserae, providing a robust data set for statistical analysis.

Effects of fault slip distribution on the geometry and kinematics of the southern Junggar fold-and-thrust belt, northern Tian Shan

At the foreland edge of fold-and-thrust belts (FTBs), tectonic shortening could either be accommodated by thick-skinned faulting and/or structural wedges developed near range-fronts, or be transferred along detachment layers further into basins, forming thin-skinned structures. Although the geometry and kinematics of each structure have been well-investigated, studies that characterize slip distribution along a specific fault as well as the effects of variations in magnitude of fault displacement on the development of fault-related folds are still lacking. In this study, we present a case study of the southern Junggar Thrust (SJT), a seismically-active fault developed in the southern Junggar FTB, northern Tian Shan with an overall vergence to the north. The results of structural interpretations of seismic reflection profiles combined with the age constraints provided by previous magnetostratigraphic studies demonstrate that: 1) The kinematics of the SJT vary along strike from purely structural wedging to classical fault-bend folding. Besides these end-member models, intermediate cases exist between them, in which fault slip is partitioned by two-stage structural deformation, i.e., fault-bend folding subsequent to structural wedging at the range front. As a result, part of the displacement is transferred northward, forming the Tugulu anticline. 2) Along-strike variations in displacement along the Tugulu fault (frontal fault of the SJT) also affect the geometry and kinematics of the Tugulu anticline, varying from detachment folding, trishear fault-propagation folding with a pure-shear fault-bend folding back-limb, and classical trishear fault-propagation folding to fault-bend folding with increasing displacement. 3) The SJT has an average Quaternary slip rate of 3.9 ± 0.4 mm/yr, serving as a principal structure accommodating tectonic shortening in the southern Junggar FTB. The results of this study are helpful for seismic hazard assessments of the study area, which may also shed light on structural evolution of fault-related folds in other FTBs.

Evidence for three Cenozoic phases of upper crustal shortening of the Xiongpo structure in the Longmen Shan fold-and-thrust belt, China: Implications for the eastward growth of the eastern Tibetan Plateau

Cenozoic crustal shortening across the Longmen Shan (LMS) fold-and-thrust belt records the eastward growth of the Tibetan Plateau. Although the structural geometries of individual structures in the region have been studied, the kinematics and rates of deformation across these structures remain unclear. In this study, we construct a geological cross section to derive the subsurface structural architecture of the Xiongpo structure in the LMS piedmont using one seismic reflection profile, and data from two boreholes. The structure displays a superimposed deep fault-bend fold and shallow structural wedge with three splay thrust faults. Using two-dimensional kinematic models, based on fault-bend folding algorithm, we reconstruct three episodes of upper crustal shortening, with 7.0 ± 0.3 km of total shortening. Combining with previously published low-temperature thermochronology data, we further estimate that the structure has experienced an average shortening rate of ∼ 0.2 mm/yr since 40–25 Ma. We infer that the structure acted as the deformation front of the LMS fold-and-thrust belt between 40 and 25 Ma and 15–10 Ma driven by the eastward growth of the Tibetan Plateau. Finally, we review the shortening rates of other five structures within the LMS piedmont, and find that these six structures (including the Xiongpo structure) individually have undergone an average shortening rate < 0.5 mm/yr on each fault, suggesting that strain is partitioned along several structures within the LMS fold-and-thrust belt. We also show that the southern LMS piedmont has absorbed more upper crustal shortening than the central LMS, ∼0.5–0.7 mm/yr, since ∼2.6 Ma.

Transformation of coseismic faults in the northern Longmenshan tectonic belt, eastern Tibetan Plateau: Implications for potential earthquakes and seismic risks

Increasingly frequent earthquakes in the Longmenshan (LMS) tectonic belt are linked to the ongoing eastward growth of the Tibetan Plateau during the Late Cenozoic. Following the devastating 2008 Ms 8.0 Wenchuan earthquake in the central LMS and the 2013 Ms 7.0 Lushan earthquake in the southern segment, an unexpected Ms 5.4 Qingchuan earthquake occurred on September 30, 2017 in the northern LMS. We synthesize focal mechanism solutions, relocated aftershocks, regional geology, and seismic reflection profiles in order to determine the pre-existing faults and active fault system in the northern LMS, thus to better assess potential seismic hazards. At a seismically resolvable scale, we identify a large structural wedge at the basement that is cross-cut by numerous multiphase and multi-behavior active faults. Our seismotectonic model indicates that the seismogenic fault responsible for the 2017 Ms 5.4 Qingchuan earthquake is a pre-existing thrust fault with a dip of ∼34° in the frontal thrust ramp. Reactivation of this fault then triggered the activity of the co-seismic strike-slip fault that was formed in the 2008 Wenchuan event. The seismic interpretation, focal mechanism solutions, and aftershock distribution indicate that two sub-segment faults with different geometries are distinguished along the northern LMS. Accordingly, a transformation of the coseismic fault with different geometry and kinematics is formed. With the constraints that the mainshock and aftershocks occurred on two different faults, we propose that both the frontal ramp thrust fault and the recently-formed strike-slip fault are tectonically active. This study highlights the importance of the influence of the pre-existing fault system on the evolution of the northern LMS during the Late Cenozoic. The rapid uplift of the eastern Tibetan Plateau and the complex fault system indicate the potential for seismic risks in the northern LMS.

Role of geological structures in seepage from Lar dam reservoir

The main objective of this paper is to estimate the water seepage from Lar dam reservoir based on a combination of the geological structure study results and identification of the flow conduits in the right bank of the reservoir. From the beginning of impounding the dam in 1980, heavy seepage was observed at two karstic springs, Haraz and Galugah, located about 9 km downstream of the dam. During the first impounding, the discharge of the Haraz spring abruptly increased from 0.5 m3/s to around 5 m3/s. The results of piezometers and dye tests indicate that seepage occurs mainly through the right abutment of the dam where there is a structural wedge between the north dipping North Tiz Kuh and the south dipping Lar Valley faults. F1, F2, and F3 faults are the most important faults in vicinity of the structural wedge. Based on the dye test results, the North Tiz Kuh and F3 faults along which caves No. 1 and 2 are formed are regarded as two isolated conduits for seepage and conveyance into Lar Valley Fault at downstream of Lar dam. After identifying the conduits, water seepage from the Lar dam reservoir has been calculated using finite element method. According to the results of numerical method, when the reservoir water level is at 2485 m a.s.l, the average of water seepage is around 8.51 m3/s (this amount of water is related to the seepage along the Lar Valley Fault). The average discharge of springs downstream of the dam has been used to verify the numerical method. The results show a very close relation between estimated and observed discharge.

Tectonometamorphic evolution along the Iapetus suture zone in Newfoundland: Evidence for polyphase Salinic, Acadian and Neoacadian very low- to medium-grade metamorphism and deformation

The Red Indian Line (RIL) in central Newfoundland is the suture, where the main tract of the Iapetus Ocean was closed at ~452 Ma during accretion of the peri-Gondwanan Victoria arc with the composite active Laurentian margin. The protracted deformation history of this soft collision started at ~471 Ma with accretion of oceanic terranes to the active composite Laurentian margin. After Iapetus closure both colliding active margins were progressively deformed and metamorphosed during Silurian and Devonian (Salinic, Acadian and Neoacadian orogenic cycles).Peak conditions of the very low- to medium-grade, heterogeneously distributed metamorphism were determined by pseudosection techniques within the range of 2–7 kbar, 230–450 °C during increase of the metamorphic field gradient from ~12 °C/km to ~32 °C/km over time. Multiple metamorphic crystallisation stages were dated by white mica 40Ar/39Ar spot and plateau ages, additional Rb-Sr mineral isochrons involving white mica and one U/Pb age of titanite. All resulting ages between 439 ± 4 Ma and 356 ± 16 Ma postdate the closure of Iapetus.Results differ along two transects: The oldest ages of 443–421 Ma (Salinic orogenic cycle) were observed along the northern transect through the RIL zone with minimal younger overprint. Hence low temperature, intermediate to high pressure conditions (4.0–7.0 kbar, 230–340 °C) achieved during Taconic-Salinic underthrusting are well preserved. During Acadian dextral transpression the Taconic-Salinic structural wedge was tilted subvertically.In contrast, rocks along the southern transect through the RIL zone mainly show Acadian ages of 408–390 Ma with local preservation of older ages. Acadian deformation occurred under low temperature/low pressure conditions (~250–450 °C, 2.5–4.6 kbar). Also Silurian terrestrial cover rocks were buried under these conditions. Acadian-Neoacadian deformation (393–340 Ma) becomes younger towards the northwest and progressively localized in transcurrent fault zones. This final foreland deformation at shallow crustal level established the Acadian/Neoacadian orogenic front in central Newfoundland slightly northwest of the RIL.

The Grenvillian Namaqua fold belt adjacent to the western Kaapvaal Craton: 2. Archaean Craton and supercontinent connections

The Namaqua sector of the Namaqua-Natal Province consists of large tectonostratigraphic terranes, some of which includes Paleoproterozoic fragments inherited from the supercontinent Columbia. Most of the terranes became part of Rodinia during the Mesoproterozoic Namaqua Orogeny (1.3–1.0Ga, Grenvillian). The terranes in the western foreland of the Kaapvaal Craton (Kheis Subprovince) form a thin-skin fold and thrust belt (5–15km depth to sole thrust). Westerly directed thrusting towards the hinterland is reminiscent of thick-skin tectonics, associated with the exhumation of high grade facies terranes, such as the granulitic and migmatitic Grünau Terrane. Upper amphibolite Upington Terrane was thrusted from the west onto thrust sheets contiguous to Kaapvaal Craton, in turn over-ridden near Prieska by the Grünau Terrane. Severity of the ductile deformation is illustrated by regional shear zones, ubiquitous recumbent isoclinal folds and allochthonous mega sheath folds in the Grünau Terrane. Vergence reversal of fold and thrust structures along a narrow (10–20km) northwesterly trending zone (Vergence Inversion Zone, VIZ), represents a crustal scale pop-up structure coincident with the Namaqua Front, a paired low-high gravity anomaly linked to a mega-scale anticline-syncline pair caused by deep-seated thrusting of the Columbian Moho.Late Columbian extension (1.6–1.3Ga, diachronous with early Rodinian events) produced parautochthonous basins with volcano-sedimentary sequences, deformed during the Namaqua Orogeny. Early to mid-Columbian (2.3–1.9Ga) rocks form basement to the mobile belt, with Steinkopf Terrane as type outcrop area (probably time-equivalent with the Grünau and Pofadder Terranes, Sperrgebiet Domain, Rehoboth structural Province and part of the Congo Craton). The western boundary of the Kaapvaal Craton (and composite Kalahari Craton) exhibits structural wedge edges of Archaean Kaapvaal basement and Ventersdorp-Vaalian basins. Columbia-Kaapvaal Detachment (COLKAD), a décollement between Kaapvaal and Columbia cratons (footwall) and Olifantshoek and other Namaqua terranes (hanging wall) extends for >600km westwards.Namaqua crust was thickened (20–25km) by both structural stacking and sheeted intrusions (Namaqua tectogenesis) with clockwise PTt-paths in the west-central Namaqua sector (Olifantshoek, Grünau, Pofadder, Bladgrond Terranes). In the western part (Okiep/Garies Terrane), crust was dominantly thickened by massive sheetlike granitoid intrusions resulting in granulite grade and anti-clockwise PTt-paths. COLKAD-rooted inter/intra-terrane thrusts are subhorizontal except where rotated by subvertical (discrete) shear zones (e.g. Tantalite Valley and Brakbos-Doringberg-Dabep). Crustal thickening and granulite-uplift stimulated temperature rise with anatexis-palingenesis of early Columbian/supracrustal rocks. Kilometer-scale granitoid bodies, derived from predominantly Columbian crust (according to isotopic source rock models) that are emplaced along shear zones, caused a rise in the brittle to ductile transition zone (i.e. mid-crustal conditions) and low-pressure granulite facies.

Geometry and kinematics of extensional structural wedges

Structural wedges in the compressive environment have been recognized and studied in different locations. However, extension structural wedges are less well-understood. Based on the normal fault-bend folding theory and inclined shear model, this paper quantitatively analyses deformations related to extensional structural wedges and builds a series of geometric models for them. An extensional structural wedge is a fault-block held by two or more normal faults, the action of which would fold its overlying strata. Extensional structural wedges of different shapes will lead to different deformation results for the overlying strata, and this paper illustrates both the triangular and quadrangular wedges and their related deformations. This paper also discusses differences between the extensional structural wedges and the normal fault-bend-folding. By analysing two seismic sections from Langfang-Gu'an Sag, East China, this paper provides two natural examples of the triangular and quadrangular extensional structural wedges, where the models can reasonably explain the overlying distinct highs and lows without obvious faults. The establishment of a geometric model of extensional structural wedges can provide reference and theoretical bases for future quantitative analysis of deformations in the extensional environment.

Late Cenozoic Deformation Sequence of a Thrust System along the Eastern Margin of Pamir, Northwest China

AbstractA thrust belt formed in the basin along the eastern margin of Pamir. The thrust belt is about 50 km wide, extends about 200 km, and includes three compressive structures from south to north: the blind Qipan structural wedge and Qimugen structural wedge, and the exposed Yengisar anticline. The thrust belt displays a right‐stepping en echelon pattern. The Qipan structural wedge dies out northward to the west of the Qimugen structural wedge, and the Qimugen structural wedge dies out northward to the west of the Yengisar anticline. Detailed analysis of seismic reflection profiles of the western Tarim Basin reveal that fan‐shaped growth strata were deposited in the shallow part of the thrust belt, recording the deformation sequence of the thrust belt. The depth of the Cenozoic growth strata decreases from south to north. The growth strata of the Qipan structural wedge is located in the middle‐lower section of the Pliocene Artux Formation (N2a), the growth strata of the Qimugen structural wedge is close to the bottom of the Pleistocene Xiyu Formation (Q1x), and the growth strata of the Yengisar anticline is located in the middle section of the Xiyu Formation (Q1x). Combined with magnetostratigraphic studies in the western Tarim basin, it can be preliminarily inferred that the deformation sequence of the thrust belt along the eastern margin of Pamir is progressively younger northward. The geometry and kinematic evolution of the thrust belt in the eastern margin of Pamir can be compared with previous analogue modeling experiments of transpressional deformation, suggesting that the thrust belt was formed in a transpressional tectonic setting.

Structural inversion, imbricate wedging, and out-of-sequence thrusting in the southern Junggar fold-and-thrust belt, northern Tian Shan, China

Accurate definition of structural style in subsurface interpretation is critically important for understanding the deformation history of fold-and-thrust belts, as well as assessing the petroleum prospectivity of structural traps. Using two- and three-dimensional seismic reflection surveys, well data, field mapping, forward models, and balanced cross sections, we describe the structural styles across the actively deforming southern Junggar fold-and-thrust belt in northwestern China, a basin undergoing petroleum exploration and development operations. Subsurface interpretations indicate several folds in the basin overlie Jurassic normal faults that were tectonically inverted in the Late Jurassic to Early Cretaceous. Following inversion, multiple detachment levels propagated northward from the Tian Shan and formed a series of imbricated fault-related folds. The most prominent fold trend in southern Junggar consists of the Tugulu, Manas, and Huoerguosi anticlines, which trap hydrocarbons in clastic Eocene reservoirs. These structures exhibit complex internal geometries, with coeval forethrusts and backthrusts forming imbricated structural wedges. In the latest stages of deformation, and continuing at present, the uppermost thrust sheet, the Southern Junggar Thrust (SJT), truncated the backlimbs of these structural traps, implying the SJT is a tectonically active, out-of-sequence thrust. From these interpretations, we present a model for how the southern Junggar fold-and-thrust belt developed from Jurassic to present. Moreover, we detail how fold growth, fault activity, and structural style affected charge histories, trap formation, and reservoir compartmentalization. Our results have direct implications for assessment of the southern Junggar petroleum system as well as other complex fold-and-thrust belts.

Coseismic and blind fault of the 2015 PishanMw6.5 earthquake: Implications for the sedimentary-tectonic framework of the western Kunlun Mountains, northern Tibetan Plateau

On 3 July 2015, the Mw 6.5 Pishan earthquake occurred in the western Kunlun Mountains front, at the northern margin of the Tibetan Plateau. To reveal the sedimentary-tectonic framework of the seismically active structure, three high-resolution seismic reflection profiles and well drilling data were collected for seismic interpretation. The western Kunlun Mountains and Tarim Basin have two gypseous detachments and one basement detachment that control the tectonic framework and structural deformation. The upper gypseous detachment (D1) is in the lower Paleocene, and the middle gypseous detachment (D2) is in the Middle to Lower Cambrian. A Neogene shallow thrust system is developing above D1 and includes the Zepu fault (F2) and Mazar Tagh fault (F3). A deep thrust system is developing between D1 and D2 and forms a large-scale structural wedge beneath the western Kunlun Mountains front. The Pishan Mw 6.5 earthquake was triggered on a frontal blind fault of this deep thrust system. The lower detachment is in the Proterozoic basement (D3), which extends into the Tarim Basin and develops another deep thrust (F4) beneath the F3 belt. D1, D2, D3, and the Tiekelike fault (F1) merge together at depth. Crustal shortening of the western Kunlun Mountains front continues for approximately 54 km. Two tectonic evolutionary stages have occurred since the Miocene according to sedimentary unconformity, axial analysis, and fault interpretation. The results of this study indicate a regime of episodic growth of the western Kunlun Mountains and Tarim Basin during the Cenozoic.

Structural model of the central Longmen Shan thrusts using seismic reflection profiles: Implications for the sediments and deformations since the Mesozoic

The structural setting and deformation history are very important aspects of understanding the frequent earthquakes and assessing the hazards. In this paper, we interpret two new seismic profiles that were collected after the Wenchuan Mw7.9 earthquakes occurred on May, 2008. Our interpretation has integrated four wells, three seismic profiles and the surface geology to reveal structural characteristics of the central Longmen Shan thrust. The drilled wells reveal that there are two detachments within the upper Triassic and the lower Triassic at the Longmen Shan thrusts. The shallow detachment fault transferred its slip to the basin and made another deformation inner the Sichuan basin. Between the shallow detachments, many faults developed and the major structures at the thrust front have formed as structural wedges. Both the drilled wells and seismic interpretation show the upper Triassic repeated in the depth. The shortening at the central Longmen Shan thrust is approximately 30%–37%. According to geological survey and the seismic interpretation, the foreland basin was developed during the late Triassic and the angular unconformity between the lower Jurassic and the upper Triassic at the Longmen Shan front. Several different tectonic events can be identified since the Mesozoic, which indicated that the Longmen Shan has a complex evolution history. In our model, the major Beichuan fault (F2) and the Pengguan fault (F3) at the central Longmen Shan dip steeply near the surface but are more gentle-dipping at depth. Both of the shallow thrusting and the basement shortening made contribution to the rapid uplift of the Longmen Shan during the Cenozoic. The co-seismic ruptured fault is like a branchy fault that developed on the major fault system. The Wenchuan Mw7.9 earthquake and the Lushan Mw6.7 earthquake may be related to the reaction of the basement structures.