Main Boundary Fault Research Articles

Quantifying the Pabu Normal Fault Scarp, Southern Tibetan Plateau: Insights into Regional Earthquake Risk

The location of the main boundary fault of the Yadong-Gulu Rift (YGR) shifts from the east side in the southern segment to the west side in the northern segment. The Nyemo Graben Group (NGG) connects the southern and northern segments of the YGR and provides clues for understanding the migration of boundary fault locations along the YGR. However, the NGG has received very little attention. In this study, we map the geometry of the Pabu normal fault, which is the boundary fault of the westernmost graben in the NGG, using high-resolution remote sensing images. We then utilized a digital elevation model (DEM) with a spatial resolution of 1 m. Morphometric parameters such as scarp height, width, and slope were obtained from elevation profiles in three typical deformation regions. Our results reveal a fault segment approximately 3 km long that links the southern and northern segments of the Pabu Fault. Each fault segment could be a major segment. Furthermore, based on regional tectonic activity, the Pabu Fault has the potential to produce an earthquake with a magnitude of around M 6.7.

Research on fault-controlled hydrocarbon accumulation in rifted lacustrine basins based on structural geological modeling: a case study of the Banqiao area in Bohai Bay Basin, China

The Banqiao area in the Bohai Bay Basin has experienced three stages of extensional deformation, leading to the formation of numerous fault-bound traps. Faults, acting as boundary conditions for these traps, play a crucial role in hydrocarbon accumulation. In this study, we conducted a 3D structural modeling of the area using high-resolution 3D seismic data and established a fault-reservoir database based on previous research. Our findings reveal four levels of faults in the Banqiao area: basin-controlling faults, boundary faults, derivative master faults, and secondary adjusting faults. The structural units can be categorized into subsag areas, slope areas, stress tran-sition zones, bifurcation and main incised fault zones, and southern block areas. The segmented growth of the main boundary faults controls the evolution of the subsags, with the subsidence center gradually shifting eastward from Rift Phase I to Rift Phase II, aligning with the distribution of source rocks. Fault-bound traps in the Banqiao area include single faults, intersecting faults, and side faults. Faults primarily act as barriers to lateral hydrocarbon migration during the process of hydrocarbon accumulation, while also providing pathways to a lesser extent. By integrating the fault-reservoir database with the fault system classification, we identified four types of fault-controlled hydro-carbon accumulation models: like-dipping fault barrier model, oppositely-dipping fault barrier model, intersecting fault barrier model, and reactivation-controlled secondary hydrocarbon ac-cumulation model. This structural geological model effectively demonstrates the spatial configura-tion of faults and their role in hydrocarbon accumulation in the Banqiao area. The fault control mechanisms presented in the model can also be applied to other blocks in the Bohai Bay Basin, laying a foundation for future petroleum exploration in continental rifted basins and facilitating the ap-plication of big data algorithms in various geoscientific research fields.

Sedimentary records of megafloods in the Yarlung Tsangpo Gorge in the eastern Himalaya since the Last Glacial Period

The megaflooding caused by outbursts from Late Quaternary glacially-dammed lakes in the Yarlung Tsangpo Gorge (YTG) potentially shaped the fluvial landscape and controlled the geodynamic evolution of the Namche Barwa Syntaxis in the eastern Himalaya. However, the sedimentary evidence for such flooding in the narrow reaches of the Yarlung Tsangpo River (YTR) valley has been lacking. In this study we conducted geomorphological and sedimentological analyses of the Motuo stretch of the YTR downstream of the Yarlung Tsangpo Gorge and combined these with dating results to identify multistage high magnitude outburst paleofloods. One megaflood event was found to have occurred at ∼5 ka, with a minimum discharge of ∼1.1–4.4×106 m3/s. Our results suggest that this event exceeded 1000 km in range, and was consistent with the age of flood deposits downstream, where the YTR becomes the Siang-Brahmaputra River. Evidence of older and younger flood deposits was also obtained. The erosion caused by this megaflood was stronger in the steep river channel that flows through the uplift center of the Namche Barwa Massif. This may have been principally a response to the differential uplift caused by the main boundary faults. The megaflood event also gave rise to the headward erosion of the YTR's northern tributaries. The megaflood sediment zone is concentrated in the middle and lower reaches of the YTR where the valley widens. Here, the gradient of the river channel is shallower, potentially reflecting weaker tectonic activity accompanied by slower fluvial erosion. Furthermore, multistage catastrophic floods from the Yarlung Tsangpo Gorge may have significantly impacted any downstream prehistoric human settlements.

New insights from conjugate transfer zones in the southern Cairo-Suez province: Implications for deformation history and mechanism

Detailed structural field mapping with the aid of high resolution satellite images were carried out to identify structural style and geometry of Middle Eocene conjugate divergent transfer zones along the southern part of Cairo-Suez Province, Egypt. Eight conjugate divergent transfer zones are recognized between the tips of oppositely-dipping main boundary faults of the mapped fault blocks. The internal geometry of the conjugate divergent transfer zones is represented by elongated fault belts of left-stepping, en echelon normal faults between the terminations of neutral or overlapped boundary faults. Also, dip polarities of the mapped en echelon faults are found to be related to the stepping of the main boundary faults. Accordingly, three main types of conjugate divergent transfer zones are identified in the present study, which are; right-stepping-, left-stepping- and hybrid-stepping-related conjugate divergent transfer zones. Structural analysis of the mapped transfer zones revealed a geometric relationship between the overstepping (Os) of the boundary faults and the total lengths (Tl) of the extended en echelon faults. In addition, the geometry of en echelon faults formed between conjugate divergent transfer zones is attributed to the reactivation of deep seated faults by dextral transtension during the Late Eocene and Oligocene-Miocene times. The interaction of the associated preexisting deep seated Riedel shears is found to have controls on sedimentation and present day architecture. The results of this work highlight the role of preexisting basement structures and boundary faults controlling the internal geometry of transfer zones. Because of the significance of transfer zones in petroleum exploration, the findings of this research provide new data that can be correlated with similar transfer zones in the subsurface and other outcrops in extensional systems.

Slope instabilities and evolution of tectonic geomorphology along the strike of the Main Boundary Thrust zone in the western Himalaya, India

The Main Boundary Thrust (MBT) and Main Boundary Fault (MBF) zones of the southeast Himachal Himalaya are susceptible to various types of mass movement. Several active and in-active landslides are observed along the strike of the MBT. The landslides are mainly controlled by the brecciated and highly fractured nature of the bedrocks, and the intersecting joint sets form wedges. The movements are taking place along the fault planes, fracture planes, and bedding planes; and the types of failure are wedge failure, planar failure, toppling, rock falls, and complex landslides. The various morphotectonic features observed in the study area include fault traces, fault scarps, strath terraces, paleochannels and structurally controlled channels. Fault traces and their associated deformed landforms are the most spectacular tectonic landforms in the area. Along the strike length of the fault traces several linearly arranged sag ponds have formed, in the western segment the fault trace is observed along the MBF and the fault scarp dips south, with a maximum height of about 34 ​m. In the eastern segment the fault trace cut across the MBT, the fault trace is also displaced by transverse faults. Paleochannels and multiple levels of strath and fill terraces collectively indicate the river channel's disequilibrium state concerning the ongoing tectonic activity. The cross-cutting relation and displacement pattern of fault traces indicate later phases of tectonic activity.

Deep Electrical Structure of the Langshan Mountain-Linhe Basin Area on the Northwest Edge of the Ordos Block, China

A series of fault depression structures have developed around the Ordos Block. The Langshan Mountain-Linhe Basin area (LLA), located on the northwest edge of the Ordos Block, is a typical, normal tension fault system. A geological survey shows that the Langshan Piedmont fault (LPF) in this area has a large slip rate and indicates risk of earthquake preparation. Broadband magnetotelluric (MT) exploration research was recently carried out across the LLA in the NW–SE direction, and the three-dimensional deep electrical structure thus obtained revealed that the LPF in the LLA is an evident electrical boundary zone on the whole crustal scale and is the main boundary fault of the primary structural block of the Alxa and Ordos Blocks. The MT results also show that the Linhe Basin and Ordos Block belong to the same tectonic basement. The Linhe and Dengkou faults belong to the internal faults of the Ordos Block. The upper crust of the Langshan Mountain on the west side of LPF is characterized by high-resistivity, the middle and lower crust have a low-resistivity layer, and the Linhe Basin on the east side has a Cenozoic low-resistivity sedimentary layer of approximately 10 km thick, which reveals that the Linhe Basin is a faulted basin with sedimentary thickness around the Ordos Block. This indicates that the LLA has experienced continuous and strong tension, normal fault depression sedimentary activities since the Cenozoic era. The current Global Positioning System velocity field shows that there is an apparent NW–SE acceleration zone in the LLA. The leveling data indicate that Linhe Basin shows a subsidence trend relative to the Ordos Block, indicating that the area is undergoing continuous NW–SE tension and faulting. It is speculated that there is a risk of earthquake preparation in the LPF.

Quantitative Analysis of Cenozoic Extension in the Qiongdongnan Basin, South China Sea: Insight on Tectonic Control for Hydrocarbon Reservoir Accumulation and Formation

Cenozoic extension rates were calculated based on 20 seismic profiles across the Qiongdongnan Basin, South China Sea. The results confirmed that the Cenozoic rifting in the Qiongdongnan Basin exhibited multistage extension and spatiotemporal variation. In terms of the N–S striking seismic profiles, the structural forms of the northern and southern sags of the basin were characterized by narrow half grabens, while the structure at the center sag of the basin was characterized by wide and gentle grabens. The fault strikes in the west of the basin were mainly northeast–southwest trending, whereas those in the east of the basin changed from northeast–southwest trending to nearly east–west trending. The extension rate in the east sag was higher than that in the west area. The extension rate in the middle part was lower relative to the east and west sags. This was because the rifting was controlled by the distribution of the main boundary fault along the basin. Temporally, the Cenozoic extension could be divided into three periods: Eocene, Oligocene, and Miocene. The amount of stretching in the three extension stages was unevenly distributed in the entire basin. The maximum was mainly in the Oligocene Lingshui and Yacheng Formations. The Oligocene extension occurred in the entire basin, and the Eocene extension was limited to the Ledong and Changchang sags. Significant fault activity could be observed during the deposition period of the Yacheng and Lingshui Formations and could be attributed to strong extensional activity. The rifting tectonics controlled the distribution of source rocks and oil-generating window as well as hydrocarbon generation, reservoir formation, and accumulation.

Tectonic Geomorphology Of The Jhelum Fault Zone And Its Contiguous Regions In Western Himalaya

The western Himalayan syntaxis represents the region where the major Himalayan structures abruptly curve and the cause of the curvature and the tectonic geomorphology of the region has not been fully explored. The lack of detailed structural maps with extensive field-based data are missing, which is mainly because of the political problems related to border sharing between Pakistan, India, and Afghanistan. However, and fortunately, the usage of satellite derived images has overcome such constrains by providing a robust platform to remotely map such regions. Therefore, the present study was aimed to supplement our previous works in the region by exploring the western portions of the Hazara-Kashmir-Syntaxis. We have used Google terrain imagery to map the evidence for active faulting that involves mapping of triangular facets, displaced and/or faulted topographic ridges, river terraces, alluvial fans and so on. The crosscutting relationships are used to date the faulting events where absolute dates are not available. Our results show that active tectonic deformation is not just limited to the previously mapped structures (e.g. the Kalabagh Fault, the Salt Range Thrust, the Mahesian Anticline, and the Jhelum Fault) but occurs on a broader deformation zone that is delimited by the Chaman fault system in the west, and the Jhelum Fault in the east. The deformation zone in 3D resembles a tectonically formed diamond shaped box that has Salt Range Thrust in front and the Main Boundary Fault at the back with sides delimited by the Chaman and the Jhelum fault systems. The earthquake centroid moment tensor data compliments our geomorphological work, and establishes that transpression is a dominant tectonic process that governs the western regions, which is in comparison to the east where transtension is the norm in the interior Himalayan with reverse and thrust faulting dominant in the frontal regions.

3D electrical structure and crustal deformation of the Lajishan Tectonic Belt, Northeastern margin of the Tibetan Plateau

• A steep high-conductivity layer exists in the middle–lower crust to the south of the belt. • The high-resistivity body on the northern side may have blocked the high-conductivity layer, which formed the positive flower structure of the Lajishan tectonic belt and resulted in intense orogenic and seismic activities. • Crustal shortening may dominate the tectonic deformation of the NE Tibetan Plateau. The Lajishan tectonic belt is the most recently active orogenic belt in the northeastern Tibetan Plateau and represents a topographical boundary between the high Tibetan Plateau and the low-lying Longzhong basin to the east. Imaging the crustal structure beneath the Lajishan tectonic belt can improve our understanding about the regional tectonic deformation pattern, mechanisms of the outward growth of the Tibetan Plateau, and seismogenic environment in this area. In this study, we conducted two magnetotelluric (MT) profile measurements across the Lajishan tectonic belt in each section. Based on our newly-collected MT data and previous dataset in this area, a three-dimensional (3D) image of the deep electrical structure of the Lajishan tectonic belt is obtained by using 3D MT inversion and imaging technologies. Furthermore, by including 3D crustal deformation field information, a tectonic deformation model of the Lajishan tectonic belt and its adjacent area was constructed. Our present results show that a steep high-conductivity layer developed beneath the Lajishan tectonic belt and the adjacent Jianzha–Xunhua Basin and West Qinling Block, which then connected with a horizontal high-conductive layer underneath the Tibetan Plateau. Findings suggest that the Lajishan tectonic belt acts as a physical property boundary zone separating a high-resistivity body to the north from a deep high-conductivity layer in the middle-lower crust to the south. The high-conductivity layer, with a shallower depth from southwest to northeast, may be related to the NE-trending compression of the northeastern Tibetan Plateau, which was blocked by the high-resistivity body underneath the Xining basin. This led to the formation of a positive flower structure in the Lajishan tectonic belt, resulting in intense orogenic and seismic activities. The south-dipping Northern Lajishan Fault is the main boundary fault of the Lajishan tectonic belt and plays an important role in the mountain formation process, as suggested by asymmetry surface topography. The absence of a large-scale NE-SW-trending continuous high-conductivity layer in middle-lower crust indicates that the tectonic deformation of the northeastern Tibetan Plateau is dominated by a continuous upper-crustal compressing and shortening, instead of the lower-crustal flow.

Reverse Faulting Within a Continental Plate Boundary Transform System

AbstractContractional deformation is common along transform plate margins where plate motion is oblique to the plate boundary. While faults that accommodate this deformation are often inferred to be subsidiary to the main plate boundary fault, we typically lack direct geometric or kinematic information. Here we investigate the timing of faulting relative to changes in the orientation of the North American‐Pacific plate boundary. Coeval with development of an oblique plate boundary segment (i.e., the “Big Bend” of the San Andreas fault), active shortening is inferred to have initiated at ∼5 Ma in the Western Transverse Ranges (WTR). However, new low‐temperature thermochronometric data yield Miocene to Pleistocene apatite (U‐Th‐Sm)/He cooling ages and partially reset zircon (U‐Th)/He ages. Inverse thermal modeling indicate that reverse faulting initiated as early as 10 Ma, several million years prior to our current understanding of the timing of the Big Bend. Reverse faults in the WTR also propagate from west to east, toward the San Andreas Fault, rather than outwards from it. New and existing thermochronometry data delineate the WTR as the locus of rapid post‐Miocene exhumation, and demonstrate that similar exhumation is not present in the broader region surrounding the Big Bend. We posit that reverse faulting is localized in the WTR because of a weak underlying lithosphere and predates the more recent geometric anomaly of the restraining bend in the transform margin.

Detecting active faults in intramountain basins using electrical resistivity tomography: A focus on Kashmir Basin, NW Himalaya

Kashmir basin in the NW Himalaya is surrounded by the main Himalayan boundary faults, has very well documented historical earthquakes and a good instrumental earthquake record. However, the causative faults of these earthquakes except the 8 October 2005 Kashmir earthquake (M7.6) are not known. One of many historical earthquakes that have struck and caused damage and destruction in the Kashmir basin is the 30 May 1885 Kashmir earthquake (~M6.3). The extensive damage due to this earthquake was reported in the NW part of the basin and as usual the causative fault is not known and mapped. As the earthquake related geomorphic features are not preserved due to the high erosion rates in the Kashmir Himalaya, we mapped certain active fault strands using high resolution digital elevation models (DEM) and the Google Earth imagery, later complemented by the field investigation in the NW Kashmir. The Electrical Resistivity Tomography (ERT) was carried out at certain identified sites in the macroseismic epicentral area of the 1885 Kashmir earthquake. The results show a local active normal fault which was named as the NW Kashmir fault. The ERT results were confirmed by excavating a trench and an already existing road cut at the ERT sites. The results show that ERT is a very useful shallow geophysical method to detect faults in the Karewas. Karewas are the Plio-Pleistocene and Holocene (reworked by rivers), fluvio-lacustrine, soft and unconsolidated, sand-clay-conglomerate sediments, deposited as distinct table-land geomorphic features in the Kashmir basin and are significantly water saturated.

Fracture characteristics and formation mechanism of Gaoliu structural belt in Nanpu depression

The deep fault system of Gaoliu structural belt in Nanpu depression is complex, especially the joint part of Gaobei and Gaoliu faults. Because of its close proximity to large faults and the poor quality of seismic data, the reliability of structural interpretation is low, which has long affected the development of oil fields. In order to verify the deformation process and genetic mechanism of Gaoliu structural belt, this study obtains a new understanding by selecting typical sections of Gaoliu structural belt to carry out structural physical simulation experiments: the normal fault trending NE and tilting NNW is the reverse extensional fault controlled by the Bogezhuang and Xinanzhuang faults, which formed in the early stage (Shahejie stage) of tension; Gaobei fault and Gaoliu fault It is a normal fault formed in the late stage of extension, which is in the same direction with the main boundary faults such as Bogezhuang and Xinanzhuang faults. It cuts the early reverse normal faults and controls the sedimentation of the strata after the Dongying Formation to different degrees.

Transformation of sediment delivery and dispersal patterns controlled by relay-ramp evolution along the boundary fault of a lacustrine rift: The Eocene Shahejie formation, Dongying Sag, Bohai Bay Basin, NE China

The boundary fault of the Dongying Sag is divided into isolated and scattered segments and small-scale relay zones, which are important geomorphic features controlling sedimentation in the early stage of rift evolution. Previous studies have shown changes in facies in deposits of thousands of meters in thickness on fault-bounded margins, where deep-water fan systems are vertically stacked with delta deposits in the same way as in many basins around the world. However, changes in sedimentation processes and sedimentary systems and their relationship with the evolution of the main boundary fault remain a matter of debate. We focus on the transition from shallow-water to deep-water systems and their responses to relay-ramp evolution in the Yanjia Subsag of the Eocene Dongying Sag in the Bohai Basin. A relatively simple assemblage of local structures provides an ideal opportunity to investigate the impact of relay-ramp evolution on the early synrift deposits. An integrated dataset of 3D seismic volumes, cores and wireline logs is used to analyze the evolution of the Chennan Fault, to delineate thirteen lithofacies, and to recognize two depositional styles (including fan deltas and nearshore subaqueous fans) and their distribution during different stages of the relay-zone evolution. In the early stage of rifting, the depositional style changes, from the development of axial and transverse systems in the early to middle period to the development of transverse systems in the late period; this is mainly controlled by the evolution of the relay ramp. The change is accompanied by a rapid decrease in the rate of sediment supply, architectural changes in transport conduit from the relay ramp to a canyon, and local geomorphic changes that control transport routes, sediment delivery, and depositional locations. Floods ultimately become dominant transport mechanism with a new system, such that hyperpycnal flows form, leading to another transformation of the sedimentary systems from a fan-delta system to a nearshore subaqueous fan system. On the basis of these transformations in sedimentary systems we are able to propose new recommendations for reservoir exploration in continental rift basins, which include exploration potential, source reservoir cap assemblage, and spatiotemporal evolution of plays.

Geological and geomorphic evidences of neotectonic activity along the Himalayan Frontal Thrust, Nahan Salient, NW Himalaya, India

In this paper, we report geomorphic signatures of neotectonic activity identified from the area around Nahan Salient along Himalayan Frontal Thrust (HFT), using remote sensing and field investigations. We document two newly discovered exposures of HFT fault zone and active fault traces along the mountain front at Khetpurali, Nahan Salient, NW Himalaya, India. We demonstrate that HFT fault zone at Khetpurali is not blind, instead it is exposed at the surface along with highly deformed, folded strata in the hanging wall and its geometry is quite variable with the possibility of along strike-segmentation. The geometry of fold and various deformational features preserved in the hanging wall indicate a process of fault-propagation-fold. This is in contrast to the previous observation of fault-bent-folding within the Nahan Salient. Two-three levels of river terraces are preserved and truncated in the hanging wall, along with the active fault scarps which are linked to uplift along HFT fault zone. The Khetpurali Anticline (KA), is a fault-propagating-fold formed due to displacement and shortening along HFT which dips 30° towards north and strike WNW-ESE. The uplift (U) along the KA is calculated to be ~73 m and accommodated 7.3% shortening of the original length, before the fault tip truncated the fold. Various deformational structures have been identified within the hanging wall around-tip damage zone of ~80 m perpendicular to HFT fault zone and in the core of KA. Northwest of Khetpurali, a south-facing hogback structure ~100 m high is formed due to erosion of forelimb of the anticline. At Khetpurali, tectonic control due to folding of strata and uplift of river terraces is evident as the Dudhgarh River forms V-shaped profile near the thrust, compared to the wide floodplain before and after crossing the HFT fault zone. Along strike segmentation of HFT and associated anticlines is observed as the surface trace of HFT is offset at Khetpurali, Trilokpur, and Sabilpur due to the strike-slip faults. The study reveals that HFT has been truncated by tear faults and has different subsurface geometries in study area which may have implication on seismic hazard in the Nahan Salient where three thrusts; Himalayan Frrontal Thrust (HFT), Nahan Thrust (NT) and Main Boundary Fault (MBF) are located at relatively shorter distance of ~6 km as compared to the surrounding recesses.

Evolution of sedimentation pattern in a continental rift basin of India, between the Late Triassic and the early Middle Jurassic: Tectonic and climatic controls

The spatial and temporal pattern of sedimentation in a rift basin is a result of complex interplay between tectonically induced vertical crustal movement and climatic condition. Though the conceptual and numerical models for tectono-sedimentary evolution of half grabens are available, the stratigraphic arrangement of lithofacies produced by the environments on hangingwall block is poorly documented till date. Additionally, the stratigraphic record resulting from the interactions between the hangingwall processes and those of the basin trough is less known. The purpose of this work is to delineate the tectonic and climatic factors controlling the distribution of sedimentary environments from the stratigraphic architecture of a Late Triassic to Early Jurassic syn-rift strata of a continental half-graben of India. This work also aims to characterize the deposits formed in the transitional environments that occur in between the fluvial system of the hangingwall dip slope and the lacustrine system at the basin trough. This work involves a detailed sedimentological study including facies analysis and palaeocurrent analysis.The studied syn-rift sediments record i) a changeover from axial to transverse fluvial system in the end of Triassic, ii) transition from fluvial to lake marginal environment in the Early Jurassic and iii) evolution of an extensive palustrine system in the Middle Jurassic. The corresponding changes in lithological and geochemical characters for the first change points towards incresed tectonic movement along the main boundary fault that caused the progradation of the hangingwall fans towards the basin trough. Later changes can be attributed to the reconfiguration of the sedimentary environments due to increase in aridity that influenced the introduction of coarse clastics to the basin.

On the Use of Aeromagnetism for Geological Interpretation: 2. A Case Study on Structural and Lithological Features in the Northern Vosges

AbstractIn many grabens of the West European Rift System, several drilling projects are targeting fluid conductive faults to produce electricity with geothermal energy. In order to reach the adequate temperatures, the targeted faults are located in the basement rocks or at the basement/cover interface. Unfortunately, the basement rock facies and structural data useful for reservoir modeling are often poorly constrained because of the lack of boreholes deeper than 2,000 m and seismic information on the deep subsurface. With the aim to outline the basement structure in the Upper Rhine Graben, two aeromagnetic surveys were conducted where the basement outcrops in the Northern Vosges, with different flight parameters and types of magnetometers. An interpolator using equivalent sources for gridding these two surveys is described by Gavazzi et al. (2020). Here, we describe the geological interpretation of these surveys. We demonstrate the consistency between the total magnetic field intensity and the outcropping lithological units organized in ENE‐WSW subvertical bands. In detail, three main blocks were delineated, separated by N‐S faults linked to the graben formation and ENE‐WSW‐inherited Hercynian faults. The latter faults, together with more local NE‐SW and NW‐SE faults, compartmentalize the N‐S‐oriented basement rocks. This is relevant for geothermal prospection where the N‐S faults are the main targets because fluid flow through other fault systems is deemed restricted. With a focus on the interface between the Northern Vosges and the Upper Rhine Graben, we also demonstrate considerable segmentation on the main graben boundary faults.

Holocene paleoearthquakes on the Tianqiaogou-Huangyangchuan fault in the northeastern boundary fault system of the Tibetan Plateau

Rupture behavior, one of the quantitative features of active fault, is fundamental for understanding crustal deformation mechanisms and assessing regional seismic risk. The 120-km-long Tianqiaogou-Huangyangchuan Fault (THF) is an important branch fault of the Qilian-Haiyuan Fault System (QHFS), the main boundary fault zone at the emerging northeastern margin of the Tibetan Plateau. Mismatching with the fast strike-slip rate, there is no M ≥ 5 earthquake recorded along the THF. To obtain more precise information of the rupture behaviors along the boundary structure, we conducted a 1:50,000 active fault mapping and paleoearthquake trenching on the THF. Two trenches respectively illustrates at least 3 rupture events combining the radiocarbon dating and the OxCal modeling. According to the overlap of their age ranges and the analysis on completeness of the sedimentary records, it is determined at least 5 paleoearthquakes occurred along the THF in the Holocene with respective age ranges of 485 ± 675 BCE, 2085 ± 295 BCE, 5590 ± 1090 BCE, 7910 ± 970 BCE, and 9975 ± 955 BCE and moment magnitudes estimated to be 7.3–7.4. Meanwhile, the detailed investigations on paleoseismology and tectonic geomorphology show the THF was not involved into the seismogenic structure of the nearby 1927 Gulang M8 earthquake. The Monte Carlo statistical analysis of these paleoearthquakes yields a mean recurrence interval of 2.4 ± 0.3 ka, illustrating a strong activity of the THF that accords with its slip rates and an important role as a tectonic boundary accommodating the eastward extruding of the northeastern Tibet relative to the Gobi Alashan block. Moreover, based on its strong activity and long elapsed time of at least 1.7 ka, we propose the THF is another seismic gap with a potential of M ≥ 7 earthquakes similar to the Tianzhu gap near the triple junction in the QHFS, showing a high seismic risk along the THF and in this region.

The evolution of a Late Cretaceous–Cenozoic intraplate basin (Duaringa Basin), eastern Australia: evidence for the negative inversion of a pre-existing fold–thrust belt

The Duaringa Basin in eastern Australia is a Late Cretaceous?–early Cenozoic sedimentary basin that developed simultaneously with the opening of the Tasman and Coral Seas. The basin occurs on the top of an earlier (Permian–Triassic) fold–thrust belt, but the negative inversion of this fold–thrust belt, and its contribution to the development of the Duaringa Basin, are not well understood. Here, we present geophysical datasets, including recently surveyed 2D seismic reflection lines, aeromagnetic and Bouguer gravity data. These data provide new insights into the structural style in the Duaringa Basin, showing that the NNW-striking, NE-dipping, deep-seated Duaringa Fault is the main boundary fault that controlled sedimentation in the Duaringa Basin. The major activity of the Duaringa Fault is observed in the southern part of the basin, where it has undergone the highest amount of displacement, resulting in the deepest and oldest depocentre. The results reveal that the Duaringa Basin developed in response to the partial negative inversion of the pre-existing Permian–Triassic fold–thrust belt, which has similar orientation to the extensional faults. The Duaringa Fault is the negative inverted part of a single Triassic thrust, known as the Banana Thrust. Furthermore, small syn-depositional normal faults at the base of the basin likely developed due to the reactivation of pre-existing foliations, accommodation faults, and joints associated with Permian–Triassic folds. In contrast to equivalent offshore basins, the Duaringa Basin lacks a complex structural style and thick syn-rift sediments, possibly because of the weakening of extensional stresses away from the developing Tasman Sea.

Analysis of multiples in onshore Niger Delta: a prelude to fault shadow phenomenon

In this study, multiples have been analyzed as a prelude to fault shadow phenomenon using 3D seismic data from onshore Niger Delta. This involved detailed velocity analysis of a 3D seismic data using semblance plots, from which depth/interval velocity models were generated for two velocity scenarios, the primaries and lower-than-normal velocities in the shadow zone. These velocity models were subsequently used as inputs in prestack depth migration. The result of migration with the lower-than-normal velocities shows that reflections within the shadow zone are coherent, continuous and better aligned than with the primary velocities. This is an indication that these lower-than-normal velocity events have been well accounted for in the shadow zone using these velocities. This result establishes the presence of lower-than-normal velocities in the shadow zone considered as the primary cause of the reflection distortions and misalignment of events observed in the zone, due to multiple reflections. This occurs beyond 3 s two-way time (3800 m), especially at locations corresponding to the footwall of main boundary faults on seismic. This assertion is made based on the fact that all sources of low-velocity events at this depth such as overpressure and anisotropy have been accounted for by previous researchers in the study area and as such, multiple effects are the only plausible low-velocity candidate for the observed reflection distortions in the shadow zone.

Seismic source zoning and maximum credible earthquake prognosis of the Greater Kashmir Territory, NW Himalaya

We present the seismic source zoning of the tectonically active Greater Kashmir territory of the Northwestern Himalaya and seismicity analysis (Gutenberg-Richter parameters) and maximum credible earthquake (m max) estimation of each zone. The earthquake catalogue used in the analysis is an extensive one compiled from various sources which spans from 1907 to 2012. Five seismogenic zones were delineated, viz. Hazara-Kashmir Syntaxis, Karakorum Seismic Zone, Kohistan Seismic Zone, Nanga Parbat Syntaxis, and SE-Kashmir Seismic Zone. Then, the seismicity analysis and maximum credible earthquake estimation were carried out for each zone. The low b value (<1.0) indicates a higher stress regime in all the zones except Nanga Parbat Syntaxis Seismic Zone and SE-Kashmir Seismic Zone. The m max was estimated following three different methodologies, the fault parameter approach, convergence rates using geodetic measurements, and the probabilistic approach using the earthquake catalogue and is estimated to be M w 7.7, M w 8.5, and M w 8.1, respectively. The maximum credible earthquake (m max) estimated for each zone shows that Hazara Kashmir Syntaxis Seismic Zone has the highest m max of M w 8.1 (±0.36), which is espoused by the historical 1555 Kashmir earthquake of M w 7.6 as well as the recent 8 October 2005 Kashmir earthquake of M w 7.6. The variation in the estimated m max by the above discussed methodologies is obvious, as the definition and interpretation of the m max change with the method. Interestingly, historical archives (∼900 years) do not speak of a great earthquake in this region, which is attributed to the complex and unique tectonic and geologic setup of the Kashmir Himalaya. The convergence is this part of the Himalaya is distributed not only along the main boundary faults but also along the various active out-of-sequence faults as compared to the Central Himalaya, where it is mainly adjusted along the main boundary fault.