Thrust Zone Research Articles

Accumulation sequence and exploration domain of continental whole petroleum system in Sichuan Basin, SW China

Based on the oil and gas exploration in the Sichuan Basin, combined with data such as seismic, logging and geochemistry, the basic geological conditions, hydrocarbon types, hydrocarbon distribution characteristics, source- reservoir relationship and accumulation model of the Upper Triassic–Jurassic continental whole petroleum system in the basin are systematically analyzed. The continental whole petroleum system in the Sichuan Basin develops multiple sets of gas-bearing strata, forming a whole petroleum system centered on the Triassic Xujiahe Formation source rocks. The thick and high-quality source rocks in the Upper Triassic Xujiahe Formation provide sufficient gas source basis for the continental whole petroleum system in the basin. The development of conventional-unconventional reservoirs provides favorable space for hydrocarbon accumulation. The coupling of faults and sandbodies provides a high-quality transport system for gas migration. Source rocks and reservoirs are overlapped vertically, and there are obvious differences in sedimentary environment, reservoir lithology and physical properties, which lead to the orderly development of inner-source shale gas, near-source tight gas, and far-source tight–conventional gas in the Upper Triassic–Jurassic, from bottom to top. The orderly change of geological conditions such as burial depth, reservoir physical properties, formation pressure and hydrocarbon generation intensity in zones controlled the formation of the whole petroleum system consisting of structural gas reservoir in thrust zone, shale gas-tight gas reservoir in depression zone, tight gas reservoir in slope zone, and tight gas–conventional gas reservoir in uplift zone on the plane. Based on the theory and concept of the whole petroleum system, the continental shale gas and tight gas resources in the Sichuan Basin have great potential, especially in the central and western parts with abundant unconventional resources.

The collapse of the Caledonian orogen in SW Norway: Insights from quartz textures

Extensional collapse is a common late- to post-collisional feature of orogens. It is particularly prominent in the SW Scandinavian Caledonides, where extensional detachments formed progressively from the initial reactivation of the basal thrust zone to the formation of hinterland-dipping extensional shear zones. A mature stage, documented here, involves the development of bivergent (both hinterland- and foreland-dipping) shear zones and associated vertical basement mobilization. The main foreland-facing extensional shear zone in the study area is the Bergen Detachment – a until recently overlooked or misinterpreted structure. This detachment overprints top-to-W mylonitic fabrics related to the earlier Devonian extension stages and developed in response to the updoming of the Baltica basement west of Bergen (the Øygarden Complex) into a late core complex. Our microstructural and textural examinations suggest that for both the Hardangerfjord Shear Zone and the Bergen Detachment, strain was localized by activation of dislocation creep in quartz through the operation of multiple slip systems in the <a> direction, predominantly prism <a> and rhomb <a>. These examinations and existing radiometric age constraints suggest that the progressive shear zone development occurred over maybe as little as 5 million years, under upper to middle greenschist facies conditions. Synkinematic cooling brought both the Bergen Detachment and Hardangerfjord Shear Zone through the ductile-brittle transition zone. The main explanation for this prolonged collapse development is 1) that the early low-angle detachment became too low-angle for continued shearing, giving rise to the first hinterland-dipping set of shear zones, and 2) that the basement weakened rheologically and mobilized gravitationally with the formation of large upright folds with new detachments along their flanks (the bivergent stage), including the Bergen Detachment.

Depositional environments and thermal maturity of the hydrocarbon source rocks in the Devonian–Early carboniferous Ora Formation from palynological organic petrographic investigations in northern and western Iraq

The study investigates the palynofacies, organic matter character, and hydrocarbon generation potential of this rock unit, based on surface and subsurface samples from the Ora Formation type-section in extreme northern Iraq and the Akkas-3 well in western Iraq, respectively. Dark, mostly oxidized, gelified platy amorphous organic matter (AOM) dominates the palynological components in the surface section, in addition to a few rounded spores and structured phytoclasts. In contrast, palynomorphs, including Ambitisporites avitus, Aneurospora spp., Vallatisporites verrucosus, Acinosporites spp., Verrucosisporites nitidus and the algae Botryococcus, are predominant in studied samples from the Akkas-3 well. Statistical cluster analysis identified three palynofacies types in the surface section based on stratigraphic variations in the particulate organic matter. These vary from high amorphous organic matter (AOM), moderate phytoclast, and low palynomorph abundances that represent proximal suboxic–anoxic shelfal environments to moderate to good AOM and low to moderate palynomorph abundances that represent distal suboxic–anoxic or distal dysoxic–anoxic shelfal environments. The organic petrographic study of the outcrop section also revealed the strong effect of oxidation, where dispersed terrigenous and amorphous organic materials in the form of granular and gelified forms dominate and reflect a terrestrial origin of these components. In the subsurface section, a mixed terrestrial and less marine or lacustrine origin characterized the studied organic matter, where land plant spores (sporinite), in addition to vitrinite and inertinite, are dominant with a few scattered liptinite macerals. The difference in thermal maturity between the outcrop and subsurface samples is likely due to the higher tectonic burial of the outcrop samples that form part of the Northern Thrust Zone of Iraq. Nevertheless, higher abundances of AOM (oil-prone kerogen type II) accumulated in the northern outcrop type section. This might imply that the Ora Formation has a higher potential for hydrocarbon production north of the Akkas field.

Kinematics Along the Qingchuan Fault and Deformation Pattern in the Eastern Tibetan Plateau

AbstractThe accommodation of the substantial eastward crustal motion of the Bayan Har block characterizes the dynamics of faults located at the eastern Tibetan Plateau. However, uncertainties persist concerning the manner and amount of deformation distributed on these faults, with slip senses and rates constituting critical factors. In the northeastern segment of the Longmenshan thrust zone, the contentious activity and the unknown geologic slip rate present challenges. This study, focusing on the Qingchuan fault, the predominant fault within the northeastern Longmenshan, employed satellite imagery interpretation, displaced fluvial terraces surveying, displacement measurements, and chronological analysis to comprehensively characterize its fault activity. Our investigation robustly demonstrates the Qingchuan fault has been active since the late Quaternary, and is primarily marked with a pronounced dextral slip at a rate of 0.6–1.0 mm/year. By quantitatively assessing the deformation rates of the faults at the eastern Tibetan Plateau, we propose that they sufficiently accommodate the entire eastward crustal movement of the Bayan Har block; thereby no additional deformation propagates beyond the Qingchuan fault. Furthermore, we introduce a subblock model to elucidate the regional crustal deformation pattern, wherein the eastward movement of the Bayan Har block transfers to the northeastward movement of the Bikou subblock. This movement results in reverse slip patterns for the Minjiang and Huya faults, while the Beichuan and Qingchuan faults predominantly experience dextral displacements. The complex strain partitioning within the northern Longmenshan range underscores the observed variations in slip patterns across different segments of the Longmenshan thrust zone, advancing our understanding of fault behavior and the orchestration of crustal deformation in this intricate tectonic framework.

Coseismic deformation and seismogenic structure of the 2024 Hualien Earthquake measured by InSAR and GNSS

On April 3, 2024, an M7.3 earthquake occurred in the offshore area of Hualien County, Taiwan, China. The seismogenic structure at the epicentral location was highly complex, and studying this earthquake is paramount for understanding regional fault activity. In this study, we employed ascending and descending orbit Sentinel-1 Synthetic Aperture Radar (SAR) data and utilized differential interferometry (InSAR) technique to obtain the co-seismic deformation field of this event. The line-of-sight deformation field revealed that the main deformation caused by this earthquake was predominantly uplift, with maximum uplift values of approximately 38.8 ​cm and 46.1 ​cm for the ascending and descending orbits, respectively. By integrating the three-dimensional GNSS co-seismic deformation field, we identified the seismogenic fault located in the offshore thrust zone east of Hualien, trending towards the northwest. The fault geometry parameters, obtained through the inversion of an elastic half-space homogeneous model, indicated an optimal fault strike of 196°, a dip angle of 30.9°, and an average strike-slip of 0.4 ​m and dip-slip of −2.6 ​m. This suggests that the predominant motion along the seismogenic fault is thrusting. The distribution of post-seismic Coulomb stress changes revealed that aftershocks mainly occurred in stress-loaded regions. However, stress loading was observed along the northern segment of the Longitudinal Valley Fault, with fewer aftershocks. This highlights the importance of closely monitoring the seismic hazard associated with this fault segment.

Advanced 3D seismic hazard analysis for active compression in the Adriatic Thrust Zone, Italy

The probabilistic seismic hazard assessment contains two ingredients: (1) the seismic source model with earthquake rates and rupture parameters for specification of the statistical distribution of earthquakes in time and space and (2) the ground motion model, for estimation of ground shaking level at a site for each earthquake rupture. The selection of these models significantly impacts the resulting hazard maps, and it can be challenging, particularly in seismotectonic regions where overlapping structures, sited at different depths, coexist. Eastern Central Italy is a well-known active compressional environment of the central Mediterranean with a complex tectonic structure with a lithospheric double shear zone. In this study, we propose a seismic hazard assessment to analyze the contribution of these two shear zones as overlapping multi-depth seismogenic volumes to ground motion at a given hazard level. We specifically focus on selecting relevant and applicable parameters for earthquake rate modeling, emphasizing the importance of defining rate computation and rupture-depth parametrization in hazard analysis. To achieve this, we utilized a seismotectonic- and catalog-based 3D adaptive smoothed seismicity approach following the methodology given by (Pandolfi et al. in Seismol Res Lett 95: 1–11, 2023). Finally, we demonstrated how this innovative 3D approach can identify with high resolution the individual sources' contribution with particular attention to the depth location of structures that strongly influence the ground motion. Moreover, combining seismotectonic data with seismicity avoids the challenges associated with structures with scarce geologic, geodetic, or paleoseismological data. Our result provides detailed insights into the seismic hazard within the Adriatic Thrust Zone.

The Kangâmiut dykes in West Greenland: markers of the tectono-metamorphic evolution of the southern Nagssugtoqidian orogen and its foreland

The general extent and structural evolution of the southern Nagssugtoqidian orogen of West Greenland were first described by Hans Ramberg who based much of his paper on the deformation of the regional Kangâmiut dyke swarm. The southern boundary is marked by a transition from undeformed, discordant dykes in the south to highly deformed dykes and host rocks to the north. Our analysis of the southern Nagssugtoqidian orogen and its southern foreland uses a comprehensive compilation of available data and covers the area from Sisimiut in the north to Alanngua, south of Maniitsoq. This represents almost the entire c. 200 km latitudinal extent of the Kangâmiut dyke swarm and encompasses the complete range of Nagssugtoqidian overprint on these dykes and their country rocks. South of Itillip Ilua (Itilleq), the structural and metamorphic overprints on the dykes exhibit a considerable range in both intensity and P–T conditions between and even within outcrops. In contrast, north of Itillip Ilua, the rocks show more systematic gradual increases in the degree of structural overprints and metamorphic grade, culminating in the Ikertooq thrust zone where granulite facies rocks are brought southwards over amphibolite facies rocks. Currently, available age data from the Nagssugtoqidian orogen permits the identification of two metamorphic episodes at c. 1850–1800 Ma and c. 1780–1720 Ma. These groups of metamorphic ages are supported by recent 40Ar–39Ar ages from dykes in the same area, which cluster at c. 1860 Ma and c. 1740 Ma, respectively. Albeit geographically sporadic, both age intervals support a subdivision of the Nagssugtoqidian structural and metamorphic overprints across the southern Nagssugtoqidian orogen and its foreland into two distinguishable temporal phases. Further geochronological investigations may well, however, find these two phases to be part of a tectonic continuum. For now, it is thought that the older event records south-directed thrusting over the foreland and concomitant loading of this crust, at least as far south as Maniitsoq. This c. 1860–1800 Ma crustal shortening and thrusting likely also closed a depositional basin located at the current latitude of Ikertooq, which could have formed during an early-orogenic extensional event that enabled and accompanied the c. 2035 Ma emplacement of Kangâmiut dykes. Up to 50–100 Ma later, a younger (c. 1780–1720 Ma) phase of shearing and thrusting mainly affected the Itillip Ilua – Ikertooq area and likely overprinted elements of the former event. This local younger overprint generated a separate trend of distinctly northward-increasing deformation and metamorphism.

Seamount Subduction Dynamics and Long‐Term Evolution of the Franciscan Active Margin

AbstractThe Snow Mountain Volcanic Complex (SMVC; northern California, USA) is a well‐preserved example of a coherently‐exhumed subducted seamount. This study reappraises the genesis and evolution of this complex and surrounding units through detailed field, petro‐structural and geochronological analyses. This work demonstrates that the SMVC (a) erupted at ∼166 Ma as a hotspot volcano on the Farallon Plate, (b) entered the Franciscan subduction trench at ∼118 Ma, and (c) was subsequently subducted to a depth of ∼20 km (within the seismogenic zone), as shown by local blueschist‐facies assemblages formed at 0.6 GPa, 240°C. Transient subduction interfaces are preserved above, within, and below the SMVC, making it an exceptional target to study seamount subduction dynamics. Like other seamounts, the subduction‐related deformation was mainly accommodated along kilometer‐scale internal thrust zones lubricated by serpentinite/metasediments, and within centimeter‐thick crack‐seal veins recording pulsed fluid flow near peak metamorphism. No unequivocal proof of seismic activity was found. The integration of other seamounts (some potentially belonging to a former seamount chain) in the Franciscan Complex suggests that exhumed seamounts are more abundant than previously thought. Moreover, pressure‐temperature‐time estimates of subduction metamorphism for the surrounding units, combined with previous work constrain the thermal maturation of the subduction zone through time and the in‐sequence emplacement of the SMVC. Rapid changes in age of the subducted oceanic plate when subducted additionally hint to the subduction of large‐offset transform faults on the former Farallon plate. Such a process might have been linked to changes in accretion dynamics and magmatic flare‐ups in the arc.

Diagenetic control on porosity types of carbonate rocks of Late Triassic Baluti Formation in Northern Thrust Zone, Iraqi Kurdistan region, Northern Iraq

The carbonate rocks of Late Triassic Baluti Formation in Sararu outcrop, Northern Thrust Zone, Northern Iraqi Kurdistan region consist of brecciated marly limestone and sandy dolomitic limestone interbedded with shale that characterized by complex heterogeneity. The microscopic description of the porosity types and diagenetic processes controlled it were studied in current work. The petrographic study of 12 thin section of Baluti carbonate displayed that the main skeletal grains are shallow marine derivative fossils and non-skeletal grains are only monocrystalline quartz. The main matrix is micrite which sometimes changed to microspar and sparry calcite. Seven pore types were distinguished in the studied rocks of the formation from both primary and secondary stages. The primary porosity are interparticle and intraparticle pores. While, the secondary types were moldic, intercrystalline, fracture, vuggy and stylolitic porosities. The formation in studied outcrop was undergone in several diagenetic processes, that enhanced or reduced the porosity of the Baluti carbonates. Early micritization protect the primary pores from destroying by compaction. While, solution was assists in creating different secondary pores in different diagenetic stage of the formation particularly the dominant moldic porosity. Calcite cements aid in reducing all pore types in the studied formation. Late dolomitization contribute to formation intercrystalline and dolomoldic porosities. stylolites with dissolution helps to creating stylolitic porosity. Neomorphism, silicification and pyritization generally reduced porosity in the formation.

Estimation of active surface deformation using PSInSAR technique of the Central Himalayan region

The Kumaun Himalaya is considered as the most active part of the Central Seismic gap in the Indian Sub-continent. In this paper, we presented active surface deformation rates of the Kumaun region from February 2017 to February 2021 using the Persistent Scatterer Interferometric Synthetic Aperture Radar (PSI) technique. The cumulative displacement that occurred during the span of 4 years is ±55 mm, whereas the Line of Sight (LOS) deformation velocity rate ranges is ±7 mm/yr. Apart from PSI, we also estimated the b-value of the Kumaun region from 1803 to 2021 (399 events) and its value is 0.54 ± 0.03. A distinct NE-SW trend of b-value is observed where earthquakes with M > 6 occurred towards NE of this trend. The PSI-derived deformation reveals that the central part of the Inner Lesser Himalaya along with the Main Central Thrust (MCT) zone is dominated by uplift. The zone between the Munsiari Thrust (MT) and MCT in the central region shows the maximum uplift ranging 5–7 mm/yr which exactly lies above the mid-crustal ramp of the Main Himalayan Thrust (MHT). Our results are well corroborated with available observations of geodetic strain and peak ground acceleration values. However, the deformation patterns and high-velocity rates in the central part of the study area between MT and MCT indicate the accumulation of high stress.

Exhumation mechanisms of the Himalayan metamorphic core in the Bhagirathi Valley, NW India: Insights from an integrated structural and metamorphic analysis

The Himalayan metamorphic core in the Bhagirathi valley (NW India) consists of low-grade rocks belonging to the Lesser Himalayan Sequence to the south, and of high-grade rocks of the Greater Himalayan Sequence (GHS), to the north. The contact between these two units is a 2–3 km thick high-strain zone known as the Main Central Thrust zone (MCTz). Structural studies in the GHS identify the superposition of foliation (S2) related to tight isoclinal fold (F2) on a pre-existing foliation (S1). S2 was further overprinted by D3 crenulations, and a set of conjugate fractures (D4) overprinted these ductile fabrics. Deformation in the GHS was characterized by top-to-the-SW shearing, which, in its upper structural level, was overprinted by top-to-the-NE extensional shear. Metamorphic analysis shows a moderate T/high P inverted metamorphic sequence with pre-to syn-kinematic, inclusion-rich garnet in the structurally lower part of the GHS and high T-moderate P prograde metamorphism with post-tectonic inclusion-free garnet and evidence of partial melting, in its upper structural level. We envisage that the uppermost part of the GHS and the lower part of the GHS, including the MCTz, are two distinct tectonic slices, separated by a tectono-metamorphic discontinuity.

Hydrocarbon generation potential and thermal maturity of the outcropped Palaeozoic formations from Khabour Valley, Kurdistan Region, Northern Iraq

In this study, 12 outcropped rock samples from the Khabour, Kaista, Ora, and Harur formations (Paleozoic) in the Khabour Valley were studied. The location is in northern Iraq, in the Imbricated Thrust Zone, northeast of Duhok City. Rock-Eval Pyrolysis was utilized to assess the thermal maturity, organic matter composition, hydrocarbon potential, and organic richness of the Khabour, Kaista, Ora, and Harur formations situated within the Khabour Valley. The content of Total Organic Carbon ranges from 0.19 to 0.37 wt.%, with an average of 0.29 wt.% for the Khabour formation, 2.07 to 3.09 wt.% for the Kaista formation, 0.58 wt.% for the Ora formation, and 0.31 to 1.55 wt.% for the Harur formation. The source rock was rated poor based on the TOC wt.% content. According to the results, the Khabour Valley formations have low amounts of S2 (average 0.12 mg HC/g Rock, range 0.01-0.29 mg HC/g Rock) and Hydrogen Index (HI) (average 15.2 mg HC/g TOC, range 2-41 mg HC/g TOC). Drawing insights from the analysis of Rock-Eval data, the formations exhibit kerogen types classified as type IV kerogen. According to the cross plot of S2 versus TOC%, all the samples show a poor potentiality to create hydrocarbons. The graph of HI vs Tmax demonstrates that Tmax ranges from immature to postmature.

Seismic Cycle in Bituminous Dolostones (Monte Camicia Thrust Zone, Central Apennines, Italy)

AbstractSeismic ruptures often propagate along fault zones cutting km‐thick sequences of carbonates (e.g., Wenchuan Mw 7.8, 2008, China; L’Aquila Mw 6.1, 2009, Italy). As a consequence, fault rock assemblages may record the seismic cycle under a wide range of loading conditions, temperatures (&gt;1,000°C during co‐seismic slip), and fluid‐rock interactions. The Monte Camicia Thrust Zone in the Italian Central Apennines is exhumed from ∼3 km depth. We studied the seismic cycle recorded by a network of faults cutting bituminous dolostones in the footwall of the Monte Camicia Thrust. These faults accommodate up to several meters of displacement. Slip zones are mm‐ to cm‐thick and bounded by ultra‐polished (“mirror‐like”) surfaces independently of their displacement. At the microscale, deformation is accommodated by cataclasis and pressure‐solution in carbonates, and viscous flow in the foliated bitumen. Some of the faults with displacements &gt;0.10 m have multiple slip zones, separated by “mirror‐like” surfaces, and include clasts of foliated bitumen and fragments of older slip zones sealed by calcite precipitation. We conclude that (a) slip zones record post‐ to inter‐seismic (foliated bitumen) and co‐seismic (fragments of bitumen) deformation in a fluid‐rich environment (calcite precipitation) and (b) mirror‐like surfaces formed during all phases of the seismic cycle.

Switching from the top-NW backthrusting to top-SE post-orogenic deformation in the Aegean domain: Insights from the calcite microfabric and siliciclastic rocks of the Amorgos Unit (SE Cyclades, Greece).

Detailed microstructural analysis was conducted in the imbricated Amorgos stratigraphy to decipher the complex deformation history of the calcitic mylonitic zones related to different tectonic events. Four major thrust zones were developed during syn-metamorphic tectonic burial by top-NW general shear deformation. Dynamic recrystallization is the dominant deformation mechanism, evolving from the SGR to the BLG field, characterizing a downward increase in strain from the highest Potamos to the lower Paradesia thrusts under low-T conditions. Mesoscopic kinematics within these thrusts reveal top-NW shearing. However, in microscale, both top-NW and top-SE kinematics are observed. In most cases, this top-SE reversal resulted from strain partitioning due to a strong pure shear component. However, the Mouros and Richti shear zones present dominant top-SE kinematics in all scales. Additionally, overprinting relationships within the metaflysch of the Mouros shear zone suggest that the top-SE kinematics of this zone post-dates the former top-NW ones. Therefore, we suggest that a switch from the top-NW general shear deformation during the syn-orogenic compressional phase to a late top-SE extensional deformation occurred in the early Miocene, resulting in the exhumation of the Amorgos Unit below 200 °C, via the Santorini Detachment System.

CPO and quantitative textural analyses within sheath folds

We investigate the intra- and inter-crystalline deformation processes involved in sheath fold development combining complementary fabric analysis techniques and 3D modelling by neutron tomography. The investigated sheath fold is a multi-layered sub-metre scale single-eye structure, developed in metapsammites from the Ben Hope Nappe, overlying the Moine Thrust Zone of NW Scotland. Crystallographic Preferred Orientations (CPOs) of quartz and biotite were acquired through a Neutron Diffractometer and an SEM-EBSD system to compare the full-fabric of the main phases and the active slip systems for an “in situ” structural control. Combined with orientation maps and grain size maps, results show that, despite the different structural positions of the investigated microdomains (upper vs lower fold limbs, inner vs outer sheath closures, distance from hinge of the sheath fold), quartz and biotite deformed uniformly, suggesting a constant differential stress and orientation of the kinematic vorticity axis. Previously recognized detachment horizons within the sampled sheath fold do not affect the fabric patterns recorded by quartz and biotite. This may be interpreted in two different ways: i) detachments formed during earlier active folding and prior to passive amplification of folds associated with more uniform flow to create the sheath fold geometries; ii) the quartz c-axis patterns are coeval with a late deformation phase (loading of the orogenic wedge) that pervasively obliterated the previous fabric and therefore did not preserve the active folding component. Several pieces of evidence reported here, such as top-to-SE normal-shear sense which is opposite to the regional kinematics, are more supportive of the second hypothesis. The analysis of mineral textures provides an improved dataset for the whole sheath fold and increases our understanding of recrystallization mechanisms active in shear zones.

Petroleum system of the fold-and-thrust belt of the United Arab Emirates: New insights based on 1D and 2D basin modeling

The hydrocarbon potential of the fold-and-thrust belt (FTB) in the United Arab Emirates (UAE)-Oman mountains has received limited attention to date, leading to a poor understanding of the petroleum systems in this region. Despite the existence of hydrocarbon fields within the FTB, the source rock potential has not been adequately studied. This study aims to address this knowledge gap using 1D and 2D basin modeling approaches to evaluate the petroleum system of the FTB. In addition, gas chromatographs are also used to correlate hydrocarbon occurrences with their source rock. This study's findings identify the Silurian, Upper Cretaceous, Paleocene–Eocene, and Oligocene formations as the primary source rocks in the study area. Silurian shales, encountered in a well in the northern UAE, are currently considered overmature. The Cenozoic source rocks exhibit a spectrum of Total Organic Carbon (TOC) content, ranging from less than 1 to as high as 2 wt%, leading to variable degrees of expulsion efficiency. The maturity of these rocks varies based on their position in relation to the FTB and foredeep, with increasing maturity towards the north. The Upper Cretaceous sequences display low TOC and Hydrogen Index, indicating very low expulsion efficiency. The present-day distribution of maturity is largely influenced by Late Cretaceous and Oligocene–Miocene compressional events that affected the northern and northeastern Arabian Plate. This analysis shows that hydrocarbon expulsion from the Silurian source rocks was initiated during the Middle-Late Jurassic. These hydrocarbons are presumed to have migrated through Upper Permian, Jurassic, and Lower and middle Cretaceous reservoirs. Westward hydrocarbon migration, towards a regional bulge, may have also occurred following compressional events that resulted in lithospheric flexure and formation of the foreland basin. Notably, certain exceptions to migration towards the bulge include structural entrapment of hydrocarbons beneath the main frontal thrust zone of FTB and some structural traps beneath the Lower Fiqa Formation.

A deep CNN-LSTM model for predicting interface depth from gravity data over thrust and fold belts of North East India

Geological interface depth modeling from the gravity field data is crucial for the exploration of oil and gas, mapping of sediment-basement interfaces and many other geological modeling studies. Two-dimensional radially averaged power spectrum analyses yield ensemble mean depth from the gravity anomaly which lacks information away from the focus of the depth point modeled. This paper aims to introduce adeep learning techniquecombining a convolutional neural networkand long short-term memory network (CNN-LSTM) with variogram modeling to model interface depths using geospatial coordinates, Bouguer gravity anomaly and altitude variations data over the thrust and fold belts of North-East India. Prior to actual data analysis, the method is tested on a synthetic model. After the optimal network selection with the ‘Relu’ transfer function and ‘Adam’ optimization, the robustness of the CNN-LSTM model is examined and the test results show that the model is robust up to the data assorted with moderate level (∼20%) of correlated noise. The CNN-LSTM model detects important geological structures, subsurface faults, and thrust zones namely, Kohima Synclinorium, Naga and Disang thrust, and Eastern boundary thrust. Comparative results suggest that the CNN-LSTM is skillful based on the mean-squared error (MSE) between the observation and the prediction (MSECNN-LSTM ∼ 0.014) relative to the other machine learning models (Adaptive-Neuro-Fuzzy-Inference System (MSEANFIS ∼ 0.020), Random Forest with Extra Tree(MSERF ∼ 0.075), Support Vector Regression (MSESVR ∼ 0.025), Bayesian Neural Network(MSEBNN ∼ 0.116), Convolutional Neural Network(MSECNN ∼ 0.022), Long Short-Term Memory Network(MSELSTM ∼ 0.023), and Gaussian Process(MSEGP ∼ 0.095)) used in the study. We, therefore, conclude that the proposed approach is robust enough to model the various geological interface depths precisely with appropriate input data of geospatial coordinates, Bouguer gravity anomaly and altitude variation. The approach used here has potential and could be explored further in other complex geological provinces around the world.

Mitigating loose rock fall and cavity formation in Adit-2 tunnel of Rammam III hydroelectric project in Himalayas, India: Challenges and Solutions.

The turbulent geological history of “The Himalayas” arguably poses the most challenging ground conditions than anywhere in the world for underground excavations works. Cavity formation, loose fall, rock busting, high ingress of water and squeezing ground conditions are common phenomenon that encountered along tunnel profile in Himalayas. The Rammam III Hydroelectric Project, situated in Sikkim Himalayan belt lies in Main Crystalline Thrust zone (MCT) and is facing its fair share of Geological challenges during the underground excavation process.The primary objective of the Rammam Stage-III Hydro Electric Project is to generate 120 MW power through 8.2 km of HRT (Head Race Tunnel).Excavation of the Adit-2 to HRT has been challenging because of its proximity to the Jhepi Khola, a fault-incised stream. The tunnel’s progress was impeded by various obstructions of small and large magnitude caused by several sympathetic shears along joints and other geological complexities.This paper will focus on the challenges faced in mitigating loose rock falls, cavity formation in Adit-2 from RD 312.0m to 320.0 m. To address the issue, an effective excavation philosophy known as DRESS (Drainage-Reinforcement-Excavation-Support-Solution) was adopted. The solution proposed is highly effective in addressing water-bearing issues in the Himalayas.The treatment method adopted not only achieved the desired level of safety and smooth construction progress, but also provides a valuable reference for future similar cases of loose fall and cavity formations.

Zircon U–Pb, Hf, and O isotopic constraints on the tectonic affinity of the basement of the Himalayan orogenic belt: Insights from metasedimentary rocks, orthogneisses, and leucogranites in Garhwal, NW India

Zircon U–Pb geochronology and Hf–O isotope geochemistry of metasedimentary rocks, orthogneisses, and leucogranites in Garhwal, NW India, provide new insights into the tectonic affinity of the basement of the Himalayan orogenic belt. Detrital zircon U–Pb ages from metasedimentary rocks in both the Lesser Himalayan Sequence (LHS) and the overlying Main Central Thrust (MCT) zone yielded dominant Paleoproterozoic ages with the youngest age peak of 1840 Ma. Along with that, the variable ɛHf (t) values (−14.56 to 8.44) indicate that these metasedimentary rocks could have been derived from the juvenile Paleoproterozoic orthogneiss basement in the Himalayas and/or the ancient Indian craton in the south. In contrast, the metasedimentary rocks in the Ediacaran Harsil Formation are dominated by the ca. 1200–800 Ma zircons with the youngest age peak of ca. 630 Ma and variable ɛHf (t) values (−16.57 to 13.32), which belong to the underlying High Himalayan Crystalline Sequence (HHCS), rather than the overlying Tethyan Himalayan Sequence. Orthogneisses in the MCT zone record ca. 1.87–1.85 Ga felsic magmatism associated with the Nuna supercontinent. The Bhagirathi leucogranites intruding the Harsil Formation were formed at ca. 496–490 Ma. These Paleoproterozoic orthogneisses and early Paleozoic leucogranites show only positive ɛHf (t) values (0 to +15) correlatable with a juvenile mantle input. However, the zircon Hf model ages older than the U–Pb ages and the oxygen isotope values (δ18O: 7.74–8.91 ‰), significantly higher than zircons crystallized from mantle-derived magma, indicate large degrees of melting and contamination of crust materials in both the granitoids. In combination with the pre-existing geochronological data, the current study indicates that the Garhwal HHCS is characterized by bimodal Neoproterozoic zircons at ca. 1100–900 Ma and ca. 860–800 Ma with wide εHf (t) values which are similar to those in the outer LHS and the Cathaysia Block (including the Jiangnan Orogen) of the South China Craton. These observations suggest that a close relationship existed between North India and South China during the Neoproterozoic since Tonian. The provenance of the Neoproterozoic HHCS is possibly a mixed source from both southern Gondwana terranes (e.g., east Antarctica) and northern South China.

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.