Tectonic Period Research Articles

Regional Geological and Tectonic Environment and Evolutionary Characteristics of the Yangbajing-Gedaxiang Basin,Tibet

Gedaxiang Basin in Tibet is located in the south-eastern section of Gangdisi structural belt, with a typical geological and tectonic environment and a commonly developed tectonic pattern in plateau area. By studying the structural geological environment of Geda Township area and analyzing its internal geology, it can be seen that: (1) Six large-scale faults developed in the area control the basic geological and tectonic structure of the working area and affect the spatial structure of the area; The fault development direction is mainly north-south, mostly brittle faults at shallow surface level, and a few of them are east-west orientated. Most of the faults in the region are caused by the development of geological and tectonic activities in the Himalayan period, and the ones that developed later are the north-south oriented positive faults formed by the extension movement in the late Himalayan period. (2) There are three periods of tectonic environment evolution in the area of the evolutionary history, since Jurassic: the first period is the late Yanshan period, tectonic activity for the extrusion environment, the formation of north-south fold deformation under the compression environment; The second stage is early to middle mountain stage, which formed large scale EW-trending nappe structure and thrust fault, and the regional stress background is compressional environment. The third stage is the late Alpine stage, forming a series of tectonic systems of grabens and normal faults with SN strike, which were mainly formed by geological activities under the extensional system. The results of this study provide a reference for distinguishing the trace of geological rock group in the tectonic evolution of the Himalayan tectonic period.

Tectonic Transition from Passive to Active Continental Margin of Nenjiang Ocean: Insight from the Middle Devonian-Early Carboniferous Granitic Rocks in Northern Great Xing’an Range, NE China

Northeast China occupies the majority of the eastern Central Asian Orogenic Belt, which mainly consists of continental blocks and accretionary terranes. The Devonian was a tectonic quiet period in the NE China region due to a lack of tectono-magmatism, but the tectonic background of this period has been unclear, especially for the Hegenshan-Heihe Suture between Xing’an and Songliao accretionary terranes, which represents the Paleozoic Nenjiang Ocean (a branch ocean of the eastern Paleo-Asian Ocean). Here we report granitic rocks from the Woluohe area, Northern Great Xing’an Range, NE China, to constrain the tectonic process of the transition from the Devonian quiet period to the Early Carboniferous active tectonic period. Three granitic rock samples produce zircon U-Pb ages of 389 Ma, 368 Ma, and 351 Ma, belonging to the Middle and Late Devonian and Early Carboniferous, respectively. They have high Si, Al, K, and Na contents, but with low Mg, Fe, and Ti contents, together with positive Hf isotopic features and low molar Al2O3/(MgO+FeOT) ratios, we suggest that they were derived from partial melting of lower crustal igneous rocks. Meanwhile, the narrow major element variation at odd with the fractionation process and their negative Nb and Ta anomalies imply the obvious contribution of crustal. Comprehensive tectonic setting analysis shows all samples are in calc-alkali magmatic series with rightward fractionated REE and trace element patterns that are enriched in LREE and LILE and depleted in HREE and HFS, indicating a subduction-related magmatic arc setting. Considering the regional tectonic setting and the small scale of the Devonian plutons, we suggest a limited subduction tectonic setting during the quiet period of the northern Great Xing’an Range, which might indicate the beginning of an initial northwestward subduction of the Nenjiang Oceanic lithosphere beneath the Xing’an Accretionary Terrane in the Middle Devonian, accelerated subduction in the Late Devonian, and bidirectional subduction in the Early Carboniferous.

Using multiple methods to better constrain the depositional age of a meta-sedimentary succession: An example of the Laoling Group from the northeastern North China Craton

The youngest detrital zircon (DZ) or groups of zircons age is usually interpreted as the maximum depositional age (MDA) and the oldest metamorphic zircon since deposition is used to resolve the minimum depositional age of a meta-sedimentary succession. However, DZ grains with multi-cycle or fingerprinting proximal sources may cause the MDAs older than the true depositional age, whereas post-depositional resetting has the opposite effect. Here, we present DZ U-Pb, Lu-Hf isotopic and trace elemental data, supplemented by whole-rock geochemistry, from the greenschist-facies metamorphosed Laoling Group consisting of clastic and carbonate rocks in the northern margin of the Jiao-Liao-Ji belt (JLJB) in the North China Craton to investigate the use of multiple methods to constrain its true depositional age. The dominant late Neoarchean and middle Paleoproterozoic DZ age peaks from the meta-sandstones, meta-siltstone, meta-mudstone, and meta-carbonate rocks are interpreted to be sourced from the Southern Jilin Complex and the JLJB, respectively. The low Th/U ratios (<0.18) and Ce/Nd ratios (<2.5) of the 1.96–1.77 Ga metamorphic zircons, which is consistent with the monazite ages of 1.95–1.76 Ga, are used to filter the possibly altered detrital magmatic zircons and the source-normalized α-dose metrics bigger than ca. 0.8 imply a predominant first-cycle origin, therefore the true depositional age of the group is constrained at 2.08–2.03 Ga. Additionally, in consideration of the strike of the basin parallel to the boundary between the JLJB and the Longgang Block, two periods of regional extensional tectonism, bimodal provenance, and depositional age coeval with the second magmatic event in the JLJB, we suggest that the Laoling Group most likely deposited in a back-arc basin in the northern margin of the JLJB. Finally, the temporal changes of zircon Hf isotope and Eu/Eu* in zircon crustal thickness proxy, especially the coeval excursion towards more radiogenic Hf signals and crustal thinning at 2.17–2.13 Ga, is consistent with the back-arc basin opening and closure model rather than the intra-continental rift model.

Migration of paleo-uplift in southwestern Tarim Basin and its implications for orogenesis and reservoir development, NW China

Based on well horizon calibration, the typical seismic profiles in southwestern Tarim Basin were interpreted systematically, regional geological sections were established, and the regional denudation thickness of each tectonic period was restored. On this basis, the plane morphology maps of ancient structures of the Cambrian pre-salt dolomites in different periods were compiled, and the spatial distribution, development, evolution and migration of paleo-uplift in the late Early Paleozoic were analyzed. In the late Early Paleozoic, there existed a unified regional paleo-uplift widely distributed in southwestern Tarim Basin, which is called the southwestern Tarim plaeo-uplift. The “Tarim SW paleo-uplift” and “Hetian paleo-uplift” proposed in previous literatures are not independent, but the result of the spatio-temporal migration and evolution of the southwestern Tarim paleo-uplift identified in this paper. The southwestern Tarim paleo-uplift emerged at the end of Middle Ordovician, and took its initial shape with increased amplitude in the Late Ordovician. During the Silurian, the southwestern Tarim paleo-uplift rose steadily and expanded rapidly to the east, incorporating Pishan—Hetian and other areas, with the structural high locating in the Pishan—Hetian area. During the Devonian, the southwestern Tarim paleo-uplift began to shrink gradually, to a limited range in the Pishan—Hetian area in the southern part of the basin. During the Carboniferous, the southwestern Tarim paleo-uplift became an underwater uplift, that is, the paleo-uplift gradually died out. The southwestern Tarim paleo-uplift belongs to the forebulge of the southwestern Tarim foreland basin in the late Early Paleozoic, and its formation and evolution are related to the early Paleozoic orogeny of the West Kunlun orogenic belt in the south of the Tarim Basin. The migration of the southwestern Tarim paleo-uplift from the northwestern part of the southwestern Tarim Basin to the Pishan—Hetian area indicates the early Paleozoic orogenic process of the West Kunlun orogenic belt, which started in the western section at the end of Middle Ordovician and extended from west to east in a “scissor” style. The migration and evolution of the southwestern Tarim paleo-uplift controlled the development of unconformities at the end of Middle Ordovician, the end of Late Ordovician, and the end of Middle Devonian, and the spatial distribution of dissolved fracture-cave reservoirs in weathered crust below the unconformities in the southwest of Tarim Basin. The migration of the structural high of the southwestern Tarim paleo-uplift also played an important role in controlling the development of dissolved fracture-cave reservoirs in weathered crust.

Interconnected multi-layer aquifer with evaporitic fossil waters in Chott-El-Gharbi endorheic basin (Western high plateaus, Algeria): Hydrochemistry, environmental and strontium isotopes

Wetland are of paramount importance for the entire province of Nâama as well as for the entire Algerian Western High Plateaus. One of them, Chott El Gharbi, is a major but poorly-known hydrogeological unit shared between Algeria (80%) and Morocco (20%). Chott El-Gharbi is a multi-layered aquifer system whose most productive layers are the Middle Jurassic (Bajocian-Bathonian dolomite and limestone) and the Cenozoic continental sediments (lacustrine intercalation of clays and limestones, marls and sands). Groundwaters exhibit depleted δ18O (−8.98 to −5.53‰) and δ2H (−69.4 to −48.0‰) isotope ratios linked to an evaporative process. Groundwaters are old, with 14C activities from 2 pmC to 58 pmC and residence times of 23 ky to 9 ky. These old recharges occurred during a more humid and colder epoch. 87Sr/86Sr ratios, coupled with major element abundances, determine that Gharbi waters resulted from the mixing of waters having been in contact with the Triassic evaporite-rich sequence (0.70810, rich in sulfates) and waters in contact with Lago Mare (late Miocene) sequence (0.70875, less rich in sulfates). Water with Triassic signature is present in the Jurassic aquifer (Bathonian-Bajocian) while water with Lago Mare or mixed signature is present in the Miocene, locally along faults in the Jurassic aquifer. 14C ages indicate that the mixing process occurred at c. 10,000 years along faults probably during an active tectonic period. Waters with modern signature are only known outside the endorheic Gharbi basin in shallow Jurassic aquifers. The Gharbi aquifer system is separated from the large Chergui aquifer system to the east.

Spatial and temporal evolution of paleo-highs/lows of the permian strata and its petroleum significance in the Sichuan basin, SW China

ABSTRACT The Sichuan basin has a great potential for hydrocarbon exploration. In recent years, Permian strata have been identified as one of the main target strata for natural gas exploration in the Sichuan basin. Not only the source rocks are abundant, but also there are good reservoirs due to the wide and complex continental oceanic interaction. However, there are still many shortcomings, especially the relationship between the tectonic evolution and the hydrocarbon accumulation model. Therefore, this research aims at understanding the characteristics of paleo-highs/lows of the Permian strata and their controlling effects on hydrocarbon migration/accumulation in the Sichuan basin. Firstly, the distribution patterns of paleo-highs/lows in the Permian strata at each critical tectonic period were constructed by integrating spatial erosion calculation. Then the temporal evolution of the Permian strata at each critical tectonic period will be revealed by restoring the upper strata sequentially. The results of this research improve the precision of paleo-highs/lows reconstruction and provide useful hints for the hydrocarbon exploration of the Sichuan basin: (i) Paleo-highs/lows directly determine the development location of the Permian key reservoirs; (ii) Tectonic evolution controls many factors and the process of hydrocarbon accumulation.

Strain Field Features and Three-Dimensional Crustal Deformations Constrained by Dense GRACE and GPS Measurements in NE Tibet

The continuing impact between the Eurasia Plate and India results in the thickening and shortening of the N-S Tibetan Plateau. There has been strong tectonic movement along the boundary of the zones of deformation of the NE corner of the Tibetan plateau (NET) since the new tectonic period, with its dynamic mechanisms remaining controversial. Here, we use observations of 39 Continuous Global Positioning System (CGPS) gauges and 451 Crustal Movement Observation Network of China (CMONOC) campaign-mode stations to detect the three-dimensional deformation of the crust in the NET. Improved processing procedures are implemented to strengthen the patterns of strain throughout the NET. The principal component analysis (PCA) technique is introduced to decompose the time series into spatial eigenvectors and principal components (PCs), and the first three PCs are used to estimate and rectify common mode errors (CMEs). In addition, GRACE observations are used to detect deformation changes that account for non-tidal oceanic mass loading, hydrological loading, and surface pressure. The rectified deformation of the crust indicates the anisotropic nature of both the subsidence and uplift, and that the highest uplift rate of the Longmen Shan fault uplift reaches 7.13 ± 0.53 mm/yr. Finally, the horizontal velocity is further used to enumerate the strain rates throughout the NET. The results show that the shear band retained property in line with the strike-slip fault along the Altyn Tagh fault, the Qilian Shan faults, the Haiyuan fault, the West Qinling fault, the East Kunlun fault, and the Longmen Shan fault. In addition, the results further indicate that the whole NET shows a strong relationship with the mean principal rates of horizontal shortening strain. Extension and compression of the crust reasonably describe its sinking and uplifting.

Key structural periods and Permian tight gas accumulation response in Hangjinqi area, Ordos Basin

Critical tectonic period is a critical time point that characterizes the genetic connection and causal relation (accumulation response) between tectonic effect and oil/gas accumulation in large superposition basins, it is also an important window for analyzing accumulation evolution and accumulation regularity of oil/gas. Based on the systematic review of the tectonic evolution history of the Ordos Basin, the tectonic evolution and deformation sequence since the Mesozoic in Hangjinqi area in the northern part of Ordos Basin were studied through the constraints of apatite fission tracks and other thermal chronological indicators. Two key structural periods including J3-K1 and K2-E1 were proposed. It was believed by combining with the understanding of the Carboniferous-Permian hydrocarbon-generating and reservoir-forming histories that the J3-K1 was the period of design finalization in the Hangjinqi area which formed a currant NE-trending structural framework high in the northeast and low in the southwest during which, lithologic gas reservoirs including the Carboniferous Taiyuan and the Permian Shanxi formations, first member of the Permian Shihezi Formation to the north of the Sanyanjing-Wulan Jilin Temple-Bo'erjiang Haizi fault were formed and corresponding to the period of great amount of hydrocarbon generated and expulsed from source rocks as well as the densification of sandstones. On the other hand, the K2-E1 period was the major stage for fault reactivation and the formation of NE-trending tectonic trap and NW-trending fault that led to the adjustment of early lithologic gas reser-voirs in the south and the formation of structural or structural-lithologic gas reservoirs in the north. The difference of accumulation on each sides of the main fault is obvious. The southern part was formed earlier than the northern part, and was dominated by lithologic gas reservoirs yet the northern part was dominated by structural-lithological gas reservoirs. The southern gas-bearing strata are dominated by deep strata, while those in the north are both shallow and deep ones. In Paleogene, lifting in the north caused loss of gas. The key accumulation and transformation periods transformed with space and time, which transformed accumulation factors differentially and finally constrained the enrichment and depletion of natural gas.

Source rocks in foreland basins: A preferential context for the development of natural hydraulic fractures

Bedding-parallel veins of fibrous calcite (also called “beef”) occur in many sedimentary basins, especially those containing low-permeability strata with organic source material for petroleum. The formation of such veins is commonly linked with fluid overpressure in these source rocks. In this review, we demonstrate that beef veins are most commonly present in foreland basins worldwide or in basins that recorded a compressive tectonic period. The formation of beef veins is related to two main phases: (1) the initiation of bedding-parallel fracture and (2) the infilling of the fracture. Previous structural studies have shown that formation of beef veins occurred during a period of compressive stress activity. This is especially the case for the Wessex Basin (United Kingdom) and the Neuquen Basin (Argentina). In this paper, we provide more observations for other basins: the Cordillera Oriental (Colombia), the Paris Basin (France), the northern Pyrenees (France), the Uinta Basin (United States), the Tian Shan Mountains (central Asia), and the Appalachian Mountains (United States). In the Paris Basin, beef vein formation is dated at 155 Ma (U/Pb calcite method) and is coeval with the compressional deformation in the eastern part of the basin. Because of the timing of generation for such veins and even if the theory and the experiments of fracturing demonstrate that bedding-parallel fractures can be generated only with a distributed fluid overpressure, the formation of beef veins seems to be a consequence of both fluid overpressures and a compressional tectonic stress.

Quaternary Tectonics as the Neotectonics in Turkey

The base of the Quaternary was formally redefined in 2009 by the International Commision on Stratigraphy at 2.6 Ma by shifting a 800 ka, formerly part of the Pliocene epoch, into the Pleistocene. It has made tremendous impression in literature. The Quaternary period is unique because of its climatic characteristics; however, it is a geochronological unit, and a redefinition of its position would affect many areas, such as the definition of neotectonic period in Turkey. The major tectonic elements of the Eastern Mediterranean region are located in there. Recent studies related to neotectonics in Turkey present a consensus on the driving mechanism and age of this tectonic period: the late Pliocene (before the formal adding of Gelasian stage into the Quaternary period) westward escape of the Anatolian plate. Following the collision of the Arabian and Eurasian plates at ~11 Ma BP, the North Anatolian Fault Zone created in the east and the escape tectonics could not occur until the propagation of the North Anatolian Fault Zone to the Aegean Arc at ~2.5 Ma BP and gaining a transform fault character with a similar slip rate as in today. The spatial distribution of active faults in Turkey also supports such an approach. The great majority of them have no relations with the pre-Quaternary formations, on the other hand are bounding the Quaternary basins. In this study, the neotectonics of Turkey discussed within the view of the redefined Quaternary. The comparison suggests the using of the term Quaternary tectonics substantially coincides with neotectonics in literature for Turkey, because the base level of this period in this region overlaps with the redefined base level of the Quaternary period.

Sediment supply on the West Greenland passive margin: redirection of a large pre-glacial drainage system

The Mesozoic–Cenozoic separation of Greenland and North America produced the small oceanic basins of the Labrador Sea and Baffin Bay, connected via a complex transform system through the Davis Strait. During rifting and partial breakup sedimentary basins formed that record the changing regional sediment supply. The onshore and offshore stratigraphy of Central West Greenland outlines the presence of a major fluvial system that existed during the Cretaceous and was later redirected in the Early Cenozoic by the formation of the West Greenland Igneous Province. Hydrological analysis of Greenland's isostatically balanced basement topography outlines two major drainage systems that likely flowed across Greenland prior to the onset of glaciation and emptied into the Sisimiut Basin within the Davis Strait, offshore West Greenland. The course of the northern drainage system suggests that it initially flowed NW into the Cretaceous/Palaeocene Nuussuaq Basin, before being redirected SW around the West Greenland Igneous Province in the Mid-Palaeocene. Moreover, characteristics of these two drainage systems suggest they acted as a single larger fluvial system, prior to the onset of glaciation, that was likely the primary source of sediment across Central West Greenland throughout the Cretaceous and Palaeogene. This scenario provides a greater understanding of the West Greenland margin's late Cenozoic evolution, which differs from previous interpretations that hypothesize a period of considerable post-rift tectonism and uplift. This work highlights the importance of large pre-glacial drainage systems across North Atlantic passive margins and their relevance when studying post-rift stratigraphy in rifted margin settings. Supplementary material: Isostatic modelling, hydrological analysis and chi mapping is available at: https://doi.org/10.6084/m9.figshare.c.5050146

Ganymede, Then and Now: How Past Eccentricity May Have Altered Tidally Driven Coulomb Failure

AbstractLaplace‐like resonances among Ganymede, Europa, and Io may have once led Ganymede to have an eccentricity (presently e = 0.0013) as high as ~0.07 (Showman &amp; Malhotra, 1997, https://doi.org/10.1006/icar.1996.5669). While diurnal stresses at Ganymede today are small (less than 10 kPa), a previous period of higher eccentricity may have allowed for an order of magnitude increase in the diurnal tidal stresses that could drive fault initiation and result in a past period of active tectonism. To investigate the role of tidal stresses on faulting, we use the numerical model SatStress (Wahr et al., 2009, https://doi.org/10.1016/j.icarus.2008.11.002) to calculate diurnal tidal stresses on Ganymede's surface assuming e = 0.05, representative of a more eccentric orbit in Ganymede's past. We resolve normal and shear stresses onto discrete mapped fault segments and assess Coulomb failure criteria along three inferred shear zones on Ganymede's surface: Dardanus Sulcus, Tiamat Sulcus, and Nun Sulci. While Coulomb failure is not expected from diurnal stressing at any of the three shear zones for a diurnal model of present‐day (low) eccentricity, we do predict Coulomb failure for a past, high eccentricity case, in isolated diurnal slip windows and limited to very shallow depths (~100 to 250 m). In these cases, this model may provide an alternative to invoking additional stresses such as nonsynchronous rotation or true polar wander, as in previous studies (Cameron et al., 2019, https://doi.org/10.1016/j.icarus.2018.09.002). Additionally, this model is in general agreement with the sense of inferred shear from imagery and structural mapping.

Paleomagnetic Constraint on the Carboniferous Paleoposition of Indochina and Its Implications for the Evolution of Eastern Paleo‐Tethys Ocean

AbstractTo better understand the rifting history and mechanism of the Indochina Block from eastern Gondwana, we report a paleomagnetic study on middle Carboniferous (~315 Ma) limestones from the southernmost Yunnan of China. Characteristic remanent magnetizations (ChRMs) were isolated from samples in 14 sites after secondary magnetizations were erased at relatively low temperature ranges, of which magnetites were interpreted to be the magnetic carriers for the former while goethites and pyrrhotites for the latter. The rock‐magnetically interpreted minerals are confirmed with a scanning electron microscope (SEM) on samples with a high concentration of magnetic minerals, of which the calcium carbonates were dissolved with a mixed solution of CH3COOH and CH3COONa. The ChRMs passed reversal and fold tests and are very likely a primary magnetization, which correspond a paleomagnetic pole of 76.1°S/181.3°E (A95 = 4.0°, N = 14) and locate the Indochina Block at 18.4 ± 4.0°S. A relatively quiet tectonic period was suggested for the North Qiangtang, Indochina, and South China blocks during the middle Carboniferous‐early Permian, and a sequential start of rapid drifting of these blocks was recognized, which is coeval with the rifting of Cimmerian terranes from Gondwana.

Oligocene molasse sedimentation in Central Taurides: Records of the onset of extensional tectonic regime

The lacustrine units have been deposited in the Ermenek, Bucakkisla, Korucuk and Camliyayla basins in the Central Taurides since early Oligocene. In this study, structural features and stratigraphical properties of Oligocene basins have been described in detail and the lacustrine units have been dated. Regional geological interpretations have been made by using tectono-stratigraphic and age data of the basins. Lacustrine Oligocene sedimentation in these basins, which were opened on the tectonic units of Taurides due to normal faults, constitute the first records of the sedimentation occurring under the extensional tectonic regime after the orogeny of the Central Taurides. All the pre-Oligocene units that had been emplaced in the region due to a north-south compressional movement in the Central Taurides, continued to be compressed until the end of Eocene, and completed their orogenic development as napped structures. The bedrock units, reaching the maximum elevation, have been subjected to “orogenic collapse” due to the strain or gravity forces as a result of an interruption or termination of the compression which lasted until the end of Eocene. Thus, “orogenic collapse” basins have started to be formed in the Central Taurides since Oligocene. The orogeny and the compressional tectonic regime forming the Taurides lasted until late Miocene in the Eastern Taurides. On the other hand, extensional basin formation since early Oligocene on the Taurus units, which had been emplaced in the region due to nappe tectonics, indicates that a new tectonic period has started in the Central Taurides.

Soft-sediment deformation structures as indicators of tectono-volcanic activity during evolution of a lacustrine basin: A case study from the Upper Triassic Ordos Basin, China

Abstract The Ordos Basin, central China, is a large-scale Late Triassic intracratonic lacustrine basin, and in which the Upper Triassic Yanchang Formation comprises interbedded sandstones, mudstones and tuffs, representing deposits of a complete lacustrine life cycle. Based on observation of extensive well cores, various types of soft-sediment deformation structures have been identified in the lacustrine deposits of the southwest basin, including deformed lamination/bedding, load casts and flame structures, pseudonodules, microfaults, autoclastic brecciation, sand dykes, dyke-sill complexes, and sandy and muddy slumps. Liquefaction and fluidization of unconsolidated permeable sediments, together with brittle fracturing of semi-consolidated sediments, are the major deformation mechanisms that generated these structures. Both the scale and prevalence of the deformed sediments, as well as the geological setting of the lacustrine basin, suggest that seismic events (synsedimentary tectonic activity) were the main trigger to initiate deformation processes, although sedimentary overloading could also have been a minor localized trigger. Many volcanic tuffs preserved in both the primary undisturbed deposits and deformed sediments serve as support for synsedimentary tectono-volcanic activities that induced deformation in these lacustrine sediments. The deformation structures usually form vertical clusters of several different types, representing active syndepositional tectonic periods. Stratigraphic correlation shows three such deformation clusters to be widespread in the area detailed herein, and likely recorded three major tectonic periods during deposition. A thick extensive oil shale, with various deformation structures, also reveals another active tectonic period. These four intervals coincide well with the transition of the sedimentary architecture in the area, indicating four phases of most intense tectonic periods and their influence on lacustrine evolution.

Geochronology of Two Mafic Intrusions in Funing, Yunnan: Implications for Its Relationships with Carlin-like Gold Mineralization

The relationship between mafic intrusion and gold deposits has always received substantial attention. Based on the petrography, mineralogy, and chronology of two mafic intrusion suites exposed in the Funing area, we found that the mafic intrusion suites have closely relationship with gold mineralization. The 255±4.9-Ma U-Pb age of the early mafic intrusion suite is much older than the 218.9±6.4-Ma (MSWD=1.2) U-Pb age of the late mafic intrusion suite. Based on previous studies on former gold mines in the area, it is believed that early mafic intrusion suite appears to have contributed to the early hydrothermal enrichment of gold deposits through the action of water-rock, together with atmospheric precipitation and stratigraphic water. Due to favorable structural conditions during the Indosinian tectonic period, the late basic rock not only extracted metallogenic materials from the stratum but also acted as an important heat source for the metallogenic fluid circulation and mineralization, together with superposed mineralization of frequent tectonic activities on early deposits, finally forming into ore deposits in favorable structures. Therefore, we believe that the mafic intrusion suite in the late period of the region are closely related to the gold mineralization in the region, and it can be used as a prospecting mark for gold deposits in the area.

Finite Element Simulation of Multi-Scale Bedding Fractures in Tight Sandstone Oil Reservoir

Multi-scale bedding fractures, i.e., km-scale regional bedding fractures and cm-scale lamina-induced fractures, have been the focus of unconventional oil and gas exploration and play an important role in resource exploration and drilling practice for tight oil and gas. It is challenging to conduct numerical simulations of bedding fractures due to the strong heterogeneity without a proper mechanical criterion to predict failure behaviors. This research modified the Tien–Kuo (T–K) criterion by using four critical parameters (i.e., the maximum principal stress (σ1), minimum principal stress (σ3), lamina angle (θ), and lamina friction coefficient (μlamina)). The modified criterion was compared to other bedding failure criteria to make a rational finite element simulation constrained by the four variables. This work conducted triaxial compression tests of 18 column samples with different lamina angles to verify the modified rock failure criterion, which contributes to the simulation work on the multi-scale bedding fractures in the statics module of the ANSYS workbench. The cm-scale laminated rock samples and the km-scale Yanchang Formation in the Ordos Basin were included in the multi-scale geo-models. The simulated results indicate that stress is prone to concentrate on lamina when the lamina angle is in an effective range. The low-angle lamina always induces fractures in an open state with bigger failure apertures, while the medium-angle lamina tends to induce fractures in a shear sliding trend. In addition, the regional bedding fractures of the Yanchang Formation in the Himalayan tectonic period tend to propagate under the conditions of lower maximum principal stress, higher minimum principal stress, and larger stratigraphic dip.

Geology of the Eastern Part of the Volcanic-Kendeng Zone of East Java: Stratigraphy, Structures and Sedimentation Review from Besuki and Situbondo Areas

The Tertiary stratigraphy of Situbondo was constructed by a series of volcanoclastic-carbonate turbidite facies of Menuran Formation with Pacalan limestone Member, and Leprak Formation. These formations formed a regional east-west trending circular anticlinorium. The Tertiary formations were covered by Quaternary volcano-clastic Ringgit Formation and subsequent younger Bagor volcanic products. The oldest Tertiary rock units are the Late Miocene-Pliocene Menuran Formation, with Pacalan Limestone Member. Formation is mainly composed of foram-rich marls and calcareous, sometimes tuffaceous sandstones, with conglomerate intercalations. Sedimentation of this formation is interpreted as to be a mixing, from proximal to distal turbidite, involving volcaniclastic and carbonate sources, in a bathyal open marine environment. The Early Pliocene Leprak Formation overlies conformably the Menuran Formation, which consists of alternating calcareous sandstones and tuff sandstones deposited in a bathyal open marine environment with proximal turbidite mechanism suggesting that basin depocenter was located to the east. Up to Late Pliocene, the region was dominated by developments proximal turbidite volcanoclastic sedimentation of The Leprak Formation, contemporaneous with increasing volcanic activity in the south. Deformation of Plio-Pleistocene in Java is believed to be the last major tectonic period, which forms the west-east trending structures. In Situbondo area, folding structures in this direction involves the Neogene Menuran Formation, Pacalan Member and Leprak Formation. Volcanic activity persists, and increases, with the activity of Ringgit-Beser volcano in Pleistocene. These late events of magmatism, volcanism and uplift were contributed to the last structural configuration of the area.Keyword : Situbondo, structural geology, volcanic-kendeng zone, stratigraphy

Geology of the Eastern Part of the Volcanic-Kendeng Zone of East Java: Stratigraphy, Structures and Sedimentation Review from Besuki and Situbondo Areas

The Tertiary stratigraphy of Situbondo was constructed by a series of volcanoclastic-carbonate turbidite facies of Menuran Formation with Pacalan limestone Member, and Leprak Formation. These formations formed a regional east-west trending circular anticlinorium. The Tertiary formations were covered by Quaternary volcano-clastic Ringgit Formation and subsequent younger Bagor volcanic products. The oldest Tertiary rock units are the Late Miocene-Pliocene Menuran Formation, with Pacalan Limestone Member. Formation is mainly composed of foram-rich marls and calcareous, sometimes tuffaceous sandstones, with conglomerate intercalations. Sedimentation of this formation is interpreted as to be a mixing, from proximal to distal turbidite, involving volcaniclastic and carbonate sources, in a bathyal open marine environment. The Early Pliocene Leprak Formation overlies conformably the Menuran Formation, which consists of alternating calcareous sandstones and tuff sandstones deposited in a bathyal open marine environment with proximal turbidite mechanism suggesting that basin depocenter was located to the east. Up to Late Pliocene, the region was dominated by developments proximal turbidite volcanoclastic sedimentation of The Leprak Formation, contemporaneous with increasing volcanic activity in the south. Deformation of Plio-Pleistocene in Java is believed to be the last major tectonic period, which forms the west-east trending structures. In Situbondo area, folding structures in this direction involves the Neogene Menuran Formation, Pacalan Member and Leprak Formation. Volcanic activity persists, and increases, with the activity of Ringgit-Beser volcano in Pleistocene. These late events of magmatism, volcanism and uplift were contributed to the last structural configuration of the area.Keyword : Situbondo, structural geology, volcanic-kendeng zone, stratigraphy

Prediction of geomechanical sweet spots in a tight gas sandstone reservoir: A case study of lower Permian strata in the southern Qinshui Basin, China

Using the tight sandstone of the lower Permian Shanxi Formation in the southern Qinshui Basin as an example, we have developed the concept of “geomechanical sweet spots” in a strongly deformed strike-slip fault zone. The evaluation elements for geomechanical sweet spots include current tectonics, paleotectonics, fractures, and rock brittleness. The high-angle basement-surface strike-slip faults that developed in the study area control the formation and development of the descending plane (upper plate) positive traction folds and the ascending plane (lower plate) isopach folds. The current tectonics and the paleotectonics in the mid-Himalayan tectonic period have a certain adjustment effect on the distribution of natural gas. We predicted the fracture development area of the target layer in the main episode of the Himalayan period based on the 3D finite-element method. The regions with a higher fracture development degree are mainly distributed in the Sitou-Houchengyao strike-slip fault zone and the northwestern and eastern parts of the study area. We also divide the brittleness index of sand group II into four grades. Based on a single-factor evaluation of the current tectonics, paleotectonics, fractures, and rock brittleness, we established a standard parameter system for evaluating the geomechanical sweet spots of tight gas sandstone reservoirs.