NE Trending Research Articles

Structural controls of fluid flow and gold mineralization in the easternmost parts of the Karagwe-Ankole Belt of north-western Tanzania

Gold mineralization in the Biharamulo region of western Tanzania is confined to the sheared, low-angle basement-cover contact between Archaean basement gneisses of the Tanzania Craton and the structurally overlying, low-grade metamorphic metasediments of the Mesoproterozoic Karagwe-Ankole Belt. Regional-scale fluid flow along this detachment is indicated by the pervasive silicification and retrogression of wall rocks to pervasively foliated phyllonites and pyritization of particularly metasediments, commonly graphite-rich, in the hanging wall of the shear zone. Gold mining centres on specific structural sites along the detachment, but also in stratigraphically higher sections in the structurally overlying metasediments. Zones of gold mineralization along the detachment correlate with NE trending ramp structures (dip angles 20°–35°) that are most ideally orientated for slip and reactivation within the low-angle phyllonitic detachment. Repeatedly overprinted auriferous quartz-vein stockworks in quartzofeldspathic gneisses immediately below the detachment indicate brittle fracturing of the competent footwall lithotypes during slip along the weaker detachment. In cases of massive silicification, up to 50m thick quartz blows are formed along the contacts between detachment phyllonites and footwall gneisses. The multiple overprinting relationships of successive quartz-vein generations in these zones of massive silicification suggests that the quartz blows acted as competent blocks in the weak detachment, causing the repeated overprint of earlier silicification by later fracturing and quartz-veining events. Gold mineralization above the detachment and in stratigraphically higher metasediments is closely associated with fold structures that form part of the low-grade metamorphic fold-and-thrust belt. Veining is particularly abundant in competent lithotypes, such as quartzite and chemically reactive ferruginous mafic sills. Overprinting relationships between quartz vein sets illustrate fluid flow during fold amplification and, importantly, the final lock-up stage of folds, during which much of the mineralization was introduced. Oxygen isotope values for quartz veins indicate fluids were likely derived from clastic, mainly metapelitic sedimentary sequences of the Karagwe-Ankole Fold Belt. The data also implies that the partially reworked Archaean granitoid-greenstone basement of the Tanzania Craton has not contributed to the fluid evolution and possibly gold mineralization. The extent (>100km) of the basement-cover detachment and associated alteration is indicative for a regional-scale fluid system. Gold mineralization is, however, controlled by local structures and lithological contrasts that require the detailed mapping and sampling of the regional structure.

Fold/cleavage relationships as indicator for late Variscan sinistral transpression at the Rheno-Hercynian–Saxo-Thuringian boundary zone, Central European Variscides

The boundary between the Rheno-Hercynian and the Saxo-Thuringian zones of the European Variscides is characterized by a NE–SW striking late orogenic fold-and-thrust belt affecting the intervening Rheic suture. Classical models used the first-order strike of this zone as an indicator for perpendicular plate convergence, i.e. NW–SE. We present structural data from both sides of the suture, focusing on fold–cleavage relationships. The statistical analysis reveals an orientation maximum of the youngest cleavage that deviates from the strike of the fold-and-thrust belt by c. 22°. The presence of clockwise transection of the folds by the cleavage (up to −16°) indicates pervasive sinistral transpression. Three types of fold–cleavage relationships are observed: NE trending folds (I) with or (II) without a transecting cleavage, and (III) non-transected ENE trending folds. We explain the occurrence of different fold–cleavage types by strain partitioning due to NNW convergence obliquely to pre-existent NE trending mechanical anisotropies. In terms of plate tectonics we propose that the classical boundary of the Rheno-Hercynian and the Saxo-Thuringian Zone represents an initial transform plate boundary that was finally affected by sinistral transpression.

Effect of strain-weakening on Oligocene–Miocene self-organization of the Australian-Pacific plate boundary fault in southern New Zealand: Insights from numerical modelling

Tectonic inheritance acquired from past geological events can control the formation of new plate boundaries. The aim of this paper is to explore the role of inherited NE and NW trending fabrics and their rheological influence on the propagation of Oligocene–Miocene strike-slip faulting that matured to become the Australian-Pacific plate boundary fault in southern New Zealand. Strain weakening plays a significant role in controlling the formation, growth and evolution of strain localization. In this study, three-dimensional thermo-mechanical models have been used to explore the effect of strain weakening on the Oligocene–Miocene self-organization of strain localization. Strain weakening is simulated through decreasing either the coefficient of friction of upper crust, its cohesion, or the rheological viscosity contrast between the inherited structures and their surrounding wall rocks. Viscosity contrast is obtained by varying the viscosity of inherited structures. Softening coefficient (α) is a measure of strain weakening. Our experiments robustly demonstrate that a primary boundary shear zone becomes mature quicker when softening coefficients are increased. Deformation is focused along narrow high-strain shear zones in the centre of the model when the softening coefficients are high, whereas the strain is more diffuse with many shear zones spread over the model and possibly some high-strain shear zones focused near one border at lower softening coefficients. Varying the viscosity contrast has less effect on the distribution of maximum finite strain.Under simple-shear boundary conditions, NW trending inherited structures make a major contribution to forming early zones of highly focused strain, up to a shear strain of about γ=3.7. During this process, most NE-trending structures move and rotate passively, accommodate less strain, or even be abandoned through time.

Lineament Length and Density Analyses Based on the Segment Tracing Algorithm: A Case Study of the Gaosong Field in Gejiu Tin Mine, China

This study used the Segment Tracing Algorithm (STA) to extract lineaments from remotely sensed images. A computer program was then written to calculate the lineament densities and lengths. In Gaosong field, 3,233 lineaments were extracted based on a 200 m × 200 m grid size. The results indicate that most lineaments lengths are between 30 m and 50 m, and the number of lineaments within each cell ranges from 1 to 6. Areas with high distributions exist on both sides of the central region. According to the contour map of lineament length, the maximum lineament length is 380 m, and the minimum length is 30 m. The contours mainly extend in two directions, including NE and NW trends. This is consistent with the prominent NE and NW strike faults that prevail in the mining area. The results are similar to those obtained in the Machishui ore block, which has become a mine production area. High values of lineament length and density in the contour map of Gaosong field may be associated with hydrothermal tin mineralization in the study area. The results of this study potentially provide a new approach to mineral exploration in the early stage of geological prospecting.

Aligned glacitectonic rafts on the floor of the central Barents Sea

Numerous mounds are found aligned in a SW-trending chain on the floor of the central Barents Sea and are interpreted as glacial rafts. Glacitectonic megablocks and rafts occur widely in glaciated lowlands, as shown by many terrestrial examples of flat-topped allogenic pieces of sedimentary rock or till (Ringberg et al. 1984; Ruszczynska-Szenajch 1987; Aber 1988). In the Barents Sea, Cenozoic–Mesozoic subcropping sedimentary bedrock appears particularly susceptible to glacitectonic deformation and erosion, as shown by the occurrence of megablocks and rafts within Pleistocene glacial sediments (Saettem 1994; Andreassen et al. 2004). Terrestrial and marine examples both show that megablocks and rafts tend to be found within till rather than at its surface and such features are, therefore, rare as seafloor landforms. About 1300 flat-topped mounds are present on the seafloor in the Bear Island Trough in the central Barents Sea, at water depths of c. 350 m (Fig. 1). The most pronounced mounds, with heights up to 28 m above the surrounding seafloor, occur aligned in a chain extending for at least 50 km. Mounds tend to be elongate with their preferred orientation varying across the area. Overall, the chain of mounds trends NE–SW, roughly parallel to …

Fault linkage and reactivation on the northern margin of the Dampier sub-basin

SUMMARY The north-west margin of the Dampier sub-basin is characterised by a strongly segmented fault pattern. NE trending faults define the edge of the Rankin Platform, and separate it from the Kendrew Trough. However a secondary set of NNE trending faults define smaller scale graben on the edge of the Rankin Platform that preserve Lower and Middle Jurassic sediments. This strongly suggests oblique reactivation of an inherited NE trending basement fabric under WNW oriented extension during Middle Jurassic extension.

Neogene oblique extensional system in the north-western Bonaparte Basin, Australia

The North-western Bonaparte Basin offers a very good opportunity to understand the nature of oblique extension system, where Neogene flexure-induced extension was superimposed obliquely to the Mesozoic rift-related structures. The Mesozoic trends strongly control the distribution and style of the younger Neogene structures, both at regional and local scale. The younger Neogene activity produced a new set of NE trending, right-stepping en echelon faults and reactivated the older faults. In addition, episodes of stratigraphic growth provide critical evidence regarding the timing of fault activity. Results demonstrate that, in the study area, main fault activity occurred in several pulses during the latest Miocene to Late Pleistocene. These episodes of fault activity correspond to recently constrained regional tectonic events i.e., the initial collision of the Australian Plate with the Banda Arc, the episodes of uplift of the Timor Island and the timing of lithospheric flexure.

The Pampe gold deposit (Ghana): Constraints on sulfide evolution during gold mineralization

The Ashanti belt, a world-renowned gold producing region in southwest Ghana, has received renewed attention in recent years. Most studies, however, focused on the major deposits situated along the Ashanti shear zone and in the adjacent Tarkwa Basin to the east, neglecting smaller yet important occurrences, such as the Pampe deposit which occurs few kilometers to the west of this shear zone, on the Akropong Trend. Nevertheless, investigating such simpler smaller-scale mineralizing systems is attractive, in that this can help shedding light on the processes that control gold deposition at the regional scale.At Pampe, gold endowment has been estimated at approximately half a million ounces with an average gold grade of 2.8g/t. The mineralization is related to two sets of quartz veins; a first set (V1), which has a NE trend and is sub-parallel to the main foliation (S1), and a second set (V2), which crosscuts this foliation. The V2 veins have a NNW–SSE trend with local conjugate geometries indicating that they formed during NNW–SSE shortening, which, regionally, is linked to major orogenic gold deposition. Gold mineralization is systematically associated with sulfides, which occur disseminated in the vein walls and in the surrounding host rocks. The ore sulfide paragenesis consists of 1) a first generation of pyrite, which is associated with V1 veins; 2) a second generation of sulfides, consisting of an intergrowth of arsenopyrite and pyrite that crystallized contemporaneously with the formation of the second vein set; 3) a late phase of pyrite growth which occurs as overgrowths on phase-2 sulfides and formed during the waning stages of V2 emplacement. Invisible (sub-microscopic) gold was detected in all sulfide generations by LA-ICP-MS. The analytical profiles for the Au signal are mimicked by those for Pb, Cu, As, Ag, Te, and Bi. Invisible gold is thus interpreted to have precipitated within sulfides in the form of nanoparticles (colloidal gold alloys). Conversely, visible native gold grains were recognized exclusively in association with arsenopyrite from late V2 veins, either as inclusions or, more commonly, at the boundary with other sulfides, as well as in micro-fractures that crosscut the sulfides. Gold precipitation was likely induced by sulfidation of the wall rock during fluid–rock interaction. The Pampe deposit exemplifies the mineralization processes that took place at larger scale in neighboring world-class deposits such as Obuasi, Bogoso and Prestea.

Distinguishing between local versus regional extension as a control on orogenic gold mineralisation: The new 2.4 Moz Castle Hill Camp, WA

The significance of regional extension for orogenic gold mineralisation in the Archaean Eastern Goldfields of Western Australia is not well established and there is a lack of consensus on both the timing and nature of extensional deformation. The 2.4Moz Castle Hill gold camp, located in the Coolgardie Domain, is spatially associated with a NW-SE trending granitic pluton (the Kintore Tonalite). This pluton intruded prior to the gold mineralisation and is coeval with emplacement of the lower succession of the Kalgoorlie greenstone sequence. The Kintore Tonalite is located at a major change in the geometry of the Kunanalling Shear Zone, where the strike of the shear zone changes from the regional NW-SE trend, to ∼N-S. This change in the geometry of the Kunanalling Shear Zone is interpreted to highlight the presence of an early transfer fault intersecting the Kunanalling Shear Zone, or step-over zone on the Kunanalling Shear Zone, that was active at the time of greenstone emplacement. This early architecture created a crustal-scale pathway that facilitated the sourcing and emplacement of the tonalitic magma and also later gold-bearing fluids from lower crustal levels. Three main deformation events that reactivated this early architecture have been delineated, D1–D3. D1 is associated with NE-SW directed compression reactivating NW trending shear zones and lithological boundaries, causing reverse shearing, folding and development of a pervasive foliation. D2 is synchronous with the main gold event which occurred during NW-SE directed far-field compression, causing sinistral shearing on NW trending structures, the development of N-S striking faults, the formation of mineralised quartz veins associated and vertical axis rotation of the Kintore Tonalite. During the D2 stage gold was deposited with tellurobismuthite assemblages over the Castle Hill region, associated with synchronous extensional and contractional features at the southern and northern ends of the Kintore Tonalite, respectively. D2 extensional structures, related to mineralisation, are local features linked to development of sinistral movement on the N-S striking faults. D3 is associated with NE-SW directed compression and the development of N to NE trending dextral faults, dominated by brittle deformation. The primary control on the regional location of the gold system is interpreted to be early pre-existing crustal architecture, marked by the major strike change that hosts the tonalite. The geometry of structures developed within the step-over zone exerts a second order control on the location of deposits within the Castle Hill camp and fluids were focussed into areas of enhanced permeability during deformation. Individual deposits in the camp are hosted in mineralised structures that locally exhibit extensional kinematics but developed in an overall compressional regime.

Mapping the 3D extent of the Northern Lobe of the Bushveld layered mafic intrusion from geophysical data

Geophysical models image the 3D geometry of the mafic portion of the Bushveld Complex north of the Thabazimbi-Murchison Lineament (TML), critical for understanding the origin of the world's largest layered mafic intrusion and platinum group element deposits. The combination of the gravity and magnetic data with recent seismic, MT, borehole and rock property measurements powerfully constrains the models. The intrusion north of the TML is generally shallowly buried (generally <1500m) with a modeled area of ∼160km×∼125km. The modeled thicknesses are not well constrained but vary from ∼<1000 to >12,000m, averaging ∼4000m. A feeder, suggested by a large modeled thickness (>10,000m) and funnel shape, for Lower Zone magmas could have originated near the intersection of NS and NE trending TML faults under Mokopane. The TML has been thought to be the feeder zone for the entire Bushveld Complex but the identification of local feeders and/or dikes in the TML in the models is complicated by uncertainties on the syn- and post-Bushveld deformation history. However, modeled moderately thick high density material near the intersection of faults within the central and western TML may represent feeders for parts of the Bushveld Complex if deformation was minimal. The correspondence of flat, high resistivity and density regions reflect the sill-like geometry of the Bushveld Complex without evidence for feeders north of Mokopane. Magnetotelluric models indicate that the Transvaal sedimentary basin underlies much of the Bushveld Complex north of the TML, further than previously thought and important because the degree of reaction and assimilation of the Transvaal rocks with the mafic magmas resulted in a variety of mineralization zones.

Influence of the continental margin on the stress field and seismicity in the intraplate Acaraú Seismic Zone, NE Brazil

The Borborema province in NE Brazil is characterized by seismic sequences with small earthquakes that can last 10 yr or more. The seismicity in this region is concentrated in three main seismic zones. In this work, we investigate the stress field in one of these zones, the Acaraú Seismic Zone, which is located in the NW part of the Borborema province. This seismic zone exhibits earthquake sequences that contain repeated earthquakes with similar waveforms and a shallow depth. Using a local network, we investigated a seismic sequence close to the town of Santana do Acaraú from December 2009 to December 2010, and we present detailed results (velocity model, hypocentres and focal mechanism) from this network. In addition, we inverted seven focal mechanisms, including six that were used in previous studies, and determined the directions of the three main axes of the regional stress field. Selecting a very precise set of 12 earthquakes, we found an active seismic zone with a depth between 3.5 and 4.8 km and with a horizontal dimension of approximately 2.5 km in the NW–SE direction (azimuth of 118°) and a strike-slip focal mechanism. The new seismic fault and some of the previous seismic faults determined in previous studies occur near the continental-scale Transbrasiliano lineament, but they exhibit no direct relationship with that ancient structure. The stress field is characterized by NW–SE trending compression and NE–SW trending extension. This result suggests that the rheological contrast between the continental–oceanic crusts created flexural stresses with maximum horizontal compression parallel to the continental margin. This stress pattern occurs along the Potiguar basin and continues west as far as the Amazon fan along the Equatorial margin of Brazil. This stress field and related seismicity may be a characteristic of this type of passive margin that is generated during the transform shearing between the South America and Africa plates and that exhibits an abrupt oceanic–continent transition, steep continental slopes and high bathymetric gradients.

Seismicity of the 24 May 2014 Mw 7.0 Aegean Sea earthquake sequence along the North Aegean Trough

The northern Aegean Sea was hit by a large size (Mw=7.0) earthquake on 2014 May 24. Centroid moment tensor solutions for 40 events with moment magnitudes (Mw) between 3.3 and 7.0 are computed by applying a waveform inversion method on data from the Turkish and Greek broadband seismic networks. The time span of data covers the period between 2014 May 24 and 2014 June 26. The mainshock is a shallow focus strike–slip event at a depth of 15km. Focal depths of aftershocks range from 6 to 30km. The seismic moment (Mo) of the mainshock is estimated as 4.60×1019Nm. The calculated rupture duration of the North Aegean Sea mainshock is 40s. The focal mechanisms of the aftershocks are mainly strike–slip faulting with a minor normal component. The geometry of focal mechanisms reveals a strike–slip faulting regime with NE–SW trending direction of T-axis in the entire activated region. A stress tensor inversion of focal mechanism data is performed to acquire a more accurate picture of the northern Aegean Sea stress field along the North Aegean Trough. The stress tensor inversion results indicate a predominant strike–slip stress regime with a NW–SE oriented maximum principal compressive stress (σ1). In the development of the North Aegean Trough in Aegean Sea is in good agreement with the resolved stress tensors. With respect the newly determined focal mechanisms, the effect of the propagating of the North Anatolian Fault into Aegean Sea is very clearly pronounced. According to high-resolution hypocenter relocation of the North Aegean Sea seismic sequence, three main clusters are revealed. The aftershock activity in the observation period between 2014 May 24 and 2014 July 31 extends from the mainshock cluster from NE to the SW direction. Seismic cross-sections indicate that a complex pattern of the hypocenter distribution with the activation of seventeen segments. The eastern cluster is associated with a fault plane trending mainly ENE–WSW and dipping vertical, while the western is related to a fault plane trending NE–SW and dipping towards NNW. The best constrained focal depths indicate that the aftershock sequence is mainly confined in the crust (depth⩽30km) and is operating in the approximate depth range from 1.5 to 30km. Consequently, Coulomb stress analysis is performed to calculate the stress transfer and correlate it with the activated region. Positive lobes with stress more than 3bars are obtained, indicating that these values are large enough to increase the Coulomb stress failure towards ENE–WSW direction.

Paleostress Reconstruction of Micangshan Anticlinorium on the Southern Margin of the Qinling Orogenic Belts, China

AbstractBrittle structures in rock of different ages can be used to establish the tectonic evolution of an orogenic belt through paleostress calculations. Micangshan is located at the southern margin of the Qinling orogenic belt, between the SE‐trending Longmenshan fold‐and‐thrust belt and the arcuate Dabashan thrust‐and‐fold belt. Structural observations revealed that the dominant structures are reverse and strike‐slip faults and folds with E–W and NE–SE trends. To increase knowledge of the tectonic evolution of the Micangshan anticlinorium, faults, joints, veins, and folds were measured at more than eighty sites. On the basis of structural analysis, it emerged that the multiphase paleostress fields became established after the oblique collision between the North and South China plates. The earliest stress field with N–S compression was established during the Micangshan uplift associated with the E–W trending faults and folds. Subsequently, a N–S extension occurred when the Qinling orogenic belt collapsed. Then NW–SE compression developed, with NE trending faults and folds forming in relation to Longmenshan thrusting toward southwest on the eastern margin of the Tibetan plateau. With the development of the arcuate Dabashan orogenic belt, the compression stress orientation of the Micangshan anticlinorium altered from NE–SW to E–W.

Accumulation conditions and exploration directions of natural gas in deep subsalt Sinian-Cambrian System in the eastern Sichuan Basin, SW China

Abstract Based on comprehensive analysis of outcrops and drilling data, and interpretation of more than 6×10 4 km 2 seismic data, this paper delineates the regional distribution of the gypsum-salt layers in Middle-Lower Cambrian in eastern Sichuan Basin, and discusses accumulation conditions for natural gas in deep subsalt Sinian-Cambrian System in eastern Sichuan Basin, and points out favorable exploration directions. There are two sets of regional gypsum-salt layers in the Middle-Lower Cambrian in eastern Sichuan Basin, which are regional detachment layers, with fault-related folds occurring above and broad anticlines occurring below. There are 6-7 rows of NNE and NE trending large traps developed under the salt. Good space configuration of source rocks, reservoirs and cap rocks provide favorable accumulation conditions for natural gas in the deep subsalt in this region: three sets of hydrocarbon source rocks including Sinian Doushantuo Formation, the third Member of Sinian Dengying Formation, and dominant Cambrian Shuijingtuo Formation; three sets of weathering crust karst reservoirs and grain beach reservoirs including the second and fourth Members of Sinian Dengying Formation, and Cambrian Shilongdong Formation; and the regional cap of gypsum-salt in the Middle-Lower Cambrian and shale in Shuijingtuo Formation. The top target in deep subsalt exploration in eastern Sichuan Basin should be the fourth Member of Dengying Formation, followed by Shilongdong Formation and the second Member of Dengying Formation, and areas on migration direction with appropriate burial depth and being far away from the edge of the basin should be the first choices in exploration. Therefore, the favorable hydrocarbon exploration areas in the near future are the Honghuadian-North Huaying Mountain, Liangshuijing-Pubao Mountain-Leiyinpu, and Datianchi structural belts.

SNeP: a tool to estimate trends in recent effective population size trajectories using genome-wide SNP data.

Effective population size (Ne) is a key population genetic parameter that describes the amount of genetic drift in a population. Estimating Ne has been subject to much research over the last 80 years. Methods to estimate Ne from linkage disequilibrium (LD) were developed ~40 years ago but depend on the availability of large amounts of genetic marker data that only the most recent advances in DNA technology have made available. Here we introduce SNeP, a multithreaded tool to perform the estimate of Ne using LD using the standard PLINK input file format (.ped and.map files) or by using LD values calculated using other software. Through SNeP the user can apply several corrections to take account of sample size, mutation, phasing, and recombination rate. Each variable involved in the computation such as the binning parameters or the chromosomes to include in the analysis can be modified. When applied to published datasets, SNeP produced results closely comparable with those obtained in the original studies. The use of SNeP to estimate Ne trends can improve understanding of population demography in the recent past, provided a sufficient number of SNPs and their physical position in the genome are available. Binaries for the most common operating systems are available at https://sourceforge.net/projects/snepnetrends/.

Structure of the Yanbu suture zone in Northwest Saudi Arabia inferred from aeromagnetic and seismological data

The present work focuses on a study of the Yanbu suture zone and related ophiolites using aeromagnetic and seismological data. The interpretation of aeromagnetic data indicates that NW–SE- and NE–SW-trending structural faults dominate the study area. These faults coincide with narrow and elongated magnetic highs, possibly caused by intrusions emplaced along the faults. The NE trend is dissected by cross-cutting NW-trending faults in some places. The NE-oriented suture zone is marked by short-wavelength high magnetic anomalies associated with ultramafic rocks (ophiolites) and is divided by the NW-oriented left-lateral Najid faulting (Qazaz shear zone) into two shifted alignments. This implies that deep-seated regional trends govern to a large extent the location and extension of the ophiolite exposures. There is a clear correlation between land earthquakes and the interpreted magnetic structures, where the majority of earthquake epicenters lie on or close to the interpreted major faults. A clear concentration of earthquakes is observed where the NE-trending faults cross the Red Sea NNW-trending faults. Here, two separate aftershock clusters occur to the north of Yanbu City, indicating the reactivation of the pre-existing Precambrian NE-trending transform fault,which crosses two parallel NNW-trending faults that have been injected by Cenozoic volcanics during the Cenozoic rifting of the Red Sea. These results indicate that the seismotectonics of the study area is strongly related to the geodynamic rifting processes acting in the Red Sea, where the relative movement between African and Arabian plates resulted in several series of normal and transform faults that run parallel and cross the Red Sea, respectively.

Paleocurrents and Paleohydraulics Studies of the Proterozoic Kolhan Siliciclastic Unit (India): A Case Study from the Chaibasa-Noamundi Basin, Jharkhand, India

Detailed palaeocurrent and palaeohydraulic analyses of the fluvio-deltaic Proterozoic Kolhan siliciclastic unit has provided new insights into the sedimentological knowledge of the fan delta deposition, allowing for the reconstruction of paleogeographic and paleoclimatic conditions prevalent in the Chaibasa-Noamundi intracratonic basin of India. The azimuth reading of cross-bed stratifications of fluvial deposits and the preferential grain orientation of different lithofacies have indicated at least two major paleo-flow systems, NW and NE trending. The local occurrence of sedimentary structures of high energy depositional environments has indicated the superimposition of retrogradational strata pertaining to an older progradational fan complex. The fluvio-deltaic Proterozoic sedimentation has followed the emplacement of volcanic dykes, basic in composition, indicating a thermal rejuvenation of the continental crust. Finally, the partial development of limestones and dolomites in the Kolhan succession has pointed out the peneplanation of the cratonic areas and the development of carbonate sedimentation on stable carbonate platform.

鄂西地区三叠纪嘉陵江组二段与三段局部平行不整合识别及沉积古地理研究

研究区位于上扬子地块的中北缘，南秦岭造山带的南侧，西侧为米仓山断裂带与大巴山弧形断裂带，是北东向构造区与近东西向构造区的叠合位置，在构造分区上具有重要的意义，构造运动在本区具有敏感的响应。前人在对本区的研究中认为晚二叠世–早三叠世的地层中均为整合接触。但在本次调查过程中，通过沉积地层、构造资料的研究对比分析，识别出了嘉陵江组二段与三段(T1-2j2/T1-2j3)之间的局部平行不整合接触关系，代表区域内又一次差异升降运动。同时将印支运动在本区活动的时限由中晚三叠世向前推至早–中三叠世。 The study area is located on the northern edge of the Yangtze Block and the south side of the Southern Qinling orogenic belt, and on the west side of this area there are Micang mountain fault belt area and Dabashan arc fault belt area. This area is an overlaying zone between NE and EW trending structure, and it shows great influences on the structure partition, with sensitive response to tectonic movement in this area. The early study believes that the formations from the Late Permian to the Early Triassic are conformities. However, this study, from the analysis of sediment formations and structure, identifies the accord unconformity between the middle and the upper Jialingjiang formation (T1-2j2/T1-2j3), which stands for another differential subsidence. The latest effect time of Indosinian movement in this area is early-middle Triassic.

Development characteristics and hydrocarbon accumulation significance of fractures in Lower Silurian Kalpintag Formation, northern slope of central Tarim Basin

Abstract Based on the data of core, microscope thin section, scanning electron microscope, cathode luminescence, and conventional and dipole acoustic logging, the development characteristics and hydrocarbon accumulation significance of fractures in the Lower Silurian Kalpintag Formation on the northern slope of the central Tarim Basin were studied. A large number of mainly high-angle or vertical fractures developed in the study area. They feature low density, long length, medium width and low filling extent. More fractures were found in the relatively weak bands of rock, such as the margins and interlayers of boulder clays. Conventional logging curves showed two peaks and one order of magnitude difference. The anisotropy of fast shear wave in dipole acoustic logging showed that fracture development was influenced by NE trending strike-slip faults and the NE-SW trending maximum horizontal principal stress. The fractures in the Kalpintag Formation played important roles in the formation of tight sand-stone reservoirs. They improved the porosity and permeability of tight sandstones, and the large-scale, high-angle or vertical fractures served as pathways for vertical migration of hydrocarbons. For the small-scale adjustment of hydrocarbon migration, the small-scale, low-angle or horizontal fractures were important.

Active faults paragenesis and the state of crustal stresses in the Late Cenozoic in Central Mongolia

Active faults of the Hangay-Hentiy tectonic saddle region in Central Mongolia are studied by space images interpretation, relief analysis, structural methods and tectonic stress reconstruction. The study results show that faults activation during the Late Cenozoic stage was selective, and a cluster pattern of active faults is typical for the study region. Morphological and genetic types and the kinematics of faults in the Hangay-Hentiy saddle region are related the direction of the ancient inherited structural heterogeneities. Latitudinal and WNW trending faults are left lateral strike-slips with reverse or thrust component (Dzhargalantgol and North Burd faults). NW trending faults are reverse faults or thrusts with left lateral horizontal component. NNW trending faults have right lateral horizontal component. The horizontal component of the displacements, as a rule, exceeds the vertical one. Brittle deformations in fault zones do not conform with the Pliocene and, for the most part, Pleistocene topography. With some caution it may be concluded that the last phase of revitalization of strike slip and reverse movements along the faults commenced in the Late Pleistocene. NE trending disjunctives are normal faults distributed mainly within the Hangay uplift. Their features are more early activation within the Late Cenozoic and the lack of relation to large linear structures of the previous tectonic stages. According to the stress tensor reconstructions of the last phase of deformation in zones of active faults of the Hangay-Hentiy saddle using data on tectonic fractures and fault displacements, it is revealed that conditions of compression and strike-slip with NNE direction of the axis of maximum compression were dominant. Stress tensors of extensional type with NNW direction of minimum compression are reconstructed for the Orkhon graben. It is concluded that the activation of faults in Central Mongolia in the Pleistocene-Holocene, as well as modern seismicity were controlled mainly by additional horizontal compression in the SW direction, which was associated with convergence of the Indian subcontinent and Eurasia. The influence of the asthenosphere flow in the SE direction at the base lithosphere is an additional factor facilitating strike-slip deformation of the crust in the study area and a possible explanation of divergent movements in the Baikal Rift, as well as the SE movement of the Amur plate. The Eastern Hangay crust is deformed under extension associated with a dynamic impact of the local mantle anomaly on the lithosphere. The boundary between the Amur plate and the Mongolian block (according to [ Zonenshain, Savostin, 1979 ]) is fragmentary expressed in the tectonic structure. It represents a rim part of the deformation zone, embracing the Mongolian block and the adjacent uplifts of the Mongolian Altai, Tuva and Eastern Sayan. Along the boundary, compressive and transpressive strain occurred in the Pleistocene-Holocene.