NE Trending Research Articles

Preliminary Analysis of Tectonic Stress Field Characteristics of Granite Rock Mass Based on Surface Joints Distribution

Tectonic stress is an important factor influencing the development of granite joints. The tectonic stress in Jijicao is preliminary analyzed based on joints measurement, structural features tracing and stereographic projection. It is considered that the NW and NE trending tectonic systems in the study area belong to the products of different tectonic processes in different times. After intrusion, Jijicao granite rock mass has been forced in normal fault type stress, and the NW steeply dipping joints under vertical maximum principal stress and horizontal minimum principal stress squeezing on NE generated. Thereafter, the in-plate action is adjusted, and the minimum principal stress is deflected from NE to NW, resulting in a steep inclined joint system with the maximum principal stress nearly perpendicular and the minimum principal stress being subjected to NW horizontal extrusion

A long term hydraulic stimulation study conducted at the Salak geothermal field

Awi 18-1 is an injection well drilled in the Cianten Caldera, near the western margin of the Salak (Awibengkok) reservoir in west Java, Indonesia. As the initial well injectivity was low, a long-term hydraulic stimulation was conducted to improve the permeability and establish a better connection to existing natural fractures. A geologic model of the area was built by integration of surface mapping, log and core data, and well performance information. The well penetrated pre- and post-caldera volcanics, the caldera ring fault intrusion and contact metamorphic zone, and marine sedimentary rocks. Permeability was found primarily near the caldera margin in pre-caldera lavas and ring fault intrusion along steeply dipping N to NW and NE trending fractures that were partially sealed by mineral precipitation. A geomechanical simulation model was developed from the geologic model to understand the injectivity evolution mechanisms and behaviours under different injection conditions, and also predict long term future injectivity performance. The model domain contains the completed interval of a deviated injection well (Awi 18-1) and covers the majority of microseismic events observed during the course of the stimulation. It simulates injectivity evolution using fully coupled processes of heat and mass transfers in poro-elastic media. The model was history-matched against the injection data (i.e. pressure) by calibrating rock mechanical and fracture properties. A few what-if scenarios under different operation conditions were simulated to evaluate the effects of injection pressure and temperature on the injectivity. These what-if scenario simulations indicate that both colder injection temperature and higher wellhead pressure lead to better injection efficiency. Also, the higher pressure had prolonged impacts while the colder temperature impacts are limited at the early time.

Structural features and age of gold mineralization in the Troia Massif, Borborema Province, NE Brazil: A Paleoproterozoic (∼2029 Ma) hypozonal orogenic gold deposit overprinted by the late Neoproterozoic Brasiliano/Pan–African orogeny

Hypozonal orogenic gold mineralization has been recently recognized in the Troia Massif, an Archean/Paleoproterozoic basement inlier within the Neoproterozoic Borborema Province, NE Brazil. The mineralization occurs in association with quartz veins hosted by the amphibolite facies Paleoproterozoic Serra das Pipocas greenstone belt. The main mineralized areas are located near parallel to lithological contacts, siting on shear zones established at the metavolcanic and metasedimentary boundary. Four deformation events are recognized: (1) Dn was responsible for the early Sn foliation, parallel to the bedding (So) of the greenstone units. (2) Dn+1 is characterized by a pervasive, southeasterly–dipping Sn+1 foliation that is axial–planar to several asymmetric, tight to isoclinal and recumbent folds. (3) Dn+2 represents a strike–slip NE–trending shear zones, and (4) the late Dn+3 event is characterized by ductile–brittle deformation. Evolution from Dn+1 to Dn+3 is interpreted as a progressive deformation, with the maximum compressive stress in the WNW–ESE direction. The main stage of gold mineralization is associated with the Dn event, and comprised by deformed quartz veins and associated high–temperature calc–silicate alteration (diopside, hornblende, titanite) and albitization of the host rocks. Lower temperature gold (±Te, Ag) occurs along late stage brittle structures (Dn+3). The U–Pb titanite age of 2029 ± 28 Ma for the calc–silicate alteration of the early gold mineralization is similar to the age of many Transamazonian/Eburnean gold mineralizations elsewhere. However, the strong Pb loss of titanite grains defines a 574 ± 7 Ma lower intercept age, indicating that early gold mineralization was affected (remobilized) by Neoproterozoic deformational events and metamorphism (Brasiliano/Pan–African orogeny). This is in agreement with the U–Pb zircon age of 575 ± 3 Ma obtained for the syn–tectonic Guaribas dikes, which bracketed the age of Dn+3. Therefore, as Dn+1, Dn+2 and Dn+3 evolved by progressive deformation; they are all of Neoproterozoic age. This two–stage gold mineralization is here interpreted as product of crustal exhumation, with (1) early hypozonal orogenic gold mineralization occurring shortly after the high–grade Paleoproterozoic metamorphism and first exhumation processes of the greenstone pile; and (2) late gold mineralization occurring at shallow levels (second exhumation process), associated with the Neoproterozoic Brasiliano/Pan–African orogeny.

Using hydrogeomorphic patterns to predict groundwater discharge in a karst basin: Lower Flint River Basin, southwestern Georgia, USA

Study regionThe Lower Flint River Basin (LFRB): a karst catchment in southwestern Georgia, USA. Study focusUsing the U.S. EPA Reach File 3 data set, we generated stream reach azimuths for all tributaries of the Lower Flint River Basin (LFRB) in southwestern Georgia, USA, then compared these results to regional bedrock jointing orientations and stream chemistry (indicating incoming groundwater discharge) in one tributary of the LFRB, Ichawaynochaway Creek. Our objective was to determine if stream bearing might be a useful predictor of increased groundwater discharge in streams of the LFRB where groundwater development has significantly impacted baseflows. New hydrological insights for the regionWe identified a dominant N-S trend in 44% of reaches in tributaries of the LFRB with lesser E-W, NNW, NW and NE trends. Bedrock joints and stream reaches in Ichawaynochaway Creek (a tributary of the Flint River) shared similar azimuth trends. When we compared stream reach orientation with known locations of enhanced groundwater inputs (previously detected by Rugel and others) we found that 55% of the time reaches in Ichawaynochaway Creek with increased groundwater discharge followed NW or NNW bearings (mean N49W). Further investigation to replicate these results in other tributaries of the LFRB is warranted and may help inform management strategies which could protect both ecological and economic interests in this region.

Geostatistical Determination of Ore Shoot Plunge and Structural Control of the Sizhuang World-Class Epizonal Orogenic Gold Deposit, Jiaodong Peninsula, China

The Jiaodong Peninsula in eastern China is the third largest gold-mining area and one of the most important orogenic gold provinces in the world. Ore shoots plunging in specific orientations are a ubiquitous feature of the Jiaodong lode deposits. The Sizhuang gold deposit, located in northwestern Jiaodong, is characterized by orebodies of different occurrences. The orientation of ore shoots has remained unresolved for a long time. In this paper, geostatistical tools were used to determine the plunge and structural control of ore shoots in the Sizhuang deposit. The ellipses determined by variogram modeling reveal the anisotropy of mineralization, plus the shape, size, and orientation of individual ore shoots. The long axes of the anisotropy ellipses trend NE or SEE and plunge 48° NE down the dip. However, individual ore shoots plunge almost perpendicular to the plunge of the ore deposit as a whole. This geometry is interpreted to have resulted from two periods of fluid flow parallel to two sets of striations that we identified on ore-controlling faults. Thrust-related lineations with a sinistral strike-slip component were associated with early-stage mineralization. This was overprinted by dextral and normal movement of the ore-controlling fault that controlled the late-stage mineralization. This kinematic switch caused a change in the upflow direction of ore-forming fluid, which in turn controlled the orientation of the large-scale orebodies and the subvertical plunge of individual ore shoots. Thus, a regional transition from NW-to-SE-trending compression to NW-to-SE-trending extension is interpreted as the geodynamic background of the ore-forming process. This research exemplifies an effective exploration strategy for studying the structural control of the geometry, orientation, and grade distribution of orebodies via the integration of geostatistical tools and structural analysis.

Mantle deformation patterns and signatures of rift, beneath Eastern Ghats Mobile Belt

The Eastern Ghats Mobile Belt (EGMB) and the adjacent Archaean Singhbhum and Bastar craton contact collisional sutures zones in southeastern India are investigated for seismic anisotropic signatures. The EGMB-craton contact boundary is a witness to the episode of rifting related to the breakup of super-continent Columbia, followed by reassembly and breakup of Rodinia, and eventual episode of collision of eastern India with Antarctica during the Pan-African age. With the intent to capture signatures of the collision, 22 broadband seismic stations were installed along two distinct profiles covering cratons and Eastern Ghats contact boundaries. Shear wave splitting analysis using core refracted phases, SKS, SKKS and PKS, waveforms resulted in 202 high-quality measurements, with variation in delay times (δt) from 0.4 s to 2.0 s, and predominant NE trend for the fast polarization directions (FPDs). The anisotropic parameters along the two profiles depict the influence of two dominant mechanisms. First, Absolute Plate Motion (APM) related strain where mantle drag at the base of lithosphere combined with other plate driving forces creates a NE dominance of fast-axis orientations in the direction of the Indian plate motion. Second, fossil anisotropy of the lithospheric origin, as fast axis is not completely in the direction of the Indian plate motion, specially across Rengali Province and Mahanadi Rift basin. Fast-axis orientations parallel to the surface geological features indicate large-scale pervasive deformation related to the collision and rifting episodes of India and East Antarctica. The composite stacked FPDs and δt in the Rengali domain is manifestation of the NE trend of the Indian Plate motion, combined with the WNW–ESE to E–W oriented fabrics associated to the EGMB-cratons contact shear zones. The Mahanadi Rift has a predominance of the rift parallel signatures due to the ancient preserved fabrics, which correlates the Mahanadi Rift with the Lambert Rift of East Antarctica, and is of key importance in plate-tectonic reconstructions.

Break-up related 2170–2120 Ma mafic dykes across the North Atlantic craton: Final dismembering of a North Atlantic-Dharwar craton connection?

Progress with supercontinental reconstructions relies on accurate age determinations for continental geological units such as mafic dyke swarms. Here we present zircon and baddeleyite U-Pb ID-TIMS isotope data of six mafic dykes from the Archean North Atlantic craton in present-day southern West and South-East Greenland. Two dykes in southern West Greenland yield crystallization ages of 2021 ± 4 Ma, for the NE-trending Hamborgersund dyke, and 2125 ± 9 Ma, for the E-trending Angissunguaq dyke. Additional age determinations of mafic dykes from South-East Greenland are 2166 ± 8 Ma and 2158 ± 8 Ma for two E–W trending dykes, herein named the Skjoldungen dykes, and 2137 ± 11 and 2124 ± 11 for two ENE and NE trending dykes, respectively. The name Ruinnæsset dykes is proposed for this slightly younger, ca. 2137–2124 Ma, generation of dykes in SE Greenland, and Nuuk dykes for coeval ca. 2125 Ma dykes in SW Greenland. The Skjoldungen and the Ruinnæsset dykes have primordial mantle geochemical signatures, with only minor LILE-enrichments. These signatures differ from other, more ‘lithospheric’, Proterozoic dykes within the region, and may reflect highly attenuated lithospheric extension during their emplacement. Coeval mafic magmatism in North Atlantic and Dharwar cratons suggests that these two fragments coexisted within a common Paleoproterozoic supercraton. A lack of younger age-matches further argue for ca. 2170–2140 Ma rifting and subsequent break-up of this Dharwar-North Atlantic connection, prior to the 2137–2124 Ma emplacement of the Ruinnæsset dykes. Other global age correlations are discussed and a likely paleogeographic reconstruction of North Atlantic craton together with the Dharwar and Superior cratons within a Paleoproterozoic supercraton is presented.

Integrating AMS data with structural studies from granitoid rocks of the Eastern Dharwar Craton, south India: Implications on successive fabric development and regional tectonics

Five successive deformation phases have been identified from structural analysis of granitoid and gneissic rocks of the study area. The magnetic foliation data of the studied granitoids show two distinct NW and NE trending sets. The NW trending magnetic foliation is found to be essentially parallel to the mesoscale planar structures of regional D1-D2 deformations and is thus correlated with the D1-D2 phases. On the contrary, the NE trending magnetic foliation appears to be kinematically compatible with the NW-SE compression direction deciphered from conjugate ductile shear zone patterns of the D3 event. Combining AMS, field and microstructural data, the granitoid outcrop have been separated into older D1-D2 and younger D3 domains. The published geochronological data from the area when compared with the established D1-D2 and D3 domains, shows a good correlation pointing tentatively the timing of the D1-D2 deformation phase to be ∼2550 Ma while the D3 phase to be ∼2524 Ma. Hence the study highlights the importance of combining AMS data with field and microstructural studies to constraint the timing of regional successive deformation imprints. It also helps to identify the chronologically separated emplacement phases of the granitoid massif in question, which would not have been possible otherwise.

Strike-slip faults and their significance for hydrocarbon accumulation in Gaoshiti–Moxi area, Sichuan Basin, SW China

Abstract Based on the three dimensional seismic data and drilling data in Gaoshiti–Moxi area, the structural characteristics and evolution process of strike-slip faults in the study area and their significance for gas accumulation were examined using the method of fault structure analysis. The results show that: (1) Transtensional strike-slip faults are developed in the Paleozoic in study area. On the profile, there are three types of structural styles: steep and erect, flower structures, “Y” and reversed “Y” type faults. On the plane, strike-slip faults developed in the Cambrian extending linearly and along the nearly EW, NW and NE direction. The faults are composed of multiple secondary faults in en-echelon arrangement; faults in the Permian are mainly along nearly EW and NW direction, and the Permian faults are distributed in the middle east part of the study area and they are more in the north than in the south. (2) The nearly EW and NW trending faults are dextral strike-slip faults, and the NE trending faults are sinistral strike-slip faults. The strike-slip strength of the nearly EW trending faults are the strongest and the maximum horizontal displacement is 550 m, while the strike-slip strength of the NE trending faults is the weakest. The Cambrian faults had stronger activities than the Permian faults. (3) The transtensional strike-slip faults were active in two periods, the early Caledonian period and the late Hercynian period, and were the products of oblique extension of pre-existing weak zones in Xingkai and Emei taphrogenesis, with certain inheritance in the main faults. (4) The strike-slip faults and surrounding fractures enhance the porosity and permeability of the reservoir in Cambrian Longwangmiao Formation and control the distribution of karst reservoirs in the Permian Qixia Formation–Maokou Formation, bringing about the situation of multiple gases bearing series in this area.

Neogene–Quaternary Magmatism of the Çaldıran Plain and its Vicinity (Eastern Turkey): an Example of Post-Collisional Transition from Subduction to Intraplate Type

This paper is aimed at studying the chronological evolution of the Neogene–Quaternary volcanic activity within the Caldiran plain and its mountainous framing (Eastern Turkey). It is shown that the last pulse of continental-margin magmatism related to the subduction and closure of Neotethys oceanic basin occurred in the Middle Miocene (13.5–12.5 Ma). The post-collision volcanism proceeding simultaneously with large-scale regional tectonic rearrangement and initiation of the long-term Caldiran fault began in the Late Miocene (7–6 Ma), and reached maximum activity in the Middle Pliocene (4.7–3.6 Ma). The Quaternary period in the region evolution was marked by the abundant within-plate magmatic activity restricted to the regional SW–NE trending zone, and the formation of Eastern Turkey’s largest Tendurek shield volcano (Late Pleistocene–Holocene). Petrological-geochemical data indicate that magmas during the overall evolution of young volcanism of the Caldiran plain was generated from a single mantle reservoir, whose composition gently one-way evolved with time. Calculations show that melting occurred in the upper part of the asthenosphere (immediately near the boundary with thinned lithospheric mantle), which was metasomatized by pre-existing long-continued subduction. The chemical variations of mantle source with time (from the Middle Miocene to Quaternary) were mainly determined by a decrease of subduction component and the presence of aqueous phases, with a general trend from E-MORB to OIB-type for generated magmas. The composition of Late Quaternary basic lavas of Tendurek Volcano in terms of most petrological-geochemical characteristics corresponds to within-plate alkaline basalts. The main trend of geochemical evolution of mantle source is correlated with a systematic change of the predominant serial affinity of igneous rocks from calcalkaline through moderately alkaline to Na-alkaline varieties. Discrete character of young magmatism within the Caldiran plain, and its subsequent evolution (sulrasubduction → post-collision → within-plate) were mainly determined by periodical large-scale changes in geotectonic setting within the Eurasian–Arabian collision zone: (1) cessation of subduction, (2) break-up and deepening of oceanic slab with its subsequent break off, (3) inferred emergence of incipient rift setting under conditions of intense submeridional compression.

Stratigraphic, magmatic and structural features of Ordovician tectonics in Sardinia (Italy): a review

The Variscan Palaeozoic successions exposed in Sardinia show clear evidences of tectonic instability during Ordovician times. In the Foreland zone (SW Sardinia), the Cambrian to lower Ordovician strata are folded giving way to an angular unconformity sealed by a Middle-Upper Ordovician syn-tectonic succession, characterized at the base by a thick conglomeratic deposit. This is indicative of a tectonic-driven uplift, followed by intensive erosion. A time-equivalent angular unconformity, sealed by Middle Ordovician volcano-sedimentary complexes, is also recorded in the External nappe zone (central–southern Sardinia). These volcanic products became rare toward NE and approaching the Inner zone, where few remnants of Ordovician magmatic rocks consist of the protoliths of small bodies of calc-alkaline orthogneisses and metabasites. Paleogeographic reconstructions generally assume that Sardinia formed a coherent crustal block since the late Cambrian; therefore, the SW–NE trend of magmatic and sedimentary features observed in the Ordovician successions has been interpreted as sequential snapshots captured along an Ordovician volcanic arc. Stratigraphic and paleontological data, sediment provenance and petro-chronology of magmatic products indicate that the Ordovician successions could have belonged to different, and possibly far paleogeographic domains. These data suggest that the actual zonation of the Palaeozoic basement of Sardinia is a puzzle resulting from extensive crustal reworking and amalgamation in Variscan times. Hence it cannot be directly referred to an Ordovician paleogeography.

Existence of new rock unit of pink dolomite within the Midland succession of Balanch area, Darchula, Far-Western Nepal

Geological setting and lithostratigraphy of Balanch area in the Darchula District, Far-western Nepal has been worked out with the aim to study the stratigraphy of the area. It has been found that the Bitule Dolomite developed in this area possibly represents the youngest rock unit of the Midland Group of the Nepal Lesser Himalaya. The Balanch area lies within the autochthonous zone of the Lesser Himalaya. The rock succession has been divided into six units: Bagadi Slate, Simar Talc Beds, Baril Dolomite, Tupkan Slate, Bitule Dolomite, and Balligad Sandstone. The Balligad Sandstone possibly belongs to the rock succession of Lower Tertiary whereas all other units belong to the Midland Group. From south to north, the area is characterized by the development of three tectonic zones. The southern zone is characterized by the development of oldest rock unit (Bagadi Slate) having NW to NE trend and dipping towards south and possibly thrusted over the middle zone. The middle zone is characterized by the development of the Simar Talc Beds, Baril Dolomite, and the Tupkan Slate, and also thrusted over rock succession of the northern zone, which is characterized by the development of the Bitule Dolomite and the Balligad Sandstone, the youngest rock units of the area. Geological setting and lithostratigraphy of Balanch area in the Darchula District, Far-western Nepal has been worked out with the aim to study the stratigraphy of the area. It has been found that the Bitule Dolomite developed in this area possibly represents the youngest rock unit of the Midland Group of the Nepal Lesser Himalaya. The Balanch area lies within the autochthonous zone of the Lesser Himalaya. The rock succession has been divided into six units: Bagadi Slate, Simar Talc Beds, Baril Dolomite, Tupkan Slate, Bitule Dolomite, and Balligad Sandstone. The Balligad Sandstone possibly belongs to the rock succession of Lower Tertiary whereas all other units belong to the Midland Group. From south to north, the area is characterized by the development of three tectonic zones. The southern zone is characterized by the development of oldest rock unit (Bagadi Slate) having NW to NE trend and dipping towards south and possibly thrusted over the middle zone. The middle zone is characterized by the development of the Simar Talc Beds, Baril Dolomite, and the Tupkan Slate, and also thrusted over rock succession of the northern zone, which is characterized by the development of the Bitule Dolomite and the Balligad Sandstone, the youngest rock units of the area.

Spatial distribution and variation of ore body, alteration and ore-forming fluid of the giant Zijinshan epithermal Cu-Au deposit, SE China: implication for mineral exploration

The Zijinshan high-sulphidation Cu-Au deposit located in the west Fujian Province is the largest active gold mine in China with total production of gold over 300 tonnes to date. The high-sulphidation ore bodies are characterized by the upper supergene Au and lower hypogene Cu zones bounded roughly by the paleowater table near 600 m elevation. Au-bearing goethite and limonite are the main ore minerals in the supergene zone whereas the hypogene Cu mineralization is dominated by covellite and anilite (or digenite) with minor enargite. The expected porphyry mineralization that coupled with Zijinshan is so far not exposed even to a depth of 1500 m from surface. However, the Luoboling Cu-Mo deposit which occurs only 3 km NE to Zijinshan is identified as a typical porphyry deposit. Current debate centered on whether the Zijinshan high-sulphidation epithermal deposit and the adjacent Luoboling porphyry deposit are genetically linked or discrete hydrothermal systems. Recent deep exploration at Zijinshan revealed an extension trend of Cu ore bodies and quartz-alunite alteration zone toward SE below 0 m elevation. This contradicts previous considered NE trend above 0 m elevation. In this study, fluid inclusion mapping based on rigorous strategy and sample selection was carried out on 20 sample suites scattered over the whole high-sulphidation Cu-Au ore bodies from −450 to 800 m elevation. Aqueous fluid inclusions of unambiguous primary origin in syn-ore miarolitic quartz, coarse-grained alunite crystals and quartz overgrowth zones were measured. Microthermometry results show the average T h and salinity of each sample suite ranging from 205 to 304°C and 2.4 to 5.5 wt% NaCl equiv., respectively. Both fluid temperature and salinity exhibit negative correlation with increasing elevation, indicating convective cooling by mixing and circulating of meteoric water has involved. The good consistence of mineralization distribution pattern and fluid evolution trend implies that the southeastward extending ore bodies probably reflect the fluid pathway which is connected to the deeper porphyry mineralization and their magmatic source. Considering the temperature condition (370–400°C) required for brittle-plastic transition which commonly characterizes porphyry deposits, the potential porphyry mineralization coupled to Zijinshan should have occurred at greater depth under higher temperature. Combined with recent exploration progress and the fluid evolution pattern, we therefore proposed that the Luoboling porphyry Cu-Mo deposit and the Zijinshan high-sulphidation Cu-Au deposit are likely discrete hydrothermal systems, and future porphyry deposit exploration at the deep SE segment of Zijinshan is recommended.

The Dihedral Angle and Intersection Processes of a Conjugate Strike‐Slip Fault System in the Tarim Basin, NW China

AbstractRecent studies, focused on dihedral angles and intersection processes, have increased understandings of conjugate fault mechanisms. We present new 3‐D seismic data and microstructural core analysis in a case study of a large conjugate strike‐slip fault system from the intracratonic Tarim Basin, NW China. Within our study area, “X” type NE and NW trending faults occur within Cambrian‐Ordovician carbonates. The dihedral angles of these conjugate faults have narrow ranges, 19° to 62° in the Cambrian and 26° to 51° in the Ordovician, and their modes are 42° and 44° respectively. These data are significantly different from the ∼60° predicted by the Coulomb fracture criterion. It is concluded that: (1) The dihedral angles of the conjugate faults were not controlled by confining pressure, which was low and associated with shallow burial; (2) As dihedral angles were not controlled by pressure they can be used to determine the shortening direction during faulting; (3) Sequential slip may have played an important role in forming conjugate fault intersections; (4) The conjugate fault system of the Tarim basin initiated as rhombic joints; these subsequently developed into sequentially active “X” type conjugate faults; followed by preferential development of the NW‐trending faults; then reactivation of the NE trending faults. This intact rhombic conjugate fault system presents new insights into mechanisms of dihedral angle development, with particular relevance to intracratonic basins.

Quantitative Analysis of Faults in Huizhou Sub-basin, Pearl River Mouth Basin

This study used 2D seismic profiles to investigate the Cenozoic evolution of faults in the Huizhou Sub-basin. It aims to define the basin structure style through describing the geometric features of the faults and quantitatively analyzing their activities. The results indicate that the boundary faults in the Huizhou Sub-basin display en echelon arrangement in plan view, which indicates that, it was caused by a kind of oblique extension. Calculating the fault slips shows long-term activities of faults occurred in the basin, and some boundary faults kept active after 5.5 Ma. The evolution history of the fault system is reconstructed. Initially, during the Eocene-Early Oligocene, mainly NNE-NE and NW trending faults and NE striking grabens and half-grabens formed in the basin and a series of faults system controlled the deposition. During the Late Oligocene-Early Miocene, the structural activities were relatively weak, the fault activity and the fault growth rate decreased sharply. Finally, in the late stage from Late Miocene to the present, the structure movement was re-activated, and some faults were also reactivated. Our study will help better understand the structural features and evolution of the petroleum-bearing basins in the northern margin of the South China Sea.

Structural characteristics and evolution of the South Yellow Sea Basin since Indosinian

Based on the seismic data gathered in past years and the correlation between the sea and land areas of the Lower Yangtze Platform, the structural characteristics of the South Yellow Sea Basin since the Indosinian tectonic movement is studied in this paper. Three stages of structural deformation can be distinguished in the South Yellow Sea Basin since the Indosinian. The first stage, Late Indosinian to Early Yanshanian, was dominated by foreland deformation including both the uplifting and subsidence stages under an intensively compressional environment. The second stage, which is called the Huangqiao Event in the middle Yanshanian, was a change for stress fields from compression to extension. While in the third stage (the Sanduo Event) in the Late Himalayan, the basin developed a depression in the Neogene-Quaternary after rifting in the Late Cretaceous-Paleogene. The long-time evolution controlled 3 basin formation stages from a foreland basin, then a fault basin to a final depression basin. In conclusion, since the Indosinian, the South Yellow Sea Basin has experienced compressional fold and thrust, collisional orogen, compressional and tensional pulsation, strike-slip, extensional fault block and inversion structures, compression and convergence. The NE, NEE, nearly EW and NW trending structures developed in the basin. From west to east, the structural trend changed from NEE to near EW to NW. While from north to south, they changed from NEE to near EW with a strong-weak-strong zoning sequence. Vertically, the marine and terrestrial facies basins show a “seesaw” pattern with fold and thrust in the early stages, which is strong in the north and weak in the south and an extensional fault in later stages, which is strong in the north and weak in the south. In the marine facies basin, thrust deformation is more prevailing in the upper structural layer than that in the lower layer. The tectonic mechanism in the South Yellow Sea Basin is mainly affected by the collision between the Yangtze and North China Block, while the stress environment of large-scale strike-slip faults was owing to subduction of the Paleo-Pacific plate. The southern part of the Laoshan uplift is a weak deformation zone as well as a stress release zone, and the Meso-Paleozoic had been weakly reformed in later stages. The southern part of the Laoshan uplift is believed, therefore, to be a promising area for oil and gas exploration.

In-situ Stress Field of Fanzhuang Coabed Methane Block in Qinshui Basin

Geostress is a crucial factor that affects the in-situ permeability and reservoir stimulation, and thus the performance of coalbed methane (CBM) recovery from coal. Therefore, a systematically investigation of in-situ geostress was made based on pressure-time records of the hydraulic fracturing of 238 CBM wells collected from Fanzhuang block in Qinshui Basin of China. The results indicate the vertical stress, maximum and the minimum horizontal principal stress range from 5.92 to 20.08 MPa, 8.03 to 41.75 MPa and 5.38 to 21.24 MPa, respectively. The gradient of the maximum horizontal stress and least horizontal stress vary from 14.16 to 77.82 kPa/m and from 11.12 to 41.43 kPa/m in respective. The relatively high horizontal principal stress zone concentrates in the NE trending within the middle of the block, while the relatively low-stress zone distributes in west, east and south of the block. The analysis of the microseismic fracture monitoring from 72 CBM wells shows that the orientation of the maximum horizontal principal stress concentrates in the NEE∼NE direction and locally distorted by the faults, which can be further used to optimize the CBM well drilling, completion and stimulation in the studied area.

Structural interpretation of aeromagnetic data for the Wadi El Natrun area, northwestern desert, Egypt

The Wadi El Natrun area in Egypt is located west of the Nile Delta on both sides of the Cairo–Alexandria desert road, between 30°00′ and 30°40′N latitude, and 29°40′ and 30°40′E longitude. The name refers to the NW-SE trending depression located in the area and containing lakes that produce natron salt. In spite of the area is promising for oil and gas exploration as well as agricultural projects, Geophysical studies carried out in the area is limited to the regional seismic surveys accomplished by oil companies. This study presents the interpretation of the airborne magnetic data to map the structure architecture and depth to the basement of the study area. This interpretation was facilitated by applying different data enhancement and processing techniques. These techniques included filters (regional-residual separation), derivatives and depth estimation using spectral analysis and Euler deconvolution. The results were refined using 2-D forward modeling along three profiles.Based on the depth estimation techniques, the estimated depth to the basement surface, ranges from 2.25 km to 5.43 km while results of the two-dimensional forward modeling show that the depth of the basement surface ranges from 2.2 km to 4.8 km. The dominant tectonic trends in the study area at deep levels are NW (Suez Trend), NNW, NE, and ENE (Syrian Arc System trend). The older ENE trend, which dominates the northwestern desert is overprinted in the study area by relatively recent NW and NE trends, whereas the tectonic trends at shallow levels are NW, ENE, NNE (Aqaba Trend), and NE. The predominant structure trend for both deep and shallow structures is the NW trend.The results of this study can be used to better understand deep-seated basement structures and to support decisions with regard to the development of agriculture, industrial areas, as well as oil and gas exploration in northern Egypt.

Feeder systems of acidic lava flows from the Paraná-Etendeka Igneous Province in southern Brazil and their implications for eruption style

In the Rio Grande do Sul State, southern Brazil, the volcanic sequence of the Paraná-Etendeka Igneous Province consists of pahoehoe and rubbly pahoehoe lava flows with basaltic and basaltic andesitic composition respectively, overlaid by acidic volcanic rocks. The acidic volcanic rocks of the Paraná-Etendeka Igneous Province exhibit textures and structures that can be related to effusive and/or explosive eruptions generating predominantly rheoignimbrites. The huge lava volume related to the emplacement of large igneous provinces implicates on efficient feeder systems that are more commonly observed in continental environments. In the Paraná-Etendeka Igneous Province, feeders of basaltic rocks are exposed in several dyke swarms (Ponta Grossa NW trending, Florianópolis/Skeleton Coast (NW Namibia) N-S trending, Serra do Mar NE trending and Henties Bay/Outjo NE trending). In contrast, the only feeder system proposed to the acidic rocks of the Paraná-Etendeka Igneous Province is the Messum complex in Namibia (Milner et al. 1995). In the study area, the opening of three quarries for the extraction of dimension stones has exposed impressive structures/textures that show the effusive emplacement and the ductile to fragile-ductile magma transition along the acidic feeder dykes. Besides that, magma mixing/mingling processes between two acidic magmas are observed along the dykes. Here we describe new occurrences of acidic feeder dykes, correlate the dykes with acidic flows and discuss their importance to understand the emplacement of the Palmas type acid units in southern Brazil.

The 2015 Mw7.2 Sarez Strike‐Slip Earthquake in the Pamir Interior: Response to the Underthrusting of India's Western Promontory

The Pamir orogen, Central Asia, is the result of the ongoing northward advance of the Indian continent causing shortening inside Asia. Geodetic and seismic data place the most intense deformation along the northern rim of the Pamir, but the recent 7 December 2015, Mw7.2 Sarez earthquake occurred in the Pamir's interior. We present a distributed slip model of this earthquake using coseismic geodetic data and postseismic field observations. The earthquake ruptured an ∼80 km long, subvertical, sinistral fault consisting of three right‐stepping segments from the surface to ∼30 km depth with a maximum slip of three meters in the upper 10 km of the crust. The coseismic slip model agrees well with en échelon secondary surface breaks that are partly influenced by liquefaction‐induced mass movements. These structures reveal up to 2 m of sinistral offset along the northern, low‐offset segment of modeled rupture. The 2015 event initiated close to the presumed epicenter of the 1911 Mw∼7.3 Lake Sarez earthquake, which had a similar strike‐slip mechanism. These earthquakes highlight the importance of NE trending sinistral faults in the active tectonics of the Pamir. Strike‐slip deformation accommodates shear between the rapidly northward moving eastern Pamir and the Tajik basin in the west and is part of the westward (lateral) extrusion of thickened Pamir plateau crust into the Tajik basin. The Sarez‐Karakul fault system and the two large Sarez earthquakes likely are crustal expressions of the underthrusting of the northwestern leading edge of the Indian mantle lithosphere beneath the Pamir.