NE-trending Faults Research Articles

An integrated geophysical study on the Mesozoic strata distribution and hydrocarbon potential in the South China Sea

A series of drilling, dredge, and seismic investigations indicate that Mesozoic sediments exist in the South China Sea (SCS) which shows a bright prospect for oil and gas exploration. In order to study the distribution of Mesozoic strata and their residual thicknesses in the SCS, we carried out an integrated geophysical study based mainly on gravity data, gravity basement depth and distribution of residual Mesozoic thickness in the SCS were obtained using gravity inversion constrained with high-precision drilling and seismic data. In addition, the fine deep crustal structures and distribution characteristics of Mesozoic thicknesses of three typical profiles were obtained by gravity fitting inversion. Mesozoic strata in the SCS are mainly distributed in the south and north continental margins, and have been reformed by the later tectonic activities. They extend in NE-trending stripes are macro-controlled by the deep and large NE-trending faults, and cut by the NW-trending faults which were active in later times. The offset in NW direction of Mesozoic strata in Nansha area of the southern margin are more obvious as compared to the north margin. In the Pearl River Mouth Basin and Southwest Taiwan Basin of the north continental margin the Mesozoic sediments are continuously distributed with a relatively large thickness. In the Nansha area of the south margin the Mesozoic strata are discontinuous and their thicknesses vary considerably. According to the characteristics of Mesozoic thickness distribution and hydrocarbon potential analyses from drilling and other data, Dongsha Uplift-Chaoshan Depression, Southwest Taiwan Basin-Peikang Uplift and Liyue Bank have large thickness of the Mesozoic residual strata, have good hydrocarbon genesis capability and complete source–reservoir–cap combinations, show a bright prospect of Mesozoic oil/gas resources.

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.

The Late Triassic Dengfuxian A-type granite, Hunan Province: age, petrogenesis, and implications for understanding the late Indosinian tectonic transition in South China

The Triassic (Indosinian) granites in the South China Block (SCB) have important tectonic significance for understanding the evolution of Eastern Asia. The Dengfuxian biotite granite in eastern Hunan Province, China, reported in this article, was recognized as Late Triassic (late Indosinian) weakly peraluminous A-type granite with a zircon laser ablation inductively coupled plasma mass spectrometry U–Pb age of 225.7 ± 1.6 Ma. It is enriched in F, Cs, Rb, Th, high field strength elements, and rare earth elements (REEs) and depleted in Ba, Sr, P, Ti, Nb, and Ta, with high Ga/Al ratios and zircon saturation temperatures. The Dengfuxian biotite granite shows high initial Sr isotope values (0.715932 to 0.716499) and negative ɛNd(t) (−10.46 to −9.67) and ɛHf(t) (−9.92 to −6.29) values, corresponding to the Nd model ages of 1.79 to 1.85 Ga and the Hf model ages of 1.65 to 1.88 Ga. It is proposed that the Dengfuxian biotite granite was derived from high-temperature partial melting of the Palaeoproterozoic lower crust undergoing granulitization. Some Late Triassic A-type granites were recently identified in the SCB with the ages between 202 and 232 Ma. These A-type granites have the same geochemical characteristics and petrogenesis as Dengfuxian A-type granite, and show A2-subtype granite affinity. The Late Triassic A-type granite formed a NE-trending granite belt, which is consistent with the main NE-trending faults in the SCB. The formation of these A-type granites was in response to the subduction of the palaeo-Pacific plate underneath the SCB, and indicates an extensional tectonic environment in the SCB. Combined with previous studies on tectonic evolution, we suggest that there may be a tectonic transition inside the SCB from compression to extension at least from 225 to 230 Ma.

A more accurate relocation of the 2013 M s7.0 Lushan, Sichuan, China, earthquake sequence, and the seismogenic structure analysis

We use a combined earthquake location technique to relocate the M s7.0 Lushan, Sichuan, China, earthquake sequence of April 20, 2013. A stepwise approach, employing three existing location methods (the HYPOINVERSE method, the Minimum 1-D model, and the Double Difference method), is used to improve location precision by iteratively revising the velocity model station corrections, and hypocenter relocations throughout the process. Our stepwise approach has significantly improved the location precision of the Lushan earthquake sequence, yielding hypocenter locations with final errors of 359, 309, and 605 m in the E-W, N-S, and vertical directions, respectively, with average travel time residuals of 0.12 s. Furthermore, we analyzed the seismogenic structure surrounding the Lushan earthquake sequence by combining the results of the relocated hypocenter distribution with new focal mechanism solutions and information from regional geological and geophysical investigations. From our analysis, we conclude that the vast majority of the aftershocks of the Lushan earthquake sequence occurred at depths of 6–9 km, near the front of the southwestern segment of the NE-trending Longmenshan fault zone. Densely aligned hypocenters clearly suggest that the seismogenic structure of the mainshock consists of a set of basal thrust faults dipping to the NW at 40–50°, at a ramp of the deep basal décollement-thrust system at depths of 7–18 km. Focal mechanism solutions suggest that the seismogenic faults have produced almost pure thrusting. At least one SE-dipping back-thrust is also observed within the basement, as indicated by the hypocenter relocations, which points to either a secondary rupture plane during the mainshock or a plane of aftershock slips. A small number of minor events in the Lushan sequence are located at depths of 0–6 km, with a distribution suggesting that the three NE-trending faults with surface traces running through or passing close to the aftershock area are confined to the upper Mesozoic sedimentary cover, making them independent of the deeper thrust faults that ruptured during the mainshock. Therefore, the 2013 M s7.0 Lushan earthquake was a blind thrust fault generated on active thrust faults within the basement of the southwestern Longmenshan fault zone, with an upper limit estimation of the rupture length, average down-dip width, and rupture area of 40, 16, and 640 km2, respectively.

Characteristics and origin of the Zhulazhaga gold deposit in Inner Mongolia, China

The Zhulazhaga gold deposit in the northern margin of the North China Craton (NCC) is a large tonnage hydrothermal deposit with a reserve of about 50t at about 4g/t Au. The deposit is characterized by an epigenetic style of mineralization in metasedimentary rocks dominated by slates, low-grade metamorphic clastic rocks and minor volcanic rocks. Gold mineralization is dominantly controlled by structures, manifested by stratabound orebodies located immediately to the east and west of a major N–S-trending fault and associated extensional fractures and NE-trending fault zones. The mineralization is also preferentially located in the fourth sub-member of first member of the Agulugou Formation. There are many auriferous quartz veins parallel to the bedding of the host rocks which contain the orebodies, and high-grade ore preferentially occurs near to these quartz veins. The structures provided pathways for transportation of gold-bearing fluids within the formation that appears to form a particularly favorable host rock and depositional setting.Hydrogen and oxygen isotope data indicate that ore-forming fluids were dominated by magmatic water mixed with minor meteoric water. Sulfur and lead isotope data indicate that some sulfur and lead came from the magmatic fluid. The metallogenic age (280Ma) of this deposit and the crystallization age (280–290Ma) of the granite porphyry in the district indicate a link between the Hercynian magmatism and gold mineralization. The Hercynian tectono-magmatism and associated hydrothermal activity remobilized gold and drove ore-forming fluids to the favorable depositional environment.The northern margin of the NCC is prospective for gold mineralization hosted in metasedimentary rocks which may be one of the most important deposit types for future gold exploration in northern China.

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.

The structural and tectonic relationships of the major fault systems of the Tan-Lu fault zone, with a focus on the segments within the North China region

The Jiashan-Lujiang, Yishu and Bohai segments have been the most tectonically active parts of the Tan-Lu fault system since the Mesozoic. Any analysis of faulting processes in the Tan-Lu system is highly dependent on the relationships among the three segments. Here we divide the Tan-Lu system into three styles of faulting; strike-slip faulting, regional extensional faulting and listric normal faulting on the basis of EMAP data, seismic profiles and geological evidence. We suggest that: (1) the strike-slip faults, which formed in the Late Jurassic, include mainly the Jiashan-Lujiang fault, the deep parts of the Chihe-Taihu Fault and the Guhe-Sanbing fault; these three faults have had the most dramatic effect on the Jiashan-Lujiang segment. The strike-slip style of faulting produced a huge positive-flower structure that extends between the deep section of the Chihe-Taihu fault and the Guhe-Sanbing fault, and overlaps with the position of the Zhangbaling ductile shear zone; (2) the regional extensional style of faulting, whose characteristics are reflected best in the Yishu fault zone, is composed of the Changyi-Dadian, Anqiu-Quxian, Yishui-Yangtou and Tangwu-Gegou faults, all of which were active during the Early Cretaceous. These extension-dominated faults extend southwards to the Jiashan and Hefei Basins and die out in front of the Dabie Mountains; to the north, the faults penetrate the deeper parts of the Weibei depression where they controlled Cretaceous deposition in “graben-horst-graben” structures; (3) the listric normal faults, which were initiated in the Paleogene, generally occur around the Bohai Bay Basin, and include a series of NE-trending faults in the Weibei depression, such as the Tandong and Tanxi faults. These listric faults differ from the regional extensional faults in their localized occurrence and association with basin formation. Our study indicates that these three styles of faulting varied not only in timing and mechanical properties but developed at different levels at different locations, resulting in different tectonic relationships and the formation of different geologic features in the different segments of the Tan-Lu master system.

3D seismic attributes for structural analysis in compressional context: A case study from western Sichuan Basin

Compressional region usually forms complex thrust faults system, which is difficult to identify using traditional migration profiles. The successful application of three-dimensional (3D) seismic attributes analysis greatly reduces the difficulty, and improves the accuracy and efficiency of seismic interpretation and structural analysis. In this paper, we took Qiongxi area in the compressional region of western Sichuan as an example, using two 3D seismic attributes, coherence and instantaneous phase, to identify fault assemblages and variations both vertically and laterally. The results show that the study area mainly consists of NS-, NE- and NEE-trending faults. The NS-trending faults are the largest and have a component of sinistral slip controlling the formation of NEE-trending faults, while the NE-trending faults are intermediate in scale, formed earlier and were cut by the NS-trending faults. Our results demonstrate that using seismic attributes for structural analysis have the following advantages: (1) more details of major fault zones, (2) highlighting minor faults which are hardly traced in seismic migration cube, and (3) easier acquisition of accurate fault systems. The application of seismic attributes provides a new idea for deciphering fine and complicated structures, and will significantly contribute to the development of objective and precise geological interpretation in the future.

Petrogenesis and Mineralization of the Tongshan Skarn Cu–Au Deposit, Lower Yangtze Metallogenic Belt

The Tongshan copper deposit in Anqing-Guichi Cu-FeAu-Mo district is a typical skarn deposit in the Lower Yangtze River metallogentic belt, which is located at the northern margin of the Yangtze craton and bordered by the Dabieshan orogenic belt to the north. The ore bodies are related to Tongshan granodiorite and quartz monzonite porphyry, which intruded mainly into Middle Carboniferous Huanglong-Chuanshan Formation, Lower Permian Qixia Formation carbonates and is characterized by porphyritic textures and carbonization. The location of ore-forming intrusions is controlled by the intersecting places between the NE and EW trending fault structures.

Characterising the nature, evolution and origin of detachment fault in central depression belt, Qiongdongnan Basin of South China Sea: evidence from seismic reflection data

Using regional geological, newly acquired 2D and 3D seismic, drilling and well log data, especially 2D long cable seismic profiles, the structure and stratigraphy in the deep-water area of Qiongdongnan Basin are interpreted. The geometry of No.2 fault system is also re-defined, which is an important fault in the central depression belt of the deep-water area in the Qiongdongnan Basin by employing the quantitative analysis techniques of fault activity and backstripping. Furthermore, the dynamical evolution of the No.2 fault system and its controls on the central depression belt are analyzed. This study indicates that the Qiongdongnan Basin was strongly influenced by the NW-trending tensile stress field during the Late Eocene. At this time, No.2 fault system initiated and was characterized by several discontinuous fault segments, which controlled a series small NE-trending fault basins. During the Oligocene, the regional extensional stress field changed from NW-SE to SN with the oceanic spreading of South China Sea, the early small faults started to grow along their strikes, eventually connected and merged as the listric shape of the No.2 fault system as observed today. No.2 fault detaches along the crustal Moho surface in the deep domain of the seismic profiles as a large-scale detachment fault. A large-scale rollover anticline formed in hanging wall of the detachment fault. There are a series of small fault basins in both limbs of the rollover anticline, showing that the early small basins were involved into fold deformation of the rollover anticline. Structurally, from west to east, the central depression belt is characterized by alternatively arranged graben and half-graben. The central depression belt of the Qiongdongnan Basin lies at the extension zone of the tip of the V-shaped northwestern ocean sub-basin of the South China Sea, its activity period is the same as the development period of the northwestern ocean sub-basin, furthermore the emplacement and eruption of magma that originated from the mantle below the Moho surface occurred at the region between Songnan-Baodao and Changchang sags, from east to west with the early-stage spreading of the South China Sea. Therefore, this study not only helps in depicting the structural features and evolution of the deep-water basin in the Qiongdongnan Basin, but also provides the geological and structural evidence for establishing a unified model of continental margin extension and oceanic spreading.

Counterclockwise rotations in the Late Eocene–Oligocene volcanic fields of San Luis Potosí and Sierra de Guanajuato (eastern Mesa Central, Mexico)

We used paleomagnetic and structural data to investigate the late Eocene–Oligocene tectonic evolution of the Mesa Central area in Mexico. The Mesa Central was affected by NW-trending faults (Tepehuanes-San Luis fault system) coeval with a Late Eocene–Oligocene ignimbrite flare-up and by post-27Ma NNE-trending grabens related to the Basin and Range. We obtained reliable paleomagnetic directions from 61 sites within the Late Eocene–Oligocene volcanic series (~30 to ~27Ma) of the San Luis Potosí volcanic field and Sierra de Guanajuato. For each site we also measured the anisotropy of magnetic susceptibility (AMS). Tilt corrections were made using AMS data for 33 sites where in situ bedding measurements were not available. Paleomagnetic directions indicate counterclockwise rotations of about 10° with respect to stable North America after 30–25Ma. Structural data suggest that the volcanic succession was mainly affected by normal faults. However, we also found evidences for oblique or horizontal striae showing a left-lateral component along NW-trending faults and a right lateral component along NE-trending faults. Both motions are consistent with a N–S extension oblique to the Tepehuanes-San Luis fault system. Previous paleomagnetic studies in northern and southern Mexico show the prevalence of minor left-lateral shear components along regional-scale transpressional and transtensional lineaments. Our paleomagnetic data may reflect thus small vertical-axis rotations related to a minor shear component coeval with the Oligocene intra-arc extension in central Mexico.

The Serra Pelada Au-Pd-Pt Deposit, Carajas, Brazil: Geochemistry, Mineralogy, and Zoning of Hydrothermal Alteration

The Serra Pelada Au-Pd-Pt deposit is located within the Carajas mineral province, which also hosts the world-class iron ore and iron oxide-hosted copper-gold deposits of the Amazon craton in Brazil. The unusual low-temperature hydrothermal mineralization at Serra Pelada is epigenetic, hosted by metasedimentary rocks of the Aguas Claras Formation and structurally localized in the Serra Pelada overturned syncline. The orebodies are controlled by the intersection of subvertical NE-trending fault zones with metasiltstones, mainly at the syncline’s hinge, with minor ore occurrences at the upper and lower limb. Intense tropical weathering over the last 70 m.y. has completely overprinted the shallow ore in and near the flooded open pit, but primary hydrothermal features are preserved in deeper drill core delineating the remaining resource. Gold, platinum, and palladium mineralization is associated with intense argillic alteration, hematite breccias, and silicification, with the highest grade ore hosted by brecciated metasiltstones that are highly enriched in amorphous carbon. Distal alteration zones comprise a reducing and oxidizing alteration front. The hydrothermal mineral paragenesis comprises kaolinite, quartz, sericite, amesite (Mg-rich Al-silicate), amorphous carbon, hematite, monazite, rutile, pyrite, and a complex assemblage of Bi-, Ag-, Pb-, Cu-, Co-, Ni-, Pt-, Pd-, and Au-bearing, chalcogenide (S, Se), and arsenide (As, Sb) minerals. Major element changes during hydrothermal alteration include C and Mg addition, K depletion, localized silica loss, and silicification with notable introduction of trace elements including light rare earth elements (LREE), Bi, Pb, U, V, Cu, Co, Ni, and As. The hydrothermal alteration and element association of the Serra Pelada deposit show geochemical similarities with unconformity-related uranium deposits, which may also be enriched in Au, Pd, and Pt and were formed by mixing of fluids that interacted with a highly oxidized cover sequence and highly reduced rock packages in structures of brittle-ductile strain.

Cenozoic inversion of the East China Sea Shelf Basin: implications for reconstructing Cenozoic tectonics of eastern China

The structural styles of the East China Sea Shelf Basin are described based on the interpretation of seismic profiles. The basin is divided into West and East Rift regions. It has undergone four phases of compressive structural inversions during the late Palaeocene Oujiang Event, the late Eocene Yuquan Event, the early Miocene Huagang Event, and the latest Miocene Longjing Event. Structural inversions have superimposed on the Cenozoic extensional rift basin during these compression events, forming regional unconformity reflectors (T4°, T3°, T2°, and T1°). The Changjiang Depression located in the north of the West Rift Region is characterized by a number of NW-, EW-, to NE-trending faults. However, the other depressions in the East China Sea Shelf Basin are dominated by NNE- to NEE-trending faults. During the Palaeocene, the Taibei Depression was in a marine environment, whereas the Changjiang Depression was terrestrial, although both depressions are located in the West Rift Region. The Hongzhouwan–Okinawa Fault separates the East China Sea Shelf Basin into south and north. The structural styles and the sedimentary environment change longitudinally and transversely, which are controlled by basement structures. The depocentre of the East China Sea Shelf Basin has gradually migrated and became younger from west to east. The structural inversion also becomes younger from west to east, but the intensity of inversion increases from south to north. These structural inversion events reveal a periodic compression and imply that the compressive uplift of eastern China resulted from subduction of the Pacific Plate as well as collision of the Luzon arc and the Asian continent rather than the remote effect of India–Asian collision.

Characteristics of Subtle Fault Zone in Jinhu Sag

Subtle fault zones are caused by the weak deformation generated in the sedimentary cover of a sag due to the influence of regional or local stress fields or basement faults. They are too subtle to be easily identified by conventional exploration methods and technologies and are thus usually ignored. Research results prove that there are two basement faults in the Jinhu sag referred to as the NE-and NW-trending basement faults. Parts of the NE-trending basement fault are intense enough to control sag formation and evolution (such as the faults in Yangcun and Shigang, etc.). However, the NW-trending and the rest of the NE-trending basement faults show weak activity and exert little influence on sedimentary cover deformation. These faults merely yield some weakly-deformed trend zones in the sedimentary cover, such as small en-echelon faults, small faults intermittently distributed along fixed directions, buried alluvial fans, zonal or stringy oil-gas traps, or linear structures (such as local folds, narrow and deep half-grabens, etc.). Apart from the two aforementioned types of subtle fault zones, intermittent and stringy NS-trending subtle fault zones are also induced by the EW-trending extrusion stress component in the sag generated by the regional dextral stress field. Keywords: Jinhu sag; basement faults; subtle fault zones; tectonic evolution; en-echelon; trap distribution

Late Ediacaran–Cambrian structures and their reactivation during the Variscan and Alpine cycles in the Anti-Atlas (Morocco)

The post-Pan-African evolution of the northern border of the West African Craton is largely controlled by the remobilisation of Late Neoproterozoic basement faults. The Upper Ediacaran volcanic and volcano-sedimentary sequences of the Ouarzazate Group show dramatic and rapid thickness changes, consistent with active extensional faulting associated with post-orogenic collapse and incipient continental rifting. The geometry and kinematics of these faults differ from west to east in the Anti-Atlas. N- to NE-trending faults dominate in western Anti-Atlas in response to E–W to NW–SE pure extension, while a transtensive opening regime characterize the central (Bou Azzer) and eastern (Saghro–Ougnate) Anti-Atlas.The marine incursion in the west-central Anti-Atlas during the late Ediacaran–Early Cambrian occurred without major geodynamical break between the continental Ouarzazate Group and marine sediments of the Adoudou Fm. Extensional tectonics went on during the Early Cambrian, being concentrated in the western and central parts of the belt. From Middle Cambrian to Lower Devonian and mainly due to thermal subsidence, the Anti-Atlas basement was buried under marine sediments with dominant south-derived detrital input. Basement faults control the distribution of subsiding versus shallow areas. During the Middle–Late Devonian, the dislocation of the Saharan platform occurred, mainly in the eastern Anti-Atlas where Precambrian faults were also remobilized during the Early Carboniferous.During the Variscan orogeny, the Paleozoic series of the Anti-Atlas basin were involved in folding tectonics, concomitant with the uplift of Proterozoic basement blocks bounded by inherited basement faults. The pre-existing rift-related faults were variably inverted across the Anti-Atlas. In the westernmost part of the belt, Variscan shortening induced positive inversions along the remobilized basement faults, but in some cases, some faults preserved an apparently normal throw. Some hidden basement faults accommodate the Variscan shortening by strike-slip movement expressed by en echelon fold pattern in the overlying cover.The Mesozoic-Cenozoic evolution of the Anti-Atlas is again marked by the reactivation of the basement faults. During the Triassic, the Anti-Atlas belongs to the uplifted shoulder of the Atlasic-Atlantic rift zone. Remobilisations of paleofaults again play a significant role in the weak burial of the Anti-Atlas during the Late Cretaceous–Eocene before the Neogene exhumation related to the Africa–Europe collision. Hence the structural evolution of the Anti-Atlas during the Paleozoic to Present times has been heavily dependent on the fault pattern inherited from the Late Ediacaran–Cambrian rifting evolution at the northern fringe of the WAC, already deeply affected by the Pan-African orogeny.

Compositional spatial zonation and 2005–2013 temporal evolution of the hydrothermal-magmatic fluids from the submarine fumarolic field at Panarea Island (Aeolian Archipelago, southern Italy)

The November 2002 submarine gas blast at Panarea Island (Sicily, southern Italy) was an unexpected reactivation event able to locally affect this hydrothermal-magmatic system whose the youngest eruptive products were dated at 20,000±2000years BP. The presence of magmatic gases (SO2 and HF) in the fumarolic gas discharges after the violent exhalative event was indicative of a magmatic input that temporary displaced the hydrothermal system. A few months later these acidic gases were indeed not detected in any of the studied fumaroles. Nevertheless, new geochemical data obtained by periodical sampling up to June 2013 suggest that the chemical–physical conditions of the hydrothermal-magmatic system at Panarea were not completely restored with respect to the geochemical data obtained in the early nineties. Thus, the 2002 gas burst has unequivocally caused a permanent modification to the fluid circulation system feeding the submarine fumaroles. In addition, strong compositional differences were observed by the 46 gases collected in 2012–2013 from submarine fumaroles located in different sites of the studied area, allowing to distinguish three different groups of fumaroles: A) H2- and CO-rich gases, which also show relatively low Ar concentrations, B) H2S-rich gases, having variable CO/CH4 ratios, and C) Ar-rich gases, having relatively low H2 concentrations. Gases from group A are distributed along NW- and NE-trending fault systems, whereas those of groups B and C discharge at increasing distance from the intersection of the two fault systems, indicating a spatial and compositional control by the local tectonic setting. The H2/CO ratios of groups A and B gases are significantly lower than those measured prior to 2012. This would imply an increase of gas pressure at depth, possibly caused by continuous addition of gas and energy from the magmatic source to the hydrothermal reservoir. Continuation of this process may lead to the occurrence of gas burst events in the next future, a hypothesis that is supported by the strong ongoing degassing activity at the surface notwithstanding a decrease of temperatures at depth. A geochemical, seismological and ground deformation monitoring of the Panarea submarine fumarolic field is highly recommended to obtain precursory signals of new strong degassing phenomena.

Structural evolution of the southern transfer zone of the Gulf of Suez rift, Egypt

We present a detailed study about the initiation and reactivations of Zeit-El Tor transfer zone, south Gulf of Suez rift, and its structural setting and tectonic evolution with respect to the Cretaceous-Cenozoic tectonic movements in North Egyptian margin. NE trending zone of opposed-dipping faults (22km wide) has transferred the NE and SW rotations of the sub-basins in central and south Gulf of Suez rift, respectively. The evolution of this zone started by reactivation of the NE oriented late Neoproterozoic fractures that controlled the occurrence of Dokhan Volcanics in the rift shoulders. Later, the Syrian Arc contraction reactivated these fractures by a sinistral transpression during the Late Cretaceous–Eocene time. N64°E extension of the Oligo-Miocene rift reactivated the NE fractures by a sinistral transtension. During this rifting, the NE trending faults forming the transfer zone were more active than the rift-bounding faults; the Upper Cretaceous reverse faults in the blocks lying between these NE trending faults were rotated; and drape-related reverse faults and the positive flower structures were formed. Tectonic inversion from contraction to extension controlled the distribution and thickness of the Upper Cretaceous–Miocene rocks.

Localization of mineralization, its age, and relationship to magmatism at the Mogot silver-base-metal deposit, North Stanovoi metallogenic zone in the southeastern framework of the North Asian Craton

The results of studying the Mogot silver-base-metal deposit located in the Dzhugdzhur-Stanovoi Superterrane are discussed in this paper. The main ore-controlling structural elements of the studied district are near-latitudinal and NE-trending faults, which are accompanied by zones of hydrothermal metasomatic potassic, propylitic, and argillic alterations, breccias with quartz and quartz-carbonate cement replacing metamorphic rocks and granitoids of the Late Stanovoi Complex. The total sulfide content in ore is 2–3%. The high Ag, Pb, and Zn contents in ore allow us to consider the Mogot deposit as silver-base-metal, since except of orebody 4, there are no silver minerals proper. This indicates that silver is incorporated into crystalline lattice of sulfides. The results of 40Ar/39Ar geochronological investigations show that the hydrothermal ore deposition dated at 127–125 Ma was related to emplacement of intrusions pertaining to the Tynda-Bakaran Complex.

Analysis of Geological and Geophysical Datasets for Radioelement Exploration in Kab Amiri Area, Central Eastern Desert, Egypt

Detailed geological mapping description of the different granitic types of Kab Amiri pluton were integrated with geophysical (ground gamma ray spectrometry and aeromagnetic) data to identify and map the distribution of radioelement concentrations as well as delineate structural trends that control the mineralized zones in the study area. The geologic studies revealed the existence of four granite varieties namely; biotite granite, garnet two mica granite, muscovite granite, and episyenite. The structural interpretation of geological and geophysical data revealed two intersecting sets of NW and NE-trending faults. The NW-trending set of faults is younger than the NE-trending set where the latter is dissected and displaced by the former. Interpretations of radiometric data revealed the presence of radioactive anomalies southwards that indicate an original increase in uranium contents in the same direction. It could be related to the prevailing NE and ENE fault trends. The southern anomalous zone is related to a very high episyenitization and uranium mobilization. It exhibits a sharp increase in the eU concentrations reaching 370 ppm resulting in a high eU/eTh ratio, approaching 4.5 and, characterized by oval shape trending in the NE direction. Some areas along the peripheral part of Kab Amiri pluton are episyenitized and show a remobilization of uranium. These areas are mostly controlled by faults mainly trending ENE. In the case of the episyenite zone, strong mobilization of uranium is expected to host epigenetic uranium occurrences because more uranium is available. It is obvious that the episyenite rocks of high permeability have higher levels of uranium and their ratios. Thus, these rocks act as an important reservoir for U- mineralization bearing solutions.

Duration of the formation and sources of the granitoids of the Litsk-Araguba Complex, Kola Peninsula

Postorogenic granitoids of the Litsk-Araguba Complex compose a chain of intrusive bodies around 850 km2 in area, which are confined to the NE-trending deep-seated fault zone. Results of U-Pb zircon dating indicate that the formation of granitoids of the Litsk-Araguba Complex lasted 28 ± 9 Ma. Note that the rocks of the first-fourth phases have similar age within (1774–1762 Ma), while quartz syenites of the fifth phase were formed much later (1746 ± 8 Ma). The study of Sm-Nd isotopic system revealed that the quartz syenites plot in the field of the Nd isotopic evolution of the lower crust represented mainly by the Paleoproterozoic garnet granulites with model ages TNd(DM) = 2.4–2.7 Ga and ɛNd(T) from −5.6 to −6.3. It was found that the near-contact syenites of the Litsk Massif contain composite zircons with an age of 1758 ± 9Ma. They differ from zircons in coeval porphyraceous granites in lowered U and Th concentrations, which are close to those in zircons from the lower crustal garnet granulites of this region. These data in combination with internal structure of the crystals determine xenogenic lower-crustal origin of zircons from syenites and confirm geochemical data on the lower crustal input in the formation of granitoid melts.