Nappe Structures Research Articles

Application of kinematic vorticity and gold mineralization for the wall rock alterations of shear zone at Dungash gold mining, Central Eastern Desert, Egypt

The use of porphyroclasts rotating in a flowing matrix to estimate mean kinematic vorticity number (Wm) is important for quantifying the relative contributions of pure and simple shear in wall rocks alterations of shear zone at Dungash gold mine. Furthermore, it shows the relationship between the gold mineralization and deformation and also detects the orientation of rigid objects during progressive deformation. The Dungash gold mine area is situated in an EW-trending quartz vein along a shear zone in metavolcanic and metasedimentary host rocks in the Eastern Desert of Egypt. These rocks are associated with the major geologic structures which are attributed to various deformational stages of the Neoproterozoic basement rocks. We conclude that finite strain in the deformed rocks is of the same order of magnitude for all units of metavolcano-sedimentary rocks. The kinematic vorticity number for the metavolcanic and metasedimentary samples in the Dungash area range from 0.80 to 0.92, and together with the strain data suggest deviations from simple shear. It is concluded that nappe stacking occurred early during the underthrusting event probably by brittle imbrication and that ductile strain was superimposed on the nappe structure during thrusting. Furthermore, we conclude that disseminated mineralization, chloritization, carbonatization and silicification of the wall rocks are associated with fluids migrating along shearing, fracturing and foliation of the metamorphosed wall rocks.

The geological and geodynamic condition on the formation of the Dabashan thrust nappe structure: Based on FLAC numerical modelling

The Dabashan thrust nappe structure at the southern margin of the Qinling orogenic belt suffered at least two stages of evolution which are Late Triassic plate subduction collisional orogeny between North China block, Qinling micro-plate and Yangtze block followed by intracontinental orogeny since the Meso-Cenozoic. A prominent topography characteristic within the Dabashan area is a southwestward extrusive arc (Bashan Arc fault) that is one of key factors to understand the geodynamic condition of the Dabashan thrust nappe structure. In this work, two-dimensional plan-view models are constructed to simulate the collisional and intracontinental orogenic movements, and the factors that may control the formation of the Bashan Arc fault are analysed. The modelling results show that the compressive stress produced by the plates collision along both north and south boundaries is the main driving force. The dextral shearing derived from the inconsistent shape on the block margins is the main controller. Rigid tectonic units such as Bikou and Hanan-Micangshan terranes, Foping and Wudang domes, as well as Shennongjia-Huangling anticline also contribute as “anchor” effects. Additionally, the rheology properties of rock material in the Dabashan area affect the shape of the arc. Condición geológica y geodinámica para la formación estructural de la falla de cabalgamiento en las montañas Dabashan basada en el modelo numérico del software FLAC ResumenLa estructura de la falla de cabalgamiento de las montañas Dabashan en el margen sur del cinturón orogénico de Qinling sufrió por lo menos dos etapas de evolución, la colisión orogénica del Triásico Superior entre el bloque de la China del Norte, la microplaca de Qinling y el bloque Yangtze, y la orogénesis intracontinental desde el Meso-Cenozoico. Una característica topográfica prominente del área de Dabashan es un arco extrusivo (falla Arco de Bashan) hacia el suroeste, que es un factor determinante para entender la condición geodinámica de la falla de cabalgamiento en las montañas Dabashan. En este trabajo se construyeron modelos bidimensionales planos para simular los movimientos de colisión e intracontinental orogénicos y se analizaron los factores que podrían controlar la formación de la falla del Arco de Bashan. Los resultados del modelado muestran que el esfuerzo de compresión producido por las placas de colisión en los límites norte y sur es la principal fuerza impulsora de la falla. La principal controladora es la fuerza de cizallamiento dextral derivada de la forma inconsistente en los margenes del bloque. Las unidades tectónicas rígidas como los terrenos Bikou y Hanan- Micangshan, el domo Foping y Wudang, al igual que el anticlinal Shennongjia-Huangling tienen funciones de ancla. Adicionalmente, las propiedades reológicas del material rocoso en el área Dabashan afectan la forma del arco.

Active faults and seismogenic models for the Urumqi city, Xinjiang Autonomous Region, China

We have studied the characteristics of the active faults and seismicity in the vicinity of Urumqi city, the capital of Xinjiang Autonomous Region, China, and have proposed a seismogenic model for the assessment of earthquake hazard in this area. Our work is based on an integrated analysis of data from investigations of active faults at the surface, deep seismic reflection soundings, seismic profiles from petroleum exploration, observations of temporal seismic stations, and the precise location of small earthquakes. We have made a comparative study of typical seismogenic structures in the frontal area of the North Tianshan Mountains, where Urumqi city is situated, and have revealed the primary features of the thrust-fold-nappe structure there. We suggest that Urumqi city is comprised two zones of seismotectonics which are interpreted as thrust-nappe structures. The first is the thrust nappe of the North Tianshan Mountains in the west, consisting of the lower (root) thrust fault, middle detachment, and upper fold-uplift at the front. Faults active in the Pleistocene are present in the lower and upper parts of this structure, and the detachment in the middle spreads toward the north. In the future, M7 earthquakes may occur at the root thrust fault, while the seismic risk of frontal fold-uplift at the front will not exceed M6.5. The second structure is the western flank of the arc-like Bogda nappe in the east, which is also comprised a root thrust fault, middle detachment, and upper fold-uplift at the front, of which the nappe stretches toward the north; several active faults are also developed in it. The fault active in the Holocene is called the South Fukang fault. It is not in the urban area of Urumqi city. The other three faults are located in the urban area and were active in the late Pleistocene. In these cases, this section of the nappe structure near the city has an earthquake risk of M6.5–7. An earthquake MS6.6, 60 km east to Urumqi city occurred along the structure in 1965.

Identification of new NE-trending deep-seated faults and tectonic pattern updating in northern Tunisia (Mogodos–Bizerte region), insights from field and seismic reflection data

The northern Tunisia is occupied by the Tellian domain constituent the eastern end of the Maghrebides, Alpine fold–thrust belt. Study area includes partially the Tellian domain (Mogodos belt) and its foreland (Bizerte region). Most of this region outcrops consist of Numidian thrust sheet flysch attributed to the lower Oligocene–Burdigalian. In the study area, the major fault systems are still subject of discussion. The Numidian nappe structure, the distribution of basalt and Triassic outcrops within and at the front of this Tellian domain deserve more explanation. In this work we intend to update the structural scheme and the tectonic evolution of the northern Tunisia, taking into account salt tectonics and magmatism. The updated tectonic evolution will be integrated in the geodynamic framework of the Central Mediterranean. For this purpose, we have analyzed morphologic, seismic and structural data. The compilation of the results has allowed the identification of new regional NE-trending faults dipping towards the NW: the Bled el Aouana–Bizerte, the Sejnane–Ras Enjla and the Oued el Harka faults. They correspond to the reactivation of deep-seated normal faults splaying on the Triassic evaporites. This fault system constitutes the main component of the northern Tunisia structural scheme and has influenced its tectonic evolution marked by the main following stages. The Tellian thrust-sheets were immobilized at the uppermost Langhian. During the major Tortonian NW-trending compressive phase, these faults were reactivated with reverse kinematics and controlled the distribution of the post-nappes Neogene continental deposits. At the early Pleistocene, a compressive NNW-trending event has reactivated again these faults with sinistral-reverse movements and deformed the post-nappes Neogene series. Late Quaternary to Actual, the tectonic regime continues to be compressive with a NNW-trending maximum horizontal stress.

Hydrocarbon exploration in the South Yellow Sea based on airborne gravity, China

Differences between the Cenozoic and pre-Cenozoic strata and structures in the middlesouth South Yellow Sea are analyzed using high-resolution airborne gravity data combined with data from offshore wells, seismic exploration and the regional geological background, using forward and inverse methods. We discuss why hydrocarbon exploration has so far failed in the South Yellow Sea, and put forward alternative future exploration plans. The results show that there are thick Cenozoic strata over Jurassic–Cretaceous continental strata in the southern basin of the South Yellow Sea, contrasted with thick Mid-Paleozoic marine strata preserved in the middle uplift area. In the mid-southern South Yellow Sea, airborne Bouguer gravity anomalies are fragmentized with scattered local anomalies. Many tensile normal faults and minor fault blocks occur in the Cenozoic strata. In contrast, reverse faults and nappe structures are found in pre-Cenozoic strata. The essential reasons for the lack of a breakthrough in hydrocarbon exploration are the complexity of geological structures, hydrocarbon accumulation environments, seismic-geologic conditions, and the difficulty of interpreting and understanding these features, rather than an imbalance of exploration framework. Hydrocarbon exploration should be targeted at Mid-Cenozoic continental strata, especially of Paleogene age, in the middle to southern parts of the South Yellow Sea. Special attention should be paid to the favorable structural belts SYI1, SYI2, SYI3 and SYI4 in the southern basin of the South Yellow Sea, where detailed seismic exploration or drilling should be carried out. Attention should also be given to Mid-Paleozoic marine strata in the middle uplift area of the South Yellow Sea. Key strategies that could lead to a hydrocarbon exploration breakthrough are improving exploration resolution, conducting detailed studies of fine structures, and accurately locating minor structures.

3D reflection seismic imaging at the 2.5 km deep COSC-1 scientific borehole, central Scandinavian Caledonides

The 2.5km deep scientific COSC-1 borehole (ICDP 5054-1-A) was successfully drilled with nearly complete core recovery during spring and summer of 2014. Downhole and on-core measurements through the targeted Lower Seve Nappe provide a comprehensive data set. An observed gradual increase in strain below 1700m, with mica schists and intermittent mylonites increasing in frequency and thickness, is here interpreted as the basal thrust zone of the Lower Seve Nappe. This high strain zone was not fully penetrated at the total drilled depth and is thus greater than 800m in thickness.To allow extrapolation of the results from downhole logging, core analysis and other experiments into the surrounding rock and to link these with the regional tectonic setting and evolution, three post-drilling high-resolution seismic experiments were conducted in and around the borehole. One of these, the first 3D seismic reflection land survey to target the nappe structures of the Scandinavian Caledonides, is presented here. It provides new information on the 3D geometry of structures both within the drilled Lower Seve Nappe and underlying rocks down to at least 9km.The observed reflectivity correlates well with results from the core analysis and downhole logging, despite challenges in processing. Reflections from the uppermost part of the Lower Seve Nappe have limited lateral extent and varying dips, possibly related to mafic lenses or boudins of variable character within felsic rock. Reflections occurring within the high strain zone, however, are laterally continuous over distances of a kilometer or more and dip 10–15° towards the southeast. Reflections from structures beneath the high strain unit and the COSC-1 borehole can be followed through most of the seismic volume down to at least 9km and have dips of varying degree, mainly in the east–west thrust direction of the orogen.

Geology of the Jiama porphyry copper–polymetallic system, Lhasa Region, China

The Jiama deposit, located in the eastern part of the well-known Gangdese Metallogenic Belt on the Tibetan Plateau, is the largest porphyry Cu–polymetallic system in the region, with the largest exploration budget, and is economically viable in the Gangdese Belt to undergo large-scale development. The deposit is well preserved and has experienced little erosion. The proven resources of the deposit are 7.4Mt Cu, 0.6Mt Mo, 1.8Mt Pb+Zn, 6.65Moz Au, and 360.32Moz Ag. The results presented in this paper are based on geological and tectonic mapping, geological logging, and other exploration work performed by members of the Jiama Exploration Project Team over a period of 6years. We propose that the Jiama porphyry Cu–polymetallic system is composed of skarn Cu–polymetallic, hornfels Cu–Mo, porphyry Mo±Cu, and distal Au mineralization. The development of skarn Cu–polymetallic orebodies at the Jiama deposit was controlled mainly by the contact zone between porphyries and marbles, an interlayer detachment zone, and the front zone of a gliding nappe structure. The hornfels Cu–Mo and porphyry Mo±Cu orebodies were controlled mainly by a fracture system related to intrusions, and the distal Au mineralization resulted from late-stage hydrothermal alteration.On the basis of field geological logging, optical microscopy, and chemical analysis, we verify that the alteration zones in the Jiama deposit include potassic, phyllic, propylitic, and argillic alteration, with a local lithocap, as well as endoskarn and exoskarn zones. The endoskarn occurs mainly as epidote alteration in quartz diorite porphyry and granite porphyry, and is cut by massive andradite veins. The exoskarn includes garnet–pyroxene and wollastonite skarn, in which the mineralogy and mineral chemical compositions display an outward zonation with respect to the source porphyry. From the proximal skarn to the intermediate skarn to the distal skarn, the garnet/pyroxene ratio varies from >20:1 to ~10:1 to ~5:1, the garnet color varies from red-brown to brown-green to green-yellow, and the average composition of garnet varies from Ad80.1Gr18.9(Sp+Py)1.0 to Ad76.3Gr23(Sp+Py)0.7 to Ad59.5Gr39.5(Sp+Py)1.0, respectively. The pyroxene is not as variable in composition as the garnet, and is primarily light green to white diopside with a maximum hedenbergite content of ~20% and an average composition of Di88.6Hd8.9Jo2.5. From the proximal skarn to the intermediate skarn to the distal skarn, the mineralization changes from Cu–Mo to Cu±Mo to Pb–Zn±Cu±Au ores, respectively. The wollastonite skarn displays no zonation and hosts mainly bornite mineralization. The Cu and Mo mineralization is closely related to the potassic and phyllic zones in the porphyry–hornfels.Zircons from four mineralized porphyries yield U–Pb ages of 15.96±0.5Ma, 15.72±0.14Ma, 15.59±0.09Ma, and 15.48±0.08Ma. The Re–Os ages of molybdenite from the skarn, hornfels, and porphyry are 15.37±0.15Ma, 14.67±0.37Ma, and 14.66±0.27Ma, respectively. The present results are consistent with the findings of previous research on fluid inclusions, isotopes, and other such aspects. On the basis of the combined evidence, we propose a porphyry Cu–polymetallic system model for the Jiama deposit and suggest a regional exploration strategy that can be applied to prospecting for porphyry-skarn mineralization in the Lhasa area.

Application of kinematic vorticity techniques for mylonitized Rocks in Al Amar suture, eastern Arabian Shield, Saudi Arabia

The rotation of rigid objects within a flowing viscous medium is a function of several factors including the degree of non-coaxiality. The relationship between the orientation of such objects and their aspect ratio can be used in vorticity analyses in a variety of geological settings. Method for estimation of vorticity analysis to quantitative of kinematic vorticity number (Wm) has been applied using rotated rigid objects, such as quartz and feldspar objects. The kinematic vorticity number determined for high temperature mylonitic Abt schist in Al Amar area, extreme eastern Arabian Shield, ranges from ∼0.8 to 0.9. Obtained results from vorticity and strain analyses indicate that deformation in the area deviated from simple shear. It is concluded that nappe stacking occurred early during an earlier thrusting event, probably by brittle imbrications. Ductile strain was superimposed on the nappe structure at high-pressure as revealed by a penetrative subhorizontal foliation that is developed subparallel to tectonic contacts versus the underlying and overlying nappes. Accumulation of ductile strain during underplating was not by simple shear but involved a component of vertical shortening, which caused the subhorizontal foliation in the Al Amar area. In most cases, this foliation was formed concurrently with thrust sheets imbrications, indicating that nappe stacking was associated with vertical shortening.

Integrated Remote Sensing and Structural Analysis Studies of Tayyib Al-Ism Area, Northwestern Arabian Shield, Saudi Arabia

Geological and structural mappings of Tayyib Al-Ism area were carried out using the rocks finite strain data, the Landsat-7 Enhanced Thematic Mapper Plus (ETM+) data and the field based observations. To analyze the finite strain in the studied rocks, the Rf /ϕ and Fry methods are applied to feldspar porphyroclasts and mafic grains from nine metavolcano-sedimentary samples (Hegaf Formation), four diorite-gabbros suite samples (Sawawin Complex), two meta-granite samples (Ifal suite) and five Zuhd alkali granite samples. The obtained data indicate traces of high to moderate level of deformation in the meta-granite and metavolcano-sedimentary rocks. The axial ratios along the XZ section range from 1.70 to 4.80 for the Rf/ϕ method and from 1.50 to 4.50 for the Fry method. A sub-vertical trend of short axes in association with sub-horizontal foliation is also observed. These informations allow us to conclude that a finite strain in the deformed granitic rocks is of the same order of magnitude as in the metavolcano-sedimentary rocks. The contacts between the metavolcano-sedimentary and granitic rocks in Tayyib al Ism area were formed during the granitic intrusions along some of the faults under brittle to semi-ductile deformation conditions. These faults have significantly influenced the geometry and style of rifting in the Red Sea during the Neogene. The finite strain was accumulated in the area during the process of deformation, which superimpose the already existed nappe structure. It indicates that the nappe contacts formed during the accumulation of finite strain. In addition to finite strain analysis, band ratio images (3/1, 5/3, 7/5) and Principal Component Analysis (PCA) technique have been used, which proved effective in mapping geological and structural features of various rock bodies exposed in the study area.

Exceptional Tl-bearing manganese oxides from Zalas, Krakow area, southern Poland

Abstract In the Permian rhyodacite quarry at Zalas near Krakow, southern Poland, thallium-bearing Mn oxides occur in a small fault zone cutting Middle Jurassic sandy limestone poorly encrusted by an oxidized polymetallic mineralization. The encrustation comprises sulphides (pyrite, chalcopyrite, chalcocite, covellite, galena, marcasite), native bismuth, hematite, goethite, cuprite, mottramite, iodargyrite, unrecognized Cu sulphates and Bi oxychlorides as supergene minerals, barite and rare tiny grains of gold. It is most likely connected with rejuvenation of Early-Paleozoic faults during the Alpine orogeny on the Oligocene–Miocene boundary. Rare Tlbearing Mn oxides occur in an outside zone of the encrustations, filling small fractures and voids in limestone forming the fault breccia. Tl contents, reaching 20.82wt% as Tl2O, exceed by more than two orders of magnitude those reported in similar minerals before, making the oxides unique on a world scale. The Tl-bearing Mn oxides from Zalas reflect intensive weathering of an older Tl-bearing sulphide mineralization in an arid climate, involving saline fluids delivered to the groundwater system as the nappe structure of the Carpathians was developing during the Sava tectonic phase Oligocene/Miocene boundary.

The Tuscan Nappe structures in the Monte Amiata geothermal area (central Italy): a review

The present knowledge on the structures affecting the Tuscan Nappe in the Monte Amiata area is here presented, highlighting that the tectonic evolution of the area is incorporated in the inner Northern Apennines framework. In fact, field and subsurface data indicate that the Tuscan Nappe is internally characterized by tectonic doublings, occurred during the collisional stage. Subsequently, as well as in the whole southern Tuscany, Miocene extension determined the lateral segmentation of the Tuscan Nappe; in the Mt. Amiata area, it resulted in three isolated geological bodies, partly cropping out. During Pliocene, the Tuscan Nappe was cross-cut by normal to oblique faults which contributed to channel the hydrothermal fluids that gave rise to the Pleistocene Hg-Sb ore deposits and Present geothermal resources. As it regards the deformation ages, since the late Oligocene-Aquitanian Macigno Fm is involved in the thrusts, the internal stacking of the Tuscan Nappe is related to the post-Aquitanian and pre-Langhian time period, on the basis of the Langhian sediments deposited in an extensional setting and presently recognised underneath the sediments of the nearby Pliocene Radicofani extensional Basin. It is therefore concluded that the switch from compression to extension occurred in a time span encompassed between early Burdigalian and early Langhian.

Ediacaran to Cambrian oceanic rocks of the Gondwana margin and their tectonic interpretation

In tectonic maps of Variscan Europe, allochthonous pieces of Cadomian basement clearly stand out with their predominant metabasic to ultrabasic elements, the so-called exotic terranes with ophiolites. Most of these domains are observed in basements of the Central Iberian Allochthone, the South Armorican domain, the nappe structures of the French Massif Central, the Saxothuringian Zone and the Bohemian Massif. Similar relics can be recognized in many Alpine basement areas, and correlations with supposedly more autochthonous basements, such as the Ossa Morena Zone and the Central Iberian basement, can be envisaged. All of these relics are thought to represent the interrupted trace of a former continuous or discontinuous structure, characterized by the presence of ocean-derived proto-Rheic rock suites. These can be interpreted as pieces of former magmatic arcs of Ediacaran to Cambrian age accreted to the Gondwana margin, which later were scattered as allochthonous units during the Variscan plate-tectonic processes. The presence of similar rock suites of Ordovician age in the Alpine realm is explained by the accretion of exotic China-derived basements and their collision with the Gondwana margin during the opening of the Rheic Ocean.

Geochronology of the Granitoids Above Nappe Structure in Nanling Scientific Drilling

Geochronology of the Granitoids Above Nappe Structure in Nanling Scientific Drilling

The Alpstein in three dimensions: fold-and-thrust belt visualization in the Helvetic zone, eastern Switzerland

To investigate the geometrical relationships between folding and thrust faulting, we built a 3D geological model of the Helvetic fold-and-thrust belt in eastern Switzerland from several existing and two newly drawn cross-sections in the Säntis area. We partly redrew existing cross-sections and validated them by checking for line length balance; to fill areas with no data we drew additional cross-sections. The model was built based on surface interpolation of the formation interfaces and thrusts between the cross-sections, which allowed generating eight main surfaces. In addition, we used cave data to validate the final model in depth. The main structural elements in the Säntis area, the Säntis Thrust and the Sax-Schwende Fault, are also implemented in the model. The result is a 3D structural model of the area, which provides an intuitive way for examining a portion of a complex structural nappe. The 3D model highlights the shapes of the main anticline-syncline pairs and how these fold trains vary laterally in amplitude and wavelength. It shows how lateral variations in fold style correlate with regional shortening gradients as determined from line-length balancing. The model also clearly shows the lateral extension, the trend, and the variation in displacement along the principal faults. The reconstruction of horizons in 3D allows the investigation of cross-sections in any given direction. The 3D model is useful for developing and understanding how the internal nappe structures, namely folds and thrust faults, change along strike due to palaeogeographic and stratigraphic variations. Lateral stratigraphy variations correlate with different deformation responses of the nappe. Changes can occur either abruptly across transverse faults or in a more gradual manner.

Finite strain analysis of metavolcanics and metapyroclastics in gold-bearing shear zone of the Dungash area, Central Eastern Desert, Egypt

The Dungash gold mine area is situated in an EW-trending quartz vein along a shear zone in metavolcanic and metasedimentary host rocks in the Eastern Desert of Egypt. These rocks are associated with the major geologic structures, which are attributed to various deformational stages of the Neoproterozoic basement rocks. Field geology, finite strain and microstructural analyses were carried out and the relation-ships between the lithological contacts and major/minor structures have been studied. The Rf/ϕ and Fry methods were applied on the metavolcano-sedimentary and metapyroclastic samples from 5 quartz veins samples, 7 metavolcanics samples, 3 metasedimentary samples and 4 metapyroclastic samples in Dungash area. Finite-strain data show that a low to moderate range of deformation of the metavolcano-sedimentary samples and axial ratios in the XZ section range from 1.70 to 4.80 for the Rf/ϕ method and from 1.65 to 4.50 for the Fry method. We conclude that finite strain in the deformed rocks is of the same order of magnitude for all units of metavolcano-sedimentary rocks. Furthermore, the contact between principal rock units is sheared in the Dungash area under brittle to semi-ductile deformation conditions. In this case, the accumulated finite strain is associated with the deformation during thrusting to assemble nappe structure. It indicates that the sheared contacts have been formed during the accumulation of finite strain.

Mechanical genesis of Henan (China) Yima thrust nappe structure

Considering the serious coal and rock dynamic disasters around the main slip plane called F16 in the coal mining area) of Henan Yima (China) thrust nappe structure, the mechanical genesis of the Yima thrust nappe structure was studied comprehensively using geomechanics, fault mechanics, elastic mechanics, and Coulomb’s law of friction. First, using the centrifugal inertia force of Earth’s rotation as a source, a mechanical model of N-S compression superimposed with W-E reverse torsion was established to explain the formation of the early Yima coal basin and Jurassic Yima Group coal measures. Second, an equation for the ultimate stress in the forming stage of F16 was derived using the plastic slip-line field theory and the parabolic Mohr failure criterion. Moreover, the distribution of ultimate stress and the geometric characteristics of the fault profile were obtained using the field model parameters. Finally, the stress field of F16 and the mechanical genesis of the large-scale reverse thrust sheet were discussed based on elastic mechanics theory and Coulomb’s law of friction. The results show that the tectonic framework of the early Yima coal basin and the formation pattern of Jurassic Yima Group coal measures given by the model are consistent with the in-situ explorations. The geometric characteristics of the fault profile obtained by numerical calculation can better reflect the shape of F16 in its forming stage, and the mechanical genesis of the large-scale reverse thrust sheet also concurred with the field situations. Thus, this work can provide a foundation for further studies on the genesis of the thrust nappe structure, the mechanism of rock bursts induced by F16, and the characteristics of the residual stress field in the Yima mining area.

Late Cretaceous Sub-Marine Fan System in Batain Mélange Zone, the Fayah Formation in Northeastern Oman

The Batain coast along the northeastern margin of Oman between Ra’s Al-Hadd and Ra’s Jibsch, is comprised of Permian to Late Cretaceous complex stratigraphy in a tectonically deformed area recording Permian rifting to late Cretaceous Tethys closure events. These rocks are thrust over Mesozoic and older autochthonous sedimentary cover in the form of a major nappe structure known as the Batain Nappe. The uppermost part of the Batain nappe is comprised of isolated outcrops of early Maastrichtian siliciclastic Fayah Formation dominated by gravity flow deposits. The Fayah Formation in the Jabal Fayah area is over four hundred meters thick and comprised of five distinct facies associations; namely, i) coarsening-up sandstone, ii) conglomerate, iii) debris- flow, iv) turbidite, and v) inter-bedded sandstone and shale lithofacies. These lithofacies associations are repeated many times in the section. The sandstone lithofacies association exhibits a coarsening-upward trend making sequences tens of meters thick in various parts of the formation. Waterscape structures are common along with occasional sandstone dykes and convolute bedding, reflecting fluidized conditions of deposition. The conglomerate lithofacies association is comprised of a series of interbedded coarsening-upward pebble to gravel size conglomerates containing chert, limestone, granite and volcanic clasts ranging a few mm to cm in diameter. Occasionally these are interbedded with sandstone lithofacies. The conglomerate lithofacies was deposited by a high-energy channelized flow in a sub-aqueous setting. The debris-flow lithofacies association is a matrix supported chaotic mixture of clay and boulders of granite, limestone and volcanic rocks, some of which are meter-sized in diameter, and possibly derived from the nearby basement rocks such as the Jabal Ja’alan basement rocks. It constitutes the most dominant part of the formation. These sediments were deposited along a slope setting, possibly as olistostrome formed due to submarine slumping and sliding. The turbidite lithofacies association is comprised of monotonous grayish-green to brown coloured clays tens of meters thick interbedded with thin, clean, well-sorted sandstone. The Interbedded sandstone and shale lithofacies association is comprised of a half to one meter thick cross-bedded, burrowed arkosic sandstone and plane laminated shale. The sandstone constitutes about 25% of the association with ripple lamination in the upper part of the unit indicating a fining-upward trend. Dewatering structures are common. This association constitutes the upper 100m of the formation. These sediments were deposited in shallow water conditions by channelized flows. Based on the lithofacies associations described above, especially the dominance of debris-flow units and turbidites, the greater part of the Fayah Formation are interpreted as having been deposited under a sub-marine fan setting. Only the upper part of the formation was deposited in a shallow water setting before the onset of overlying carbonate deposits. The sub-marine fan system was active during the last stages of the Tethys Ocean closure at the time of onset of the Batain nappe.

Kinematic analysis of the Migif area in the Eastern Desert of Egypt

The use of porphyroclasts rotating in a flowing matrix to estimate mean kinematic vorticity number (Wm) is important for quantifying the relative contributions of pure and simple shear in penetratively deformed rocks. The kinematic vorticity number determined for high temperature mylonitic gneisses in the Migif area in the Eastern Desert of Egypt range from ∼0.6 to 0.9. The results from vorticity and strain analyses indicate that deformation in the area deviated from simple shear. It is concluded that nappe stacking occurred early during the thrusting event, probably by brittle imbrications, and that ductile strain was superimposed on the nappe structure at high-pressure as shown by a penetrative subhorizontal foliation is developed subparallel to the tectonic contacts with the under- and overlying nappes. The accumulation of ductile strain during underplating was not by simple shear but involved a component of vertical shortening, which caused the subhorizontal foliation in the Migif area. In most cases, this foliation was formed during thrusting of the nappes onto each other, suggesting that nappe stacking was associated with vertical shortening.

The Late Mesozoic tectonic evolution and magmatic history of west Zhejiang, SE China: implications for regional metallogeny

The granitoids and related polymetallic mineralization in the Zhejiang Province at the southeast margin of the Yangtze Block in China provide an important window to evaluate metallogeny associated with convergent margin magmatism. Here, we present geochronological, geochemical, and isotopic data from the granitic rocks of west Zhejiang, to constrain the timing of transformation of the tectonic setting of this region from volcanic arc to intra-plate during Late Mesozoic and its bearing on regional metallogeny. The granitic rocks in west Zhejiang can be geochemically subdivided into two groups. The first group is characterized by relatively steep rare earth element (REE) patterns with slight Eu anomalies, high Sr, low Yb, and negative Nb–Ta–Ti (NTT) anomalies, indicating a volcanic arc environment with a thickened crust in a convergent setting. The second group is featured by flat REE patterns with prominent negative Eu anomalies, low Sr, high Yb, and weak NTT anomalies, suggesting an intra-plate extensional environment with a thin crust. The geochronology of granitic rocks in west Zhejiang, combined with ages of regional tectonic basins and nappe structures, constrains the timing of the tectonic transformation to be in the range from 150 to 140 Ma. Sr–Nd isotopic data and a positive correlation displayed by oxygen fugacity (fO2), and La/Sm and Ba/Th ratios (proxies of subducted sediments and slab dehydration fluids) suggest that the high oxygen fugacity is probably related to the melting of subducted sediments and slab dehydration. From 180 to 80 Ma, due to the increasing dip angle of the subducted Izanagi Plate, the volcanic arc belt migrated oceanward, leaving most of the interior of Zhejiang Province under an intra-plate environment where insufficient subducted components and upwelling mantle generated reduced magmas which were not favorable for Cu–Mo mineralization. Our model provides a plausible explanation for the absence of Cu–Mo porphyry deposits in the adjacent region of Zhejiang, Jiangxi, and Anhui provinces (Zhe-Gan-Wan region) after 140 Ma.

Structural analysis for metavolcanics and their metapyroclastics at gold deposit of the Mahd Ad Dahab area, Arabian Shield, Saudi Arabia

The present study focuses on the gold mining in Mahd Ad Dahab region of Saudi Arabia. The study aims to assess the spatial relationship between tectonic contacts in Mahd Ad Dahab area and to provide a meaningful hypothesis relating gold metallogeny to the evolution of the Arabian Shield. Distribution and localization of gold occurrences in the study area was envisaged based on the different styles of microstructures and the major deformation phases affecting the area. The detailed petrographical and mineralogical investigations indicate that the metavolcanic rocks at the Mahd Ad Dahab gold mine area can be classified into metabasalt, metaandesite, and the felsic varieties (metadacite, metarhyodacite and metarhyolite) associating their metapyroclastics (conglomerate and tuffs). Furthermore, quartz forms allotriomorphic crystals which exhibit wavy extinction, deformational lamina and foliation due to subsequent deformations. Furthermore, we conclude that finite strain in the deformed rocks is of the same order of magnitude for all units of metavolcano-sedimentary rocks. Furthermore, contacts formed during intrusion of plutons with some faults in the Mahd Ad Dahab area under brittle to semi-ductile deformation conditions. In this case, finite strain accumulated during superimposed deformation on the already assembled nappe structure. It indicates that the contacts formed during the accumulation of finite strain.