ENE WSW Research Articles

The Effects of Structural Elements on Groundwater of Wadi Yalamlam, Saudi Arabia Using Integration of Remote Sensing and Airborne Magnetic Survey

The present study aimed to develop an integrated approach that utilized field geology, remote sensing and airborne magnetic data to identify the Quaternary aquifers impounded by dykes and explain the dykes/lineaments relationship and their effects on the storage and flow of the groundwater of Sadiyah sub-basin, lower part of Wadi Yalamlam basin, Western Arabian Shield, Saudi Arabia. Field visits supported by Landsat ETM + ratio images (5/7, 5/4 and 5/1) revealed the presence of three main dyke types that can be chronologically arranged from older to younger as: Sheared mafic and acidic dykes (D1); Damm dyke complex (D2); and Ghumayqah dykes (D3). Two dykes referred to the first type (D1) were traced with depth using airborne magnetic survey crosscutting the main course of Wadi Yalamlam around Sadiyah village at high angle. According to water table elevation measurements, the aquifer of lower Wadi Yalamlam area is divided into three parts namely I, II and III. The water table elevations decreased from 158 m in the first part (I) to reach 66 m in the third part (III) of the aquifer. This drop is attributed to the effect of dykes which act as natural barriers. The study area was affected by three major fault systems as confirmed by field study and total magnetic map. Two of these systems have NE–SW to ENE–WSW and N–S to NNW–SSE directions, whereas the third one represents the Najd fault system trends in NW–SE direction. The present study identifies two locations valid for agricultural expansion lie just to the north of Sheikh Said and Abu Helal Farms.

Investigating deep geological reservoirs using seismic reflection and well logs, Tawila oil field, Yemen: Implications for structural setting and reservoir properties

Seismic reflection data were used to evaluate the deep geological structures in Tawila oil field, Masila rift basin, Yemen. A combined set of 2D seismic profiles and geophysical well logs were used to produce structural mapping at the top of the clastic Qishn reservoir and the several overlying and underlying horizons. We have investigated the distribution of reservoir fluids in relation to the identified structures. The seismic horizons with their intersecting faults have been determined to be very important in the hydrocarbon distribution. Two key fault systems have been identified to control the hydrocarbon entrapment in the study area: major, deep, E–W normal faulting that affected the northern area, which caused extensive structural deformation, and a much shallower ENE–WSW fault system to south. The shale type and distribution and the basic reservoir properties, were enhanced through analysing the logging data using the standard clastic models. The reservoir is composed of three clastic compartments (S1, S2 and S3) which are separated by thin shale beds. No hydrocarbons were found in the S1 top unit which had considerable shale volume of a dispersed shale type. Considerable to good hydrocarbon content up to 57% and 61%, associated with a structural uplift, was identified in front of the S2 and S3 units, respectively. A dispersed/laminated shale model was assigned for these two units with a laminated style priority. The hydrocarbon entrapment in the study area was caused by the sealing characteristics of the overlying layers as well as the juxtaposition of the reservoir against other seal rocks. The distribution maps show an increase in hydrocarbon saturation toward the west, north and northeast which coincided with areas of low seismic-derived properties of velocity, acoustic impedance and negative reflection coefficient. An elongated structural horst of good hydrocarbon potentiality was identified in the upthrow of the major fault. This prospective structural high is a key to explore hydrocarbons in nearby areas and other basins of similar geologic characteristics.

Topographic divides formed by active flexural folding in the NE marginal zone of the Tibetan Plateau

Topographic divides along the northeastern marginal zone of the Tibetan Plateau separate tributary drainage systems that flow into the Yellow River. Field investigations and topographic analysis indicate that: i) Late Quaternary–Holocene sedimentary sequences are deformed into a series of left-stepping WNW–ESE-trending en echelon flexural folds; ii) the folds form topographic divides that separate drainage systems that flow into the Yellow River; and iii) the regional axis of maximum compressive stress, as inferred from flexural folds and active fault-related structures, trends ENE–WSW, consistent with geophysical and seismic data. These results demonstrate that the topographic divides formed through active flexural folding and faulting in the northeastern marginal zone of the Tibetan Plateau, corresponding to the ongoing northeastward shortening of the plateau accommodating the Eurasia-India continental collision.

New insights into the geology and tectonics of the San Dimas mining district, Sierra Madre Occidental, Mexico

The San Dimas district is a world-class silver-gold low-sulfidation epithermal deposit located in the central part of the Sierra Madre Occidental of Mexico, within the eastern part of the Gulf of California extensional province. Previous works assumed a single period of mineralization between ∼38.5 and 31.9 Ma, which is at odds with the existence of vein systems with two different orientations and Ag/Au ratios. We present a re-evaluation of this district based on new zircon U/Pb and apatite fission-track ages as well as petrographic and field observations of mineralization styles. Our study also includes two new prospective areas of Causita and Mala Noche located to the south of the main district.Within the Lower Volcanic Complex, we identify a Late Cretaceous volcanic succession (∼77 to 69 Ma) correlative with the Tarahumara formation from southern Sonora and coeval with the San Ignacio batholith exposed to the west. This volcanic succession hosts Au-rich mineralized sub-volcanic felsic bodies yielding slightly younger ages and is covered by Paleocene intermediate lavas with hypabyssal intrusions with ages around 48 Ma. A voluminous intrusive suite (Piaxtla batholith and associated dike swarms) was emplaced in the region between ∼49 and 44 Ma. Early extensional basins were filled by a continental sedimentary sequence (Palmas formation), which yielded U/Pb age peaks at 66 and 56 Ma from detrital zircons and a maximum depositional age of ∼43 Ma. The last magmatic activity, as in the rest of the Sierra Madre Occidental, consists of silicic ignimbrites and less basaltic lava flows clustered in two pulses of ∼31.5–29 Ma and 24–20 Ma.NNW–SSE extensional fault systems expose the mineralization and tilted all the succession prior to the emplacement of a ∼24 Ma ignimbrite package. This late Oligocene extension, associated to the early stage of the Gulf of California rift, is confirmed by apatite fission-track dating of samples from the Piaxtla batholith, which consistently indicate an episode of cooling at 25–23 Ma followed by a second episode at ∼12.5 Ma. Our absolute ages and geologic mapping allow to infer that an older, WSW–ENE trending normal fault system with up to 1 km of displacement must exist between the San Dimas district and the Causita area to the south. This fault system, currently buried beneath Oligocene–Miocene ignimbrites, may have controlled the intrusion of the Piaxtla batholith and played a crucial role in the preservation of large vein systems of San Dimas in a tectonic depression setting.The main epithermal mineralization is associated with two kinds of structures: Ag/Au veins with WSW–ENE to E–W orientation and Au/Ag veins with NNW–SSE to N–S orientation. The dominant Ag/Au veins slightly post-date the Piaxtla intrusive suite and partly recycled older felsic intrusion with porphyry mineralization. The NNW–SSE to N–S Au/Ag veins are most probably associated with the Oligocene silicic volcanism.

Structural and sedimentological analysis of the Pliocene–Late Miocene deposits from the Romanian Black Sea offshore based on borehole electrical imaging

This study presents the results of the structural and sedimentological interpretation of borehole electrical imaging data recorded in a gas exploration well from the Romanian offshore area of western Black Sea. The well intercepted on the investigated interval Pliocene to Late Miocene deposits, including a gas reservoir hosted in Early Pliocene shallow marine sands and silts. A total of 824 features were picked on the electrical image, such as bedding, soft sediment deformations, open (conductive) fractures and closed (resistive) fractures. An analysis of the dip angles and azimuths data identified four structural domains within the study interval: two are quasi-horizontal, one shows slightly inclined bedding dips and one is dominantly deformed (slump features and possibly sediment creep). The open fractures show a preferential WNW–ESE strike, whereas the closed fractures show, besides a WNW–ESE strike, mostly ENE–WSW to N–S strikes. The opposite directions of the two fracture sets suggest a relationship to the regional tectonic stress, with open fractures being nearly parallel to maximum horizontal stressSHmaxdirection and closed fractures being nearly normal toSHmaxdirection. The variable bioturbation intensity observed on the electrical image may reflect salinity fluctuations in the Black Sea basin.

Facies architecture of the Cambrian succession at the western margin of Baltica in the Podlasie region (E Poland)

Sedimentary features of the Cambrian-age succession transected in seven borehole cores sited in the Podlasie region document vertical and lateral variations of shallow-marine sedimentary facies, deposited at the rifted western margin of the Baltica Palaeocontinent. The boreholes are distributed along two lines of cross-section (E–W and NE–SW) running roughly perpendicular to the margin of the palaeocontinent. The easternmost borehole represents a proximal setting located on a relatively stable, shallow basement in the east; the remaining boreholes document conditions of deposition in the subsiding shallow-marine basin, extending towards the SW. Fourteen sedimentary facies defined on the basis of their lithological and sedimentary features are interpreted in terms of the sedimentary environments they represent. Strata deposited upon the stable craton in the east document a stratigraphically condensed succession of proximal facies, 240 m thick, whereas a sequence three times thicker is positioned distally, 170 km to the west. Facies associations in the proximal section represent the lower to upper shoreface in the lower part of the section and evolve upwards to the intermediate shoreface. Facies complexes in the remaining, intermediate and distal areas form a symmetrical megasequence, composed of a positive (i.e., fining-upwards – FU) transgressive sequence, overlain by a negative (coarsening-upwards – CU) regressive sequence. The vertical arrangement of the sedimentary subenvironments during the transgression indicates a tidally influenced shoreline followed by oscillations between the swash zone, the upper, intermediate and lower shoreface, and the offshore. The symmetry of the megasequences and the rhythmic pattern of the component facies complexes indicate that the intensity of supply in the terrigenous material and the efficiency of its reworking and redistribution within the basin were similar during the transgression and the regression. The facies types and variations within the basal part of the succession reflect syndepositional movements of tectonic blocks parallel to the rifted basin margin. Differences in total thickness and facies associations between the two lines of cross-section approximately perpendicular to the basin margin indicate that sedimentation was also influenced by a synsedimentary hinge fault, extending in a WSW–ENE direction.

The Sangmelima granite-greenstone belts (South Cameroon): Integration of remote sensing and aeromagnetic data for structural interpretation

The integration of geology, geophysics and remote sensing data was used to map the Sangmelima granite-greenstone belt terrain located in the South Cameroon. The region was not easily accessible, because of dense vegetation and thick lateritic overburden. Aeromagnetic data were help in mapping new possible small greenstone belts and structural features in this area. Many of the identified lineaments are interpreted to represent fracture/fault zones, some of strike-slip faults. The NE–SW, NW–SE, N–S and E–W are the main revealed from remote sensing processing; while the NE–SW, E–W to ESE–WNW, NW–SE and N–S are the dominant structural trends revealed from aeromagnetic data. We distinguish three deformation events within the Sangmelima area: D1 is recorded by a sub-vertical penetrative S1 foliation bearing steeply plunging stretching lineations. D2 overprints D1 structures with tight-to-isoclinal folds with associated foliation, N160°−175°E and N05°−25°E ductile shear zones, indicating ENE–WSW shortening. The D3 strike slip shearing is likely the result of the Eburnean deformation phase. The mafic dykes are striking NE–SW, N–S and NW–SE, and field evidence and airborne magnetic data suggest that some of them intrude preexisting structures. Many faults/fractures might have originated during previous deformation phases and they were only reactivated during the pan-African events.

Mapping magnetic lineaments and subsurface basement beneath parts of Lower Benue Trough (LBT), Nigeria: Insights from integrating gravity, magnetic and geologic data

In this study, we present the analysis of the aeromagnetic data of parts of the Lower Benue Trough. Lineament analysis of the aeromagnetic data demonstrated four tectonic trends of the basement terrain. The lineaments are in the northeast to southwest (NE–SW), east, northeast to west, southwest (ENE–WSW), north to south (N–S), and east, southeast to west, northwest (ESE–WNW) directions. The NE–SW and ENE–WSW are the most dominant whereas the N–S and ESE–WNW are the minor trends. The estimated magnetic basement using spectral analysis vary between 3.5 and 5 km and the shallow magnetic sources (depth to top of intrusions) vary between 0.24 and 1.2 km. The result of the basement estimation from the magnetic data is comparable with the previous results from other studies as well as with the basement depth estimated from the gravity data of part of the present study area are incorporated in the study. From the gravity data, we identified sub-basin around Makurdi and basement of the sedimentary basin (5 km) is estimated using GPSO algorithm and Oasis Montaj (Geosoft).

Major fault zones in the Austroalpine units of the Kreuzeck Mountains south of the Tauern Window (Eastern Alps, Austria)

The Kreuzeck Mountains are located between the Tauern Window in the north and the Southalpine unit in the south. They expose an Eoalpine (Cretaceous) nappe stack of Adria derived continental crust. The Koralpe–Wolz Nappe System in the footwall partly experienced Cretaceous eclogite-facies metamorphism, while the Drauzug-Gurktal Nappe System in the hanging wall remained at anchizonal to greenschist-facies conditions in Cretaceous time. From the Late Cretaceous onward the Austroalpine units in the Kreuzeck Mountains were affected by brittle/ductile shearing and brittle deformation. This contribution is based on mapping of the southeastern part of the Kreuzeck Mountains at the 1:10000 scale in combination with structural and petrological investigations and Rb–Sr biotite dating. The new data indicate that the boundary between the Koralpe–Wolz Nappe System and the overlying Drauzug-Gurktal Nappe System consists of three segments. It includes (1) the south dipping Wallner Normal Fault representing a more than 500 m wide shear zone responsible for the Cretaceous exhumation of the eclogite-bearing Polinik Complex to shallow crustal levels (2) the Oligocene to Miocene Ragga-Teuchl Fault and (3) the Groshalsgraben Fault. The latter is mechanically related to the Molltal Fault and shows a dextral offset. The Drauzug-Gurktal Nappe System is from bottom to top composed of the Strieden, the Gaugen and the Goldeck complexes, which are transgressively overlain Permo-Triassic (meta)sediments. It is dissected by the steeply north dipping Lessnigbach Shear Zone with a dominant top to the south reverse sense of shear and the ENE–WSW trending, sub-vertical Blassnig Shear Zone showing sinistral kinematics. Both faults were active in the Late Cretaceous, but might have been initiated in the Late Jurassic.

Structural pattern recognition applied on bathymetric data from the Eratosthenes Seamount (Eastern Mediterranean, Levantine Basin)

The scope of the present work is to apply tested techniques of pattern recognition on bathymetric data from the Eratosthenes Seamount to (a) detect topographic highs and linear-curvilinear lineaments present on the seabed, (b) evaluate the morphostucture of the seamount and its relation with the degree of deformation the region has suffered, and (c) to check if shallow salt movements are associated with specific patterns on bathymetric data. Our scope is implemented by applying a two-step procedure on bathymetric data. Initially, the topographic highs in the study area are detected and their geomorphologic attributes (orientation, average slope, eccentricity, and shape complexity) associated with the degree of seabed deformation are computed. Then, quantitative attributes (slope, aspect, and their derivatives) are combined to automatically detect seabed morphologic structures (lineaments, highs, or depressions), which are potentially related to the presence of geological faults or triggered by other subsurface processes. Based on the analysis in this work, two major segments (the northern and the southern) are identified on the Eratosthenes Seamount and related to E–W trending oblique faults. These two segments are highly deformed according to the quantitative estimation of the geomorphologic attributes. Furthermore, ENE–WSW to NE–SW and WNW–ESE to NW–SE lineaments are identified and limit this prominent topographic high. Smaller topographic highs accompanied by topographic depressions, north and southwest of the Eratosthenes Seamount are also detected, showing medium to strong deformation and reflecting a similar geodynamic activity in this area. As a corollary, a comparison of the bathymetric patterns shows that the present seabed morphology north of the Eratosthenes Seamount relates to shallow salt movements, which create a chaotic pattern. This pattern is expressed by short and curved normal faults, shear zones, possible small-scale reverse faults, massive extrusion of salt bodies (possibly allochthonous), and Pliocene–Quaternary minibasins or peripheral sinks.

Tectonically controlled multiple stages of deformation along the Yangsan Fault Zone, SE Korea, since Late Cretaceous

We describe the structural characteristics and tectonic evolution of the NNE–SSW-striking Yangsan Fault, Korea. The surface trace of the fault extends for over 170 km on land, displaying ∼20–30 km of dextral offset. It transects mainly Mesozoic–Cenozoic sedimentary and igneous rocks. Our field observations suggest that the style of deformation within the fault core is controlled by the ductility of the protoliths. Where sedimentary rocks have been faulted, fault cores tend to be relatively wide, having hosted multiple events. Internal fault core structures are characterized by networks of multiple anastomosing strands of fault gouge and breccia, which enclose lenses of fractured protolith. However, where igneous (crystalline) rocks have been faulted, slip tends to be much more localized within narrow fault cores comprising cataclastic rocks. Combining our new data with previous research results, we infer that four main movement phases have occurred on the fault: (1) Late Cretaceous sinistral slip faulting with a component of extensional deformation, under a regime of NW–SE compression; (2) late Paleogene intense dextral slip faulting, which occurred under a regional NE–SW compressional regime; (3) middle Miocene weak sinistral slip faulting, during NNW–SSE compression; and (4) local Quaternary dextral slip faulting with a reverse component, under ENE–WSW to E-W compression. The most intense phase of deformation appears to have occurred during the late Paleogene due to a major reorganization of plate motion in and around the NW Pacific area. Our findings highlight that the fault has been a preferred zone of weakness for episodic reactivations having significant implications for the changing tectonic environments of East Asia from Late Cretaceous to Quaternary, and that its segments underwent distinguishable deformations at each phase probably due to their geometrical features and fault rock properties.

Numerical modeling of flow and transport in the Bari industrial area by means of rough walled parallel plate and random walk models

Abstract. Modeling fluid flow and solute transport dynamics in fractured karst aquifers is one of the most challenging tasks in hydrogeology. The present study investigates the hotspots of groundwater contamination in the industrial area of Modugno (Bari – southern Italy), where the limestone aquifer has a fractured and karstic nature. A rough walled parallel plate model coupled with a geostatistical analysis to infer the values of the equivalent aperture has been implemented and calibrated on the basis of piezometric data. Using the random walk theory, the steady-state distribution of hypothetical contamination with the source at the hotspot has been carried out, reproducing a pollution scenario which is compatible with the observed one. From an analysis of the flow and transport pattern it is possible to infer that the anticline affecting the Calcare di Bari formation in direction ENE–WSW influences the direction of flow as well as the propagation of the contaminant. The results also show that the presence of nonlinear flow influences advection, in that it leads to a delay in solute transport with respect to the linear flow assumption. This is due to the non-constant distribution of solutes according to different pathways for fractured media which is related to the flow rate.

The “Sete Fontes” groundwater system (Braga, NW Portugal): historical milestones and urban assessment

Water resource assessment is essential to bring about sustainable growth of urban development and overall economic progress. Climate change and population growth pressure are important drivers in the available water resources. The crucial point of water management should be to promote an integrated water management in a river catchment and associated groundwater systems. The ancient drinking water system from Sete Fontes (Braga, northern Portugal) was built in the mid-eighteen century, and was the main water supply system of the city until 1913. This water system is located in a valley, with a NE–SW trend, promoting a favourable gentle slope to water–gravity transport into the city. The Sete Fontes aquifer is characterized by Silurian metasediments intruded by Variscan granites, with two main fracturation trends—ENE–WSW and NW–SE. The main hydrogeochemistry composition is Na–Cl facies and locally Na–HCO3 facies. The groundwater is poorly mineralized, suggesting meteoric water contribution, with a relatively shallow circuit. During the Roman period, Sete Fontes system was probably the main source of water for the city. The economic, and demographic expansion of Braga lead to the increasing use of other water resources. Presently, the city’s primary delivery system is only the surface water supplied from Cavado catchment. Natural groundwater characteristics from Sete Fontes show that it is still suitable for human consumption and also for non-potable uses. The assessment also suggests that this is a sustainable water resource, that can constitute an important backup solution in case of failure of the main delivery system due to scarcity or catastrophe.

Miocene fan delta conglomerates in the north-western part of the Danube Basin: provenance, paleoenvironment, paleotransport and depositional mechanisms

Abstract The Blatné Depression located in the NW part of the Danube Basin represents the northernmost sub-basins of the Pannonian Basin System. Its subsidence is associated with oblique collision of the Central Western Carpathians with the European platform, followed by the back-arc basin rifting stage in the Pannonian domain. The conglomerates recognized in the Cífer-2 well document the latest Burdigalian–early Langhian deposition in fan delta lobes situated above the footwall and hanging wall of a WSW–ENE trending fault system, the activity of which preceded the opening of the late Langhian–Serravallian accommodation space with a NE–SW direction. The provenance area of the “Cífer conglomerate” was linked to the Tatric Super-unit complexes. Similar rocks crop out in the southern part of the Malé Karpaty Mts. and are also present in the pre-Cenozoic basement of the Danube Basin. Documented extensive erosion of the crystalline basement and its sedimentary cover lasted until the early/middle Miocene boundary. The “Cífer conglomerate” has distinct clast composition. The basal part consists of poorly sorted conglomerate with sub-angular clasts of metamorphic rocks. Toward the overlying strata, the clasts consist of poorly sorted conglomerates with sub-rounded to well-rounded carbonates and granitoids. The uppermost part consists of poorly sorted conglomerates with sub-rounded to rounded clasts of carbonate, granitoid and metamorphic rock. Within the studied samples a transition from clast to matrix supported conglomerates was observed.

Automated Geological lineaments mapping for groundwater exploration in the basement complex terrain of Akoko-Edo area, Edo-State Nigeria using remote sensing techniques

This study aims to map geological structures (lineaments) which are indications of sustainable groundwater supply in a hard rock terrain of Akoko-Edo local government area of Edo State, Nigeria. Landsat-OLI imagery, topographical map and geological map of Nigeria were used for this study. The methodology involves subjecting the imagery to preprocessing algorithms, linear/edge enhancement, directional filtering, contrast stretching using ENVI 4.7 software. The lineaments were extracted automatically using PCI Geomatica software while ArcGIS and Rockworks software were used to plot the lineament density map and lineament trend respectively. The result shows the total number and length of lineaments to be 197 and 278.85 km respectively. Shorter lineaments constitute about 40% of the total number of lineaments. The lineaments density varies from 0 to 2.40 km/sq.km, and areas with 1.32–2.40 km/sq.km and 0.82 km/sq.km reflect high and moderate degree of rock fracturing which makes these areas suitable target for groundwater exploitation as they possess more lineaments. The lineament trend results showed two prominent trends E–W, N–S and ENE–WSW. Other minor trends are also observed in the study area. These trends validate with results of earlier local studies and with directions of prominent geological structures and features of the study area. The discovery and delineation of the geological structures (lineaments) will serve as direct guides for decision makers to accurately site productive boreholes that will solve the problem of water scarcity in the study area, thereby promoting good developmental policy.

Structural evolution of two‐stage rifting in the northern East China Sea Shelf Basin

This study focuses on the stratigraphic correlation and structural evolution of the Ieodo and Jeju basins in the northern East China Sea Shelf Basin (ECSSB), which are compared to the Changjiang and Xihu depressions, respectively. Based on multi‐channel seismic reflection data, Cenozoic sedimentary successions resting on the acoustic basement in both basins can be divided into multiple syn‐ and post‐rift units. Basement‐involved structures exhibit different orientations, that is, E–W (ENE–WSW) and NE–SW trending structures in the Ieodo and Jeju basins, respectively. They represent the development of two‐stage rifting. The first rift stage began in the Palaeocene and was only restricted to the Ieodo Basin located west of the Hupijiao Rise. The second rift stage subsequently occurred in the Jeju Basin located east of the Hupijiao Rise during the late Eocene to Miocene. The Eocene post‐rift unit of the Ieodo Basin was uplifted while the Jeju Basin was subsided by bounding faults. The sequential development of rift structures in the basins suggests eastward migration of the Cenozoic rifting in the northern ECSSB. The NE‐trending rift structure of the Jeju Basin parallel to the subduction zone of the Pacific Plate suggests that the second rift stage possibly evolved under the influence of the back‐arc extensional regime. The Hupijiao Rise between the Ieodo and Jeju basins was possibly a geologic basement in the Palaeocene and was an uplifted remnant caused by footwall exhumation during the second rift stage of the Jeju Basin.

Integrated calcareous nannofossil and ammonite data from the upper Barremian–lower Albian of the northeastern Transdanubian Range (central Hungary): Stratigraphical implications and consequences for dating tectonic events

The transition of post-obduction Neotethyan contraction to Eo-Alpine (“Austroalpine”) nappe stacking that took place during the Early Cretaceous is an important event in the Cretaceous history of the Alpine-Carpathian-Dinaridic orogenic system. The Transdanubian Range (TR) in Hungary has been shown to have been impacted by both events. Dating of this transition has proved to be difficult; several interpretations were published during the last decades. Uniquely exposed in and around the Gerecse Mountains, the Lábatlan Sandstone Formation (LSF) and the Vértessomló Aleurolite Formation (VAF) bracket this time interval. The last phase of Neotethyan contraction occurred after the deposition of the LSF, and the Eo-Alpine nappe stacking started during the deposition of the VAF. Earlier stratigraphical studies provided valuable data for our understanding of geodynamics, but precise bio- and chronostratigraphy – to constrain the interval of deformations – became possible only with systematic correlation of macrofossil-poor outcrops to ammonite-rich series with the help of calcareous nannofossil data. Here new nannofossil data, and whenever still available, a re-examination of the original smear slides yields new constraints on the age of the younger part of the sequence. The nannofossil and ammonite record is now combined to create a local chronostratigraphical correlation, with records tied in to the integrated Tethyan nannofossil and ammonite biozonation, which, in turn, is numerically calibrated by international zonal standards and radiometric ages. Our study demonstrates that the youngest part of the sandstone-dominated LSF is in nannozone NC7B of late Aptian age. The overlying Köszörűkőbánya Conglomerate Member has a closely similar age in NC7A/B, noticeably older than previously suggested. On the other hand, the VAF clearly is of early Albian age (NC8). This latter unit is not represented in the inner Gerecse Mountains, in contrast to what has been suggested in previous studies. The observed earlier two sub-phases of Neotethyan compression with N–S to ENE–WSW shortening possibly are latest Aptian in age and are within nannofossil subzone NC7C, while the following Eo-Alpine deformation has an early Albian age (NC8) and is marked by W–E to NW–SE compression. The Aptian/Albian boundary (around 113 Ma) may indicate the switch of the TR from lower to upper position with respect to Neotethyan subduction to “Austroalpine” nappe stacking. Our data on deformational ages may support the idea that the onset of “Austroalpine” transpressional deformation at c. 114–112 myr ago could be related to the start of Penninic subduction, or, alternatively, to the onset of intra-oceanic subduction within the Austroalpine realm.

40Ar–39Ar ages from the Sabongari and Nana igneous complexes within the central part of the Cameroon Line (Central Africa)

The Sabongari and Nana volcano–plutonic complexes are two of about sixty complexes that outcrop along the Cameroon Line. In both complexes, granites and rhyolites are the prevalent rock types among plutonic and volcanic rocks, respectively. The Nana complex has never been dated, while data ranging from 77 to 49 Ma have been reported for the Sabongari intrusive rocks. These ages appeared inconsistent with field relationships, the oldest ones 77 and 74 Ma being obtained on microgranites that form a WSW–ENE alignment of small plugs within the main granite. We performed 40Ar–39Ar laser step–heating analyses on a plutonic and a volcanic rock for each complex. The age obtained on Sabongari granite, 57.70 ± 0.44 Ma, is about 1 Ma older than the age obtained on a related trachyte, and consistent with field relationships. Rocks from the Nana complex are younger than those from Sabongari, ranging from 52 to 49 Ma. This is the second paper with 40Ar–39Ar data on the Cameroon Line anorogenic complexes after that on the Hossere Nigo complex, sited at about 240 km to the NE of the study area. Both results challenge earlier K–Ar and Rb–Sr model ages along the Cameroon Line. More 40Ar–39Ar data on a large number of complexes are necessary to better understand their spatial relationships over time.

Petrology, geochemistry, radioactivity, and M–W type rare earth element tetrads of El Sela altered granites, south eastern desert, Egypt

The southern part of the Eastern Desert of Egypt—about 30 km southwest of Abu-Ramad city—is mainly covered by ophiolitic rocks (Sul Hamed), island arc assemblage, younger granites (muscovite granites of Qash Amer and two mica granites of El Sela), and various acidic and basic dikes. Field, petrological, and geochemical studies indicate that the El Sela shear zone has been subject to hydrothermal and supergene alteration such as kaolinization, albitization, sericitization, and hematitization. It is invaded by quartz ENE–WSW veins associated with hydrothermal alteration accompanied by radioactive mineralization. The investigated younger granitic rocks had very low contents of compatible elements, such as Cr, Ni, and Co; and high contents of incompatible elements, such as Zr, and large ion lithophiles, such as Sr, especially in the El Sela shear zone. Major oxide and trace element analyses revealed calc-alkaline affinity and peraluminous character. These highly differentiated granitic rocks’ lower Zr/Hf and higher Y/Ho than the normal ratio are consistent with hydrothermal alteration. Most samples had rare earth element (REE)-patterns with an M-type tetrad effect in the first and fourth segments and a W-type tetrad in the third segment. The average ∑REE in the studied granites was lower than the world granite average; the ratio of light to heavy REEs greater. The main radioactive, uranium-bearing, and uraniferous Fe and Mn minerals are uranothorite, autunite, uranophane and autunite as compounds, kasolite, columbite, xenotime, uranophane-bearing zircon and jarosite, silver-bearing pyrite, hematite, and autunite-bearing pyrolusite.

South China Sea crustal thickness and oceanic lithosphere distribution from satellite gravity inversion

Inversion of satellite-derived free-air gravity-anomaly data has been used to map crustal thickness and continental lithosphere thinning in the South China Sea. Using this, we determine the ocean–continent transition zone structure, the distal extent of continental crust, and the distribution of oceanic lithosphere and continental fragments in the South China Sea. We construct a set of regional crustal cross-sections, with Moho depth from gravity inversion, spanning the South China Sea from offshore China and Vietnam to offshore Malaysia, Brunei and the Philippines to examine variations in ocean–continent transition structure and ocean-basin width. Our analysis shows a highly asymmetrical conjugate margin structure. The Palawan margin shows a narrow transition from continental to oceanic crust. In contrast, the conjugate northern margin of the South China Sea shows a wide region of thinned continental crust and an isolated block of continental crust (the Macclesfield Bank) separated from the Chinese margin by a failed oceanic rift. The Dangerous Grounds are predicted to be underlain by fragmented blocks of thinned continental crust. We use maps of crustal thickness and continental lithosphere thinning from gravity inversion together with free-air gravity- and magnetic-anomaly data to identify structural trends and to show that rifting and the early seafloor-spreading axis had an ENE–WSW trend while the later seafloor-spreading axis had a NE–SW trend.