ENE WSW Research Articles

Combined analysis of remote sensing, gravity and magnetic data across Moyar Bhavani Shear Zone, Southern Granulite Terrane (SGT), India: appraisals for crustal architecture and tectonics

We present a combined analysis of remote sensing, gravity and magnetic data across Moyar Bhavani Shear Zone (MBSZ), Southern Granulite Terrane (SGT), India to study crustal architecture and tectonics. The horizontal gradient of gravity data shows strong density inhomogeneity across the MBSZ. The magnetic data and residual gravity analysis shows corroboration of a NE–SW trending linear magnetic anomaly zone and residual gravity high strip along the MBSZ. The low magnetic anomalies dominate the northern part of the MBSZ, while the southern part is dominated by moderate to high anomalies. The integration of potential data and remote sensing data clearly reveals that ENE–WSW is the dominant structural trend that coincides with the Mettur Shear Zone (MeSZ) trend. The joint gravity and magnetic modelling suggest that the MBSZ, is characterized by the disposition of mélange of moderate to high-density rocks of diamagnetic nature.

The Amami Rift: Clarifying the roles of rifting and volcanism in the central Ryukyu Arc

Arc volcanism in the Ryukyu subduction system shows an along-arc variation characterized by the development of subaerial volcanoes in the northern arc and submarine ones in the southern arc. The central arc is a transitional area in arc volcanism, but its characteristics and tectonic controls remain largely unknown. This paper presents detailed new data on the tectonic and volcanic features of the Amami Calderas in the central Ryukyu Arc based on high-resolution bathymetry and a seismic reflection profile. Ship-based bathymetry reveals that ENE–WSW to E–W trending normal faults form a local rift structure which is ~60 km long, ~10 km wide and ~ 400 m deep. Small volcanic edifices and crater-like depressions are scattered in and around the rift. A number of bubble plumes were detected by shipboard multibeam sonar in the rift. AUV near-bottom mapping confirmed positive temperature anomalies near the bubble plumes, indicative of hydrothermal venting. Our observations demonstrate that the previously named Amami Calderas are not volcanic in origin, but are instead tectonic, and we therefore re-name these features the Amami Rift. The Amami Rift is the northernmost active rift along the central Ryukyu Arc and its unique location provides new insights into the interaction of rifting and arc volcanism. Yokoate-jima Island is a large volcanic edifice close to the Amami Rift whose surrounding seafloor shows no distinctive rift structures and few diffuse lineaments. In contrast, the Amami Rift consists of small scattered volcanic edifices and a prominent focused rift. This sharp volcano-tectonic contrast between Yokoate-jima Island and Amami Rift suggests that focused rifting makes arc volcanism scattered rather than focused in the area, with relatively small volcanic edifices. Another notable feature of the Amami Rift and surrounding area is that volcanic edifices, indicators of hydrothermal activity and bubble plumes are aligned not parallel to the rift, but in the cross-arc direction, indicating the presence of cross-arc volcanism. Cross-arc volcanism has been recently reported in the northern Ryukyu Arc, but this is the first time it has been observed in the central arc. Our observations combined with previous studies suggest a combination of cross-arc volcanism and crustal extension characterizes the spatial distribution of active magmatism in the central Ryukyu Arc.

Structural and mineral exploration study at the transition zone between the North and the Central Eastern Desert, Egypt, using airborne magnetic and gamma-ray spectrometric data

The transition zone between the North and the Central Eastern Desert of Egypt has been investigated using airborne magnetic and radiometric data to better characterize the subsurface structures, the tectonic framework, and mineral occurrences in the area. The magnetic data were subjected to several filtering and depth estimation techniques including edge detectors, Euler deconvolution, 2D forward modeling and 3D inversion. Statistical trend analysis of the lineaments obtained from geological and filtered magnetic maps shows that the major tectonic trends are NNE–SSW, NE–SW, and NW–SE, in addition to less predominant ENE–WSW and N–S trends. Two parallel shear zones trending NE–SW were detected. The causative magnetic structures were imaged at depths varying from the surface down to 2200 m. A number of strong uranium anomalies associated with Younger Granites and Dakhla, Duwi, and Quseir formations were identified which are worth being considered for further ground investigations.

Current stress field and its relationship to tectonism in a coal mining district, central China, for underground coal energy exploration

Adopting 47 sets of in situ stress measurements taken at depths ranging from 417 to 1123 m in a coal mining district in central China by the overcoring method, we characterize the magnitude and orientation of the current stress field and its relationship to tectonism in this district to determine the controls on in situ stress in underground coal energy exploration and seismogeology. The measurements reveal that this district is in a relatively high horizontal stress environment. The stress regimes are generally favorable for strike-slip faulting and reverse faulting and are characterized by σH >σv >σh and σH >σh >σv, revealing that the regional stress field is absolutely dominated by horizontal tectonic stress and reflects a typical tectonic stress field type. The maximum horizontal principal stress is predominantly oriented between the ENE–WSW and WNW–ESE directions and averages nearly E–W,​ which is closely related to the tectonic plate movement and crustal deformation and agrees well with the direction derived from focal mechanism solutions and other stress indicators in and around the region. The current stress field in this district generally inherited the characteristics of the tectonic stress field in the third stage and partially retained the traces generated by the tectonic stress field in the first and second stages. In addition, the frictional parameter μm, which is calculated using the derived stress data, is used as a strong predictor to reflect the capability of the level of crustal stress accumulation. This region is in a state of critical or subcritical equilibrium, and regional faults at shallow depths have a medium to slightly high frictional strength, indicating that some well-oriented faults may theoretically approach or reach frictional equilibrium and that frictional sliding may occur.

Active Tectonics of the North Tunisian Continental Margin

AbstractA poorly defined boundary between the Nubia and Eurasian plates runs along the Northern Tunisian continental margin. The Tunisia margin is deformed by a slow NW–SE trending convergence resulting in a diffuse deformation zone with scarce and scattered seismicity compared to the seismic activity into the neighboring regions to the east and west along the boundary. The area has been poorly studied and therefore its recent evolution is almost unknown, particularly offshore. Here, we present a structural analysis of the active tectonics in this submarine continental margin. The data used for this analysis are high‐resolution bathymetric maps together with parametric echosounder images which have allowed to obtain a map of active faulting with unprecedented detail. The structural analysis supports a dominantly transpressive to compressive component of faulting, resulting from the current regional NW–SE trending compressive regime between plates. The North‐eastern Domain of the study region contains the highest number of active faults with numerous pockmarks aligned along them. This study shows that the plate boundary across the North Tunisia margin is incipient and poorly developed, which may be due to the fact that deformation is partitioned over a large number of structures, each accommodating a small percentage of convergence, with the exception of the Hayat fault system. The Hayat reverse fault, striking WSW–ENE, is the largest fault system that comparatively may accommodate a greater amount of displacement, and is probably responsible for the uplift of the North‐eastern Domain of the continental margin.

Two Distinct Back-Arc Closure Phases of the East Sea: Stratigraphic Evidence From the SW Ulleung Basin Margin

This study focuses on revisiting the tectostratigraphic framework of the Ulleung Basin and conceptualizing neotectonics around the western East Sea margin. Based on the analysis of 2D and 3D multi-channel seismic reflection data and offshore drill wells, we divided the entire sedimentary successions of the Ulleung Basin into four tectostratigraphic sequences, named TS1 (c. 23–16 Ma), TS2 (c. 16–9 Ma), TS3 (c. 9–4 Ma), and TS4 (c. 4 Ma–present), in ascending order. The results show that each sequence has been deformed once or multiple times in different periods by juxtaposing two major compressional structures named the Dolgorae Thrust-Fold Belt and the Gorae Anticline. Interpretation of the stratal deformation and termination patterns of the syn- and post-deformational sequences of each structures suggests that the thrusting and folding of the Dolgorae Thrust-Fold Belt was active from c. 16 Ma to c. 9 Ma under the NNW–SSE compressional stress regime (Stage-2), whereas the Gorae Anticline was active from 4 Ma to the present under the ENE–WSW compressional stress regime (Stage-4). Between these two compressional events, there was an intervening period of regional slow subsidence driven by thermal contraction of the back-arc lithosphere and isostatic sedimentary loading (Stage-3). Based on the stratigraphic and structural reconstruction, we propose a 4-stage tectonic model: Stage-1) back-arc opening stage associated with the southward drift of the Japanese islands (c. 23–16 Ma), Stage-2) tectonic-inversion stage in association with the reorganization of the Pacific and Philippine Sea plates and clockwise rotation of SW Japan (c. 16–9 Ma), Stage-3) post-inversion stage with regional thermal and isostatic subsidence (c. 9–4 Ma), and Stage-4) neotectonic stage in which embryonic subduction is nucleating on the East Sea margins under the E–W compressional stress regime (c. 4 Ma–present).

Application of 3D Static Modelling in Reservoir Characterization: A Case Study from the Qishn Formation in Sharyoof Oil Field, Masila Basin, Yemen

AbstractThree‐dimensional (3D) static modelling techniques are applied to the characterization of the Qishn Formation (Fm.) in the Sharyoof oil field locating within the Masila basin, southeastern Yemen. The present study was initiated by the seismic structural interpretation, followed by building a 3D structural framework, in addition to analysing well log data and from these, 3D facies and petrophysical models are constructed. In the Sharyoof oil field, the Qishn Fm. exhibits depth values within the range of 400–780 m below sea level, with a general increase towards the SSE. A set of high dip angle normal faults with a general ENE–WSW trend dissect the rocks. The strata are also folded as a main anticline with an axis that is parallel to the fault trend, formed as a result of basement uplift. According to the facies models, the Qishn Fm. comprises 43.83% limestone, 21.53% shale, 21.26% sandstone, 13.21% siltstone and 0.17% dolomite. The Qishn Carbonates Member has low porosity values making it a potential seal for the underlying reservoirs whereas the Upper Qishn Clastics S1A and C have good reservoir quality and S1B has fair reservoir quality. The Upper Qishn Clastics S2 and S3 also have fair reservoir quality, while the Lower Qishn Clastics zone has good reservoir quality. The water saturation decreases towards the west and east and increases towards north and south. The total original oil in‐place (OOIP) of the Upper Qishn clastics is 106 million STB within the S1A, S1C and S2 zones. Drilling of development wells is recommended in the eastern study area, where good trapping configuration is exhibited in addition to the presence of a potential seal (Upper Qishn Carbonates Member) and reservoir (Qishn Clastics Member) with high porosity and low water saturation.

Interactions between large-scale strike-slip intersecting faults: Implication from the Tan–Lu and Xiangfan–Guangji fault zones in eastern China

ABSTRACT The large-scale strike-slip Tan–Lu Fault Zone (TLFZ) and Xiangfan–Guangji Fault Zone (XGFZ) both terminate in the southeastern corner of the Dabie Orogen at an angle of almost 90°, and this corner therefore provides a very good natural laboratory for understanding the mechanism by which large-scale strike-slip faults terminate. We present new geochronological and structural data for the southeastern tip of the XGFZ and the southwestern tip of the TLFZ. The NW–SE-striking XGFZ records ductile shearing in its northwestern segment, characterized by discrete dextral shear zones that formed at temperatures of 350–400°C, as indicated by quartz c-axes fabrics and microstructures. In the southeastern segment of the XGFZ, WNW–ESE-trending thrusts are displayed. The NE–SW-striking TLFZ is characterized by discrete NE–SW-trending sinistral ductile shear zones in the Qianshan–Tongcheng segment, brittle left-lateral strike-slip faults in the Taihu–Qianshan segment, and thrusts to the south of Taihu. The trends of these thrusts change progressively southward from NE–SW to ENE–WSW and E–W. New zircon U–Pb dating results and previous cooling biotite 40Ar/39Ar ages constrain the timing of shearing in the XGFZ to 112–102 Ma (late Early Cretaceous), which is the same as the age of faulting in the TLFZ (110–102 Ma). The large-scale strike-slip TLFZ and XGFZ both terminate at their tips as thrusts. We suggest that interactions between the two faults led to the kinematic changing from strike-slip to thrusting, with this playing an important role in controlling the termination of these two large-scale intersecting strike-slip faults. The dextral shearing of the NW–SE-trending XGFZ, the sinistral shearing of the NE–SW-trending TLFZ, and the nearly E–W-trending thrust faults all indicate continental-scale N–S compression in eastern China during the late Early Cretaceous. This compression resulted from rapid NNW-ward oblique subduction of the Paleo-Pacific Plate.

Morpho-tectonic Assessment of the Abu-Dabbab Area, Eastern Desert, Egypt: Insights from Remote Sensing and Geospatial Analysis

The Abu-Dabbab area, located in the central part of the Egyptian Eastern Desert, is an active seismic region where micro-earthquakes (≈ML < 2.0) are recorded regularly. Earthquake epicenters are concentrated along an ENE–WSW trending pattern. In this study, we used morphological indexes, including the valley floor width-to-valley floor height ratio (Vf), mountain front sinuosity (Smf), the asymmetry factor index (Af), the drainage basin shape index (Bs), the stream length–gradient index (SL), hypsometric integral (Hi) water drainage systems, and a digital elevation model analysis, to identify the role of tectonics. These indexes were used to define the relative tectonic activity index (RTAI), which can be utilized to distinguish low (RTAI < 1.26), moderate (RTAI = 1.26–1.73), and high (RTAI > 1.73) tectonic activity signals all over the study area. Firstly, our results indicate low to medium tectonic activity and general anomaly patterns detected along the major tectonic zones of the study area. Secondly, based on most of the low to medium tectonic activity distributed in the study area and the detected anomalies, we discuss two potential drivers of the seismicity in the Abu-Dabbab area, which are fault-controlled and deep-rooted activities.

ANALYSIS OF LINEAMENT MAP OF PART OF MARU SCHIST BELT, NORTHWEST, NIGERIA AND ITS IMPLICATION FOR MINERAL EXPLORATION

Lineament are extended mappable linear or curvilinear feature of a surface which can be utilized in mineral, oil, gas, and underground water studies. They are obvious in satellite images, aerial images or aerial photographs. The aim of this study is to analyse an automatically generated lineament from part of the Maru schist belt using a combination of Digital Elevation Model (DEM) data and Aeromagnetic data. The area of study is within the Maru schist belt in Zamfara State. A shaded relief map was produced from the two data sources and the lineament was extracted automatically from them. The extracted lineament was combined to obtain a final lineament map of the area. Trends such as N-S, NW–SE, ENE–WSW and NW–SE, with very little E-W were all present. The areas of very high, high, moderate and low population of lineaments was delineated. Regions with very high lineament intersections and lineament concentration, covers an area of 153.6 km2 and accounts for 16.1% of the study area, these regions are known to harbour gold and iron ores from literature reviews. These target regions are located at the Northern and South part of the study area. Also, the gold artisan miners have dominated these same regions and they have only started from exposed vein of gold mineralization. With this rather fast and easier approach of obtaining such a map, it could be used as a start point for planning a more detailed geophysical or geological survey

Recent Activity and Kinematics of the Bounding Faults of the Catanzaro Trough (Central Calabria, Italy): New Morphotectonic, Geodetic and Seismological Data

A multidisciplinary work integrating structural, geodetic and seismological data was performed in the Catanzaro Trough (central Calabria, Italy) to define the seismotectonic setting of this area. The Catanzaro Trough is a structural depression transversal to the Calabrian Arc, lying in-between two longitudinal grabens: the Crati Basin to the north and the Mesima Basin to the south. The investigated area experienced some of the strongest historical earthquakes of Italy, whose seismogenic sources are still not well defined. We investigated and mapped the major WSW–ENE to WNW–ESE trending normal-oblique Lamezia-Catanzaro Fault System, bounding to the north the Catanzaro Trough. Morphotectonic data reveal that some fault segments have recently been reactivated since they have displaced upper Pleistocene deposits showing typical geomorphic features associated with active normal fault scarps such as triangular and trapezoidal facets, and displaced alluvial fans. The analysis of instrumental seismicity indicates that some clusters of earthquakes have nucleated on the Lamezia-Catanzaro Fault System. In addition, focal mechanisms indicate the prevalence of left-lateral kinematics on E–W roughly oriented fault plains. GPS data confirm that slow left-lateral motion occurs along this fault system. Minor north-dipping normal faults were also mapped in the southern side of the Catanzaro Trough. They show eroded fault scarps along which weak seismic activity and negligible geodetic motion occur. Our study highlights that the Catanzaro Trough is a poliphased Plio-Quaternary extensional basin developed early as a half-graben in the frame of the tear-faulting occurring at the northern edge of the subducting Ionian slab. In this context, the strike-slip motion contributes to the longitudinal segmentation of the Calabrian Arc. In addition, the high number of seismic events evidenced by the instrumental seismicity, the macroseismic intensity distribution of the historical earthquakes and the scaling laws relating to earthquakes and seismogenic faults support the hypothesis that the Lamezia-Catanzaro Fault System may have been responsible for the historical earthquakes since it is capable of triggering earthquakes with magnitude up to 6.9.

The role of intracratonic crustal reworking in the tectonic compartmentalization of the Juruena Terrane, SW Amazonas Craton: the Roosevelt-Guariba Transpressive Belt

The multiscale structural analysis carried out in the NNW sector of the Juruena Terrane, SE of the State of Amazonas combines structural mapping, microtectonic analysis and aeromagnetic data analysis. It shows a complex structural and metamorphic history composed of successive deformation events, responsible by the formation of its tectonic framework. Event D2 (1520-1460 Ma) is responsible for the main structure of the NNW sector of the Juruena Terrane, and it generates a dextral transpressive shear zone NW-SE, called Roosevelt-Guariba Transpressive Belt (RGTB). This is an intracontinental high-temperature, medium-pressure tectonothermal event, compatible with the conditions of the upper amphibolite to granulite facies. The RGTB controls the tectonic compartmentation of the region, particularly affecting the Granitic Gneiss Domain (1760-1750 Ma). Other sectors, such as the Supracrustal Rock Domain (1800-1730 Ma) and the Pre-Juruena geological-structural association (>1830 Ma), are subordinately affected. Event D2 overlaps the regional structure E-W (ENE-WSW) attributed to Event D1 (1690-1630 Ma), which is restricted to the deep basement in the NNW sector, but dominant in the south central sector of the Juruena Terrane. There are also two late brittle-ductile to brittle deformation events (events D3 and D4). Event D3 (1320-1300 Ma) is represented by NE-SW cataclastic and shear zones, commonly serving as conduits for hydrothermal fluids relative to gold mineralizations. Event D4 is constituted by normal faults and fractures that reactivate RGTB shear zones, limiting Neoproterozoic and Paleozoic-Mesozoic sedimentary exposures. The tectonic structural model proposed in this study reinforces the interpretation that the Juruena Terrane evolved from a foreland basin, which was formed during the Juruena Orogen and whose basement is the Tapajos Crust. This basin was preserved from the tectono-thermal effects of the end of the Statherian (D1), and it was only during event D2, after emplacement of the RGTB, that it underwent high-grade deformation and metamorphism, whose driving force is attributed to the propagation of stresses in pericratonic regions, associated with Calymmian orogens. The successive events are related to the cratonization of the Juruena Terrane, represented by late crust reactivation (D3) and neotectonic events (D4).

Slumping as a record of regional tectonics and palaeoslope changes in the Satpura Basin, central India

Abstract Soft-sediment deformation structures play an important role in interpreting regional tectonics and basin evolution during slumping events. The Satpura Basin is interpreted as pull-apart with a monoclinal northerly palaeoslope throughout its evolution. The basin formed as a result of sinistral strike-slip faulting, induced by the ENE–WSW-trending Son-Narmada South fault in the north and the Tapti North fault in the south. We have analysed the slump folds within the basalmost Talchir Formation and related these to regional tectonics and palaeoslope changes in the Satpura Basin. The glaciofluvial strata of the Talchir Formation, exposed in the southern part of the Satpura Basin, record intricacies of folds created during slumping. Several fold styles can be distinguished, within alternations of competent sandstone and incompetent shale layers, some of which indicate buckling. Upright folds, resulting from pure shear, underwent rotation of their axial planes and fold axes during simple shear-dominated progressive deformation when the slump moved downslope. The soft-sediment deformation structures that we have studied show refolding patterns that closely resemble comparable folds known from lithified rocks. These layers with refolded structures are overlain by unde-formed sediments, which proves that they are the product of a single ongoing slumping process, rather than of successive deformation events. Our analysis of their fold axes and axial planes, together with fold vergences and thrust directions within the slumps, suggests a mean slumping direction towards the southwest. Analyses of slump folds and their relationship with regional tectonics have allowed us to reinterpret basin evolution history. The southwesterly trending palaeoslope of the basin suggest that the slope of the basin was not uniform throughout its evolution. At the opening, the oblique slip fault, which trended NE–SW, generated due to movement along the ENE–WSW basin bounding faults, was more active and triggered slumping event within the Talchir deposits in the basin. With progressive overlapping of the basin-bounding faults, the Satpura Basin gradually tilted towards the north.

Brittle Deformation in the Neoproterozoic Basement of Southeast Brazil: Traces of Intraplate Cenozoic Tectonics

The basement of southeast Brazil is traditionally interpreted as the result of Neoproterozoic and early Paleozoic orogenic cycles. Wide regions of the Atlantic Plateau (southeast Brazil) are characterized by rocks and tectonic structures of Precambrian age. According to the classical literature, these regions have not been affected by tectonics since the Miocene, despite the fact that they rest close to Cenozoic basins, which have suffered recent tectonic deformation. The objective of this research is to evaluate the role of neotectonics in the Atlantic Plateau. This task is accomplished through a multiscalar approach which includes lineament domain analysis from regionally sized digital elevation models and structural geology field surveys. Lineaments are automatically detected and statistically analyzed. Azimuthal analyses of data on faults and fractures by a polynomial Gaussian fit enables the identification of the main structural trends. Fault-slip direct inversion by means of the original Monte Carlo approach allows one to compute the multiple paleostresses that produced the measured fault population. The results show the presence of a principal ENE–WSW lineament domain, related to an old shear zone possibly reactivated since the Miocene. One of the paleostresses computed from fault-slip inversion is in agreement with the neotectonic stress-field proposed by other authors.

GNSS Constraints to Active Tectonic Deformations of the South American Continental Margin in Ecuador.

GNSS observations constitute the main tool to reveal Earth’s crustal deformations in order to improve the identification of geological hazards. The Ecuadorian Andes were formed by Nazca Plate subduction below the Pacific margin of the South American Plate. Active tectonic-related deformation continues to present, and it is constrained by 135 GPS stations of the RENAGE and REGME deployed by the IGM in Ecuador (1995.4–2011.0). They show a regional ENE displacement, increasing towards the N, of the deformed North Andean Sliver in respect to the South American Plate and Inca Sliver relatively stable areas. The heterogeneous displacements towards the NNE of the North Andean Sliver are interpreted as consequences of the coupling of the Carnegie Ridge in the subduction zone. The Dolores–Guayaquil megashear constitutes its southeastern boundary and includes the dextral to normal transfer Pallatanga fault, that develops the Guayaquil Gulf. This fault extends northeastward along the central part of the Cordillera Real, in relay with the reverse dextral Cosanga–Chingual fault and finally followed by the reverse dextral Sub-Andean fault zone. While the Ecuadorian margin and Andes is affected by ENE–WSW shortening, the easternmost Manabí Basin located in between the Cordillera Costanera and the Cordillera Occidental of the Andes, underwent moderate ENE–WSW extension and constitutes an active fore-arc basin of the Nazca plate subduction. The integration of the GPS and seismic data evidences that highest rates of deformation and the highest tectonic hazards in Ecuador are linked: to the subduction zone located in the coastal area; to the Pallatanga transfer fault; and to the Eastern Andes Sub-Andean faults.

Plio–Quaternary Structural Evolution of the Outer Sector of the Marche Apennines South of the Conero Promontory, Italy

Some new results and preliminary remarks about the Plio–Quaternary structural and evolutionary characteristics of the outer Marche Apennines south in the Conero promontory are presented in this study. The present analysis is based on several subsurface seismic reflection profiles and well data, kindly provided by ENI S.p.A. and available on the VIDEPI list, together with surface geologic–stratigraphic knowledge of Plio–Quaternary evolution from the literature. Examples of negative vs. positive reactivation of inherited structures in fold and thrust belts are highlighted. Here, we present an example from the external domain of the Marche Apennines, which displays interesting reactivation examples from the subsurface geology explored. The study area shows significant evolutionary differences with respect to the northern sector of the Marche region previously investigated by the same research group. The areal distribution of the main structures changes north and south of the ENE–WSW oriented discontinuity close to the Conero promontory. Based on the old tripartite classification of the Pliocene, the results of this work suggest a strong differential subsidence with extension occurring during the Early Pliocene and principal compressive deformation starting from the Middle Pliocene and decreasing or ceasing during the Quaternary. The main structure in this area is the NNW–SSE Coastal Structure, which is composed of E-vergent shallow thrusts and high-angle deep-seated normal faults underneath. An important right-lateral strike–slip component along this feature is also suggested, which is compatible with the principal NNE–SSW shortening direction. As mentioned, the area is largely characterized by tectonic inversion. Starting from Middle Pliocene, most of the Early Pliocene normal faults became E-vergent thrusts.

Multi-scale analysis and modelling of aeromagnetic data over the Bétaré-Oya area in eastern Cameroon, for structural evidence investigations

Abstract. This study was carried out in the Lom series in Cameroon, at the border with Central African Republic, located between the latitudes 5∘30′–6∘ N and the longitudes 13∘30′–14∘45′ E. A multi-scale analysis of aeromagnetic data combining tilt derivative, Euler deconvolution, upward continuation, and 2.75D modelling was used. The following conclusions were drawn. (1) Several major families of faults were mapped. Their orientations are ENE–WSW, E–W, NW–SE, and N–S with a NE–SW prevalence. The latter are predominantly sub-vertical with NW and SW dips and appear to be prospective for future mining investigations. (2) The evidence of compression, folding, and shearing axis was concluded from superposition of null contours of the tilt derivative and Euler deconvolution. The principal evidence of the local tectonics was due to several deformation episodes (D1, D2, and D4) associated with NE–SW, E–W, and NW–SE events, respectively. (3) Depths of interpreted faults range from 1000 to 3400 m. (4) Several linear structures correlating with known mylonitic veins were identified. These are associated with the Lom faults and represent the contacts between the Lom series and the granito-gneissic rocks; we concluded the intense folding was caused by senestral and dextral NE–SW and NW–SE stumps. (5) We propose a structural model of the top of the crust (schists, gneisses, granites) that delineates principal intrusions (porphyroid granite, garnet gneiss, syenites, micaschists, graphite, and garnet gneiss) responsible for the observed anomalies. The 2.75D modelling revealed many faults with a depth greater than 1200 m and confirmed the observations from reduced-to-Equator total magnetic intensity (RTE-TMI), tilt derivative, and Euler deconvolution. (6) We developed a lithologic profile of the Bétaré-Oya basin.

Crustal Strain and Stress Fields in Egypt from Geodetic and Seismological Data

The comparison between crustal stress and surface strain azimuthal patterns has provided new insights into several complex tectonic settings worldwide. Here, we performed such a comparison for Egypt taking into account updated datasets of seismological and geodetic observations. In north-eastern Egypt, the stress field shows a fan-shaped azimuthal pattern with a WNW–ESE orientation on the Cairo region, which progressively rotated to NW–SE along the Gulf of Aqaba. The stress field shows a prevailing normal faulting regime, however, along the Sinai/Arabia plate boundary it coexists with a strike–slip faulting one (σ1 ≅ σ2 > σ3), while on the Gulf of Suez, it is characterized by crustal extension occurring on near-orthogonal directions (σ1 > σ2 ≅ σ3). On the Nile Delta, the maximum horizontal stress (SHmax) pattern shows scattered orientations, while on the Aswan region, it has a WNW–ESE strike with pure strike–slip features. The strain-rate field shows the largest values along the Red Sea and the Sinai/Arabia plate boundary. Crustal stretching (up to 40 nanostrain/yr) occurs on these areas with WSW–ENE and NE–SW orientations, while crustal contraction occurs on northern Nile Delta (10 nanostrain/yr) and offshore (~35 nanostrain/yr) with E–W and N–S orientations, respectively. The comparison between stress and strain orientations over the investigated area reveals that both patterns are near-parallel and driven by the same large-scale tectonic processes.

Upper mantle anisotropy from shear wave splitting of teleseismic earthquakes in the Kumaun-Garhwal and adjoining area of NW Himalaya

Teleseismic earthquakes data recorded by 8 broadband digital seismic stations deployed in the Kumaun-Garhwal and adjoining area of NW Himalaya are analyzed to investigate the seismic anisotropy in the upper mantle. Shear-wave splitting parameters (ɸ and ∂t) derived from the analysis of core-refracted SKS/SKKS phases provide first hand information about seismic anisotropy and deformation in the upper mantle beneath the region. A total, 209 good measurements of seismic anisotropy have been obtained from the two years of collected high-quality teleseismic data in the study region. The analysis shows considerable strength of anisotropy (delay time(∂t) 0.6–2.8 s) with average ENE–WSW oriented fast polarization orientation at most of the stations while other one either along with the absolute plate motion (APM) of the Indian plate or parallel to strike of the Himalayan mountain belt. The Fast axis direction observed at stations close to between the MBT and MCT aligns parallel to the strike of local geological faults and orthogonal to absolute plate motion (APM) direction of the Indian plate. The large variation in the splitting strength and fast polarization trends around the Kumaun-Gharwal and adjacent in NW Himalaya have invoked both lithospheric and asthenospheric anisotropy.

Post-Impact Faulting of the Holfontein Granophyre Dike of the Vredefort Impact Structure, South Africa, Inferred from Remote Sensing, Geophysics, and Geochemistry

Better characterization features borne from long-term crustal modification processes is essential for understanding the dynamics of large basin-forming impact structures on Earth. Within the deeply eroded 2.02 Ga Vredefort Impact Structure in South Africa, impact melt dikes are exposed at the surface. In this study, we utilized a combination of field, remote sensing, electrical resistivity, magnetic, petrographical, and geochemical techniques to characterize one such impact melt dike, namely, the Holfontein Granophyre Dike (HGD), along with the host granites. The HGD is split into two seemingly disconnected segments. Geophysical modeling of both segments suggests that the melt rock does not penetrate below the modern surface deeper than 5 m, which was confirmed by a later transecting construction trench. Even though the textures and clast content are different in two segments, the major element, trace element, and O isotope compositions of each segment are indistinguishable. Structural measurements of the tectonic foliations in the granites, as well as the spatial expression of the dike, suggest that the dike was segmented by an ENE–WSW trending sinistral strike-slip fault zone. Such an offset must have occurred after the dike solidified. However, the Vredefort structure has not been affected by any major tectonic events after the impact occurred. Therefore, the inferred segmentation of the HGD is consistent with long-term crustal processes occurring in the post-impact environment. These crustal processes may have involved progressive uplift of the crater floor, which is consistent with post-impact long-term crustal adjustment that has been inferred for craters on the Moon.