ENE WSW Research Articles

Dependence of earthquake faulting type on fault strike across the Korean Peninsula: Evidence for weak faults and comparison with the Japanese Archipelago

We re-evaluate the stress state across the Korean Peninsula by analyzing the stress tensor inversion results, strike–rake relationships, and faulting-type indicator–magnitude relationships of published earthquake focal mechanism data. Previous studies have shown that the stress state is characterized by a strike-slip faulting stress regime with ENE–WSW maximum compression, NNW–SSE minimum compression, and vertical intermediate stress. The differential stress magnitudes are estimated to be on the order of 100 MPa under the assumption of the strong-fault earthquake with a high Coulomb friction coefficient of 0.85. Conversely, other studies have shown that the weak-fault model is applicable across the Japanese Archipelago, where the differential stress level is only a few tens of MPa. We, therefore, review previous studies, evaluate the focal mechanism data across the Korean Peninsula, and infer the absolute stress level. Our comparison of the geological and tectonic histories of the Korean Peninsula and Japanese Archipelago suggests that the NNW–SSE tensional stresses in the Korean Peninsula are caused by slab rollback of the Philippine Sea Plate at around 5 Ma, west of Kyushu Island, southwestern Japan. We also find that the earthquake faulting types across the Korean Peninsula are closely related to their fault strikes, confirming a spatially uniform distribution of the strike-slip stress regime with nearly equal tensional and compressional tectonic stresses. However, this situation leads to the apparent paradox of strike-dependent fault strength. We eliminate this paradox by proposing a model whereby reverse- and normal-faulting events are caused by local stress heterogeneities around the edges of strike-slip faults under a uniform low-strength strike-slip stress state for all faulting types. We also highlight the usefulness of strike–rake and faulting-type indicator–magnitude diagrams in considering stress states.

Gravity interpretation for delineating subsurface structures and depth of basement at El Moghra area, North Western Desert, Egypt

ABSTRACT El Moghra area represents a vital part of the national reclamation area at the northern part of Western Desert, Egypt. Delineating subsurface structures (faults & basins) and depth to the basement is the base of planning the area. Basins can be water aquifers or oil traps. Gravity data has been used to tackle such aims. The gravity (Bouguer) anomaly map is separated into regional components (deep sources), and residual component (shallower sources). The residual gravity map was used to delineate subsurface structural features dissecting the investigated area. Also, Euler deconvolution and Tilt Derivative methods have been used for delineating the structural elements. The gravity interpretation indicates that the most main tectonic have E–W, ENE–WSW, and N–S trends for the major structures, while the minor structures are aligned in NNE–SSW, NE–SW, NNW–SSE, and NW–SE. The results of depth estimation using 2D and 3D modelling show that the eastern and southwestern parts of the area under study exhibit a deep basement structure, reaches more than 7000 m (basin). However, the central and northwestern parts of the investigated area show shallower depth of basement rocks (4100 m).

Arc‐parallel extension in preparation of the rotation of southwest Japan: Tectonostratigraphy and structures of the Lower Miocene Ichishi Group

AbstractOpening of the Japan Sea back arc basin was accompanied by extensional tectonics in the drifting southwest Japan arc. Various trends of Early Miocene grabens in the arc suggest multi‐directional rifting, which necessarily involved strike‐slip components of some of basin‐margin faults. However, such components are not well understood. In this work we conducted a field survey in the Early Miocene Ichishi basin on the northern side of the Median Tectonic Line, central southwest Japan. We found that the basin was a compound of grabens that were formed along normal and sinistral strike‐slip faults, the latter of which had northeast–southwest trends. The block faulting in this phase produced basement highs between sub‐basins, which were filled with the lower part of the Ichishi Group. We found a low‐angle angular unconformity at a middle horizon in the group, with which we define the upper and lower part of the group. The upper part onlapped both the basement highs and the lower part. It means that the transtensional basin formation ceased sometime between 18 and 17.5 Ma in the Ichishi area. The Ichishi basin turned subsequently into a sag basin subsided due to normal faulting probably along the Nunobiki‐sanchi‐toen fault zone. The transtension and the basin sag were driven by ENE–WSW extensional stress. This arc‐parallel extension produced grabens various areas including Ichishi in the Early Miocene. The extensional deformation was eventually localized to the deep rift along the Fossa Magna to make the lithosphere under southwest Japan decoupled from that under northeast Japan. The decoupling allowed the rapid rotation of southwest Japan from ~17.5 Ma. The cluster of those grabens around the Ise bay probably determined the southeastern margin of the Kinki triangle.

The 25 October 2018 Zakynthos (Greece) earthquake: seismic activity at the transition between a transform fault and a subduction zone

SUMMARYThe properties of the Mw = 6.7 earthquake that took place on 25 October 2018, 22:54:51 UTC, ∼50 km SW of the Zakynthos Island, Greece, are thoroughly examined. The main rupture occurred on a dextral strike-slip, low-angle, east-dipping fault at a depth of 12 km, as determined by teleseismic waveform modelling. Over 4000 aftershocks were manually analysed for a period of 158 d. The events were initially located with an optimal 1D velocity model and then relocated with the double-difference method to reveal details of their spatial distribution. The latter spreads in an area spanning 80 km NNW–SSE and ∼55 km WSW–ENE. Certain parts of the aftershock zone present strong spatial clustering, mainly to the north, close to Zakynthos Island, and at the southernmost edge of the sequence. Focal mechanisms were determined for 61 significant aftershocks using regional waveform modelling. The results revealed characteristics similar to the main shock, with few aftershocks exhibiting strike-slip faulting at steeper dip angles, possibly related to splay faults on the accretionary prism. The slip vectors that correspond to the east-dipping planes are compatible with the long-term plate convergence and with the direction of coseismic displacement on the Zakynthos Island. Fault-plane solutions in the broader study area were inverted for the determination of the regional stress-field. The results revealed a nearly horizontal, SW–NE to E–W-trending S1 and a more variable S3 axis, favouring transpressional tectonics. Spatial clusters at the northern and southern ends of the aftershock zone coincide with the SW extension of subvertical along-dip faults of the segmented subducting slab. The main shock occurred in an area where strike-slip tectonics, related to the Cephalonia Transform Fault and the NW Peloponnese region, gradually converts into reverse faulting at the western edge of the Hellenic subduction. Plausible scenarios for the 2018 Zakynthos earthquake sequence include a rupture on the subduction interface, provided the slab is tilted eastwards in that area, or the reactivation of an older east-dipping thrust as a low-angle strike-slip fault that contributes to strain partitioning.

Late Cenozoic paleostress reconstruction and stress regimes in Taiyuan basin of the Shanxi rift, North China

The Taiyuan basin, initiated during the late Cenozoic, is part of the left-stepping en-echelon graben systems of Shanxi. The state of stress in this area and its evolution over time is a significant concern given the existence of high shear strain and seismic risk along the basin boundary faults. In the present study, the inversion of fault-slip data through the right dihedron and the rotational optimization methods led us to identify 21 significant paleostress tensors. The maximum principal stress orientation shows two modes trending NE–SW and ENE–WSW with most of stress tensors having stress ratio values within a wrench regime. The extensional sites reveal NW–SE and NE–SW directions. Three tectonic stages with different kinematics were successfully identified since the late Cenozoic. The oldest event has been identified in our analysis since the late Miocene to late Pliocene and belongs to NW–SE extension/strike-slip stress regime. The next was generated in the early Pleistocene under the NE–SW extension stress regime, and the youngest activity recorded belongs to the NNW–SSE extension/strike-slip regime developed since the late Pleistocene. These events were dynamically driven by the northeastward extrusion of the Tibetan Plateau in response to the plate boundary motion involving the Indian and Eurasian plates and lesser by the NW subduction of the Pacific plate. The present-day stress state is characterized by ENE–WSW compression and NNW–SSE extension derived from earthquake focal mechanisms.

Outcrop-scale manifestations of reactivation during multiple superimposed rifting and basin inversion events: the Devonian Orcadian Basin, northern Scotland

The Devonian Orcadian Basin in Scotland hosts extensional fault systems assumed to be related to the initial formation of the basin, with only limited post-Devonian inversion and reactivation. However, a recent detailed structural study across Caithness, underpinned by published Re–Os geochronology, shows that three phases of deformation are present. North–south- and NW–SE-trending Group 1 faults are related to Devonian ENE–WSW transtension associated with sinistral shear along the Great Glen Fault during the formation of the Orcadian Basin. Metre- to kilometre-scale north–south-trending Group 2 folds and thrusts are developed close to earlier sub-basin-bounding faults and reflect late Carboniferous–early Permian east–west inversion associated with dextral reactivation of the Great Glen Fault. The dominant Group 3 structures are dextral oblique NE–SW-trending and sinistral east–west-trending faults with widespread syndeformational carbonate mineralization (± pyrite and bitumen) and are dated using Re–Os geochronology as Permian ( c. 267 Ma). Regional Permian NW–SE extension related to the development of the offshore West Orkney Basin was superimposed over pre-existing fault networks, leading to local oblique reactivation of Group 1 faults in complex localized zones of transtensional folding, faulting and inversion. The structural complexity in surface outcrops onshore therefore reflects both the local reactivation of pre-existing faults and the superimposition of obliquely oriented rifting episodes during basin development in the adjacent offshore areas. Supplementary material: Stereographic projections of compiled structural data from individual fieldwork localities are available at https://doi.org/10.6084/m9.figshare.c.5115228

The influence of geological fractures on karstic sinkhole development in eastern areas of Centurion, South Africa

Centurion is a fast developing area facing severe karstic sinkhole development. The area is predominantly underlain by fractured dolomite formations that include Oaktree, Monte Christo, Lyttelton, and Eccles of the Malmani Dolomite Group (~ 2600 and ~ 2500 Ma). Fractures of the underlying dolomite formations were investigated as one of the most influencing factors in karstic sinkhole formation using fieldwork mapping, petrology, and remote-sensing techniques (automated lineament extraction methods). Fieldwork results indicate that the study area has two prevalent fracture orientations trending N–S and ENE–WSW with average orientations of 178°/86° E and 073°/67° N. The fieldwork mapping results coincide with those acquired using remote-sensing techniques by automated extraction of lineaments from colour-shaded image and from four Sobel directional filtered images. Remote-sensing techniques indicate that there are extra two prevalent lineament directions; NE–SW and E–W in the study area and majority of the sinkholes develop in areas with high lineament density. Eccles, Monte Christo, and Oaktree dolomite formations exhibit a unidirectional fracture system while Lyttelton dolomite formation exhibit bidirectional fracture system. It is, therefore, concluded that (1) geological lineaments extracted using remote sensing and fractures present in the underlying dolomite formations play a significant role in controlling fluid circulation and thus influencing karstic processes such as dissolution, and therefore, (2) sinkhole development is primarily structurally controlled; however, it is exacerbated by anthropogenic activities due to the influence of water ingress.

Tectonic, volcanic and hydrothermal features of a nascent rift graben in the southern Okinawa Trough

High-resolution geophysical mapping using an autonomous underwater vehicle (AUV) and surface vessel revealed tectonic, volcanic and hydrothermal features of a nascent rift graben in the southern Okinawa Trough. Ship-based bathymetry identified ENE–WSW trending lineaments at a water depth of 1900–2000 m that form a rift graben, the Ishigaki Rift, which is ~50 km long, ~7 km wide and has relief of ~100 m. Shipboard EM122 multibeam sonar detected acoustic bubble plumes and high-backscatter intensity on the seafloor along the rift. The AUV high-resolution bathymetry (2 m) and sidescan imagery revealed the presence of a ropey texture with high acoustic reflectivity, interpreted to be relatively young lava flows. Evidence of hydrothermal venting was observed at two previously unknown sites along the rift. Steep-sided spires up to 23 m in height and a mound discharging smoke-like acoustic plumes were identified. The AUV temperature sensor observed sharp temperature increases of up to ~0.13 °C around the plumes, even at the vehicle's altitude of ~100 m above the seafloor. The presence of relatively young lava flows and indicators of high-temperature hydrothermal venting and gas bubbles suggest the Ishigaki Rift is magmatically active. Compared with adjacent active back-arc rifts, the Ishigaki Rift has a shorter length, narrower width and lower relief, and occurs at a shallower water depth, suggesting that the rift is relatively young and immature. Considering that the Ishigaki Rift is located near the arc volcanic front and that other mature rifts are typically located farther from the front, it is suggested that a new axial rift system is evolving within the Okinawa Trough and that back-arc rifting processes may be gradually migrating arcward in the trough during the Quaternary.

Relocation, Focal Mechanisms and Stress Field Characteristics of Earthquake Swarms in the Songyuan Area of Jilin Province, China

Two earthquake swarms, the Qianguo swarm and Ningjiang swarm, have been observed since 2013 in the Songyuan area, Jilin Province, China. Using the seismic data of broadband permanent and temporal stations that recorded a larger number of earthquakes in the two swarms, we analyze the distribution of aftershocks and the focal mechanisms of these earthquakes and investigate the stress field and faulting properties of the Songyuan area. The relocated hypocenters of the Qianguo earthquakes are distributed along a NNW direction at focal depths of 4–13 km, while the dominant distribution of relocated Ningjiang earthquakes is along a NE direction at depths of 8–16 km. The focal mechanism solutions of 36 earthquakes with ML ≥ 3.6 demonstrate a combined strike-slip and thrust faulting mechanism with centroid depth concentrated at 3–7 km for the Qianguo swarm and a strike-slip mechanism with centroid depth at 5–7 km for the Ningjiang swarm. The full moment tensors of seven moderate–strong earthquakes show that these earthquakes contain a high percentage of non-double-couple components. The regional stress field is inverted using the focal mechanism parameters of the 36 earthquakes. In the Qianguo area, the horizontal maximum principal compressive stress ( $$\sigma_{1}$$ ) is in the E–W direction with a dip angle of 14.1°. In the Ningjiang area, the $$\sigma_{1}$$ is in the ENE–WSW direction with a dip angle of 9.2°. The $$\sigma_{1}$$ directions of the two areas are slightly different, but both approximately along an E–W direction. The comprehensive results of this study show that the Qianguo swarm is likely caused by the movement of a buried fault in the NNW direction oblique to the Changling–Dashanzi fault, whereas the Fuyu–Zhaodong fault in the NE direction is responsible for the Ningjiang swarm. Both the Qianguo and Ningjiang swarms are probably closely related to the thrusting effect of the subduction of the Pacific Plate into the Northeast Asian continental margin and the complex geological conditions in the Songyuan area.

Structural and hydrothermal alteration mapping related to orogenic gold mineralization in part of Kushaka schist belt, North-central Nigeria, using airborne magnetic and gamma-ray spectrometry data

This paper presents an interpretation of airborne magnetic and gamma-ray spectrometry data over a part of the Kushaka schist belt of Nigeria with a view to assess the sub-surface structural framework and hydrothermal alteration haloes that are related to orogenic gold mineralization in the study area. The geophysical analyses performed to improve the quality of the aeromagnetic data for better understanding of the subsurface geology were reduction to the equator, upward continuation, total horizontal derivative, analytical signal, source edge detection, tilt-angle derivative of total horizontal derivative. The analyses performed on the airborne gamma-ray spectrometry data were made on K/eTh, F parameter and deviation of ideal K values (Kd). Deep-seated, brittle–ductile shear/fracture zones with characteristic depth of 428.0–607.6 m and area extent of 15.5 km by 6.5 km that were delineated in the area coincide with beehive of artisanal miners exploiting gold deposits. In this study, subsurface geologic structures, specifically dykes/faults, that trend in the ENE–WSW direction constitute a first-order control on orogenic gold mineralization in the study area. The hydrothermally altered zones mapped in the area indicated they are structurally controlled and closely associated with orogenic gold mineralization. Apart from geologic structural evidences that are controlling orogenic gold mineralization, this study has indicated that hydrothermal alteration mapping is equally important for a promising orogenic gold mineralization mapping.

Geometry and kinematics of the Baza Fault (central Betic Cordillera, South Spain): insights into its seismic potential

The geometry and kinematics of active faults have a significant impact on their seismic potential. In this work, a structural characterization of the active Baza Fault (central Betic Cordillera, southern Spain) combining surface and subsurface data is presented. Two sectors are defined based on their surface geometry: a northern sector striking N–S to NNW–SSE with a narrow damage zone and a southern sector striking NW–SE with a wide damage zone. A kinematic analysis shows pure normal fault kinematics along most of the fault. Geometric differences between the northern and southern sectors are caused by i) a heterogeneous basement controlling the fault geometry at depth and in the cover; ii) different orientations of the Baza Fault in the basement with respect to the regional extension direction and iii) interaction with other active faults. We use this structural characterization to analyse the segmentation of the Baza Fault. According to segmentation criteria, the entire Baza Fault should be considered a single fault seismogenic segment. Consequently, the seismic potential of the fault is defined for a complete rupture. Magnitude for the Mmax event is calculated using several scale relationships, obtaining values ranging between Mw 6.6 and Mw 7.1. Recurrence times range between approximately 2,000 and 2,200 years for Mmax events and between 5,300 and 5,400 years for palaeo-events. A geodetic scenario modelled for an Mmax event of Mw 6.7 shows permanent vertical displacements of more than 0.40m and an overall WSW–ENE extension during entire ruptures of the Baza Fault.

Geological control of the eastern Great Wall: Mountain-basin relationships in the eastern North China Craton

The E–W trending Yanshan belt, an intraplate fold-thrust belt located in the northern North China Craton, has experienced several episodes of Mesozoic deformation, which resulted in the widely distributed magmatism and mountain-basin tectonics that completely re-shaped the topography of the eastern North China Craton. The eastern part of the famous Chinese Great Wall was built on the high range of the southeastern Yanshan mountain belt juxta-posed to the plain, which directly relates to the Bohai Bay basin. Our study focuses on which tectonic processes created such mountain-basin couple in Mid-Late Mesozoic times. The U-Pb LA-ICP-MS dating of zircons yield ages of 114 to 201 Ma for various granites and 115 to 116 Ma for volcanic rocks from Yixian and Jiufotang Fms. The detrital zircons from the Lower Cretaceous sandstones yield four age groups of 2587 to 2460 Ma, 2222 to 1828 Ma, 297 to 190 Ma and 187 to 100 Ma, which are all sourced from the Qinglong and surrounding areas and indicating that the Qinglong area started to uplift after the Middle Jurassic. The Qinglong area underwent multiple deformation by NE–SW compression in the Middle-Late Jurassic times, WNW–ESE compression in the Late Jurassic to Early Cretaceous, ENE–WSW extension in the Early Cretaceous and NNW–SSE compression in the Late Cretaceous during the final stage of the Yanshanian orogeny. Meanwhile, widely distributed granite intrusions and emplacement of the Upper Jurassic-Lower Cretaceous volcanic rocks indicate a large amount of magma input into the area. The Qinglong area with the Great Wall along its southern margin close to adjacent plain to the south was uplifted to form the Qinglong highland and surrounding related basins by combination of the following three processes: the multiple tectonic Late Jurassic and Late Cretaceous shortening processes related to Yanshanian orogeny, Early Cretaceous regional extension triggered by slab-retreat of the Paleo-Pacific ocean and inflation of large amount magma at depth during Jurassic and Early Cretaceous.

Insight on the Crustal Stress State in Faial and Pico Islands (Azores), from Analysis of Aftershocks of the 1998 Earthquake

The seismicity in the Azores plateau can be of volcanic or tectonic origin, the latter usually more destructive, especially in the central group of islands. In this study we aimed to obtain crustal stress indicators from seismic data collected in July 1998, following the July 9th M5.8 earthquake that struck near Faial and Pico islands. The maximum horizontal stress (SHmax) azimuth was obtained from two methods. For the seismogenic volume, the SHmax was obtained from single and composite focal mechanisms, whereas the state of stress beneath the seismic network was derived from shear-wave splitting analysis of local earthquakes. The composite focal mechanisms were obtained from a waveform clustering method applied to identify “identical” earthquakes. The seismic anisotropy analysis could be applied to only six stations, the polarization of the first S arrival being stable, and with a secondary perpendicular pulse arriving 0.6–0.24 s later. The focal mechanisms provided a NW–SE SHmax with azimuth range of 96°–114°, and an associated transition of normal-fault to strike-slip dominant regime consistent with the estimated regional field of 110°–120°, and coherent with movement along three fault systems. Inland the SHmax azimuths present greater variability. In Faial, the stations located north of the Caldeira present a stable azimuth of 151°–153° and are consistent with the effect of a NNW–SSE fault system present in the islands, rotating to 50°–77° in the eastern sector of the Pedro Miguel Graben, probably due to a complex interaction of the WSW–ENE dominant fault system with old eroded volcanic complexes that segmented the island into several blocks. In Pico island, the SHmax direction azimuth ranges from 93° to 135°, in a radial pattern around the island's main stratovolcano, overlapping the regional stress field.

Interpretation of Airborne Magnetic and Geo-electric Data: Resource Potential and Basement Morphology of the Ikom–Mamfe Embayment and Environs, Southeastern Nigeria

The Ikom–Mamfe Embayment, which cuts across southeastern Nigeria and southwestern Cameroon, is a bifurcation of the Benue Trough and the NW–SE splay component of the NE–SW trending trough. The Nigerian sector of the embayment, a sedimentary province bounded to the north and south by basement terrains, is considered a relatively frontier basin with little information about its resource potential. Here, airborne magnetic, resistivity and self-potential (SP) data were integrated in studying the Nigerian sector of the embayment and its surrounding localities. This study aimed to delineate (possible) mineral-bearing lineament structures and their corresponding trend orientations, to evaluate their relationship to tectonic fabric, to estimate depth of perturbing causative sources, to map ore bodies, and to identify probable locations for detailed petroleum system analysis. For the first time, 2D electrical resistivity and SP methods were deployed in the sub-surface characterization of the embayment. Results of aero-magnetic data analysis revealed the presence of high magnetic intensities predominantly in the western and northeastern parts of the study area. The high intensities observed at the western part are due to mineral ore bodies present within a sedimentary host. In addition to the presence of mineral ores, the noticeable high magnetic signatures at the northeastern part are associated with the presence of outcropping units of basement complex rocks that comprise mainly of migmatitic metasediments. The large variation in magnetic intensities within the embayment is attributed to the combined effect of igneous intrusives and mineral-bearing lineaments emplaced within the overlying sediments. Analysis of structural lineaments depicts E–W and ENE–WSW as the dominant orientations with minor NW–SE, NE–SW and NNE–SSW trends. The NE–SW and ENE–WSW structures are aligned parallel to the Benue Trough and the Cameroon Volcanic Ranges, and are genetically related. These lineaments occur in the form of faults, fractures and veins, and they are densely distributed within some areas of moderate to high intensities and analytical amplitudes. This suggests a possible relationship between structural lineaments and metalliferous deposits. These lineaments were marked for ground geophysical surveys. The geophysical traces derived from these areas reveal numerous underlying mineral bodies of contrasting resistivity and potential with their host formation. These bodies are shallow magnetic sources exposed in the first and second vertical derivative images. These show remarkable agreement between the geo-electric data and the aero-magnetic data. The results of quantitative assessment show shallow basement depth and, hence, thin sedimentary layer (0.2–2.4 km) within and around the embayment, which is insufficient for the concealment of hydrocarbon resource. In addition, the presence of numerous intrusive rocks implies a high temperature environment favorable for “cooking” of potential source rocks beyond the hydrocarbon generating window. This renders the area disadvantageous for hydrocarbon exploration. However, we discover that the sedimentary thickness (ranging between 3 and 4 km) mapped at the southwestern part of the area could favor the formation and accumulation of hydrocarbon fluids. This area, characterized by moderate igneous intrusion, lies approximately northeast of the prolific Niger Delta oil province and shows fair to good sub-surface structural conditions required for hydrocarbon entrapment. The basement morphology, sedimentary thickness and structural disposition call for a detailed petroleum system analysis within this area.

Mapping of subsurface structural features in some parts of Anambra Basin, Nigeria, using aeromagnetic data

Aeromagnetic data of some parts of Anambra Basin, Nigeria, covering Illushi, Nsukka, Igumale, Onitsha, Udi and Nkalagu areas were digitally processed, analyzed and interpreted qualitatively and quantitatively. The study was carried out to delineate the subsurface structural lineaments/trends that provide the locales for mineral deposit and hydrocarbon resources and to estimate the depth to magnetic sources/sedimentary thickness of the area. The data were qualitatively interpreted using reduction to equator-inverted (RTE_INV), vertical derivatives (VD), tilt derivative (TDR) filter, lineament density filter and Rose plot. Quantitative interpretation was done using source parameter imaging and Euler deconvolution methods. The results depicted that the study area has enormous structural lineaments especially in the northeast and northwest that showed a high cluster of these lineaments. The results showed that the trends of the structural lineaments have a dominance of NE–SW direction, with ancillary trends in E–W, NW–SE, N–S, NNE–SSW and ENE–WSW directions. The SPI depth result ranges from 318.45 m for shallow magnetic sources to − 11,253 m for sedimentary thickness. The 3D Euler deconvolution depth estimate revealed depths to magnetic sources ranging from − 400 to − 4300 m. The results show good evidence of the presence of the essential factors for the exploration of both mineral deposits and hydrocarbon resources in the study area.

Late Quaternary transpressional earthquakes on a long-lived intraplate fault: A case study of the Southern Yangsan Fault, SE Korea

The NNE–SSW-striking Yangsan Fault in southeastern Korea has been regarded as one of the most prominent seismogenic structures in the Korean Peninsula on the basis of instrumental and historical seismicity, and paleoseismic records along the fault zone. Its seismic behavior is, however, still uncertain due to long recurrence intervals of strong earthquakes and insufficient historical and geologic records. We conducted a detailed paleoseismic investigation, including an 8 m-deep excavation, in order to understand recent earthquake-faulting events along the Southern Yangsan Fault. Our observations indicate that at least two and possibly four dominantly strike-slip surface-faulting events have occurred along a subsidiary fault in the eastern boundary of the fault valley since the late Pleistocene. Stratigraphic features and OSL ages of the stratigraphy exposed in the trenches indicate that the most recent rupture(s) occurred after 29.2 ± 1.4 ka and the timing of earlier ruptures are constrained to between 70.0 ± 3.7 ka and 29.2 ± 1.4 ka. Using a contact of the oldest dated unit that has at least 7.5 m of vertical separation, we calculate a minimum vertical slip rate during the late Quaternary of 0.11 mm/yr. The minimum horizontal slip rate is presumed to be two times that of the vertical slip rate based on striations observed on clasts next to the fault core. We also propose that the late Quaternary earthquake-faulting kinematics along the Yangsan Fault, expressed as contractional dextral slip with east-side-up geometry, is strongly dependent on a pre-existing fault zone architecture with a strike of N10–20°E that dips to the east, and the direction of neotectonic maximum horizontal stress (ENE–WSW to E–W).

Contribution of bedding to the petrophysical characterization of naturally fractured reservoirs: Example of the Matachines fields, Upper Magdalena Valley (Valle Superior del Magdalena – VSM) Colombia

The Matachines field, located in the Girardot subbasin, Upper Magdalena Valley (Valle Superior del Magdalena – VSM) basin, has a morphology characterized by antiforms arranged along a N–S axis. These antiforms involve multiphase tectono-sedimentary sequences with half-graben geometries associated with Mesozoic sequences reactivated by at least three Andean phases. Bedding surfaces are mechanically active planes that contribute to the flexural slip that is generated by the folding of multilithologic sequences. In the Matachines field, the bedding planes help to improve the petrophysical characteristics of the reservoirs by incorporating a significant number of discontinuities, which complement the fluid connectivity. Thus, the subhorizontal planes are linked to subvertical fractures preferentially arranged in the ENE–WSW direction, generated by horizontal compressive stress. In particular, these effects are magnified on the western flank and hinge zone of the anticline structure, where they contribute greatly to the connectivity of the production wells. In naturally fractured reservoirs, the correct 4D technical and economic evaluation of the quality and arrangement of discontinuities is essential to determine their actual contribution to improving the petrophysical properties of such reservoirs. Accordingly, many methods often used to study wells from core samples or images identify, along with different types of fractures, the presence of bedding surfaces, although they evaluate them separately, so they overlook petrophysical factors that affect the porosity and permeability of the wells. The present study discusses the genetic and morphological aspects of bedding surfaces that enhance the petrophysical potential of reservoirs, which usually have limited primary values of permeability and porosity, to meet the economic expectations of specific resources in hydrocarbon exploration and/or production activities.

Structural fabric over the seismically active Kachchh rift basin, India: insight from world gravity model 2012

The Kachchh rift basin (KRB), located in the western continental margin of India, is very important due to repeated large magnitude occurrence of intraplate earthquakes. The present study elucidates the subsurface structural features using wavelength filtering, total horizontal derivative (THD) and 2.5 dimensional (2.5D) density modelling of the gravity data. Power spectrum analysis of the gravity data gives the cut-off wavelength of 100 km that has been used for wavelength filtering for regional-residual separation. The residual gravity anomaly significantly enhances the shallow features (like Allah-Bund fault, Kachchh mainland uplift, Wagad uplift, and Pachham uplift) and gives an NW–SE subsurface structural trend. The predominant direction for major lineaments derived by the THD of gravity data shows the ENE–WSW as well as NE–SW and NW–SE directions. The 2.5D density model of the regional gravity anomaly reveals the presence of high-density magmatic underplating layer and intrusive mafic body in the lower crust that is in accordance with the previous studies. The regional slope of the Moho in the KRB is towards the SW direction. An intrusive mafic body in the lower crust and volcanic plugs in the upper crust act as stress concentrator which is responsible for seismicity in the KRB. Several new lineaments have been identified based on the present study, which may be accountable for seismicity over the different part of KRB.

Relative Abundance and Compositional Variation of Silicates, Oxides and Phosphates in the W-Sn-Rich Lodes of the Panasqueira Mine (Portugal): Implications for the Ore-Forming Process

Panasqueira is a world-class W-Sn-Cu lode-type deposit located in Portugal. It consists of a dense swarm of subhorizontal quartz lodes criss-crossed by several ENE–WSW and N–S fault zones, bordering Late Variscan granite and hosted in Late Ediacaran—Early Cambrian metasediments. The relative abundance and compositional variation (assessed with EPMA) of the main silicates, oxides and phosphates forming the quartz lodes and their margins were examined, aiming to explore: (i) mineral and geochemical zonation at the mine scale; and (ii) some conclusions on the chemical nature of prevalent fluid inflows and T-conditions of mineral deposition. Quartz lodes nearby or far from the known greisen-granite cupola display significant differences, reflecting multiple fluid influxes of somewhat distinct composition related to various opening and closing events extending for several My, ranging from an early “oxide–silicate stage” (OSS) to a “main sulfide stage” (MSS), and further on to a post-ore carbonate stage (POCS); however, a rejuvenation event occurred after MSS. The onset of OSS was placed at ca. 299 ± 5 Ma and the rejuvenation event at ca. 292 Ma. The OSS was confined to ≈500 ≤ T ≤ 320 °C, following rutile and tourmaline growth under ≈640 ≤ T ≤ 540 °C (depending on aSiO2). The rejuvenation event (≈440–450 °C) preceded a late chlorite growth (≈250–270 °C) and the progression towards POCS.

Curvature interpretation of gravity data for delineating structural features across the transition zone of Narmada–Son lineament in central India

Potential-field anomalies can be converted to a common function which describes the ridge, peaks over an isolated source. In this study, attempt has been made to interpret curvature attributes of equipotential surface using gravity gradients for delineating thrust–fault locations in one of the seismically active transition zones of Narmada–Son Lineament (NSL) in central India. The NSL is oriented in the ENE–WSW direction, divides the area into two geological provinces. The northern part of this NSL comprises Vindhyan formations and southern part covers Gondwana formations. It has been observed that, the northern part (around Jukehi) of the area has higher elevation with lower gravity anomaly and the southern part (around Khamaria and Kataria) has lower elevation with higher gravity anomaly. The tectonic activities like intrusions, volcanisms and earthquake occurrences were the sources of crustal resettlement during the past in this area. Various researchers commented about the structural configuration; however, the present study helps to understand the structural features using various curvature analysis like maximum, minimum, most-positive, most-negative, mean, dip (profile), strike (tangential), contour (plan), curvedness, shape index and Gaussian curvatures. Out of these curvatures most positive, maximum, mean, dip and curvedness show more effective results compared to other curvatures. However, shape index states domes, ridge, flat, valley, bowl shape type structures. The results are also correlated with the conventional horizontal gravity gradient, analytic signal and tilt derivative analysis. The colour contrast variation of these differential curvatures make in-depth understanding with extensive multiplicities of integrity for delineating subsurface structures.