SE Faults Research Articles

INSIGHTS INTO THE MOHO DEPTH VARIATIONS AND CRUSTAL CHARACTERISTICS IN THE GUELMA–CONSTANTINE BASIN, NORTHEASTERN ALGERIA: A SHORT-PERIOD SEISMIC-RECEIVER-FUNCTION PERSPECTIVE

Teleseismic receiver functions (RF) were extracted from data collected at eight short-period, three-component seismic recording stations over the Guelma–Constantine Basin, northeastern Algeria, to improve the understanding of crustal structure and geodynamic processes. The H-κ stacking method was used to determine the Moho depths and average vP/vS ratios at each station. Careful linear inversion of RF was performed to determine the most appropriate average shear-wave and P-wave velocity profiles at each site. Both methods have yielded highly congruent results, with Moho depths showing robust correlations with previous seismological and geophysical studies. The previously observed pattern of the increasing Moho depth from north to south in the Tell Atlas has been confirmed. Furthermore, the identified transitional nature of the Moho in the Constantine Basin is consistent with a recent study. In addition, we identify a low-velocity zone (LVZ) at approximately 20 km depth within the southern Guelma Basin, confirming previous results in the Constantine Basin and suggesting an eastward elongation of the LVZ, at least into the southern periphery of the Guelma Basin. Examination of data from the northern tip of the Hammam Debbagh–Roknia NW–SE fault, the western boundary of the Guelma pull-apart basin, revealed a shallow Moho depth (22 km), less than the basin average depth of 25 km. The LVZ observed in the lower crust (12 km) suggests the presence of partial melts, consistent with gravimetric and chemical analyses of hydrothermal sources in the area. The extensional tectonic activity along this boundary, coupled with the low-viscosity zone and low average vP/vS ratio, is potentially associated with delamination processes. The effectiveness of our approach underscores its potential as a viable alternative or complementary method for investigating variations in the Moho depth.

Geologic causatives of the stratigraphic and structural heterogeneties of Baharyia formation, based on their seismic interpretation, Razzak field, Western Desert, Egypt

Evaluation of seismic attributes and seismic reflection characteristics analysis of Razzak Field was undertaken by subdividing the study area rock units. There are four seismo-facies units of varying parameters from top to bottom. These units are comparable to the Apollonia and Khoman Formations; the Abu-Roash Formation; Baharyia Formation and Alamein Formation. The seismo-facies unit 1 of the Apollonia and Khoman Formations reveal parallel to sub-parallel layering, with thickness increases northwards. The lithological distribution consists of limestone with minor clay intercalations with facies varying from middle neritic graded to outer neritic. Unit 2 of the Abu-Roash Formation shows divergent layering, with a thickness increasing gradually northwards and eastwards. The lithological distribution shows predominance of carbonate, sandstone occurring in minor portions and laterally graded into shale with littoral facies ranging to inner and middle neritic facies. Unit 3 of the Baharyia Formation reveals chaotic and oblique layering with a time thickness increases towards northeast and southwest parts, the lithological distribution exhibited sand and shale with minor limestone streaks, with facies varied changed from continental to inner-middle neritic, unit 4 of the Alamein Formation shows variation from parallel-subparallel orientation to oblique and chaotic with thickness increasing towards northwards and eastwards, the lithological distribution exhibited dolomitic limestone with minor streaks of limestone with facies varied from middle neritic facies to outer neritic and bathyal. Finally, the results are integrated to build up a seismo-stratigraphic model of the evaluated area of the northwestern Desert of Egypt. Seismic interpretation involves the construction of structure contour maps, in terms of time and depth, on the tops of the Apollonia, Abu-Roash, Baharyia and Alamein Formations. These maps show three structural closures, due to folding, that are dissected by NW–SE faults, analysis of relevant structural and stratigraphic seismic attributes such as root-mean-square amplitude, local structural dip, variance, ISO frequency component, sweetness and acoustic impedance average energy applied on reservoir tops, to enhance the visibility of faults, geological interpretation and the physical parameters of the subsurface related to lithology and stratigraphy for reservoir characterization. Finally, the results obtained are used to construct a seismic structural model. Integration of seismic and stratigraphic data is used to build a geological model of the northwestern Desert of Egypt.

Orogenic lode-gold deposits and listvenization processes in the El-Barramiya area, Eastern Desert, Egypt

Gold mineralization in the El-Barramiya region of the Eastern Desert, Egypt, is connected to the post-accretionary stage throughout the Central Eastern Desert. It is represented by quartz, quartz-carbonate veins and disseminations in listvenite rocks. The thrust contact between rock units in El-Barramiya area played an imperative part in gold mineralization where the obduction of ophiolitic rocks over the metasediments and metavolcanics caused shear zones. Mineralization in the study area formed along shear zones and the gold mineralization prefers to precipitate along the transition zone between low-grade regional metamorphic area which is represented by metasediments and high grade which is represented by actinolite schist. The gold mineralization lode of El-Barramiya gold mine area is situated in E–W trending quartz and quartz-carbonate veins along a shear zone located in the intersections between faults trend in NE–SW (Najd fault), NW–SE and thrust faults trend in NEE–SWW in metavolcanic and metasedimentary host rocks. Porphyry granite in the mine area played an important role in hydrothermal alteration process where it represents the source of K, listvenite formed when fluids rich in CO2 and bearing-K permeate and alter the previously altered ultramafic rocks, usually serpentinites of the ophiolitic mélange rocks. The listvenitization process includes silicification and carbonatization metasomatic processes, tectonized serpentinites are altered to listvenite as the carbonatization becomes more intense close to dipping transpressive faults. Geochemical studies of listvenite and mineralized veins helped to determine the ultramafic genesis of listvenite and gold transformed as gold bisulfide. The whole rock geochemical data from El-Barramiya and elsewhere indicate that the transformation of serpentinite into listvenite involves profound metasomatic modification of the bulk-rock geochemistry. The chemical changes during alteration of serpentinite to listvenite are dominated by the addition of CO2, the removal of H2O, and the redistribution of SiO2, MgO and CaO as carbonate minerals and silica replace serpentine. All listvenites at El-Barramiya lode gold deposit are enriched in CaO, Fe2O3 and K2O, but depleted in MgO compared with associated serpentinite that is presumed to represent their protoliths. The chemical changes during alteration of serpentinite to listvenite are dominated by the addition of CO2, the removal of H2O, and the redistribution of SiO2, MgO and CaO as carbonate minerals and silica replace serpentine. Alteration also caused redistribution of trace elements, with some being locally remobilized within the rock, some being added from a fluid phase, and others being leached out of the rock. Petrographic investigation and geochemical studies show different types of alterations (carbonatization and silicification) and mineralization. Mineralizations are represented by gold and sulfides (pyrite, arsenopyrite and smaller quantities of chalcopyrite, sphalerite, galena, tetrahedrite and gersdorffite) found in auriferous quartz veins and disseminated in listvenite. The area exposed to brittle–ductile deformation in addition to different types of structures such as faults and fractures controlling on the formation of mineralization and act as hydrothermal channels ways for fluid flow. Fluid inclusions studies revealed that gold mineralization was formed from heterogeneous trapping of H2O–CO2 fluids at a temperature of 280–340 °C and pressure within the range of 1.5–1.9 kbar, which is consistent with the mesothermal conditions.

Three-Dimensional Electromagnetic Imaging of Geothermal System in Gonghe Basin

To better understand the geothermal system of the Gonghe Basin, we deployed 471 magnetotelluric survey points with an average distance of 2~3 km, covering the eastern and southern areas of the Basin. We used ModEM inversion software to carry out 3D inversion of 431 survey points and established a 3D-electrical model at a depth of 50 km in the area. The resistivity model shows that the low resistivity in the shallow part of the basin is related to the Cenozoic loose sedimentary cover, while the resistivity values of the mountains around the basin and the magmatic rock uplift zone are higher. The electrical model also shows that the high-conductivity layer is widely distributed in the middle and lower crust (15~35 km) of the basin, and direction of the high-conductivity layer is consistent with that of NW–SE fault in the basin. These high-conductivity layers may be the principal reason for the high heat flow values in the Gonghe Basin. Our resistivity model also shows that there is an obvious discontinuity between high- and low-resistivity blocks at different depths in the middle and upper crust. These discontinuities are consistent with the faults observed on the surface, which are related to the strong topographic relief. Our electrical model shows that these faults in the middle and upper crust are connected with the high-conductivity layer as the channel of heat transfer to the shallow part. Finally, the heat energy is enriched in the Triassic granite to form dry hot rock (HDR). The 3D-magnetotelluric imaging results depict the 3D-distribution characteristics of the geothermal system in the eastern and southern parts of the Gonghe Basin.

An Application of Softsign Function (SF) Filter to Low-Latitude Aeromagnetic Data of Tafawa-Balewa Area, Northern Nigeria for Geostructural Mapping and Tectonic Analysis

A recently proposed edge detection technique – Softsign Function (SF) filter was applied to a synthetic magnetic data to illustrate its application. It was then applied in combination with 3D Euler deconvolution and 2D magnetic modelling techniques to the low-latitude aeromagnetic data of Tafawa-Balewa Area, Northern Nigeria, aimed at subsurface geostructural mapping and tectonic analysis of the study area. This was corroborated with mapping and analysis of surface lineaments from Landsat-9 image of the study area. The SF filter depicted edges of magnetic sources from the synthetic magnetic and aeromagnetic data as distinct peaks. The 3D Euler deconvolution solutions and 2D magnetic model showed two dominant depth ranges ∼ 10 m to ∼ 400 m (D1) and ∼ 400 m to ∼ 1000 m (D2) of magnetic sources. The subsurface and surface lineaments from the aeromagnetic data and Landsat-9 image, respectively, fairly agreed in location, trends, even distribution, and density. The trends of mapped lineaments are majorly ENE – WSW direction, and the minor trends are WNW – ESE, NE – SW, NNW – SSE, NNE – SSW, and NW – SE directions. These linear structures, consistent with regional tectonics, are suspected to be strike-slip faults generated by the conjugate system of strike-slip NE – SW and NW – SE faults, including the Romanche and Chain fault systems, produced by the oceanic-continental transcurrent movements which affected the Nigerian basement complex. Explicitly, Softsign Function (SF) filter is a valuable edge detection technique in mapping geological structures from a real low-latitude aeromagnetic data reduced to the geomagnetic equator for tectonic analysis.

A single multi-scale and multi-sourced semi-automated lineament detection technique for detailed structural mapping with applications to geothermal energy exploration

Semi-automated algorithms incorporating multi-sourced datasets into a single analysis are increasingly common, but until now operate at a fixed pixel resolution resulting in multi-sourced methods being limited by the largest input pixel size. Multi-scale lineament detection circumvents this issue and allows increased levels of detail to be captured. We present a semi-automated method using a bottom-up Object-Based Image Analysis approach to map regional lineaments to a high level of detail. The method is applied to onshore light detection and ranging (LiDAR) data and offshore bathymetry around the Land's End Granite (Cornwall, UK). The method uses three different pixel resolutions to extract detailed lineaments across a 700 km 2 area. The granite displays large-scale NW–SE fault zones that are considered analogous to those being targeted as onshore deep geothermal reservoirs (2–5 km in depth). Investigation of the lineaments derived from this study shows along-strike variations from NW–SE orientations within granite to NNW–SSE within slate and reflects structural inheritance of early Variscan structures within Devonian slates. This is furthered by analysing these major structures for reservoir potential. Lineaments proximal to these broadly NW–SE features indicate that a damage zone c. 100–200 m wide is present. These observations provide a preliminary understanding of reservoir characteristics for fault-hosted geothermal systems. Thematic collection: This article is part of the Remote sensing for site investigations on Earth and other planets collection available at: https://www.lyellcollection.org/topic/collections/remote-sensing-for-site-investigations-on-earth-and-other-planets Supplementary material: A description of the OBAI method and additional figures are available at https://doi.org/10.6084/m9.figshare.c.6309629

Investigation of the Ayrobera geothermal field using 3D magnetotelluric data inversion, Afar depression, NE Ethiopia

The Afar Depression, a region of active extensional tectonics and volcanism in the northeastern section of the Ethiopian Rift, hosts a high-enthalpy geothermal area, the Ayrobera geothermal field. High temperatures recorded from wells drilled in this area suggested the existence of this geothermal system. In this study, magnetotelluric data acquired from 199 stations in Ayrobera at different times since 2007 were inverted to produce a new resistivity distribution model to image geothermal structures. Our new 3-D magnetotelluric model revealed three main resistivity features: a near-surface conductive first layer (C1) interpreted as clay-rich sediment at shallow depths down to 600m and basaltic rocks (600–1200 meters) corresponding to the smectite–zeolite zone, followed by a high resistivity region (R1 and R2) below the C1 conductive layer, associated with high-temperature alteration minerals such as chlorite and epidote. The chlorite and epidote were detected below 800 meters in the deep well TD-3, drilled in the southern section of the Ayrobera geothermal field. Fumaroles with high temperatures recorded on the surface near the NW–SE faults (F1 and F2) and hot ground anomalies from shallow ground temperature measurements above the resistive zones (R1 and R2) indicate an upflow region underneath the Ayrobera field. This also implies the possibility of two reservoirs within the resistive zones (R1 and R2) inside the fractured Afar Stratoid Series. At depth, the resistivity model reveals another conductive region (C2) below 8 km, interpreted as a partial melt (heat source) based on previous studies including seismic and geochemical analysis.

Polyphased brittle deformation around a crustal fault: A multi-scale approach based on remote sensing and field data on the mountains surrounding the Têt hydrothermal system (Eastern Pyrénées, France)

Studying hydrothermal systems in basement environments requires knowledge of fault and fracture network distributions. This study addresses this through multi-scale structural analysis of the Têt fault and its surrounding fracture systems (Eastern Pyrénées) using remote sensing and field data. This study aims to achieve this through: 1) precisely mapping and describing the brittle fault network, 2) analysing the distribution of lineaments and outcrop-scale fractures related to these faults, 3) making comparisons to fault-kinematic evidence combined in a new regional review, 4) examining the relations between fractures features and lithology, 5) applying statistical analysis to highlight relations between different scales of deformation. The complex fault network is inherited from consecutive tectonic stages (Hercynian, Pyrenean compression, Neogene extension) and has been reactivated since the middle-Miocene. NE–SW secondary faults are abundant at the regional scale, even away from the Têt fault. Major NW–SE faults are constituted by 10s-m wide brittle core zones, and NW–SE secondary faults are concentrated around the Têt fault, attesting that they had formed at shallow crust levels after the Oligo-Miocene extension. N–S fractures, formed during Pyrenean compression, are part of the background fracturing and are scattered throughout the study area. Intersections of fault and fracture networks provide efficient permeable pathways for meteoric and hydrothermal fluids. Finally, a dislocation model reveals a lithological control on fracture apertures in crystalline rocks, which appears more preserved at depth than in metasediments. All of these elements are integrated in a global model of the hydrothermal system establishment in accordance with the faulting sequence, with the damage distribution and with the lithology. This distributed fault system could represent the surface expression of the crustal thinning revealed by recent geophysical data. The realized identification of the lithological and structural characteristics of the surrounding mountains, allowing hydrothermal circulation to establish itself, provides a better understanding of the orogenic-belt related hydrothermal systems necessary to the geothermal exploration.

Geological characterization of the Patterson CO2 storage site from 3-D seismic data


 Approximately 26 square miles of new 3-D seismic data were acquired in July 2019 over the Patterson Site (Kearny County, Kansas) to assess its potential for carbon dioxide (CO2) storage. Seismic interpretation revealed that the Patterson Site contains multiple structural closures that lie on uplifted fault blocks, bounded by two reverse faults that strike nearly perpendicular to each other. These faults offset Precambrian through Pennsylvanian sections, including several primary reservoir and seal intervals. Fault displacements are maximum at the Precambrian basement and decrease upward. Data indicated a range of structural and combination traps exists at the Patterson Site in the Cambrian-Ordovician Arbuckle through Mississippian Osagian reservoirs. The three-way closures along the NW–SE fault have structural relief of ~130 ft (40 m), and the four-way closures contain relief of ~60 ft (18 m). Erosional surfaces and multiple basement fractures also are observed on the top of the Precambrian. A Mississippian-aged incised valley system also was observed at the Patterson Site. The incised valleys formed during the Meramecian-Chesteran Stages with an incised depth up to 250 ft (76 m). The motion of the reverse faults likely captured existing meandering and linear channels, causing the current deeply incised morphology. The incised valleys observed at Patterson are similar in age, structural style, shape, incision depth, and seismic attribute properties to incised valleys observed by other workers at Pleasant Prairie South, Eubank, and Shuck oil fields (southwest Kansas). Further research should focus on estimating reactivation tendency and sealing characteristics of the reverse faults to evaluate the seal integrity of the saline reservoirs. This will reduce uncertainty concerning the risk of CO2 migration during injection and storage. Further reservoir description, modeling, and simulation are also underway to characterize the storage potential at the Patterson Site.

Mapping the Groundwater Potentiality of West Qena Area, Egypt, Using Integrated Remote Sensing and Hydro-Geophysical Techniques

The integrated use of remote sensing imagery and hydro-geophysical field surveys is a well-established approach to map the hydrogeological framework, and thus explore and evaluate the groundwater potentiality of desert lands, where groundwater is considered as the main source of freshwater. This study uses such integrated approach to map the groundwater potentiality of the desert alluvial floodplain of the Nile Valley west of Qena, Egypt, as alternative water source to the River Nile. Typically ground gradient, faults and their stress field, lateral variation of rock permeability, drainage patterns, watersheds, rainfall, lithology, and soil types are the main factors believed to affect the groundwater recharge and storage from the infiltration of present-time and paleo-runoff. Following this generally accepted approach, different remote sensing data sets (SRTM DEM, Landsat-8, ALOS/PALSAR-1, Sentinel-1, and TRMM) as well as auxiliary maps (geological and soil maps) were used to identify and map these factors and prepare thematic maps portraying the different influences they exert on the groundwater recharge. These thematic maps were overlaid and integrated using weights in a GIS framework to generate the groundwater potentiality map which categorizes the different recharge capabilities into five zones. Moreover, the aeromagnetic data were processed to map the deep-seated structures and estimate the depth to basement rocks that may control the groundwater occurrence. In addition, the vertical electrical sounding (VES) measurements were applied and calibrated with the available borehole data to delineate the subsurface geological and hydrogeological setting as well as the groundwater aquifers. Different geoelectric cross-sections and hydro-geophysical maps were constructed using the borehole information and VES interpretation results to show the lateral extension of the different lithological units, groundwater-bearing zones, water table, and the saturated thickness of the aquifer. The GIS model and geophysical results show that the southwest part of Nag’a Hammadi-El-Ghoneimia stretch has very high recharge and storage potentiality and is characterized by the presence of two groundwater-bearing zones. The shallow groundwater aquifer is located at a depth of 30 m with a saturation thickness of more than 43 m. However, there are NW–SE faults crossing the study area and most likely serve as recharge conduits by connecting the shallow aquifer with the deeper ones. Such aquifers connection has been confirmed by investigating the chemical and isotopic composition of their groundwater.

The nature and age of Mesoproterozoic strike-slip faulting based on Re–Os geochronology of syntectonic copper mineralization, Assynt Terrane, NW Scotland

In ancient basement regions such as the Lewisian Complex, NW Scotland, the ages of brittle deformation events are commonly poorly constrained owing to a lack of datable fills. An array of NW–SE sinistral and antithetic east–west dextral faults related to a regionally recognized episode of brittle shearing cut Neoarchean gneisses and c. 2.25 Ga quartz–pyrite veins close to the trace of an unexposed, regional-scale NW–SE fault. Copper–iron mineralization occurs at an intersection between an antithetic dextral fault and an older c. 2.25 Ga quartz vein. Optical microscopy, SEM and XRD analyses reveal an array of intergrown, co-genetic copper–iron sulfides, hematite and barite. Complex millimetre-thick zoned alteration rims rich in epidote occur at contacts between the sulfides and gneisses. Rhenium–osmium copper–iron sulfide geochronology yields an age of c . 1.55 Ga for the hydrothermal mineralization event associated with faulting. Fault movements demonstrably overlap with mineralization based on the asymmetric fibrous growth forms of these minerals within local dextral shears, which acted as local channelways for mineralizing fluids during and after faulting. We tentatively propose that this regionally recognized strike-slip faulting, previously termed the ‘Late Laxfordian’, should be referred to as the ‘Assyntian’ to distinguish it from kinematically distinct Laxfordian events.

Chiswick and Kew fields, Blocks 49/4a, 49/4b, 49/4c, 49/5a and 49/5b, UK North Sea

Abstract The Chiswick Field is a Carboniferous gas field located in UK Blocks 49/4a and 49/4b in the Southern North Sea, approximately 18 km NW of the Markham Field, close to the UK–Netherlands median line. The Kew Field is situated approximately 3 km NE of the Chiswick Field. The Kew structure is a NW–SE-trending horst separated from the Chiswick Field, a large anticlinal domal structure, by a major NW–SE fault and a structural low. The productive reservoir units are Carboniferous (Westphalian A and B) fluvial sandstones. Both fields are situated on the eastern edge of the Silverpit Basin (part of the Southern Permian Basin). The initial exploration drilling had Leman Sandstone Formation as the primary objective, but the first wells encountered a tight Permian reservoir with gas-bearing Carboniferous reservoirs, subsequently appraised and developed. The current estimate for the gas initially in place of Chiswick and Kew is respectively 687 bcf and 85 bcf in the Carboniferous reservoir. The fields to date (Q4 2018) have produced respectively 220 bcf and 33 bcf sales gas. Gas recovery is through natural depletion from hydraulically fractured, horizontal development wells.

Transpression and Exhumation of Granitoid Plutons along the Northern Part of the Nehbandan Fault System in the Sistan Suture Zone, Eastern Iran

Geometry and kinematics of deformation across the Nehbandan Fault System (NFS) have been studied in the Bibimaryam and Damdameh areas in the Sistan Suture zone (SSZ). Nearly vertical fault planes with sub-horizontal lineations and correlation of these faults analysis with anisotropy of magnetic susceptibility (AMS) indicates a predominantly transpressive regime including right-lateral strike-slip faults with reverse components in the region. An assemblage of en-echelon and doubly plunging folds with slip indicators, such as duplexes in their limbs, confirms progressive transpressive deformation. Accompanying hydrothermal alteration has created ideal conditions for reactivating shear fractures during late Cenozoic times that record the last phase of tectonic activity. Intrusive bodies such as the Bibimaryam and Damdameh granitoid plutons are exposed along the NW‒SE faults that branch off the Nehbandan Fault System (NFS). Fault analysis there shows three directions for compression (σ1): N085°, N074°, and N024° from the Late Cretaceous to the Plio-Quaternary respectively, on the borders of the NFS. Our results are consistent with recent paleostress studies, which revealed counterclockwise rotation of σ1 during the Late Cenozoic in eastern Iran. Redistribution of σ1 directions along the NFS in the context of general N‒S striking indicates a decreasing pure shear component, and consequently an increasing simple shear component from the Late Cretaceous to Plio-Quaternary. This reconfiguration of σ1 likely triggered the exhumation of the Tertiary Bibimaryam and Damdameh granitoid plutons along the fault zones. Field evidence and lithofacies indicate that exhumation of the intrusive bodies cannot have preceded the late Eocene. Hence, the late Cenozoic uplift and exhumation of the Sistan Belt may largely have resulted from the development of a dextral transpressive regime along the NFS in the SSZ.

Scattering and absorption imaging of a highly fractured fluid-filled seismogenetic volume in a region of slow deformation

Regions of slow strain often produce swarm-like sequences, characterized by the lack of a clear mainshock-aftershock pattern. The comprehension of their underlying physical mechanisms is challenging and still debated. We used seismic recordings from the last Pollino swarm (2010–2014) and nearby to separate and map seismic scattering (from P peak-delays) and absorption (from late-time coda-wave attenuation) at different frequencies in the Pollino range and surroundings. High-scattering and high-absorption anomalies are markers of a fluid-filled fracture volume extending from SE to NW (1.5–6 ​Hz) across the range. With increasing frequency, these anomalies approximately cover the area where the strongest earthquakes occurred from the sixteenth century until 1998. In our interpretation, the NW fracture propagation ends where carbonates of the Lucanian Apennines begin, as marked by a high-scattering and low-absorption area. At the highest frequency (12 ​Hz) the anomalies widen southward in the middle of the range, consistently marking the faults active during the recent Pollino swarm. Our results suggest that fracture healing has closed small-scale fractures across the SE faults that were active in the past centuries, and that the propagation of fluids may have played a crucial role in triggering the 2010–2014 Pollino swarm. Assuming that the fluid propagation ended at the carbonates barrier in the NW direction, fractures opened new paths to the South, favoring the nucleation of the last Pollino swarm. Indeed, the recently active faults in the middle of the seismogenic volume are marked by a high-scattering and high-absorption footprints. Our work provides evidence that attenuation parameters may track shape and dynamics of fluid-filled fracture networks in fault areas.

Geological and structural setting of the Vinodol Valley (NW Adriatic, Croatia): insights into its tectonic evolution based on structural investigations

The area of the Vinodol Valley and Bakar Bay represents a NW‒SE oriented valley in the NW Adriatic characterised by prominent historical and instrumentally recorded seismicity. As part of the greater geodynamic domain including the Ilirska Bistrica–Rijeka–Senj seismogenic fault zone, new geological and structural data addressing the tectonic evolution of the area were collected in order to better understand the focal mechanisms of previous earthquakes and to enable identification of potential seismogenic sources. Mapped informal lithostratigraphic units mostly correspond to the Upper Cretaceous, Palaeogene and Quaternary successions described in other parts of the External Dinarides. However, a shorter stratigraphic range of the Gornji Humac fm., the youngest Cretaceous unit in the study area, was determined and suggests that the uplifted area in the central NW part of the Adriatic Carbonate Platform already comprised several thousand km2 (from W and NW Istria to Krk and Vinodol area) at the end of the Turonian. Structural measurements of the fault planes in the study area generally correspond to the existing structural model of the tectonic evolution of the Dinarides. However, in contrast to the SW vergences typical of the Dinarides, NE-vergent reverse structures are common, especially along the SW margin of the Vinodol Valley. Cross-cutting relationships suggest that transpressional (NW–SE and NE–SW striking dextral and sinistral faults) and extensional features (NW–SE and NE–SW striking normal faults) are structurally concurrent or younger than the reverse faults, suggesting a change in the palaeostress field during the Neogene–Quaternary, with prevalent transpression and radial extension. Comparison of results of the palaeostress field analysis and the constructed synthetic focal mechanisms on one side, with available focal mechanism solutions for earthquakes within the Ilirska Bistrica–Rijeka–Senj seismogenic fault zone on the other, shows a favourable orientation of the observed NW‒SE and NE‒SW striking faults with respect to the recent compressional/transpressional stress field (N‒S oriented P-axis), indicating these as potential seismogenic sources within the study area.

Gravity and magnetic field interpretation to detect deep buried paleobasinal fault lines contributing to intraplate earthquakes: a case study from Pohang Basin, SE Korea

SUMMARY Potential field interpretation is a powerful method to locate deep buried tectonic fault lines that contribute to intraplate earthquakes. A magnitude 5.4 earthquake (2017, M5.4_PO) occurred in the Middle-Miocene Pohang Basin (PB), SE of the Korean Peninsula on 15 November 2017, in the area where no fault lines appear on geological and tectonic maps. To constrain fault locations, we calculate the gravity effect of the current basin fill with a gravity stripping method and used curvature analysis to identify former geological and tectonic structures, assumed formed in the Early-Miocene. The Early-Miocene PB is divided into two subregions (northern- and southern sub-basins) by a modelled NW–SE fault line, similar to the other Early-Miocene basins (e.g. Eoil basin). Fault line trends are NE–SW in the northern sub-basin, and NNE–SSW in the southern sub-basin. 2017M5.4_PO arose from a tectonic movement along the eastern boundary of the northern sub-basin, the cross-over area from the isolated high-magnetic/low-gravity region to low-magnetic/high-gravity region. The largest aftershock of the 2017M5.4_PO occurred along the NW–SE fault line bordering the northern- and southern sub-basin.

Neoproterozoic hydrothermal activity in the West Australian Craton related to Rodinia assembly or breakup?

The timing of final assembly and initiation of subsequent rifting of Rodinia is disputed. New rutile ages (913 ± 9 Ma, 900 ± 8 Ma and 873 ± 3 Ma) and published zircon, monazite, titanite, biotite, muscovite and xenotime geochronology from the Capricorn Orogen (West Australian Craton) reveal a significant early Neoproterozoic event characterized by very low to low metamorphic grade, abundant metasomatism, minor leucogranitic and pegmatitic magmatism and NW–SE fault reactivation episodes between ca. 955 and 830 Ma. Collectively, these are termed the ca. 955–830 Ma Kuparr Tectonic Event. An age range of ca. 955–830 Ma is concomitant with the final stages of Rodinia assembly and the initial stages of its attempted breakup. Very low- to low-grade metamorphic and structural geological evidence favor a distal north–south compressional regime as the driver for hydrothermal activity during ca. 955–830 Ma. Nearby continental collision or accretion from the west (e.g., South China and/or Tarim) are ruled out. The cessation of metasomatism and magmatism in the West Australian Craton after ca. 830 Ma is concomitant with the emplacement of the Gairdner–Amata dyke swarm and associated magmatic activity in South China and Laurentia, the inception of the Adelaide Rift Complex and the deposition of the Centralian Superbasin. We posit that the cessation of hydrothermal activity in the Capricorn Orogen was caused by a tectonic switch from compressional to extensional at ca. 830 Ma. Magmatic and hydrothermal fluids were transferred away from the Capricorn Orogen to the incipient Adelaide Rift Complex, terminating metasomatism in the West Australian Craton. Ultimately, the Kuparr Tectonic Event marked the final stages of Rodinia assembly and its cessation marks the initial stages of its attempted breakup.

The active faults of the Mitidja basin (North Central Algeria): what does the seismic history of the region tell us? A review

The Africa–Eurasia plate boundary runs along Northern Algeria, which is thus an active seismic zone. However, surface ruptures from large earthquakes are rarely observed, which makes it difficult to define the characteristic parameters of any active fault. In this paper, we combine the results obtained in recent investigations of historical and instrumental seismicity with field observations and available geological maps and measurements of neotectonic ruptures to help comprehend the complexity of the active deformation of the Mitidja basin (MB). Our analyses reveal the tectonic characteristics of the Mitidja basin and indicate that the seismic activity is essentially concentrated along the system of the boundary faults. The general structural shape of the seismogenic Mitidja basin suggests that the southern fault system (Blida faults) would mimic the northern fault system (Sahel fault), and we observe a highest seismicity rate at the junction point of the NW–SE fault and the south thrust fault system.

Neotectonic and Geomorphological Characteristics of the Ma Pi Leng Area (Ha Giang Province, North Vietnam): Implications for Improving the Worth of Vietnamese Geological Heritage

Ma Pi Leng area is the core part of Dong Van karst plateau geopark, and is a region of high diversity of geology and tectonics in Vietnam. This region has experienced strong uplift motions that have created three grades of regional relief: 1400–1600 m; 1100–1200 m and 500–600 m. This geomorphology is in turn strongly eroded by the stream systems and divided into different structural blocks controlled by the faults. During the neotectonic stage, several modes of faulting have occurred in Ma Pi Leng such as strikeslip, normal, extension, riverse faults. The NW–SE fault system had a decisive role in determining the structural pattern of the region. The tectonic fractures, at the largest scale, are hundreds of meters high and up to a kilometer long. Tectonic activities, together with exogenic processes, have made regional relief strongly diverse with strips of horsts and grabens as well as laddersteps-shaped escarpments, inverse topography, high cliffs and deep canyons. Besides that there are also the caves at varying levels of height, chains of sinkholes, and pyramidal mountainous peaks. Ma Pi Leng area is truly majestic and worthy of tourist attractions both in terms of scientific value and natural landscape.

Geochemistry, geochronology, isotope and fluid inclusion studies of the Kuh-e-Zar deposit, Khaf-Kashmar-Bardaskan magmatic belt, NE Iran: Evidence of gold-rich iron oxide–copper–gold deposit

The Kuh-e-Zar deposit is located in the central part of the Khaf-Kashmar-Bardaskan Tertiary magmatic belt, NE Iran. The prevailing stratigraphic unit is composed of Cenozoic volcanic rocks (rhyolitic to andesitic in composition), which are intruded by subvolcanic plutons. Intrusive rocks spatially close to mineralization include metaluminous to peraluminous, calc-alkaline and I-type diorite, granodiorite, quartz monzonite, quartz monzodiorite, and syenogranite. The quartz monzonite and quartz monzodiorite have identical zircon U–Pb ages of ca. 40.7–41.2Ma. The intrusions are characterized by enrichment in large-ion-lithophile elements (LILEs) and light rare-earth-elements (LREEs), depletion in heavy rare-earth-elements (REEs, LaN/YbN≈7 to 9.07) and high-field-strength-elements (HFSEs) and εNd(t) ranging from −0.06 to −2.93 at 86Sr/87Sr(t)=0.7054–0.7064 and 206Pb/204Pbi=18.6–18.71. The intrusive rocks of Kuh-e-Zar are products of crustal assimilation by melts derived from the metasomatized mantle wedge above the subducting Neotethyan Ocean slab beneath SW Eurasia.Field work and mineralogical, petrological studies show that, the hydrothermal alteration produced a zone of silicification, propylitic alteration, and albitization associated with minor sericitic-argillic alteration. The mineralization of Kuh-e-Zar occurs in veins, stockworks, and breccias, which is located near or within NE−SW and NW−SE faults cutting through volcanic rocks and wall rocks. The ore minerals are dominated by specular hematite and gold with small amounts of pyrite, chalcopyrite, and galena. Hematite, goethite, malachite, covellite, and cerussite are secondary minerals. The main gangue minerals include quartz, siderite, chlorite, and albite. Microthermometric study of fluid inclusions shows homogenization temperatures at medium-high temperature of 248 to 491°C. Salinities of ore-forming fluids are medium-low ranged from 4 to 19.2wt% NaCl equivalent. The sulfur isotope composition of chalcopyrite (δ34S=−2.5 to 0.9‰) suggests that sulfur was derived mainly from igneous rocks. The oxygen isotopic data (δ18Owater=7.4–7.9‰) indicate that the ore fluids were in magmatic origin.The mineralogy, alteration, geochemistry, stable isotopes, and petrogenesis of intrusions of the Kuh-e-Zar deposit indicate it is an Eocene gold-rich iron oxide-copper-gold (IOCG) deposit.