Synclinal Structure Research Articles

Fine interpretation of magnetic data for a concealed mineral deposit: A case study of the Fe-polymetallic deposit from Western China

We interpret the magnetic anomalies for exploration and characterization of the Galinge iron polymetallic deposit, which is the largest iron polymetallic deposit in Western China. This undeveloped deposit is completely concealed by the thick Quaternary sediments, presenting an exploration challenge. A lot of drillholes have been implemented focusing on the strong magnetic anomaly centers, while the relationship between magnetite-bearing and non-magnetite-bearing drillholes is unclear, and the metallogenic regularity in the whole area needs to be further studied. The edge detection is applied on the downward continued field to resolve individual magnetite ore bodies and enhance possible concealed ore bodies. Three anomaly belts trending NE-SW with a total extension length of ∼11 km are first revealed. Combining the sharper images from the enhanced magnetic anomaly and the drillholes, we infer that the deposits would be related to the syncline structure. Forward modeling confirms the inferred three belts of the ore bodies and also shows possibly two layers deposits in which the deep layer would be located at a depth of ∼1000 m. Although the deeper deposit results in a very smooth and low amplitude magnetic anomaly, the forward modeling shows that the anomalies from both the shallow magnetite and the inferred deep one fit the observed anomaly better. One deep drillhole reveals the fact that the magnetite is existed at a depth of ∼1000 m which highlights the interpretation of the deep deposit. The results show the contribution of the fine processing techniques which would be used to distinguish zones of potentially concealed mineralization.

Electrical Structure between the Main and Eastern Deposits of the Bayan Obo Mine: Results from Time-Domain CSEM Methods

Bayan Obo is a well-known polymetallic deposit containing significant quantities of rare earth elements, niobium, thorium, and iron. However, the epoch in which mineralization occurred and the mineralization process are still debated due to the complex nature of its mineralization and geological evolution. Inadequate geophysical exploration has further contributed to this lack of clarity surrounding critical issues, such as the deep link between the main orebody and the eastern orebody, the form and distribution of the extensive dolomite, and the geologic structures in the area. Therefore, we implemented the time-domain controlled-source electromagnetic method (CSEM) to acquire electrical structures at depths down to 2.5 km between the Main and Eastern mines. According to the inverted resistivity structure, in conjunction with existing geological and drilling data, we classified the main lithologies and faults based on their resistivity characteristics. Overall, the mineralized carbonatite reflects high to moderately high resistivity. The mineralized carbonatite dips overall from north to south, with a maximum extension depth not exceeding 1.5 km, and its range of occurrence is controlled by nearly east–west-striking faults distributed along the bounding line between the roof and floor rocks. The Main and Eastern mines are connected at depth, but the morphology and position of the ore bodies have significantly changed due to multiple phases of tectonic activity. The electrical structure does not reveal any obvious syncline structures, further refuting the traditional view that the Bayan syncline controls ore formation.

The Age and Origin of the Rocks of the West Azov Group (Lozuvatka Antiform, the Ukrainian Shield)

Geochronological studies of amphibole-biotite gneisses of the Verkhnii Tokmak Unit and amphibolites and Banded Iron Formation (BIF) of the Kaiinkulak Unit of the West-Azov Group in the Lypovets area of the Lozuvate antiform in the West Azov block of the Ukrainian Shield were carried out. A multi-stage history of the formation of amphibole-biotite gneisses has been established. In terms of chemical composition they correspond to quartz diorites. Rare earth elements are differentiated — (La/YbN = 7.54), with a significant negative europium anomaly — Eu/Eu* = 0.66. The REE distribution indicates that the initial melt for quartz diorites was produced in a crustal magmatic source. In zircon crystals from amphibole-biotite gneisses, relics (cores) of older zircon and mantles that surround them were distinguished. Relict zircon is coarse-zoned, cracked, and its mantles have a thin-zoned structure. A small amount of transparent non-zonal zircon crystals was also found. Based on the results of zircon dating by the LA-ICP-MS method, it was determined that the age of the relict zircon is 3.16 Ga. The thin-zonal mantles have an age of 2.9-2.8 Ga, which probably corresponds to the time of crystallization of quartz diorites. Non-zonal transparent zircon with an age of 2.1 Ga corresponds to the time of dynamometamorphism (collision). BIF of the Lypovets deposit in the West Azov iron ore province, which belongs to the Kaiinkulak Unit of the West Azov Group, has a high Ni/Fe×10–4 = 0.5-2.1 ratio, typical for the Archean BIFs of the Algoma type. This deposit is confined to an Archean greenstone belt in a synclinal structure composed of metamorphosed basaltic komatiites and sedimentary rocks. Thus, the Verkhnii Tokmak Unit comprises Mesoarchean quartz diorites of crustal genesis, while the Kaiinkulak Unit probably represents heavily metamorphosed rocks of an Archean greenstone belt.

An integrated approach of advanced methods for mapping geologic structures and sedimentary thickness in Ukelle and adjoining region (Southeast Nigeria)

High-resolution aeromagnetic data were enhanced using recent and advanced filters to map the geologic structures of the Ukelle and adjoin region (Southeast Nigeria). Aeromagnetic data were reduced to the equator (RTE) and upward continued to 100 m. Subsequently, enhancement operations like the tilt angle of the horizontal gradient (TAHG), logistic function of the horizontal gradient (LTHG), and fast sigmoid function (FSED) operations were carried out. The results from these filters indicated that the ENE-WSW, NE-SW, NNE-SSW, and NNW-SSE orientations dominate the structural pattern of the Ukelle region. In addition, the edge filters delineated NE-SW trending synclinal structures that match the location of thick (500-1400 m) sedimentation obtained by the tilt-depth (TD) method. Furthermore, the structural map obtained from remote sensing data validated the lineament orientations and position of the NE-SW trending synclinal structure. The results also showed that the study location’s southeastern and northwestern flanking portions, controlled by extensive Santonian igneous intrusions and metamorphisms, are characterized by high lineaments and thin (0-500 m) sedimentation. The observed thin sedimentation is believed to be caused by widespread Santonian tectonic events in the area. At the same time, related geologic structures served as migration pathways and accumulation zones for rift mineralization.

Seismogenic structures and earthquake mechanisms in the Changning area, China: Insights from seismicity and tomography

The Changning area, located in southwestern Sichuan Basin, China, was previously known to be a seismically inactive region. However, in recent years, the region has experienced an increase in the rate of seismic activity. To gain a better understanding of the seismogenic structures and earthquake mechanisms in the region, we deployed 298 short-period seismic stations and collected 47 days of continuous waveform data following the 2019 Ms 6.0 Changning earthquake. Using a machine-learning-based earthquake location workflow (LOC-FLOW; Zhang et al., 2022), we detected and located over 60,000 seismic events. These events were then used for three-dimensional Vp and Vs inversion using a double-difference tomography method (tomoDD). The results show that the earthquake distribution has distinct zoning characteristics with different mechanisms. In the Changning-Shuanghe anticline, most earthquakes are distributed in areas with large velocity and Vp/Vs gradients, typically occurring in regions with high velocity and low Vp/Vs, where tectonic stresses are prone to accumulate. Long-term water injections may promote fault activation in the southeast of Changning anticline. Earthquakes around the salt mining well were found to occur in sedimentary cover, indicating that the basement fault has not been activated but rather acts as a fluid-flowing path. In the Changning shale gas block, earthquakes showed different triggering mechanisms, accompanied by different velocity characteristics. The spatial variations in earthquake locations and velocity characteristics can be explained by the terrain at shallow depths, syncline structures, and the distribution of fluids and gas reservoirs. Our study provides new insights into the detailed seismogenic structures and earthquake mechanisms in the Changning area. These findings can be used to improve earthquake hazard assessment in the region.

2-Dimensional joint inversion modelling of the Niger – Delta basement using potential field data

Sub-surficial models predict the occurrence and geometry of strata, geological structures, inclusions, tectonic settings and so on. Robust data including geologic information, airborne gravity, and magnetic data, with constraining drill–depth data are here integrated to model the topography of the basement that underlies the Niger Delta sedimentary sequence. The aim was to elucidate the existence of a petroleum system from the unexplored depths of the delta near the basement, hypothetically controlled by the basement structures. Airborne total magnetic intensity (TMI) and Bouguer gravity anomaly (BGA) of the Niger Delta, obtained from the Nigerian Geological Survey Agency that were noise–filtered and corrected are used. The gradient of the geomagnetic field was removed from the magnetic data and the data was reduced to the equator (RTE). As precursory operations, sub-surficial lineament structures were delineated from these grids using the Tilt Derivative filters, and the depths were determined using the Source Parameter Imaging (SPI) technique. Subsequently, basement topography models were generated across four (4) profiles in a gravity-magnetic joint inversion scheme. Consequently, the strata that constitute the Niger – Delta ‘cake’ from the Cretaceous to Recent were modeled. The results showed that the Late Cretaceous Synrift Clastics (LCSC) formation presumed to directly overlie the basement may vary in thickness from 2.15 to 2.74 km. The lineament structures were found to dominantly trend in NE – SW direction. The depth to the basement, from the 2-dimensional models, ranges approximately from 13.18 to 19.35 km. It is postulated that while the Tertiary Niger Delta is characterized by lineaments engendered by diapiric instability of the over–pressured Paleocene – Pliocene marine shales, there exist other lineament structures beneath with NE – SW as the dominant trends, offshoots of Mid – Atlantic fracture zones including Chain and Charcot. Basement undulations, which are relicts of the vast subsidence Passive margins undergo during their formation are observed. These resultant synclinal structures create accommodation for the extensive deposition of the LCSC. The juxtaposition of this reservoir facie, the lineament-laden undulating Passive basement escarpment, and, the Paleocene – Pliocene Marine Shale formation could make for another petroleum system.

Favorable tectonic patterns for shale gas preservation and enrichment in lower Paleozoic, southeastern Chongqing, China

Marine shale reservoirs have experienced intense tectonic movements in southeastern Chongqing, resulting in widely developed folds, faults and bedding slip deformation. The gas content is of a wide range while the organic matter content and thickness are sufficient for shale gas accumulation. Here, we review previous work and present outcrop and core data from 11 shale wells around southeastern Chongqing to analyze and summarize tectonic types and their combinations, as well as their influence on shale gas preservation and enrichment. Fault and bedding slips are prevalent in shale reservoirs, and they apparently influence shale gas content. Bedding slip crush is usually formed at the bottom interval where the total organic content is high from intense horizontal compression. Bedding slip crushing and well-developed high-angle natural fractures form a net fracture system where shale gas is enriched in an anticline structure or lost to air in a syncline structure laterally. Strata dip is the key geological factor that controls bedding slip deformation in shale reservoirs in certain areas. According to the established bedding slip geological model, when bedding slip occurred, the critical strata dips of the lower Silurian Longmaxi Formation were 13.34°, respectively. Bedding slip crushed anticlines without faults and synclines without faults and bedding slip crushing are the two tectonic patterns that are favorable for shale gas preservation and enrichment, which should be the focus of further explorations of lower Paleozoic shale gas in southeastern Chongqing.

Predicting zones of increased water inflows in local folded structures

Purpose. To predict zones of increased water inflows based on geological exploration and mining-geological data in the real conditions of an operating coal mine using the method of predictive evaluation of prospects of local structures for the presence of gas accumulations. Methodology. The research was carried out by constructing a map of local structures using trend analysis. The synclinal fold is allocated in the plan and its main parameters – amplitude and width – are determined. Based on these parameters of the structure, the authors performed calculations of relative linear deformations and relative volume deformation of the studied fold. Findings. According to the deviation of the hyposometry of layer (“Samarska” mine) from the approximating surface, a local synclinal structure is allocated on the map. Based on calculated data on the volumetric deformation of rocks (siltstones and sandstones) and average values of open porosity, fracture porosity and absolute permeability were calculated. The obtained data indicate the formation of a fractured zone with improved filtration and capacity properties of rocks within the studied area. According to the actual mining and geological data, within the defined area, there are increased water inflows into the mine workings, which further confirms the results of the prediction. Originality. For the first time, the algorithm of predictive evaluation of the prospects of local anticlinal structures for the presence of gas accumulations was used to predict areas of increased water inflows. Practical values. In the real conditions of an operating mine, in the area of mining operations, the method for predicting zones of increased water inflows was tested according to a predictive evaluation of the prospects of local folds for the presence of gas accumulations.

Constraints from the Ntaba Mhlope Syncline and the Mkomozane Homocline, Moodies Group (3.22 Ga), Eswatini, on the deformation of the southern margin of the Barberton greenstone belt

The Barberton Greenstone Belt (BGB) has been classically thought to be a first-order synclinal structure, therefore constraining vertically-dominated models of early Earth lithosphere (de-)formation. Geological mapping, strain analysis, U-Pb zircon age dating, and Raman spectroscopy of carbonaceous matter (RSCM) of the Ntaba Mhlope Syncline (NMS) and the Mkomozane Homocline (MH) close to the southeastern linear margin of the BGB in Eswatini assess this interpretation by constraining the type and timing of the dominant late-phase deformation in this region. Geologic mapping shows that the NMS is a steeply doubly plunging, highly strained syncline composed of basal Moodies conglomerates overlain by sandstones and subordinate shales and siltstones. Deformation of conglomerate clasts is markedly constrictional and of uniform orientation throughout the syncline, suggesting that regional deformation occurred in zones of ductile strain. RSCM indicates maximum peak paleotemperatures of ca. 500 °C, somewhat elevated compared to temperatures measured throughout the BGB. Shortening may have occurred contemporaneously with lower crustal extension.

Numerical simulation of the palaeotectonic stress field and prediction of the natural fracture distribution in shale gas reservoirs: A case study in the Longmaxi Formation of the Luzhou area, southern Sichuan Basin, China

Natural fractures have an important relationship with shale gas production. This study selected the reservoir of the Wufeng‐Longmaxi Formation in Luzhou in southern Sichuan as an example. Based on the seismic reflection map of the formation, using the mechanical parameters obtained from the core sample experiment and using the finite element numerical simulation method, a three‐dimensional geological model is established, the tectonic stress field is inverted and the fracture distribution is predicted according to the Mohr‐Coulomb fracture criterion of the rock fracture, which provides theoretical support for the subsequent well pattern layout and is conducive to improving the efficiency of shale gas exploration and development. The analysis shows that the study area is dominated by NE reverse faults, with an inclination of more than 40°. The core observation results show that the filling degree of fractures is relatively high, and the width is mostly less than 1 mm, mainly caused by tectonic shear fractures. The tectonic stress in the sedimentary belt where the syncline structure is deeply buried is higher, the stress in the northwest and southeast tectonic ridges is relatively low, and the differential stress in the northeast tectonic ridge and near the fault is high. The NE fault and the surrounding rock mass have the highest fracture degree; the fracture proximity coefficient is >1.5 and the fractures are the most developed. The degree of fracture development in the area with a gentle structure is relatively low, but it is far from large faults, which is conducive to preserving shale gas.

The Mio-pliocene aquifer system of the South Auresian Trough, Low Septentrional Sahara, Algeria: aquifer geometry and groundwater characterization

In the South auresian trough, Low Septentrional Sahara, Algeria, the sands aquifer is contained in the Mio-pliocene deposits in which several sedimentary sets have identified and contain two independent aquifer levels. Their relationship is determined by data correlation of several cross-sections surveyed on outcrops and boreholes that permit us a 2D construction of the multi-layer aquifer formations and to specify its geometry. The latter shows an asymmetrical synclinal structure with greater depth to the south and an unconformity of the upper unit (upper aquifer) on the base unit (lower aquifer) with a reduction in thickness from south to north. The upper aquifer, commonly designated “undifferentiated Mio-pliocene aquifer” is the most exploited. Its water is moderate to strongly charged and presents two families at evolutional hydrochemical facies. Their spatial distribution is strongly influenced by the natural context and the aquifer geometry. The hydrochemical analysis reveals that the dissolution of carbonate (CaCO3 or CaMgCO3), sulfate (CaSO4) and salt (NaCl) were the major sources of ions in the studied groundwaters. In this region deprived of treatment means and softening of water, the exploitation of the study results can help the decision makers, preferentially, to choose the captive fields principally destined for potable water supply in sectors containing the water less charged.

Iron quartzites of the Simandou mountain (Republic of Guinea)

In this article, we for the first time fully describe the geology and petrography of the iron quartzite formation, localized in Southeast Guinea in the Simandou mountain range. In this region, a meridional distribution belt of banded magnetite quartzites (itabirite) of the Lower Proterozoic Simandou metamorphic series has been mapped. The rocks form an extended (210 km) and narrow (0.4-7 km) complex synclinal structure in the background of the Archean granite-gneiss basement. The structure of the Simandou metamorphic series includes the lower sequence of metaterrigenous rocks up to 400-500 m thick (metasandstone, quartzite, phyllite, mica schist) and the upper itabiritic sequence up to 220-450 m thick. The metamorphism of the rocks of the series increases from north to south from the greenschist to epidoteamphibolite facies. Members of two varieties of banded iron rocks are distinguished in the itabiritic stratum - itabirite and amphibole itabirite. The composition of itabirite is (wt %): magnetite 51-55, quartz 48-36, and the rest is cummingtonite and mica. The composition of amphibole itabirite is (wt %): magnetite 39-47, quartz 31- 30, amphibole (cummingtonite) 24-15, the rest is hedenbergite, mica and calcite. Rich martite and hematitegoethite ores (60-66% Fe) are formed in the lateritic weathering crust after itabirite, forming manto deposits with a depth of 9 to 350 m.

Shallow shear wave velocity structure of the Dongshan sag area using surface wave data in a deep reflection profile of the Yuanmou area of Yunnan province, China

Deep reflection seismic data contain abundant surface wave information for which the velocity structure in the near-surface shallow underground can be obtained using multichannel analysis of surface waves (MASW). This paper uses the MASW method to process deep reflection data in the Yuanmou area in Yunnan province, China. The MASW method uses the phase velocity spectral transformation of Rayleigh waves to obtain its dispersion curve. The initial model for different lithology classifications is established to invert the underground velocity structure. This study obtains a two-dimensional shear wave velocity structure profile to investigate the shear wave velocity structure. Generally, the study results have shown that the shear wave velocity structure obtained by the MASW method corresponds well with the surface conditions and accurately reflects faults' changes, folds, and lithology. Moreover, the formation sand content calculation shows a large difference in Dongshan sag's formation sand content. The sand contents of Cretaceous and Middle Jurassic strata are over 30% similar, and the sandstone content of underlying Early Jurassic strata is mostly 30% lower. The stratum buried depth of Early Jurassic strata is between 0– 800 m. The dip angle of LZJF at this location is 90°. The west side of the LZJF is based on Proterozoic uplift, and the Quaternary sedimentary thickness exceeds 300 m above the uplift stratum. Whereas on the east side of the LZJF, the stratum of the Dongshan sag section is mainly a synclinal structure.

Role of an advanced gravity data analysis in improving the geologic understanding of the northern Tebessa region, Northeastern Algeria

The Tebessa region in the extreme northeastern part of the Saharan Atlas of Algeria is characterized by a succession of anticline and syncline structures, diapirs, grabens, and a thick sedimentary cover underlying the Paleozoic basement. The region constitutes a complex structural system. The extensional structures (i.e., grabens and basins) in the Tebessa region belong to the foreland grabens of the North Africa Alpine Belt and their evolution is related to the tectonic history of the western Mediterranean region. The Tebessa region has been the subject of several geologic studies in the last decades. However, the subsurface structural configuration remains unknown, resulting in uncertainty about the exact origin of the grabens present in the Tebessa region.The current research involves gravity data analysis in order to understand the geologic subsurface system. Accordingly, gravity data are used to delineate the fault systems and the tectonic framework by using the 2-D Source Edge Detection (SED) technique, Centre for Exploration Targeting (CET) grid analysis, Euler Deconvolution, multiscale edge detection method (Worming), and 3-D modeling.Results indicate that the main tectonic trends within the study area are NE-SW, E-W, ENE-WNW, and NW-SE directions, making the northern Tebessa a complex structural region. The eastern part of the Tebessa region is characterized by a basement uplift which contributes to the positive gravity anomalies of the diapirs. In addition, the grabens have a sedimentary deposit thickness of about 2–8 km and underlain by Triassic evaporite layers that facilitated their opening by providing rheological weakness.The grabens in the Tebessa region have different mechanisms that have been responsible for their formation. However, the Early Mesozoic extensional tectonic regime initiated some grabens by creating the bounding normal faults, followed by Cenozoic strike-slip motions, when grabens opened as pull-apart basins.

Three-dimensional density model of the Tarasivka structure of the Golovanivsk suture zone of the Ukrainian Shield

For the first time, based on the data of three-dimensional density modeling, a diagram of the density distribution on the surface of the basement of the Tarasivka mafic-metamafic structure with an area of 2.5×5.5 km, extending to a depth of no more than 3—4 km, was constructed. The distribution of density to a depth of 5 km has been studied in detail, and the depths of occurrence of geological bodies have been determined. New in methodological terms is the use of a bypass step-like boundary from below, the form of representation of which is determined by the expected depth of occurrence of bodies with different densities. Thus, the depth of bodies with increased density near the basement surface is 2 km, of granitoids — 3 km, of undivided charnockites and enderbites — 4 km. It is shown that apogabbroids and aponorites with an average density of 2,80—2,90 g/cm3 (slightly altered rocks of mafic-metamafic composition), which form the central part of the Tarasivka mafic-metamafic structure, extend to a depth of 2 km without changing their angle fall, which is confirmed by seismic and electrical survey data. Based on the complex of available data, the selection and substantiation of the density of the host rocks of the charnockite-enderbite series, which are characterized by a density of 2,75—2,76 g/cm3 and form a ledge of about 1—2 km within the Tarasivka structure, were made. The marginal parts of the structure extend deeper than the central ones, which testifies against its synclinal structure. Along the latitudinal strike-slip fault, which passes through the central part, the Tarasivka structure is divided into two parts: the northern, less dense, and the compacted southern one. It is shown that in the eastern and northeastern parts of the structure, the fault zones are fragmented and compacted.In some of them, the density is 2,77 and 2,80 g/cm3 throughout the section, which can be explained by the intersection of high-density rocks by faults in such a places.The absence of supply channels and the shallow depth of the Tarasivka structure can be explained in two ways: either the channels of the mafic intrusion that forms the structure, most likely, have a small diameter (or diameters), that is why they cannot be fixed by gravimetry; or powerful strike-slip processes, which are fixed within the Golovanivsk suture zone, led to the formation of a detachment at a depth (modern) of 3—4 km, as a result of which the upper part of the Yatra block, together with the Tarasivka mafic-metamafic structure, moved quite strongly to the south, tearing it from root part. The last statement is considered the most probable.

The Dynamics of Forearc – Back‐Arc Basin Subsidence: Numerical Models and Observations From Mediterranean Subduction Zones

AbstractThe subsidence history of forearc and back‐arc basins reflects the relationship between subduction kinematics, mantle dynamics, magmatism, crustal tectonics, and surface processes. The distinct contributions of these processes to the topography variations of active margins during subduction initiation, oceanic subduction, and collision are less understood. We ran 2D elasto‐visco‐plastic numerical models including surface and hydration processes. The models show the evolution of wedge‐top and retro‐forearc basins on the continental overriding plate, separated by a forearc high. They are affected by repeated compression and extension phases. Compression‐induced subsidence is recorded in the syncline structure of the retro‐forearc basin from the onset of subduction. The 2–4 km upper plate negative residual topography is produced by the gradually steepening slab, which drags down the upper plate. Trench retreat leads to slab unbending and decreasing slab dip angle that leads to upper plate trench‐ward tilting. Back‐arc basins are either formed along inherited weak zones at a large distance from the arc or are created above the hydrated mantle wedge originating from arc rifting. Back‐arc subsidence is primarily governed by crustal thinning that is controlled by slab roll‐back and supported by the underlying mantle convection. High subduction and mantle convection velocities result in large wavelength negative dynamic topography. Collision and continental subduction are linked to the uplift of the forearc basins; however, the back‐arc records ongoing extension during a soft collision. During the hard collision, both the forearc and back‐arc basins are ultimately affected by the compression. Our modeling results provide insights into the evolution of Mediterranean subduction zones.

Deep groundwater circulation in a syncline in Rucheng County, China

Deep groundwater circulation is closely related to the earth surface processes of water, energy and matter, and imposes great influences on the earth critical zone. It is still unknown whether a deep groundwater circulation occurs in a syncline structure. Hydrogeochemical investigations of hot spring, cold spring and shallow groundwater were carried out in a syncline basin in Rucheng County, China. The water source, circulation depth, reaction temperatures, and mixing processes were evaluated by water chemistry and thermodynamic methods to reveal the pattern and controlling factors of the deep groundwater circulation. Results showed that two deep circulation groundwater systems with meteoric origin were developed in the syncline. The Tangkou system is related with the northwest wing of the syncline, the main fault F2, and the RS01 thermal spring. The Luoquan system is related with the southeast wing, the main fault F1, and the RS02 thermal spring. The main faults are water-blocking faults, separating the two systems. The mean elevations of recharge areas of the two systems were 1200m and 850m, respectively. The deep groundwater circulation depths of two systems reached 5.5 ± 0.5 and 3.6 ± 0.3 km, respectively. Within the geothermal reservoirs, deep circulating groundwater was heated up to 156°C for RS01 and 108°C for RS02. Before arriving at the ground, cold shallow groundwater mixed into the deep circulation systems. The mixing ratios of deep circulating groundwater were 14.8% at RS01 and 35.6% at RS02. The HCO3-Ca type deep circulating groundwater was slightly mineralized karst or karst-related water. Carbonate dissolution was the dominant water-rock interaction. The mixing obviously diluted the deep circulating groundwater, but didn’t alter its water type. In our case, the large-scale synclinal structure is favorable to deep groundwater circulation, which is closely related with the carbonate stratum and faults. The depth of the deep groundwater circulation can be determined by recharge area elevation and geothermal reservoir depth, which is a simple and effective method.

Geologic map of central Panamint Range, California, USA

This detailed geologic map and supplemental digital data set1 examine and demonstrate the complex deformational history and reactivation relationships of the Panamint Range (California, USA), from active transtension of the Walker Lane belt, Miocene extension of the Basin and Range, to multiple Mesozoic events related to subduction, and Neoproterozoic extension. This collection of map data focuses on the geometry, kinematics, and relative timing of deformation to understand the deformation history and effects of structural reactivation. A minor portion of this geologic mapping data was presented in the analysis and figures of Andrew and Walker (2009). The Neogene extension and subsequent dextral transtension deformation has created a complex network of faults via partial reactivation of Mesozoic and Neoproterozoic structures. Structural data show oblique normal slip overprinting earlier normal slip along the western range flank fault of the western Panamint Range. Jurassic and Cretaceous deformation is localized along the western range on the Goldbug fault. The hanging wall of this fault preserves migmatitic fabrics and intense deformation due to Jurassic contraction. The Goldbug fault places Paleoproterozoic to Mesoproterozoic rocks over Neoproterozoic rocks. The Jurassic contraction has top-to-the-northeast relative transport and the more discrete Cretaceous thrust faulting has top-to-the-east transport. A set of Late Cretaceous plutonic rocks and mylonitic gneisses derived from them, occur along the Goldbug fault and demonstrate the reactivated nature of this fault in the Late Cretaceous. New data for the Butte Valley fault show that this fault cuts Late Jurassic plutonic rocks and has normal slip. The Butte Valley fault ends northward at the linked sinistral slip Warm Spring Canyon fault, which was previously interpreted to be an intrusive contact. A previously unrecognized rim syncline structure occurs along the boundary of the Late Jurassic Manly Peak quartz monzonite. Neoproterozoic deformation is difficult to discern due to the overprinting deformations. Numerous Neoproterozoic deformation-related mass wasting deposits can be seen within this formation, including a set of conspicuous allochthonous deposits and clasts of older Beck Spring Dolomite that appear to be frozen in the process of breaking away from intact, normal thickness beds in the Surprise–Happy Canyons divide. This detailed geologic mapping and collection of structural data for the rocks in the central Panamint Range were created using digital in-the-field geographic information systems software running on a field-hardened laptop computer combined with an earlier set of field data that were digitized into the digital georeferenced database. This map is a simplification of detailed geologic mapping data collected at 1:2000–1:6000 scales and reduced to 1:20000 scale. Structural data include kinematic and relative timing of deformation information.

Regional and local controls of hydrothermal fluid flow and gold mineralization in the Sheba and Fairview mines, Barberton Greenstone Belt, South Africa

High-grade gold-quartz-sulphide lodes of the Sheba-Fairview Complex (SFC) of gold mines in the Barberton Greenstone Belt of South Africa describe a wide range of geometries, orientations, controls, and a number of different and often contradicting models have been postulated to account for their formation. This paper aims to present a holistic model that describes the focussing of a regional-scale fluid flow into narrower, mineralized structures during the late-stage refolding and tightening of the first-order folds that host the mineralization.The auriferous reefs of the SFC are hosted by well-bedded, low-grade metamorphic, clastic sedimentary rocks of the Paleoarchaean Fig Tree and Moodies groups that are preserved in two regional-scale, strongly curved and refolded synclinal structures along the northern margin of the belt. The majority (85 %) of presently and historically mined orebodies occur in the broad hinge zone of the refolded synclines. This suggests the regional-scale fluid redistribution from fold limbs into the first-order fold hinge following regional hydraulic gradients during refolding. Ubiquitous bedding-parallel quartz (-carbonate-sulphide) veins and associated alteration underline the significance of bedding anisotropies and bedding-parallel flexural slip for regional-scale fluid flow during refolding. The focussing of these fluids and economic-grade gold mineralization occurs preferentially along contacts between lithologies with pronounced rheological contrasts. Lithological contacts combine the effects of strain localization in weaker units and the formation of fracture permeabilities in competent units across these contacts. Examples include kilometer-scale hinge-parallel ore shoots that are controlled by undulations (jogs) along low-displacement slip horizons between retrogressed ultramafic rocks and competent metasediments. In contrast, low-angle thrusts or steep reverse faults form accommodation structures related to the late-stage tightening of the first-order folds. These structures transect bedding at high angles and are, thus, ideally orientated to collect fluid from the regional system of bedding-parallel fluid flow.Mineralization in the SFC may be explained by the spatial and temporal coincidence of (1) late-stage NW-SE shortening and refolding, of (2) a well-bedded, anisotropic wall-rock sequence, associated with (3) regional-scale fluid redistribution along bedding planes during flexural slip, leading to (4) fluid focussing into either jog geometries or bedding discordant structures. The combination of these factors is unique to the SFC and accounts for the location of the main Barberton gold mines along the northwestern margin of the greenstone belt.

Delineation of Subsurface Structures Using Gravity Data of the Shallow Offshore, Lamu Basin, Kenya

Lamu Basin is located in South Eastern Kenya and covers about 170 000 km2 both onshore and offshore. Kenya’s Lamu Basin is hitherto underexplored even though there have been notable oil and gas discoveries along the margin of East Africa. This study focuses on the shallow section of the Lamu offshore bounded by 39°E to 43°E by 2°S to 6°S, whereby, unfortunately, some of the wildcat wells turned out to be dry although expensive. Gravity interpretation techniques such as spectral analysis and first horizontal derivative were applied to the reduced gravity data to delineate and model structures to minimize the high investment risks. The gravity data used in this study were sourced from the International Gravity Bureau (BGI) and National Oil Corporation of Kenya (NOCK) digital data courtesy of companies like Woodside Energy, Anadarko Kenya Limited, and Total Exploration and Production companies. The obtained reduced gravity data were gridded to produce the gravity anomaly grids (Free air, Bouguer, and Isostacy), which were consequently drawn into maps. From spectral analysis, depths to shallow sources and deep sources were estimated. These depths were used to set regional and residual separation filters using the Gaussian filter. The first horizontal derivative (FHD) applied to the regional Isostatic gravity anomaly map yielded features that were inferred as intrasediment fractures/faults trending in NW-SE and NE-SW directions. The features like the ridges, troughs, and faults mainly trending in the NW-SE direction are discernable from the regional anomaly map. The developed models show the basement highs and lows with a possibility of anticlinal and synclinal structures and thick sedimentary successions likely to represent good hydrocarbon source kitchens.