NW-trending Faults Research Articles

Exploring geothermal resources with the CSAMT and microtremor methods: a case study in Tangquan, Jiangsu Province, China

The development of geothermal energy has received extensive attention because of global energy scarcity and environmental pollution. The Tangquan area is in the north of the Yangtze fold belt, and the lack of magmatic rock development has resulted in a low-temperature type of geothermal reservoir. There are sporadic hot springs and cold springs exposed in the study area, but the uncertain deep geological structure, unknown geothermal reservoir formation mechanism, and lack of detailed exploration data seriously restrict the exploration and development of geothermal resources in the area. Filling underground faults with water can significantly reduce electrical resistivity, while traps filled with warm water can cause a decrease in S-wave velocity. Thus, a new integrated geophysical method, including the controlled source audio-frequency magnetotelluric (CSAMT) and microtremor methods, is applied for geothermal exploration in the region. The combination of CSAMT and microtremor methods can determine thermal-controlled and water-conducting structures more effectively and locate geothermal storage more accurately. The source, channel, storage, and cover of regional geothermal reservoir formation are analyzed using geophysical and geological data. That is, the regional NE- and NW-trending faults are explained as thermal-controlled and water-conducted structures, respectively. The deep Sinian Dengying Formation supplies a heat- and water-bearing space. The tectonic intersection area, especially the tensional fault zone, is found to be closely related to the existence of geothermal reservoirs, which is well verified by later drilling results. Finally, a geothermal reservoir model is established to comprehensively understand the distribution of geothermal energy in the region.

Formation Mechanism of NW-Trending Faults and Their Significance on Basin Evolution in Zhu III Depression of the Pearl River Mouth Basin, SE China

Formation Mechanism of NW-Trending Faults and Their Significance on Basin Evolution in Zhu III Depression of the Pearl River Mouth Basin, SE China

Formation mechanism of the small-angle X-type strike-slip faults in deep basin and its controlling on hydrocarbon accumulation: a case study from the Tabei Uplift, Tarim Basin, NW China

In the central Tarim Basin, numerous hydrocarbon deposits were found along ultra-deep strike-slip faults, and its evolving progress and formation mechanism are research hotspots. The Paleozoic small-angle X-type strike-slip fault in the Tabei Uplift is the research subject in this article. Based on high-precision three-dimensional seismic data, three structural deformation layers were revealed: the rift system, weak strike-slip deformation and salt tectonics in the deep structural layer (Sinian–Middle Cambrian), the strong strike-slip deformation and karst-dissolution structure in the middle structural layer (Upper Cambrian–Middle Ordovician), and echelon normal faults in the shallow structural layer (Upper Ordovician–Carboniferous). The formation and evolution of strike-slip faults is jointly controlled by the distribution pattern of basement rift and the activities of surrounding orogenic belts, which can be divided into three stages. In the Middle to Late Cambrian, the initial subduction of the Paleo-Asian and Proto-Tethyan oceans precipitated the emergence of two sets of small-angle X-type strike-slip faults, striking NW and NE above the grooves of Precambrian rifts, influenced by local weak compressive stress. Affected by the closure of peripheral paleo-ocean, strike-slip faults deformed considerably in the Middle–Late Ordovician and were reactivated in the Silurian–Carboniferous, forming en-echelon normal faults in the shallow layer. The layered deformation structure of the strike-slip faults significantly affects the accumulation of hydrocarbons. The differential hydrocarbon enrichment of faults in the Tabei Uplift is collectively influenced by the distribution of source rocks and the migration of oil and gas. The topographical features of the Tabei Uplift, along with the distribution of strike-slip faults across tectonic units, have rendered the NE direction the preferential pathway for hydrocarbon migration. Additionally, impacted by the development of en echelon faults, the NE-trending faults offer superior conditions for hydrocarbon preservation and charging condition, compared to the NW-trending faults.

Late quaternary faulted landforms and recent activity of the eastern segment of the Nanbei River–Boli Fault

The Nanbei River–Boli Fault is one of the largest NW-trending faults in Northeast China. However, limited data on the style and timing of recent earthquake activity on this fault greatly restrict the accuracy of seismic risk assessment. Here, we provide evidence of late Pleistocene to Holocene activity of the eastern segment of this fault for the first time. Specifically, the eastern segment of the Nanbei River–Boli Fault has experienced multiple earthquakes since the late Quaternary. Trench excavations indicate a recent strong earthquake at 7.28–2.11 ka, with approximately 1 m of co-seismic vertical displacement and preliminary estimates of the minimum vertical slip rates of 0.07–0.12 mm/a. According to a comprehensive analysis, we estimate that the eastern segment of the Nanbei River–Boli Fault can generate strong earthquakes of magnitude 7.0. This research provides important information for seismic risk assessments and seismic zoning, which can help seismic hazard assessment and mitigate their effects in Northeast China.

Geology, fluid inclusions and C−O−S−Pb isotopic compositions of the Chahmileh Pb-Zn deposit, Central Iran: Implications for ore genesis

AbstractThe Chahmileh Pb–Zn deposit, located northwest of the Central Iran Zone, is a sediment-hosted Pb–Zn deposit in the ‘Yazd-Anarak Metallogenic Belt’. It is hosted in Middle Triassic carbonate rocks and is mainly controlled by NW-trending faults. The main ore minerals are galena and sphalerite with minor chalcopyrite, pyrite, and quartz, dolomite, along with minor calcite and baryte as gangue minerals. Cerussite, hemimorphite, wulfenite, mimetite, smithsonite, malachite and iron oxy-hydroxides are the main non-sulphide ore minerals. The main styles of mineralization are vein-veinlet, breccia, disseminated and replacement in association with silicification and dolomitization. Microthermometry of fluid inclusions in dolomite and quartz indicates that the ore precipitated from a warm to hot basin-derived saline fluid. Dolomite samples have δ13CVPDB and δ18OVSMOW values of −0.99 to +1.99‰ and +20.74 to +25.48‰, respectively, and are plotted in the marine carbonate rocks field. These isotopic values suggest that CO2 in the hydrothermal fluids mainly originated from marine carbonate rock. The δ34S values range from +6.3 to +8.2‰ for galena, +5.9 to +6.2‰ for sphalerite, +1.4 to +3.4‰ for chalcopyrite and +15.0 to +17.4‰ for bayite are compatible with a predominant thermochemical sulphate reduction process, and with sulphur sourced from Triassic seawater. Galena samples have a homogeneous Pb isotopic composition that is indicative of a continental crustal reservoir as the main source of lead and probably for the other ore metals. Based on geology, mineralogy, texture and fluid characteristics, the Chahmileh deposit is classified as a carbonate-hosted Mississippi Valley-type deposit.

Application of integrated geological and structural investigations to the discovery of new mineral zones related to the Daping gold deposit, Southwest China

The Daping gold deposit is one of the largest gold deposits in the Ailaoshan gold belt of SW China. It has proven gold reserves of more than 70 tonnes and has been mined for 36 years. Finding new orebodies to extend the mine life has become urgent since the main gold orebodies are almost depleted. Ore deposit types in the Daping area include quartz-vein and porphyry gold types of mineralization. The quartz-vein ores are hosted in late Proterozoic diorite and Silurian limestones and controlled by NW trending faults. We have studied the ore-controlling structures to determine palaeo-stress directions of V1-2-3 and V37 orebodies, and the distribution of gold mineralization in the V1-2-3 orebodies. Based on the field observation, we identified six stages of structure in the Daping gold deposit, i.e., pre-mineralization (stage 1), syn-mineralization (stages 2 to 4), and post-mineralization (stages 5 and 6). The spatial distribution of gold orebodies at Daping is mainly controlled by the compressional faults of stage 1 that is associated with the Ailaoshan orogenic belt, and overprinted by structures of later stages. Orebodies from gold deposits in orogenic belts commonly have greater vertical dimensions than horizontal dimensions, suggesting that V1-2-3 orebodies may extend to deeper area.Our data indicate that higher gold grades and the thickest parts of the orebodies are associated with deeper parts of the V1-2-3 orebodies (up to 800 m in depth), and are likely related to a concealed mineralized porphyry which was intersected in drill core and identified as xenolith in lamprophyre dikes in the underground mine workings. The quartz-vein type gold mineralization at the Daping gold deposit was possibly overprinted by the porphyry gold mineralization, as indicated by the discovery of the new quartz-vein-associated orebodies beneath the current levels of Daping gold deposit and associated porphyry gold orebodies at Daping gold area.

Holocene Activity of the Wudaoliang–Changshagongma Fault of the Eastern Tibetan Plateau

The Wudaoliang–Changshagongma fault is one of the NW-trending faults located within the southern Bayan Har Block of the Tibetan Plateau in China. In this paper, we used high-resolution imagery and digital elevation model data to study the geomorphological and geological characteristics of the fault. Furthermore, the result also determined the fault trace and estimated the average horizontal slip rate of the fault since the late Quaternary to have been 2.6 ± 0.6 mm/a. This slip rate is approximately equivalent to that of the Awancang, Madoi–Garde, and Dari faults, which are also located within the block. Furthermore, the slip rates of these faults obtained by remote sensing and geological methods are consistent with GPS observations. It indicates that tectonic deformation within the block is continuous and diffuse. Using trenching study results and sedimentary radiocarbon dating, we identified four paleoearthquake events that occurred at 42,378–32,975, 33,935–20,663, 5052–4862, and after 673–628 cal BP, respectively. The recurrence intervals of large earthquakes on the faults within the block are much longer than those of the boundary faults, and the slip rates are also smaller, indicating that faults within the block play a regulatory role in the tectonic deformation of the Bayan Har Block.

Extensional tectonics, structural architecture modeling and geodynamic evolution in the Cretaceous Tinghir-Errachidia-Boudenib basin (Pre-African Trough, Morocco)

Structural evolution of the Cretaceous Tinghir-Errachidia-Boudenib foreland basin in the south front of the Moroccan High Atlas belt is documented here. Fieldwork, lithostratigraphic and new structural analysis coupled with borehole and geoelectrical data identify two ENE-to NE-trending sub-basins in the large Cretaceous Tinghir-Errachidia-Boudenib (TEB) basin. One named Errachidia sub-basin is situated in the west and the other Boudenib sub-basin is located in the east. They are separated by a major NE-SW trending anticlinal, inherited from a paleohigh structure, where the thickening of the Cretaceous series increases towards the center of the sub-basins and in their northern borders with the central-eastern High Atlas. The Cretaceous series clearly show a considerable thickness variation, angular unconformities and lateral facies change that was related to the syn-sedimentary normal faulting activity. The Albian-Turonian period is characterized by the development of ENE- and NW-trending fault systems. These latter are linked to an extensional tectonics event responsible for half-graben basin geometry associated with significant sedimentary rates. This considerable thickness depocenters geometry is related to normal fault coupled with the intra-basin faults. The intensity of subsidence increases from south to north and from west to east, with a high rate in the Boudenib sub-Basin. The compressive Cenozoic events caused the inversion of these extensional faulting and the folding of the High Atlas area with ENE-trending structures.

Adjacent Iron Oxide-Apatite and Iron Oxide Copper-Gold Mineralization in the Acarí District, Peru: The Magmatic to Hydrothermal Transition

Abstract Hierro Acarí and Cobrepampa are adjacent, Lower Cretaceous iron oxide-apatite (IOA) and iron oxide copper-gold (IOCG) districts, respectively, emplaced in the Coastal Cordillera of southern Peru, separated by a regional NW-trending fault. Mineralization in both districts is hosted mainly by Lower Cretaceous plutons of the Coastal batholith that intrude sedimentary and volcanic sequences of the inverted Cañete basin. The deposits form part of a region that includes the much larger Marcona and Pampa del Pongo IOA/skarn and the Mina Justa IOCG deposits, although Marcona is 60 m.y. older than the others. Hierro Acarí contains fracture-controlled steeply dipping, shallow-plunging N- and NE-trending magnetite veins in a 101.7 ± 1.4 Ma dioritic intrusive, and hematite mantos in a quartzite roof pendant. The magnetite ore shoots show colloform and microporous textures and grade downward within 150 m to actinolite and carbonates, which replace the magnetite. The Cobrepampa district comprises numerous NW-striking chalcopyrite-pyrite ± hematite veins and oxidized equivalents in a 99.7 ± 0.4 Ma monzonitic intrusive. Fluid inclusions indicate that the copper mineralization was deposited at 166° to 266°C from a fluid with low to moderate salinity of 3.8 to 16.5 wt % NaCl equiv. Colloform and microporous textures in magnetite are interpreted in comparable Chilean deposits to indicate colloidal transport of magnetite microlites, crystallized from a magma, which attach to bubbles in the magmatic-hydrothermal fluid in a process resembling froth flotation. Magnetite textures at Hierro Acarí suggest that magnetite was transported by this late magmatic process and deposited in the roof zone of the host intrusive. The hydrothermal fluid may have evolved to deposit hematite as mantos in the overlying sediments under more oxidizing conditions and may then have deposited the Cobrepampa copper and hematite veins in distal NW-trending faults. Recent studies into the geochemistry of Cañete basin volcanic rocks suggest that a mid-ocean ridge was subducted at the latitude of the Raúl-Condestable IOCG deposit, 350 km northwest of the Acarí district, at approximately 130 Ma, predating current Andean subduction below southern Peru. That situation could result in a configuration resembling the Gulf of California, where a series of fault-terminated pull-apart basins focus hydrothermal fluids at intervals along the main trough. IOCG and IOA/skarn mineralization in the Acarí district occurred during the early stages of Late Cretaceous basin inversion and intrusion of the Coastal batholith. However, the Marcona IOA deposit in the same districts was emplaced during an earlier, unrelated Jurassic to Early Cretaceous mineralizing episode.

A study of faults in the Superior province of Ontario and Quebec using the random forest machine learning algorithm: Spatial relationship to gold mines

This paper uses the random forest (RF) algorithm to produce two maps of orogenic gold prospectivity (MPM) across the entire Superior geologic province of Ontario and Quebec, Canada. One MPM of this entire study area is based on a compilation of mapped faults while the other is based on a manual interpretation of lineaments from airborne magnetic data. In addition, we have created four MPMs over the Abitibi region as the MPM map of the entire Superior is characterized by a mapping bias due to more intense mapping of faults over the Abitibi portion of the study area with respect to the rest of the Superior province. There are three RF maps generated over the Abitibi region based on faults, fault density and fault intersection density, faults + magnetic data and faults + gravity + magnetic data. To reduce the effect of the fault mapping bias, we developed a fourth map based on a knowledge driven (weighted sum technique). Statistically the best MPM is based on the faults, gravity and magnetic data. The major predictors of gold are NW-SE, NE-SW, EW trending faults, fault intersection density between EW and NW trending faults and to a lesser extent fault density and a magnetic vertical gradient image. In addition, we compare two greenstone belts in three dimensions with respect to their potential for gold exploration. We conclude that the Larder Lake greenstone belt is more fertile with respect to gold mineralization than the Swayze greenstone belt due to deeper penetrating faults which have greater potential for tapping gold mineralized fluid from the mantle or lower crust. Our final MPM map has identified prospective areas that contain known gold mines as well as areas that do not contain any known gold mines. These prospective areas may be prime for orogenic gold exploration.

Role of metasomatized mantle lithosphere in the formation of giant lode gold deposits: Insights from sulfur isotope and geochemistry of sulfides

The Wulong deposit is one of the largest quartz vein-type gold deposits with at least 80 tons of identified gold reserves in the eastern part of the Liaodong Peninsula. Gold orebodies are mainly hosted in the Late Jurassic gneissic two-mica granite and Early Cretaceous diorite dykes, and are structurally controlled by the NNE- and NW-trending faults. Gold mineralization mainly occurs as veins with lenticular shapes and is closely associated with sulfides and Bi minerals. Previous studies on the deposit mainly focused on its geological characteristics, fluid inclusions and the timing of gold mineralization. However, the extreme enrichment mechanism of gold, the geodynamic setting and ore source of the gold deposit remain equivocal. Here we present results from geochemical studies of pyrite and pyrrhotite at Wulong to address these issues. Pyrrhotite can be grouped into euhedral (Po1) and anhedral (Po2) varieties based on the textures. Four generations of pyrite were identified based on petrographic studies: Py1 is coarse-grained subhedral to euhedral crystal coexisting with milky quartz, and Au-depleted in composition; Py2 is represented by medium-grained, subhedral to anhedral pyrite coexisting with euhedral Po1, smoky quartz and minor chalcopyrite; Py3 occurs as fined-grained anhedral grains in the quartz-polymetallic sulfide veins; Py4 occurs as veinlets or aggregates in the quartz-calcite veins and contains the lowest Au concentrations. The time-resolved LA-ICP-MS ablation depth profiles of sulfides combined with detailed petrological observations indicate that gold mainly occurs either as invisible solid solution within the crystal lattice of sulfides, to some extent, or as small blebs of native gold and electrum. The close temporal and spatial relationships among native bismuth, native gold and Bi-Te-S minerals suggest that Bi-rich melts may have significantly sequestered gold from the hydrothermal fluids in the Wulong gold deposit. Sulfides from auriferous quartz veins have a restricted Δ33S range (−0.2‰ to +0.2‰) and a mean δ34S value of +1.25‰ with a feature of the mass-dependent fractionation (MDF) of sulfur isotopes, implying that the ore-forming materials were potentially derived from the mantle lithosphere that was metasomatized and fertilized by subduction-related fluids released from slab during the Early Cretaceous. This study demonstrates the Bi-rich melts as significant gold scavengers in As-deficient ore-forming fluid, and highlights the fundamental role of the metasomatized and fertilized mantle lithosphere in the formation of giant lode gold deposits in the eastern North China Craton at ca. 120 Ma.

Spatial distribution of porphyry copper deposits in Dehaj terrain: Implications for exploration

Magmatism is triggered and controlled by tectonic systems, and thus these processes play an irrefutable role in the deposition and emplacement of hydrothermal mineral systems. It is, therefore, paramount to recognize the tectonic processes that are genetically associated with hydrothermal mineral systems. This study seeks to address this caveat by recognizing the main tectonic processes that have controlled the distribution of porphyry copper deposits in Dehaj terrain, Central Iran. For this purpose, the spatial association of 31 known porphyry copper deposits, faults, and fractures were evaluated by fry, fractal, and distance-distribution analyses. The results of fry analysis revealed that mineralization has distributed through three different trends, namely NE, NW, and NS, which shows a clear association with the fault systems presented in the area. Also, the fractal method applied demonstrated that structural controls on mineralization have operated on two different scales, regional and local scales. Distance-Distribution analysis was further used to assess the spatial correlation between known porphyry Cu deposits and fault traces, supplementing the results of fry and fractal analyses by quantitative measurements. The synthesis of the results of these three methods shows that the NW-trending faults have plausibly controlled the magmatism at a regional scale; nevertheless, NE- and N-trending faults have probably operated at local scales controlling the channeling and emplacement of mineral-bearing fluids.

Cenozoic evolution of the Yangjiang-Yitong’ansha fault zone in the northern South China Sea: Evidence from 3D seismic data

The Yanjiang-Yitong’ansha Fault Zone (YYFZ) traverses the Pearl River Mouth Basin (PRMB) and plays an essential role in basin formation and hydrocarbon accumulation in the PRMB. Because of the lack of seismic data, its distribution, evolution, and effect on the basin evolution in the Cenozoic are poorly known. Based on a detailed interpretation of 3D multi-channel seismic data and previous research results, the YYFZ was identified and characterized. It comprises a series of NW-trending strike-slip faults that exhibit horsetail and en echelon structures in the map view and flower-like or Y-shaped structures in the profile view. By comprehensively analyzing the Paleogene migration of depocenters, activity characteristics of the YYFZ-related faults, and the relationships between faults and sedimentary sequences along the YYFZ, we propose the sinistral motion of the YYFZ initiated at about 35 Ma. The YYFZ served as a transfer zone during the intense rifting in the PRMB from 65 to 35 Ma. Then due to the combined effect of the Indian-Eurasian collision to the west, the Pacific subduction to the east, and the proto-SCS slab-pull to the south, it became a sinistral transtensional fault from 35 Ma to 16.5 Ma. Our results highlight the significant role of the NW-trending fault systems in the basin formation and the regional tectonic evolution of the PRMB.

Geology, geochemistry, fluid inclusions, and H–O–C–S–Pb isotope constraints on the genesis of the Atash-Anbar epithermal gold deposit, Urumieh–Dokhtar magmatic arc, central-northern Iran

The newly discovered Atash-Anbar gold deposit, with ∼2 Mt of ore grading 2.13 g/t Au (locally up to 14 g/t), is located in the Buin-Zahra Range, central-northern Iran. Hosted by Middle Eocene (ca. 39.0 Ma) volcanic-subvolcanic rocks, the Au-polymetallic orebodies occur as NW− and NE − trending quartz-sulfide veins that are structurally controlled by NW-trending faults. The Middle Eocene host rocks are characterized by high-K calc-alkaline, peraluminous signatures, and moderately Eu negative anomalies (Eu/Eu* = 0.2–0.6). Four primary paragenetic stages of veining have been recognized: (I) smoky-grey quartz-chalcopyrite stage, (II) grey-white quartz-polysulfide stage, (III) white-pinkish barite-sulfide stage, and (IV) late quartz-carbonate stage. Fluid inclusion investigations coupled with laser Raman analyses indicate that the ore-forming fluids were formed in a NaCl − H2O ± CO2 system with two types of FIs: two phase, liquid-rich inclusions (type I) and two phase, vapor-rich inclusions (type II). The primary coexisting types I and II inclusions are observed in gold-bearing ore-stage II, sharing similar homogenization temperatures in the range of 233–286 °C and 243–281 °C, but contrasting salinity values of 10.7–14.8 and 14.6–15.5 wt% NaCl equivalent, respectively. Fluid inclusion examinations show that fluid phase separation occurred concurrently with gold precipitation during the ore-stage II. The δ18Ofluid values calculated from the δ18OSMOW and δDSMOW values of the inclusions in quartz veins are 4.9–11.1 ‰ and −84.6 ‰ to −68.3 ‰, respectively. δ13CPDB and δ18OSMOW values of Fe-dolomite are −6.5 ‰ to −5.0 ‰ and 3.1 ‰–4.9 ‰, respectively. H–O–C isotope data indicate magmatic origin of initial ore-forming fluids with minor addition of meteoric water through time. The δ34S values (−3.1 ‰ to −1.2 ‰, avg. = −2.0 ‰) of the ore-stage II sulfides suggest that sulfur comes from a homogeneous magmatic source. The Pb isotopic compositions of sulfides (207Pb/204Pb = 15.420–15.525, 206Pb/204Pb = 18.423–18.536, 208Pb/204Pb = 37.135–38.208) indicate that the Pb in the Atash-Anbar gold deposit is a mixture of crust and mantle components. We conclude that (1) the Atash-Anbar is an IS epithermal system that (2) resulted from Eocene subduction of the NeoTethys Ocean plate beneath the central Iran plate and thus (3) ore-forming fluid and its metal components (i.e., Ag, Pb, and Zn) were emanated through crystallization of the final phase of a granite porphyry intrusion.

Revealing the geomorphic parameters of neotectonics based on geological and seismic data at Wadi Beda basin, NW Gulf of Suez, Egypt

The current study used remote sensing, GIS, and seismic data to determine the geological and morphometric parameters of the Wadi Beda Basin area, which is located in the northwestern part of the Gulf of Suez Province. The geomorphic parameters related with the development of neotectonics are deduced using the Shuttle Radar Topography Mission (SRTM), DEM-derived catchments, and drainage networks of the research area. The most important geomorphic parameters applied to the current study area are valley floor width to height ratio (Vf), stream length gradient index (Sl), transverse topographic symmetry index (T), hypsometric curve and integral (HI), and drainage basin shape (Bs). Based on the obtained geomorphic parameter results, the Wadi El-Noqra and Wadi Umm Athala basins exhibit moderate tectonic activity and moderate symmetry and are considered to be at their mature stage. The main basins of Wadi Umm Elda and Wadi Beda are moderately symmetrical and reflect the youth stage. However, the Bs index suggests that the Beda main basin has moderate tectonic activity, whereas Wadi Umm Elda has strong tectonic activity. By integrating the geomorphic and geological data, it is possible to determine that the majority of the study area (83.4%) is categorized by a moderate risk. Low-risk and high-risk zones, on the other hand, make up 11.1% and 5.5%, respectively, of the study area. The majority of the wadis in the Wadi Beda Main Basin are controlled by NW-trending faults, and the perturbation sites are concentrated there where these faults cross with E-W-trending faults, making them high-risk areas.

Fault characteristics and their control on oil and gas accumulation in the southwestern Ordos Basin

3D seismic data recently acquired from the Ordos Basin shows three sets of regularly distributed fault systems, which overrides previous understanding that no faults were developed in this basin. Seismic interpretation suggests that the faults in the southwestern Ordos Basin have three basic characteristics, namely extreme micro-scale, distinct vertical stratification, and regularity of planar distribution. These NS-, NW-, and NE-trending fault systems developed in the Meso-Neoproterozoic – Lower Ordovician strata. Of these, the NS-trending fault system mainly consists of consequent and antithetic faults which show clear syndepositional deformation. The fault systems in the Carboniferous – Middle-Lower Triassic strata are not clear on seismic reflection profiles. The NW- and NE-trending fault systems are developed in the Upper Triassic – Middle Jurassic strata. Of these, the NW-trending fault system appears as a negative flower structure in sectional view and in an en echelon pattern in plan-view; they show transtensional deformation. A NE-trending fault system that developed in the Lower Cretaceous – Cenozoic strata shows a Y-shaped structural style and tension-shear properties. A comprehensive analysis of the regional stress fields at different geologic times is essential to determine the development, distribution direction, and intensity of the activity of fault systems in the Ordos Basin. Current exploration suggests three aspects in which the faults within the Ordos Basin are crucial to oil and gas accumulation. Firstly, these faults serve as vertical barriers that cause the formation of two sets of relatively independent petroleum systems in the Paleozoic and Mesozoic strata respectively; this is the basis for the ‘upper oil and lower gas’ distribution pattern. Secondly, the vertical communication of these faults is favorable for oil and gas migration, thus contributing to the typical characteristics of multiple oil and gas fields within the basin, i.e. oil and gas reservoirs with multiple superimposed strata. Finally, these faults and their associated fractures improve the permeability of Mesozoic tight reservoirs, providing favorable conditions for oil enrichment in areas around the fault systems.

Seismic analysis of the Xiluodu reservoir area and insights into the geometry of seismogenic faults

The Xiluodu (XLD) reservoir is the second largest reservoir in China and the largest in the Jinsha River basin. The occurrence of two M > 5 earthquakes after reservoir impoundment has aroused great interest among seismologists and plant operators. We comprehensively analyzed the seismicity of the XLD reservoir area using precise earthquake relocation results and focal mechanism solutions and found that the seismicity of this area was weak before impoundment. Following impoundment, earthquake activity increased significantly. The occurrence of M ≥ 3.5 earthquakes within five years of impoundment also appear to be closely related to rapid rises and falls in water level, though this correlation weakened after five years because earthquake activity was far from the reservoir area. Earthquakes in the XLD reservoir area are clustered; near the dam (Area A), small faults are intermittently distributed along the river, while Area B is composed of multiple NW-trending left-lateral strike-slip faults and a thrust fault and Area C is composed of a NW-trending left-lateral strike-slip main fault and a nearly EW-trending right-lateral strike-slip minor fault. The geometries of the deep and the shallow parts of the NW-trending fault differ. Under the action of the NW-trending background stress field, a series of NW-trending left-lateral strike-slip faults and NE-trending thrust faults in critical stress states were dislocated due to the stress caused by reservoir impoundment. The two largest earthquakes in the XLD reservoir area were tectonic earthquakes that were directly triggered by impoundment.

Lateral and vertical growth and linkage of normal faults in Bozhong8-4 structure of Western Bozhong Sag, Bohai Bay Basin, China

Many previous studies have shown that normal faults are formed by growth and linkage of shorter fault segments through time and have suggested various fault growth models. Although these models have yielded useful information on the evolution of normal fault systems in map view, the three-dimensional (3D) evolution of normal faults is poorly understood. Here, we employ an integrated 3D seismic reflection and well dataset to investigate the 3D growth process of fault zone F1 in the Bozhong 8–4 (BZ8-4) structure of the Western Bozhong Sag (WBS). We i) constructed 3D structural models of individual fault surfaces, ii) calculated the dip angles and dip azimuth distributions on fault surfaces, iii) produced D-d profiles and throw contour maps of fault surfaces, and iv) investigated the 2D/3D segmentation characteristics of geometry and displacement distribution. We found that Fault F1 exhibits segmentation characteristics of geometry and displacement distribution both along strike and dip oriented directions. Vertically, fault surfaces can be divided into three areas (lower, middle and upper parts) that correspond to three stages of regional tectonic evolution (intense faulting, relatively quiescent faulting and post-rifting). The coalescence zone of the upper and lower fault surfaces shows as a sub-horizontal anomaly of geometry and displacement distribution that is sub-parallel to the traces of the horizons. The lower and upper parts of fault surfaces also show segmentation characteristics along strike, which are identified by vertically anomalous zones of geometry and displacement distribution. However, the origins of the segmentation in this case are different. The segmentation of the lower part of the fault developed during the rifting stage is caused by the intersection of faults with different strikes, whereas the upper part of the fault is the result of the lateral growth and linkage of overlapping fault segments. Therefore, not all fault segmentation characteristics along strike can be attributed to fault growth and linkage. Based on 3D fault segment identification, we therefore establish a 3D evolution model of fault zone F1. During the deposition of the Shahejie Fm up to the member (Mbr) 3 of the Dongying Fm in the Paleogene, the faults in this area were highly active, with the development of the NE-trending Fault F1, the NEE-trending Fault Fa and the NW-trending Fault F2. The intersection of Faults Fa and F2 with Fault F1 results in Fault F1 exhibiting segmentation in geometry and displacement distribution. From the deposition of the Mbr 2 of the Dongying Fm, these faults ceased to be active. Since the Pliocene (5.2 Ma), under the influence of transtensional stress field, an en-echelon fault zone was developed in the Neogene strata above fault zone F1 active in the rifting stage. While these faults grow and link laterally, their tip-lines propagate along dip-oriented directions and activate the lower faults and link vertically with them to form the present fault zone F1.

The Maluntou Au deposit: A transitional mineralization type between low- and intermediate-sulfidation epithermal styles

The Maluntou Au deposit is located in Southeast Coastal Volcanic Belt in South China. Its orebodies are hosted by the Cretaceous intermediate to silicic volcanic rocks and controlled by tensional NW-trending faults. The types of ore-related wall rock alterations consist of silicification, argillization, pyritization, and weak chloritization. Based on detailed field and microscopic petrography observation, three mineral paragenetic stages can be divided, i.e. PbZn stage, AuAg stage and post-ore stage.Electron microprobe analysis reveals slightly different composition of metal sulfides between PbZn stage and AuAg stage. The Fe content of Maluntou sphalerite is generally <2 %, indicating a typical Fe-poor sphalerite. Fluid inclusions microthermometric analysis of the early PbZn mineralization stage and late AuAg stage yielded homogenization temperatures (Th, 205–260 °C, 138–234 °C) and salinities (1.0–4.0 wt% NaCl equiv., 0.3–3.4 wt% NaCl equiv.), respectively. The low-moderate Th and low salinity ranges indicate an epithermal mineralization environment. From early stage to late stage, δDV-SMOW values show similar ranges from −69.4 ‰ ~ −64.3 ‰ to −68.2 ‰ ~ −64.9 ‰, with δ18OH2O‰ values ranges from −4.6 ‰ ~ −2.6 ‰ to −5.6 ‰ ~ −4.3 ‰, indicating ore-forming fluids derived from a magmatic and meteoric mixed source. The microthermometric data and H − O isotopic compositions of ore fluids suggest fluid mixing contributed to metal precipitation. The δ34SV–CDT values of sulfide minerals (i.e. pyrite, sphalerite, galena, chalcopyrite; −3.1 to 3.4 ‰) indicate a magmatic sulfur source. The relatively homogeneous lead isotopic compositions of the ore minerals (206Pb/204Pb = 18.4203–18.4814, 207Pb/204Pb = 15.6712–15.6853, and 208Pb/204Pb = 38.7730–38.8636) suggest a crustal origin.All the above results imply that the Maluntou Au deposit is a transitional type epithermal deposit between low- and intermediate-sulfidation styles which occurred distal to the magmatic hydrothermal system. Considering the close time and space relationship of this type deposit with other epithermal type and porphyry type mineralization, our results also suggest a possible exploration potential for the epithermal-porphyry type mineralization in the surrounding altered volcanic rocks in Zhenghe–Jian'ou area.

Effects of Seawater Recharge on the Formation of Geothermal Resources in Coastal Areas and Their Mechanisms: A Case Study of Xiamen City, Fujian Province, China

The southeast coastal areas of China have abundant geothermal resources. Most especially, seawater-recharged geothermal systems in the coastal areas have large quantiles of recharge but suffer water salinization and low water temperature. Moreover, the geothermal water development in these areas may induce seawater intrusion. Understanding the genetic patterns of geothermal resources is significant for rational exploration and protection. This study analyzed the hydrochemical and environmental isotopic characteristics of geothermal water, groundwater, and surface water samples collected in the area with geothermal resources in Xiamen Province in the southeast coastal areas of China. Based on this, the recharge of geothermal water circulation and the genetic patterns of geothermal resources were revealed. The results of this study indicate that the geothermal water in mountainous areas and piedmonts in Xiamen is mainly recharged by rainfall infiltration. In contrast, the geothermal water in coastal areas in Xiamen is recharged by seawater mixing to different extents, as indicated by hydrochemical types, isotopic characteristics, and the C1-/Br- ratio of geothermal water. As revealed by the calculation results using the Cl−mixing model, 10 of 13 geothermal fields in Xiamen are recharged by seawater mixing, with a mixing ratio of up to 73.20% in the Pubian geothermal field. After being recharged by rainfall in the low mountainous areas, geothermal water migrates toward deep parts along NW-trending faults. Then, it converges with regional NE-trending deep faults to absorb heat conducted from deep parts to form deep geothermal reservoirs. The deep geothermal reservoirs were estimated to be 185–225°C using the silica-enthalpy mixing model. The geothermal water is mixed with cold water or seawater while rising along faults. The temperature of shallow geothermal reservoirs was estimated to be 71–145°C using SiO2geothermometers.