NW-trending Faults Research Articles

The Dihedral Angle and Intersection Processes of a Conjugate Strike‐Slip Fault System in the Tarim Basin, NW China

AbstractRecent studies, focused on dihedral angles and intersection processes, have increased understandings of conjugate fault mechanisms. We present new 3‐D seismic data and microstructural core analysis in a case study of a large conjugate strike‐slip fault system from the intracratonic Tarim Basin, NW China. Within our study area, “X” type NE and NW trending faults occur within Cambrian‐Ordovician carbonates. The dihedral angles of these conjugate faults have narrow ranges, 19° to 62° in the Cambrian and 26° to 51° in the Ordovician, and their modes are 42° and 44° respectively. These data are significantly different from the ∼60° predicted by the Coulomb fracture criterion. It is concluded that: (1) The dihedral angles of the conjugate faults were not controlled by confining pressure, which was low and associated with shallow burial; (2) As dihedral angles were not controlled by pressure they can be used to determine the shortening direction during faulting; (3) Sequential slip may have played an important role in forming conjugate fault intersections; (4) The conjugate fault system of the Tarim basin initiated as rhombic joints; these subsequently developed into sequentially active “X” type conjugate faults; followed by preferential development of the NW‐trending faults; then reactivation of the NE trending faults. This intact rhombic conjugate fault system presents new insights into mechanisms of dihedral angle development, with particular relevance to intracratonic basins.

Seismogenic structures of the 2006 ML4.0 Dangan Island earthquake offshore Hong Kong

The northern margin of the South China Sea, as a typical extensional continental margin, has relatively strong intraplate seismicity. Compared with the active zones of Nanao Island, Yangjiang, and Heyuan, seismicity in the Pearl River Estuary is relatively low. However, a ML4.0 earthquake in 2006 occurred near Dangan Island (DI) offshore Hong Kong, and this site was adjacent to the source of the historical M5.8 earthquake in 1874. To reveal the seismogenic mechanism of intraplate earthquakes in DI, we systematically analyzed the structural characteristics in the source area of the 2006 DI earthquake using integrated 24-channel seismic profiles, onshore–offshore wide-angle seismic tomography, and natural earthquake parameters. We ascertained the locations of NW- and NE-trending faults in the DI sea and found that the NE-trending DI fault mainly dipped southeast at a high angle and cut through the crust with an obvious low-velocity anomaly. The NW-trending fault dipped southwest with a similar high angle. The 2006 DI earthquake was adjacent to the intersection of the NE- and NW-trending faults, which suggested that the intersection of the two faults with different strikes could provide a favorable condition for the generation and triggering of intraplate earthquakes. Crustal velocity model showed that the high-velocity anomaly was imaged in the west of DI, but a distinct entity with low-velocity anomaly in the upper crust and high-velocity anomaly in the lower crust was found in the south of DI. Both the 1874 and 2006 DI earthquakes occurred along the edge of the distinct entity. Two vertical cross-sections nearly perpendicular to the strikes of the intersecting faults revealed good spatial correlations between the 2006 DI earthquake and the low to high speed transition in the distinct entity. This result indicated that the transitional zone might be a weakly structural body that can store strain energy and release it as a brittle failure, resulting in an earthquake-prone area.

Geology and fluid inclusion geochemistry of the Zijinshan high-sulfidation epithermal Cu-Au deposit, Fujian Province, SE China: Implication for deep exploration targeting

The giant Zijinshan Cu-Au deposit in the Zijinshan orefield, Fujian Province, southeastern China, is the first high-sulfidation epithermal deposit identified in mainland China. The Cu and Au orebodies occur as veins and pods in the NW-trending faults and breccias zones. Intensive and pervasive alteration is characterized by downward and outward zoning from intensively leached silicic alteration (or vuggy quartz, Q), through alunite-quartz-pyrite alteration (advanced aragillic alteration, Q-Alu) and the quartz-alunite alteration overprinting the sericite alteration (Q-Alu-Di-Srt), to sericite (or phyllic) alteration zone. Gold mineralization mainly occurs in the silicic alteration zone, while the copper mineralization is confined in the alunite-quartz alteration zone. On the basis of detailed petrographic study, an early porphyry type mineralization stage, characterized by chalcopyrite-bornite-molybdenite-pyrite-sericite assemblage is recognized at Zijinshan, which is subsequently and strongly overprinted by the sulfate alteration and high-sulfidation Cu-Au mineralization stages. A supergene stage is also identified at shallow depth of the deposit, with gold largely enriched but copper commonly leached. The ore-forming fluids related to the early sericite alteration and possibly porphyry type mineralization are high-temperature, high-salinity magmatic water, characterized by the presence of the halite-bearing inclusions. The CO2-H2O (C-type) inclusions are mostly vapor-rich and abundantly identified in the samples with advance argillic alteration (alunite alteration). They are regarded to be the buoyant vapor phase by fluid boiling of a single-phase, low- to moderate-salinity magmatic fluid at depth, where a separated saline phase and related porphyry mineralization might be expected. A group of secondary inclusions coexisting with enargite grains are recognized in the samples with alunite alteration and suggested to be trapped from the ore-forming fluids of the Cu-Au mineralization stage. The total homogenization temperatures and salinities of the secondary inclusions are below 300°C (peaking at 260–280°C) and under 10wt% NaCl eqv., respectively. Intensive fluid boiling is the major mechanism for the formation of the giant high-sulfidation Cu (covellite-, digenite-dominated) orebodies in the Zijinshan deposit. It is a deep-seated high-sulfidation epithermal deposit according to the estimated depth of 1.4–2.1km from the C-type inclusions in quartz grains from the alunite alteration zone. By fluid inclusion mapping of a nearly NW-trending cross section profile, the isotherms are extrapolated using the average total homogenization temperatures and the possible heat source is suggested. It is indicated that the heat source and possible concealed porphyry mineralization nearly coeval to the high-sulfidation Cu-Au mineralization at Zijinshan might be located at the southeastern Zijinshan deposit or the northern area between the Zijinshan and Wuziqilong deposits, where deep drilling is encouraged.

Brittle Shear Tectonics in a Narrow Continental Rift: Asymmetric Nonvolcanic Barmer Basin (Rajasthan, India)

Our field studies emphasizing brittle shear P- and Y-planes along the margins of the Barmer basin (Rajasthan, India) support its two-phase (NW-SE, followed by NE-SW) extension during Early Cretaceous and Late Cretaceous–Paleocene periods. We also document nearly NE-trending megascale transfer zones along the northern margin of the Barmer basin. Preexisting brittle planes in the Malani basement rocks guided the relay structures here. Structures at the western basin shoulder margin indicate NE-SW extension, and the crosscut relation connotes the relative timing of the two extension phases. The crosscutting conjugate fault sets are non-Andersonian. The NW-trending faults produced by the second-phase extension and the inherited NNW-trending brittle features are dominantly dip-slip. Prior fractures of the Malani rocks at ≥45° to the NE-SW principal extension direction extended the Barmer basin obliquely during the Late Cretaceous–Paleocene period. The asymmetric nature of the rift, too, connotes its oblique rifting. The extension direction of the first phase probably rotated clockwise. This is derived mainly from WSW-trending faults cutting NE-trending faults. Brittle planes of shear and fracture significantly promoted fluid flow, as understood from secondary hydrothermal mineral deposits (Barmer hill area) and pre-Deccan basalts (Sarnoo area). Reverse slip detected along subvertical faults on the western and eastern rift shoulders are probably due to isostatic flexure–related contraction or might be related to the far-field effect of ridge-push forces. The Mesozoic subsurface stratigraphy there and elsewhere within the Barmer basin requires more study to substantiate the potential for structural entrapment of hydrocarbon.

Geology, fluid inclusions, and geochemistry of the Zhazixi Sb–W deposit, Hunan, South China

The Zhazixi Sb–W deposit in the Xuefeng uplift, South China, exhibits a unique metal association of W and Sb, where the W orebodies are hosted by interlayer fractures and the Sb orebodies are contained within NW-trending faults. This study proposes that the W and Sb mineralization took place in two separate periods. The mineral paragenesis of the W mineralization reveals a mass of quartz, scheelite and minor calcite. The mineral assemblage of the Sb mineralization developed after W mineralization and consists of predominantly quartz and stibnite, and small amounts of native Sb, berthierite, chalcostibnite, pyrite, and chalcopyrite. Fluid inclusions in quartz and coexisting scheelite are dominated by two-phase, liquid-rich, aqueous inclusions at room temperature. Microthermometric studies suggest that ore-forming fluids for W mineralization are characterized by moderate temperatures (170–270°C), low salinity (3–7wt% NaCl equiv.), low density (0.75–0.95g/cm3), and moderate to high pressure (57.2–99.7MPa) and these fluids experienced a cooling and dilution evolution during W mineralization. Ore-forming fluids for Sb mineralization are epithermal types with low temperatures (150–230°C), low salinity (4–6wt% NaCl equiv.), moderate density (0.82–0.94g/cm3), and high pressure (42.2–122.5MPa) and these fluids display an evident decline in homogenization temperature during Sb mineralization. Laser Raman analyses of the vapor phase indicate that the ore-forming fluids for both W and Sb mineralization contain a small amount of CO2.The ore-forming fluids for Sb mineralization are identified as predominantly originating from the continental crust, as suggested by the low 3He values (0.009×10−12cc.STP/g) and 3He/4He ratios (0.002–0.056Ra) as well as high 36Ar values (1.93×10−9cc.STP/g) and 40Ar/36Ar ratios (909.5–2279.7). The source of S is identified to be the Neoproterozoic Wuqiangxi Formation, as traced by the δ34SV-CDT values of stibnite (3.1–9.4‰). The 208Pb/204Pb (37.643–40.222), 207Pb/204Pb (15.456–15.681), and 206Pb/204Pb (17.093–20.042) ratios suggest a mixture of lower crustal and supracrustal Pb sources.It is thus concluded that the ore genesis of the Zhazixi Sb–W deposit is related to the intracontinental orogeny during the early Mesozoic. Fluid mixing is considered to be the critical mechanism involved in W mineralization, whereas a fluid cooling process is responsible for Sb mineralization. Furthermore, the absence of Au is attributed to the low Σas content in Sb-mineralizing fluids.

Structures of the Zhazixi Sb–W deposit, South China: Implications for ore genesis and mineral exploration

The Zhazixi Sb–W deposit is located in the Xuefeng uplift belt, South China. It is a structurally controlled hydrothermal vein-type deposit with unique metal associations of Sb and W. Field investigations suggest that structures in the Zhazixi Sb–W deposit are dominated by NW-trending faults and interlayer fractures. The former are critical to Sb mineralization, which is simplified by quartz-stibnite veins. In contrast, the latter are closely associated with W mineralization, which is predominantly composed of quartz-scheelite veins. Kinematic analyses suggest that the NW-trending faults experienced a sinistrally transpressional tectonic movement before Sb mineralization and converted to an extensional kinematic regime during Sb mineralization. Rose diagrams and stereographic projection of W-bearing fractures indicate that the interlayer fractures may form under compressional stress in a NW–SE direction. Ore genesis is discussed based on the geological and geochemical features, which indicate the metal associations of Sb and W are a superosition of two separate ore-forming periods that took place during the early Mesozoic intracontinental orogeny. Finally, integrated with the trend surface of Sb, the ore grade distribution regularities are discussed and favorable mineral exploration areas of Sb and W are proposed in the Zhazixi Sb–W deposit.

Sulfur isotope analysis of carbonate-hosted Zn–Pb deposits in northwestern Guizhou Province, Southwest China: Implications for the source of reduced sulfur

To determine the source of reduced sulfur in the Zn–Pb deposits in northwestern Guizhou Province in the Sichuan–Yunan–Guizhou (SYG) Zn–Pb triangle, Southwest China, sulfur isotope analyses were carried out on the coexisting sphalerite–galena pairs of the main mineralization stage from four Zn–Pb deposits: the Shanshulin, Qingshan, Yadu, and Tianqiao deposits. These neighboring deposits are controlled by adjacent parallel NW-trending faults. The results of the sulfur isotope analysis showed δ34S values ranging from 11.2‰ to 20.3‰ for sphalerite, and from 8.3‰ to 17.4‰ for galena. The δ34S value of the sphalerite is greater than that of the galena in each coexisting sphalerite–galena pair, which indicates that sulfur isotope equilibrium was reached between the sphalerite and galena. The total δ34S of the ore-forming fluids was determined to be 13.0–19.0‰, and the calculated equilibrium temperature ranged from 120°C to 280°C. The relatively large values of total δ34S combined with the relatively high temperatures indicate that the reduced sulfur most likely originated from thermochemical sulfate reduction (TSR) of seawater sulfates. The results of age-dating studies combined with the composition of ore–forming fluid indicate that brine from the Youjiang Basin accounted for the mineralization of the SYG Zn–Pb triangle (Li et al., 2002; Zhang et al., 2015). Several indices indicate that the Youjiang Basin developed a thermal anomaly of 200–350°C during the Middle Triassic. This thermal anomaly may have resulted in the relatively high temperature of the basin brine and oil cracking to produce methane, which further triggered TSR and reduced the aqueous sulfate in the basin brine. Expelled by the uplift of the Youjiang Basin in the Late Triassic, the basin brine carrying the reduced sulfur likely migrated through the Yadu–Ziyun NW-trending fault and supplied the reduced sulfur for Zn–Pb mineralization along secondary NW–trending faults in northwestern Guizhou Province.

Three-dimensional structural model of the Qaidam basin: Implications for crustal shortening and growth of the northeast Tibet

AbstractThe Qaidam basin, bounded by the Altyn Tagh fault in the north, is located in the northeast of the Tibet plateau, and it has important implications for understanding the history and mechanism of Tibetan plateau formation during the Cenozoic Indo-Eurasia collision. In this study, we constructed the main geological structures and surfaces in three dimensions through the interpolation of regularly spaced 2D seismic sections, constrained by wells data and surface geology of the Qaidam basin in northeast Tibet. Meanwhile the Cenozoic tectonic history of the Qaidam basin was reconstructed and the uplift mechanism of the Tibetan plateau was discussed. This study presents the subsurface data in conjunction with observations and analysis of the stratigraphic and sedimentary evolution. The Cenozoic deformation history of the Qaidam basin shows geologic synchroneity with uplifting history of the Tibet Plateau. It is therefore proposed that the deformation and uplifting in the south and north edges of the Tibet Plateau was almost synchronous. The total shortening and shortening rate during Cenozoic reached 25.5 km and 11.2% respectively across the Qaidam basin, indicating that the loss of the left-lateral strike slip rates of the Altyn Tagh fault has been structurally transformed into local crustal thickening across NW-trending folds and thrust faults. Meanwhile there is an about 11° vertical component along the strike-slip Altyn Tagh fault, the block oblique slip shows one more growth mechanism of the northeast Tibet.

Geology and geochemistry of the Early Permian Axi low-sulfidation epithermal gold deposit in North Tianshan (NW China)

The Axi gold deposit containing 12Mt of ore with an average Au grade of 5g/t is the largest low-sulfidation epithermal gold deposit in Chinese Tianshan. It is hosted by Late Devonian to Early Carboniferous continental arc volcanic rocks in the Tulasu volcanic-sedimentary Basin in North Tianshan (NW China). The orebody is controlled by annular and radial faults associated with volcanic crater, and later NW-trending faults. Four stages of ore-formation could be identified based on petrographic studies: (I) chalcedony; (II) gold-bearing pyrite-quartz veins and breccias; (III) gold-bearing marcasite-quartz-carbonate veins; and (IV) quartz±barite-calcite veins. Electrum precipitated in stage II (285–195°C, logfS2=−6.7 to −13) and stage III (190–95°C, logfS2=−15.8 to −25.6). The chalcedony formed in stage I is strongly enriched in LREE (La/Yb=4.5–36.2) compared with later gold-bearing pyrite-quartz veins and quartz±barite-calcite veins (La/Yb=1.2–2.0). O–H isotopic data indicate that fluid mixing is a major mechanism causing electrum precipitation in stage II. Hydrothermal breccias consisting of chalcedony, altered wall-rocks and gold-bearing pyrite-quartz veins (stage II) are cemented by gold-bearing marcasite-quartz-carbonate assemblages (stage III), which implies that brecciation plays an important role in stage III ore-formation.Re-Os isotopic analyses of gold-bearing pyrite samples yield an 187Re/188Os–187Os/188Os isochron age of 299±35Ma with an initial 187Os/188Os ratio of 0.57±0.37. K-Ar ages of four intensively sericitized andesitic wall rocks range between 286 and 293Ma. These Early Permian ages are much younger than the Late Devonian-Early Carboniferous host volcanic rocks, but overlap the age range of postcollisional granitoid in the Tulasu Basin. The Re-Os isotopic compositions and the ages suggest that mantle-derived components were involved in the Axi ore-forming system in a postcollisional setting.

Origin and model of transform faults in the Okinawa Trough

Transform faults in back-arc basins are the key to revealing the opening and development of marginal seas. The Okinawa Trough (OT) represents an incipient and active back-arc or marginal sea basin oriented in a general NE-SW direction. To determine the strikes and spatial distribution of transform faults in the OT, this paper dissects the NW- and NNE-SN-trending fault patterns on the basis of seismic profiles, gravity anomalies and region geological data. There are three main NW-trending transpressional faults in the OT, which are the seaward propagation of NW-trending faults in the East China Continent. The NNE-SN-trending faults with right-stepping distribution behave as right-lateral shearing. The strike-slip pull-apart process or transtensional faulting triggered the back-arc rifting or extension, and these faults evolved into transform faults with the emergence of oceanic crust. Thus, the transform fault patterns are inherited from pre-existing oblique transtensional faults at the offsets between rifting segments. Therefore, the OT performs the oblique spreading mechanism similar to nascent oceans such as the Red Sea and Gulf of Aden.

Ore-controlling factors and prospecting direction of Feimo Cu-Mo Polymetalic deposit in Yuanyang, Yunnan province, China

Major ore prospecting breakthroughs have currently been made in the Feimo Cu-Mo polymetallic deposit from the middle section of the Ailaoshan Orogeny Belt. Controlling factors of the ore body have the following distinctive characteristics: NNE and NW trending faults are the major mineralization passageways and ore-containing structures; the orebody exists in the Feimo section of the Along Formation within the Ailaoshan Group; and the concession around the granitic massif has multi-period dyke intrusions and is the preferred prospecting target. Through studies of orebody controlling factors, ore prospect orientations are determined in the Feimo deposit. The Feimo Cu-Mo polymetallic deposit extends along strike and dip directions and has considerable resource potential. In addition, there is considerable potential from the deep-seated Cu-Mo orebody in the area and the possible occurrence of a rich Cu-Mo ore body in surrounding areas such as Caishanping and Niujiaozai.

The new gold deposit in the Lower Amur region: Chulbatkan, Khabarovsk region

The main characteristics of the Chulbatkan gold deposit are considered. This deposit consists of steeply dipping plate- and lens-shaped bodies with dessimination-veinlet mineralization in Late Cretaceous plagiogranites (Lower Amur complex) controlled by the NE- and NW-trending faults. The host rocks are affected by hydrothermal and metasomatic processes (beresitization, silicification, and carbonation). The only useful component in low-grade sulfide and non-rush ore is gold, while the silver content is insignificant. The resource potential of the deposit is estimated at 50 tons.

白坭–沙湾断裂特征及活动性研究

白坭–沙湾断裂是一条贯穿珠江三角洲中心部位的北西向大断裂。它北起花都白坭，向南经南海区官窖、松岗、大沥、顺德平洲、陈村至番禺沙湾，沿洪奇沥水道没入伶仃洋。该断裂带的展布及其活动性是影响珠江三角洲中部区域稳定性的重要不利因素。本文通过调查断裂带展布位置、露头特征等，对其整体活动性进行了初步分析评价。晚第四纪以来，此断裂主要以高角度正断方式活动，断裂最新一次较强烈活动的时间可能发生于距今10~20 ka之间。 Baini-shawan fault zone is an important NW trending fault across central area of Pearl River delta. It starts from Baini town of Huadu city in the north, extends southward across Guanyao, Songgang and Dali in Nanhai, and then across Pingzhou, Chencun in Shunde, Shanwan in Panyu area, ends in hongqili River channel within Lingdingyang bay. The fault and its activity become a negative factor of regional crust stability within central Pearl River delta area. Based on a large amount of available data, this paper uncovered outcrop features of Baini-shawan fault, studied the structural feature and the activities of the fault. Since late Quaternary, it faulted mainly as normal faults with steep fracture planes, the latest faulting event might happen probably during 10 - 20 ka B.P.

Structural development of northwest Saudi Arabia using aeromagnetic and seismological data

High resolution aeromagnetic and seismological data constrained by field-based structural investigations have been used to map and delineate the structural elements that affected and shaped the Midyan area in the northwest part of Saudi Arabia. The area was divided into four major domains defined by NNE, NNW, NW and ENE trending faults identified by trends, patterns and intensity of magnetic anomalies. The ENE trending left-lateral strike-slip faults intersected by NNE trending faults are the predominant tectonic features in the Gulf of Aqaba coastal area and stop at the boundary of a central domain characterized by complexity in the pattern and intensity of magnetic anomalies, that may be attributed to heterogeneity of basement rocks containing complex igneous rock suites including diorite, gabbro, ultramafic and alkali granitic rocks. This domain is characterized by the presence of narrow linear magnetic anomalies that extend for kilometers in an NNW direction, indicating dikes intruded through NNW trending faults. These dikes become WNW-oriented near their northern termination by transfer of movement to WNW-oriented faults marking the northern termination of the Red Sea rift. It is believed that this fault zone is still experiencing neotectonic activity, as evident from recorded seismicity. The aeromagnetic structural results coincide with fault plane solutions for the largest earthquakes, confirming aeromagnetic interpreted trends and illustrating mixed mechanisms between extensional and strike-slip faulting. Thus the study area displays different mechanisms associated with different tectonic trends which show clearly in the structural patterns of the area.

Identifying potential mineralisation targets through airborne geophysics - The Western Papua New Guinea Case study

Over the last few years exploration activity in Papua New Guinea (PNG) has declined to alarming lows that the Mineral Resources Authority (MRA) has introduced initiatives to acquire new geo-scientific datasets to enhance exploration in the country. One such initiative has been the 30,000 plus line kilometre airborne magnetic and radiometric survey over the western part of the country. The airborne survey was strategically located between the large Ok Tedi porphyry copper-gold mine and the advanced Frieda River porphyry copper-molybdenum prospect. The airborne datasets were acquired at a north-south line spacing of 500m with a Tie-line spacing of 5km and at nominal terrain clearance of 100m. The main aim of the airborne geophysical survey was to identify magnetic and radiometric anomalies that may be further investigated for their mineral potential. Preliminary results of the airborne survey show large deep-seated NW-trending fault systems cross-cut by north-east trending transfer structures that may have contributed to mineral deposition in the region. Several magnetic and radiometric anomalies located within the periphery of existing prospects also have the potential to substantially increase resource values, while isolated anomalies may indicate new targets for further exploration. While this paper highlights some of the interesting magnetic and radiometric features defined from the WPAGS, the release of the datasets now ensures investors have an extra layer of information to build on the mineral potential of the surveyed area. It is anticipated that the results from the survey will not only boost interest to explore in the area, but also drive exploration activities in the area to new heights, hopefully resulting in the identification of new mineral prospects.

Sedimentological architecture, shelf-edge trajectories and evolution of an Oligocene reservoir, East Nile Delta

Abstract3D seismic data and wireline logs from 17 wells in the onshore East Nile Delta were utilized to study the stratigraphic architecture and evolution of Oligocene deposits. The Oligocene is readily recognized on seismic cross-sections by a series of northward prograding clinoforms. The Oligocene sequences show lateral facies changes from fluvial to deepwater via a shelf-edge delta. Such deltaic features include distributary channels, interdistributary bay fill, and a delta plain and front, gradually changing downslope into prodelta and slope deposits. The slope deposits include three main depositional elements: a deeply incised channel, a complex of laterally switched leveed channels and lobes, and crevasses and frontal splays. These depositional elements are formed in the main western slope basin and local eastern intraslope ponded sub-basin. The reactivated E–W- and NW-trending faults as well as NE–SW inverted structures are the main controls on slope physiography that formed the main corridors for Oligocene sediment distribution. Two main globally correlated Oligocene sequences are encountered being separated by the major drop in global eustatic sea level at 28.1Ma. The lower Oligocene sequence has two main stages: (1) the formation and filling of a basal canyon; and (2) the formation and progradation of shelf deltas to form shelf-edge deltas, and then progradation with an ascending shelf-edge trajectory (clinoforms 1). The upper Oligocene sequence is characterized by accretionary shelf-edge deltas with descending and then flat trajectories (clinoforms 2–4). The frontal splay and lobe complex exhibits potentially favourable reservoir continuity and areal coverage followed by the slope channels.

Seismogenic tectonics of the Qian-Gorlos earthquake in Jilin Province, China

The Qian-Gorlos earthquake, which occurred in the Songliao basin in Jilin Province in 1119 AD, was the largest earthquake to occur in NE China before the 1975 Haicheng earthquake. Based on historical records and surface geological investigations, it has been suggested previously that the earthquake epicenter was in the Longkeng area. However, other workers have considered the epicenter to be in the Halamaodu area based on the landslides and faults found in this region. No seismogenic structure has yet been found in either of these two regions. We tried to detect active faults in the urban areas of Songyuan City, where the historical earthquake was probably located. One of the aims of this work was to clarify the seismogenic structure so that the seismic risk in the city could be more accurately evaluated. The area was investigated and analyzed using information from remote sensing and topographic surveys, seismic data from petroleum exploration, shallow seismic profiles, exploratory geological trenches on fault outcrops, and borehole data. The geophysical data did not reveal any evidence of faults cutting through Cretaceous or later strata under the Longkeng scarp, which has been suggested to be structural evidence of the Qian-Gorlos earthquake. The continuous fault surfaces on the back edge of terraces in the Halamaodu area stretch for >3.5 km and were probably formed by tectonic activity. However, results from shallow seismic profiles showed that the faults did not extend downward, with the corresponding deep structure being identified as a gentle kink band. A new reverse fault was found to the west of the two suggested epicenters, which presented as a curvilinear fault extending to the west, and was formed by two groups of NE- and NW-trending faults intersecting the Gudian fault. Three-dimensional seismic and shallow seismic data from petroleum exploration revealed its distinct spatial distribution and showed that the fault may cut through Late Quaternary strata. Exploration boreholes and later geomorphological studies provided further proof of this. Based on these results and analysis, the Gudian fault was confirmed as having been an active fault since the Late Quaternary, with the possibility of earthquakes of magnitude >7 in the future. The Qian-Gorlos earthquake was most probably the result of breakage on one or two sections of this 66-km-long fault.

The ‘Helmut’ geophysical anomaly: A regional left-lateral transtensional shear zone system connecting Santos and Campos basins, southeastern Brazil

In the ultradeep water of Santos and Campos basins, eastwards from the area of present day pre-salt exploration in southeastern Brazil, there is a nearly 1000 km long, curved magnetic anomaly here named ‘Helmut’. On a magnetic map, the anomaly orientation switches from NE–SW in Santos basin to nearly N–S in Campos, mimicking the curved Neoproterozoic shear zone pattern of the basement exposed onshore. From the comparison between the position of such anomaly and gravity-derived modeling and residual maps, it is suggested that the magnetic anomaly also corresponds to a major gravimetric boundary, separating different crustal domains. This boundary is slightly oblique to the rift-related normal fault trend in Santos basin. Based on interpreted 3D seismic data, the NNE trending normal faults are dragged to the NE direction close to the magnetic anomaly in southern Santos basin, where an en echelon array of oblique magnetic anomalies occurs, which resulted mainly from the left lateral component of an overall transtensional displacement field.In regional 2D deep reflection seismic profiles, the ‘Helmut’ anomaly is correlated to major eastward dipping, down-to-the basin stepped faults sometimes arranged in a fan-like fashion in Santos basin, and as a landward dipping normal fault zone in Campos basin, north of the E–W Araruama transfer zone. In some areas, the fault zone displaces the base salt reflector and cross-cuts the entire stretched continental crust. The analysis of potential field and seismic data suggests that the ‘Helmut’ anomaly may be interpreted as a crustal-scale shear zone system, active after salt deposition during the Late Aptian. Regionally, the ‘Helmut’ anomaly connects two approximately N–S trending branches of the South Atlantic Rift System: the northern branch that runs from Espírito Santo basin northwards up to the Camamu (south of the Recôncavo-Tucano-Jatobá aborted trough), and the southern branch running southwards from Santos basin through Pelotas basin in Brazil as far as the basins offshore Uruguay and Argentina basins southwards. Extensional faulting of the salt reflector along early NNE rift-related normal faults and dextral transtension on the NW trending fault zones are all consistent with a sinistral transtensive motion on the NE-trending ‘Helmut’ anomaly during the Late Aptian.

New Approach of Structural Setting of Gold Deposits in Birimian Volcanic Belt in West African Craton: The Example of the Sabodala Gold Deposit, SE Senegal

The Sabodala gold deposit in the Mako volcanic Belt (SE-Senegal, West African Craton) occurs in mafic and ultramafic rocks overlain by clastic sedimentary rocks and felsic volcanic-volcaniclastic sequences. It is a structurally controlled mesothermal vein gold deposit developed in deformed rocks units near the Falombo pluton. The Sabodala Goldfield comprises several faults systems striking N-S to NE-SW. New structural evolution consisting of both thrusting and transcurrent events has been defined for the Sabodala Goldfield. The majority of the gold at Sabodala occurs on the western flank of a large, doubling plunging dome (the Sabodala Antiform) and is also associated with sinistral wrenching that overprints early structures within the Sabodala Mine. The gold bearing veins are hosted by flat lying and NW-trending shear zones. The NW-trending faults acted as transfers syn-gold mineralization, although only discrete segments of these faults were active during the main stage of gold mineralization. Late exhumation resulted in extension along fault zones was responsible for the juxtaposition of medium and low grade metamorphic rocks. The granitic suite of the Falombo pluton is linked to exhumation. Final dextral transpression postdating exhumation produced shearing and folding. Gold lodes developed during brittle deformation are associated with high fluid pressures in a northwest-southeast shortening regime. These are termed mainly the Sabodala deposit, and secondary the Niakafiri and Masato deposits.

Isotopic analysis of the super-large Shuangjianzishan Pb–Zn–Ag deposit in Inner Mongolia, China: Constraints on magmatism, metallogenesis, and tectonic setting

The super-large Shuangjianzishan Pb–Zn–Ag deposit is a newly discovered deposit located in the Huanggang–Ganzhuermiao polymetallic metallogenic belt of Inner Mongolia, NE China. The deposit's resource includes 0.026Mt Ag, 1.1Mt Pb, and 3.3Mt Zn. The deposit is controlled by a NW-trending ductile shear zone and NE- and NW-trending faults in black pelite assigned to the lower Permian Dashizhai Formation. LREE enrichment, HREE depletion, Nb, Ta, P, and Ti depletion, and Zr and Hf enrichment characterize felsic magmatic rocks in the Shuangjianzishan Pb–Zn–Ag district. The ages of porphyritic monzogranite, rhyolitic crystal–vitric ignimbrite, and porphyritic granodiorite are 254–252, 169, and 130Ma, respectively. Pyrite sampled from the mineralization has Re–Os isochron ages of 165±7Ma, which suggest the mineralization is associated with the ca. 169Ma magmatism in the Shuangjianzishan district.Zircons extracted from the porphyritic granodiorite yield εHf(t) values of −11.34 to −1.41, with tDM2 dates of 1275–1901Ma. The εHf(t) values of zircons in the rhyolitic crystal–vitric ignimbrite and the ore-bearing monzogranite porphyry are 7.57–16.23 and 10.18–15.96, respectively, and their tDM2 ages are 177–733 and 257–632Ma, respectively. Partial melting of depleted mantle resulted in the formation of the ca. 254–252Ma ore-bearing porphyritic monzogranite and the ca. 169Ma rhyolitic crystal–vitric ignimbrite; dehydration partial melting of subducted oceanic crust resulted in the formation of the ca. 130Ma porphyritic granodiorite. The porphyritic monzogranite was emplaced during the late stages of closure of the Paleo-Asian Ocean during the transformation from a collisional to extensional tectonic setting. The ca. 170 and ca. 130Ma magmatism and mineralization in the Shuangjianzishan district are related to subduction of the Mongolia–Okhotsk Ocean and subduction of the Paleo-Pacific Ocean Plate, respectively.