NW-striking Faults Research Articles

Spatial analysis of gold occurrences in the Zuru Schist Belt, northwest Nigeria

ABSTRACT The Zuru Schist Belt in Northwest Nigeria is one of the auriferous schist belts occurring in the western half of Nigeria. The gold mineralization largely hosted within Neoproterozoic metasedimentary rocks is considered orogenic. In an attempt to understand the pattern of occurrence of gold mineralization, their spatial distribution and association with regional structures, spatial analysis including point pattern, fractal, Fry, and distance distribution analyses were carried out to determine and quantify spatial pattern, spatial association, and regional structural control. Point pattern analysis reveals that the gold occurrences in the Zuru Schist Belt have a clustered pattern. Fractal analysis shows that the structures that controlled the distribution of gold occurrences can be divided into two categories: those that operated at a spatial scale of below 8 km, and those that operated at around 8 km and more. Fry analysis shows that the gold occurrences in the Zuru Schist Belt are aligned in a NNE-NE direction. This is parallel with the NNE-NE striking faults, establishing these faults as the main controlling structures. Fry plot of orientations of translated pairs within an 8 km distance shows that in addition to NNE-NE striking faults, folds with ESE trending axial trace and NW striking faults also exert some control on these known gold occurrences. The influence of these structures on the gold occurrences is further established by the result of distance distribution analysis. The results of spatial analysis using point pattern, fractal, Fry, and distance distribution analyses established the NNE-NE striking faults as the major structures that controlled the distribution of gold occurrences in the belt, and NW striking faults as important structures at a local scale. In addition to NW striking faults, this study further identified folds with axial trace oriented in ESE as additional secondary control structures for gold mineralization in the Zuru Schist Belt.

Cenozoic structure of the Zhu 3 Depression in the Pearl River Mouth Basin and its response to the dynamic background of the South China Sea

The Zhu 3 Depression is located at the transition between the South China Sea and the South China Block. The Cenozoic structure reflects the dynamic background of the South China Sea. We used 3D seismic and log data to study the structural evolution of the Zhu 3 Depression. We defined the characteristics of Cenozoic rift deformation and the migration of the depocentre based on fault activity and the distribution of sediment thickness. The spatial and temporal differences in the Cenozoic structure of the Zhu 3 Depression are due to the influence of pre-existing faults and regional stresses. Our results show that there are three populations of faults in the Zhu 3 Depression, striking NE, east–west and NW. The NE-striking faults are mainly large-scale boundary faults. The east–west-striking faults are small and were activated in the later stages of deformation. The NW-striking faults are continuous at depth, but are en echelon and parallel to each other at shallow depths. Based on these results, we suggest that the NE- and NW-striking pre-existing faults divide the Zhu 3 Depression into different structural zones and controlled the differences in deformation in the basin. The regional stress direction, which is controlled by regional plate interactions and the Red River Fault Zone, changed clockwise from NW-trending during the Paleocene to NE-trending during the Mid-Miocene. These findings will contribute to a better understanding of the evolution of the entire South China Sea. Thematic collection: This article is part of the Emerging knowledge on the tectonics of the South China Sea collection available at: https://www.lyellcollection.org/topic/collections/south-china-sea

Structural control on the alteration and fluid flow in the lithocap of the Allumiere-Tolfa epithermal system

Zones of advanced argillic alteration with general low permeability (i.e., lithocaps) are common place in the shallow parts of porphyry and epithermal ore deposits and active geothermal systems. The study of structural control on alteration distributions is of paramount importance for exploitation purposes as it really influences the caprock efficiency. We present the results of a structural-mineralogical study carried out in the lithocap of the Allumiere-Tolfa epithermal system (Northern Apennines). We characterized the composition, textures and physical properties (i.e., in-situ permeability; relative rock strength) of alteration facies. We then integrated field structural analysis with analysis of a virtual outcrop model to reconstruct the geometry of principal fluid-corridors. It resulted that advanced argillic alteration was promoted by circulation of highly reactive fluid(s) along a complex network of NE- and NW-striking faults and fractures dissecting the acidic volcanic dome. Such structures likely developed in response to a local disturbance of the regional stress field due to the extrusion of the Tolfa dome, which controlled the syn-extensional mineralization of the Allumiere-Tolfa area.

Tectonic-magmatic-hydrothermal interactions in a hot dry rock geothermal system: The role of the transfer and normal faults in the Acoculco caldera (Mexico)

In the Acoculco caldera (hot dry rock system) two geothermal boreholes were planned to intersect fracture systems in the carbonate basement. Even though the caldera is located in an area of active deformation with high rainfall, a scarce permeability was recognized. To shed light on this apparent mismatch, we investigate the area by integrating a structural and kinematic dataset on faults and fractures with the volcanological data. The results highlighted two main coeval fault systems, NW- and NE striking respectively, and two associated minor N-S and E-W striking sets, affecting the area of the Acoculco caldera and controlling its volcanic evolution. The kinematic analysis on fault-slip surfaces demonstrated that the NW-striking faults are characterized by two superposed movements: the first comprises right strike-slip to right lateral oblique-slip, while the second is dominantly normal. In contrast, the NE-striking structures display only dominantly normal movements. We explain the kinematic evolution by two recurring events, determined by the interplay between regional extension and uplift: when a crustal extension was dominant in the region (first kinematic event), the NW- and NE-trending structures played the role of transfer and normal faults, respectively; in contrast, when uplift predominated (second kinematic event) both the pre-existing NW- and NE-striking faults acted with normal movements. The location of eruptive centers is controlled by the permeability along the regional faults and their step-over zones. A closer analysis carried out in the surroundings of the geothermal boreholes suggests that deformation migrated northwards, and that fractures are sealed in this sector of Acoculco, thus strongly reducing permeability. In this view, the most promising areas for potential permeability at depth might occur northwards, beyond of the Acoculco caldera, and along the NW-trending structures. In conclusion, the fault kinematics is influenced by periods of increased partial melting of the lithosphere, the latter determining anomalous heat flux and uplift. This process interacts with the fluid pathways that are in turn controlled by the tectonic segmentation, and this is a new model to explain the tectonic-magmatic and hydrothermal interaction of this region.

Evolution of Miocene normal and dextral faulting in the lower Colorado River region near Blythe, California, USA

Abstract The evolution of strain in nascent continental plate boundaries commonly involves distributed deformation and transitions between different styles of deformation as the plate boundary matures. Distributed NW-striking faults, many with km-scale right-lateral separation, are prevalent near Blythe, California, and have been variably interpreted to have accommodated either Middle Miocene NE-SW extension as normal faults or Late Miocene to Pliocene dextral shear as strike-slip faults. However, with poor timing and kinematic constraints, it is unclear how these faults relate to known domains of Neogene deformation and the evolution of the Pacific–NorthAmerica plate boundary. We present kinematic data (n = 642 fault planes, n = 512 slickenlines) that demonstrate that these faults dominantly dip steeply northeast; ~96% of measured faults record normal, dextral, or oblique dextral-normal kinematics that likely reflect a gradational transition between normal and dextral oblique kinematic regimes. We constrain fault timing with 11.7 Ma and 7.0 Ma 40Ar/39Ar dates of rocks cut by faults, and laser ablation–inductively coupled plasma–mass spectrometry U-Pb dating of calcite mineralized during oblique dextral faulting that demonstrates fault slip at ca. 10–7 Ma and perhaps as late as ca. 4 Ma. This Late Miocene dextral oblique faulting is best compatible with a documented regional transition from Early to Middle Miocene NE-directed extension during detachment fault slip to subsequent NW-directed dextral shear. We estimate 11–38 km of cumulative dextral slip occurred across a 50-km-wide zone from the Palen to Riverside mountains, including up to 20 km of newly documented dextral shear that may partly alleviate the regional discrepancy of cumulative dextral shear along this part of the Late Miocene Pacific–North America plate boundary.

The 4 January 2020 offshore Pearl River Estuary earthquake sequence in South China: Implications for seismic potential of the NW-striking local faults

The complexity of intraplate earthquakes and their challenge to current earthquake models persist as an important scientific and social problem, both about their causes and the resulting hazards. The Guangdong-Hong Kong-Macao Greater Bay area (GHMGB) is located in the coastal seismic zone of South China. Due to the huge residential population and important economic status, seismic hazard in the GHMGB often causes widespread concern. In this work, waveform data of the 2020 Pearl River Estuary (PRE) earthquake (ML3.5) and its aftershocks are carefully analyzed using about 6-month data recorded at 124 local seismic stations. A total of 8 aftershocks are identified.We determine the hypocentral parameters of the PRE mainshock and its aftershocks, as well as their focal mechanisms.Our results show that the 2020 PRE earthquake sequence is distributed in a NW-SE direction with a lateral extent of about 3.5 km, which is consistent with the location of the NW-striking Qi'Ao-Guishan fault. The 2020 PRE earthquake and its aftershocks exhibit similar focal mechanisms. According to the stereographic projection results of the P and T axes, the orientation of the principal compressive stress is oblique to the NW-striking Qi'Ao-Guishan fault, hence these earthquakes are mainly characterized by left-lateral strike-slip faulting. In combination with many previous geological and geophysical results, we suggest that the NW-striking faults could have similar seismogenic importance as the NE-striking faults in the GHMGB and even the whole coast of the northern South China Sea, especially at the intersection of the NW- and NE-trending faults.

The Mesozoic tectonic evolution of the East China Sea Basin: New insights from 3D seismic reflection data

The Mesozoic is a significant period in the East China Sea Basin (ECSB), since it recorded the tectonic transition from the Triassic Tethys tectonic domain to the subsequent Jurassic-Cretaceous Paleo-Pacific domain. However, the Mesozoic tectonic evolution of the East China Sea Basin is not well defined, and it has been highly debated in recent years. Here high-quality 3D seismic reflection data, information about fault timing, and fault maps were employed to analyze Mesozoic faulting and evolution of the ECSB. This study significantly improves our understanding of the Mesozoic tectonic evolution of the ECSB. Our results demonstrate that two sets of Mesozoic faults developed in the ECSB: (1) NW-striking faults are typical negative inversion structures and initially formed as low-angle thrusts as the result of multiphase plate convergence during Triassic. Then, the thrusts reactivated as extensional or transtensional faults during Cretaceous, which are interpreted to be related to NNW-SSE oblique extension associated with roll-back of the subduction zone of Paleo-Pacific plate. (2) The NE-striking faults are inferred to be thrust structures formed during the Early to Middle Jurassic. Flat-slab subduction of the Paleo-Pacific plate is introduced to explain the compression event during this stage. The NE-striking reactivated and newly-formed extensional faults were then developed extensively in the ECSB during the Cretaceous. The NNW-SSE oblique extension played a dominant role in this faulting event, which is related to roll-back of the subduction zone of Paleo-Pacific plate. Furthermore, the NE-striking faults continued to propagate cut through the Cenozoic sequence, since the strike of these faults is perpendicular to the Cenozoic extension stress field.

Structural analysis, mineralogy, and cassiterite U–Pb ages of the Wuxu Sb-Zn-polymetallic district, Danchi Fold-and-Thrust belt, South China

The Danchi fold-and-thrust belt, Guangxi, South China, contains the Mangchang, Dachang, and Wuxu large and giant Sn-polymetallic districts. It hosts more than 1.2 Mt of ore grading 1 % Sn, 6.23 Mt Zn at 2.4 %, and 1.13 Mt Sb at 1.3 %. Wuxu district is one of the Sb-Zn dominated hydrothermal districts without magmatic rocks exposing in the south of the Danchi belt. Recent drilling and exploring mining have identified a partially Sn mineralization associated with the Zn-Sb mineralization. These works provide an opportunity to evaluate the potential of tin mineralization in the south of Danchi belt and relationship among Sn, Zn, and Sb mineralization. Structural analysis suggests that the Wuxu district underwent at least three deformational phases since the Triassic. The first NE-SW shortening deformation controlled the structural framework of the Wuxu district and is expressed by NW-verging thrusts and folds, axial-plane cleavages around the core of the Wuxu anticline, transverse joint sets on the limbs of the Wuxu anticline, and dilational zones in the bedding-fractures. Afterward, an E-W extension stress field produced NS-striking normal faults and reactivated pre-existing NNW-striking faults which were expressed by sinistral top-to-the north sense of shear. During this deformation, some low order dilational spaces of the NW-striking faults and bedding-fractures formed and a syn-kinematic deposition of minerals in these hosting structures to form the ore shoots. The NNW-striking faults, axial-plane cleavages, and large dilational jogs controlled the subvertical large veins in the Jianzhupo deposit. The conjugate joints and inter-bedding fractures in the limb of the Wuxu anticlinorium controlled the branching veinlets and stratiform, paleokarst infilled ores in the Bawang deposit. The post-ore EW-striking faults are not obvious and displaced preceding two-phase structures and ores. Three primary paragenetic stages of hydrothermal process have been recognized: (I) Sphalerite dominated + massive pyrrhotite with euhedral pyrite porphyroblasts, (II) Cassiterite + siderite + arsenopyrite dominated with minor scheelite + quartz + fluorite, and (III) Jamesonite + stibnite + sulfosalt + carbonates. Cassiterite U-Pb dating provided a lower intercept age of 90.2 ± 3.1 Ma and 90.6 ± 4.6 Ma for the Jianzhupo and Bawang deposits, respectively. These two ages are consistent with the chronological data of tin-sulfides and ore-related felsic intrusions in the Danchi fold-and-thrust belt, South China. Our studies coupled with previous gravity data suggests a distal magmatic-hydrothermal infilling tin-polymetallic district and indicates that the Wuxu district has great potential for additional tin mineralization.

Clay Minerals in the Deep Reservoir of the Cerro Pabellón Geothermal System (Northern Chile)

This study focuses on hydrothermal alteration, and more specifically clays, in the geothermal reservoir of Cerro Pabellón in Andean Cordillera, Northern Chile. Although it is the first commercial geothermal power plant in South America with 48 MWe installed and an additional 33 MWe during 2022, the alteration mineralogy of the deep reservoir remains poorly investigated. Cuttings from two deep wells drilled between 500 and 3000 m depth were sampled. The fine-grained clay fraction was analyzed using X-ray diffraction to observe the mineral distribution and variations of crystal structure of the clays. Scanning electron microscopy coupled with energy dispersive spectroscopy allowed us to perform microanalysis of the clays. The results suggest a high heterogeneity of the reservoir in term of alteration: a pervasive alteration dominated by trioctahedral clays was observed in all the samples, whereas a fracture-controlled alteration was only observed locally, dominated by dioctahedral clays. Illite-smectite mixed layers with more than 90% illite typically occur in permeable fracture and fault as a typical clay signature. This study completed an integrated conceptual model of the high enthalpy reservoir (>250 °C). Circulations of a neutral pH Na-K-Cl fluid occur in a wide fracture system connected to highly dipping and NW-striking faults and could extend beyond the Pabelloncito graben.

Numerical modeling of NE Brazil coastal aquifer: Fault controlled conduits for seawater intrusion

Saline intrusion is a problem that mostly affects coastal aquifers around the world, either owing to anthropic and/or natural factors. The Barreiras Aquifer, which is located on Brazil's northeastern coast and of an unconfined hydraulic nature, is subject to this due to a water demand increase. This aquifer is located in an area structurally controlled by neotectonic activity. Increased complexity occurs in the form of variations in saturated thickness, hydraulic conductivity and exploited flow rates. This study aimed to construct a numerical model of dependent density flow for the Barreiras Aquifer using the SEAWAT software (USGS) and simulate saline intrusion over 20 years of pumping wells that currently exploit approximately 750 m³/h. The results can contribute to local hydrogeological management and assist managers. Simulation results with variation of recharge between dry and rainy periods and reduction of well exploitation flow rates were also obtained. The two main local faults, Boa Cica and Tabatinga, striking NW-SE and NE-SW, respectively, were simulated in a calibrated transient hydrogeological model as zones of high hydraulic conductivity (conduits) as well as in the inverse form (barriers) and offered a better fit considering the faults as zones of relatively higher hydraulic conductivities and preferred pathways for saltwater intrusion progression. A two-dimensional resistivity cross-section was used to constrain the electrical property of the NW striking fault, whether in the form of a resistive anomaly or conductive anomaly. The interpretation of the 2D geoelectric model is consistent with the conductive geoelectric anomaly associated to the Boa Cica Fault, where its transtensional nature favors wider saline contamination.

Recent Seismic Sequences and Activation of Normal Fault Systems in the Mugello Basin and Surrounding Areas (Northern Apennines, Italy)

The Mugello Basin (North-Eastern Tuscany) is an intermontane basin of the Northern Apennines (Italy) with a well-documented record of seismicity; the two major historical earthquakes occurred in 1542 (Mw = 6.0) and in 1919 (Mw = 6.4). In this study, we integrate different seismic catalogs spanning the 2005–2019 time interval, and complement these data with phase arrival times from a temporary network that specifically operated in the area throughout the 2019–2021 time interval. The subsequent relocation of this data set with a double-difference algorithm allows for accurate analyses of the most relevant seismic sequences which affected the study area in 2008, 2009, 2015–2017, and 2019. These sequences are associated with the activation of adjacent segments of larger NW-striking fault systems, one of which bounds the NE margin of the Mugello Basin (Ronta Fault System). For each seismic sequence, best-fit fault surfaces are derived from orthonormal regression of relocated hypocenters, yielding consistent results with that derived from fault plane solutions. The four sequences mark a significant increase in the seismicity rate with respect to what was recorded in the previous decades. This suggests that, following the 2008 renewal of seismicity, static or dynamic stress changes, or both depending on the case, played a role in advancing the time of failure of the fault segments activated subsequently.

Fault Activation in Central Mongolia during the Holocene: Results of Study of the Mogod Earthquake Ruptures

Abstract —In order to determine the seismotectonic activity of faults in the Holocene, we performed trench studies of the ruptures produced by the catastrophic Mogod earthquake (5 January 1967, M = 7.5–7.8, I0 = 9–10) in the junction zone of the N–S striking Hulzhin Gol fault and the NW striking Tullet fault. Paleoseismic interpretation of seismic-deformation sections and radiocarbon dating of the samples allowed determining the kinematics and obtaining, for the first time, the absolute ages of paleoevents preceding the Mogod earthquake. Analysis of the tectonic conditions for realization of earthquake sources has shed light on the complex structure of ruptures in the area of the Mogod earthquake epicenter, within which three segments differing in the displacement amplitudes and kinematics have been identified. The research data indicate the repeated activation of the Tulet and Hulzhin Gol faults in the Late Pleistocene–Holocene. The absolute age of the latest activation is 596–994 AD for the Tulet fault and 11,379–6235 BC for the Hulzhin Gol fault. The cumulative deformation from paleoearthquakes in the trench sections in the Tulet fault zone points to at least two displacements of thrust kinematics, with the latest of them having an amplitude of 2.8 m. The paleoearthquake in the Hulzhin Gol fault zone is characterized by the presence of lateral slip. The amplitudes of deformations attest to earlier earthquakes similar in energy to the 1967 Mogod event or even stronger in the fault node. The obtained data on the timing of these earthquakes and the amplitudes of the accompanying displacements made it possible to estimate slip rates along the faults: 0.2–0.3 m/kyr horizontal-slip rates on the Hulzhin Gol fault and 0.5–0.7 m/kyr vertical-slip rates on the Tulet fault.

Early Permian lode gold mineralization in the northern North China Craton: Constraints from S-Pb isotope geochemistry and pyrite Re-Os geochronology of the Chaihulanzi deposit

• A newly recognized Early Permian gold deposit in the Chifeng-Chaoyang gold district. • The Chaihulanzi deposit is an atypical orogenic gold deposit. • A metasomatized lithospheric mantle derivation for the ore metals. The Chaihulanzi gold deposit is located in the Chifeng-Chaoyang region in the northern margin of the North China Craton. It contains auriferous quartz veins and disseminated mineralization in hydrothermal alteration zones, controlled by NW-striking faults in mica schist of the Neoarchean-Paleoproterozoic Dayingzi Formation. Pyrite and pyrrhotite are the predominant ore minerals. Gold mineralization is closely associated with pyritic and phyllic alteration. The ore-forming process can be divided into four stages involving stage I quartz-pyrite; stage II quartz-pyrite-pyrrhotite-chalcopyrite; stage III quartz-polymetallic sulfides; and stage IV quartz-calcite. Gold generally occurs in fissures or as inclusions within stage II pyrite and quartz. Stage II pyrite yields a well-defined 187 Re/ 188 Os versus 187 Os/ 188 Os isochron age of 291.8 ± 3.4 Ma (MSWD = 2.1). This is the first record of Early Permian gold mineralization in the Chifeng-Chaoyang district. Sulfur and lead isotope data of stage II pyrite and pyrrhotite show that ore metals are principally derived from metasomatized lithospheric mantle, with few involvements of crustal materials. On the basis of available evidence, we suggest that the Chaihulanzi deposit is an atypical subduction-related orogenic gold deposit. These new findings might have important implications for future gold exploration in the area.

Two-phase late Paleozoic magmatism (~\u2009313\u2013312 and\u2009~\u2009299\u2013298\xa0Ma) in the Lusatian Block and its relation to large scale NW striking fault zones: evidence from zircon U\u2013Pb CA\u2013ID\u2013TIMS geochronology, bulk rock- and zircon chemistry

Late Paleozoic (Variscan) magmatism is widespread in Central Europe. The Lusatian Block is located in the NE Bohemian Massif and it is part of the Saxothuringian Zone of the Variscan orogen. It is bordered by two major NW-trending shear zones, the Intra-Sudetic Fault Zone towards NE and the Elbe Fault Zone towards SW. The scarce Variscan igneous rocks of the Lusatian Block are situated close to these faults. We investigated 19 samples from Variscan plutonic and volcanic rocks of the Lusatian Block, considering all petrological varieties (biotite-bearing granites from the Koenigshain and Stolpen plutons, amphibole-bearing granites from three boreholes, several volcanic dykes, and two volcanites from the intramontane Weissig basin). We applied whole-rock geochemistry (18 samples) and zircon evaporation dating (19 samples). From the evaporation data, we selected six representative samples for additional zircon SHRIMP and CA–ID–TIMS dating. For the Koenigshain pluton, possible protoliths were identified using whole-rock Nd-isotopes, and zircon Hf- and O-isotopes. The new age data allow a subdivision of Variscan igneous rocks in the Lusatian Block into two distinct magmatic episodes. The spatial relation of the two age groups to either the Elbe Fault Zone (298–299 Ma) or the Intra-Sudetic Fault Zone (312–313 Ma) together with reports on the fault-bound character of the dated intrusions suggests an interpretation as two major post-collisional faulting episodes. This assumption of two distinct magmatic periods is confirmed by a compilation of recently published zircon U–Pb CA–ID–TIMS data on further Variscan igneous rocks from the Saxothuringian Zone. New geochemical data allow us to exclude a dominant sedimentary protolith for the Koenigshain pluton as supposed by previous investigations. This conclusion is mainly based on new O- and Hf-isotope data on zircon and the scarcity of inherited zircons. Instead, acid or intermediate igneous rocks are supposed as the main source for these I-type granitoids from the Koenigshain pluton.

Preface to Advances in the Jurassic of Argentina

Mocho-Choshuenco Volcanic Complex (MCVC) is one of the most hazardous volcanoes in Chile's Southern Volcanic Zone (SVZ). It is located at 40°S 72°W in Los Ríos Region, Chile, where the SVZ coexists with the margin-parallel Liquiñe-Ofqui Fault System (LOFS) and with NW-striking Andean Transverse Faults (ATF). The LOFS is 1200 km-long intra-arc strike-slip fault system located between 38° and 46°S, and partially controls the occurrence, spatial distribution and geometry of major stratovolcanoes. The fault-fracture networks induced by crustal deformation plays an essential role in the generation of pathways, facilitating the transport and emplacement of magmas. Thus, the Chilean Andes provides one of the best natural laboratories in the world to assess the link between tectonics and volcanism. In this study, we provide new structural, kinematic and dynamic insights, which would condition the formation and evolution of the MCVC and laying foundations for future works.Structural field data was collected and analyzed from regional and local scales. The strain analysis shows heterogeneous deformation. The stress field analysis at a regional scale indicates a strike-slip dominated transpressional regime with N61°E-trending σ1 and N26°W-trending σ3, which is consistent with the actual oblique convergence. Deformation driven by this oblique convergence is partially partitioned, where the margin-parallel component originates the dextral strike-slip in LOFS. Results are consistent with reactivation of preexisting NW-striking faults under the regional stress field, facilitating hydrothermal fluid flow and magma migration, associated with volcanoes, dikes and minor eruptive centers present in the study area. As a final result, we propose a simplified, three-dimensional model for the MCVC's basement, contributing to the understanding of the system and the way in which the different factors interact.

Dynamic and kinematic characterization of the basement structures of the Mocho-Choshuenco Volcanic Complex, Southern Andes, Chile

Mocho-Choshuenco Volcanic Complex (MCVC) is one of the most hazardous volcanoes in Chile's Southern Volcanic Zone (SVZ). It is located at 40°S 72°W in Los Ríos Region, Chile, where the SVZ coexists with the margin-parallel Liquiñe-Ofqui Fault System (LOFS) and with NW-striking Andean Transverse Faults (ATF). The LOFS is 1200 km-long intra-arc strike-slip fault system located between 38° and 46°S, and partially controls the occurrence, spatial distribution and geometry of major stratovolcanoes. The fault-fracture networks induced by crustal deformation plays an essential role in the generation of pathways, facilitating the transport and emplacement of magmas. Thus, the Chilean Andes provides one of the best natural laboratories in the world to assess the link between tectonics and volcanism. In this study, we provide new structural, kinematic and dynamic insights, which would condition the formation and evolution of the MCVC and laying foundations for future works.Structural field data was collected and analyzed from regional and local scales. The strain analysis shows heterogeneous deformation. The stress field analysis at a regional scale indicates a strike-slip dominated transpressional regime with N61°E-trending σ1 and N26°W-trending σ3, which is consistent with the actual oblique convergence. Deformation driven by this oblique convergence is partially partitioned, where the margin-parallel component originates the dextral strike-slip in LOFS. Results are consistent with reactivation of preexisting NW-striking faults under the regional stress field, facilitating hydrothermal fluid flow and magma migration, associated with volcanoes, dikes and minor eruptive centers present in the study area. As a final result, we propose a simplified, three-dimensional model for the MCVC's basement, contributing to the understanding of the system and the way in which the different factors interact.

Latest Quaternary Active Faulting and Paleoearthquakes on the Southern Segment of the Xiaojiang Fault Zone, SE Tibetan Plateau

AbstractThe Xiaojiang fault zone (XJFZ) is an important part of the Xianshuihe-Xiaojiang fault system, acting as the eastern boundary of the Chuan-Dian block on the southeastern margin of the Tibetan Plateau and accommodating the lateral extrusion of the block. The faulting activity and paleoseismic history on the southern segment of the XJFZ remain poorly understood. Here, trench excavations and radiocarbon dating revealed that four recent surface-rupturing paleoearthquakes have occurred on the Jianshui fault (JSF) in the southern segment of the XJFZ since ~15370 yr BP. The ages of these events, labeled E4-E1 from oldest to youngest, are limited to the following time ranges: 15360-12755, 10845-6900, 1455-670, and 635-145 yr BP, respectively. The most recent event E1 was most likely the 1606 Jianshui earthquake. These events appear to occur unregularly in time. The time interval between the last two events is 726±235 yr, and the average recurrence interval for all four events is 4589±3132 yr. The deformed strata show that the JSF is characterized kinematically by transtension, which likely respond to the apparent change in the direction of clockwise rotation of the Chuan-Dian block around the eastern Himalayan syntaxis. Combined with the analysis of the neighboring NW-striking faults, our study suggests that the south-southeastward motion of the Chuan-Dian block is likely to be firstly accommodated in part by the right-lateral shear and dip-slip motions of the Qujiang and Shiping faults and continues across the Red River fault zone, then is transmitted southward along the Dien Bien Phu fault. Therefore, the southern segment of the XJFZ plays a dominant role in the tectonic deformation of the southeastern Chuan-Dian block, with a high seismic hazard.

The relationships among faults, geology and geophysical data in the southwestern Korean Peninsula, including the Haenam area, and their application for the interpretation of earthquakes

The southwestern Gyeonggi Massif features many faults. The main faults are NE-striking faults, including the Jeonju and Kwangju faults, and they are cut by N-S- and NNE-striking faults. Additionally, E-W-, WNW-, and NW-striking faults occur as minor faults. Historical earthquakes with magnitudes greater than 5.0 have occurred on or near these faults. Earthquakes with focal depths shallower than 12 km have generally occurred on the NE-, NNE-, and N-S-striking faults, while those with hypocenters deeper than 12 km have occurred on the NW-, WNW-, and NE-striking faults or do not show a relationship with any faults. The instrumental earthquakes with magnitudes greater than 5.0 in the Korean Peninsula mainly have hypocenters deeper than 12 km, and their epicenters form a NW-oriented trend. These data effectively reflect two seismogenic layers with a boundary at ca. 12 km, as suggested in a previous study. The recent earthquake in the Haenam area occurred in the lower seismogenic layer. The shallower earthquakes seem to be the result of the reactivation of pre-existing faults. In comparison, the deeper earthquakes tend to occur on recently formed or newly formed faults related to the present regional stress of the Korean Peninsula. The shear-wave velocity tomography of the southwestern Gyeonggi Massif at 1 km corresponds well to the geology. In contrast, the shear-wave velocity variations at 5–9 km generally correspond to the locations of brittle faults. The depth of the ~3.5 km/s isovelocity line is quite irregular and is deeper in areas with active fault movement. In the Gyeongsang basin, the relationship between the shear-wave velocity and faults is somewhat unclear because the thickness of low-density sedimentary rocks is up to 8 km. Shear-wave velocity tomography at 13 km can be used to identify areas with high shear stress. These data indicate that shear-wave velocity tomography may provide different information depending on the depth and that the brittle faults in this region may not extend to a depth of 13 km.

PRIMORYE EARTHQUAKE on April 12, 2014, KP=11.9 (Far Eastern Russia)

The article presents instrumental and macroseismic data of the earthquake that occurred on April 12, 2014 in the Primorye Region of the Far Eastern Federal District of the Russian Federation. Primorye refers to areas with a weak of shallow seismic activity. This relatively small magnitude M=4.5 earthquake is a rare occurrence in this region. It caused a significant macroseismic effect over an unexpectedly large area. The highest seismic intensity as large as 5 degrees was observed in the settlements nearest to the epicenter – Mezhgorye, Krylovka and Maryanovka. 36 minutes after the main event, an aftershock was recorded with an epicenter 6.5 km southeast of the main shock location, felt by the inhabitants of the settlement of Krylovka. According to the data obtained, the focal mechanism of the earthquake might be treated as the strikeslip fault type with the nodal planes of the sublatitudinal and sublongitudinal extension. In view of tectonics, the earthquake and its aftershock epicenters might be related to a nameless NW striking fault located near Mezhgorye Settlement and linking the Krylovsky and Chernorechensky faults.

FAULT SYSTEMS IN THE UPPER CRUST OF THE FENNOSCANDIAN SHIELD, THE EAST EUROPEAN PLATFORM

Directions of 683 faults located in the southeastern part of the Fennoscandian (Baltic) shield were statistically analyzed, and three orthogonal associations of fault systems were identified in the study area. According to the dynamic analysis of the fault systems and their associations, the main NW-striking faults belong to the fault network originating mainly from the early Paleoproterozoic. These faults functioned in the Paleoproterozoic during four main deformation stages: D1 – sinistral shear transtension and asymmetric rift genesis (2.1–1.9 Ga); D2 – sinistral shear transpression under oblique accretion and convergence (1.9 Ga); D3 – sinistral shear transpression under oblique collision (1.89–1.80 Ga); D4 – dextral strike-slip displacements at the background of complex escape tectonics of the late collision stage (1.80–1.78 Ga). The regional stress field changed as follows: D1 – northeast- or east-trending extension; D2 – northeast compression; D3 – sub-latitudinal compression; D4 – sub-meridian compression. Changes in dynamic loading conditions led to multiple kinematic inversions of the fault networks. Widespread transtension and transpression settings in the southeastern parts of the Baltic Shield give evidence of asymmetric rifting, oblique accretion and collision in the Paleoproterozoic, which must be taken in to account in geodynamic reconstructions.