Detachment Zone Research Articles

Using Resistivity Structure to Study the Seismogenic Mechanism of the 2021 Luxian Ms6.0 Earthquakes

Over the past few years, there has been a noticeable change in the occurrence of seismic disasters in Sichuan, China. The focus has shifted from Western Sichuan to the previously more stable Southeastern Sichuan. The recent Ms6.0 earthquake in Luxian, Southeastern Sichuan, on 16 September 2021, has once again captured the interest of scholars, who are closely examining the seismogenic environment and potential seismic hazards in the region. We conducted a magnetotelluric (MT) array survey in the Luxian earthquake area to explore the deep seismogenic environment of the 2021 Luxian Ms6.0 earthquake zone and understand the potential effects of industrial extraction on seismic activities. Here are the insights we obtained: Underneath the anticline in the Luxian Ms6.0 earthquake area, there is a structure that mainly exhibits high resistance. On the other hand, beneath the syncline, a structure with medium to low resistance is observed. The epicenter of the mainshock was identified near the intersection of high- and low-resistance media within the Fuji syncline area. Smaller aftershocks that followed the mainshock were mainly concentrated in the low-resistance layers at depths of 3–5 km in the Fuji syncline area. MT survey results have confirmed the existence of a detachment zone in the shallow crust near the epicenter of the Luxian Ms6.0 earthquake. It is believed that this detachment layer played a significant role in the seismogenic process of the Luxian Ms6.0 earthquake. During different stress conditions, this layer became active and caused the compression and faulting of a hidden fault below, resulting in the Luxian Ms6.0 earthquake. After the main earthquake, a series of smaller aftershocks with varying focal mechanisms occurred as the stress fields continued to release. It is important to note that the Luxian Ms6.0 earthquake highlights the ongoing high stress levels in the southern region of the Sichuan Basin. This emphasizes the need for continued monitoring and consideration of potential seismic hazards in the southern Sichuan area.

The collapse of the Caledonian orogen in SW Norway: Insights from quartz textures

Extensional collapse is a common late- to post-collisional feature of orogens. It is particularly prominent in the SW Scandinavian Caledonides, where extensional detachments formed progressively from the initial reactivation of the basal thrust zone to the formation of hinterland-dipping extensional shear zones. A mature stage, documented here, involves the development of bivergent (both hinterland- and foreland-dipping) shear zones and associated vertical basement mobilization. The main foreland-facing extensional shear zone in the study area is the Bergen Detachment – a until recently overlooked or misinterpreted structure. This detachment overprints top-to-W mylonitic fabrics related to the earlier Devonian extension stages and developed in response to the updoming of the Baltica basement west of Bergen (the Øygarden Complex) into a late core complex. Our microstructural and textural examinations suggest that for both the Hardangerfjord Shear Zone and the Bergen Detachment, strain was localized by activation of dislocation creep in quartz through the operation of multiple slip systems in the <a> direction, predominantly prism <a> and rhomb <a>. These examinations and existing radiometric age constraints suggest that the progressive shear zone development occurred over maybe as little as 5 million years, under upper to middle greenschist facies conditions. Synkinematic cooling brought both the Bergen Detachment and Hardangerfjord Shear Zone through the ductile-brittle transition zone. The main explanation for this prolonged collapse development is 1) that the early low-angle detachment became too low-angle for continued shearing, giving rise to the first hinterland-dipping set of shear zones, and 2) that the basement weakened rheologically and mobilized gravitationally with the formation of large upright folds with new detachments along their flanks (the bivergent stage), including the Bergen Detachment.

Numerical and experimental examination of performance curve instabilities of a centrifugal fan

Abstract This paper investigates the performance curve instability of centrifugal fans used in industrial applications such as sintering or pelletizing plants. The study focuses on experimental and numerical analyses of the fan’s performance curve during operation. Model measurements of an optimized fan reveal instabilities in the characteristic curve around 73 % and 42 % of the volume flow at best efficiency point (Q/Qref) resulting in a sudden drop in pressure coefficient and the onset of low-frequency noise emission. Numerical investigations using ANSYS CFX predict the flow pattern accurately. Excellent agreement is observed between measured and calculated data for flow conditions, with deviations below 1 % in pressure coefficient. Notably, the calculated instability range occurs earlier at lower flow rates, leading to a 6 % shift in the data. The flow exhibits a detachment zone on the blade suction side and a complete blockage of a single impeller passage, corresponding to the pressure drop in the performance curve. Further investigations reveal that rotor-stator interaction causes the abrupt impeller rotating stall phenomena, significantly impacting pressure generation at lower flow rates. A FFT analysis confirms the presence of rotating stall, with its dominant frequency observed and multiple harmonics. The comparison of separation frequency with existing literature supports the assumption that rotating stall is the cause of the instability.

Non-invasive follow-up for intracranial aneurysms treated with contour neurovascular system-comparison of digital subtraction angiography (DSA) to magnetic resonance imaging (MRI) and spectral computed tomography angiography (CTA) in vitro.

The contour neurovascular system (CNS) is an intrasaccular flow-disrupting device designed for the treatment of intracranial wide-necked bifurcation aneurysms. Metal artifacts limit magnetic resonance imaging (MRI) accessibility after implantation. The purpose of this in vitro study was to evaluate non-invasive imaging alternatives to digital subtraction angiography (DSA). Three aneurysms of patients originally treated with CNS were three-dimensional (3D)-printed (one at the basilar tip and two at the middle cerebral artery bifurcation). CNS devices were implanted under fluoroscopic control into the 3D models. Post-implantation two-dimensional-DSA, flat panel computed tomography angiography (CTA), MRI, and spectral CTA were performed. Time of flight angiography and T1 weighted sequences showed large susceptibility artifacts at the detachment zone of the devices. A thin-sliced T2 weighted sequence in cross-sectional orientation to the aneurysm allowed visualization of the aneurysm dome, but the aneurysm neck and parent vessel could not be assessed. Focused spectral CTA, especially a 40 keV reconstruction with a metal artifact reduction algorithm (orthopedic metal artifact reduction (OMAR)), showed only minor artifacts at the detachment zone. This approach achieved a very similar result to DSA and flat panel computed tomography, enabling the assessment of the device structure, aneurysm perfusion, and parent vessel perfusion. For non-invasive follow-up of CNS, focused 40 keV CTA with OMAR seems to be a valuable option. MRI can be valuable for larger aneurysms to assess the aneurysm dome, but was not suitable for evaluating the parent vessels and aneurysm neck after CNS implantation in this study.

Lithospheric resistivity structure of the Xuefeng Orogenic Belt and its implications for intracontinental deformation in South China

The Xuefeng Orogenic Belt (XFOB), located in the central part of the South China Block, is a typical Mesozoic intracontinental orogen in the central Jiangnan Orogenic Belt. By collecting magnetotelluric (MT) data across the XFOB, we obtained the resistivity structure of the lithosphere, which sheds light on the Mesozoic intracontinental orogenic processes in the XFOB. The resistivity structure reveals a low-resistivity body (<10 Ω∙m), beneath the XFOB, dipping southeast wards from a depth of 10 km to the bottom of the crust. This conductor is interpreted as a relic of the lower detachment zone, which coincides with low-density areas obtained from joint inversion of seismic models. It is believed to result from mineral fluids migrating along the thrust fault and squeezing sulfides into folds. Four low-resistivity bodies were identified at three extensional locations along the Jiangshan-Shaoxing Fault and at the Cili-Baojing Fault. The low-resistivity body (<10 Ω∙m) at the junction of the Shaoyang and the Hengyang Basin is located at the point where the Moho depth thins. The variation trend of the terrestrial heat flow values, with this low-resistivity body as the plate boundary, is consistent with the average variation of the terrestrial heat flow values within the block. We propose that the low-resistivity body under the Qidong-Yongzhou-Guilin fault conforms to the characteristics of the suture zone in the resistivity structure. Its existence indicates that the missing location of the Jiangshan-Shaoxing suture zone of the Yangtze and Cathaysia Block in the middle-southwest section of the South China Block is the Qidong-Yongzhou-Guilin fault. The Yangtze Block and the Hengyang Basin show high resistivity, the depth of which reaches 100 km and 40 km, respectively. Based on the resistivity model and geological data, the XFOB experienced Triassic compression, leading to basement decollement, thrusting, and nappe structures due to low-angle Paleo-Pacific Plate subduction. This compression also led to the uplift of the orogenic belt. Moreover, under the tension caused by the high-angle retreat of the Paleo-Pacific Plate, the Cretaceous extensional tectonics led to detachment along the thrust faults, forming half-graben and basin structures along the margins.

The Early Mesozoic NE–SW Extensional Model and Exhumation Processes at the Southeastern Margin of the Central Asian Orogenic Belt: Insights from the Strain and Kinematic Vorticity Analysis of the Sonid Zuoqi Ductile Detachment Zone

AbstractThe Sonid Zuoqi ductile detachment zone is located at the southeastern margin of the Central Asian orogenic belt (CAOB), striking EW and dipping to the S. The major rock type of the Sonid Zuoqi ductile detachment zone is mylonite derived from granite. The sequence of mylonite features is: (1) S and C foliations of mylonite, and (2) extensional crenulation cleavage (ecc) or C and the kinematic vorticity (Wk) value changed from 0.70 to 0.95 and from 0.37 to 0.69, respectively; the strain type of the mylonites within the Sonid Zuoqi ductile detachment zone is compressional to planar strain. The strong deformation mylonite and Halatu plutons yielded a zircon U‐Pb age of 244 Ma and a zircon (U‐Th)/He age of 214 Ma, respectively. Based on the strain and kinematic vorticity analysis, together with the zircon U‐Pb and zircon (U‐Th)/He ages and the regional tectonic background, the study area experienced three stage evolution: tangential simple‐shear (244 Ma), simple‐shear‐dominated general shear represented by upper crustal extension (224 Ma) and pure–shear–dominated general shear represented by the Halatu pluton doming (214 Ma), which constrained the early Mesozoic NE–SW crustal extension at the southeastern margin of the CAOB. This NE–SW extension probably originated from the post‐orogenic extensional collapse of the CAOB, subsequent exhumation being controlled by the far afield effects of the closure of the Mongol–Okhotsk belt.

Mountain permafrost landslides: Experimental study investigating molard formation processes

Landslides triggered by degrading mountain permafrost pose an increasing risk to life and infrastructure in mountain regions due to current climate change trends. Because discontinous mountain permafrost is not directly detectable by remote sensing, permafrost molards offer a way of indirectly detecting its location and state. Permafrost molards are cones of loose debris that are associated with landslide deposits. They originate from ice-cemented blocks of sediment that are transported downslope with the landslide. These ice-cemented blocks are fragmented parts of the permafrost initially located in the landslide source area. Over time, these blocks degrade into conical mounds with diameters typically ranging from decimeters up to 40 m. They indicate the presence of an area of degrading permafrost at the level of the detachment zone. The physical processes that lead to the formation of molards from the ice-cemented blocks have not yet been studied in detail. Therefore, we perform an experimental study to recreate molards in a controlled laboratory environment and investigate the key formation processes. We downscale the molards to an initial cube side length of ~30 cm, and reduced the model's complexity by using two different types of gravel with a defined granulometry and lithology. We quantified the degradation phase by using a novel photogrammetric time-lapse system to detect changes in the digital elevation model between half-hourly time-steps. For the first time, we successfully recreated morphologies resembling molards under controlled laboratory conditions on a decimeter scale. We find the main processes to produce the final molard shape are cascades of grainfall for slightly cohesive sediment, and individual grainfall for non-cohesive sediment. Our experiments reveal three possible cross-section shapes (bell-shaped, triangular, trapezoidal) that correspond to molards that can be found in the field. Along with these field observations, we suggest that it may be possible to identify the cohesion of a molard's sediment based on its morphology. Together with future field data and experiments investigating the granulometry, ice and clay contents of the initial ice-cemented block, we aim to use molards as a record to better understand the landslide dynamics and state of the mountain permafrost.

The Ottawa River Gneiss Complex revisited: definition of the metamorphic core and detachment zone of a large Grenvillian metamorphic core complex

Using new and published data, we synthesize the tectonic evolution of the Ottawa River Gneiss Complex (ORGC), the metamorphic core and detachment zone of a large mid- to late-Ottawan metamorphic core complex in the western Grenville Province. Field and petrologic data indicative of retrogression and exhumation, combined with maps and schematic crustal-scale sections, are used to document spatial and temporal relationships of multi-scale structures developed during its formation, of which the largest, termed mega-cross-folds and megaboudins, occur within and define the detachment zone. Mega-cross-folds, orogen-normal structures up to 70 km in length with coaxial constrictional fabrics in their hinge-lines, formed in a single phase of deformation during retrogression and exhumation. A cluster of asymmetric megaboudins, individually from 10–50 km long with granulite-facies cores and high-strain amphibolite-facies rims, similarly formed during syntectonic retrogression and exhumation of granulite-facies precursors. We argue the mega-cross-folds developed in a regime of regional transtension, whereas the megaboudin cluster formed by extensional inversion of an anastomosing early-Ottawan thrust system, with the strain patterns of both suggesting the detachment zone was the site of intense ductile flow between the stronger metamorphic core and cover. Comparison of these results with generic numerical models of extensional collapse of overthickened continental crust suggests the first-order tectonometamorphic features of the ORGC developed during necking of the upper crust and associated large-scale extensional flow of the mid and lower crust into the domiform necked region during collapse of the early-Ottawan thrust stack.

The Proterozoic–Cambrian salt detachment zone at Lakhar Kuh, Iran: a seismic-scale analogue for salt stringers, detachment folding and multiphase structural development

Abstract In the Middle East, significant evaporite units formed in the latest Precambrian–Cambrian, Triassic, Jurassic, Cretaceous and Cenozoic. The Precambrian–Cambrian period gave rise to the Ara Salt carbonate stringer plays, southern Oman and the giant Zagros anticline traps (Hormuz Formation). While outcrops of salt diapirs are common, basal detachment exposures are extremely rare. The Lakar Kuh area of Central Iran reveals the Precambrian–Cambrian basal salt detachment, on satellite images, in natural cross-section view through the entire Phanerozoic sedimentary section east of Ravar. This view illustrates how older diapiric structures (pillows and normal faults) were the focus of later contractional folds, particularly in the Jurassic and Cenozoic. The salt detachment zone contains many floating blocks (stringers) of clastic, carbonate and igneous rocks. Some blocks were stoped from the overlying beds, while most were layers originally interbedded with the evaporites. Block size, distribution and orientation is highly variable, and folding is infrequent. Lakar Kuh encompasses several key themes generally pertinent to structural geology and salt system research: the presence of sedimentary stringers within evaporites, multiple detachment levels within a thick (&gt;5 km) stratigraphic section, detachment folding, multiphase salt activity, reactivation of older structures by newer ones and multi-stage development of salt bodies.

Falling film flow and heat transfer on a horizontal tube under the shearing of vertical vapor

Falling film evaporators are commonly employed in a wide range of industrial applications, with a particular emphasis on the management of vapor generated during operations. It is widely recognized that vapor streams have a significant influence on falling film flow and heat transfer. In this paper, we conducted numerical investigations of this issue in the presence of vertical vapor streams and validated the present numerical results against previous experimental data. The results indicate that the length of the liquid tongues and the interaction time between two liquid films are affected by both downward and upward vapor streams. The effect of vertical vapor on the film thickness is dependent on the vapor’s orientation and position on the tube. Typically, is decreased by 25 % at z* = 0 and vg = 4.0 ms−1 and is increased by 20 % at z* = 0 and vg = 4.0 ms−1. The downward vapor enhances heat transfer on the top half of the tube’s periphery while weakening it on the bottom half, while the upward vapor contributes to heat transfer enhancement mainly in the detachment zone. Overall, the vertical vapor stream causes substantial redistributions of the heat transfer coefficient, velocity vector, and liquid film thickness in three distinct zones. Downward and upward vapor streams predominantly impact the upper and lower parts of the tube, respectively. Within the current velocity range, it is evident that vertical vapor streams consistently lead to an average enhancement in heat transfer.

Dynamic stress prediction for a Pump-Turbine in Low-Load Conditions: Experimental validation and phenomenological analysis

To meet the increasing demand for higher flexibility in hydropower, an operating range extension in an existing hydroelectric powerplant is considered. To this end, the dynamic flow and structural behavior of the low-head pump-turbine is investigated in turbine mode in partial load and deep partial load conditions, to evaluate the feasibility of this flexibilization due to the increased high cycle fatigue damage. In a first step, different numerical simulation approaches that combine fluid dynamics and structural dynamics are applied and compared to in-situ dynamic strain and pressure measurements on the impeller. As a result, the modelling requirements for a numerical workflow that leads to unmatched accuracy in predicting the broad-band dynamic stresses of deep partial load are identified. In a second step, the operating-point-dependent fatigue crack initiation spots are identified based on dynamic stresses. Herein, the emergence of an additional dominant critical spot near the blade leading edge is observed, which is unexpected for low-head pump turbines. Thanks to the detailed numerical flow and structural analyses, the root cause for the dynamic stress concentration is studied and the emergence of the new critical spot is understood. It turns out in deep partial load, that the main source of pressure oscillations on the impeller blade surfaces is the vortex-dominated flow detachment zone around the blade leading edge, which, in combination with the geometry of the investigated impeller, translates into the main stress oscillations. The results of this work were later used to extend the operating range of the hydroelectric powerplant from initially 25% to 100% power output to now 0% to 100%.

Surface-derived fluid percolation along detachment systems enhanced by syn-kinematic granites: uranium mineralization as an application

Detachment zones are privileged areas for the interaction between surface-derived fluids and rocks, potentially leading to ore deposition. However, the hydrodynamics of detachments and specifically the way by which surface-derived fluids reach crustal depths, remain enigmatic. This question is even more puzzling when the heating caused by the emplacement of a syn-kinematic granite increases the buoyancy of fluids, thus impeding their descent. Here, 2D hydrothermal numerical models are performed. The geometry comprises a detachment and secondary normal faults in the hanging wall. Sensitivity tests were carried out to assess the impact of topographic gradients, syn-tectonic magmatic activity and the depth-dependent permeability contrast between the detachment and the crust. Several flow indicators, integrated over time and combined with particle tracking, enable us to highlight the main controls of fluid circulations. Our study reveals that the infiltration of surface-derived fluids into detachment zones is enhanced by the presence of a heat source at depth, such as a syn-kinematic pluton. Secondary faults are the main percolation path for surface-derived fluids infiltrating the detachment. Plume-like thermal anomalies have been spotted between these faults. The dynamic permeabilities of magmatic intrusions, which depend on sub-solidus temperatures, spatially and temporally reproduce the conceptual model of uranium mineralization in the South Armorican Variscan Domain, which is used as an example.

Вероятность формирования оползней на норвежской континентальной окраине

Numerous landslides are located on the passive margin of Norway. According to the landslides number and the extent of their detachment zones, the margin can be divided into three segments (from south to north) — Scandinavian, Barents Sea and Svalbard. The fourth segment (Arctic) is the transition area located north of the Spitsbergen archipelago. In the Scandinavian segment, about forty large submarine landslide bodies have been identified on the continental slope and deeper. The Barents Sea segment is dominated by deep-sea fan deposits and relatively small landslides. No large landslides were found in the Svalbard segment. Analysis of published and original geological and geophysical data indicates that the formation of new landslides may occur in the Svalbard segment, as well as on the Vestnesa Ridge.

Deep structural controls on the distribution of carbonate reservoirs and overburden heterogeneity in Central Luconia province, offshore Borneo revealed by 3D anisotropic inversion of regional controlled-source electromagnetic and magnetotelluric profile data

Understanding the deep structural controls on reservoir distribution and overburden heterogeneity is important for near-field exploration and green-field development of the Miocene to Holocene carbonate buildups in Central Luconia province in offshore northwest Borneo, Malaysia. Data from 59 stations along an approximately 180 km long controlled-source electromagnetic (CSEM) and magnetotelluric (MT) survey line with three segments recorded using different CSEM transmitter-towing directions were available. We applied three-dimensional (3D) anisotropic resistivity inversion with cross-gradient constraint and verified the resulting models using resistivity logs from nearby wells and the acoustic basement interpreted from seismic data. Our anisotropic resistivity models reveal a fragmented carbonate-rich zone atop a segmented basement comprising electrically resistive horsts coinciding with the northerly Mega-platform, Central and Southern Field Highs, separated by steep conductive zones coinciding with the West, East, and Southeast Troughs previously interpreted from seismic data. The structural highs correlate with the spatial distribution of the known carbonate buildups implying a genetic link. The carbonate bodies are overlain and underlain by persistent layers (C2 and C3) of low resistivity and high anisotropy, which we interpret as indicating compressional deformation or detachment zones. Overburden layer C2 (whose base coincides with the top of a key sedimentary package, Cycle V in seismic data) is thinnest over the East Trough and is discontinuous at the eastern margin of the West Trough, which are locations where drilled wells did not find hydrocarbons implying that the seal rocks are inefficient or breached at those localities. We used these observations to refine the existing seismic-based interpretation of carbonate play-types along our transect indicating how 3D joint CSEM-MT imaging can potentially contribute to derisking or optimizing future exploration and/or development work in this province.

Calcite fabric development in calc-mylonite during progressive shallowing of a shear zone: An example from the South Tibetan Detachment system (Kali Gandaki valley, Central Himalaya)

Calcite-rich lithologies within the Annapurna Detachment zone, a strand of the South Tibetan Detachment System in central Himalaya (Kali Gandaki region, Western Nepal) have been characterized for the superimposition of microstructures to unravel deformation variations during syn-collisional exhumation. Finite-strain, grain size, twinning and crystallographic preferred orientations have been combined to define the contribution of dynamic recrystallization and twinning in calcite in the overall deformation. Dynamic recrystallization and twinning occurred respectively at temperatures of c. 400–550 °C and <250 °C. The comparison of calcite-based paleopiezometers indicates that cooling occurred along with an increase in differential stress from c. 4–19 MPa up to c. 118–154 MPa at decreasing strain rates. Flow estimates with the subsimple shear (30–50% of simple shear) regime for both fabrics support a single progressive deformation where the plastic regime progressively changed. We interpret the changes in intracrystalline deformation mechanisms in calcite and their differences in differential stress records, deformation temperature, and strain rate as linked to the exhumation of the rocks induced by the detachment, from deeper to upper crustal levels. Results for the Annapurna Detachment zone have been compared with the literature database for the regional continuation along the Himalaya, the South Tibetan Detachment System, which is defined by a lower main shear zone and an upper brittle fault. Data comparison led us to link our early stage of ductile shearing to that elsewhere recorded in the main lower branch of the system. Conversely, data for the late shearing are not found for the main branch when affecting quartz-bearing rocks, as strain hardening resulted in an upward partitioning of the deformation and in the localization of a brittle fault. Therefore, this study highlights how variations in lithology in regional-scale shear zones influences the exhumation path and the overall architecture of shear zones.

Petrogenesis of oxide-apatite mineralization associated with proterozoic anorthosite massifs at Lac Mirepoix, Quebec, Canada: A multi-injection model for Fe-Ti-P mineralization in the Central Grenville Province

The Lac Mirepoix Fe-Ti-P mineralization presents several mineralized lenses that outcrop near the border of the 1080 (±2) Ma Vanel and the 1016 (±2) Ma Mattawa Anorthosite massifs, in the Central Grenville Province, Quebec, Canada. The mineralization is (hemo)-ilmenite-dominated, accompanied by magnetite and apatite. It is subdivided in three different zones due to the appearance of different cumulate phases: zone I comprises mainly massive oxide (>70 % hemo-ilmenite ± magnetite) layers hosted in anorthosite. Towards the center of the mineralization (zone II), massive oxide layers are less common whereas apatite-bearing cumulates appear, forming massive nelsonite (50–70 % magnetite ± ilmenite and 25–30 % apatite) and oxide apatite norite (OAN, 15–25 % hemo-ilmenite ± magnetite and 8–20 % apatite). Finally, zone III is marked by the alternance of OAN layers (10–25 m), richer in magnetite, in addition to (hemo)-ilmenite and apatite, and an absence of massive oxide and nelsonite. In-situ trace element analysis of plagioclase, apatite and oxides by laser ablation ICP-MS, reveals cryptic variations related to magma differentiation and multiple injections of ferrodiorite parental magmas of similar composition. In-situ U-Pb dating of zircon from the OAN mineralization itself indicates two different crystallization ages between zone III (1048 ± 8 Ma) and zone I (964 ± 9 Ma), favouring a model of multiple injections of similar Fe-Ti-P-rich melts of similar composition over a period of 80 Mys rather than in-situ crystallization of a single intrusion, which is supported by trace-element geochemistry. The Lac Mirepoix mineralization records the following fractional crystallization sequence of a high-Ti-P magma, residual after the anorthosite formation: first, massive oxides of hemo-ilmenite crystallized (high Ti/Fe) by oxide settling, with primitive compositions, similar to the world-class Lac Tio deposit. The oxide-apatite mineralization (nelsonite and OAN) crystallized from the residual liquid (lower Ti/Fe, evolved compositions) in which magnetite and apatite were more abundant, similar to nearby mineralization in the area (e.g. Lac à l’Orignal Fe-Ti-P deposit). The observed decrease in Ti content of the evolving melt is supported by the liquid line of descent of several ferrodiorite dykes within the host-rocks and mineralization. Finally, we propose a model of multiple injections of residual Fe-Ti-P-rich liquids drained from or filter-pressed within diapirs of plagioclase-rich mushes that were emplaced over 80 Mys apart along the same crustal detachment zone.

Analogue modelling of the Ainsa oblique zone in the Southern Central Pyrenees, Spain

The Pyrenees are a collisional orogen striking WNW-ESE. However, in the Ainsa oblique zone in the south-central part of the Pyrenees, faults and folds rotate to ∼N–S over a westward thinning evaporitic layer. The influence of lateral change in basal friction due to declining evaporitic layer thickness in a fold-and-thrust belt is studied by analogue modelling simulating the Ainsa oblique zone. In the experiments the transition zone is simulated by a viscous silicone mixture sheet that laterally changes in thickness adjacent to frictional-plastic quartz-sand. The models confirm that a transition between weak and strong basal friction causes oblique structures to develop. Weak detachment zones are prone to fault propagation whereas strong detachment zones are prone to imbrication. At the transition zone those structural styles are combined, resulting in a complex domain. The differences in deformation style cause dextral shear and clockwise rotation in the transition zone and larger horizontal deformation propagation in the weak basal friction zone. Structures produced in the analogue models are compared to structures in the Ainsa oblique zone and found to be representative. Therefore, this study shows that transition between weak and strong detachment layers can explain the regional structural deformation style in the Ainsa oblique zone.

Ascertaining Potential Causes of Hillslope Failure Associated to Human Settlement: A Case Study from Alaknanda Valley, Uttarakhand, NW Himalaya, India

Abstract Growing human population along the river valleys in hilly terrain particularly on the fluvial sediments poses increasing risk of terrace instability and subsequent failure. Such instability and failure result in frequent loss of settlement, agricultural lands, and often lives. Alaknanda River valley in Uttarakhand comprises many fluvial terrace slopes that accommodate human settlement and hence one such fluvial terrace slope was taken as a case study area. On Feb. 28, 2022, Saari (or Sari) village, situated on the fluvial sequence, witnessed a slope failure collapsing 3 houses but no casualties. The hillslope with a total disturbed area of ~3889±5.0 m2 and failed material volume of ~16858±4.3 m3 partially dammed a tributary of Alaknanda River, which passes through the toe of the failed slope. The present study is an attempt to understand the instability that led to such failure because there was no extreme rainfall or earthquake prior to this failure. Preand post-failure topography of slope was used to demarcate the detachment and deposition zones. Pre-failure topography was used to perform the Finite Element Method (FEM) based slope stability simulation. In order to evaluate the possible contribution of exposed rockmass in failure, kinematic analysis was also performed. Results revealed the development of displacement pattern, particularly due to anthropogenic loads that must have initiated this failure. Such studies are primary requisites for an effective disaster mitigation in the NW Himalaya where growing human population on fragile hillslopes are at risk.

Crustal structure beneath the Jiamusi Block and the Nadanhada Terrane from seismic and magnetotelluric studies: Implications for the subduction of the Pacific Plate

The Nadanhada terrane is considered to accrete to the eastern end of the Jiamusi Block during the Pacific Plate subduction. Constraints on the deep structure of the Nadanhada terrane and Jiamusi Block are essential to study the evolution, transformation and current activity of the eastern Asian continental margin. To reconstruct the accretion processes, two magnetotelluric sounding sections and a seismic reflection section were conducted in the eastern part of Jiamusi Block, the middle, and the southern end of the Nadanhada terrane respectively. Geophysical data revealed resistant stable structures and west-dipping discontinuous reflections with a thickness of around 15 km in the upper crust of Jiamusi Block. In contrast, the lower crust of the Jiamusi block exhibited high conductivity and short sub-horizontal reflections that suggest upwelling of mantle material under Pacific Plate subduction. Moreover, the low resistivity zone observed connecting the shallow crust with the upper mantle in the Tongjiang-Yuejinshan fault was interpreted as the boundary line between the Jiamusi block and Nadanhada terrane. The Nadanhada terrane can be subdivided into the Yuejinshan and Raohe complexes based on their distinct geophysical features. The Yuejinshan Complex is characterized by irregular bodies with high resistivity and short arc reflections with a thickness of about 10 km. In contrast, the Raohe Complex displays high-resistivity blocks and west-dipping thrust nappe strong reflections with a thickness of about 5 km. Additionally, the high-conductivity layer located between the Raohe Complex and the underlying basement is believed to be the detachment zone. Since the Cretaceous, the continuous subduction of the Pacific Plate has caused magmatism that has led to the high conductivity crust. The observation of low-resistivity channels and arcuate reflections within the crust supports the notion of upwelling of mantle-derived materials brought about by the subduction of the Pacific Plate in the Cenozoic era.

An oceanic core complex preserved in the Squanga Lake ophiolite, northern Atlin terrane, Yukon

The Squanga Lake ophiolite of southern Yukon exhibits a non-Penrose pseudostratigraphy with mantle tectonites separated from upper crustal rocks by an extensional detachment zone (Squanga Lake detachment zone, SLDZ), compatible with formation in an oceanic core complex. Spinel, orthopyroxene and clinopyroxene mineral compositions indicate dynamic re-crystallisation during exhumation of ophiolitic mantle rocks. The rocks and structures of the SLDZ also preserve evidence of exhumation: 1) clasts of primitive olivine-cumulates in the SLDZ mélange represent excised lower oceanic crust; 2) structures in the SLDZ indicate ductile to brittle conditions; 3) temperatures along the SLDZ span from upper- to lower-amphibolite facies conditions. We obtained 248.77 ± 0.22, 248.53 ± 0.41, and 248.84 ± 2.09 Ma U-Pb zircon ages from upper crustal gabbro, a gabbro fragment within the SLDZ mélange and a dyke within the SLDZ implying syn-kinematic magmatism. Calc-silicate alteration associated with cross-cutting diabase dykes yielded a UPb titanite age of 247 ± 10.5 Ma. All igneous rocks show similar geochemical signatures with ubiquitous subduction input. Our data indicate that Squanga Lake ophiolite preserves an oceanic core complex, formed in a back-arc basin environment.