Fault Core Research Articles

Study on Water Inrush Characteristics of Hard Rock Tunnel Crossing Heterogeneous Faults

Fault water inflow is one of the most severe disasters that can occur during the construction of hard and brittle rock tunnels. These tunnels traverse brittle fault breccia zones comprising two key components: a damage zone dominated by low-strain fractures and an internally nested high-strain zone known as the fault core. Structural heterogeneity influences the mechanical and hydraulic properties within fault breccia zones, thereby affecting the evolving characteristics of water inflow in hard rock faulting. Based on the hydraulic characteristics within hard rock fault zones, this paper presents a generalized dual-porosity fluid-solid coupling water inflow model. The model is utilized to investigate the spatiotemporal evolution patterns of water pressure, inflow velocity, and water volume during tunneling through heterogeneous fault zones in hard rock. Research findings indicate that when tunnels pass through the damage zones, water inrush velocity is high, yet the water volume is low, and both decrease rapidly over time. Conversely, within the core regions of faults, water inflow velocity is low, yet the water volume is high, and both remain relatively stable over time. Simulation results closely align with the water inflow data from China’s largest cross-section tunnel, the Tiantai Mountain Tunnel, thus validating the accuracy of the evolutionary model proposed in this paper. These findings offer a new perspective for devising effective prevention strategies for water inflow from heterogeneous faults.

Diagenesis and lateral sealing types of fault zones in the Lishu Depression, Songliao Basin, NE China

The Lishu Depression in the Songliao Basin is characterized by well-developed faults, and the accumulation of oil and gas is closely related to fault sealing. However, there remains a dearth of comprehensive investigation into the diagenesis of fault zones and their influence on lateral sealing. This study employed electron probe analysis, cathodoluminescence, and carbon and oxygen isotope techniques to investigate the diagenetic processes in fault zones. This research also reconstructed the evolution of porosity and displacement pressure of fault rocks, and identified the lateral sealing characteristics of fault zones and their impact on oil and gas accumulation. The majority of faults in the study region formed earlier, and the fault zones experienced intricate diagenesis, characterized by compaction, cementation, replacement, and dissolution. The displacement pressure of fault rocks is higher during fault intermittent and static periods compared to active periods, and it is influenced by diagenesis. Late-stage calcite and pore-filling chlorite were deposited subsequent to the period of fault activity, filling dissolution pores. This led to relatively higher displacement pressure and improved lateral sealing of fault zone. The kaolinite cement exhibits a relatively elevated potassium concentration, due to its connection with the dissolution of feldspar, resulting in lower displacement pressure and poor sealing of fault zone. Based on the evolutionary characteristics of displacement pressure in fault rock and reservoir during various transportation stages, this study identified six types of lateral sealing of fault zones. Notably, the sealing types characterized by "fault core and damage zone sealing during static periods" and "fault core and damage zone sealing during intermittent and static periods" exhibit superior lateral sealing properties, effectively controlling the accumulation of oil and gas in the sand bodies on either side of the fault zone.

Physical properties variations in a shaly formation across a fault core

Summary Faults in general, and in clay materials in particular, have complex structures that can be linked to both a polyphased tectonic history and to the anisotropic nature of the intact rock. Drilling through faults in shaly materials allows measuring properties such as the structure, mineralogical composition, stress orientation and physical properties. We combine different petrophysical measurements on core samples retrieved from a borehole drilled perpendicularly to a fault zone affecting Toarcian shales from the Tournemire underground research laboratory (France). The borehole is cross-cutting the entire fault thickness which is of the order of ten meters. We perform several types of measurements: density, porosity, saturation directly in the field, and P-wave velocities together with P-waves anisotropy on core samples taken at regular intervals. Special protocols were developed to preserve as much as possible the saturation state of the samples. From our measurements, we were able to track the increase of damage, characterized by a smooth decrease in elastic moduli from the intact zone to the fault core. We then calculated Thomsen's parameters to quantify the elastic anisotropy evolution across the fault. Our results show strong variations of the elastic anisotropy with the distance to the fault core as well as the occurrence of anisotropy reversal.

Decomposing significant factors of Coulomb stress and its components in injection-induced seismicity

Injection-induced seismicity has been a focus of industry for decades as it poses great challenges to the associated risk mitigation and hazard assessment. The response surface methodology is integrated into the geo-mechanical model to analyze the effects of multiple factors on induced seismicity during the post shut-in period. We investigate the roles of poroelastic stress and pore pressure diffusion and examine the differences in the controlling mechanism between fault damage zones and the fault core. A sensitivity analysis is conducted to rank the selected factors, followed by a Box‒Behnken design to form response surfaces and formulate prediction models for the Coulomb stress and its components. Reservoir properties significantly affect the potentials of induced seismicity in the fault by changing pore pressure diffusion, which can be influenced by other factors to varying degrees. Coulomb stress is greater in pressurized damage zones than in fault cores, and the seismicity rate exhibits a consistent variation. Poroelastic stress plays a similar role to pore pressure diffusion in the stability of the fault within the pressurized damage zones. However, pore pressure diffusion dominates in the fault core due to the low rigidity, which limits the accumulation of elastic energy caused by poroelastic coupling. The slip along the fault core is a critical issue to consider. The potential for induced seismicity is reduced in the right damage zones as the pore pressure diffusion is blocked by the low-permeability fault core. However, poroelastic stressing still occurs, and in deep basements, the poroelastic effect is dominant even without a direct increase in pore pressure. The findings in this study reveal the fundamental mechanisms behind injection-induced seismicity and provide guidance for optimizing injection schemes in specific situations.

Multi-stage fault formation and REE distribution in the surrounding Devonian dolomites in the south-eastern part of the Holy Cross Mountains (Poland)

Metallogenic studies carried out in the Holy Cross Mountains indicate a relationship between mineralization and fault tectonics in Devonian formations. The impact of fault formation on the geochemistry of host rocks has not yet been studied. Mineralogical and geochemical studies of fault core gouges and damage zones in the fault walls of Devonian dolomites in the Budy and Jurkowice quarries were carried out. In the clay-carbonate filling of the fault fissure, the presence of two generations of Fe sulfides, the increased content of Zr, Nb, U and Th in relation to the surrounding rocks was noted. In the fault walls of the dolomites, iron sulfide and hematite mineralization were found. Research on the REE content indicates that it is lower in the fault walls than in those located far from it, while it is clearly higher in the fault gouge, especially in terms of the content of “heavy” elements (HREE). This indicates both the supply of some components to the fault zone from external sources and their displacement from the surrounding rocks. It was also found that the fault was renewed before and after the Neogene at least twice (Badenian)

Reconstruction of the deformation history of an active fault: implications from the Atera Fault, Central Japan

Atera Fault clay gouges were collected for age dating near Kawaue, Nakatsugawa City, Central Japan, and the results integrated within its complex geological history. The results form an internally consistent data set constrained by extensive geochronological data (AFTA, ZFTA, CHIME) and support the application of gouge dating in constraining timing of brittle deformation in Central Japan. The Atera illite age data complete recently obtained limited illite fault gouge age data from underground exposure in the Toki Granite; the new illite age data are identical within error. The age of the heterogenous welded tuff breccia zone (Atera 1) ranges from 40.6 ± 1.0 Ma to 60.0 ± 1.4 Ma, whereas ages of the fault core gouge sample (Atera 2) range from 41.8 ± 1.0 Ma to 52.7 ± 1.2 Ma. The finest < 0.1 µm fraction of the fault breccia and fault core gouge yield ages around 41 Ma, identical within error. The new illite age data indicate brittle faulting and a following geothermal event occurring in the Paleogene–Eocene, similar to the nearby Toki Granite area and confirm they were both synchronous with a post-intrusive pluton exhumation. The Atera Fault illite age data provide additional insights into an integrated, regional-scale record of the tectonic displacement of Central Japan and might be influenced by large-scale tectonic processes such as the Emperor sea mount kink around 55 to 46 Ma with fault initiation around 50 Ma and brittle fault cessation or reactivation around 40 Ma in the Eocene.Graphical

Seismic faulting and CO2-rich fluid interactions: Evidence from carbonate spherulitic grains in ultramafic fault damage zones

Faults and shear zones in the metamorphic ultramafic rocks of the Voltri Massif (Ligurian Alps, NW Italy) are often accompanied by intense or complete carbonation of the host rocks. One of these fault zones, hosted in peridotite and serpentinite, is characterized by distinctive structural features that can be related to a fluid-assisted multistage brittle deformation, indicative of paleoseismic activity. The fault rocks contain cataclasite and breccias with layers of coseismic spherulitic grains of dolomitic composition, silica/calchedony veins and cement, and saponite bearing gouge. The fault is part of a system of regional structures associated with gold mineralization in quartz veins and various stages of hydrothermal alterations.Here we present a multiscale study of this fault zone combining field observations, microstructural analysis, SEM-EDS, and electron backscattered diffraction (EBSD). Focus is on the microstructures of the fault core and the significance of the characteristic carbonate spherulite layers in association with silica/chalcedony cement and veins. Our results show that these structures are indicative of the interaction between CO2-rich fluids released during coseismic and interseismic phases of faulting, i.e., during cycles of fluid pressure build-ups, faulting, fluid flushing, and mineral precipitation and sealing during seismic failure of the fault.

Quantitative damage evaluation of tunnel subjected to a subsequent strong seismic after a quasi-static reverse faulting

An individual fault dislocation or seismic effect is often considered to investigate the deformation and failure of a fault-crossing tunnel, while emerged in high-intensity seismic zones, it will be likely to subject to a superimposed effect from fault dislocation and strong seismic excitation. In this study, a three-dimension numerical model to simulate fault dislocation and subsequently dynamic response characteristics of fault-crossing tunnel was established, in which fault dislocation and earthquake levels at the site of interest. The nonlinear damage feature of tunnel was considered to furtherly reveal damage states of tunnel under the superimposed effect. And then, a quasi-static large-scale model test was performed to verify the correctness of pre-order fault dislocation calculation results. The results showed that only when the tunnel was subjected to fault dislocation, a prominent relative deformation of tunnel exists in the fault fracture zone and tension and compression zone were noticeably identified at the convex and concave zones respectively. After superimposing the seismic loading, acceleration and equivalence plastic strain amplification of tunnel near the fault fracture zone is great of significance and the damage of the tunnel under the fault dislocation was concentrated at the fault core. More importantly, the fault-crossing tunnel is more of sensitivity to the initial fault dislocation from the obvious rising on the tension damage index, and when PGA is greater than 0.2 g, the tunnel emerged in the host rock mass should also be paid more attentions, not only the tunnel in the potential fault-affected zone.

Petrological Evolution and Mass Redistribution in Basaltic Fault Zones: An Example From the Faroe Islands, North Atlantic Igneous Province

AbstractFault rock petrology exerts an important influence on the permeability structure and mechanical properties of fault zones. Slip‐related deformation on upper‐crustal faults in basaltic rocks is closely associated with fluid‐rock interaction, altering the distribution of physical properties within the fault. Here, we present quantitative descriptions of the geochemical and petrological evolution of basalt‐derived fault rocks from three passively exhumed fault zones in the Faroe Islands. Fault‐rock petrology is determined by optical petrography and automated phase identification based on micrometer‐scale chemical maps from scanning electron microscope X‐ray spectroscopy. Geochemical evolution is assessed from major and trace element composition measured by X‐ray fluorescence. The fault rocks show intense fluid‐mediated alteration from a tholeiitic basalt protolith in the damage zones, and mechanical mixing in the fault cores. Pervasive alteration occurs early during fault zone evolution, with incipient fault damage increasing permeability and allowing along‐fault percolation of carbonated meteoric water, increasing fluid‐rock ratios. Our results suggest that the only mobile species within the fault zones are Ca, Si, and Al, which are leached during the hydrolysis of volcanic glass and plagioclase, and CO2, which is added by percolating waters. These species are transported from the damage zones into the fault cores, where they precipitate as zeolite and calcite cement in veins and hydrothermal breccias. We propose that solutes are replenished by cement dissolution through pressure‐solution during cataclastic creep, during repeated cycles of hydrofracture and cementation. The fault zones are natural reactors for fluid‐mediated alteration by CO2 and water, while other species are redistributed within the fault zones.

Mineralogical and frictional properties of fault rocks from the Laohushan creep segment of the Haiyuan fault, Northeastern Tibet

Shallow creep has been observed in the Laohushan segment which lies within a seismic gap between the co-seismic ruptures of the 1920 Haiyuan M8.5 earthquake and the 1927 Gulang M8.0 earthquake. However, there has been a lack of geological and mechanical evidence to explain the mechanism behind this shallow creep. In this study, we carried out detailed fieldwork along the Laohushan fault, analyzed the mineral compositions and microstructures of the fault rocks, and performed frictional experiments on fault gouges under various temperature and pressure conditions in the laboratory. Our findings revealed that clay minerals are prevalent in the fault rocks along the Laohushan creep segment, especially in the fault gouge at the fault core. Microstructural observations showed that extensive development of clay foliation in the fault gouges, along with carbonate veins healing cracks and minerals dissolution in the fault rocks. The frictional experiments indicated that the clay-rich gouges showed low frictional coefficients of 0.46–0.58, with positive a-b values of 0.0035–0.0052, reducing to 0–0.0015 at 200 °C. The experimental samples exhibited microstructural evolution of pervasive well-developed clay foliation and fewer clast fractures, which controls the velocity-strengthening sliding behavior of fault gouges. Our results suggest that the enrichment of weak clay minerals in the fault zone generates a well-foliated microfabric, which, as demonstrated by frictional experiments, controls the Laohushan shallow creep along the Haiyuan fault at depths of less than 8 km. This finding is consistent with the shallow creep inferred from geodetic data and repeating earthquakes.

Anatomy of a km-scale fault zone controlling the Oligo-Miocene bending of the Ligurian Alps (NW Italy): integration of field and 3D high-resolution digital outcrop model data

We report the first description of a ~ 15 km long NE-SW-striking transtensive fault network crosscutting the metamorphic units of the Ligurian Alps. The main fault zone, hereby named Horse Head Fault Zone, is up to 250 m thick, involves quarzite, metarhyolite, marble and alternation of dolostone and limestone and minor pelite. Relatively narrow (~ 1–3 m-thick) fault cores are characterized by gouge and cataclasites, surrounded by brecciated damage zones as thick as tens to hundreds of meters. Damage zones show widespread evidence for dilation in the form of dilation breccia, large calcite crystals and aggregates, and centimeter- to meter-thick veins. Moreover, the fault zone contains a multitude of polished slip surfaces with multiple sets of slickensides and slickenfibers. Oblique to strike-slip kinematics dominates over the large part of the fault mirrors and both overprint and are overprinted by down-dip slip surfaces. The fault network includes dominant NE-SW right-lateral faults with a minor normal component and NW–SE left-lateral steep faults with a negligible reverse component, consistent with a km-size dextral NE-SW-striking Riedel shear zone, in turn representing an antithetic R’ of the regional sinistral shear zone constituted by the Ligurian Alps after the nappe stacking. The Horse Head Fault Zone accommodated km-scale displacement before the Early Miocene, as it is sealed by the sedimentary deposits of the Finale Ligure Basin, thus predating the Corsica-Sardinia drifting. Results of this work constraint the bending of the Ligurian Alps as part of the Western Alpine arc as accomplished through two consecutive, late Oligocene and Early Miocene, stages driven by the combination of Adria rotation and the rollback of the Apennine subduction.

3D topological analysis in deformation bands: Insights for structural characterization and impact on permeability

This study uses a 3D exposure of a fault zone in the Rio do Peixe Basin (NE Brazil) to investigate the impact of deformation band connection on permeability in porous sandstones. We analyze the topological deformation band distribution pattern and in situ permeability measurements. The results show that more Y-nodes dominate the fault core, while the damage zone registers more Y- and X-nodes. The I-nodes are associated with the highest permeability values, while X-nodes have lower values close to the fault core. In the damage zone, permeability acquisition on segment intersections (nodes) varies according to the type of node and directions (horizontal and vertical) of measurements. The vertical section (horizontal permeability) shows maximum values for I-nodes and minimum for X-nodes, whereas the horizontal section (vertical permeability) exhibits higher permeabilities for I-nodes and the lowest for Y-nodes. Therefore, this study contributes with new perspectives on the influence of deformation band connections on the permeability of fault zones and has implications for the modeling and flow simulation of siliciclastic reservoirs with deformation bands in the damage zones.

A simplified analytical method for the tunnels’ mechanical behaviour under the action of active fault zones

A fault zone consists of a fault core and a fault damage zone, and its width varies from tens to hundreds of metres. The permanent displacement of a stratum during rupture of a fault is obviously nonlinear, which would result in significant damage to the tunnels crossing active fault zones. At present, a series of mechanical models have been proposed for the preliminary design of tunnels under the action of active fault zones. However, in the current analytical models, the fault zone is simplified to a thin line, the nonlinear permanent stratum displacement due to rupture of a fault is simplified as a uniform displacement, and the mechanical characteristics of different fault types are not fully considered. As a result, the application of these analytical models is limited. In this study, a simplified procedure for analysing the mechanical behaviour of tunnels under the action of active fault zones is established. Specifically, the effect of the fault zone width on the mechanical behaviour of a tunnel is considered by reducing the stratum stiffness. In addition, the nonlinear displacement of a stratum under the action of five different fault types is considered by using Okada’s model (Okada, 1992), and the differential equations for tunnel displacement are solved by employing the finite difference method. Then, the proposed model is verified by comparisons with both 3D solid and beam-spring numerical models, and the results are in excellent quantitative agreement. Finally, the parameter analysis shows that the mechanical response of the tunnel is greatly affected by the width and mechanical properties of the fault zone, and ignoring the effect of the fault zone (as assumed in the current analytical models) would cause an overestimation of the internal forces on the tunnel. The proposed model can be used as an efficient tool for the design of tunnels that cross active fault zones.

Geophysical characteristics of a fault system in the northern Central Range of Taiwan and its applications for geothermal energy exploration

The northern Central Range of Taiwan is a high-potential geothermal region. Since the formations are mainly tight metasandstone and slate, permeable structures associated with faults are commonly considered as conduits of geothermal fluids. This study determines the characteristics and orientations of the permeable fault zones by analyzing the geophysical logs and microresistivity formation image log (FMI) of the JT-4 well in Jentse, an important geothermal area in the northern Central Range. Between 720 and 1480 m measured depth (MD), the effective porosity of the intact host rock is mostly below 3% calculated by the geophysical log. Zones with porosity greater than 5% are only clustered within a few thin intervals. The FMI interpretations show these porous zones are in the interior of the fractured and faulted intervals. These porous fault zones comprise fault damage zones with a high density of open fracture planes and fault cores with porous fault breccias. There is a highly brecciated fault core in 1334–1339 m MD, which would be the most permeable interval of the well. Additionally, some healed fault zones with sealed fractures are observed. The picked drilling-induced tensile fractures signify that the direction of the present-day maximum horizontal principal stress is N40–50°E, and most of the open fractures also strike parallel to the NE–SW direction. The study results show that the open fractures are concentrated in the four fault zones belonging to one major normal fault system. After integrating the orientations and locations of the fault zones, we propose that the permeable normal fault system is about 200 m wide, trends N50–70°E, and dips 70–80° to the NW. The development of the open fractures and the permeable fault system in the northern Central Range may be controlled by the current rifting of the Okinawa Trough offshore northeastern Taiwan. The study exhibits the characteristics of fractured fluid conduits of the regional geothermal system, which will benefit future geothermal exploration in northeastern Taiwan.

A numerical model for gas CO 2 migration in a fault zone

Understanding whether fractures and faults impact the CO 2 migration through the overburden is critical in the evaluation and monitoring of CO 2 geological storage sites. We present a numerical model and workflow to describe the hydraulic behaviour of a fault located in the shallow part of the overburden. This helps to evaluate the sealing potential of the system in case of unwanted CO 2 migration toward the surface and to design an efficient monitoring plan. The model configuration is representative of several experiments performed at real sites under quite shallow conditions (50–500 m). The model results, applied to a selected fault outcropping in the Apennines (Italy), show that most of the gas migrates through the high permeable footwall damage zone. A significant amount of gas then dissolves into the water, emphasizing the importance of accurate modelling to assess the hazard of CO 2 leakage into near-surface aquifers or to the surface. The occurrence of pressure buildup close to the fault core points out that detailed modelling of the migration conditions is required to predict gas path through a fault zone. Thematic collection: This article is part of the Fault and top seals 2022 collection available at: https://www.lyellcollection.org/topic/collections/fault-and-top-seals-2022

Fluid Influx Promotes Local Strengthening of the Creeping Xianshuihe Fault, Eastern Tibet

AbstractWhile the Xianshuihe fault displays continuous creeping behavior, it is also the most seismically active fault in the eastern Tibetan Plateau, and its earthquake mechanisms remain unclear. Here, we aim at the stick‐slip portion of the creeping Qianning segment of the Xianshuihe fault to determine the characteristics of fault rocks and how fluids at depth influence fault behavior. Field survey, optical and scanning electron microscope observations, X‐ray diffraction and fluorescence analyses, as well as carbon and oxygen isotope analyses were performed on the collected samples. The fault core consists of 3–5 cm‐thick black fault gouge and ∼2.5 m‐thick breccia, surrounded by ∼12 m damage zone. In contrast to fault breccia (1–8 cm in diameter), the black fault gouge, which represents the principal slip zone of repeated seismic events, contains angular quartz particles (∼10 μm on average) and clays dominated by illite. The fluid‐rock interactions altering silica minerals into illite, and the thermal decomposition of carbonate minerals, passively increase the relative content of quartz and feldspar (total 63%–73%) in the fault gouge. The deeply sourced CO2 (from mantle and metamorphic degassing) within the hydrothermal fluids causes carbonate precipitation in breccias (21%–53%), composed of calcite, dolomite, and aragonite. These fluid‐assisted reactions lead to more abundant strong mineral phases (quartz, feldspar, and carbonates, 64%–87%) than weak clays (12%–36%) within the fault core, and locally strengthen the fault, which inhibits slow release of stress at shallow depth and promotes seismic rupture of the fault.

The modeling of fault activation, slip, and induced seismicity for geological [formula omitted] storage at a pilot-scale site in the Janggi Basin, South Korea

We investigate the effects of possible fault activation and slip during geological CO2 storage operations numerically. To this end we develop a numerical simulator by coupling a multiphase flow simulator with an existing geomechanics simulator in order to model fault slip and seismic events induced by reservoir pressurization. After validation and verification of the developed coupled simulator, we apply it to a pilot-scale site of geological CO2 storage located in the Janggi Basin, South Korea. We design a fault structure with a fault core and surrounded damage zones, assuming the fault to be activated along the fault core only. Zero-thickness interface elements are employed in the numerical model to represent the fault core. We perform numerical tests with different injection scenarios to evaluate the impact of pore pressure on fault activation and its post-failure behavior. We model occurrence and the magnitude of seismic events, identifying sudden changes of the moment magnitude at the onset of fault activation. Specifically, we take CO2 injection with 100 , 200, 400 ton/day for 20 years, from which the cumulative moment magnitudes (Mw’s) of 1.46, 2.36 and 3.11 are estimated, respectively. However, the maximum instantaneous changes of Mw occur at the onset of fault activation for all the injection cases (i.e., approximately 0.1, 0.36, 0.75, respectively). As the injection rate increases, fault activation occurs early and the magnitude of induced seismicity becomes large, but substantial reduction of the injection rate can weaken following induced seismicity. Interestingly, the location of maximum surface lift does not correspond to the injection well due to heterogeneity of geology and geomechanics. We thus expect relatively low risk of induced seismicity overall within the range of injection rates considered in this study, but monitoring surface uplift and seismic signals is still necessary to prevent any unexpected events due to uncertainty of parameters.

Near-surface termination of upward-propagating strike-slip ruptures on the Yangsan Fault, Korea

We present a new example of the termination of strike-slip paleoearthquake ruptures in near-surface regions on the Yangsan Fault, Korea, based on multi-scale structural observations. Paleoearthquake ruptures occur mostly along the boundary between the inherited fault core and damage zone (N10–20°E/> 75°SE). The ruptures propagated upward to the shallow subsurface along a < 3-cm-wide specific slip zone with dextral-slip sense, along which the deformation mechanism is characterized mainly by granular flow in near-surface region. The ruptures either reach the surface or are terminated in unconsolidated sediment below the surface. In the latter case, the rupture splays show westward bifurcation, and their geometry and kinematics show a change to NNW-strike with low-angle dip and dextral-reverse oblique-slip sense in the strata. We suggest that the upward termination of the contractional strike-slip ruptures is controlled by the inherited fault geometry that is unfavorable with respect to the stress field (ENE–WSW σHmax) at basement depths in terms of movement on the fault, and the lack of extension of the fault into shallow subsurface; a depth-dependent change in stress from σHmax > σv > σHmin to σHmax > σHmin > σv at depth of a ~ 200 m; and the physical properties of unconsolidated sediment, which have low inter-granular cohesion, resulting in distributed deformation.

Spatial variation of surface erosion rate in a fault zone and its controlling factors

Topographic relief and bedrock fractures caused by fault activity can accelerate surface erosion, suggesting that erosion rates increase with decreasing distance to the fault core. To test this hypothesis, we used 10Be cosmogenic nuclides to quantify catchment-wide erosion rates at various distances from a fault in the footwall of the Daqing Shan fault system, a 200-km-long active normal fault in Inner Mongolia, northern China. In addition, in order to examine the influence of topography and bedrock fractures on the surface erosion processes in the fault-weakened zone, their relationships with hillslope erosion, fluvial sediment grain size and transport efficiency, erosion rates, and erosion coefficients were studied. The resulting catchment-wide erosion rates ranged from 0.01 ± 0.00–0.13 ± 0.01 mm yr−1. These erosion rates and coefficients were unaffected by distance from the fault. The analysis of topography, bedrock fractures, and hillslope erosion yielded distinct hillslope erosion processes at different distances from the fault. Rapid mass wasting events, such as landslides, occurred at <15 km from the fault in steep, rocky hillslopes with abundant bedrock fractures. In contrast, at >15 km from the fault, hillslopes gradually become more gentle and soil-mantled. These spatial-slope configuration differences result in a decrease in sediment grain size delivered to river channels with increasing distance from the fault. Once sediments enter river channels, they impact fluvial transport capacity by influencing the initial motion threshold of sediment. Correlation analyses revealed that sediment transport efficiency in channels directly controls the catchment-averaged erosion rate and coefficient. This mechanism explains why the erosion rate remains insensitive to variations in topography and bedrock fractures in fault-weakened zones.

Active ages of metallogenic structures in the Liangyan lead–zinc deposit, northwestern Guizhou: Constraints from geometric, kinematic, and sphalerite Rb-Sr dating analyses

More than ten lead–zinc deposits formed under the control of the Yadu-Mangdong fault (YMF) are known to have undergone multi-stage activity. However, ore control processes in such fault zones are poorly understood, such as in the Liangyan deposit in northwestern Guizhou. The internal geometrical structure, mineralization, alteration, tectonic stress, and kinematics of the YMF were investigated in three sections (1605, 1655, and 1720) of the Liangyan deposit. The ore-forming age (214.8 ± 4.3 Ma) was obtained by rubidium-strontium dating of sphalerite. The YMF is defined by double-fault cores and superimposed faults that indicate at least three stages of deformation since the mineralization period. The first stage is characterized by the ore-bearing dolomitization ancient fault core formed in the Indosinian period. The principal compressive stress direction was northwest to southeast (NW-SE). The second stage of Yanshanian was the tectonic breccia fault stage (new fault core). The principal compressive stress was northeast to southwest (NE-SW). The third stage was dominated by superposed faults that destroyed the new fault core in the Himalayan period. The principal compressive stress direction was east to west (E-W). In summary, we established a model of evolution for the YMF since the metallogenic period. In conclusion, based on the above research, we consider that the YMF turning point of the Liangyan deposit is prone to orebodies. Additionally, the prospecting target should be concentrated in the footwall (NW-trending) and hanging wall (NE-trending) of the Liangyan deposit YMF model.