Fault Breccia Zones 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.

Preliminary appraisal of deformation structures from Great Nicobar Island, Andaman-Sumatra arc system

The Andaman and Nicobar Group of Islands represent a topographic high of forearc sediments in an actively rising accretionary prism on the Andaman subduction margin. Preliminary observations of deformation structures recorded in tuff are reported from Great Nicobar Island. Active compression led to the formation of meso-scale deformation structures such as a schuppen zone with horse units; sole and roof thrusts; fault breccia zone; high angle reverse faults and pop-up structures. Offset of tuff beds up to a few centimetres are observed across these faults.

Identification and evaluation of fault-fracture reservoirs in buried hills of the Lower Paleozoic, Chengdao area, China

The Bohai Bay Basin is a Meso-Cenozoic rifted basin where the Paleozoic buried hills with great hydrocarbon potentials are well developed. The reservoir space types are complex and diverse due to tectonic activities, making fracture distribution highly heterogeneous. Reservoir identification and mapping is challenging due to their large burial depth and poor resolution of seismic data. An integration of well-logging, seismic data interpretation and core observation is applied to identify three structural unit types in the study area, that is, fault breccia zone, fault cataclastic zone, and fault massive rock zone. A comprehensive well-logging identification template and a comprehensive discriminant function M for the reservoir are established based on the well-logging response characteristics. A M value greater than 0.12 indicates a fault breccia zone, that between 0.04 and 0.12 marks a fault cataclastic zone, and that in the range from 0.02 to 0.04 represents a fault massive rock zone. A seismic prediction method with multi-parameter fusion is proposed in the study. The large-scale fractures are mapped by coherence-clutter parameters, while small fractures are predicted via waveform indication inversion. The spatial distribution of “fault-fracture reservoirs” is precisely mapped by frequency fusion technology. It is found that the fault breccia zones usually occur close to the fault planes, while the fault cataclastic zones are slightly away from the fault planes. The hydrocarbon abundance of the breccia zones is greater than that of the fault cataclastic and fault massive rock zones.

Spatial variations in damage zone width along strike-slip faults: An example from active faults in southwest Japan

Field investigations reveal spatial variations in fault zone width along strike-slip active faults of the Arima–Takatsuki Tectonic Line (ATTL) and the Rokko–Awaji Fault Zone (RAFZ) of southwest Japan, which together form a left-stepping geometric pattern. The fault zones are composed of damage zones dominated by fractured host rocks, non-foliated and foliated cataclasites, and a fault core zone that consists of cataclastic rocks including fault gouge and fault breccia. The fault damage zones of the ATTL are characterized by subsidiary faults and fractures that are asymmetrically developed on each side of the main fault. The width of the damage zone varies along faults developed within granitic rocks of the ATTL and RAFZ, from ∼50 to ∼1000 m. In contrast, the width of the damage zone within rhyolitic tuff on the northwestern side of the ATTL varies from ∼30 to ∼100 m. The fault core zone is generally concentrated in a narrow zone of ∼0.5–∼5 m in width, consisting mainly of pulverized cataclastic rocks that lack the primary cohesion of the host rocks, including a narrow zone of fault gouge (<0.5 m) and fault-breccia zones either side of the fault. The present results indicate that spatial variations in the width of damage zone and the asymmetric distribution of damage zones across the studied strike-slip faults are mainly caused by local concentrations in compressive stress within an overstep area between left-stepping strike-slip faults of the ATTL and RAFZ. The findings demonstrate that fault zone structures and the spatial distribution in the width of damage zone are strongly affected by the geometric patterns of strike-slip faults.

Internal structure of the Wenchuan earthquake fault zone, revealed by surface outcrop and WFSD-1 drilling core investigation

Fault zones record a series of faulting events that have occurred under different physical conditions during their evolution. Therefore, it is essential to understand the internal structures of fault zones in order to better understand the mechanical behavior of faults. The internal structure of the Wenchuan earthquake fault zone that prevailed at the Bajiaomiao outcrop and in the WFSD-1drilling cores, located along the southern segment of the Yingxiu–Beichuan surface rupture in the Hongkou area, is described in details in this paper. Based on field surveys, X-ray diffraction analysis, microstructure and analysis of the drilling cores, an ~ 240 m-wide fault zone was confirmed as the Yingxiu–Beichuan fault zone (YBF) at the Bajiaomiao outcrop, corresponding to the ~ 100 m fault zone in the WFSD-1 drilling cores. Fault rocks, including fault breccia, fault gouge and cataclasite were identified in both the outcrop and drilling cores, while pseudotachylyte was only present at the outcrop. Two different types of gouge veins, formed by thermal pressurization and fluidization respectively, are observed in this area. The YBF possesses the characteristics of a multiple core model, and consists of 5 different fault rock units. From top to bottom, these are cataclasite zone, black fault gouge–breccia zone, gray fault breccia zone, dark-gray fault breccia zone and black fault gouge–breccia zone. Outcrop investigation and drilling core research show that the slip zone of the Wenchuan earthquake does not completely follow the ancient fault slip zone. The Wenchuan earthquake fault is a high angle thrust fault which crosses the YBF obliquely. The multi-layered fault rocks displayed in the research area might indicate that the YBF comes from the long-term fault activity and evolution over the last ~ 15–10 Ma.

Mineralogical and geochemical studies on the Gowd-e-Morad (Ni, Co, As–Cu) mineral deposit, Anarak (central Iran)

The Ni, Co, As, and Cu deposit of Gowd-e-Morad is located 20 km northwest of Anarak in Central Iran. In this hydrothermal deposit, mineralization occurs as veins in a fault breccia zone hosted by the Chahgorbeh (schist and metabasite) complex. The main ores are made up of Ni, Co, and Cu arsenides. Petrologic studies and results obtained from geochemical analyses have indicated that the Ni, Co, As, and Cu are derived from ultramafic rocks while Pb and Zn are likely to be derived from schist. Based on the geochemical evidence, particularly the high correlation between Ni, Co, and As, it is proposed that this deposit be categorized as a “five elements” mineral deposit. Fluid inclusion studies have shown homogenization temperatures (TH) in the range 113−206 ˚C and salinity 3−13.5 % wt eq. NaCl. Therefore this “five elements” mineral deposit has been determined as a low temperature, epithermal deposit type. It is proposed that the low fluid temperatures are a result of an environment of formation which was distal to a volcanogenic source systems and the major influence of meteoric waters in the hydrothermal system.

경주시 양북면 단층암의 원소거동과 광물조성 특성

단층활동 과정에서 암석 내 화학조성과 광물 조성 간의 연관성을 이해하기 위한 목적으로 경주시 양북면 용당리에 발달하는 단층암에 대하여 XRF, ICP, XRD, EPMA/BSE 분석을 시행하여 단층암 내의 광물 조성 및 원소의 거동 경향성을 파악하였다. 단층암의 대표적인 주성분으로서 <TEX>$SiO_2$</TEX>는 61.6~71.0%의 범위로 가장 높은 함량을 나타내며, <TEX>$Al_2O_3$</TEX>는 10.8~15.8%이다. <TEX>$Na_2O$</TEX>는 0.22~4.63%, <TEX>$K_2O$</TEX>는 2.02~4.89%이고, <TEX>$Fe_2O_3$</TEX>는 3.80~12.5% 범위로 나타나 단층암 내에서 특히 이들 원소의 편차가 크다. 각력대에서 비지대로 갈수록 감소하는 경향을 나타내는 원소에는 <TEX>$Na_2O$</TEX>, <TEX>$Al_2O_3$</TEX>, <TEX>$K_2O$</TEX>, <TEX>$SiO_2$</TEX>, CaO, Ba, Sr 등이 있고, 증가하는 경향을 나타내는 원소에는 <TEX>$Fe_2O_3$</TEX>, MgO, MnO, Zr, Hf, Rb 등이 있다. 이러한 화학적 거동은 각력대가 대부분 석영, 장석류와 같은 주요 조암광물로 구성되는 반면, 비지대는 일라이트와 같은 점토광물이 다량 함유되는 것과 밀접한 관련성이 있다. 본 단층대에서는 기존 광물의 변질과 동시에 점토광물이 생성되는 데 있어 단층 활동에 수반된 유체활동이 큰 영향을 끼쳤다. 단층활동 초기에는 파쇄대가 열수의 통로로 작용하는 과정에서, 주원소 및 미량원소, 희토류원소는 기존 광물로부터 용탈되어 높은 원소 유동성을 나타내었으나, 파쇄대 중심부가 비지대로 점차 변화하는 과정에서는 이차적으로 형성된 점토광물로 인해 단층대의 투수성이 감소하고, 점토광물이 풍부해진 비지대 내에 농집되므로 낮은 원소 유동성을 가진 것으로 해석된다. This study is focused on element behaviors and mineral compositions of the fault rock developed in Yongdang-ri, Yangbuk-myeon, Gyeongju City, Korea, using XRF, ICP, XRD, and EPMA/BSE in order to better understand the chemical variations in fault rocks during the fault activity, with emphasis on dependence of chemical mobility on mineralogy across the fault zone. As one of the main components of the fault rocks, <TEX>$SiO_2$</TEX> shows the highest content which ranges from 61.6 to 71.0%, and <TEX>$Al_2O_3$</TEX> is also high as having the 10.8~15.8% range. Alkali elements such as <TEX>$Na_2O$</TEX> and <TEX>$K_2O$</TEX> are in the range of 0.22~4.63% and 2.02~4.89%, respectively, and <TEX>$Fe_2O_3$</TEX> is 3.80~12.5%, indicating that there are significant variations within the fault rock. Based on the chemical characteristics in the fault rocks, it is evident that the fault gouge zone is depleted in <TEX>$Na_2O$</TEX>, <TEX>$Al_2O_3$</TEX>, <TEX>$K_2O$</TEX>, <TEX>$SiO_2$</TEX>, CaO, Ba and Sr, whereas enriched in <TEX>$Fe_2O_3$</TEX>, MgO, MnO, Zr, Hf and Rb relative to the fault breccia zone. Such chemical behaviors are closely related to the difference in the mineral compositions between breccia and gouge zones because the breccia zone consists of the rock-forming minerals including quartz and feldspar, whereas the gouge zone consists of abundant clay minerals such as illite and chlorite. The alteration of the primary minerals leading to the formation of the clay minerals in the fault zone was affected by the hydrothermal fluids involved in fault activity. Taking into account the fact that major, trace and rare earth elements were leached out from the precursor minerals, it is assumed that the element mobility was high during the first stage of the fault activity because the fracture zone is interpreted to have acted as a path of hydrothermal fluids. Moving toward the later stage of fault activity, the center of the fracture zone was transformed into the gouge zone during which the permeability in the fault zone gradually decreased with the formation of clay minerals. Consequently, elements were effectively constrained in the gouge zone mostly filled with authigenic minerals including clay minerals, characterized by the low element mobility.

Structures and high-velocity frictional properties of the Pingxi fault zone in the Longmenshan fault system, Sichuan, China, activated during the 2008 Wenchuan earthquake

Abstract This paper reports on the internal structures of the Pingxi fault zone near the northeastern end of the coseismic surface ruptures associated with the 2008 Wenchuan earthquake. The fault zone consists of a 50–80 cm wide fault core and several-meter wide fault breccia zones. The fault core contains yellowish gouge (YG), gray-blackish gouge (GBG), black gouge (BG) and gray-blackish fault breccia. A coseismic slip during the Wenchuan earthquake occurred within GBG in a slip zone of 24 ± 9 mm in width. High-velocity friction experiments were conducted at normal stresses σn of 0.45–2.50 MPa and slip rates v of 0.0014–2.09 m/s. YG and GBG exhibit dramatic slip-weakening at constant high slip rates that can be described as an exponential decay from peak friction coefficient μp to steady-state friction coefficient μss over a slip-weakening distance Dc. Specific fracture energy EG for YG and GBG was 0.37–2.02 MJ/m2 of the same order of magnitude as the fracture energy estimated for natural earthquakes. Empirical equations describing the changes in μp, μss and Dc in terms of σn and v indicate that EG decreases with increasing σn, so that higher-normal stress experiments will lead to EG lower than seismic values. Thus our experiments probably reproduced deeper seismic fault motion under dry conditions with respect to EG. Frictional behavior of BG is characterized by a sharp initial weakening, subsequent strengthening to the second peak friction, and slip weakening towards the steady-state friction. The initial weakening and strengthening seem to be caused by the formation of a very sharp slip zone (~ 5 μm in width) and subsequent formation of a widely deformed zone. A comparison of our data with reported results for other outcrops indicates that the high-velocity frictional properties of the Longmenshan fault system may be quite uniform. The observed dramatic high-velocity weakening must have promoted dynamic rupture propagation during the Wenchuan earthquake.

Internal structure of Longmenshan fault zone at Hongkou outcrop, Sichuan, China, that caused the 2008 Wenchuan earthquake

This paper reports the internal structures of the Beichuan fault zone of Longmenshan fault system that caused the 2008 Wenchuan earthquake, at an outcrop in Hongkou, Sichuan province, China. Present work is a part of comprehensive project of Institute of Geology, China Earthquake Administration, trying to understand deformation processes in Longmenshan fault zones and eventually to reproduce Wenchuan earthquake by modeling based on measured mechanical and transport properties. Outcrop studies could be integrated with those performed on samples recovered from fault zone drilling, during the Wenchuan Earthquake Fault Scientific Drilling (WFSD) Project, to understand along-fault and depth variation of fault zone properties. The hanging wall side of the fault zone consists of weakly-foliated, clayey fault gouge of about 1 m in width and of several fault breccia zones of 30–40 m in total width. We could not find any pseudotachylite at this outcrop. Displacement during the Wenchuan earthquake is highly localized within the fault gouge layer along narrower slipping-zones of about 10 to 20 mm in width. This is an important constraint for analyzing thermal pressurization, an important dynamic weakening mechanism of faults. Overlapping patterns of striations on slickenside surface suggest that seismic slip at a given time occurred in even narrower zone of a few to several millimeters, so that localization of deformation must have occurred within a slipping zone during coseismic fault motion. Fault breccia zones are bounded by thin black gouge layers containing amorphous carbon. Fault gouge contains illite and chlorite minerals, but not smectite. Clayey fault gouge next to coseismic slipping zone also contains amorphous carbon and small amounts of graphite. The structural observations and mineralogical data obtained from outcrop exposures of the fault zone of the Wenchuan earthquake can be compared with those obtained from the WFSD-1 and WFSD-2 boreholes, which have been drilled very close to the Hongkou outcrop. The presence of carbon and graphite, observed next to the slipping-zone, may affect the mechanical properties of the fault and also provide useful information about coseismic chemical changes.

Frictional strength of fault gouge in Taiwan Chelungpu fault obtained from TCDP Hole B

Three fault zones (FZB1136, FZB1194 and FZB1243) are found in core samples in Hole B borehole penetrating through the Chelungpu fault, central Taiwan that activated at the 1999 Taiwan Chi-Chi earthquake. Each fault zone is composed of a black gouge zone (BGZ) and a gray gouge zone (GGZ) surrounded by a fault breccia zone (BZ) and/or fracture-damaged zone (FZ). We conducted tri-axial friction experiments on samples collected from subzones of the above three fault zones under the 1 km depth ambient condition with axial shortening rates of 0.1 to 10 µm/s to investigate frictional strength and its velocity dependence. We found that the coefficient of friction is 0.3 for the BGZ at FZB1136 (1136BGZ), which is lower than those at other fault zones. This result indicates that fault slip at the Chi-Chi earthquake occurred at the 1136BGZ. The estimated frictional strength profile across FZB1136 was that the frictional strength was the lowest (the coefficient of friction was 0.3) for the BGZ and increased up to 0.5 for undeformed host rock with increasing distance from the BGZ. Presence of the intermediate frictional strength zone consisting of the GGZ and the BZ suggests a development of off-fault damage zone where deformation occurred not only in the BGZ but also in the intermediate zone. This feature should be important to estimate total fracture energy released during earthquakes. We also suggest that coefficients of friction and the velocity dependences of the samples are closely related to localization of shear deformation and clay content, respectively.

Meso- and Microstructural Analysis of Coseismic Shear Zone of the 1999 MW 7.6 Chi-Chi Earthquake, Taiwan

The surface rupture of the 1999 M w 7.6 Chi-Chi (Taiwan) earthquake was controlled primarily by the pre-existing Chelungpu fault zone, which is distributed in a wide zone of up to 60 m, although the main coseismic slip is localized in a narrow shear zone of <0.3 cm. The Chelungpu fault zone is composed of cataclasite, fault breccia, and gouge zones that are well observed in both the fault outcrops and the drill cores taken throughout the fault zone. The foliations developed in the cataclasite and fault breccia zones are oriented parallel to that of the fault gouge zone where the main coseismic slip occurred. The structural analyses of the shear zone and fault rocks show that the Chelungpu fault zone has slipped as a thrust fault with a significant left-lateral slip component since its formation in the Pleistocene. This oblique thrust motion is caused by the southeast–northwest oblique convergence between the Philippine Sea plate and the Eurasian plate.

Principal fault zone width and permeability of the active Neodani fault, Nobi fault system, Southwest Japan

The internal structure and permeability of the Neodani fault, which was last activated at the time of the 1891 Nobi earthquake (M8.0), were examined through field survey and experiments. A new exposure of the fault at a road construction site reveals a highly localized feature of the past fault deformation within a narrow fault core zone. The fault of the area consists of three zone units towards the fault core: (a) protolith rocks; (b) 15 to 30 m of fault breccia, and (c) 200 mm green to black fault gouge. Within the fault breccia zone, cataclastic foliation oblique to the fault has developed in a fine-grained 2-m-wide zone adjacent to the fault. Foliation is defined by subparallel alignment of intact lozenge shaped clasts, or by elongated aggregates of fine-grained chert fragments. The mean angle of 20°, between the foliation and the fault plane suggests that the foliated breccia accommodated a shear strain of γ<5 assuming simple shear for the rotation of the cataclastic foliation. Previous trench surveys have revealed that the fault has undergone at least 70 m of fault displacement within the last 20,000 years in this locality. The observed fault geometry suggests that past fault displacements have been localized into the 200-mm-wide gouge zone. Gas permeability analysis of the gouges gives low values of the order of 10 −20 m 2. Water permeability as low as 10 −20 m 2 is therefore expected for the fault gouge zone, which is two orders of magnitude lower than the critical permeability suggested for a fault to cause thermal pressurization during a fault slip.

Geological setting, nature of ore fluids and sulphur isotope geochemistry of the Fu Ning Carlin‐type gold deposits, Yunnan Province, China

AbstractCarlin‐type gold deposits in southern China are present in Palaeozoic to Mesozoic siliciclastic and carbonate rocks. The border region of Yunnan, Guizhou and Guangxi Provinces contains gold deposits on the south‐western margin of the Pre‐Cambrian South China Craton in south‐eastern Yunnan Province. The Fu Ning gold deposits host epigenetic, micron‐sized disseminated gold in: (i) Middle Devonian (D1p) black carbonaceous mudstone at the Kuzhubao gold deposit and (ii) fault breccia zones at the contact between Triassic gabbro (β\mathrm{\mu} ^{1}_{5}) and the Devonian mudstone (D1p) at the Bashishan gold deposit. The deposits are associated with zones of intense deformation with enhanced permeability and porosity that focused hydrothermal fluid flow, especially where low‐angle N‐S striking thrust faults are cut by NW striking strike‐slip and/or NE striking normal faults. Major sulphide ore minerals in the Fu Ning gold deposits are pyrite, arsenopyrite, arsenic‐rich pyrite, stibnite and minor iron‐poor sphalerite. Gangue minerals are quartz, sericite, calcite, ankerite and chlorite. Hypogene ore grades range from 1 to 7 g t−1 Au and up to 18 g t−1 Au at the Kuzhubao gold deposit and are generally less than 3 g t−1 Au at the Bashishan gold deposit. Sub‐microscopic gold mineralization is associated with finely disseminated arsenic‐rich pyrite in the Stage III mineral assemblage. Two types of primary fluid inclusions have been recorded: Type I liquid–vapour inclusions with moderate‐to‐high liquid/vapour ratios, and Type II inclusions containing moderate liquid/vapour ratios with CO2 as determined from laser Raman analysis. Temperature of homogenization (Th) data collected from these primary fluid inclusions in gold‐ore Stage III quartz ranged from 180 to 275°C at the Kuzhubao gold deposit and 210 to 330°C at the Bashishan gold deposit. Salinity results indicate that there were possibly two fluids present during gold deposition, including: (i) an early fluid with 0.8–6.5 wt.% NaCl equivalent, similar to salinity in shear‐zone‐hosted gold deposits with metamorphic derived fluids; and (ii) a late fluid with 11.8–13.4 wt.% NaCl equivalent, indicating possible derivation from connate waters and/or brine sources. CO2 and trace CH4 were only detected by laser Raman spectrometry in gold‐ore‐stage primary fluid inclusions. Results of sulphur isotope studies showed that δ34S values for pyrite and arsenopyrite associated with gold‐ore mineralization during Stage III at the Kuzhubao and Bashishan gold deposits are isotopically similar and moderately heavy with a range from +9 to +15 per mil, and also fall into the range of δ34S values reported for Carlin‐type gold deposits. Sulphur isotopes suggest that the Fu Ning gold deposits were formed from connate waters and/or basinal brines. Fluid geochemistry data from the Fu Ning gold deposits suggest a Carlin‐type genetic model, involving fluid mixing between: (i) deep CO2‐rich metamorphic fluids, (ii) moderately saline, reduced connate waters and/or basinal brines; and (iii) evolved meteoric waters.

特集 火山活動と地殻応力場 中生代玄武岩岩脈の岩脈連結断層と火山性地震の関係 群発地震の化石か？

In the coastal region of the State of Rio de Janeiro, Brazil, Jurassic (120 Ma) basaltic dykes are present intruding in late Precambrian (600 Ma) gneissic rocks. They are typically 10 m wide, constituted by thoreiitic gabbro, and frequently have sinistral en-echelon setting. Intrusion depth of the present surface exposure is estimated to be 4 km based on apatite fission track dating and regional uplift history. The en-echelon dyke terminals are often linked by a short oblique path, showing extended “Z” -shaped dyke appearance on outcrops. Some of the link paths of wide dykes (>4 m) are accompanied by well-consolidated narrow (<1 m) brecciated zone on both sides, composed uniquely of angular host gneiss fragments. The fragment size is relatively fine (1 cm) on dyke link path side (inner), and gross (10 cm) on host rock side (outer). Large fragments are very angular, and show auto-brecciated texture. The lithologic characteristics of the breccia correspond to those of fault breccia.Geometric setting and kinematic relation between these fault breccia zones and the dykes are similar to those of mid-ocean ridges and transform faults. Dyke dilatation due to magma injection may cause shear stress at the dyke junction zones. If the stress is sufficiently great, the host rock will rupture to form fault fracture zones, and they are successively filled by magma to result the link paths. If the stress is insufficient, there small cracks may be created. Such type of faults, namely dyke linkage faults, are primary and monogenic ones, which are generated in uncracked host rock body with no repeated stick slips.The movement mechanism of dyke linkage faults is similar to the Hill's model, but differs a little from it especially in case of the faults with link path filled by magma. Total displacement of dyke linkage fault is defined by dyke width, Even after the magma fill on the fault plane, fault displacement will continue aseismically. The basaltic dykes in this region commonly have well-developed horizontal columnar joints, but the cooling joints at link paths are pertubated and platy-like joints take place, indicating that the link paths were submitted to shear deformation during magma cooling. In this sense, the earthquakes are more likely to occur only at the top of the intruding dykes. The fault length is defined by en-echelon dyke off-set at junction zones, being several times more than the dyke width. In comparison with tectonic faults and earthquakes, the diplacement is very large relative to the length.The earthquakes generated by dyke linkage fault formation are expected to have strike-slip focal mechanism, linear epicentre trend, nodal plain oblique to the trend, and a high frequency spectrum. Some Japanese volcanic earthquake swarms have such characteristics.

Fluid inclusion and stable isotope studies of gold-silver-bearing hydrothermal vein deposits, Yeoju mining district, Republic of Korea

The Yeoju gold-silver mineralized district is located approximately 60 km southeast of Seoul within the Precambrian Gyeonggi metamorphic belt of the Korean peninsula. Mines of the district are located along gold-bearing hydrothermal quartz veins that crosscut early Proterozoic augen gneiss and Mesozoic granites. Mineralization occurs in three stages (I, II, and carbonate) which fill preexisting fault breccia zones. Fluid inclusion data suggest that quartz sulfide-bearing stages I and II evolved from initial high temperatures (near 350 degrees C) to later lower temperatures (near 180 degrees C). Fluid inclusion data from the postore carbonate stage reflect much cooler (220 degrees -190 degrees C), more dilute (4-5 equiv wt % NaCl) hydrothermal fluids.Fluid inclusion and stable isotope evidence show that electrum, jalpaite, argentite, galena, and sphalerite were deposited at temperatures between 285 degrees and 185 degrees C from fluids with salinities between 14.0 and 2.6 equiv wt percent NaCl. Fluid inclusion evidence of boiling suggests pressures of less than 100 bars during portions of stage I and II mineralization, corresponding to depths between 500 and 1,250 m, assuming lithostatic and hydrostatic loads.Sulfur isotope compositions of stage I sulfide minerals decrease systematically with paragenetic time from calculated delta 34 S (sub H 2 S) values of 7.7 to 0.7 per mil. A gradual increase in the sulfate/sulfide ratio of the fluid, likely due to H 2 S loss during boiling coupled with declining temperatures, not only resulted in the systematic decrease of delta 34 S (sub H 2 S) values with time but may also have caused gold deposition by the breakdown of Au(HS) (super -) 2 .The H and O isotope values of hydrothermal fluids in the Yeoju district are consistent with meteoric water dominance approaching unexchanged meteoric water values. Comparison of these values with those of other Korean Au-Ag deposits reveals a relationship between depth and water to rock ratios: Jurassic Au systems are mesothermal and have ratios of 0.001 to 0.01 whereas Cretaceous Au-Ag systems are epithermal with ratios of 0.02 to 0.5.

Stable isotope and fluid inclusion studies of gold- and silver-bearing hydrothermal vein deposits, Cheonan-Cheongyang-Nonsan mining district, Republic of Korea; Cheonan area

The Cheonan gold-silver-mineralized area is located approximately 80 km south-southeast of Seoul within the Precambrian Gyeonggi metamorphic belt of the Korean peninsula. The Cheonbo, Ilbo, Seonggeo, Daeheung, and Chungnam mines which comprise the area are each located along gold-bearing hydrothermal quartz veins that crosscut early Proterozoic gneiss and Jurassic and Cretaceous granites. Mineralization can be separated into three distinct stages (I, II, and III) which fill preexisting fault breccia zones. Stages I and II are terminated by the onset of fracturing and brecciation events. Fluid inclusion data suggest that quartz-sulfide-bearing stages I and II each evolved from initial high temperatures (near 400 degrees C) to later lower temperatures (near 200 degrees C). Each of these stages represents a separate mineralizing system which cooled and largely abated prior to the onset of the next stage. Fluid inclusion data from stage III, a postore carbonate stage of mineralization, indicate a much cooler (240 degrees - 115 degrees C), more dilute (3-4 equiv wt % NaCl) hydrothermal system which was the result of increasing influx of meteoric waters.Sulfur isotope and fluid inclusion evidence suggests electrum-sphalerite-galena ore mineralization was deposited during stage I at temperatures between 170 degrees and 255 degrees C from fluids with salinities between 3 and 9 equiv wt percent NaCl. Fluid inclusion evidence of boiling suggests pressures of less than 100 to 300 bars. This range of pressure is consistent with a depth of mineralization near 1 km under pressure conditions that alternated between hydrostatic and lithostatic. Sulfur isotope compositions of sulfide minerals are consistent with an igneous source of sulfur with a delta 34 S (sub Sigma S) value near 5.0 per mil.Measured and calculated delta D and delta 18 O values of hydrothermal fluids in the Cheonan area ore deposits are: stage I, -122 to -127 and 6.2 to 5.9 per mil; stage II, -135 to -143 and 3.6 to 3.2 per mil; stage III, -134 to -137 and 3.4 to -7.4 per mil, respectively. These values are consistent with progressive meteoric water involvement (increasing water/rock ratios) with increasing paragenetic time. Comparison of these values with those of a deeper (2-3 km) Jurassic gold deposit (delta D = -78--102; delta 18 O = 7.7-3.8ppm) suggests a relationship between depth and water/rock ratio in Korean Au-Ag-bearing vein deposits: the Jurassic Au system has ratios of 0.001 to 0.01; the Cretaceous Au-Ag system, 0.02 to 0.5. The relationships among depth, metal contents, and water/rock ratios in these Korean Au-Ag deposits suggest differences in the hydrodynamics and postmagmatic evolutions of granitic hydrothermal systems and may be indicative of the manner in which gold and silver are scavenged from cooling plutons.

Copper-bearing hydrothermal vein deposits in the Gyeongsang Basin, Republic of Korea

The hydrothermal vein-type deposits which comprise the Sambong, Samsan, and Seongji mines are primarily copper deposits, but they have associated lead, zinc, and silver mineralization. The deposits occur within Upper Cretaceous volcanic rocks of the Gyeongsang basin of the southern Korean peninsula. Mineralization can be separated into three distinct stages (I, II, and III) which fill preexisting fault breccia zones. Each stage is terminated by an onset of fracturing and brecciation events. Fluid inclusion data suggest that stages I and II each evolved from initial high temperatures (near 350 degrees C) to later lower temperatures (near 200 degrees C). Each stage represents a separate mineralizing system which cooled and largely abated prior to the onset of the next stage. Fluid inclusion data from stage III, a postore, carbonate stage of mineralization, indicate a much cooler (190 degrees -120 degrees C), more dilute hydrothermal system which was probably the result of increasing influx of meteoric waters.Sulfur isotope and fluid inclusion evidence suggests that ore minerals were deposited at temperatures between 350 degrees and 250 degrees C from fluids with salinities ranging from 3 to 17 equivalent weight percent NaCl. Fluid inclusion evidence of boiling suggests pressures of less than 100 bars during portions of stage I and II mineralization. This pressure corresponds to depths at the time of mineralization between 500 and 1,250 m.Sulfur isotope compositions of sulfide minerals are consistent with an igneous source of sulfur with a delta 34 S (sub Sigma S) value near 4.0 per mil. Carbon and oxygen isotope compositions of carbonate-stage calcites suggest that meteoric water dominated the hydrothermal system at temperatures below 200 degrees C.The similarity of the features of the Sambong, Samsan, and Seongji mines to those of other copper-bearing hydrothermal deposits in Korea and elsewhere suggests a genetic tie between granitic magmatism and the development of the observed Cu-Pb-Zn-Ag mineralization and parageneses.

On the granitic rocks and their collapse at Kamo District, Shimane Prefecture

The geology, petrographic characters of the granites, and the types of collapses of the rocks caused by heavy rain fall in the Kamo District, Shimane Prefecture, are described to clarify the relation between rock types and collapsing phenomena in the granite mountain area. The granitic rocks comprise granodiorite and biotite granite, and both are weathered remarkably in most parts. From the genetic point of view, the collapses are classified into six types. 1) Collapse, closely related to the joint structure of the rocks; they occur characteristically at the foot of slopes in the region of weathered granodiorite. 2) Collapse of the weathered zone (uppermost layer covered with vegetation) of slopes, caused by difference of physical properties to the ground water between the weathered zone and the basement rocks. 3) Collapse with characters of (1) and (2). 4) Collapse closely related with dikes in granite. 5) Collapse accompanying landslide in fault breccia zones. 6) Collapse or resliding of the talus deposit, made by an older land slide of the adjoining rock masses. The majority of the collapses in this district belong to types (1), (2) and (3). Types (1) and (2) predominate in granodiorite and (3) in biotite granite.