Stable Tectonic Setting Research Articles

Provenance of sandstone on the western flank of Anambra Basin, Southwestern Nigeria

Petrographic and heavy mineral studies were carried out on clastic deposits that crop out in Ikpeshi, Auchi and Fugar localities in order to determine the provenance of the sediments and the tectonic setting of the source area. The localities are in the western flank of the Anambra basin, southwestern Nigeria. Petrographic study shows that the sandstone deposits are composed of variable amounts of quartz, feldspars and lithic fragments with minor occurrence of authigenic silica and chlorite cements. Quartz is the predominant detrital mineral in all the samples accounting for over 87 % of the total framework composition. The representative samples from all the localities contain 88-94 % quartz, 1-8 % lithic fragment, 1-4 % cement and 0-4 % matrix. Feldspar is present in small amount only in samples from Fugar locality. The sandstones each classify as quartz arenites and sublitharenites. Both opaque and transparent heavy minerals occur in the sandstones. The opaques are magnetite, ilmenite and leucoxene. Together, they account for 35 – 55 % of the total heavy mineral abundance. The transparent heavy minerals are zircon, tourmaline, rutile, staurolite, and apatite listed in order of decreasing abundance. The heavy mineral assemblages in the localities are similar except for the absence of rutile from the sediments of Ikpeshi locality. The heavy mineral grains are subangular to rounded. The zircon-tourmaline-rutile (ZTR) index for the sediments from the three localities are generally high and range from 90.5 to 100% indicating high degree of mineralogical maturity of the sandstones. The heavy mineral suites and the petrographic signatures of the sandstones suggest derivation mainly from acid igneous rocks, gneisses and older sandstones in both stable tectonic and recycled orogenic settings. However, the absence of rutile in sandstone from Ikpeshi may suggest derivation from lithologically more restricted provenance.

Tight sandstone gas accumulation mechanism and development models

Tight sandstone gas serves as an important unconventional hydrocarbon resource, and outstanding results have been obtained through its discovery both in China and abroad given its great resource potential. However, heated debates and gaps still remain regarding classification standards of tight sandstone gas, and critical controlling factors, accumulation mechanisms, and development modes of tight sandstone reservoirs are not determined . Tight sandstone gas reservoirs in China are generally characterized by tight strata, widespread distribution areas, coal strata supplying gas, complex gas–water relations, and abnormally low gas reservoir pressure. Water and gas reversal patterns have been detected via glass tube and quartz sand modeling, and the presence of critical geological conditions without buoyancy-driven mechanisms can thus be assumed. According to the timing of gas charging and reservoir tightening phases, the following three tight sandstone gas reservoir types have been identified: (a) “accumulation–densification” (AD), or the conventional tight type, (b) “densification–accumulation” (DA), or the deep tight type, and (c) the composite tight type. For the AD type, gas charging occurs prior to reservoir densification, accumulating in higher positions under buoyancy-controlled mechanisms with critical controlling factors such as source kitchens (S), regional overlaying cap rocks (C), gas reservoirs, (D) and low fluid potential areas (P). For the DA type, reservoir densification prior to the gas charging period (GCP) leads to accumulation in depressions and slopes largely due to hydrocarbon expansive forces without buoyancy, and critical controlling factors are effective source rocks (S), widely distributed reservoirs (D), stable tectonic settings (W) and universal densification of reservoirs (L). The composite type includes features of the AD type and DA type, and before and after reservoir densification period (RDP), gas charging and accumulation is controlled by early buoyancy and later molecular expansive force respectively. It is widely distributed in anticlinal zones, deep sag areas and slopes, and is controlled by source kitchens (S), reservoirs (D), cap rocks (C), stable tectonic settings (W), low fluid potential areas (P), and universal reservoir densification (L). Tight gas resources with great resource potential are widely distributed worldwide, and tight gas in China that presents advantageous reservoir-forming conditions is primarily found in the Ordos, Sichuan, Tarim, Junggar, and Turpan-Hami basins of central-western China. Tight gas has served as the primary impetus for global unconventional natural gas exploration and production under existing technical conditions.

Detrital zircon U–Pb ages of the Proterozoic metaclastic-sedimentary rocks in Hainan Province of South China: New constraints on the depositional time, source area, and tectonic setting of the Shilu Fe–Co–Cu ore district

The Shilu Fe–Co–Cu ore district, located at Hainan Province of South China, is well known for high-grade hematite-rich Fe ores and also two Precambrian host successions, i.e. the Shilu Group and the overlying Shihuiding Formation. This district has been interpreted as a banded iron formation (BIF) deposit-type, but its depositional time, source area and depositional setting have been in debate due to poor geochronological work. Detrital zircon U–Pb dating aided by cathodoluminescence imaging has been carried out on both the Shilu Group and Shihuiding Formation. Most of the zircon grains from both the successions are subrounded to rounded in morphology and have age spectra between 2000Ma and 900Ma with two predominant peaks at ca. 1460–1340Ma and 1070Ma, and three subordinate peaks at ca. 1740–1660Ma, 1220Ma and 970Ma. The similar age distribution suggests the same depositional system for both successions. Linked to the geological and paleontological signatures, the Shihuiding Formation is better re-interpreted as the top, i.e. Seventh member of the Shilu Group, rather than a distinct Formation. The youngest statistical zircon age peaks for both successions, i.e. ca. 1070–970Ma may define the maximum depositional time of the Shilu Group and interbedded BIFs. At least two erosional sources are required for deposition of the studied detrital zircons, with one proximal to provide the least abraded zircons and the other distal or recycled to offer the largely abraded zircons. The predominance of rounded or subrounded zircons over angular zircons probably implies a relatively stable tectonic setting during deposition. Given the Precambrian tectonics of Hainan Island, a retro-arc foreland basin is proposed for the deposition of the Shilu Group and interbedded BIFs. In comparison with those from the South China and other typical Grenvillian orogens, the detrital zircon age populations reveal that Hainan Island had crystalline basement similar to neither the Yangtze nor the Cathaysia Blocks. Combined with the tectonothermal events, we propose that Hainan Island was independent of South China at least before the late Ordovician and most likely attached or close to northwestern Laurentia before the breakup of Rodinia.

Geologic controls on bedrock channel width in large, slowly-eroding catchments: Case study of the New River in eastern North America

We have investigated the geologic controls on hydraulic geometry of bedrock rivers using a single large catchment, the New River, from a stable tectonic setting with variable, resistant lithology but spatially stable climate. Our survey of channel width at 0.5km spacing along 572km of the river shows major variation that only roughly fits the expected scaling relationships between width, drainage area, and slope. Considerable variations in width, including steps in trends and large spikes, relate to physiogeologic boundaries that the river passes through. A large fraction (15%) of the river's length classifies as bedrock reach, showing that it behaves more like a bedrock river than an alluvial river. Unlike established trends, the channel is wider in bedrock than in alluvium. Field observations show that aspect ratio (width to depth) is not constant, but fluctuates systematically with width from wide, shallow reaches to narrower, deeper reaches. Our observations of bedrock properties suggest that susceptibility to fluvial plucking versus abrasion may control this anomalous channel morphology. One end member form with aspect ratio as high as 500, which we term the incision plain, is associated with very closely spaced discontinuities (~10cm) in otherwise hard rock. We propose that the closely spaced discontinuities enable efficient plucking that leads to widening by lateral erosion. This morphology locally occurs in other passive margin rivers and may be a fundamental fluvial form that is similar to, but the inverse of, slot canyons. The other end member, which we term channel neck, is narrower and deeper with complex flow paths through blocky bedrock. This form occurs where discontinuity spacing is longer (>0.5m) and erosion is abrasion dominated. These results imply that changes in channel width do not necessarily reflect variations in uplift rate, but instead may result from complex response to bedrock properties.

Geochemistry of the Eocene Jijuntun Formation oil shale in the Fushun Basin, northeast China: Implications for source-area weathering, provenance and tectonic setting

Within the fault-bound Fushun Basin of northeastern China, the Eocene Jijuntun Formation hosts extensive deposits of thick lacustrine oil shale. Systematic sampling and geochemical analysis of these deposits has revealed that the parent rocks of the oil shale underwent moderate chemical weathering; and that its mineralogy and trace and rare earth element geochemistry were mainly controlled by parent rock composition, with no synsedimentary changes in the source terrain. Based on source rock and tectonic setting discrimination diagrams, we concluded that the parent rocks of the oil shale were mostly basalts of the Paleogene Laohutai Formation. These basalts originated in a continental back arc environment and contain abundant nutrient elements such as Fe, P, Ni, Cu and Zn, all of which are essential for the growth of aquatic photoautotrophs in lakes. Continuous, high primary productivity in the Jinjuntun lacustrine depocentre, combined with a stable tectonic setting and underfilled sedimentary environment, were key factors in the genesis of its oil shale.

Site Classification for Earthquake Hazard and Risk Assessment in Australia

While site response phenomena are well recognized globally, the influence of regolith properties on ground motion has not previously been investigated for much of Australia. In an effort to characterize the regolith in terms of its ability to modify earthquake ground shaking, this validation study assesses the national site classification map of Australia, which has been developed for application in first‐order earthquake hazard and risk assessment. In the absence of regolith thickness and fundamental period data, site classes are assigned based on a method developed in California that uses the relationship between geological material and the shear‐wave velocity of the upper 30 m ( V S 30). The classification scheme is then adjusted to suit the Australian geological environment, including a correction for the presence of weathered in situ regolith commonly encountered in this stable tectonic setting. The results are validated using geophysical and geotechnical data from a variety of Quaternary sedimentary environments in the Newcastle, Sydney, and Perth urban areas, and from bedrock‐dominated environments at a range of sites across Australia. The results of these analyses demonstrate the utility, but also highlight the limitations, of applying a depth‐limited shear‐wave velocity method in site classification for seismic hazard assessment.

Provenance and tectonic setting of the Triassic Yidun Group, the Yidun Terrane, Tibet

The Yidun Group extends from the Shangri-La region to the south and the Changtai region to the north, and is an important component of the Triassic Yidun arc in the eastern Tibetan plateau. It is composed of the Lieyi, Qugasi, Tumugou and Lanashan Formations from the base upward. Both the Lieyi and Lanashan Formations consist dominantly of black or gray slate and sandstone, whereas the Qugasi and Tumugou Formations have variable amounts of mafic to felsic volcanic rocks and tuffs accompanied with gray slate and sandstone.Sandstone from the Yidun Group has variable CIA values from 55 to 76, indicative of mild to moderate weathering condition for the source rocks. All the sandstones define a general weathering trend nearly parallel to the A-CN boundary in the A-CN-K triangular diagram, implying limited effect of diagenetic and post-depositional K-metasomatism. Dominant detrital quartz and feldspar grains of the sandstones suggest predominantly felsic sources. Relatively high Y/Ni and low Cr/V ratios of sandstones from the Yidun Group indicate more contribution from felsic than mafic sources. Similarly, the Yidun sandstones have Co/Th and La/Sc ratios generally similar to upper continental crust (UCC) and cluster between UCC and felsic sources, indicating felsic rocks as primary sources. Granodiorite represents the average chemical composition of sources as evaluated by extending the predicted weathering trend back to the feldspar join in A-CN-K diagram.Prominently high Zr/Sc ratio or Hf concentration and Paleoproterozoic Nd modal ages (1.94–2.21 Ga) point to input of recycling components derived from old sedimentary source in a relatively stable tectonic setting.

The “coal–bauxite–iron” structure in the ore-bearing rock series as a prospecting indicator for southeastern Guizhou bauxite mines

Bauxite deposits in southeastern Guizhou occur in the lower Permian Liangshan Formation. The rock series bearing the sedimentary bauxite exhibit a typical “coal–bauxite–iron” structure, in which the lower part consists of bauxitic shale intercalated with siderite concretions, the middle part consists of bauxitic rock intercalated with multilayer lenticular or earthy bauxite, and the upper part consists of carbonaceous shales and sandstones intercalated with coal seams. The paralic, coastal and paludal depositional environments at one time had a stable tectonic setting. By studying the elemental geochemistry of the ore-bearing rock series, it can be seen that the common presence of the “coal–bauxite–iron” structure has resulted from (1) the extensive desilication and iron depletion during the formation of the sedimentary bauxite due to the varying physical and chemical environment, and (2) the sufficient supply of organic matter by the external environment. Such geological anomalies that resulted from the physical and chemical changes during the formation of the layered structure in the ore-bearing rock series can serve as a mineralization indicator in prospecting for new deposits.

Major element geochemistry of purple soils/rocks in the red Sichuan Basin, China: implications of their diagenesis and pedogenesis

Major element compositions and chemical weathering features of the purple soils and their corresponding rocks in the Red Sichuan Basin, China were studied in this paper to infer the diagenetic features of the purple rocks and the pedogenetic features of the purple soils. The results showed that all of the rock and soil samples, except those from Yibin, have strongly similar major element compositions and the distribution patterns of their major element compositions are similar to those of the upper continental crust. Chemical index of alteration, chemical index of weathering and Na2O/K2O indicate that the purple soils and rocks have similar chemical weathering intensities, whereas the Al2O3–CaO* + Na2O–K2O (A–CN–K) relationship, suggests that chemical weathering was strong during diagenesis of the purple rocks, but weak during pedogenesis of the purple soils. Variations in major element compositions of the purplish rock samples, except that from Yibin, do not primarily reflect differences in compositions of the sediment source, sediment recycling and potassium metasomatism, and therefore the effects of chemical weathering on the major element compositions were mainly dependent on climate condition and the residence time of material exposed to chemical weathering. Chemical weathering of the purple rocks reached only moderate degrees under the general influence of warm and humid climate during Jurassic and Cretaceous. Warmer and more humid climate conditions partly resulted in stronger chemical weathering of rocks in the southern region of the basin than in the central and western region, whereas shorter residence time of material in upslope position partly resulted in weaker chemical weathering of rocks in the eastern region than in the central and western region. The same climate and stable tectonic setting led to comparable chemical weathering intensities of rocks in the Ziliujing and Jiaguan Formations. Gradually colder and dryer climate from Early Jurassic to Middle Jurassic then to Late Jurassic and shorter residence time in tectonically active setting of Middle Jurassic and late Late Jurassic resulted in the following order of chemical weathering intensity of the purple Jurassic rocks: Ziliujing Formation ≫ Shaximiao Formation ≈ Suining Formation > Penglaizhen Formation.

Modal and Geochemical Compositions of the Lower Silurian Clastic Rocks In North Qilian, Nw China: Implications For Provenance, Chemical Weathering, and Tectonic Setting

Abstract The North Qilian orogenic belt records the transition from Ordovician marine back-arc basin to Early Devonian terrestrial molasse basin as the Central Qilian Block was sutured to the North China Craton. The detrital framework and geochemical composition of Lower Silurian sedimentary rocks within the North Qilian Belt record the provenance, tectonic setting, and a variety of sedimentary processes that operated during transport from source to sink. The petrological and geochemical characteristics of sandstones, siltstones, and shales in the western segment of the North Qilian Belt indicate a generally low degree of chemical weathering of source area(s) and limited sorting prior to deposition. These characteristics suggest an active tectonic setting. In contrast, samples studied from the eastern part of the North Qilian Belt indicate low to moderate chemical weathering and low to moderate hydraulic sorting. Such characteristics imply a more stable tectonic setting. Geochemical data including Eu/Eu*, Ti/Nb, Th/Sc, and Co/Th ratios suggest contrasting source regions for the western and eastern segments of the North Qilian Belt. Sources in the west included basement rocks of the Central Qilian Block and the North China Craton. In addition, western sources may have included mafic rocks located within the North Qilian arc. In contrast, data derived from eastern samples suggest a felsic provenance that probably formed part of the basement complex in the Central Qilian Block and the North China Craton. Overall, detrital and geochemical data, combined with the regional geology, suggest a foreland basin setting that formed during collision of the Central Qilian Block with the North China Craton.

Subduction initiation by thermal–chemical plumes: Numerical studies

Prior suggestions for the initiation of subduction have included sediment loading, compression, and plate reconfiguration as potential triggers. Here, we investigate the possibility of subduction initiation by the interaction of the lithosphere with a buoyant mantle plume. Numerical testing of this hypothesis has been conducted in 2D with the I2VIS thermo-mechanical code accounting for phase transitions and a viscoplastic model of a thin oceanic lithosphere hit by a partially molten thermal–chemical or purely thermal plume. We demonstrate that a mantle plume can break the lithosphere and initiate self-sustaining subduction, provided the plume causes a critical local weakening of the lithospheric material above it. The intensity of the required weakening depends on the plume volume, plume buoyancy, and the thickness of the lithosphere and is the highest for the least buoyant purely thermal plumes. Another necessary condition is the presence of high-pressure fluids at the slab upper interface, reducing the effective friction coefficient there to very low values. Based on our results, we suggest that sheet-like instabilities of the Archean mantle convection could have initiated subduction on Earth where ocean was already present in less stable tectonic settings, provided that mantle plumes (sheets) at that time were rich in water and melt, which could drastically reduce the effective friction coefficient in the lithosphere above the plume. Our numerical models are also in good agreement with suggested concepts for corona formation on Venus.

Deep drill hole for the New Madrid Seismic Zone

A 5‐ to 6‐kilometer‐deep scientific drill hole is the ultimate goal of Fault Observatory for the Central U.S. (FOCUS), a new multidisciplinary, multi‐institutional working group that formed at a workshop held at the University of Memphis, in Tennessee. Beneficial shortand intermediate‐term science goals are also part of establishing this borehole geo‐observatory in the most prominent, widely known intraplate earthquake zone in the world.The midcontinent has a long, complex history of tectonism. Precambrian basement rocks record intense tectonic deformations that preceded the accumulation of thick sedimentary sequences of sediments that constitute the present‐day Mississippi Embayment. Despite its location far from the active North American plate margins, the New Madrid seismic zone of the central United States exhibits a persistent pattern of seismicity. What causes this intraplate seismicity? How does the intraplate lithosphere support the forces that maintain earthquake productivity in this supposedly stable tectonic setting? Is there evidence for spatial or temporal migration of seismicity in the continental interior?

SHRIMP U–Pb Zircon geochronology of the Altai No. 3 Pegmatite, NW China, and its implications for the origin and tectonic setting of the pegmatite

The “Altai No. 3 granitic pegmatite” in NW China is the largest pegmatite in the world that has been found in an orogenic belt; it forms one of the largest muscovite deposits in Asia and is most famous for its concentric-ring structure. Although it has been mined for more than 70 years, the age of formation and mineralization of the pegmatite has not been well determined, despite numerous previous geochronological studies. Here we present new results of SHRIMP U–Pb zircon dating. Three weighted mean 206Pb/ 238U ages of 220 ± 9 Ma, 198 ± 7 Ma and 213 ± 6 Ma were obtained for pegmatite zones 1, 5, and 7, respectively. The analyses were made on clear, recrystallized domains and overgrowth rims of large, opaque high-U zircon grains. We interpret the age of 220 ± 9 Ma for zone 1 to approximately represent the initial time of formation for No. 3 pegmatite. An additional date of 213 ± 2 Ma was determined on zircon from a small subordinate vein formed in the late-stage phase of the No. 3 pegmatite. This age set the late age of emplacement of the pegmatite. The time span of 213 to 198 Ma may reflect late stage hydrothermal alteration. In addition, a weighted mean 206Pb/ 238U age of 277 ± 6 Ma was obtained from small, clear, transparent low-U zircon grains which are more probably xenocrysts. Our new age data indicate that the pegmatite has no genetic relationship with the wall granitic rocks (409 Ma), but is likely related to the early Mesozoic (220 Ma) granitic magmatism that was just recognized. The No. 3 pegmatite was formed in an early Mesozoic anorogenic extensional regime. A stable tectonic setting was undoubtedly required for the formation of the large pegmatite.

Long-term stability of flow-path structure in crystalline rocks distributed in an orogenic belt, Japan

Most evaluations of the contaminant retardation processes likely to be important in geological disposal (e.g. for high level radioactive waste (HLW)) consider only the present characteristics of fractures and associated mineral infills. Relatively little attention has been given to possible long-term changes in these features, and their influence on groundwater flow. The work reported here seeks to provide analogous evidence that such changes are not likely to be important and hence to improve confidence in the presently adopted evaluation methodology and its long-term applicability. In the orogenic belt that is formed by the Japanese islands, there are wide areas of crystalline rock. The rocks in each area have a distinctive age sequence which is partly reflected in the characteristics of the fracture systems and associated mineral fillings that occur. These characteristics generally imply that groundwater and solutes can be conducted through fracture networks, except in the cases of fault zones or crushed zones. The structural and mineralogical features of these networks readily illustrate how certain contaminants might react and be retarded by the fracture fillings and open pore geometry, due to chemical sorption and/or physical retardation. Here, we describe the fracture systems developed in crystalline rocks with different ages that are intruded into the Japanese orogenic belt. The aim is to build a model for the long-term fracturing process and hence to evaluate fracture ‘stability’. In particular, the comparisons are made between the fracture geometries and the frequencies observed in the 1.9–0.8 Ma Takidani Granodiorite (the youngest exposed pluton in the world), the ca. 67 Ma Toki Granite and the ca. 117 Ma Kurihashi Granodiorite located in central to northwest Japan. The observations show that all these crystalline rocks have similar fracture frequencies, with 1 to 2 fractures per meter in the massive part of rock bodies. Mineralogical studies and dating analyses of fracture fillings also suggest that fractures are relatively physically stable. Major new fractures tend not to be created in the massive part of rock bodies even when a pluton has been subjected to the regional stresses of plate movements with a duration of about 100 Ma. The results show the unique characteristics of the fracture forming process and the relatively stable geometries of fracture network systems in crystalline rocks distributed within the orogenic belt. This analogue also enables us to provide a model to build confidence in a technical approach applicable for modeling of hydrogeology and geology over long time scales under the orogenic stress field present in Japan. The model may also be useful for other stable tectonic settings as well as for a characterizing sites in crystalline rocks for the possible geological disposal of HLW and other toxic wastes.

Tectonic and stratigraphic controls of hydrocarbon systems in the Ordos basin: A multicycle cratonic basin in central China

The Ordos basin is the oldest and still an important hydrocarbon province in central China. It is a typical cratonic basin developed on the Archean granulites and lower Proterozoic greenschists of the North China block. The development of the Ordos basin during the Paleozoic–Mesozoic can be divided into three evolutionary stages: Cambrian–Early Ordovician cratonic basin with divergent margins; Middle Ordovician–Middle Triassic cratonic basin with convergent margins; and Late Triassic–Early Cretaceous intraplate remnant cratonic basin. Two hydrocarbon systems are present in the basin: the Paleozoic gas and Mesozoic oil systems. In the Paleozoic gas system, the Lower Ordovician marine carbonates and Pennsylvanian–Lower Permian coal measures serve as source rocks. The Lower Ordovician karst-modified dolomites and Pennsylvanian bauxitic mudstones form a significant reservoir-seal association, and the Pennsylvanian–Lower Permian deltaic sandstones and Upper Permian lacustrine mudstones form another effective reservoir-seal association. In the Mesozoic oil system, the Upper Triassic lacustrine mudstones are mature source rocks. The Upper Triassic deltaic sandstones and overlying shallow-lacustrine and swamp mudstones form a reservoir-seal association, and the Lower Jurassic fluvial sandstones and overlying shallow-lacustrine and swamp mudstones form another reservoir-seal association. In both hydrocarbon systems, the stratigraphic variations provide the principal traps. The Ordos basin is characterized by a stable tectonic setting that controlled the distribution of depositional systems and the development of erosional surfaces and ultimately governed the distribution of oil and gas fields and trap types.

Recurring tide-dominated sedimentation in Kyonggi Bay (west coast of Korea): similarity of tidal deposits in late Pleistocene and Holocene sequences

Up to 40 m of tidal deposits occur in the inner part of Kyonggi Bay, west coast of Korea (Yellow Sea). A prominent paleosol in the upper part of the succession separates two depositional sequences, of late Pleistocene and Holocene age. Identical tidal facies, including tidal rhythmites, occur within both sequences. The rhythmites in the late Pleistocene sequence demonstrate tidal periodicities ranging from semidiurnal to monthly that are similar to those of the modern tides. The fine-grained texture, accentuated monthly tidal cycles, facies associations, and stratal architecture indicate that the rhythmites are of upper intertidal origin. The predominance of such tidal facies throughout the considerable thickness of both sequences indicates that considerable vertical aggradation occurred and that a tide-dominated (presumably macrotidal) estuarine environment was sustained in Kyonggi Bay throughout the late transgression and early highstand of both the late Pleistocene and Holocene sequences. The broad, shallow shelf and stable tectonic setting of the Yellow and East China seas facilitated the repetition of tide-dominated sedimentation in Kyonggi Bay over several sea-level cycles. The system does not appear to be strongly sensitive to changes in relative sea level or coastal configuration. Ancient sedimentary successions that record repeated episodes of tide-dominated sedimentation must also have formed in settings where the paleogeography changed relatively little between sea-level cycles, and/or that the changes were insufficient to affect the tidal dynamics.

Geological and geotechnical investigation in a metro project in Nanjing, China

Currently, there are six cities constructing their metro systems in China, among which, the Nanjing metro project is characterized by its complicated and unfavorable geological conditions, such as varied topography, rugged bedrocks, many faults passing through, rapid variation in lithology and geotechnical properties, soft ground, high groundwater table and multiple groundwater types. Therefore, the geological and geotechnical investigations play a vital role in the project. This paper describes and discusses the investigations taken in the project. From a seismological point of view, the project is in a relatively stable tectonic setting, the major regional active faults are a little far. Among many faults found in the city area, the NWW Dinghuaimen-Gulou fault and the NNW Nanjing-Hushu fault merit attention to their influences on the foundation stability. High groundwater table and multiple groundwater types feature the hydrogeological conditions. The paper describes the hydrogeological characteristics and parameters on the basis of groundwater type division. The inflow prevention of abundant pore water in ancient riverbed, fault water in Gulou and karst water in Xiaohongshan are difficult. Along the metro axis, the local morphology can be divided into five sections: three sections of tectonic denudated low-rise hills and two sections of ancient channel alluvial plains. As many as 50 of Quaternary soil formations have been encountered. They are too many for architects to conceive and establish the engineering geological model. Therefore, generalization and division of engineering geological strata groups is necessary. The Quaternary deposits at the Metro site were divided according to their origin and relevant age into four engineering geological strata groups and subdivided on the basis of their composition, physical state and geotechnical properties into distinct subgroups. Data from this study indicate that the division is feasible and the subgroup II2(CS) and III2(CS), which are soft cohesive soils of Holocene and late Pleistocene respectively, represent the most sensitive and weak geotechnical unit, and the Holocene sandy soils subgroup II4(SL) is of the most potential danger of liquefaction. None of the current rock mass classification systems has been found to originate from and be adequate for the shallow metro tunnel. The CGIMC, Vp and BQ systems have been combined in this study. The study indicates that the combination of multiple systems, qualitative and quantitative, is helpful to ensure the reliability of rock masses evaluation. To overcome the deficiency of the rock mass intactness index Kv and the BQ system in the national standard, their determination methods were modified. The paper describes in details the modified Kv and BQ procedure taking account of the characteristics of the rock masses at the metro site. The rock masses are categorized into very poor, poor and fair quality. The poor conditions of the rock masses are largely due to intensive weathering and dense fracturing. Along the metro axis, eight geotechinal zones were divided according to their identical characters of topographical, engineering geological and hydrogeological conditions and construction methods. The paper highlights the different geotechnical problems and challenges that will be encountered in different geotechnical zones. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.

An analogue of long-term stability of flow-path structure in crystalline rocks distributed in the orogenic belt, Japan

In the orogenic belt, in the Japanese islands, crystalline rocks from the youngest to older ages and of different orders have been identified which have formed massive areas. The fracture system observed within these rock masses implies that the groundwater and solute can be conducted through the fracture's network. It is expected that the nuclides can be retarded due to chemical sorption and/or physical retardation by the fracture fillings and fracture open pore geometry. Most of the evaluation framework of the nuclides retardation process in the geological disposal of high level radioactive waste (HLW) is, however, basically taken into account in the present geological state, without changes of structural and mineralogical features, and in its influence on the groundwater flow system over a long period of time. This paper seeks analogous evidence that can provide the confidence of such evaluation methodology and its long-term applicability. Here, we describe the fracture system developed in the crystalline rock with the different ages intruded in the orogenic belt in order to build the long-term fracturing and its "stability" model. In particular, comparisons with the rock of 1.9–0.8 Ma Takidani Granodiorite (the youngest pluton in the world), ca. 67 Ma of Toki Granite and ca. 117 Ma Kurihashi Granodiorite located in central to northwest Japan suggest a unique characteristic of the fracture forming process and their relatively stable geometrical changing. This analogue enables us to provide a model to build the confidence of a safety context applicable for the geological setting under the orogenic field with a long-term scale. The model may also be useful for other stable tectonic settings as well as for a site characterisation methodology of crystalline rock for HLW geological disposal.

A Metallogenic Model of Gold Deposits of the Jiaodong Granite‐Greenstone Belt

Abstract An analysis of trace elements and isotopic geochemistry suggest that the ore‐forming materials of gold deposits in the Jiaodong granite‐greenstone belt have multiple sources, especially the mantle source. Seismic wave, magnetic and gravity fields show that the crust‐mantle structure and its coupling mechanism are the fundamental dynamic causes for the exchange and accumulation of materials and energy in the metallogenic system. Considering the evolution history of the structural setting, the tectono‐metallogenic dynamics model of the area can be summarized as follows: (1) occurrence of the greenstone belt during the Archean‐Proterozoic—the embryonic form of Au‐source system; (2) stable tectonic setting in the Paleozoic—an intermittence in gold mineralization; (3) intensive activation and reformation of the greenstone belt in the Mesozoic—tectono‐mineralization and tectono‐diagensis; (4) posthumous structural activity in the Cenozoic—destruction of orebodies in the later stage. In the middle and late Indosinian, the Tancheng‐Lujiang fault zone cut deeply into the upper mantle so that the ore‐bearing fluids migrated to higher layers through the crust‐mantle interaction, resulting in alteration and mineralization.

Late Paleoproterozoic Climate, Tectonics, and Metamorphism in the Southern Lake Superior Region and Proto–North America: Evidence from Baraboo Interval Quartzites

Red, supermature quartzites of the Baraboo interval of the Lake Superior region contain detrital zircon that ranges in age from 1782 to 1712 Ma. Deposition clearly occurred after the geon 18 Penokean orogeny. These late Paleoproterozoic sedimentary rocks consist largely of quartz, kaolinite or pyrophyllite, and hematite; detrital feldspar and muscovite are rare or absent. Their Chemical Index of Alteration ranges from 96.8 to 98.6, among the most chemically mature clastic sediments in the geological record. The quartzites are underlain by mature, feldspar‐free paleosols, accounting for the absence of feldspar in the overlying sediments and indicating the presence of first‐cycle quartzose detritus. Such physical and chemical characteristics imply that late Paleoproterozoic deposition in the Lake Superior region occurred in a stable tectonic setting with subdued topographic relief in a warm, humid climate. Folding and low‐grade metamorphism of the quartzites is thought to reflect ∼1630 Ma foreland deformation related to the Mazatzal orogeny. Younger hydrothermal alteration is widespread in the Baraboo and Sioux quartzites and, based on 40Ar/39Ar dating of low‐temperature minerals, is attributed to the migration of fluids along permeable channels in response to the thermal effects of magmatism associated with the 1465 Ma Wolf River batholith. Much of the Paleo‐ and Mesoproterozoic crust of Proto–North America may have been affected by areally extensive, but stratigraphically restricted, hydrothermal alteration related to the influence of geon 14 transcontinental A‐type granitic magmatism.