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1.
The chemical and isotopic compositions of clay minerals such as illite and chlorite are commonly used to quantify diagenetic and low-grade metamorphic conditions, an approach that is also used in the present study of the Monte Perdido thrust fault from the South Pyrenean fold-and-thrust belt. The Monte Perdido thrust fault is a shallow thrust juxtaposing upper Cretaceous–Paleocene platform carbonates and Lower Eocene marls and turbidites from the Jaca basin. The core zone of the fault, about 6 m thick, consists of intensely deformed clay-bearing rocks bounded by major shear surfaces. Illite and chlorite are the main hydrous minerals in the fault zone. Illite is oriented along cleavage planes while chlorite formed along shear veins (<50 μm in thickness). Authigenic chlorite provides essential information about the origin of fluids and their temperature. δ18O and δD values of newly formed chlorite support equilibration with sedimentary interstitial water, directly derived from the local hanging wall and footwall during deformation. Given the absence of large-scale fluid flow, the mineralization observed in the thrust faults records the P–T conditions of thrust activity. Temperatures of chlorite formation of about 240°C are obtained via two independent methods: chlorite compositional thermometers and oxygen isotope fractionation between cogenetic chlorite and quartz. Burial depth conditions of 7 km are determined for the Monte Perdido thrust reactivation, coupling calculated temperature and fluid inclusion isochores. The present study demonstrates that both isotopic and thermodynamic methods applied to clay minerals formed in thrust fault are useful to help constrain diagenetic and low-grade metamorphic conditions.  相似文献   

2.
In orogenic systems, thrust faults play a major role in stacking different tectonic units and may act as conduits for the expulsion of large amounts of fluid of different origins (metamorphic, diagenetic, meteoric). This study focuses on the Monte Perdido thrust unit emplaced in the Paleogene Jaca thrust-sheet-top basin, in the SW-central Pyrenees. We aim to decipher the mechanisms and P-T conditions of deformation in fault zones and characterize the related fluid involvement, through combined microstructural, geochemical and microthermometry analyses. Two thrust faults cutting platform limestones, marls and siliciclastic turbidites of the lower part of the basin-fill (Paleocene–lower Eocene) have been studied. The fault zones are characterized by metre-thick shear zones with highly deformed, foliated clay-rich sediments. Foliation is underlined by preferentially oriented phyllosilicates. Several generations of shear and extension calcite, quartz and chlorite-bearing veins attest to fluid-rock interactions during a multi-stage deformation. Microstructural observations and stable isotope analyses on calcite from veins and host sediments suggest that deformation was aseismic and dominated by diffusive mass transfer from pressure solution sites along cleavage and stylolites to the precipitation sites in veins, with mineralizing fluids in equilibrium with the host sediments. Our results suggest an essentially closed hydrologic system, and imply the absence of significant fluid flow along the studied fault zones. Microthermometric study on fluid inclusions present in calcite and quartz veins, and calcite-quartz oxygen isotopic fractionation determined for the first generation shear veins, allow a geothermal gradient of 34 °C/km to be estimated. Analytical results demonstrate an evolution of the fault zones in three stages. The first stage was related to the emplacement of the Monte Perdido thrust unit during the middle Eocene at a temperature of ～208 °C and a burial depth of ～5.7 km. The second stage corresponds to a fault reactivation at a temperature of ～240 °C and a burial depth of ～6.5 km. The latter deformation may have been related to folding of the Monte Perdido thrust unit during the emplacement of the underlying Gavarnie thrust unit during the late Eocene–early Oligocene, with deeper burial resulting from aggradation of the thrust-sheet-top basin-fill. The last event corresponds to the formation of a dilatant vein system likely related to the exhumation of the massif.  相似文献   

3.
The Restefond fault, located in the Late Eocene-Early Oligocene Alpine foreland basin, affects the well lithified and low porosity Grès d??Annot. The fault core zone is characterized by the occurrence of highly deformed sandstone lenses. Deformation inside the lenses corresponds to mm to sub-mm-spaced cleavage planes rich in phyllosilicates and up to cm-thick and dm-long quartz-calcite pure extensional veins. The cleavages are mostly composed of newly-formed synkinematic white mica and chlorite. By using thermodynamic thermometers based on the chemical composition of chlorite, a temperature of 200?±?20?°C of fault activity was computed. This temperature shows that the Restefond fault was active at burial conditions comprised between 6.5 and 8?km, assuming a mean geothermal gradient between 25 and 30?°Ckm?1. The petrophysic properties of sandstones from the core zone and in the hanging and foot wall of the fault were determined on drilled plugs following three spacial directions. The permeability of the highly deformed sandstone from the core zone is about one order of magnitude higher than in the host rock. This increase in permeability occurs in the direction parallel to the S?CC structures and is explained by the occurrence of well-connected micropores localized between platy phyllosilicates. This study shows that the fault petrophysic properties are mostly controlled by the precipitation of synkinematic phyllosilicates under deep burial conditions.  相似文献   

4.
The influence of tectonic strain on the diagenetic degree and illitization process of mixed-layers illite–smectite at shallow crustal conditions was studied. For this purpose, the modal composition of clay fraction and illite FWHM parameters of argillites deformed by a regional-scale fault zone were studied in detail by XRD, chemical analyses and by SEM observations. Analyses were performed on deformed samples of the fault rock and compared with the non-deformed rocks off the fault zone. In addition, this paper reports a detailed comparative analysis of deformed (shear surfaces and cleavage domains) and non-deformed domains (lithon cores) of a scaly fabric in the fault rock. A systematic increase in illite concentration, a decrease of Kübler index and FWHM(002) values, and an enrichment of K+ ions were observed in cleavage domains with respect to the non-deformed sediments off the fault zone and the lithon cores within the fault rock. Migration of K+-rich fluids along scaly cleavage domains causes progressive conversion of smectite-rich I–S to illite-rich I–S and thickening of illite crystallites along the c-direction. Changes in mineralogical and crystallographic parameters, therefore, seem to be strongly controlled by shear plane development in highly sheared rocks.  相似文献   

5.
Fault affecting silicoclastic sediments are commonly enriched in clay minerals. Clays are sensitive to fluid–rock interactions and deformation mechanisms; in this paper, they are used as proxy for fault activity and behavior. The present study focuses on clay mineral assemblages from the Point Vert normal fault zone located in the Annot sandstones, a Priabonian-Rupelian turbidite succession of the Alpine foredeep in SE France. In this area, the Annot sandstones were buried around 6–8 km below the front of Alpine nappes soon after their deposition and exhumed during the middle-late Miocene. The fault affects arkosic sandstone beds alternating with pelitic layers, and displays throw of about thirty meters. The fault core zone comprises intensely foliated sandstones bounding a corridor of gouge about 20 cm thick. The foliated sandstones display clay concentration along S–C structures characterized by dissolution of K-feldspar and their replacement by mica, associated with quartz pressure solution, intense microfracturation and quartz vein precipitation. The gouge is formed by a clayey matrix containing fragments of foliated sandstones and pelites. However, a detailed petrographical investigation suggests complex polyphase deformation processes. Optical and SEM observations show that the clay minerals fraction of all studied rocks (pelites and sandstones from the damage and core zones of the fault) is dominated by white micas and chlorite. These minerals have two different origins: detrital and newly-formed. Detrital micas are identified by their larger shape and their chemical composition with a lower Fe–Mg content than the newly-formed white micas. In the foliated sandstones, newly-formed white micas are concentrated along S–C structures or replace K-feldspar. Both types of newly formed micas display the same chemical composition confirmed microstructural observations suggesting that they formed in the same conditions. They have the following structural formulas: Na0.05 K0.86 (Al 1.77 Fe0.08 Mg0.15) (Si3.22 Al0.78) O10 (OH)2. They are enriched in Fe and Mg compared to the detrital micas. Newly-formed chlorites are associated with micas along the shear planes. According to microprobe analyses, they present the following structural formula: (Al1,48 Fe2,50 Mg1,84) (Si2,82 Al1,18) O10 (OH)8. All these data suggest that these clay minerals are synkinematic and registered the fault activity. In the gouge samples, illite and chlorite are the major clay minerals; smectite is locally present in some samples.In the foliated sandstones, Kubler Index (KI) ((001) XRD peak width at half height) data and thermodynamic calculations from synkinematic chlorite chemistry suggest that the main fault deformation occurred under temperatures around 220 °C (diagenesis to anchizone boundary). KI measured on pelites and sandstones from the hanging and footwall, display similar values coherent with the maximal burial temperature of the Annot sandstones in this area. The gouge samples have a higher KI index, which could be explained by a reactivation of the fault at lower temperatures during the exhumation of the Annot sandstones formation.  相似文献   

6.
The Lower Silurian??Lower Devonian Arisaig Group (Antigonish Highlands) in the Canadian Appalachians is a sequence of shallow marine strata deposited after the accretion of Avalonia to Baltica during the closure of the Iapetus Ocean. Deformation of the strata is widely attributed to the Devonian Acadian orogeny and produced shallowly plunging regional folds and a cleavage of varying penetrativity. Phyllosilicate minerals from the finest-grained rocks exhibit very low-grade (diagenetic-anchizone) metamorphic conditions. X-ray diffraction study reveals that the sampled rocks contain quartz, K-white mica, chlorite, and feldspars; illite?Csmectite and chlorite?Csmectite mixed-layers are common but Na?CK mica and kaolinite occur only in some samples. The identification of illite?Csmectite mixed-layers in diagenetic samples, with Kübler Index >0.50 ??°2?? and the highly heterogeneous b-cell dimension of the K-white micas are in agreement with the variable chemical composition of dioctahedral micas, which present low illitic substitution and variable phengitic content. The spatial variation in the above crystal-chemical parameters was plotted along a NW?CSE composite cross section across the regional folds. No correlation was found between the metamorphic conditions and either the stratigraphic depth or the strain values measured by phyllosilicates orientation analyses, as a function of the penetrativity of the cleavage. However, the metamorphic grade generally increases towards the Hollow Fault, and is highest in samples located within a 1?km corridor from the fault surface. Incipient cleavage is observed in the anchizonal samples located in the vicinity of the Hollow Fault and in some of the diagenetic samples, indicating cleavage development under low temperatures (<200?oC). These relationships, together with regional syntheses, suggest low-grade metamorphism post-dated regional folding and was coeval with Late Carboniferous dextral movement along the Hollow Fault. Fluid circulation associated with movement along this major fault may be the driving mechanism for the increasing metamorphism towards it.  相似文献   

7.
Alastair Beach 《Tectonophysics》1977,40(3-4):201-225
Vein arrays and pressure-solution cleavages are common in the sandstone units of the deformed flysch. Both structures were established prior to the folding of the sediments, and they continued to evolve during this folding. Tensile fractures and conjugate sets of shear fractures (of wrench fault type) were formed. En-echelon arrays of veins are closely associated with the formation of these principal veins. The geometry and relations of the veins are described in detail and their relation to the principal stresses at the time of formation is discussed.

Forking at vein terminations, branch fractures and offset structures in veins are described. The analogies between these and structures produced in rock deformation experiments and found in magmatic dykes are discussed. The branch fracture provides a record of the orientation of the maximum principal stress at the time of its formation.

The veins are infilled with quartz and siderite displaying drusy growth fabrics, indicating that crystal growth occurred into a fluid-filled cavity. It is suggested that the veins originated as hydraulic fractures in a flysch sequence that had developed high pore-fluid pressures during sedimentation. The material in the veins was derived by pressure-solution activity in the sandstone units, which produced a spaced pressure-solution cleavage throughout the region. The relations between veins and pressure-solution cleavages are described. Both small- and large-scale solution transfer of material was involved.  相似文献   


8.
Gold mineralization in the West Hoggar shear zone,Algeria   总被引：1，自引：0，他引：1  
The Amesmessa gold prospect is located along a vertical N-S-trending crustal-scale ductile shear zone; stretching lineations are subhorizontal. This major shear zone is a Late Pan African dextral strike-slip fault of the Pharusian Belt of the Tuareg Shield (Algeria). The Amesmessa shear zone is asymmetric: strong thermal and deformational gradients are present along its western border where biotitic ultramylonites are in contact with a rigid Archean complex (In Ouzzal block), whereas there is a progressive gradation, through mylonite then protomylonite, to the Proterozoic gneiss of the Eastern block which displays co-axial Pan African structures. The Amesmessa shear zone is characterized by the presence of a felsic dike complex emplaced during shearing, and forming the most important parent material for ultramylonites. Basic magmas and carbonatites also intruded within the shear zone. The gold-rich quartz veins are located within the ultramylonitic western part of the shear zone. These N-S-trending laminated quartz veins formed during the late increments of shearing (plastic/brittle transition), by repeated syntectonic hydraulic fracturing along zones of rheological contrast parallel to foliation. The ore mineral association (pyrite, galena, native gold, sphalerite) crystallized in the deformed quartz matrix along late shear planes. Undeformed E-W trending banded quartz veins are present in the mylonitic eastern part of the shear zone; their gold content is low and no native gold has been observed. A strong hydrothermal alteration resulted in the development (along the walls of the N-S gold-bearing quartz veins) of a 5-m-wide carbonate-sericite-albite-pyrite secondary mineral association which implies an important CO2 supply and moderate temperature conditions. There is no alteration halo around the E-W quartz veins. Ultramylonites, hydrothermally altered rocks and quartz veins display similar REE patterns characterized by strong LREE enrichments. Shear-related fluids could be likely parental fluids for the Amesmessa gold mineralization and the associated hydrothermal alteration. Hydrothermal fluids were drawn into dilation zones and filled opening fractures along the main planar discontinuity of the most deformed rocks. The supply of CO2 may come from a deep-seated source as suggested by the presence of carbonatite dikes in the shear zones and the existence of CO2-H2O-rich fluid inclusions in quartz. The location of the gold-bearing quartz veins in the western part of the shear zone can be explained by the presence of strong thermal and rheological gradients.  相似文献   

9.
The Vado di Corno Fault Zone (VCFZ) is an active extensional fault cutting through carbonates in the Italian Central Apennines. The fault zone was exhumed from ∼2 km depth and accommodated a normal throw of ∼2 km since Early-Pleistocene. In the studied area, the master fault of the VCFZ dips N210/54° and juxtaposes Quaternary colluvial deposits in the hangingwall with cataclastic dolostones in the footwall. Detailed mapping of the fault zone rocks within the ∼300 m thick footwall-block evidenced the presence of five main structural units (Low Strain Damage Zone, High Strain Damage Zone, Breccia Unit, Cataclastic Unit 1 and Cataclastic Unit 2). The Breccia Unit results from the Pleistocene extensional reactivation of a pre-existing Pliocene thrust. The Cataclastic Unit 1 forms a ∼40 m thick band lining the master fault and recording in-situ shattering due to the propagation of multiple seismic ruptures. Seismic faulting is suggested also by the occurrence of mirror-like slip surfaces, highly localized sheared calcite-bearing veins and fluidized cataclasites. The VCFZ architecture compares well with seismological studies of the L'Aquila 2009 seismic sequence (mainshock MW 6.1), which imaged the reactivation of shallow-seated low-angle normal faults (Breccia Unit) cut by major high-angle normal faults (Cataclastic Units).  相似文献   

10.
A comparison is made between the Gavarnie thrust and the Mérens Fault in the Axial zone of the Pyrenees. The former has a gentle dip and quite a large displacement (at least 12 km) but does not cut through either Hercynian or Alpine isograds. The latter has a smaller displacement (~ 5 km) but dips steeply and cuts through both Hercynian and Alpine isograds at a high angle. On this basis and on the basis of shear zone geometries immediately north of it, it is proposed that the Mérens Fault nucleated as a steeply (65°–80°) dipping structure, while the Gavarnie thrust nucleated with a shallow attitude. The Mérens Fault is not a backward-rotated thrust fault, nor is it the root zone for any major nappe structure. Similar steep ductile structures occur within the Gavarnie nappe and may reflect considerable internal strain in basement lithologies.The relationship between steep and shallow structures is not yet clear; the shear zones may pre-date the thrusting in which case they may be thick-skinned structures affecting the whole lithosphere, or they may be contemporary with thrusting reflecting only local thickening above a décollement.Rheological models can be used to test proposed geometrical and kinematic models for the lithosphere-scale evolution of the Pyrenees. Suggested models are dominated by a cool, rigid, high-level mantle wedge beneath the North Pyrenean zone which probably controlled the location of north-dipping thrust faults. Thick-skinned shortening is possible in thick crust in the Axial zone but is very unlikely in the North Pyrenean zone where steeply rooted structures would have to cut through the strongest part of the lithosphere.  相似文献   

11.
本文选取秦岭商丹断裂带以南,南秦岭刘岭群以北的弧前沉积体为研究对象,研究扬子板块向北秦岭俯冲的运动学特征。这套弧前沉积体产状北倾,矿物拉伸线理为倾向线理,岩片强烈褶皱,断裂发育,表现出由北向南逆冲的运动学特征。利用糜棱岩与云母石英片岩中矿物变形温度计,结合石英C轴组构估算岩石变形温度集中在400℃～550℃之间。结合其所处的构造位置,本文认为这套强烈变形的岩石构造组合体形成于秦岭古岛弧弧前沉积背景,在扬子板块向北秦岭俯冲过程中,北秦岭沿着商丹带由北向南逆冲形成。后期又遭受商丹韧性剪切带左行走滑活动的改造。  相似文献   

12.
Phyllosilicates occurring as replacements of olivine, clinopyroxene and interstitial materials and as veins or fracture-fillings in hydrothermally altered basalts from DSDP Hole 504B, Leg 83 have been studied using transmission and analytical electron microscopy. The parageneses of phyllosilicates generally change systematically with depth and with the degree of alteration, which in turn is related to permeability of basalts. Saponite and some mixed-layer chlorite/smectite are the dominant phyllosilicates at the top of the transition zone. Chlorite, corrensite, and mixed-layer chlorite/corrensite occur mainly in the lower transition zone and upper levels of the sheeted dike zone. Chlorite, talc, and mixed-layer talc/chlorite are the major phyllosilicates in the sheeted dike zone, although replacement of talc or ohvine by saponite is observed. The phyllosilicates consist of parallel or subparallel discrete packets of coherent layers with packet thicknesses generally ranging from< 100 Å to a few hundred Å. The packets of saponite layers are much smaller or less well defined than those of chlorite, corrensite and talc, indicating poorer crystal-linity of saponite. by contrast, chlorite and talc from the lower transition zone and the sheeted dike zone occur in packets up to thousands of Å thick. The Si/(Si+Al) ratio of these trioctahedral phyllosilicates increases and Fe/(Fe+Mg) decreases in the order chlorite, corrensite, saponite, and talc. These relations reflect optimal solid solution consistent with minimum misfit of articulated octahedral and tetrahedral sheets. Variations in composition of hydrothermal fluids and precursor minerals, especially in Si/(Si+Al) and Fe/(Fe+Mg) ratios, are thus important factors in controlling the parageneses of phyllosilicates. The phyllosilicates are generally well crystallized discrete phases, rather than mixed-layered phases, where they have been affected by relatively high fluid/rock ratios as in high-permeability basalts, in veins, or areas adjacent to veins. Intense alteration in basalts with high permeability (indicating high fluid/rock ratios) is characterized by pervasive albitization and zeolitization. Minimal alteration in the basalts without significant albitization and zeolitization is characterized by the occurrence of saponite±mixed-layer chlorite/smectite in the low-temperature alteration zone, and mixed-layer chlorite/corrensite or mixed-layer talc/chlorite in the high-temperature alteration zone. Textural non-equilibrium for phyllosilicates is represented by mixed layering and poorly defined packets of partially incoherent layers. The approach to textural equilibrium was controlled largely by the availability of fluid or permeability.Contribution No. 488 from the Mineralogical Laboratory, Department of Geological Sciences, The University of Michigan  相似文献   

13.
Alpine deformation in the Grimsel granodiorite (Aar massif, Central Alps) at greenschist facies conditions (6.5 ± 1 kbar for 450°C ± 25°C) is characterized by the development of a network of centimetre to decametre localized shear zones that surround lenses of undeformed granodiorite. Localization of deformation is assumed to be the result of a first stage of extreme localization on brittle precursors (nucleation stage) followed by a transition to ductile deformation and lateral propagation into the weakly deformed granodiorite (widening stage). A paradox of this model is that the development of the ductile shear zone is accompanied by the crystallization of large amounts of phyllosilicates (white mica and chlorite) that maintains a weak rheology in the localized shear zone relative to the host rock so that deformation is localized and prevents shear zone widening. We suggest that chemical processes, and more particularly, the metamorphic reactions and metasomatism occurring during re‐equilibration of the metastable magmatic assemblage induced shear zone widening at these P–T–X conditions. These processes (reactions and mass transfer) were driven by the chemical potential gradients that developed between the thermodynamically metastable magmatic assemblage at the edge of the shear zone and the stable white mica and chlorite rich ultramylonite formed during the first stage of shear zone due to localized fluid infiltration metasomatism. P–T and chemical potential projections and sections show that the process of equilibration of the wall rocks (μ–μ path) occurs via the reactions: kf + cz + ab + bio + MgO + H2O = mu + q + CaO + Na2O and cz + ab + bio + MgO + H2O = chl + mu + q + CaO + Na2O. Computed phase diagram and mass balance calculations predict that these reactions induce relative losses of CaO and Na2O of ～100% and ～40% respectively, coupled with hydration and a gain of ～140% for MgO. Intermediate rocks within the strain gradient (ultramylonite, mylonite and orthogneiss) reflect various degrees of re‐equilibration and metasomatism. The softening reaction involved may have reduced the strength at the edge of the shear zone and therefore promoted shear zone widening. Chemical potential phase diagram sections also indicate that the re‐equilibration process has a strong influence on equilibrium mineral compositions. For instance, the decrease in Si‐content of phengite from 3.29 to 3.14 p.f.u, when white mica is in equilibrium with the chlorite‐bearing assemblage, may be misinterpreted as the result of decompression during shear zone development while it is due only to syn‐deformation metasomatism at the peak metamorphic condition. The results of this study suggest that it is critical to consider chemical processes in the formation of shear zones particularly when deformation affects metastable assemblages and mass transfer are involved.  相似文献   

14.
The meta-anorthosite is locally deformed by brittle shear fracturing, which progressively increases from isolated fractures with little cataclasite to many generations of closely spaced fractures, the intervening rock being highly deformed, in both a plastic and brittle way. In most cases an E-W compression on gently dipping to steep reverse shear planes occurs, which we relate to a Caledonian thrust zone.In places, the highly deformed rock is cut by pseudotachylite veins, which locally form networks. The pseudotachylite is generally intrusive, but does not appear to be related to movement on major slip surfaces. Very locally it may have formed in situ. Pseudotachylite only occurs in highly deformed rock, is only very occasionally deformed itself and, thus, generally represents at each locality the last stage of a complex deformation history, as if its presence welded the rock and prevented further deformation. These striking differences from the country-rock gneisses (in which pseudotachylite occurs on well developed fractures in very slightly deformed rock) are considered to be due to the low anisotropy of the meta-anorthosite, to its lower shear strength and to the easy propagation and branching of the shear fractures in plagioclase. The source of the heat necessary to generate the pseudotachylite melt is not clear—it may come from crack propagation as well as frictional sliding.  相似文献   

15.
The Finne fault zone, located in central Germany to the southwest of the Harz mountains, was studied by means of detailed map analysis, investigations of fault displacement and balanced cross-sections for the most strongly deformed area in the center of the fault zone (ca. 50?% of total fault zone length). The system of the Finne fault zone shows a nearly 100-km-long straight flexure that symbolizes the morphological and geological northeastern border of the Thuringian basin. In the central part, which should be surveyed here, the fault zone corresponds to a distinctive narrow band of highly deformed Triassic sedimentary rocks. The northwestern and especially the southeastern parts of the research area are developed as several parallel branch faults. In the southeastern elongation of the fault zone, which is not part of our survey, the sedimentary cover is missing. Here, it is possible to gain insight to the fact that the basement is also involved to the kinematics of the fault zone. Based on our results, we propose a subdivision of the fault zone into four sectors. From the northwest to the southeast, we interpret the structure of these sectors to reflect (1) a compressional flexure, (2) an overthrust graben, and (3) a partially inverted and folded half graben. In the extreme southeast (4), the fault zone is characterized by an anticline with some strike-slip movement parallel to the fold axis. This segmentation is caused by a thrust fault system whose strike direction deviates slightly from that of the earlier formed graben system. The structural configuration can be explained by a two-phase deformation, in which the contractional strain exceeded the preceding extensional deformation. In the investigated area, the horizontal shortening attains a maximum of ca. 1?km. The present study confirms many earlier hypotheses, presents new results on the deformation history of the fault zone, and attempts to evaluate the deformation in a regional geological context. The results of earlier studies were refined and revised using modern methods, and a unified structural and kinematic model of the Finne fault zone was created.  相似文献   

16.
Abstract The Shangdan fault in the Qinling Orogenic Belt of China is an important boundary between the Caledonian North Qinling Fold Belt and the Hercynian South Qinling Fold Belt. In the Danfeng area, the fault zone strikes WNW–ESE and comprises four strongly deformed zones and three weakly deformed domains parallel to each other. The fault zone has a complex history of multiple deformation and each domain has a different tectonic style that was formed at different stages of the deformation.
The rocks exposed in the weakly deformed domains belong to the Qinling, Danfeng and Liuling Groups. In this paper, the mineral chemistry and mineral assemblages are used to infer the metamorphic conditions and the P–T paths of these units. The metamorphic units in and near the fault zone have different metamorphic conditions and histories that are correlated with the tectonic evolution of the fault zone. Caledonian–Hercynian uplift and southward thrusting of the Proterozoic Qinling Group, over the Danfeng and the Liuling Groups, produced the main metamorphic and tectonic features of the fault zone. Folding of both the Liuling Group and the thrust faults during the Hercynian–Indosinian was accompanied by northward thrusting.  相似文献   

17.
Orogenic Gold Mineralization in the Qolqoleh Deposit, Northwestern Iran   总被引：1，自引：1，他引：1  
The Qolqoleh gold deposit is located in the northwestern part of the Sanandai‐Sirjan Zone, northwest of Iran. Gold mineralization in the Qolqoleh deposit is almost entirely confined to a series of steeply dipping ductile–brittle shear zones generated during Late Cretaceous–Tertiary continental collision between the Afro‐Arabian and the Iranian microcontinent. The host rocks are Mesozoic volcano‐sedimentary sequences consisting of felsic to mafic metavolcanics, which are metamorphosed to greenschist facies, sericite and chlorite schists. The gold orebodies were found within strong ductile deformation to late brittle deformation. Ore‐controlling structure is NE–SW‐trending oblique thrust with vergence toward south ductile–brittle shear zone. The highly strained host rocks show a combination of mylonitic and cataclastic microstructures, including crystal–plastic deformation and grain size reduction by recrystalization of quartz and mica. The gold orebodies are composed of Au‐bearing highly deformed and altered mylonitic host rocks and cross‐cutting Au‐ and sulfide‐bearing quartz veins. Approximately half of the mineralization is in the form of dissemination in the mylonite and the remainder was clearly emplaced as a result of brittle deformation in quartz–sulfide microfractures, microveins and veins. Only low volumes of gold concentration was introduced during ductile deformation, whereas, during the evident brittle deformation phase, competence contrasts allowed fracturing to focus on the quartz–sericite domain boundaries of the mylonitic foliation, thus permitting the introduction of auriferous fluid to create disseminated and cross‐cutting Au‐quartz veins. According to mineral assemblages and alteration intensity, hydrothermal alteration could be divided into three zones: silicification and sulfidation zone (major ore body); sericite and carbonate alteration zone; and sericite–chlorite alteration zone that may be taken to imply wall‐rock interaction with near neutral fluids (pH 5–6). Silicified and sulfide alteration zone is observed in the inner parts of alteration zones. High gold grades belong to silicified highly deformed mylonitic and ultramylonitic domains and silicified sulfide‐bearing microveins. Based on paragenetic relationships, three main stages of mineralization are recognized in the Qolqoleh gold deposit. Stage I encompasses deposition of large volumes of milky quartz and pyrite. Stage II includes gray and buck quartz, pyrite and minor calcite, sphalerite, subordinate chalcopyrite and gold ores. Stage III consists of comb quartz and calcite, magnetite, sphalerite, chalcopyrite, arsenopyrite, pyrrhotite and gold ores. Studies on regional geology, ore geology and ore‐forming stages have proved that the Qolqoleh deposit was formed in the compression–extension stage during the Late Cretaceous–Tertiary continental collision in a ductile–brittle shear zone, and is characterized by orogenic gold deposits.  相似文献   

18.
《Geodinamica Acta》2013,26(1-2):53-69
Modern convergent zones at tropical latitudes are characterized by subduction of carbonate sediment. Although carbonate response to deformation is different from clay, they are commonly treated as having a similar rheology. This approximation, though, is inadequate since carbonate behavior is complicated by cementation and pressure solution. Our goal, here, is to focus on the deformation of carbonate sediment in the footwall of a major fossil thrust zone and compare it with a possible scenario in a modern environment. The focus area is the Monti Sibillini Thrust, in the Umbria-Marche sector of the Northern Apennines where the pelagic, Late Eocene-Oligocene carbonates of the Scaglia Cinerea Formation are underthrusted beneath the late Cretaceous-Middle Eocene Scaglia Rossa Formation.

The data collected during this meso-structural study allowed for an interpretation of the temporal and spatial relationships between the observed deformation structures (S-C deformation bands, shear veins and stylolites). These results allow the development of a dynamic model for the deformation. The proposed model explains the observed features through cycles of fluid-pressure build up, hydraulic fracturing, and consecutive collapse, producing pressure-solution and stylolitization.  相似文献   

19.
We examined small-scale shear zones in drillcore samples of abyssal peridotites from the Mid-Atlantic Ridge. These shear zones are associated with veins consisting of chlorite + actinolite/tremolite assemblages, with accessory phases zircon and apatite, and they are interpreted as altered plagiogranite melt impregnations, which originate from hydrous partial melting of gabbroic intrusion in an oceanic detachment fault. Ti-in-zircon thermometry yields temperatures around 820°C for the crystallization of the evolved melt. Reaction path modeling indicates that the alteration assemblage includes serpentine of the adjacent altered peridotites. Based on the model results, we propose that formation of chlorite occurred at higher temperatures than serpentinization, thus leading to strain localization around former plagiogranites during alteration. The detachment fault represents a major pathway for fluids through the oceanic crust, as evidenced by extremely low δ18O of altered plagiogranite veins (+3.0–4.2‰) and adjacent serpentinites (+ 2.6–3.7‰). The uniform oxygen isotope data indicate that fluid flow in the detachment fault system affected veins and adjacent host serpentinites likewise. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

20.
新疆胜利达坂金矿区金矿化特征   总被引：2，自引：0，他引：2  
胜利达坂金矿区,发育近EW向的韧性剪切带。该剪切带是区内金矿的主要控矿构造,控制了矿床、矿体及矿化体的分布。矿体主要沿韧性剪切带糜棱面理分布。矿石的主要类型是浸染糜棱岩型和变形石英脉型。成矿的最有利部位是剪切带内应力梯度大的地带。区内乳白色变形石英脉具有很大的找金潜力。该区金矿属韧性剪切带型金矿。  相似文献   

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