Microcosmic analysis of ductile shearing zones of coal seams of brittle deformation domain in superf

Science in china ser d earth sciences 2004 vol 47 No, 5 393-40493Microcosmic analysis of ductile shearing zones of coalseams of brittle deformation domain in superficiallithosphereJU Yiwen, WANG Guiliang JIANG Bo& HOU Quanlin1. Graduate School, Chinese Academy of Sciences, Beijing 100039, China;2. College of Mineral Resources and Geosciences, China University of Mining Technology, Xuzhou 221008, ChinaCorrespondenceshouldbeaddressedtoJuYiwen(email:juyw@sohu.com.cn)Received July 8, 2002; revised August 26, 2003Abstract The ductile shearing zones of coal seams in a brittle deformation domain in superficial lithosphere are put forward based on the study on bedding shearing and ductile rheology ofcoal seams. The macrocosmic and microcosmic characteristics include wrinkle fold, mymonitizedzones and ductile planar structure of coal seams, etc. while the microcosmic characteristics mayalso include different optic-axis fabrics and the anisotropy of vitrinite reflectance as well as thechange of chemical structure and organic geochemistry components. The forming mechanism isanalyzed and the strain environment of ductile shearing zones of coal seams discussed. Theresult indicates that, in the superficial brittle deformation domain, the coal seams are easy to bedeformed, resulting in not only brittle deformation but also ductile shearing deformation under theaction of force. Because of simple shearing stress, the interlayer gliding or ductile rheology maytake place between coal seams and wall rocks. Therefore, many ductile shearing zones comeinto being in superficial lithosphere(<5 km). The research on ductile shearing zone of brittle deformation domain in superficial lithosphere is significant not only theoretically for the study ofductile shearing and ductile rheology of the lithosphere but also practically for the structuralmovement of coal seams, the formation and accumulation of coal-bed methane, and the preven-tion and harness of gas burst in coal mineKeywords: brittle deformation domain, interlayer gliding, ductile rheology, ductile shearing zones of coal seams, coal mrymonite, vitrinite reflectance anisotropyDOI:10.1360/02vd0291For many years, the study on ductile shearing ductile deformation" are rather a familiar phenomezones mainly focuses on the strain analysis and the non in nature researching widely on ductile deforma-characteristics of microcosmic structure of felsic duc- tion in deep layers, structural geologists have also paidtile shearing zones in deep layers(8-10 km). Discus- some attention to probing into the brittle deformationsions on the process of the development of small-type in superficial中国煤化 tile deformationand mini-size ductile shearing zones have been re- domain in bemported in recent years[1.21. "Ductile deformations in banded duct.CNMHbrittle fracturesbrittle deformation" and brittle deformations in of cap rocks in superficial lithosphere. Sun Yan et alCopyright by Science in China Press 2004Science in China ser d earth sciencestaking the brittle fault of cap rocks in the south of JiThe ductile shearing zones of coal seams are theangsu Province as an example, studied the ductile intense strain zones between the coal seams and theirdeformation domain of simple shearing of rocks; Liu roofs and floors or in the place where the coal litho-Junlai et al. studied the brittle-ductile deformation types are different from each other they have macro-mechanism of a low temperature calcitization tectonite scopically a clear shearing zone which extends along ain the faulted zone of superficial lithosphere Chen certain special layer in coal seam, such as the middleBolin et al. researched the ductile deformation in of coal seam. The structure features on both sides ofepimetamorphism detrital rocks and held that the the shearing zone usually have an obvious inconsistentcourse of deformation is a kind of ductile deformation phenomenon. The coal seam in shearing zone breaksunder the condition of low temperature and pressure in up and has various small folds and obvious wrinklesuperficial lithosphere/. The paper, based on the phenomena); and the intensively deformed coalstudy of ductile shearing zones of rocks made by pre- mymonite may be formed there. This zone is severalvious scientists, mainly probes the ductile shearing centimeters to several meters in width, having andeformation of coal seams in ductile deformation do- abruptly changing relation with the country-rocks ofmain of coal measures in superficial lithosphereboth sides. Ductile shearing zone is either developed1 Advance of the problemon the base of earlier brittle shearing broken face] orit has no clear shearing face between coal seam andAbout the mini-size and the big ductile shearing country-rock, the latter is dominated by plastic defozones in coal measures in superficial lithosphere, Hou mation, while the roof and the floor have only a littleQuanlin et al. reported the mini-size ones found in deformation, this may also be called the rheology ofPermian sandstone in southwest Fujian Province, and coal seam. Therefore, ductile rheology is the maindiscussed their characteristics and forming mecha- form of ductile shearing zones of coal seamsnism; Wang Guiliang et al. had found some swordshaped folds which should be formed in deep layers in2 Characteristics of macrocosmic and microcosthe lower Permian coal measures in Fujian where the mic deformations in ductile shearing zones of coalthrust and overlap structures are well developed inseamssuperficial lithosphere %. 10. Now that the ductileThe characteristics of macrocosmic and micro-shearing zones can exist in coal measures, is there any cosmic deformations in the ductile shearing zones ofductile shearing zones in coal seams in the brittle coal seams are mainly wrinkle fold, mymonitizedstructural domain of superficial lithosphere? And what zones, bedding planar structure, etcis the forming mechanism2.1 Wrinkle fold of coal seamsThe organic material is quite sensitive to geologicenvironmental factors such as temperature, pressureThe layer gliding along the roof of coal seamsforms rheological folds such as inharmonious fold. Asetc. The investigated instances show that there exist a kind of special form of inharmonious fold, coal en-great differences in deformation between coal and the closures are well developed in coal mines. For in-felsic and carbonatite rocks, even if a small strain may stance, the rheology of No. 744 track roadway in coalcause an obviously shearing deformation in coal seams. seams of the Haizi coal mine causes an intense changeHovever, in the typical brittle deformation domain of of thickness of coal seam, resulting in the knotty coalthe earth's surface, the ductile deformation such as the bodies bulging out from the roof and rapidly thinningntense rheology, etc. can also be induced in coal from 6 m to 0.4 m along both sides of the coal seamsseams, forming a layer-gliding structurThe forming aV山中国煤化工g1().Theinformation of frequent and intense structure-heat wrinkle foldCNH G(ig.I(b)andevents will certainly be saved in the composition and complex in lye, sacer as Icvelsc, Iclining isocline,structure of coal seamsflat-lying, etc. The scale of the biggest wrinkle fold56m∠358°∠6881,3m6.0m2stope face-ls∠3002m(d)国1國2圈3回4图囪6■8团910Fig. 1. Sketch map of ductile shearing zones of coal seams in the Haizi coal mine. (a) Coal enclosures formed by wrinkle fold of coal seams in No744 track roadway. (b) Rheological fold of coal seams in No 716 wind roadway.(c)Recumbent fold by bedding slipping stream within coal seam inNo 1024 mechanized roadway. (d)The coal mymonite zone of layer-slip and its cleavages in No 716 mechanized roadway. I, Mudstone; 2, layer-slipgouges; 3, mymonitized zone of layer-slip coal; 4, scale structure coal; 5, granulitic: 6, cataclastic: 7, coal; 8, clearage; 9, fault; 10, layer-slip faultnay be dozens of meters, while the small one is only fully filled with clay mineral are deformed to formseveral centimeters. The middle or small type of duc- disharmonic folds (fig. 2(b), the fusinoid constituenttile shearing fold in No. 1027 mechanized roadway in displays as wide gentle linear- flow foldsthe haizi coal mine in the northern anhui province areRamsayllbl holds that the earlier-formed foldshown in fig. 2(a). Some schistosities are foundfolds of coal seams in some mines, and many folds areevolved to sheath-shaped fold by shearing, and themade up of coal seams with schistosities, showing theprogressive shearing of nonpipe -like fold among layintense plastic deformation. In the middle of the coalrs might form the sheath-shaped fold. Thecoal seamseam in the Haizi coal mine. the small-type recumbent n crosscut in-740 m levels of the Kekeng coal mine,fold is formed in coal layer between two gliding facesShangjing of Fujian, forms some intense ductile folds(fig. 1(c)by shearing. The sheath-shaped folds in the directionbeveling the a-axis are found, under the microscope,In micro-scale, the plastic deformation is obyin the coal from the crosscut(fig. 2(c))ously exhibited, the shape of fold is changeful andnearly all macrocosmic characteristics can be seen in2.2 Mymonitized zones of coal seamsmicrocosmic condition. In the deformation structuresIn the bedding shearing zone, some differentof microcosmic folds of tectonic coal, the cutinite types of tectonic coal are often seen in bandinbands show a strong folding deformation, its shape is shaped distribution, thin or thick, generally as layer orof obligue-distorted thick-roof. The matrix vitrinite nearly layer shape, or lenticular shape 3. Coal my-among the cutinite bands and clay mineral produce monite commonly called soft layer is a part of coalplastic flowage, making the thickness of the hinge end seams and lies in a certain positon of coal seams suchbe as three times as that of its limb, and the pull-apart as the hinge end of fold or fault belt; and the wholephenomenon is also seen in the limb. In the middle coal seam might be a soft layer near the beddinof No. 1024 face of No. 10 coal seam in the Haizi coal gliding中国煤化工 dark gray andmine, the clay layer in coal seams is compressed to a has a looseCNMH Gng structure oftight mini-type fold and the wide gentle disharmonic alternative lignt and dark. when the structure of coalfold in the nne direction. The cell cavities of fusinite seam is complex and has some wild coals, the bouding)Fig. 2.(a)Small-to-middle ductile shearing folds of coal seams in No 1027 mechanized roadway of the Haizi coal mine( pointed by a folding rulerb) Disharmonic folds of coal in the ductile shearing zone of No 1024 face of the haizi coal mine, reflected lightcope,X32.(c)Intensiveductile shears of crosscut at -740 m level of the Kekeng coal mine in Shangjing of Fujian, the sheath-shaped folds beveling the direction of axis aeflected light, microscope, X10. (d) Mymonite coal in the ductile shearing zone of No 716Haizi coal mine: reflected light, microscope,X32.(e) Strong bedding glide along the layer surface of coal seams and ductile rheology of中国煤化工 nine in Shangjing ofFujian:;reflected light, microscope, X50. (f) Eyed structures of ductile deformation coal of Ieflected light, micro-X50.(g)C-s structure, whose whole shape is fold, formed due to the s-shaped planarCNMHsheared by fabricC in No. 2 producing area at-700 m level in the Kekeng coal mine in Shangjing of Fujian; reflected light, microscope, X50.(h) Planar twin crystalof Permian system coal seams in Shangjing of Fujian; TEM, X33000Ductile shearing zones in coal seamsstructure may be seen in an asymmetric lozenge or particles of coal, flowing around the porphyroid plaslenticular shape on transverse section, indicating the tically. The flow structure of matrix is exhibited byexistence of shearing, and the shearing direction can directional distribution of coal gel, which is the sheabe judged according to their shapes and other marks. ing face formed in a continuous deformation in ductileLi Kang et al. hold that the coal mymonite is the result shearing zone. The direction of other kinds of planarof ductile deformation 7. The coal mymonitie layers, structure is the preferred direction such as the direc-either wide or narrow are banded distributed in coal tion of rotation of mineral or the direction of theseams, such as the shearing coal my monitized zone of macro-axis of neogenic mineral, recrystallization mincoal seams in No. 716 mechanism roadway of the Ha- eral and aggregate, representing the principal axis ofizi coal mine. The coal mymonitie has not only a strain. These two kinds of planar forms a sharp anglecharacteristic of tiny particles of the usually-called in maceral, equivalent to the typical C-s structure innymonitie(fig. 2(d)), but also a planar structure(flow ductile shearing zone. Fig. 2(g) explains that thestructure)with a directional arrangement of macerals maceral has experienced a simple shearing deforma(fig. 2(e). Being compressed and stretched, the vitrinite tion. The space among planes are not only tight andof coal mymonitie is linear-banding shaped under op- compacting, but also disconnected, so the planes aretical microscope, the organic and mineral substances separated and the pores of maceral are poorly commu-are alternatively arranged or the macerals are in a d- nicatedectional arrangement. Tight arrangement among par-3 Optical and fabric characteristics of ductileticles and the tiny banding structures being parallel to shearing zones of coal seamseach other can also be seen under the semBesides the above-mentioned characteristics the2.3 The ductile planar structure of coal seamsductile shearing zones have also other micro-characIn bedding shearing zones, the bedding flow teristics such as the optical characteristic and the ani-cleavages are usually seen and most of them are dis- sotropy of vitrinite reflectancetributed outside the small fold groups near the shear3. 1 Optical characteristics of coal in ductile shearining zone. The nearer the place to the sliding face, thezonesmore schistosities are developed, and the more intensethe strain is, and a banded plastic yielding zone will beThe phenomenon of structural deformation offormed because of sliding, showing that the ductile macerals is well known, the coal seam and mac-deformation does exist in the bedding shearing zones. eral produce not only tectonic deformations suchThis banding distribution of structural features indi- folds(wrinkle), fracture, break etc. but also the mi-cates that shearing zone from the center to the outside crocosmic tectonic deformation of macerals as wavyis deformed from ductile to brittle(fig. l(d))extinction (fig. 2(h)), which is the micro-deformationsimilar to the plastic deformation of tectonic rock 24)Under the optical microscope, the characteristicsof flow cleavages and solid-state rheology can also beThe special physical mechanics properties of coal areseen. Those flow structures includes two parts: por-likely to be a favorable indicator ofphyroid and matrix. The porphyroid is some quittion. The wavy extinction or cloud extinction borigid palimpsest, involving broken blocks of palimpof the strain shadow around the micro-vitrain(such assest coal, pyrite grain, etc. They are composed ofhard components, pyrite, sclerotinite and so on, figasymmetric eyed structure (ig. 2(f)). At the two ends2(f)and the biaxial optical character of sandstone orf eyed structure, an asymmetric tail composedlow rank coal are the proofs of optical strains for tecne-clastic particles of porphyroid is formed with the tonic stress中国煤化工convergent direction parallel to the plane. The partThedCNMHG Structure mayaround the porphyroid is micro-wave-shaped matriappear in tectonic coal under a certain condition (ewhich is composed of some smaller and finer(