Joint Pattern in Precambrian Rocks Around Galudih (India): Implications for Fold Mechanism

Transcription

1 Gondwana Research, V. 7, No. 2, pp International Association for Gondwana Research, Japan. ISSN: X Gondwana GR Research Joint Pattern in Precambrian Rocks Around Galudih (India): Implications for Fold Mechanism Manish A. Mamtani*, A. Ghosh, A.K. Chaudhuri and D. Sengupta Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur , India * Corresponding author: mamtani@gg.iitkgp.ernet.in (Manuscript received June 26, 2003; accepted November 20, 2003) Abstract In this paper, analysis of joints developed in folded quartzites of Galudih area, Jharkhand State (India) is presented. The joint pattern indicates that NE SW joints are the dominant set. These have developed perpendicular to the NW-SE trending axial planes and hinges of the folds in quartzites implying that they are cross-joints. Plumose structures are observed on NE SW joint surfaces that indicate their formation by extension perpendicular to the joint face. It is inferred that this extension in NW-SE direction is related to NE SW compressional stresses that resulted in folding of the quartzite by flexural-slip mechanism. Key words: Joints, fold mechanism, plumose structure, quartzite, joint pattern. Introduction Joints are fracture surfaces across which the material has lost cohesion and no appreciable movement has occurred (Ramsay and Huber, 1987; Twiss and Moores, 1992). They are products of brittle failure, and they form when the tensile strength of stressed rock is exceeded. Three fundamental modes of fracturing that can result in joints have been described in literature. These are Mode- I (extension/opening mode), Mode-II (sliding mode) and Mode-III (scissors mode) (e.g., Davis and Reynolds, 1996). The three modes of jointing are highlighted in figure 1. Joints and fractures are the most common of all geologic features but the study of the geological history of fractures is very difficult. Pollard and Aydin (1988) reviewed the developments in study of joints and criteria that can help in deciphering the mode of fracture formation in rocks and establishing a relationship between joints and other associated structures. However, though a considerable amount of research has been done on joints, especially with reference to their origin and mechanics, little work has been done on the genetic relationship between folding and jointing in naturally deformed rocks. Some of the most significant works that explore the relationship between regional structure and jointing are Hancock (1964, 1965). In the present paper, the authors provide joint data from the folded quartzites that occur in the Precambrian rocks around Galudih, Jharkhand State (India). The relationship between the orientation of the fold and the joints is documented and in light of this the possible fold mechanism is discussed. Geology of the Study Area The Galudih area belongs to the Proterozoic mobile belt of Eastern India and is situated between Dalma synclinorium in the north and the E-W trending Singhbhum Shear Zone in the South. Inset in figure 2 shows the location of the study area that lies 295 km to the west of Kolkata. The reader is referred to the works of Dunn and Dey (1942) for the older accounts of the geology of the area; Saha (1994) presents a thorough and more recent review. The area around Galudih and the nearby town of Ghatsila (12 km east of Galudih) comprises schists with intercalated bands of quartzite that are stratigraphically considered to belong to Proterozoic age Singhbhum Group (Saha, 1994). Around Galudih two quartzite bands are clearly traceable and Naha (1965) worked in details upon the structure of these rocks. Based on presence of primary structures like asymptotic current bedding, he worked out that the eastern quartzite band is younger while the western one is older. The rocks have been folded into a series of southeasterly plunging antiforms and synforms with subvertical NW-SE trending axial planes. Near the village of Chorinda, in the north, a major fault that brings younger and older bands together

2 580 M.A. MAMTANI ET AL. Fig. 1. Schematic diagram showing the three modes of jointing. (a) Mode-I (extension/opening mode), (b) Mode-II (sliding mode) and (c) Mode-III (scissors mode). has been mapped. Besides this, a few minor faults are also mappable in between Chorinda and Galudih town. The trend of all these faults is NW SE. Figure 2 highlights the structure of the studied area. Joint Data and Ornamentation The major objective of the study was to collect joint data from the folded quartzite bands lying between Galudih and Chorinda. Two to three sets of joints have been observed in almost every outcrop. NE-SW and NW- SE striking joints are the most common with some joints also having other intermediate trends. Orientations of a total of 312 joints were recorded from the study area. Figure 3 is a rose diagram of the entire data set that was prepared for a statistical analysis. It is clear from Figure 3 that NE-SW joints have the maximum frequency and hence are the dominant set to have developed in the area. A close examination of several joint faces in the field revealed presence of interesting ornamentation. The most prominent one is the presence of origin and hackles (Fig. 4). These are structures that have a definite relationship with the process of fracture formation (Hodgson, 1961). It is known that the origin of a joint is the site of initial propagation of the joint surface and is analogous to the focus of an earthquake. In other words, it is the place where energy is first released to form the break. Hackles are linear to systematically curved markings that radiate and diverge from the origin. Collectively, the hackles generally display a featherlike pattern that is referred to as plumose structure (Hodgson, 1961; Pollard and Aydin, 1988). An interesting observation was that these plumose structures were present only on the NE-SW striking joints. Fold Mechanism: Discussion As documented above, NE-SW trending joints are the dominant set and these are clearly perpendicular to the hinge direction of the folds, which strike in NW-SE direction. Thus, the joints can be classified as cross joints (Davis and Reynolds, 1996). Although folds and joints form under quite different pressure-temperature conditions, the above documented relationship between the orientation of joints and fold hinges in the quartzites of Galudih implies that there might be a relationship between the folding and joint development. Thus, it is worth exploring the fold mechanism that is likely to have given rise to these joints. Folds can develop by different mechanisms, the most important of which are (a) Flexural folding (b) Passiveshear/flow folding (Twiss and Moores, 1992). Each mechanism results in different types of folds. In case of flexural folding of a single layer, the orthogonal thickness of the layer remains constant during folding as a result of which class-1b folds of Ramsay (1967) are produced. Two kinematic processes can be involved in flexural folding viz., orthogonal flexure and flexural shear. In orthogonal flexure, all lines that were perpendicular to the layer before folding remain perpendicular to the layer after folding and the orthogonal thickness of the fold remains constant all around the fold. In the profile plane, the surface of the layer on the convex side of a fold is stretched, and the surface on the concave side is shortened. The surface within the layer that does not change length during the folding is called neutral surface. In case of flexural-shear, there is simple shear parallel to the layer and no stretching or shortening at the extrados and intrados of the fold respectively. Flexural folding is a characteristic mechanism of competent lithologies. Passive-shear/flow folding is a characteristic of incompetent layers. The deformation takes place by inhomogeneous simple shear on shear planes that crosscut the layer and the amount and sense of shear vary systematically across the shear planes to produce the folded geometry. This fold mechanism gives rise to Class- 2 folds of Ramsay (1967). In case of multilayer rocks, where the competency contrast between individual layers is not significant, flexural-slip folding is the dominant mechanism. In flexuralslip folding, each competent layer folds by orthogonal flexure, wherein, the intrados undergoes shortening and the extrados extension. As a result, across the bedding plane, the layer on the convex side slips towards the fold hinge relative to the concave side. As a result, each individual layer has Class-1B geometry and the group of competent layers also have Class-1B geometry. However, if the multilayer sequence is made of alternating layers of contrasting competencies, then on the whole, a Class-1C geometry develops. In such a case, the competent layers as a group deform by flexural slip, with each individual layer undergoing orthogonal flexure, as explained above, while the incompetent layers deform by flow-folding. Besides the above important fold mechanisms, progressive homogeneous flattening of folds normal to

3 JOINT PATTERN IN PRECAMBRIAN ROCKS AND FOLD MECHANISM 581 Fig. 2. Structural map of the study area around Galudih, Jharkhand state (India). The folded quartzite bands are shown with grey shades while the country rock is schist. Inset: ND New Delhi and K Kolkata. the axial surface can modify the fold geometry from Class- 1B to Class-1C. In an existing Class-1B fold, due to homogeneous flattening normal to the axial surface, the limbs rotate to a high angle from the shortening direction as a result of which they undergo thinning. Conversely, in the hinge part, the layer is parallel to the direction of shortening as a result of which the hinges undergo thickening. Because of such superimposition of homogenous flattening on earlier folds, the fold geometry changes from Class-1B to Class-1C, wherein the limbs are thinner than the hinges. As stated earlier in this paper, the study area around Galudih comprises alternating sequence of quartzites and schists. Thus, it is a case of multilayer sequence wherein there is a competency contrast between adjacent layers. Therefore, as discussed above, it can be envisaged that during folding each individual quartzite layer must have had a tendency to fold by orthogonal flexure and the group of quartzite layers would deform by flexural-slip folding. The interlayer slip between the quartzite and schist layer would be accommodated by deformation of the schist layer by flow folding. As a result, the entire quartzite/schist assemblage folded to give rise to Class-1C geometry, which may have been enhanced further by homogeneous

4 582 M.A. MAMTANI ET AL. N joints helps infer the fold mechanism in the rocks around Galudih. It could be argued that joints can also develop subsequent to folding due to gravitational effects/unloading. But the authors would like to argue here that in such a case, the gravitational force would be directed downwards and the direction of extension would Origin Fig. 3. Rose diagram for 312 joints recorded in the quartzites of the study area. Class interval for the data set is 5 o. NE-SW trending joints are the most dominant set. shortening. It is known that during orthogonal flexure, there can be a space problem in the hinge part of the folds because the extrados undergoes extension while the intrados compression. This room problem is accommodated by hinge parallel extension and during this process cross joints develop (Davis and Reynolds, 1996). As has been described earlier, the joints in Galudih are dominantly cross joints with their orientation being perpendicular to the hinge of the folds in quartzites (Fig. 5). Moreover, the plumose markings, wherever observed, are always on the NE SW trending cross joints; this trend is also the major trend of joints in the area. According to Hodgson (1961) and Pollard and Aydin (1988), plumose markings always develop during Mode-I (extension) mode of fracturing and are oriented perpendicular to the direction of least principal stress σ 3. Based on the above evidence, it is thus concluded that the NE SW trending joints developed in the quartzites are cross joints; these developed due to hinge parallel extension during flexural folding of the rocks and have formed perpendicular to σ 3 direction of stress. Thus, NE SW directed σ 1 resulted in development of folds with axial plane oriented in NW SE direction. This folding took place by flexural-slip and during the late stages of folding, there was hinge parallel extension in the σ 3 direction (NW SE) to give rise to the NE SW striking cross joints. Thus, the joint data generated from the rocks and the ornamentation observed on the NE SW trending Fig. 4. (a) Photograph documenting presence of plumose structure (demarcated by the box) on a joint surface in quartzite. (b) Line drawing of the plumose structure shown in (a). a b

5 JOINT PATTERN IN PRECAMBRIAN ROCKS AND FOLD MECHANISM 583 be perpendicular to the axial plane, i.e., NE-SW. As a result, the joints would be oriented in the axial planar direction (NW-SE) and would be categorized as release joints/longitudinal joints; this is clearly not the case in the quartzites of Galudih and the joints do not post-date folding. The dominant trend of joints is NE-SW and this is perpendicular to the hinge as well as axial plane of the folds. Therefore, on basis of the present study it is inferred that the joints around Galudih are related to the flexural slip folding mechanism. It is essential to highlight here that Hancock (1964) carried out a study on the Palaeozoic rocks of Pembrokeshire that were folded and faulted during the Armorican orogeny. He introduced the term Kathetal Joints for joints that are normal to the bedding surface when orientation of joint is a function of orientation of bedding. He showed that such joints cut across axial-tracefractures and other minor structures associated with formation of folds. However, the structural as well as geological setting of the rocks studied by Hancock (1964) was quite different from the present one and although he analysed the time relationship between jointing and folding, he made no attempt to correlate their origin with the folding/fold mechanism. In contrast to the work of Hancock (1964), the data shown in the present study provides an opportunity to try and analyse the joint orientation with folding mechanism. It is inferred that whilst the joints in Galudih formed during late stage of folding, their development was controlled by the same stress that caused the folding and this jointing is a direct result of the fold mechanism. Conclusions Based on the present study the following conclusions are arrived at: (1) The joints in the Precambrian quartzites of Galudih are dominantly NE-SW oriented. This is perpendicular to the orientation of the hinges of the folds, which have a NW-SE direction. As a result, the joints of Galudih are classified as cross joints. (2) Plumose structure, whenever observed, is always present on NE-SW oriented joint faces, which implies that these are extension joints. (3) It is envisaged that flexural-slip folding of the rocks resulted in hinge parallel extension and was responsible for development of NE-SW trending joints. Acknowledgments The authors are grateful to H.B. Srivastava and an anonymous reviewer for comments. Thanks are due to M. Jayananda for editorial assistance. Fig. 5. Schematic diagram highlighting the relationship of the dominant joint set (NE-SW joints) of Galudih area with the folds. Small arrows on the extrados and intrados represent extension and compression respectively. See text for detailed interpretation. HPE = Hinge Parallel Elongation. References Davis, G.H. and Reynolds, S.J. (1996) Structural geology of rocks and regions. John Wiley & Sons Inc., New York, 776p. Dunn, J.A. and Dey, A.K. (1942) The geology and petrology of eastern Singhbhum and surrounding areas. Geol. Sur. India, Mem. v. 69, pp Hancock, P.L. (1964) The relations between folds and late-formed joints in south Pembrokeshire. Geol. Mag. v. 101, pp Hancock, P.L. (1965) Axial-trace-fractures and deformed concretionary rods in South Pembrokeshire. Geol. Mag., v. 102, pp Hodgson, R.A. (1961) Classification of structures on joint surfaces. American Journal of Science, v. 259, pp Naha, K. (1965) Metamorphism in relation to stratigraphy, structure and movements in parts of east Singhbhum, eastern India. Qr. Jr. Geol. Min. Met. Soc. Ind., v. 37, pp Pollard, D.D and Aydin, A. (1988) Progress in understanding jointing over the past century. Geol. Soc. Amer., Bull. v. 100, pp Ramsay, J.G. (1967) Folding and fracturing of rocks. McGraw- Hill Book Company, New York, 560 p. Ramsay, J.G. and Huber, M.I. (1987) The techniques of modern structural geology - v. 2, Academic Press, London, 391p. Saha, A.K., (1994) Crustal Evolution of Singhbhum North Orissa, Eastern India. Geol. Soc. India, Mem. No. 27, 341p. Twiss, R.J. and Moores, E.M. (1992) Structural geology, W.H. Freeman & Company, New York, 532p.