Small-Scale Deformational Structures as Significant Shear-Sense Indicators: An example from Almora Crystalline Zone, Kumaun Lesser Himalaya

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1 e-journal Earth Science India, Vol. I (III), 2008, pp Small-Scale Deformational Structures as Significant Shear-Sense Indicators: An example from Almora Crystalline Zone, Kumaun Lesser Himalaya K. K. Agarwal and R. Bali Centre of Advanced Study in of Geology, University of Lucknow, Lucknow kamalagarwal73@hotmail.com Abstract Small-scale deformational structures e.g. folds, asymmetric boudins, thin veins in an en-echelon pattern, small-scale duplex, shear zones and shear bands are the most common structural elements used as reliable shear-sense indicators in crystalline rocks. Most of these structures are observed in the Almora Crystalline Zone (ACZ) in Kumaun Lesser Himalaya, which is disposed in the form of a huge asymmetric synform. The shear-sense observed on the northern and southern flanks is found to be top to north and top to south respectively. Introduction The study of small-scale structural elements in the recent years has drawn much attention especially to be used as reliable shear-sense indicators (Simpson and Schmid, 1983; Weijermars and Rondeel, 1984; Brunel, 1986; Ildefonse and Caron, 1987; Mawer, 1987; Choukroune et al., 1987; Agarwal, 1994; Passchier and Trouw, 1996; Bali and Agarwal, 1999) in deformed and tectonically transported rocks as thrust sheets. The Almora Crystalline Zone (ACZ) occurs as a thrust-sheet over the Precambrian-Lower Palaeozoic sedimentary sequences of Lesser Himalaya (Heim and Gansser, 1939; Gansser, 1964; Valdiya; 1980). The two tectonic contacts it has with the surrounding sedimentaries are the North Almora Thrust (NAT) and the South Almora Thrust (SAT), which dip to the south and to the north respectively (Fig. 1). The characteristic asymmetric nature of the ACZ and the two thrusts have been discussed by many earlier workers (Misra and Sharma, 1972; Mehdi et al., 1973; Vashi and Merh, 1976; Srikantia, 1988; Agarwal, 1994). The present paper is an attempt to understand the significance of the smallscale deformational features as reliable shear-sense indicators based on the field observations along a complete north-south traverse across the ACZ.

Small-Scale Deformational Structures as Significant Shear-Sense Indicators: An Example from Almora Crystalline Zone, Kumaun Lesser Himalaya: K. K. Agarwal and R. Bali Fig.

2 Small-Scale Deformational Structures as Significant Shear-Sense Indicators: An Example from Almora Crystalline Zone, Kumaun Lesser Himalaya: K. K. Agarwal and R. Bali Fig.1: Simplified sketch map of the Almora Crystalline Zone (ACZ), Kumaun Lesser Himalaya, showing the location of the ACZ in the map of India (inset) (based on Valdiya, 1980) Deformational structures as shear-sense indicator Several small-scale structures are found to be very good indicators of shearsense in the orogenic belts, as these not only provide the information about the direction of tectonic transport but also help to understand the later tectonic adjustments. In the ACZ, the dominant lithologies are slates, phyllites, various types of schists, gneisses and mylonites (Bhattacharya and Agarwal, 1985). The structural features that have been used as shear sense indicators in the ACZ are: folds, enechelon veins, boudins (also pinch and swell structures), shear band foliation, stretching lineation, small shear zones, small duplex and ramp and flat structures. The sense of shear is deciphered by determining first the direction along which the observation is to made, and the plane along which the shear-sense indicator is observed; this can be referred to as the sense-of-shear plane ( SOS plane, Davis and Reynolds, 1996). A brief description of the shear-sense indicators observed in the ACZ is as follows: Small-scale Folds: Asymmetric folds are observed at many places, which vary in size (ranging from few meters to a few cm in size). The fold vergence is used as shear-sense indicator (cf. Mawer, 1987) as these tend to have rotated towards the dominant lineation present in the rocks. The folds observed on the southern flank essentially verging to the SW to SSW. These folds are normally seen in thin quartzites

e-journal Earth Science India, Vol. I (III), pp. 119-124 http://www.earthscienceindia.info/ interbedded with schists and are mainly isoclinal close to tight folds.

3 e-journal Earth Science India, Vol. I (III), pp interbedded with schists and are mainly isoclinal close to tight folds. The folds found on the northern flank commonly show similar geometry but verging mainly to the NE direction. Boudins: These structures are commonly formed due to layer parallel extension or layer perpendicular shortening in the rocks with considerable difference in the viscosity across the layering. In the present area boudins and pinch and swell structures are found developed in the sequences with interbedded character of relatively soft (schists) and hard (quartzites) rocks. The orientations of the boudins at several locations (Fig. 2, 3D) reveal a top to south sense of displacement on the southern flank of the Almora synform. These have developed parallel to S 1, and the S 1 leans over the sense of shear (Davis and Reynolds, 1996). Shear zones: Small-scale shear zones which at places oriented both synthetic and antithetic, especially in the mylonites (Bali and Agarwal, 1999), are used as shearsense indicators (Weijermars and Rondeel, 1984; Mawer, 1987). Shear zones in the present area indicate again a top to south movement on the southern limb of the ACZ and on the northern limb give a top to the north movement (Fig. 3A). Fig.2:Diagrammatic sketch of the structural features as exposed on the southern limb of the ACZ (19 km south of Almora town, on the main highway). Note the development of boudins; the foliation is dipping to the north (to the right) Stretching lineations: Stretching lineations in deformed rocks are most often marked by the development of a new mineral (mostly sericite and chlorite in the present case) on the foliation (XY) plane with gentle plunge down the dip. Though these have not been suitably used to determine the shear sense but are good indicators of the tectonic transport direction in the orogenic areas (Brunel, 1986; Ildefonse and Caron, 1987; Daly, 1988, Jain and Anand, 1988; Agarwal, 1994; Passchier and Trouw, 1996).

4 Small-Scale Deformational Structures as Significant Shear-Sense Indicators: An Example from Almora Crystalline Zone, Kumaun Lesser Himalaya: K. K. Agarwal and R. Bali In the ACZ the stretching lineations are observed throughout the section and also studied on the foot-wall-rocks (the surrounding sedimentaries) in the north as well as in the south. Along the two tectonic contacts viz. NAT and SAT, the stretching lineations are oriented almost perpendicular to the traces of these thrusts, and plunge down the dip. The attitude of the stretching lineations in the southern side varies between NE-ENE to NW-NNW, whereas in the northern side it is SW (cf. Agarwal, 1994). Fig.3: Field photographs of few structural features. (A) Small-scale shear zones on the northern limb (near the NAT, on the Almora-Kausani highway), note the shear movement is top to the north (right side of the photograph); (B) Fine tension veins arranged in en echelon pattern, the shear movement is top to the N (to the left of the photograph); (C) Ramp flat structure within the small duplex on the southern limb, the shear-sense is top to south (to the left); (D) Boudins developed on the southern limb of the ACZ (location as in fig.2) En echelon veins: These veins result from the filling-up of the tension gashes or fractures (Davis and Reynolds, 1996), and are very reliably used as shear-sense indicators. These veins are observed at many places, and give top to north sense of shear consistently for the northern flank (Fig. 3 B). Duplex and ramp and flat structures: These help a great deal in interpreting the bulk shear displacement in the foldthrust belts. Duplex results from the progressive cutting of the foliations by later

5 e-journal Earth Science India, Vol. I (III), pp shear planes. Most often these planes show the development of slickenside striations and fibers. Duplex of varying dimensions (ranging from a few dcm. to a few meters) have been observed throughout the ACZ. The floor thrusts are exposed almost everywhere (Fig. 3 C), while the roof thrusts are rarely seen. The individual horses are not much thick (~ a meter), and are bounded between the s plays of the main basal thrust. Discussion and Conclusions A variety of small-scale deformational structures has been observed in the ACZ and is used as shear-sense indicators. The ACZ occurs as a large thrust sheet with its roots along the Main Central Thrust (MCT, Misra and Sharma, 1972; Mehdi et al., 1973; Vashi and Merh, 1976; Valdiya, 1980; Srikantia, 1988; Agarwal, 1994), and the major tectonic transport direction remains as N-S. The shear-sense as revealed by the field analysis of the structural features indicate that there is a distinct top to the south shear movement and top to the north shear movement on the southern and the northern flanks respectively of the Almora synform. This is due to the late stage tectonic adjustment of this crystalline thrust sheet after it was emplaced over the sedimentaries. There has been a long pending debate over the synformal character of these crystalline thrust sheets occurring in the Lesser Himalaya. The top to south and top to north shear sense observed on the two limbs supports the back thrusting model proposed earlier by Agarwal (1994). Acknowledgements: The authors are grateful to Prof. A. K. Jauhri, Head, Centre of Advanced Study in Geology, Lucknow University, Lucknow, for providing working facilities, and to Prof. A. R. Bhattacharya for fruitful discussions. Thanks are also due to the anonymous referee for suggesting corrections/modifications in the earlier version of the ms. References Agarwal, K. K. (1994) Tectonic Evolution of the Almora Crystalline Zone, Kumaun Lesser Himalaya: A reinterpretation. J. Geological Society of India, v. 43 (1), pp Bali, R. and Agarwal, K. K. (1999) Microstructures of Mylonites in the Almora Crystalline Zone, Kumaun Lesser Himalaya. Gondwana Research Group Memoir, 6, Bali, R. and Bhattacharya, A. R. (1988) Geological and Structural studies of the rocks of the Dwarahat-Chaukhutia area, Kumaun Lesser Himalaya with special reference to the North Almora Fault. Geoscience Journal, v. ix, pp Bhattacharya, A. R. and Agarwal, K. K. (1985) Mylonites from the Kumaun Lesser Himalaya. Neues Jahrbuch fur Mineralogie Abhandlungen, v. 152 (1), pp Brunel, M. (1986) Ductile thrusting in the Himalayas: Shear sense criteria and stretching lineations. Tectonics, v. 5, pp Choukroune, P., Gapais, D. and Merle, O. (1987) Shear criteria and structural symmetry. J. of Structural Geology, v. 9, pp Coward, M. P., Windley, B. F., Broughton, R., Luff, I., Petterson, M. U., Pudsey, C., Rex, D. and Khan, M. A. (1986) Collision Tectonics in NW Himalaya. In: Coward, M. P. & Ries, A. (eds.), Collision Tectonics. Geological Society of London, Special Publication, no. 19, pp Daly, M. C. (1988) Crustal shear zones in Central Asia: A kinematic approach to Proterozoic tectonics. Episodes, v. 11, pp. S-11. Davis, G. H. and Reynolds, S. J. (1996) Structural Geology of rocks and regions. 2 nd edition, John Wiley & Sons, Inc. New York, 776p. Gansser, A. (1964). Geology of the Himalayas. Interscience, London, 289p. Heim, A. and Gansser, A. (1939) Central Himalayas: Geological observations of the Swiss Expedition Mem. Soc. Helv. Sci. Nat., v. 73, pp

6 Small-Scale Deformational Structures as Significant Shear-Sense Indicators: An Example from Almora Crystalline Zone, Kumaun Lesser Himalaya: K. K. Agarwal and R. Bali Ildefonse, B. and Caron, J. M. (1987) The significance of stretching lineations in terms of progressive deformation and finite strain. Geodinamica acta, v. 1 (3), pp Jain, A. K. and Anand, A. (1988) Deformation and strain pattern of intra-continental collision ductile shear zone an example from the Higher Himalayas. J. Structural. Geol., v. 7, pp Mawer, C. K. (1987) Shear criteria in the Grenville Province, Ontario, Canada. J. Structural. Geol., v. 9, pp Mehdi, S. H., Kumar, G. and Prakash, G. (1973) Tectonic evolution of eastern Kumaun Himalaya; A new approach. Him. Geol., v. 2, pp Merh, S. S. and Vashi, N. M. (1976). The problem of the South Almora Thrust. Him. Geol., v. 6, pp Misra, R. C. and Sharma, R. P. (1972). Structure of Almora Crystalline, Lesser Kumaun Himalaya. An interpretation. Him. Geol., v. 2, pp Passchier, C. W. & Trouw, R. A. J. (1996). Microtectonics. Springer-Verlag, Heidelberg. 289p. Simpson, C. and Schmid, S. M. (1983) An evaluation of criteria to deduce the sense of movement in sheared rocks. Bulletin Geological Society of America, v. 94, Srikantia, S. V. (1988) Himalayan Thrusts and structural belts. J. Geological. Society of India, v. 31, pp Valdiya, K. S. (1980) Geology of the Kumaun Lesser Himalaya. Wadia Institute of Himalayan Geology, Dehradun, 291p. Weijermars, R. and Rondeel, H. E. (1984) Shear band foliation as an indicator of sense of shear: Field observations in central Spain. Geology, v. 12, pp

MESOSCOPIC STRUCTURES FROM THE AREA AROUND SATENGAL, LESSER GARHWAL HIMALAYA

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