STUDY OF ROCK DEFORMATION

IN A SHEAR ZONE AROUND

AJAPUR MOTA,

GUJARAT

Final Stage Dissertation Report

Submitted in partial fulfillment of the requirements for the degree of

Master of Science In

Applied Geology By

WAHENGBAM DHANACHANDRA

(03506012)

Under the guidance of

Prof. T.K. Biswal

DEPARTMENT OF EARTH SCIENCES

INDIAN INSTITUTE OF TECHNOLOGY, BOMBAY 2005

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CONTENTS Name Page no. List of Tables 2 List of Figures 2 ACKNOWLEDGEMENT 2 1. INTRODUCTION 3 1.1 Regional Geology of the Aravalli Mountains 3 1.2 Lithostratigraphy 4 1.3 Structure 6 2. STUDY AREA 6

2.1 Location and Accessibility 6 2.2 Climate 6 2.3 Physiography of the Study Area 7 2.4 Previous Work 7 2.6 Objective of the Study 7 2.7 Methodology 8

3. LITHOLOGY OF THE AREA 8 3.1 Rock types 8 4. STRUCTURAL STUDY OF THE SHEAR ZONE 11

4.1 Ductile Shear Zone 11 4.2 Field structures 12

4.2.1 Mylonitic foliation 12 4.2.2 Stretching lineation 13 4.2.3 Joint plane 13 4.2.4 Fault 13

4.3 Microscopic Features 13 4.3.1 S-C fabric 13 4.3.2 Asymmetric porphyroclast 14 4.3.3 Intragranular fault 15 4.3.4 Bookshelf gliding 15 4.3.5 Mica-fish 15 4.3.5 Asymmetric fold 15

4.4 Map interpretation 15 5. ANALYSIS OF SHEAR ZONE 16 5.1 Sense of shear 16

5.1.1 Stereonet Plotting 16 5.1.2 Asymmetric winged Porphyroclast 17 5.1.3 Intragranular fault 17 5.1.4 Result 17

5.2 Strain analysis 17 5.2.1 S-C Fabrics 17 5.2.2 Graphical scatters diagram 17 5.2.3 Observations 18 5.2.4 Result 20

CONCLUSION 20 REFERENCES 21

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List of Table: Page no. 1. Stratigraphic succession of Aravalli Mountains (After Gupta et al.,) 4 2. Showing S^C angle and corresponding shear strain. 18 3. Showing data from Sample no. B1. 19 4. Showing data from Sample no. A3. 19 5. Showing data from Sample no. C5. 20 6. Showing data from Sample no. D3. 21 7. Showing Mean and Standard Deviation (σ) Strain value. 21 List of Figures: 1. Geological map of the Aravalli region, after Gupta, et. al., 1980 3 2. Location map of the study area 7 3. Lithological map of the study area near Ajapur Mota 8 4. Microphotographs of Basic granulite (left) and deformed gabbro-norite rocks (right) 11 5. Structural map of the shear zone near Ajapur Mota 12 6. Types of imbricate fault 13 7. Microphotographs showing S-C fabrics (left) and σ-type porphyroclast(right) 14 8. Microphotograph showing Bookshelf gliding (left) and field photograph of mylonite (right) 16 9. Stereogram showing foliation plots 17 10. Stereogram showing lineation plots 17 11. Scatters diagram of sample B1 19 12. Scatters diagram of sample A3 20 13. Scatters diagram of sample C5 20 14. Scatters diagram of sample D3 21

ACKNOWLEDGEMENT First, I want to express my sincere gratitude and regards to my guide Prof. T. K. Biswal for his constant guidance, inspiration and encouragement through out the course of my dissertation. I would also like to convey my regards to Prof. H. S. Pandalai, Head of the Department, Earth Sciences, IIT-Bombay, for extending the best possible departmental facilities including the computer lab facilities for the completion of the project. I am also thankful to Dr. S. C. Patel for giving suggestions to improve the project during the first stage presentation. Thanks to Dalviji, Vengurlekarji and Ramuji on their valuable assistance in thin- section preparation and Truptidi for her valuable assistance in XRD report as well as Mr. Arijit Chattopadhyay for his help while taking microphotographs. I wish to convey my acknowledgement to Mr. Amit Srivastava, my senior and field partner for his assistance during the fieldwork and lab and I may not reach this level of work without him. I thank to Mr.Aloke Kumar Shah and Mr. Haris Ahuja for being a good support and help. I also sincerely acknowledge the sincere co-operation and help of the people of Birampur and Ajapur mota areas especially Jeep drivers during my fieldwork. I would like thank my seniors, viz. Mrs. Sunayana Sarkar and Mr. Yengkhom Kishorejit Singh for their valuable help including correction during the preparation of this report. I, last but not the least, am thankful to my classmates for their co-operation and encouragement during various stages of my work.

Sincerely,

WAHENGBAM DHANACHANDRA

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1. INTRODUCTION The broadest geomorphic divisions of India are the Peninsular India, the extra-peninsular India and the Indo-Gangetic Plain which are also the three broadest tectonic divisions of India.The Peninsular India comprises the Indian Shield and its Proterozoic and Phanerozoic covers. The extra-peninsula constitutes apart of the Alpine-Himalayan Tertiary mountain belt The indo-Gangetic Plain extends from the mouth of Indus river in the west to the great deltaic Sunderban in the east. The Indian Peninsula comprises four major cratons viz, Dharwar, Bhandara, Singbhum and Bundelkhand that are bordered by Aravalli-Delhi, Satpura Singbhum and Eastern Ghat mobile belts. The Aravalli mountain range is the northern crustal segment of the Indian peninsula trending northeast- southwest. 1.1 REGIONAL GEOLOGY The Aravalli Region (fig.1) forming the northwestern part of the Bundelkhand Craton represents a classic record of the Precambrian supracrustals. It is delineated by the boundary faults of the Himalayas in the North, the Cambay Graben in the southwest and the Son-Narmada lineament in the south and southeast. The northwestern crustal segment of the Indian peninsula is marked by the NE-SW trending Aravalli Mountains that covers an area of about 100,000 sq. km stretching over 700 kilometers through states of Delhi, Rajasthan and North-Eastern part of Gujarat. The terrain is considered to be very important geologically because of the development of complete succession of rocks ranging from Achaean (3.2Ga) to Neoproterozoic (0.8Ga). Further, it provides complete outcrop preservation and accessibility due to dessert climate. Therefore, knowledge on the various geological aspects of the terrain is comparatively clearer than other terrains of India. The Precambrian rocks of Rajasthan and NE Gujarat exhibit complex structural geometry due to overprinting of multiple tectonic events. Metamorphism of the sediments and intrusions of granitic, basic and ultrabasic rocks, which are generally associated with various deformational episodes, have been clearly worked out. Eleven successive deformational episodes, four in the Bhilwara, four in the Aravalli and three in the Delhi Geological Cycles, have been identified respectively. These supracrustals based on their tectonic setting, lithostratigraphy, deformational history, magmatic events, metamorphism and radiometric dating have been assigned to three geological cycles viz., the Bhilwara (> 2500 Ma), the Aravalli (2,500-2,000 Ma) and the Delhi (2000-700 Ma). The metasediments along with concordant and discordant intrusive and extrusive phases corresponding to these geological cycles have been designated as the Bhilwara, the Aravalli and the Delhi Supergroups. These are further sub-divided into groups and formations as per the Code of stratigraphic nomenclature of India and have been named after the prominent villages/towns or cultural features to facilitate their description.

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Fig.1. Geological Map of Arravalli Region (After Gupta et al., 1980) 1.2 LITHOSTRATIGRAPHY Bhilwara Supergroup: The basement rocks of Rajasthan are referred to as Banded Gneissic Complex (BGC) (Heron, 1953), where it is best exposed in the south of Mewar. These BGC essentially contains igneous rocks and multiple cycles of metasediments that form streaky and banded composite gneiss. Zircons derived from the basement gneiss yielded an age of 3.5Ga age (Venkoba rao et al., 1958) suggesting an Archaean age for the gneisses. This has been further supported by the age of Untala granites (2.9Ga) which are intrusives within the BGC (Chaudhary et al. 1981). The basement rocks occur in an arcuate terrain between the Aravalli hill ranges in the west and the Vindhyan plateau in the East. The BGC has the shape of a V pointed eastward. The Delhi Fold belt bound its northern limb. Foliated biotite and chlorite schist are the oldest members of the BGC (Heron. 1953).

Table 1. Revised Stratigraphy of Aravalli Region (After Gupta, 1980) _____________________________________________________________________________ Era/Period Geological Cycles Rank of Lithological Unit Lithology Upper cretaceous Deccan Traps Basalt to Paleocene Lower Paleozoic Marwar Supergroup Sandstone & shale, limestone Upper Proterozoic Vindhyan Supergroup Sandstones, shales, limestone Malani suite Rhyolites and granites Erinpura granite Middle Proterozoic (8 2 ▲ D S G Punagarh/Sindreth/Sirohi Group Pilow lava,slate 0 0 │ E U R Sendra-Ambaji granite Granite 0 0 │ L P O Godhra granite Granite to 0│ H E U Kumbhalgarh/Ajabgarh Group Phyllites limestones 0) │ I R P Calc-gneisses, calc-schists my ▼ Gogunda/Alwar Group Conglomerate,quartzites, grits -----------------------------------------Unconformity-------------------------------------------------------- Early Proterozoic (2 ▲ A S G Champaner Group Phyllites,quartzite 0 │ R U R Lunavada Group Phyllites,dolomite 0 │ A P O Jharol/dovda/Nathwara Group Marble,quartzite 0 │ V E U migmatite 2 │ A R P Bari Lake/Kankroli Group Meta-basic volcanic 5 │ L Hornblende schist 0 │ L Udaipur Group Phyllites,greywacke,gneiss 0)my ▼ I Debari Group Meta-arkose,dolomite -------------------------------------------Unconformity------------------------------------------------------- Archaean (> ▲ B S G Ranthambhar Group Slate,shale, quartzite 2 │ H U R Berach granite Granite,gneiss 5 │ I P O Rajpura-Dariba/Pur-banera Dolomitic marble.calc- 0 │ L E U Jahazpur/Sewar Group Schist,quartzite 0 │W R P Hindoli Group/Mangalwar Slate, shale,phillite ) │A /Sandmata complex Migmatite,gneiss,mafic m y │R Ultra mafic,acidic body ▼A

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Now Geological Survey of India has modified BCG in 1980. Part of BGC, Berach Granite and eastern tract of Aravalli were grouped as Bhilwara Super group. The Bhilwara Supergroup (Raja Rao et.al, 1967) includes the Banded Gneissic Complex (BGC) exposed in Mangalwar and Bandanwara areas, calcareous rocks of the Raialo now called as Jahazpur group as well as the Hindoli sequence (flysch-type) which was earlier included in the Aravalli System by Heron (1936) as eastern limb of Aravalli super group. Gupta et al.1980 postulated that the Hindoli sequence belonged to Archean Age. This was because the Berach granite was found intrusive in the eastern tract of Aravalli (Heron, 1953) now known as Hindoli Group; further, also it is overlain unconformably by Jahazpur group which belongs to Archean era (Gupta, 1965 and Sahai, 1966) so it should be in Bhilwara Group. The status of the Bhilwara Group has been enhanced to a status of a Supergroup comprising older Hindoli-Mangalwar-Sandmata assemblage and younger Rajpura-Dariba, Pur-Banera, Jahazpur and Sawar Groups followed by the youngest Ranthambhor Group representing the molasse sequence. The base of the Bhilwara Supergroup is not exposed and is believed to be concealed under the Vindhyans. The grade of metamorphism shows gradual increase from greenschist facies in the Hindoli Group in the east to granulite facies in the Sandmata complex, in the west. The Berach Granite, earlier considered to be the basement for the erstwhile Banded Gneissic Complex, is now regarded as late tectonic intrusive granite. Aravalli supergroup: The rocks of Aravallis are well-exposed from beyond Kankroli in the north to Champaner in the south, over a distance of 350 km; the width of the belt in the north is about 40 km, gradually fanning out to 150 km in the south. Aravalli rocks unconformably overlie the Bhilwara metasediments. This break in sedimentation has been interpreted to represent the Archaean-Proterozoic boundary (2500 Ma) in NW India. The various lithostratigraphic units of this Supergroup are classified on the basis of their lithology, structure and metamorphic history. This Supergroup is sub-divided into the Devari, Udaipur, Kankroli, Bari Lake, Dobda, Nathwara, Lunavada and Champaner Group in order of decreasing age. This supergroup shows regional metamorphism in greenschist facies with polyphase deformations associated with the synorogenic granitic intrusions. The intrusive granites within the Aravalli Supergroup, namely, the Udaipur, the Salumbar and the Darwal Granites and related bodies are interpreted to be syn- to late-orogenic intrusive exhibiting discordant relationship with the surrounding rocks. All these granites with the exception of the Darwal Granite were earlier considered as equivalent to the post-Delhi Erinpura Granite. Earlier Heron (1953) identified Raialo series which was intermediate in position between the Delhi System and Aravalli System. This series consists principally of a limestone about 2000 feet in thickness, with at the base a thin quartzite or sandstone, which is occasionally conglomeratic. But now calcareous rocks of the Raialo is called as Jahazpur group and as a result, Raialo series is no longer in the stratigraphic sequence of Aravalli mountain ranges. Delhi Supergroup: The rocks of the Delhi Supergroup shape the main Aravalli Mountain Range. These are exposed over a strike length of nearly 700 km from Himmatnagar in Gujarat in the SW having a width of about 150 km to Delhi in the NE with a width of 25 km. The Delhi Supergroup in the north rests over the older Bhilwara Supergroup with a marked unconformity whereas in the SW, a structural discordance is noted in relation to the underlying Aravalli rocks. Along the main Aravalli mountain range, the Delhi Supergroup is divisible into the Gogunda and the Kumbhalgarh Groups. The Gogunda Group is lithologically same as the Alwar Group in the NE Rajasthan which is dominantly arenaceous, while the Kumbhalgarh Group corresponds to the Ajabgarh Group of NE Rajasthan which is dominantly of calcareous facies. The Punagarh, the Sindreth and the Sirohi Groups are considered to represent the youngest members of the Delhi Supergroup. These groups occur to the west of the main Aravalli Mountain Range. The grade of the regional metamorphism varies from greenschist facies in the Gogunda Group in the east, to granulite facies in the Kumbhalgarh Group, in the west, associated with synorogenic granitic intrusions. The syntectonic granites (Sendra-Ambaji Granites) which were earlier grouped under the Erinpura Granite Suite are now distinguished from the latter (Gupta et.al 1980) and occur SW of the Kui-Chitraseni shear zone, which trends NE-SW. This shear zone separates from the Erinpura granite. The granites occurring in the western foothills of Sendra-Ambaji Granites of the Aravalli range are retained as Erinpura Granites, whereas granites occurring in the core of Delhi Supergroup are described as Sendra-Ambaji granites. Post Delhi orogenic Sequences: The Erinpura Granite is now interpreted as syn- to post-orogenic, with reference to the Delhi Orogeny. This granite suite occupies a wide area to the west of the main Aravalli Mountain Range. The Mt. Abu

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Granite massif which was earlier regarded as the Erinpura-type is now grouped with the Malani Volcanic Suite as its plutonic equivalent. The Malani Volcanic Suite covers wide areas in western Rajasthan and appears restricted to the west of the main Aravalli Mountain Range.The Vindhyan Supergroup is on the SE, Marwar Supergroup on the NW and the Deccan Traps on the extreme SE of the main Aravalli range. 1.3 STRUCTURE Plate tectonic theory has been applied to the Aravalli Mountain (Sinha Roy, 1984a). Most of the classification of the Aravalli Mountain is based on the Plate Tectonic theory. The Delhi and Aravalli Supergroups were believed to be deposited in separate basins formed in the Bhilwara Supergroup crust (Biswal et. al., 1988). These basins are bounded by Thrust faults which appear as lineaments on the map. The basins were closed by subduction and collision forming mobile belts. The Bhilwara Supergroup forms the craton. Part of the craton has been remobilized during Delhi and Aravalli orogeny. Aravalli orogeny is during Early Proterozoic and Delhi orogeny is during late Proterozoic. The rocks of the Aravalli Supergroup (Aravalli System of Heron 1953) overlie the basement gneiss with a significant unconformity. A time range of 2.5Ga to 1.8Ga is suggested for the Aravalli Fold belt (AFB), hence early Proterozoic in age. The AFB is a part of the hill range which stretches from North Gujarat to South Haryana across Rajasthan. It is surrounded by the basement rocks in the east and the Delhi fold belt in the west. It has an inverted V-shape and its apex is located near Nathdwara and the belt gradually widens southwards. The general orientation of the AFB axis takes a clockwise turn from NW -SE in south to ENE-WSW at Nathadwara (Sinha-Roy 1998). The Delhi Fold Belt is a narrow linear fold belt which fans out from the main orographic axis of the Aravalli Supergroup in the north, northeast and south directions (Sinha-Roy 1992). It occurs as several independent and fault bound basins. They show a fair degree of continuity towards the central region i.e. south of Ajmer. The rocks of the Delhi fold system are middle to upper Proterozoic in age. The Delhi Fold belt occurs in two belts namely, 1) The North Delhi Fold Belt (NDFB) 2) The South Delhi Fold Belt (SDFB). The NDFB is developed along three sub basins in Alwar, Bayana and Khetri areas while the SDFB is developed along the Aravalli hill range in central Rajasthan. In the south the two belts are separated around Ajmer and in the north at Khetri (Gupta et al. 1980). The Raialo, Alwar and the Ajabgarh groups form the NDFB and these do not extend beyond the Sambhar-Jaipur-Dausa wrench fault (Sinha-Roy 1998). The rocks of the SDFB are deposited in two sub-basins. The eastern sub basin contains rocks of the Rajgarh Group and that of the Bhim group. The western basin contains rocks of the Barotiya and the Sendra groups which contain basic and felsic volcanics. The contacts between the different sequences are defined by prominent ductile shear zones and thrusts (Gupta et al. 1980). 2. STUDY AREA 2.1 Location and Accessibility: The area of interest falls under Survey of India toposheet number 45D/11 covering around 19 sq. km. of Banas-Kantha (Palanpur) District of Gujarat (Fig.2). The area is well connected by a network of roads. The NH-8 is the main lifeline for road communication to this area. The area is served by the meter-gauge network of Western Railways. Udaipur and Baroda are the only civil airports in the area. The M.Sc. Dissertation project work comprises detailed structural study of a shear zone on the Delhi Supergroup near Ajapur mota village (72°34'E to 72°36'E: 24°16'N to 24°19'N). The important towns around this locality are Ambaji (24°21'N, 72°51'E) to the east and Palanpur (24°10'15’’N, 72°26'E) to the SW. Ambaji in fact is an important pilgrimage of Gujarat where Amba Mata temple is situated. Ajapur mota, a small village is 37km south-east from Ambaji and can be reached by jeep through undulating terrain. 2.2 Climate: The study area comes under tropical, warm climate zone and moreover it is in close proximity with the Thar Desert in north-west Rajasthan. As a result the area experiences a wide diurnal fluctuation in temperature. During daytime the temperature goes upto 50°C during summer. Even during winter the temperature remains nearly 30°C during daytime but at night it may fall to 10°C. The average annual rainfall is very less, about 5-10 cm.

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Fig.2. Location map of study area (Map not in scale). 2.3 Physiography of the study area: The area can be grouped in three domains namely (i) the Western Sandy Plains (ii) the Aravalli Hill Ranges and (iii) the Eastern Plains. These divisions are based on the existing relief-features and provide a basis for the study of geomorphic evolution of the terrain, which has been sculptured through number of erosional cycles represented by various surfaces. The region is characterised by mature topography with more or less flat-topped mountain ridges, escarpment, inselberg surfaces, river valley, intramontane valleys and vast stretches of plains. The whole area is undulating in nature and dominated by hilly terrains. The most interesting geomorphologic feature in this area is a hill composed solely of loose clayey and sandy sediments within the rocky hill ranges. 2.4 Previous Work: Middlemiss (1921), Hacket (1887) and many others have pioneered the work on Rajasthan geology. Studies by Coulson (1933), Gupta (1934), Gupta and Mukherjee (1938) and Heron (1917a & b, 1936, 1953) helped to established the basic framework of the Precambrian geology of Rajasthan. Heron (1938) included the metasedimentary rocks and igneous intrusions present in the study area, within in the Delhi System belonging to the Alwar and Ajabgarh series. Later these metasediments were included in Kumbhalgarh Group of the Delhi Supergroup by Gupta et.al (1980). They described the basic and ultrabasic rocks as ophiolites and the granites as Sendra-Ambaji granite. Biswal et.al (1998) worked on the shear zone under consideration for the present study and concluded that the area has a continental arc setting. 2.5 Objective of the study: The main objectives of the study are the following: 1. Structural study of the shear zone near Ajapur Mota. 2. Mapping of outcrops of the shear zone with the help of compass and Survey of India toposheet no. 45D/11 3. Preparation of detailed structural and lithological map of the study area. 4. Collection of field data of the shear zone and the surrounding area and collection of oriented samples along and across the strike of the shear zone. 5. Petrographic study of the rocks of the shear zone.

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6. Microstructure study of the mylonites from the shear zone.including Microfabric analysis and determination of sense of shear. 8. Calculation of shear strain. 2.6 Methodology The following methodology is being adopted in the present study: 1. Collection of literature of the study area to undertake regional geological set-up and to have a broad idea on the present state of the affairs. 2 Field work carried out with the help of enlarged toposheet No.45 D/11 in the scale of 1:10,000, a 30 meter measuring tape, a Clinometer compass, hammer, hand lens and other necessary equipments. 3. Preparation of lithological and geological mapping done at the field itself with the collected data, traversing mainly across the strike direction and later modified with the help of AutoCAD software with the help of existing data. 4. Structural features are noted with their attitudes with the help of clinometer compass and Systematic field measurements and sample collection done wherever variation is seen. 5. The nature of the sense of shear has been analyzed with the help of Stereonet plotting and Petrofabric analysis of L-sections in the shear zone. 3. LITHOLOGY 3.1 Rock Types of the Study Area The main rock types (Fig.3), exposed in and around the shear zone are A. Granite B. Gabbro-Norite-Basic Granulite Suite C. Mylonite

Fig. 3. Lithological map.

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A. Granite: Field structures: This is the main rock type in the study area. They were earlier classified as Erinpura granite by Heron (1938), later to be grouped under Sendra- Ambaji granite by Gupta et al. (1980). This Sendra-Ambaji granite is predominantly coarse grained, composed of quartz, alkali feldspar, and plagioclase feldspar with minor amount of biotite, thereby imparting a pinkish appearance, which can be identified through hand lens. The grain size varies from coarse to very coarse. It encompasses the whole area beyond the boundary of the shear zone. These rocks show no deformation. Spheroidal weathering is very common and large boulders of granite shows exfoliation features. Microscopic texture: Under Microscope, granite appears coarse-grained, inequigranular with hypidiomorphic to xenomorphic granular texture. The grains show interlocking grain boundary. The component minerals are microcline, plagioclase, biotite and quartz. Microcline showing perthitic texture with albite patches. Optical properties of minerals: 1. Microcline: sub-idioblastic, colourless grains with low relief. They show first order grey interference colour, parallel extinction and presence of cross-hatched twinning. 2. Plagioclase: prismatic grain, moderate relief, colourless with one set cleavage. Presence of lamellar twinning, first order grey interference colour. 3. Biotite: tabular, sub-idioblastic grains. They show moderate relief and presence of one- set perfect cleavage, showing pleochroism (yellow to brown). Interference colour is masked by body-colour, mottled extinction with respect to cleavage. 4. Quartz: low relief, colourless, first order grey interference colour and presence of wavy extinction. B. Gabbro-Norite-Basic Granulite suites: Field structures: The Gabbro-norite-basic granulite suite (fig.4) of rocks occurs as a host rock to granite and mylonite and is exposed in low relief area in the study area. It shows an intrusive contact with the overlying granite and mylonite. These suites occur as underlying basement to the granite and mylonite rocks with discordant relationship. It appears dark in color and is composed mainly of mafic minerals like pyroxene, plagioclase, amphiboles with little amount of quartz and K-feldspar, thereby imposing a dark appearance to the rock. The rock has a high colour index and appears dark in colour, medium to coarse grained, granular in texture. The rock mainly comprises pyroxene and plagioclase as seen in hand specimens. Some injection veins of very dark color and very fine grain size are found to be intruded into these rocks along prominent joint planes or fractures. Microscopic textures: Under the microscope, this suite shows wide variation. The samples that have been designated as gabbro, have plagioclase and diopside-augite as the principal constituents. Subophitic texture is common in these rocks. Subordinate amounts of K-feldspar (mainly microcline) and quartz are present. Secondary alteration has resulted in the formation of sericite. In some areas within the, Shear zone, this rock shows porphyritic texture with coarse grains of pyroxene and plagioclase; the latter grains are kinked and contain inclusions which are preferentially arranged. At places, the plagioclase adjacent to the pyroxene grains show kinking, suggesting that some sort of shearing motion has taken place. Another variation of this suite is designated as norite, due to its typical composition of hypersthene, subordinate augite and plagioclase. The texture is subhedral granular. Grain size is coarse. The hypersthenes show stronger pleochroism than other orthopyroxenes of identical composition. The hypersthene grains are generally pleochroic, pale greenish in colour. The margins are highly fractured and altered. Patches of fibrous talc-serpentine are present. Biotite flakes are seen. Some elongated subgrains of quartz are observed that are typical features of dynamic recrystallization. Recovery has lead to the subdivision of the quartz grain into elongate subgrains. The rare K- feldspar grains have perthitic texture. The basic granulites are characterized by abundant plagioclase and not so abundant olivine. This rock type is found in the contact of basic plutons. The grain size in the basic granulites present in this area is coarse with the plagioclase occurring as laths, the composition of which is typically bytownite. The rock shows inequigranular texture with interlocking grain boundaries. It shows a granulitic texture comprising orthopyroxenes, clinopyroxenes, hornblende, biotite, plagioclase and quartz. Biotite shows kinking and quartz shows patchy extinction due to deformation. Pyroxene containing inclusions of quartz and some plagioclase grains meet at triple point. Association of orthopyroxenes, clinopyroxenes, hornblende and biotite is shown. Serpentinization is also noted. In all the rocktypes mentioned above, serpentinization is common. This alteration necessarily involves the introduction of water and silica.

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These suites present in the area do not show prominent foliation because main components pyroxene, plagioclase and amphiboles are resistant to shearing. But it contains coarser linear feldspar grains which show some alignment as well as deformation features like kinking and fracturing indicating effect of shearing in the body. Optical properties of minerals: 1. Clinopyroxene: prismatic grain, moderately to high relief, colourless to pale green, two sets of cleavage, second order blue green interference colour and inclined extinction up to 38°. 2. Orthopyroxene: prismatic grain, high relief, colourless to green, two sets cleavage, first order grey/blue green interference colour and straight extinction. 3. Hornblende: large prismatic grain, two sets cleavage (56°&124°), showing pleochroism pale yellowish green to dark green, 2nd order yellow to green interference colour, inclined extinction maximum upto 30°. 4. Plagioclase: prismatic grain, moderate relief, colourless with one set cleavage. Presence of lamellar twinning, first order grey interference colour. 5. Biotite: tabular, sub-idioblastic grains. They show moderate relief and presence of one-set perfect cleavage, showing strong pleochroism (light brown to dark brown). Interference colour is masked by body-colour, mottled extinction with respect to cleavage. 6. Quartz: low relief, colourless, first order grey interference colour and wavy extinetion. As both kind of pyroxene i.e. orthopyroxene and clinopyroxene has been observed in thin section hence it may be inferred that these rocks are might be two pyroxene bearing granulite.

BASIC GRANULITE

Fig.4. Microphotographs of Basic granulite (left) and deformed gabbro-norite rocks(right).

C. Mylonite: Field Structure: Mylonite (8) is the main rock type of the shear zone. This rock type is mainly composed of quartz, K-feldspar, mica and the mineralogy is similar to the granite from which it has evolved. Large feldspar clasts of flesh pink color are found to float in glassy as well as quartzo-feldspathic and micaceos matrix and the rock is broadly designated as mylonite. Fragmentation and recrystallization of rocks take place extensively in this zone. Rocks are crushed into fine-grained and recrystallized to a flinty rock which often surrounds fragments of uncrushed country rock in the deformation zone of mylonite. The mylonitic foliations swerve around the augen shaped porphyroclasts floating in the finer recrystallized matrix and form the eye shaped structures (Fig.7). During extreme deformation all the fragments in the deformation zone are crushed to fine-grained and recrystallized to a form a finer-grain nature, banded rock known as ultramylonite. At places the mylonitic foliations are folded due to late stage deformation. In the present shear zone we observe large variation in the size of clasts present within the Mylonite. Variation of protomylonite {10%-50% matrix) to ultramylonite (>90% matrix) through mylonite (50%-90%matrix) is observed as we go from the wall toward the center of the shear zone. The overall trend of the mylonitic foliations of the area is N320◦, dipping 30◦ SW (Fig.9). a) Protomylonite: The dynamic recrystallization is less within these rocks with least protolith modification. Mylonitic foliations are very poorly developed here and defined by a differential preferred orientation of the polycrystalline quartz and quartzo-feldspathic aggregates within a matrix of predominantly phyllosilicates and quartz. Some quartz ribbons occur as detached entities and most of them are attached to the porphyroclasts. Dynamically recrystallized quartz grains comprising broadly polygonal subgrains, separated by serrated margin predominantly constitute the ribbons. Porphyroclasts

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are composed of polycrystalline aggregate of quartz and feldspars. Perthitic texture is found in some feldspars. Phyllosilicates are mainly biotite and some muscovite in sericite. b) Mylonite: These rocks are the product of a high strained rock than the protomylonites. The degree of recrystallization is more than that of protomylonites. Mylonites have 50%-90% matrix and 10% to 50% porphyroclasts. Quartz ribbons are very prominent within the, fine grain matrix. The mylonite layers are demarcated by the phyllosilicates like biotite. In some places warping of mylonitic foliations around porphyroclasts are also present. The quartz ribbons constitute about 30% of the each layer. In some places the ribbons shows pinch and swell structures. The longer dimension of the recrystallized grain within the ribbon is oriented at a constant angle to the ribbon boundary as well as mylonitic foliation. A distinctive pattern of subgrain formation occurs in ribbons within the region of homogeneous foliation. The porphyroclasts are composed of individual crystal or crystal fragments of feldspar or quartz. Along the periphery of the porphyroclasts there are recrystallized grains. The smaller grains are concentrated laterally, forming the pressure shadow zone. Tails of porphyroclasts are formed of polycrystalline aggregate of the recrystallized mantle material. But tails are not always present in all porphyroclasts. c) Ultramylonite: This kind of granite contains less than 10% porphyroclasts and is dominated by very fine grained matrix (>90%). The matrices are finely ground recrystallized quartz, and phyllosilicates. Degree of recrystallization is of the highest order here. The quartz ribbons are distinct and within the ribbons the recrystallized grains are distinctly elongated with the longer axis at low angles to the ribbon boundary. Along the boundary the feldspars are altered and converted to sericite. They also demarcate the mylonitic foliation. Microscopic Texture: Under microscope, mylonite rock shows eye shaped, coarse-grained feldspar floating in fine grained quartz and micaceous matrix. This is characterised by light and dark color bandings; darker bands are composed of micaceous minerals, lighter bands are composed of recrystallized quartz and are called quartz ribbons. Micaceous minerals, mainly biotite, are very fine grained and define the C-fabbric. Within the quartz ribbons, small recrystallized quartz sub-grains are aligned making around 30○ angle to C-fabric; this is the called S-fabric. They show deformation twinning and kinking. Remnant country rock feldspar grains are coarse and surrounded by micaceous and quartz matrix which forms wings or tails. According to the shape of the porphyroclast with matrix, “σ”, “θ” types are seen but “δ” type is rare in the study area. These features are used to identify the sense of shear. Optical properties of minerals: 1. Orthoclase: colourless grains with low relief, no pleochroism, low birefringence,usually untwined, first order grey interference colour. 2. Microcline: sub-idioblastic, colourless grains with low relief. They show first order grey interference colour, parallel extinction and presence of cross-hatched twinning. 3. Plagioclase: prismatic grain, moderate relief, colourless with one set cleavage. Presence of lamellar twinning, first order grey interference colour. 4 Biotite: tabular, sub-idioblastic grains. They show moderate relief and presence of one- set perfect cleavage, showing pleochroism (yellow to brown). Interference colour is masked by body-colour, mottled extinction with respect to cleavage. 5. Quartz: low relief, colourless, first order grey interference colour and presence of wavy extinction. 4. STRUCTURAL STUDY OF THE SHEAR ZONE 4.1 Ductile Shear Zone A ductile shear zone is a tabular, planar band of definable width in which there is considerably higher strain than in the surrounding rock. The total strain within a shear zone typically has a large component of simple shear, and as a consequence, rocks on one side of the zone are displaced relative to those on the other side. In its most ideal form, a shear zone is bounded by two parallel boundaries outside of which there is no strain. It is found in middle to lower crust and asthenosphere, mostly under metamorphic conditions. Ductile shear zones are common in basement terranes, that is high grade metamorphic rocks and are zones of weakness and represent localised strain softening. Strain softening is associated with the formation of mylonite. Shear zones generally have parallel sides. 4.1.1 Dynamics of mylonite development: Mylonite zones show high finite strain values compared to the wall, indicating the incompetency of the material in the mylonite zone, in comparison to the wall rock. However, mylonite zones develop within rocks where wall rocks and mylonitic rock have nearly same chemical and mineral compositions. The

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change in the rheology of material in a ductile shear zone after its nucleation and the effect is known as softening or strain softening. The mechanism contributing strain softening includes: 1. A decrease is grain size, which enhances activity of grain size- dependent deformation mechanisms such as diffusion creep and grain boundary sliding. This decrease in grain size is caused by the fact that the size of new grains formed by dynamic recrystallisation is a function of differential stress. 2. Grain boundary migration–recrystallisation which replaces hardened crystal by new, easily deformable crystals without dislocation tangles. Notice that subgrain rotation recrystallisation will not lead directly to softening since new grains have the same dislocation density as the old ones. 3. Growth of new minerals which are more easily deformable than the host rock (reaction softening). The replacement of feldspars by aggregates of white mica and quartz is an example. 4. Development of lattice preferred orientation of mineral grains which places them in a position for easy dislocation glide (geometry softening). 5. Enhanced pressure solution due to decrease in grain size and opening of voids and cracks. 6. ‘Hydrolithic’ weakening of minerals due to diffusion of water into the lattice.

Fig.5. Structural Map.

4.1.2 Field structures The characteristic macroscopic structures of mylonites are well exposed in the study area. Mylonitic foliations, stretching lineations, S-C fabric and asymmetric folds are the main macroscopic structures present here. The mylonites are replete with large K-feldspar porphyroclasts, showing rounded, sygmoidal and deltaic structures. They are mostly floating within a matrix which appears glassy in field. The percentage of clasts in mylonitic layers increases towards the boundary of the shear zone. As we proceed towards the central portion of the shear zone, number and size of the clasts are reduced, indicating increase in shearing. 4.1.2.1 Mylonitic Foliation: Foliation is defined as a structure which penetrates the whole rock body. A mylonite is a foliated and usually lineated rock that shows evidence for strong ductile deformation and normally contains fabric elements with monoclinic shape symmetry. Mylonites occur in high strain zones known as mylonite zones, interpreted as exhumed, ‘fossil’ ductile shear zones. The foliation plane in the study area is the S0 bedding plane defining C fabric (Fig.8).

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4.1.2.2 Stretching Lineation: The linear fabric element of mylonite is known as a linear shape fabric or stretching lineation. Large orthoclase feldspar grains are stretched giving rise to this linear feature in the rock. Stretching lineation is observed on the foliation plane itself. The lineations observed have an average plunge of 27°→140°. Lineations are very prominent towards the center of the shear zone. 4.1.2.3 Joint Plane: Within the shear zone, one conjugate set of joint planes are very prominent at most places. Orientations of the planes are N60°/58°NW and N50°/67°SE. First plane has developed orthogonal to the stretching lineations and the second plane dips in the same direction as the plunge of the lineation but at a higher angle. They might have formed due to extension. But the σ1 direction is vertical so it contradicts the present sense of shearing which has been inferred from stereonet analysis that σ1 is horizontal giving rise to strike slip fault. Hence they might be result of a late stage deformation. The main mesoscopic structures present in the area are as follows: 4.1.2.4 Fault: Faults are fractures along which there is visible offset by shear displacement parallel to the fracture surface. It is formed in the upper crust in relatively rapid strain rates where Temperature-Pressure is low i.e. low deviatoric Pressure and is characterized by breccia, fractures, faults, fault gouge etc. There are two parallel faults F’ and F” in the study area which trend N50○ and cut across the shear zone. Fault F” is one of the type of imbricate fault (Fig.6). Since these faults displace the shear zone, they are inferred to be younger than the shear zone (Fig.5).

Fig.6. showing the type of imbricate fault-leading imbricate fan fault (left) and trailing imbricate fan fault (right). 4.1.3 Microscopic Features: Microscopic structures are very important in determination of the nature of strain, shear sense and amount of throw. The microscopic structures are studied in detail on L and T sections of the mylonite samples collected from field. This study reveals the variation in microstructure in the shear zone as well as, variation in strain characteristics. It is found that with the progressive increase of distance from the shear zone wall there is variation in structure. Initially there were fractured grains of feldspar and recrystallized grains of quartz. Then there was strain concentration, which caused polygonization of grains in places. At the most interior part there are quartz ribbons showing recrystallization showing grain boundary migration or grain boundary rotation. This total process comes under dynamic recrystallization. The important microstructures found in the rocks are described below. 4.1.3.1 S-C Fabric: This is the major feature of the shear zone. In thin sections the mylonitic foliation or the C-fabric is demarcated by phyllosilicates like biotite. The quartz grains are flattened, to give a ribbon-like appearance. The recrystallized grains are oriented obliquely with C-plane. These are the products of dynamic recrystallization. Orientations of those recrystallized grains give the orientation of S-fabric. Here undulose extinction of the quartz grains, strain concentration as well as subgrain development can be seen. Undulose extinction is due to deformation in the crystal lattice during deformation. In these

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regions smaller recrystallized quartz show S fabric (Fig.7). From the S ^ C, left lateral movement of the shear zone is recognized.

Sample no.D1. Sample no.D1.

Fig.7. Microphotographs showing S-C fabrics (left) and σ-type porphyroclast(right)

4.1.3.2 Asymmetric porphyroclast: Porphyroclasts are the larger grains present within the mylonite. These grains are relict in nature grains within the recrystallized matrix and carry strain effects of shearing. Porphyroclasts are the residual parts of the protoliths. Porphyroclasts in mylonites are flanked by tapering grain aggregates, which form a structural unit with porphyroclasts. If such aggregates have the same mineral composition as the porphyroclasts then the smaller grain aggregates are called the mantle and combination of two is called mantled porphyroclast. If the flanking minerals are different from the porphyroclast then it is known as pressure shadow or pressure fringe. Mantled porphyroclasts are interpreted to be a consequence of crystal plastic deformation and storage of dislocation triangles in the rim of porphyroclasts in response to flow in the matrix. The mantled tails in the core and mantle structure then extend on both sides parallel to the shape fabric in the mylonite. So the mantled porphyroclasts in mylonite have an internal monoclinic shape of symmetry defined by tails of dynamically recrystallized minerals. From the thin section study the porphyroclasts are classified into three categories, i.e. "σ" type, "δ" type and “θ" type on the basis of their geometry. The "σ" type and “δ” type porphyroclasts are mantled porphyroclast having wings (Passchier and Simpson, 1986) and they are important in terms of determination of shear sense. Another type of porphyroclasts "θ" type has no mantle (Hooper and Hatcher, 1988). Thick tails develop when rotation of porphyroclasts is minimum, where as when the rotation increases the tails become thinner. The tails lie at different levels of elevation, referred to as Stair Stepping (Lister and Snoke, 1984). The geometry of these porphyroc1asts and tails are called porphyroc1ast system. This "θ" type variety is not so useful for determining shear sense. 1. "σ" type porphyroclast: "σ" type porphyroclast systems are most commonly developed in thick mantled porphyroclasts and at low strain conditions in protomylonites and mylonites. Polycrystalline aggregates of recrystallized material, similar in composition to the host porphyroclasts, form asymmetrical tails more or less parallel to mylonitic foliation. At the initial stage there occurs recrystallization at the peripheral part of porphyroclasts (Fig.7). With increase of shear strain the recrystallized grains become oriented in linear fashion. Then due to rotation of porphyroc1asts these asymmetric wings are developed. Due to tail asymmetry the tails "step up" in the asymmetric direction (Simpson and Schmid, 1983). If a symmetric axis is considered within the porphyroclast, the axis will be at an angle to mylonitic foliation and the acute angle will indicate the shear movement from the "step up" principle. 2. "δ" type porphyroclast: "δ" type porphyroc1ast systems develop in thin winged porphyroclasts at high shear strain condition. They are most commonly found in mylonite layers. Here fine tails of recrystallized material; extend into the mylonitic foliation with a distinctive asymmetry that can determine sense of shear. After formation of the "σ" type porphyroclast if there is continuous shearing then porphyroclast will rotate further. As a result the wings will be rotated and finally they become oriented at an angle to C-planc. Again from the position of the median line of the tail with reference to the shear plane, shear direction can be determined (Simpson and Schmid, 1983). "δ" type porphyroclasts are not so common in the field area.

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3. "θ" type porphyroclast: "θ" type porphyroclasts are those which have no visible tail but the mylonitic foliations are found to wrap around the porphyroclast producing asymmetrical perturbations. During coaxial deformation phase a paired set of perturbed zone develops but during non-coaxial deformation the matrix tends to pile up on opposite sides of the object. As a result a single asymmetrical zone develops. The apex of intersection between the perturbed zone and mylonitic cleavage, points in the shear direction (Hooper and Hatcher, 1988). 4.1.3.3 Intragranular fault: In thin section study of protomylonite and mylonite, the porphyroclasts of feldspar show fracturing. Orientations of fracture planes show the principal compression direction to be northwesterly. It is also found that fractures are mostly concentrated on the brittle grains like feldspar where as the ductile grains suffering same amount of stress show ductile deformations. Here recrystallizations of larger grains are taking place. Some inter granular fractures inside porphyroclasts show displacement of adjoining blocks due to rotation of the clasts during shearing. These are termed as intragranular faults. Antithetic kind of motion is noted. Antithetic motion is found in faults at high angle with C-plane. Some of the fracture zones show infilling by the same material and these newly crystalline grains show oblique orientation. The sense of motion indicates left lateral movement of the shear zone.

Sample no.C3.

Fig. 8. Microphotograph showing Bookshelf gliding (left) and field photograph of mylonite (right).

4.1.3.4 Bookshelf gliding: When broken crystals are subjected to shear then the individual parts of that crystal rotates in the direction of shearing. The feature developed due to this shear motion between the fragments is much like collapsing of books (Fig.8). The sense of movement of the fragments can be antithetic or synthetic and depends not only on the bulk shear sense, but also on the shape of the original porphyroclast, the kinematic vorticity number of flow and on the initial orientation of the microfaults, which may be partly controlled by crystallographic directions in the porphyroclasts. 4.1.4 Map interpretation: The shear zone near Ajapur mota is tabular in shape extending in a southeast-northwest direction along N320º strike. This shear zone is characterised by prominent mylonitic foliation and stretching lineations. The mylonitic foliation is dipping more or less vertical of about 65º-80º. Most of the foliations are dipping towards southwest but some of the data are opposite to it, i.e. northeast. This variation in dip direction of mylonite foliation is due to nonuniformity of stress distribution during deformation which is common in shear zones. So slight deflection might take place in highly dipping foliation from the normal orientation. The lineations are plunging in two opposite directions, namely towards southeast which is found in southeastern part of the shear zone and another one is plunging towards northwestern part of the area which is divided by Fault F” in the map. There are two parallel faults in this area striking N50º but direct evidence of fault is not seen. Nevertheless, geomorphology supports it as there is a long straight Nala across the hill terrain. The divergent plunging direction of lineation from the fault F” suggests the possibility of rotation of northwestern block about a horizontal axis of attitude 8ºNE-N50º. So, it is inferred that fault F” is ‘Imbricate fault’. This is only possible when compressive stress is applied. This explanation is valid because this shear zone is obducted to Kui-chitraseni Shear Zone where Phulad ophiolite is found. So, the resistance generated from this obduction might give rise to

15

compressive force. Fault F’ has no structural significance except that the boundary is shifted towards southwest while tracing the boundary of shear zone. For the shear zone, it is interpreted that the northeastern granitic block is uplifted and southwestern block has gone down based on the attitude of the stretching lineation as well as mylonitic foliation and sense of shear of this shear zone. For the fault F’, the northwestern block is uplifted relative to the southeastern block which is indicated by southwestern shift of boundary. For the fault F”, the relative movement is same as that of fault F’. About the geological succession, gabbro norite basic igneous intrusion took place, and then acidic granitic igneous intrusion emplaced within the gabbro-norite intrusive body with formation of amphibole by metasomatism. This is overlain by granite in this area as it is believed that granite is laterally expanded. Thereafter, the entire formation was deformed and as a result mylonite was formed from granite and basal contact between granite and gabbro-norite metamorphosed to granulite facies i.e, basic granulite which shows foliation. But basic rock is showing not so prominent feature because of its mineralogy. This is based on the thin section study and X-ray diffraction report. 5. ANALYSIS OF SHEAR ZONE 5.1 Sense of shear: 5.1.1 Stereonet Plotting: 68 Mylonitic foliations data are plotted on stereogram with contours of 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11% (fig.9) which shows a maximum concentration in the NE part of the stereogram but some are found to be in the SW region of the stereonet. This is because foliations planes are dipping at a very high angle so during deformation or shearing, deviation of the foliation plane might take place in some part of the study area.That is why some data are fallen in the SW region of the stereonet.

Fig. 9

61 Mylonitic lineations data are plotted on stereonet with contours of 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%,9%,10%,11%,12%,13%,14% (fig.10) which shows a maximum concentration in the SSE part of the stereogram. This indicates that majority of the lineations plunge towards SSE, i.e. nearly parallel to the strike of the mylonitic foliation. This indicates the shear zone to be a low oblique-slip to strike-slip one.

Fig. 10

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5.1.2 Asymmetric winged Porphyroclast: Asymmetric porphyroclasts have been observed in most thin sections. Following the stair stepping principle (Passchier and Simpson 1986), a left lateral strike slip motion of study area is found. 5.1.3 Intragranular fault: Intragranular faults are developed within the porphyroclasts along the fracture zone. Thrusting or synthetic displacement occurs for the low angle fractures; where as normal faults or antithetic displacements are found on high angle fractures. In the study area, antithetic displacement is common. In few places displacements are not found rather some fracture filling materials are found. The infilled materials are oriented obliquely in few cases. Orientations of these oblique grains show the antithetic motion by following the same principle as the movement of the block. This feature also indicates a left lateral strike slip motion of study area. 5.1.4 Result: The shear zone is left lateral strike to slightly oblique slip shear zone which is based on the stereonet plotting and microscopic features which indicate sense of shear. 5.2 Strain analysis: 5.2.1 S-C Fabrics: L-sections are useful for study of S-C fabric. The position of acute angle between the S-plane and the C-planes determine the sense of shear (Ramsay and Huber, 1987, ). Most of the thin section show excellent development of S-C fabric and show consistent anticlockwise rotation (Fig.7). Now to estimate the shear strain (γ) in the study area, γ is calculated by using the S-C angle (θ) which is measured from thin section. The relation is given (Ramsay and Graham, 1970) as, γ = 2cot 2Ө Angular displacement (ψ) is calculated from a relation given as, Ψ = Tan -1γ Table 2. Showing S^C angle and corresponding shear strain.

Sample no. from SE to NE

S^C Angle (θ in degree)

Shear Strain (γ) Angular Displacement (ψ in degree)

D1(a) 30 1.1547 49.11 D1(b) 28 1.3490 53.45 E2(a) 26 1.5626 57.38 A1 31 1.0634 46.76 A3 27 1.4531 55.46 B1 31.5 1.0191 45.54 E3 25 1.6782 59.21 D3 24 1.8008 60.95

5.2.2 Graphical Scatters diagram: Ramsay (1967, pp. 202-211) suggested a method for determining the shape and orientation of the finite-strain ellipse in two dimensions from an aggregate of deformed grains. The method depends upon certain assumptions about the undeformed state of the rock, viz: (1) That in the undeformed aggregate the grain shapes can be approximated to spherical or ellipsoidal shapes; (2) That the long axes of the undeformed grains were initially randomly oriented; (3) That the ductility contrast between the grains and the intergranular matrix (Gay, 1968) is zero; and (4) That the strain is homogeneous on the scale of the deformed specimen. When such an aggregate is subjected to homogeneous strain, then each grain will deform to a new grain shape and orientation in a manner determined by its original grain shape and orientation, and the orientation and shape of the imposed finite-strain ellipsoid. In a section of a deformed specimen a plot of the axial ratios of an aggregate of deformed grains (Rf) against the angle (Φ) which the long axes of the grains make with a specified direction, will yield a restricted field of points whose shape will be symmetric about the line representing the orientation of the long axis of the finite strain ellipse in that section; the shape of the field will be characteristic for the axial ratio of the finite-strain ellipse. Standard curves for Rf-Ф thus allow the axial ratio of the finite-strain ellipse to be read off from plots of Rf-Φ

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measured in deformed rocks. The relations are given below to calculate Rt (Strain after deformation) and Ri (Initial Strain), Rt = (Rf max.*Rf min.)½ ; Ri = (Rf max./ Rf min.)½ when Rt>Ri. 5.2.3 Observations: Table 3: Showing data from Sample no. B1.

Sample no. X-axis (mm) Y-axis (mm) Angle Φ in degree

Axial Ratio (Rf)

B1 0.174 0.380 0 2.184 0.300 0.208 80 1.442 0.300 0.180 25 1.667 0.272 0.160 -12 1.700 0.256 0.152 -47 1.684 0.218 0.120 -38 1.817 0.416 0.200 -5 2.080 0.640 0.294 -5 2.179 0.174 0.380 0 2.184

Scatters Diagram of sample B1

0

0.5

1

1.5

2

2.5

-100 -50 0 50 100

Angle of long axis to a reference line (C-plane)

Axial Ratio

Rfmax=2.184

Rf min=1.442

Fig.11. Representative Feldspar-grain scatters diagrams from sample no. B1.

From the above graph, Rf max=2.184, Rf min=1.442 Rt = 1.775 Ri =1.231 Table 4: Showing data from Sample no. A3.

Sample no. X-axis (mm) Y-axis (mm) Angle Φ in degree Axial Ratio (Rf) A3 0.218 0.128 0 1.703 0.242 0.172 -13 1.407 0.218 0.128 0 1.703 0.430 0.254 4 1.693 0.820 0.450 9 1.822 0.360 0.260 16 1.385 1.050 0.704 18 1.491 0.264 0.108 20 2.444 0.300 0.120 5 2.5 0.334 0.200 -32 1.67 0.218 0.134 -34 1.627 0.150 0.104 -15 1.442

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Scatters Diagram of sample A3

0

0.5

1

1.5

2

2.5

3

-40

-30

-20

-10

0 10

20

30

Angle of long axis to a reference line (C-plane)

Axial Ratio (R)

Rf max=2.5

Rf min=1.385

Fig.12. Representative Feldspar-grain scatters diagrams from sample no. A3.

From the above graph, Rf max=2.5, Rf min=1.385 Rt = 1.861 Ri =1.344 Table 5: Showing data from Sample no. C5.

Sample no. X-axis (mm) Y-axis (mm) Angle Φ in degree Axial Ratio (Rf) C5 0.340 0.180 0 1.889

0.772 0.380 -12 2.032 0.500 0.294 -41 1.701 2.100 1.252 -45 1.677 0.514 0.300 9 1.713 0.800 0.472 17 1.695 1.112 0.658 38 1.69 0.480 0.280 73 1.714 0.340 0.180 0 1.889

Scatters Diagram of sample C5

0

0.5

1

1.5

2

2.5

-60 -40 -20 0 20 40 60 80 Angle of long axis to a reference line (C-plane)

Axial RatioRf max=2.032

Rf min=1.677

Fig.13. Representative Feldspar-grain scatters diagrams from sample no. C5.

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From the above graph, Rf max=2.032, Rf min=1.677 Rt = 1.846 Ri =1.101 Table 6: Showing data from Sample no. D3.

From the following graph,

Sample no. X-axis (mm) Y-axis (mm) Angle Φ in degree Axial Ratio (Rf) D3 0.300 0.190 0 1.579

0.760 0.360 32 2.111 1.320 0.600 -6 2.2 0.580 0.260 -37 2.231 0.360 0.200 -10 1.8 0.300 0.190 0 1.579

Rf max=2.231, Rf min=1.579 Rt = 1.877 Ri =1.189

Scatters Diagram of Sample D3

0

0.5

1

1.5

2

2.5

-60 -40 -20 0 20

Angle of long axis with a reference line (C-plane)

Axial Ratio

40

Rf min=1.579

Rf max=2.231

Fig.14. Representative Feldspar-grain scatters diagrams from Sample no. D3. Table.7 Showing Mean and Standard Deviation (σ) Strain value.

Sample no.

Rt (Xi) Mean (Xm)

(Xi-Xm)2 σ=√∑(Xi-Xm)2/N

Ri (Xi) Mean (Xm)

(Xi-Xm)2 σ =√∑(Xi-Xm)2/N

B1 A3 C5 D3

1.775 1.861 1.846 1.877 1.83975

0.0041925 0.0004515 0.0000390 0.0013875

0.03896

1.231 1.344 1.101 1.189 1.21625

0.0002175 0.0163200 0.0132825 0.0007425 0.08741

5.2.4 Result: Based on the above two different analyses, it is shown that the shearing strain in this shear zone is more or less same throughout the area but varies in small proportion from one place to another across the shear zone. The mean and standard deviation of tectonic strain (Rt) of the shear zone are 1.83975 and 0.03896 respectively and of initial strain (Ri) of the shear zone are 1.21625 and 0.08741 respectively. It is therefore inferred that the shear zone is moderately deformed. CONCLUSION The Study area, ductile shear zone is a tabular, planar band of definable width in which there is considerably higher strain than in the surrounding rock. The total strain within a shear zone typically has a large component of simple shear, and as a consequence, rocks on one side of the zone are displaced relative to those on the other side. Ductile shear zone is found in middle to lower crust and

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asthenosphere, mostly under metamorphic conditions. Study area is lithologically composed of granite, gabrro-norite-basic granulite and mylonite (quartzo-feldspathic mylonite) where shear zone is extending NW-SE having more or less parallel boundary. Gabbro-norite basic igneous intrusion took place, and then acidic granitic igneous intrusion emplaced within Gabbro-norite intrusive body during late phase of Delhi orogeny and contact zone between these two intrusives was metamorphosed to granulite facies, i.e mafic granulite with emplacement of amphibole by metasomatism in this transition zone. These basement rocks are overlain by granite in this area as believed that granite is laterally extended. Then the entire formation is deformed and as a result mylonite was formed from granite in the shear zone but contact zone between granite and gabbronorite experienced deformation in small proportion which has shown in thin section but not so prominent that has been seen in mylonite because of its mineralogy that resists to shearing. This is based on the thin section study. This shearing took place as a subsidiary to the Kui-chitraseni shear zone deformation during Delhi orogeny. There are two faults cross cutting the shear zone which are parallel trending N50º. The direct field evidence is distinct but Geomorphology and shifting of shear zone boundary highly support it. These faults are very late phase deformation as it can only happen when all the formation is exhumed so that brittle deformation took place. The mylonitic foliation is steeply dipping towards southwest and the northeastern granitic block is uplifted and southwestern block is gone down based on the attitude of the stretching lineation as well as mylonitic foliation and sense of shear on this shear zone. For the fault F’, the northwestern block is uplifted relative to the southeastern block which is indicated by southwestern shift of boundary. For the fault F”, the relative movement is same as that of fault F’. The sense of shear zone is left lateral strike slip to slightly oblique in nature based on the analysis that has shown in previous chapters. The deformation that took place in this shear zone is moderate. REFERENCES: Biswal, T. K., (1988), Polyphase deformation in Delhi rocks, south-east Amirgarh, Banaskantha district,

Gujarat. In: A. B. Roy (Ed) Precambrian of the Aravalli Mountain. Mem. Geol. Soc. India, v. 7, pp.267 -277.

Biswal, T. K., Gyani, K.C., Parthasarathy, R. and Pant, D. R, (1998) Tectonic Implication of Geochemistry of Gabbro-Norite Basic Granulite Suite in The Proterozoic Delhi Supergroup, Rajasthan, India. J. Geol. Soc. India, v.52, pp. 721- 732.

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Gupta, B.C., (1934). The Geology of Central Mewar. Mem. Geol. Surv. Ind. V.65(2). Gupta, B.C. and Mukherjee, P.N. (1938). Geology of Gujarat and southern Rajputna. Rec. Geol. Surv.

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Bundi Dist.,Rajasthan. Rep. (unpublished) Geol. Surv. Ind.(fs 1964-65) Gupta, S.N., Arora, Y. K., Mathur, R. K., Iqbaludin, Balmiki Prasad, .Sahai, T. N. and Sharma, S. B.

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Hacket, C.A., (1977). Aravalli series in northeastern Rajputna. Rec. Geol. Surv. Ind. V.10(2). Heron, A. M. (1917a). Geology of northeastern Rajputna and adjacent Dists. Mem. Geol. Surv.

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203 Heron, A. M. (1936), Geology of southeastern Mewar, Rajputna.Mem. Geol. Surv. India, v. 68 (1). Heron, A. M. (1938), Physiography of Rajputna. Proc. 25th Indian sci. Congr. Presidential address, pt. 2,

pp.119-131. Heron, A. M., 1953. The geology of central Rajputna. Mem. Geol. Surv. Ind. Pp. 79. Hooper, R.J. and Hatcher, R.D. (1988). Mylonite from Towaliga fault zone, central Georgia: Products of

heterogeneous non-coaxial deformation. Tectonophysics, v.152,pp1-17. Lister, G.S. and Snoke, A.W. (1984). S-C mylonites, Jour. Struc.Geol.v.1, pp.283-297. Middlemiss, (1921). The Geology of Idar state. Mem. Geol. Surv. India. V.44(1), pp. 1-166.

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Passchier, C. W. and Simpson, C. (1986). Porphyroclast systems as kinematics indicatiors. Jour. Struc. Geol., v.8, pp 831-844.

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Bhilwara dist., Rajasthan. Rep. (unpublished) Geol. Surv. Ind. (F.S. 1965-66). Simpson, C. and Schmid, S.M. (1983). An evaluation of criteria to determine the sense of movement of

sheared rocks. Jour. Geol. Soc. Am.,v. 94, pp. 1281-1288. Sinha-Roy, S., (1984a). Precambrian crustal interaction in Rajasthan, NW India. In: Proc. Sem. On

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Sinha-Roy, S., (1984a). Precambrian folded unconformity in rajasthan, Curr. Sc., v. 53, pp. 1205-1207. Sinha-Roy, S., Guha, D.B. and Battacharya, A.K., (1992). Polymetamorphic granulite facies politic

gneiss of the Precambrian Sandmata Complex. Rajasthan. Indian Min., v.46(1), pp. 1-12. Sinha-Roy, S., Malhotra, G. and Mohanty, M., (1998). Geology of Rajasthan. Geol. Soc. India. pp. 278. Venkoba Rao, V.R., (1958), geological mapping in parts of Bhilwara dist., Rajasthan. Rep.

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