mafic dykes around ramagiri schist belt

____________________ *e-mail: [email protected]

INDIAN DYKES: Geochemistry, Geophysics and Geochronology Editors: Rajesh K. Srivastava, Ch. Sivaji and N. V. Chalapathi Rao © 2008, Narosa Publishing House Pvt. Ltd., New Delhi, India

Mafic Dykes Around Ramagiri Schist Belt, East Dharwar Craton, South India: Possible Palaeo Stress Regimes T. YELLAPPA and T. R. K. CHETTY* National Geophysical Research Institute, Hyderabad 500 007, India Abstract Mafic dykes are episodic and widespread in many Precambrian cratons and represent important strain and time markers. They show a wide span of igneous intrusion events between 2.4 Ga to 0.650 Ga in the East Dharwar Craton, Southern India. Remote Sensing studies of Landsat, Aerial photograph interpretation and field observations, reveal that three important major dyke trends NW-SE, E-W, and NE-SW around Ramagiri schist belt. A set of parallel brittle to ductile NW-SE trending sinistral shear zones with in the basement gneissic complex and the schist belt reveal many tectonic events, which are closely associated with mafic intrusions. It has been inferred that the dyke emplacements which have been correlated with collision processes at the craton boundaries, sinistral shear zones, block rotation tectonics and basement fabrics. Keywords: Dharwar craton; Palaeostress; Shear zones; Dyke emplacement, Collision, Block rotation. Introduction Dharwar Craton in South India (Fig. 1) covers an area of about 40,000 sqkm with Archaean lithological units of 3.3 to 2.7 Ga age of Tonalite Trondhjemitic Granodiorite (TTG), 3.5 to 2.6 Ga age of volcano sedimentary greenstone belts, 2.7 to 2.6 Ga age of granodiorite plutons (juvenile magmatism), 2.5 Ga Closepet granite intrusions and Proterozoic mafic-ultramafic and sedimentary sequence. The craton is divided into two distinct blocks West Dharwar Craton (WDC) block and East Dharwar Craton (EDC) block by a north-south trending Closepet granitic intrusion or Chitradurga thrust-fault boundary (Drury and Holt 1980; Chadwick at al., 2000). This boundary coincides with the boundary between the 2600 Ma old low-pressure granite-rich terrain in the eastern block and 3000Ma old intermediate pressure gneiss dominated terrain in the western block (Ramakrishnan, 1994). The age of volcano sedimentary supracrustals, grades of metamorphism, magmatism, crustal thinning and temporal evolution are slightly different in WDC from those of the EDC. The craton is

Indian Dykes; Editors: Rajesh K. Srivastava, Ch. Sivaji and N.V. Chalapathi Rao

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bordered by a broad network of Proterozoic major ductile shear zones in the south as well as in the east marking the Precambrian fossil plate boundary (Leelanandam et al., 2006). The present study deals with the tectonic setting of different mafic dyke swarms in a small part of the EDC, particularly around Ramagiri schist belt; located slightly away from the SW margin of the Cuddapah Basin.

Figure 1: Geological map of southern India. Ra- Ramagiri schist belt, Ko- Kolar schist belt, Sa-

Sandur schist belt, Hu- Hutti schist belt, and Ch- Chitradurga schist belt. M-mylonitized zone on the eastern margin of the Chitradurga Schist Belt (After Chadwick et al., 1996). Inset shows the location of Dharwar Craton.

Mafic dyke swarms of different generations in different geological intervals all over the

world are well known in Precambrian terrains. The Dharwar craton has been well studied for its crustal evolution and regional distribution of mafic dykes by many workers (e.g. Murty et al., 1987; Murthy, 1995; Radhakrishna et al., 2007 and the references there in). Mafic Dyke swarms in the EDC are widespread and have intruded various lithologies such as gneisses, greenstone belts and granitoids. Three major dyke trends can be noticed that include, NW-SE; E-W; and NE-SW. In general, some of these dykes are quartz tholeiites that are common in E-W, NE-SW trending dykes, while more alkaline often with olivine are significantly seen in NW-SE trending dykes (Rao and Puffer, 1996). Mafic dykes of this region are predominantly doleritic to tholeiitic nature followed by older amphibolite dykes (Rao and Puffer, 1996). These dykes are melanocratic in colour, aphanites to sub-phaneritic in texture,

Palaeo Stress Regimes in Mafic Dykes Around Ramagirt: T. Yellappa and T.R.K. Chetty

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with grain sizes ranging from 0.2 to 1.55mm. Magnetite, plagioclase feldspar and pyroxenes are common minerals and are easily recognisable. Two dolerite types occur: one enriched, and other depleted in Fe, Ti, and and P. HFTP population is further characteriSed by lesser average values for MgO, CaO, Cr, Ni, Rb, Sr, Zr, Ba and Nd than LFPP.

Recognizing the importance of Precambrian dykes in recent years, many workers (e.g. Drury, 1984; Murthy, 1995; Murty, 1987; Chatterji and Bhattacharjee, 2001; Mallikarjuna Rao et al., 1995) have addressed several issues related to the study of dykes in the Precambrian Indian Peninsula, but the emplacement mechanism of these magma intrusives still remains as an enigma. Further, a review of the published literature on these dykes reveals that a large quantity of regional mapping, geochemistry and some geochronological data have become available over the last two decades. However, detailed structural framework of these dykes has been lacking. In the light of the above, a small region around Ramagiri schist belt, a south central part of East Dharwar Craton has been chosen for detailed mapping of dykes by making use of remote sensing data and aerial photographs. The study aims at understanding the spatial locations of dyke swarms and their directional properties which would provide important information on the controlling stress field at the time of their emplacement. Systematic studies of dykes are thus one of the best methods for explaining the paleostress field and its variation in space and time in the EDC. This study also focuses on the connectivity between the emplacement mechanisms of dykes and associated tectonics. Regional Tectonic Set Up Drury and Holt (1980) based on LANDSAT imagery brought out the structural framework of the EDC revealing the presence of a NW-SE trending major shear zones traversing the region. These shear zones have been recently described as sinistral transpressive shear zones (Chadwick et al., 2000; Chardon et al., 2006). The tectonic architecture compiled for the Eastern Dharwar Craton (EDC) involving remote sensing, field observations and published geological (quadrangle) maps reveals the presence of NW-SE trending Trans-Dharwar shear systems. These shear systems occur between the greenstone belts and plutonic complexes spaced at ~ 20km and are often interconnected by numerous relatively narrow sigmoidal shear belts, consistent with strike-slip deformation. A few N-S trending dextral shear zones are also common at some places. Imbricate shear zones and duplex structures are well evident in the region.

Based on the geometry and kinematics of NW-SE trending sinistral shear zones in the northern parts of the EDC (Chadwick et al., 2000), extended their late Archaean sinistral transpression model to the EDC and that they consider that the contact zone of the EDC as an arc formed against the WDC in a context of modern-like sinistral-oblique plate convergence. While the Closepet batholith was described as an intrusive along N-S trending dextral crustal-scale shear zone (Newton, 1990; Moyen et al., 2003; Jayananda and Mahabaleswar, 1991; Bouhallier et al., 1995) had noted the synkinematic character of the Closepet batholith with respect to the development of the regional conjugate shear zone pattern. The study area surrounding the Ramagiri schist belt has been extensively traversed by these shear zones (Fig. 2)


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Figure 2: Simplified tectonic map of Ramagiri schist belt and adjoining region derived from the

structural interpretation of Landsat data and quadrangle geological maps of GSI (1995; Geol. Surv. India, Geol. Quadrangle map of 57F, 140-150N/ 770-780E).

Geology of the Study Area The NNW-SSE trending linear Ramagiri-Penakacherla schist belt occurs in the south central part of EDC extending over a length of about 120km with width ranging from 0.25m upto 2.7km (Fig. 2). The schist belt comprises volcanoclastic metasedimentary rocks of amphibloite-chlorite sericite schists, trurbiditic metagreywackes/argillites, phyllites, siliceous shale, auriferous quartz veins, minor gabbroic intrusions and surrounding with banded ferruginous quartzites (BFQ). The schist belt has been divided in to three different blocks (Zachariah et al., 1997, Mishra and Rajamani, 1999), the eastern, western and central blocks. In the central part of the schist belt, late Archean juvenile magmatism is represented by granodiorite/grey granite plutons, which are intensely deformed with well developed quartz


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roddings. The western part comprises pillowed metabasalts with chlorite actinolite schist, BFQ with strong deformation contacts. Well foliated and significantly recrystalliSed amphibolites and minor amphibolites and BFQ are common in the eastern part. The schist belt is bordered by highly deformed and mylonitised granitic gneisses often extending over 100km long zones of high strain exposing brittle to ductile narrow shear zones. These shear zones occur in several parts of basement gneissic complex around the schist belt and are commonly associated with well developed S-C surfaces, pronounced stretching lineations etc. Other shear sense indicators like Z shaped folds, pinch and swell structures, augen feldspar, quartz and rotated asymmetric porphyroblasts are also well developed along the shear zones. Well developed felsic and quartz veins occur parallel to the mylonitic foliation with in the shear zone. The basement peninsular gneiss at the southern end of the schist belt is transformed in to phyllonite (Fig. 3A) at several places indicating high strain. In general the foliations in eastern part are trend NW-SE to N200E with steep dips (700-800) to the west.

The NW-SE striking foliation shows moderate to steep westerly dips. However, the central part of the schist belt exhibits NE-SW to ENE-WSW trending foliations with easterly dips. Lineations are poorly developed, which are characteriSed by elongate aggregates of hornblende, biotite and feldspars and are steeply plunging.

Figure 3: Field photographs showing (A) Phyllonites at the southern end of the schist belt near

Penukonda. (B) thin dyke, intruding the basement gneiss at Nagasamudram, east of the schist belt.


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Mapping of Dykes (a) Remote Sensing Remote Sensing images (downloaded from USGS site www.landsat.org) of Path-144 and Row-50 of Landsat ETM+ data, 2000’s (Enhanced Thematic Mapper) and Landsat TM data 1990’s (Thematic mapper) are used for the present study (Fig. 4). Visual interpretation has been carried out on 1:250,000 and 1:50,000 scales all around the Ramagiri schist belt (N140-N150 and E770-E780). This study has mainly focused on recognition of major lineament/faults/shear zones and major dyke systems. Some of the thin dykes (Fig. 3B) could not be recogniSed.

Figure 4: Landsat imagery (TM) of Ramgari-Penakachela schist belt and adjoining regions.

Remote sensing studies in conjunction with published information reveal the major geological units such as basement peninsular gneisses, Ramagiri schist belt in the central part, Proterozoic Cuddapah basin in the east and the 2.6 Ga N-S trending Closepet granites in


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the west (Fig. 2). The equivalents of Closepet granites are also distributed all over the margin in the form of isolated and elevated hill ranges. The other striking feature is the delineation of regional shear zones extending for hundreds of kms. They mostly occur at the contact zones between the schist belt and the adjacent peninsular gneisses. In general, the shear zones trend NW-SE with distinct sinistral displacements, while a few of them trend N-S and exhibit dextral kinematics. In the southern part of region nearly E-W trending dyke swarms across the schist belt is distinct, which are often either displaced by the shear zones or terminated abruptly at the shear zone boundary. There are other dykes exhibiting different orientations and strike lengths all over the region. Some of them often show conjugate pairs. The gneissic foliation in the basement is predominantly NW-SE with the eastward steep dips, while a few westward dips are also common mostly at the lithological contact zones. The dykes at the proximity of shear zones show deflection, rotation and drag effects. The dyke pattern has been further studied in detail in some of the selected parts through aerial photos as they are best suited for mapping of dyke pattern in a complex Precambrian cratonic region.

Figure 5: Aerial photographic interpretation of Ramagiri schist belt. (b) Aerial photographs The study of aerial photographs is limited to the central part of the Ramagiri schist belt and the surrounding region (see Fig. 2). We have used black and white aerial photographs on 1:30,000 scale in an area of 15x30 sq km (from N140 15’-14030’ to E770 15’-77045’) for


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detailed mapping of dykes (Fig. 5). A total of 110 aerial photographs have been interpreted. Mafic dykes of (thick and thin dykes) different directions have been recogniSed. Their orientation is measured along with lineaments that represent fractures and faults. However, published geological maps (GSI quadrangle) of the study area are also used here for general guidance.

Figure 6A: Dyke pattern, east of the Ramagiri schist belt.

Aerial photo interpretation clearly delineates the schist belt, younger granitoid intrusions, basement peninsular gneisses and the criss-cross pattern of dyke swarms. The details are presented in two drawings (Fig. 5A, B) with some of longitudinal overlap between the two. The schistose rocks belong to the central part of the Ramagiri schist belt representing highly NE-SW elongated features with central parts occupied by deformed migmatites and diorites. Relatively younger Archaean granitoid intrusives are also recorded. The structural trends are also interpreted from the surrounding gneisses in the western part of the region. They show predominant N-S trends, which are isoclinally folded with their axial-planes also trending conformably with the general trend of foliation. The foliations in the gneisses lie


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parallel to the schistosity of schistose rocks. In the eastern part, the foliation trends NE-SW throughout. While the foliations dip steeply to the east in the western part, they are steeply dipping westwards in the eastern part with an arm of the schist belt demarcating the two. This suggests that the schist belt occupies synclinal keels. However, the schist belt dips westward with moderate to steep values. Further, the schist belt also presents NE-SW trending possible mega strike-slip duplex structures bounded by shear zones.

Figure 6B: Dyke pattern, north of the Ramagiri schist belt (around Kanaganaplle/ Mutkapuram).

Dyke swarms in the study region are predominantly along E-W direction and are steeply

dipping or nearly vertical. They are very long, segmented and discontinuous and show more than ~ 3 kms spatial separation. These dykes often show gently sinuous shape for the whole intrusion. The next predominant direction of dykes is NW-SE to N-S. These are relatively shorter and are mostly deformed and deflected parallel to the shear zones. Some of the E-W dykes are abruptly terminated or displaced by NW-SE trending dykes or shear/fault zones. Another set of relatively well developed parallel thin dikes trend NE-SW. These are more closely spaced and their intensity of occurrence is more in the eastern part. These are distinctly discontinuous. Closely spaced fractures are also observed adjacent to the NE-SW dyke swarms and spacing of fracture sets also varies. The intensity of NE-SW dykes varies from place to place. There are several other small scale dykes trending in different directions. Based on the intensity of dyke occurrence, the area has been subdivided into three blocks, A, B and C for a detailed study (see Fig. 5). We have measured the strike directions of dykes and lineaments and plotted in rose diagrams separately for these three blocks i.e. East of Ramagiri schist belt (Block-A), around Kanganapalle/Mutkapuram village (Block-B) and west of Ramagiri schist belt (Block-C). The details are described below.


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Figure 6C: Dyke pattern, west of the Ramagiri schist belt. Block-A This block occurs at the SE sector of the region exhibiting the dominant nearly E-W trending dykes (Fig. 6A). The gneissic foliation shows predominantly NW-SE direction. The E-W trending dykes show discontinuous, often sinuous and branching features. These are intercepted by NW-SE and N-S trending dykes. Some of the E-W dykes are abruptly terminated at some of the inferred lineaments, which may represent shear zones. NW-SE and a few N-S trending dykes are also common. These dykes show thickness of 80-100m. The other prominent set of dykes, trending NE-SW direction, are straight, linear and thin (10-20m thick). However, these are relatively more closely spaced (~500mts). There are two sets of N-S trending and NE-SW trending lineaments. The N-S trending lineaments abruptly terminate some of the east-west trending dykes. It is possible to show the relative age relationships from the field data. But, the age data is not clear as there are different age groups of E-W trending dykes. We suggest that the longer ones could be older compared to the short ones, which are confined to lineament boundaries. Often, some of the NE-SW dykes are also displaced or terminated by the N-S/NW-SE trending lineaments.


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Figure 7: Rose diagram showing the orientation of dykes and structural lineaments in different blocks

around the schist belt:- (a) thick dykes, (b) thin dykes (c) lineaments, east of the schist belt; (d) thick dykes, (e) thin dykes, (f) lineaments around Knaganaplle/ Mutkapuram; and (g) thick dykes, (h) thin dykes (i) lineaments, west of the schist belt.

The rose diagrams (Fig. 7a, b, c) are drawn to present the dominant directions of thick

dykes, thin dykes and lineaments, which were interpreted from aerial photographs for the block A. The E-W to ENE-WSW dykes show predominant direction while NW-SE is subordinate (Fig. 7a). Thin dykes clearly show their intensity along NE-SW with out any deviations (Fig. 7b). Interestingly, the lineaments show two dominant directions: N-S and NE-SW, while the NW-SE is subordinate (Fig. 7c).

Block-B This block represents the north eastern corner of the study region. Two arms of the schist belt converge in the central part of the block. The gneissic foliation is N-S, while the general trend of the schist belt is NNE-SSW. The NE-SW trending thin dykes are restricted to the


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southern part. The E-W as well as NW-SE trending dykes are highly discontinuous, sinuous and disturbed (Fig. 6B). Branching of dykes is also a common feature. It is also interesting to note that the thick dykes are often becoming thin and contiguous. The intensity of the dyke distribution can be seen in two clusters in this block separated by a sliver of the schist belt.

In the rose diagram prepared for this block (Fig. 7d) shows thick dykes in three major directions: NE-SW, NW-SE and E-W in that order of importance. Thin dykes are seen only along NE-SW (Fig. 7e) while the lineaments are also observed along three major directions (Fig. 7f) similar to that of thick dykes. Block C This block represents NW part of the study area. The aerial photo interpretation reveals the occurrence of the western limb of the Ramagiri Schist belt. The gneissic foliations in the basement show N-S trends and are deformed and often dragged parallel to the schist belt boundary. The distribution of dykes is relatively less in this block. E-W trending dykes are predominant with a few N-S trending dykes and thin dykes are also relatively less (Fig. 6C). However, the NE-SW striking lineaments are predominant. Some NW-SE trending lineaments lie sub-parallel to the trend of the schist belt, commonly terminating some of the dykes of other orientations. Interestingly, some of the NW-SE trending dykes, in this part, are also thin in their width and are relatively more deformed. These dykes may belong to a different age compared to the dykes of the same orientation.

The rose drawing (Fig. 7g) for this block clearly shows thick dykes in two sets of major directions: E-W or ENE-WSW with another NW-SE direction. Thin dykes invariably show NE-SW direction (Fig. 7h). The lineaments show NNE-SSW dominant direction with a subordinate NE-SW direction (Fig. 7i). NW-SE trending lineaments are almost insignificant.

All the data described so far in the region indicate that there are three major groups of dykes striking E-W, NW-SE, and NE-SW. While the NW-SE and E-W directions represent thick dykes, the NE-SW trending dykes are almost invariably thin. There are also small and short dykes showing different morphologies including conjugate sets have been described within the shear zones. Many of the short dykes show many geometries but restricted to the presence of major shear zones and block boundaries (Chetty, 1995). Dykes of all orientations commonly display sheared and amphibolitised margins implying that these dykes were emplaced along or controlled by minor shear/fault zones (Drury, 1984).Horizontal alignment of amphiboles along NE-SW and NE-SW dykes suggest strike-slip movements and the presence of vertical linear fabrics are significant along the E-W dykes. Discussion There is no obvious correlation between the dykes and the lineaments and there are variations in their correspondence. It is well known that the area is transected by a series of NW-SE trending major sinistral shear zones and NW-SE trending dykes may be related and are possibly emplaced parallel to the shear zones. Some of the NE-SW lineaments expose dykes in some parts for small distances. However, the presence of shear zones/fault planes is scarce


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when compared to the number of dykes in that direction. It is also not clear whether or not the dykes and lineaments of the same direction were developed simultaneously or during different deformational events.

The available geochronological data (Table 1) indicate that the dyking episodes in the EDC span between 2400 Ma to 640 Ma (Radhakrishna and Joseph, 1996; Poornachadra Rao, 2005). Based on geochemical and geochronological data, three major episodes of dyke emplacement have been reported i.e. during 1900-1700Ma, 1400-1330Ma and 1200-1000Ma and the dominant emplacement being between 1330-1400Ma (Murty et al., 1987; Rao and Puffer, 1996). The E-W dykes cutting Ramagiri schist belt yielded Sm-Nd age of 2145±100Ma (Zachariah et al., 1995). Further, E-W trending dykes, south of Bangalore, yielded Rb-Sr age of 2370±230 (Ikramuddin and Stuber, 1976). The Sm-Nd data from three NW-SE trending mafic dykes situated at NW of the Cuddapah basin, yielded an age of 2173±64 Ma (Pandey et al., 1997). Some NE-SW trending granophyre dykes in the same area yielded an age of 646±24 Ma by K-Ar method (Dayal & Padmakumari, 1987). These age data described above imply that the mafic intrusive activity continued in the Dharwar craton up to the end of Proterozoic.

From cross cutting field relations together with whole rock K-Ar and paleo magnetic data at least four ages of dyke swarms have been proposed in different areas of the craton (Anjanappa, 1975; Gokhle and Waghmere, 1989; Radhakrishna and Joseph, 1996; Poornachandra Rao, 2005). On a more regional scale from aerial photos, aeromagnetic and field studies as many as five different episodes of dyke intrusions have been proposed around Cuddapah basin by Hargraves and Bhalla (1983). They have also suggested three different intensities of magnetiSation and polarities for several dykes at the western margin of the Cuddapah basin indicating three different episodes of dyke activity.

Variable distribution of dykes throughout the Dharwar craton and the differential intensities in their occurrence from place to place and the predominance of particular orientation in an area; all clearly suggest significant variations in Paleostress regimes during the protracted Proterozoic period. The field orientations and the available age data reveal that the stress variation is not only spatial but also temporal. Wide spread parallel dykes are generally considered to develop parallel to the contemporary maximum horizontal principal stress, a hypothesis established by both theoretical and experimental studies (Pollard, 1987). The dyke swarms, in general, show their trends parallel to the regional compressive stress direction, while their trends are perpendicular to the extension direction (e.g. Gudmundsson, 1995). The orientation and density of dyke swarms can be used to determine the nature of Paleostress field, defining the horizontal principal stress direction and relative magnitude (Hock and Seitz, 1995). However, the pattern of dyke swarms, orientation and the age constraints from the Dharwar craton, in general, and the study area in particular suggest contrasting tectonic scenarios and associated stress patterns responsible for the emplacement of dykes. Considering our results form the present study and the foregoing description of dykes regarding their geometry, distribution and associated tectonic features and their emplacement episodes in the above region, the following tectonic episodes and the associated responsive development of dykes in different evolutionary stages are envisaged and described below.


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Table 1: Geochronology of different dykes around the schist belt. Location Rock type Direction Method Age (Ma) Author

Bidai- Harahalli Dolerite& alkaline dyke - Rb-Sr 2370±240

&840±31 Ikramuddin and Stuber, 1976

Mahabubnagar Mafic dykes NW-SE NW-SE

Rb-Sr 2.5-2.4Ga 2.2-2.1Ga 2028±141

Pandey et al., 1997

Mahaboobnagar Dolerite dyke NW-SE Sm-Nd 2184±141 2184±232 Pandey et al., 1997

Mahabbob angar south of Gndumal Gabrroic dyke NW-SE Rb-Sr 2221±206

2301±172 Pandey et al., 1997

Mahaboob nagar Gabrroic dyke NW-SE Sr-Sr 1653±75 Pandey et al., 1997

Mahaboobnagar Dolerite Meta pyroxenite -

Sm-Nd Rb-Sr

1952±156 1474±176 2028±141 2221±206

Pandey et al., 1997

Around Mahaboobnagar Gabbro - Sm-Nd Pb-Pb

2356±482 2031±172 2085±252

Pandey et al., 1997

Central Karnataka Mafic dyke - Sm-Nd 1700 Drury, 1984

South of Bangalore Mafic dyke E-W Rb-Sr 2370±230 Ikramuddin and Huber, 1976

North of Penukonda Mafic dyke E-W Sm-Nd 2454±10 Zachariah et al., 1995

NW of Cuddapah Basin Mafic dyke E-W Sm-Nd 2173±64 Pandey et al., 1997

SE of Cuddapah basin Alkaline dykes N-S Rb-Sr 810±25 Ikramuddin and Stuber, 1976

Chelima Lmproites Lamproites - Ar-Ar 1401±4.6 -1417±8.2 Chalapti Rao et al., 1999

SW of Cuddapah basin Mafic dykes E-W Ar40-Ar39 1879±5 Chattarjee and Battacharjee, 2001

SW of Cuddapah basin Granophyre dyke NE-SW K-Ar 646±23 Dayal and Padmakumari, 1987

W of Dhone Basaltic Radial dykes NW-SE K-Ar 1454±56 Murthy et al., 1987

W of Dhone Basaltic Radial dykes NW-SE Ar40-Ar39 1489±5 Murthy et al., 1987

W of Dhone Basaltic dyke NE-SW K-Ar 1157±41 Murthy et al., 1987

N of Dhone Basaltic dyke NE-SW K-Ar 1335±49 Murty et al., 1987

South of Ramagiri Mafic dyke NW-SE Sm-Nd 2454±100 Zachariah et al., 1995

NW of Cuddapah basin Mafic dyke NW-SE Sm-Nd 2173±64 Pandey et al., 1997

Kolar schist belt Dolerite dyke N-S Ar40-K40 1636±27 Sarkar et al., 1995 Kolar schist belt, Nandidurg mines Dolerite dyke E-W Ar40-K40 1659±33 Sarkar et al., 1995

Mulbagal Dolerite dyke E-W Ar40-K40 1681±32 Sarkar et al., 1995 Kolar schist belt- Nandidurg mines Basaltic dyke NE-SW Ar40-K40

1652±26 Sarkar et al., 1995

Kolar schit belt Basaltic dyke E-W Ar40-K40

2086±37 Sarkar et al., 1995


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Stage 1: Collision Tectonics Around the Dharwar Craton The East Dharwar craton (EDC) has become rigid after the cratonisation at around 2.6 Ga giving rise to upper brittle crust and lower ductile crust. The NNW-SSE trending gneissosity in the craton suggest nearly west directed compression. Recent multi-disciplinary studies suggest the presence of Precambrian collisional boundary in the south along the Cauvery Shear Zone system (e.g. Rao and Prasad, 2006; Harinarayana et al., 2006; Singh et al., 2006; Chetty and Bhaskar Rao, 2006) and in the east along the Sileru Shear Zone (Chetty and Murthy, 1994; Leelanandam et al., 2006). The age of these collision zones is not clear and is debatable. Some opine it is late Archaean and for some, it is Neoproterozoic. The lithospheric warping and the east west trending continental flexure along N130 latitude must have developed during and after the collision from south. In that process, the upper crustal parts would have been subjected to N-S extension giving rise to crustal scale E-W trending wide extensional fractures into which the magma has been emplaced giving rise to extensive E-W dyke swarms. It is also evident from the decrease in the density of these dykes progressively northwards. It is presumed here that it could be late Archaean, most probably at around 2400Ma. We infer that E-W dykes are the resultant products that were developed as a north-south compression at lithospheric depths and upper crustal north-south extension. Some of the ages of these dykes are around 2.4 to 2.0Ga. A broad, pre-Cuddapah warping of regional Archean structural trends about an east-west axis could be a reflection of doming and north-south compression during the initiation of the precursor of the Cuddapah basin. It is also pertinent to take the cognizance of Paleo-Plate boundary and associated collision to the east of Cuddapah basin. Tectonic scenario appears to be very complex during this period. We also infer that during the same deformational episode, it is possible that some of the dykes trending NW-SE must have developed simultaneously taking advantage of the preexisting weak planes in the craton, north of the study region. Our inferences regarding the origin of the E-W trending dyke swarms in the southern part of the region is consistent with crustal up warp at the southern part of the Indian shield described by Drury (1984). Further it is not conformity with the suggestion that the extensive igneous activity around Cuddapah region in the form of dyke intrusion in the upper crust may be related to the basin evolution (Chatterji and Bhattacharji, 2001). Stage 2: Development of NW-SE and NE-SW Shear Zones in the Craton Major shear zones must have been developed during the post collision with large scale sinistral displacements. After this, there would have been a lull or quiescence for some time. Subsequent deformation, perhaps during 2.0-1.6Ga, was mostly accommodated by these shear zones in terms of sinistral transpression and continued deformation has given rise to NE-SW trending long, Riedel fractures which were mostly filled with magma, generating closely spaced thin dykes. The structural fabric of the craton was well defined by then with the generation of NW-SE and NE-SW trending shear zone s forming a mosaic of block structure. These NW-SE and NE-SW structural features in the form of dykes, shear/fault zones and or lineaments, often represent the block boundaries. The details about the


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geometry, kinematics and associated stress pattern related to block rotation tectonics have been provided in our earlier publication (Chetty, 1995).

Stage 3: Block Rotation Tectonics The mechanics of block rotation cause spatial and temporal heterogeneities resulting in the development of heterogeneous fault and fracture systems. Local stress variations will largely be responding to local kinematic constraints of block rotation and fault reactivation. The sinistral transpressional deformation in the region especially along the major NW-SE trending shear zones had direct bearing on the rotation of blocks and their internal deformation giving rise to short, small and a variety of geometrical forms of dykes, probably during 1.4 to 1.2 Ga. Most of these dykes that were formed during this process are restricted to the block boundaries with variable stress pattern inside the block. The observation such as termination of dykes at the block boundaries, attaining zig-zag fashions and curvature, sinuous shapes, small scale displacements, variation in thickness along the strike are probably indications of fracturing events with simultaneous development of dykes. These small dykes in association with other dykes of the same orientation creates complex picture in order to classify the dykes in terms of location and orientation. Therefore, it is possible to have different ages for the dykes of the same orientation and the same age for the dykes of different orientations. Dykes of the same age are not restricted to a particular field orientation; more than one period of dyke intrusions are seen in a particular field direction indicating the role of repeated reactivation tectonics in the region during the protracted Proterozoic period. Stage 4: Neoproterozoic tectonics This is further complicated by the Neoproterozoic deformation that has significantly affected the southern parts of the Indian shield. Imprints of this deformation are also evident in the development of dykes with ages around 600Ma. All the above features suggest that the tectonic forces between 2.4 and 0.6 Ga such as collisional tectonics at the margins of the EDC, sinistral transpressional tectonics along the major shear zones and the block rotation tectonics must have played significant role in the development of dykes and their emplacement mechanisms with in the EDC. It is pertinent to examine similar dyke swarms in other cratons such as western Canadian shield (Buchan and Halls, 1990), North China (Guiting Hou et al., 2006), which would be useful in the reconstruction of a mantle super plume position of super continental break up for the correlation of mafic dyke swarms different from Precambrian cratons of the world. Conclusions

1. Mapping of dykes through Remote sensing and aerial photographs has been proved to be a valuable tool in a Precambrian cratonic terrain.


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2. Intensity and clustering of dykes and field orientations of dykes clearly indicate variation in Palaeostress regimes,

3. Based on field relationships and the available geochronological data, four stages of development of dykes could be identified and associated tectonic processes, which were responsible for the generation of different dyke swarms. Dyke emplacement has been controlled by pre-existing lithospheric structures such as craton boundaries, shear zones, mobile belt orientation and other basement fabrics.

4. The dykes in the region are amply proved to be both spatial and time markers in the complex protracted Proterozoic deformational history of the East Dharwar Craton.

5. Our study is only an attempt to highlight some apparent trends and tectonic associations in order to set the stage for future researches. The present model of successive tectonic processes presents a deviation from the existing views and it does not represent a final synthesis on the emplacement mechanisms of dyke swarms of the Dharwar craton of the Indian shield.

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