Chitradurga Shear Zone Research Articles

Dyke emplacement under mixed loading conditions: Insights from the Dharwar Craton, India

Dykes are intrusive igneous bodies that play crucial role in the supply and ascent of magma to the Earth’s crust. Magma can intrude along pre-existing anisotropies such as fractures or foliations present within the host rock or it may create its own path by fracturing the host rock. In the latter scenario, when fractures are formed by the pressure exerted by the invading magma, a dyke’s outcrop shape and geometry are diagnostic of the conditions under which it evolved. Here, we report mafic dykes emplaced within the younger granites of Dharwar Craton, peninsular India. Outcrop attributes of these dykes are characteristic of emplacement under conditions of mixed mode loading. We discuss different discrete modes of fracture formation and their possible combinations to understand the generation and eventual emplacement of dykes under mixed mode loading. This leads to the development of a comprehensive sequence of progressive dyke evolution under mixed mode I-III loading and thereby distinguishing incremental orders of dyke horn formation. We further apply this knowledge along with collected field evidence on dyke body geometries to propose an evolutionary model of dyke formation and emplacement within the Chitradurga granite under varying regional stress fields of the Chitradurga Schist Belt, Western Dharwar Craton, India. We infer, that the dykes initiated as extensional fractures within an earlier NE-SW directed compressive stress field and were subsequently sheared sinistrally by the effect of the adjacent Chitradurga Shear Zone on account of a later E-W to ESE-WNW directed compression.

Deep crustal structure and compositions for tectonic and geodynamic implications of the Dharwar Craton (Southern India) inferred from 3-C wide-angle seismic data

The 3-C deep seismic study with integrated interpretation of Bouguer gravity, heat flow, geological and geochronological studies along the Perur-Chikmagalur profile of the Dharwar Craton reveals important crustal elements with compositional distinctions of different geological structures such as horst and grabens , major shear zones like BSZ, CSZ, prominent batholiths as CG , granitic and gneissic complexes, presence of LVL, plutons , magmatic intrusives, thick underplating in WDC and significant Moho upwarping towards EDC due to plume head and komatiite magmatism . • Crustal V P and V S models are derived using 3-C seismic data of the Dharwar Craton. • Rock compositions are constrained by V P / V S , Poisson’s ratio, density and heat-flow. • Imaged a major suture zone CSZ dividing EDC and WDC blocks of the Dharwar Craton. • The LVL forms a detachment zone sandwiching the supracrustals and Archean gneisses. • Moho upwarping and underplating justify plume-arc settings for the Dharwar Craton. The Dharwar Craton of southern India is an important stable cratonic province of the world with complex geology and tectonic settings. Extensive studies provide insights of crustal velocity structure for the tectonic and geodynamic evolution of this Archean craton. This region has experienced several tectonically disturbed zones like Chitradurga Shear Zone (CSZ), Bababudan Shear Zone (BSZ) and Closepet Granites (CG). We have developed a comprehensive geologically plausible tectonic model using both P- and S-wave velocity structures to image major structural elements like shear zones and decipher the compositional distinctions of different rock assemblages of Western Dharwar Craton (WDC) and Eastern Dharwar Craton (EDC) part using 3-C wide-angle seismic data acquired along the 200-km long Perur-Chikmagalur deep seismic profile. The tectonic model show large compositional changes of subsurface rocks with anomalous high V P , V S , V P / V S , Poisson’s ratio ( σ ) and density ( ρ ) forming a major tectonic divide or suture zone called CSZ between EDC and WDC blocks. Significant crustal thinning (37–41 km) is observed due to Moho upwarping towards the Neo-Archean EDC block mainly composed of felsic granites and granodiorites. The WDC block show relatively thick crust (48–50 km) due to mafic underplating and mantle plume activity below CSZ forming Meso-Archean greenschist-facies-gneisses with dominant mafic/ultra-mafic compositions. Hence, crustal velocity, density, heat-flow, geology and geochronology studies support a plume-arc model with evidence of thick magmatic underplating of the lower-crust, complex subduction and development of highly strained shear zones like CSZ as suture juxtaposing EDC and WDC blocks.

Lithospheric architecture and geodynamics of the Archean Dharwar craton and surrounding terranes: New insights from satellite gravity investigation

The construction and destruction of continents, cratons and supercontinents on the globe involved multiple events of plume, rifting, subduction-accretion, and collision through the long history of evolution of our planet. Archean cratons preserve the records of continent building in the Early Earth and understanding their lithospheric architecture is fundamental to building geodynamic models. The Dharwar craton in southern India is among the major cratons on the globe. Recent geological and geochronological studies indicate that the craton formed through the assembly of a number of micro-blocks, although the terrane boundaries and lithospheric structure have not been well-defined. Here, we analyze satellite gravity data, covering the entire Dharwar craton and its surrounding regions, with a view to understand the regional lithospheric architecture and the geodynamic evolution. Our results on the residual gravity field, based on finite element approach, can clearly demarcate the boundaries of the major geotectonic units in the Dharwar craton and surrounding regions. We identify the presence of a conspicuous residual gravity low (−55 mGal) that runs north–south for about 700 km, paralleling the western margin of the Dharwar craton. This gravity low passes through the Chitradurga schist belt and covers the Koyna seismogenic region in the north to Coimbatore in the south. The gravity patterns observed over this region, might suggest a deep-seated paleo- rift zone, which we term as ‘Koyna-Coimbatore rift’. The prominent gravity low associated with this rift structure, can be explained by the presence of a 70 km thick low velocity/low density zone (LVZ) in subcrustal lithospheric mantle, suggesting asthenospheric flow into the upper mantle. The Western Dharwar Craton (WDC) might extend further north encompassing the Koyna-Warna seismic zone. Another major finding from this study is delineation of the Central Dharwar Cratonic (CDC) block between the Chitradurga shear zone and the Kolar greenstone belt, which is characterized by a prominent residual gravity high zone (+20 mGal), coinciding with the exposures of the Closepet granitoids and their equivalents, possibly marking a collisional suture between two microblocks. The 2½D residual gravity modeling across an E-W profile, which cuts all the major geotectonic features of WDC, CDC and Eastern Dharwar Craton (EDC), reveals popping-up of the CDC terrane, below which Moho has upwarped to a depth of 36 km and the lithosphere is considerably thinner at about 120 km, compared to 48 km and 175 km respectively in the WDC. This feature corresponds to lithospheric destruction of almost 100–120 km below the Dharwar craton, with magma underplating. On the eastern side of the craton, the positive residual gravity contours that correspond to the Eastern Ghats Belt rocks, take a sharp turn into the adjacent offshore region northeast of Ongole, and continue further south, until the granulite terrane north of Chennai, indicating the absence of the Eastern Ghats Belt rocks east of Cuddapah Basin. Our gravity model is able to demarcate three collisional boundaries between (i) WDC and CDC, (ii) CDC and Cuddapah Basin, and (iii) Nellore Schist Belt and adjoining east coast terrane. The extensive lithospheric destruction with removal of nearly 100 km of the cratonic root is correlated to prolonged subduction during Paleo-Neoproterozoic, together with mantle plumes at different times playing a subordinate role, thus indicating combined mechanical, thermal and chemical erosion.

Control of pre-existing fabric in fracture formation, reactivation and vein emplacement under variable fluid pressure conditions: an example from Archean greenstone belt, India

Abstract. Most of the upper crustal fluid flows are strongly influenced by the pre-existing fractures/foliations in the rocks under a certain state of tectonic stress and fluid pressure condition. In the present study, we analyzed a wide range of crosscutting fractures that are filled with quartz veins of variable orientations and thicknesses, from the gold-bearing massive metabasalts (supracrustals) of the Chitradurga Schist Belt adjacent to the Chitradurga Shear Zone (CSZ), Western Dharwar Craton, southern India. The study involves the following steps: (1) analyzing the internal magnetic fabric, using anisotropy of magnetic susceptibility (AMS) studies, and determining strength of the host metabasalts, (2) quantifying the fluid pressure condition through lower hemisphere equal area projection of pole to veins by determining the driving pressure ratio (R′), stress ratio (ϕ), and susceptibility to fracturing, and (3) deciphering the paleostress condition using fault-slip analysis. We interpret the NNW–SSE to NW–SE (mean 337/69∘ NE) oriented magnetic fabric in the rocks of the region as having developed during regional D1/D2 deformation on account of NE–SW shortening. However, D3 deformation manifested by NW–SE to E–W shortening led to the sinistral movement along CSZ. As a consequence of this sinistral shearing, fractures with prominent orientations formed riedel shear components, with CSZ as the shear boundary. Subsequently, all the pre-existing fabrics along with the riedel shear components were reactivated and vein emplacement took place through episodic fluid pressure fluctuation from high to low Pf at shallow depth (∼ 2.4 km). However, NNW–SSE orientations were prone to reactivate under both high- and low-Pf conditions, thereby attaining maximum vein thickness along these orientations. The deduced paleostress from fault-slip analysis along with the kinematics of the fractures and veins are in good agreement with previously estimated regional tectonics. Thus, integrating multiple domains of studies helps in the logical interpretation of fluid flow conditions and vein emplacement mechanisms in the study area that has not been ventured before.

Paleostress field reconstruction in the western Dharwar craton, south India: Evidence from brittle faults and associated structures of younger granites

We present new fault slip data to decipher the tectonic stress regime in younger granites (2.61 Ga) of western Dharwar craton (WDC), south India. The study is based on paleostress reconstruction of the terrain using fault-slip data from the small scale oblique slip normal faults of Chitradurga granite. Paleostress analysis using Right Dihedron and Rotational Optimization methods reveal that these faults have developed under NNE-SSW directed extension. Similar results are also obtained when left-lateral and right-lateral oblique-slip normal faults are treated as separate entities. It is envisaged that all faults in Chitradurga granite have developed under a single tectonic event and are considered to be extensional shear fractures. We interpret that the oblique-slip normal faults in granite oriented along NNW-SSE to N-S (P shear), NW-SE (R shear), NE-SW (R′ shear), and WNW-ESE (T) are defined as the respective shear components of the regional riedel shear system. It developed due to sinistral movement along the Chitradurga Shear Zone (CSZ) during late D3 deformation at shallow crustal level. The fault planes act as primary or secondary shear planes, depending on their proximity to the CSZ and kinematically fit well with regional far-field compression. The present study addresses the formation of oblique slip normal faults in the Chitradurga granite and its implication in reconstructing the paleostress field in the Precambrian south Indian shield.

Evidence of Archaean metamorphism from the Yerrabali schist belt of Eastern Dharwar craton from EPMA dating of monazite

The Dharwar craton is divided in to the Western and Eastern Dharwar craton (WDC and EDC) by the N–S trending Chitradurga Shear zone. The EDC mainly consists of late Archean granites interspersed with N–S trending linear schist belts. The Yerrabali Schst belt (YSB) is located at the north-eastern corner of the EDC, near the Karimnagar granulite belt. The YSB is metamorphosed in middle-upper amphibolite facies and consists of Banded Magnetite Quartzite (BMQ), ferruginous quartzite, ultramafics, quartz–cordierite–gedrite ± garnet bearing quartzite, dolomite, amphibolite, deformed meta-pyroxenite, tremolite schist, sillimanite–garnet–cordierite schist, and dolerite dykes. We have dated monazites in the sillimanite–garnet–cordierite schist, exposed at the SW part of the YSB by EPMA chemical method. So far no dates are available from the YSB. The monazite chemical ages with three distinct populations, viz., 2658 ± 42, 2817 ± 19 and 3097 ± 34 Ma, probably correspond to three distinct metamorphic phases. The peak metamorphism at around 2800 Ma corresponds with the garnet and cordierite growth. A temperature of 673°C and a pressure of 4.7 kb have been estimated for peak metamorphism.

Crustal and lithospheric mantle conductivity structure in the Dharwar craton, India

The vertical extension and structure of the sub-continental lithospheric mantle beneath the Archean Dharwar craton is the main attraction of the work presented here. To delineate the electrical conductivity structure of the Dharwar craton, a magnetotelluric study is carried out. This study comprises magnetotelluric data at 22 stations along a west-east slanting profile. Inter-station spacing is approximately 15 km. This magnetotelluric study is initiated from Dandeli (in the west) to Sindhanur (in the east side). The preferable geoelectric strike directions for the crust and lithospheric mantle are N3°E and N16°E respectively. A 2-dimensional (2-D) resistivity model derived by using the crustal and lithospheric mantle strike azimuths, identified conductive features in the stable continental Dharwar craton. In the crust, prominent conductors are present in the eastern and western part of the profile. A conducting feature is present in the deeper crust associated with the Chitradurga shear zone (CSZ). The study infers a thick lithosphere beneath Dharwar craton as a preserved cratonic nucleus on the eastern and a few conductive anomalies in the western part of the Dharwar craton. The model shows two separate conductors in the depth range of 110–250 km. This study shows, the possibility of presence of kimberlite melt in the western Dharwar craton in the depth range of 110–150 km.

Magnetotelluric investigation of lithospheric electrical structure beneath the Dharwar Craton in south India: Evidence for mantle suture and plume-continental interaction

Broad-band and long period magnetotelluric measurements made at 63 locations along ∼500 km long Chikmagalur-Kavali profile, that cut across the Dharwar craton (DC) and Eastern Ghat Mobile Belt (EGMB) in south India, is modelled to examine the lithosphere architecture of the cratonic domain and define tectonic boundaries. The 2-D resistivity model shows moderately conductive features that intersperse a highly resistive background of crystalline rocks and spatially connect to the exposed schist belts or granitic intrusions in the DC. These features are therefore interpreted as images of fossil pathways of the volcanic emplacements associated with the greenstone belt and granite suite formation exposed in the region. A near vertical conductive feature in the upper mantle under the Chitradurga Shear Zone represents the Archean suture between the western and eastern blocks of DC. Although thick (∼200 km) cratonic (highly resistive) lithosphere is preserved, significant part of the cratonic lithosphere below the western DC is modified due to plume-continental lithosphere interactions during the Cretaceous–Tertiary period. A west-verging moderately conductive feature imaged beneath EGMB lithosphere is interpreted as the remnant of the Proterozoic collision process between the Indian land mass and East Antarctica. Thin (∼120 km) lithosphere is seen below the EGMB, which form the exterior margin of the India shield subsequent to its separation from East Antarctica through rifting and opening of the Indian Ocean in the Cretaceous.

Lithospheric architecture in the Archaean Dharwar craton, India: A magnetotelluric model

The lithosphere beneath the Archaean Dharwar craton was investigated with an east–west oriented, 280 km long profile (from Yellapura to Sindhanur) with 22 magnetotelluric stations. Regional strike directions, estimated were −5° and 13° for the crust and the lithospheric mantle respectively. Our results indicate in western Dharwar craton, presence of low resistivity zones in the crust besides two significant upper mantle conductive features within the highly resistive Archaean lithosphere. We analyze the available geophysical data that include heat flow, seismic tomography and magnetotellurics (MT) from the Dharwar craton. Our inference supports to the existence of a thick lithosphere. A thickness of more than 200 km is estimated for the lithosphere beneath the Dharwar craton by our magnetotelluric model. The study has brought out the presence of lithospheric upper mantle conductive features in the depth range of 100–200 km bounded to the west part of the magnetotelluric profile. Significant variations in conductivity are seen on either side of the Chitradurga shear zone. The conductive feature in the depth range 120–150 km is related with kimberlite melts and the conductive nature in the depth range 160–200 km is explained by refertilization process, as craton passed over the Marion (ca. 90 Ma) hotspot.

Fractured micro-granitoid enclaves: A stress marker

Systematic, parallel, and steeply dipping tensile fractures within rounded to sub-rounded micro-granitoid enclaves are analyzed to decipher the stress condition at Chitradurga granite (Dharwar craton, south India). The study is performed in two steps: 1) Assessing the fracturing condition through thermo-mechanical model as fractures in granite may be related to thermal stresses during its cooling or to tectonics stresses. 2) Performing the plane strain mechanical solution for circular rigid inclusions to fractured enclaves, where rock mechanics concepts of fracturing are integrated with kinematic analysis. We interpret that the parallel, systematic fractures in the micro-granitoid enclaves developed due to the stress amplification inside enclaves when the host pluton was at shallow depth of ∼2.4 km. The derived paleostress orientation fits well with the orientation and sense of movement to the adjacent Chitradurga Shear Zone. The deduced paleostress conditions from fractured micro-granitoid enclaves are also in a good agreement with the previously revealed regional tectonics. We conclude that the fractured micro-granitoid enclaves are reliable stress indicator.

A magnetotelluric study from over Dharwar cratonic nucleus into Billigiri Rangan charnockitic massif, India

The electrical resistivity structure of the crust beneath the Dharwar craton in southern India was investigated by magnetotelluric method. In the present study, a northwest-southeast oriented 220 km long profile of 18 stations with a station spacing of ∼10–15 km is used. The profile extends from Dharwar cratonic nucleus in the north to Billigiri Rangan charnockitic massifs in the south. Time series data are processed to get the apparent resistivity and phase. The dominant geoelectric strike direction (75°) was calculated in a period range of 0.01–1000 s. The data are rotated to 75° strike direction. Two-dimensional inversion is carried out by using the non-linear conjugate gradient scheme for both apparent resistivity and phase. Our inversion results show a conductor in the northern side of the profile and two distinct prominent conductors in southern part of the profile. The mid-lower crust in southern part of the profile shows less resistive (<300 ohm-m) nature in the depth range of 20–50 km and is related with the Chitradurga shear zone and Billigiri Rangan charnockite massif. These zones were interpreted as CO2 flushed terranes. Regional-scale carbonation occurred in Late Archaean is associated with Chitradurga shear zone and in Late Proterozoic is associated with Salem-Attur shear zone. The CO2 rich fluids derived during that time might have exhausted in dehydration reactions. Later events such as the Indian plate passing over several hotspots and the metasomatized fluids associated with the Cretaceous-Tertiary magmatism in the region is the reason for observed low resistivity near Billigiri Rangan massif and surrounding regions in the south.

Evolution of fabric in Chitradurga granite (south India) - A study based on microstructure, anisotropy of magnetic susceptibility (AMS) and vorticity analysis

In this paper, the fabric in massive granite (~2.6Ga) from the Chitradurga region (Western Dharwar Craton, south India) is analyzed using microstructure, anisotropy of magnetic susceptibility (AMS) study and kinematic vorticity analysis. The microstructural investigation on the granite shows a progressive textural overprint from magmatic, through high-T to low-T solid-state deformation textures. The mean magnetic foliation in the rocks of the region is dominantly NW-SE striking which have developed during regional D1/D2 deformation on account of NE-SW shortening. The plunge of the magnetic lineation varies from NW to vertical to SE, and interpreted to be a consequence of regional D3 deformation on account of NW-SE to E-W shortening. The vorticity analysis from magnetic fabric in the region reveals that the NW-SE oriented fabric formed under pure shear condition during D1/D2 regional deformation. However, some parts of the region particularly close to the adjacent Chitradurga Shear Zone show that the magnetic fabrics are oblique to the foliation as well as shear zone orientation and inferred to be controlled by simple shearing during D3 regional deformation. The shape preferred orientation (SPO) analysis from oriented thin sections suggest that the shape of the recrystallized quartz grains define the magnetic fabric in Chitradurga granite and the degree of the SPO reduces away from the Chitradurga Shear Zone. It is interpreted that the change in magnetic fabrics in some parts of the granite in the region are dominantly controlled by the late stage sinistral shearing which occurred during the development of Chitradurga Shear Zone.

U-Pb SHRIMP Ages of Detrital Zircons from Hiriyur Formation in Chitradurga Greenstone Belt and its Implication to the Neoarchean Evolution of Dharwar Craton, South India

Abstract: We report newly obtained U-Pb SHRIMP ages of detrital zircons from metagreywackes in the Hiriyur Formation (Chitradurga Group, Dharwar Supergroup) from the central eastern part of the Chitradurga greenstone belt. U-Pb analyses yield three major Neoarchean age populations ranging from 2.70–2.54 Ga with some minor age population of Mesoarchean. The maximum age of deposition is constrained by the youngest detrital zircon population at 2546 Ma. This is the first report of the occurrence of supracrustal rocks less than 2.58 Ga in the central part of Chitradurga greenstone belt. Close evaluation of detrital ages with the published ages of surrounding igneous rocks suggest that the youngest detrital zircons might be derived from rocks of the Eastern Dharwar craton and the inferred docking of the western and eastern Dharwar cratons happened prior to the deposition of the Hiriyur Formation. The Chitradurga shear zone, dividing the Dharwar craton into western and eastern blocks, probably developed after the deposition. Furthermore, the lower intercept is interpreted as evidence for the Pan-African overprints in the study area.

SHRIMP U–Pb zircon ages of granitoids adjacent to Chitradurga shear zone, Dharwar craton, South India and its tectonic implications

SHRIMP U–Pb zircon ages of granitoids adjacent to Chitradurga shear zone, Dharwar craton, South India and its tectonic implications

Tectonic evolution of Chitradurga shear zone and the discovery of Pan-African orogenic imprints in Dharwar craton, southern India

Tectonic evolution of Chitradurga shear zone and the discovery of Pan-African orogenic imprints in Dharwar craton, southern India