In the eastern part of Ladakh, the right-lateral Karakoram Fault Zone (KFZ) bifurcates into two s... more In the eastern part of Ladakh, the right-lateral Karakoram Fault Zone (KFZ) bifurcates into two strands called the Pangong Strand and the Tangtse Strand. These two strands bound a region called the Pangong Transpression Zone (PTZ), which consists of migmatitic dioritic gneiss, calc-silicates and the Durbuk Pluton; a pluton of two-mica leucogranite. Outcrop scale observations suggest pervasive migration of leucogranitic melt through the existing tectonic structures of the gneiss. Magnetic fabric from both the tectonized and undeformed parts of the Durbuk Pluton show parallelism with the tectonic fabric of the host gneiss, which, along with pervasive melt migration, indicates syn-kinematic relationship between deformation along the KFZ, leucogranite magmatism and emplacement of the Durbuk Pluton. UePb geochronology of zircons from the dioritic gneiss yields a crystallization age of 63.6 1.5 Ma and also shows younger zircon growth down to w13 Ma, which suggest arc magmatism at w65 Ma followed by partial melting and leucogranite magmatism in the KFZ till w13 Ma. One two-mica leucogranite sample from the Durbuk Pluton gives a crystallization age of 22.7 0.5 Ma. As the Durbuk Pluton is syn-tectonic with deformation along KFZ, it is inferred that the KFZ initiated at least w23 Ma ago.
Since granites do not preserve easily mappable foliations, lineations, and strain markers, determ... more Since granites do not preserve easily mappable foliations, lineations, and strain markers, determining the degree of shape preferred orientation (SPO) in them is challenging. The aim of this paper is to present a methodology for determining variation in the degree of SPO in granites and to test the feasibility of correlating the same with regional strain. The case of the Godhra Granite located in the southern parts of Aravalli Mountain Belt (India) is taken as an example. Degree of SPO is determined using two different techniques (a) anisotropy of magnetic susceptibility (AMS) and (b) strength of mineral lineation determined by calculating the concentration parameter (k) of von Mises distribution by digital image analysis of biotite (kbi) and feldspar (kf) in thin sections prepared parallel to the magnetic foliation plane. SPO data obtained using the above two techniques from 20 samples distributed across the entire granite are analysed. Samples with a higher k also have a stronger magnetic fabric (magnetic lineation, L and degree of magnetic anisotropy P0). Also, the degree of SPO is greater for granites from the southern parts as compared to the northern parts. It is argued that this variation in degree of SPO cannot be attributed to rheological differences or strain resulting from difference in aspect ratios. Regional strain is inferred as the dominant factor. Since the emplacement of the Godhra Granite is synchronous with the tectonic rejuvenation along the Central Indian Tectonic Zone (CITZ) that lies to its south, the higher degree of SPO in the southern parts is attributed to its proximity to the CITZ. Whilst caution needs to be exercised to directly apply magnetic data as a measure of strain-intensity on a regional scale, it can be a useful guide to select samples for detailed SPO analysis using some alternative technique, especially for granites and other magmatic rocks that are devoid of mesoscopic strain markers.
Anisotropy of magnetic susceptibility data for a
ferromagnetic granite (Godhra Granite, NW
India)... more Anisotropy of magnetic susceptibility data for a ferromagnetic granite (Godhra Granite, NW India) are presented and it is shown that the degree of magnetic anisotropy (P9) is not controlled by the mean susceptibility (Km). Analyses carried out across a high-strain zone lying between granite and adjacent gneiss show that P9 values are highest in samples that lie close to the contact and decrease away from it. Based on these results it is concluded that if P9 is not controlled by Km, then the former can be used to gauge strain-intensity variations in ferromagnetic granites.
Magnetomineralogical, petrographic and whole-rock
geochemical studies on the Palaeo-proterozoic W... more Magnetomineralogical, petrographic and whole-rock geochemical studies on the Palaeo-proterozoic Wangtu Gneissic Complex (WGC) of the Himachal Lesser Himalaya have been carried out to understand the tectonic setting of the northern Indian continental margin during the Palaeo-proterozoic. Petrography and magnetomineralogy suggest that, although the WGC is dominantly composed of S-type/two-mica granitoids having low magnetic susceptibility (< 500 × 10–6 SI units), part of the complex consists of hornblende– magnetite and biotite–magnetite-bearing I-type granitoids having susceptibility greater than 500 × 10–6 SI units. Comparison of magnetic susceptibility with major element concentration reveals that the high susceptibility (> 500 × 10–6 SI units) granites are low in silica content and enriched in ferro-magnesian content. Tectonic discrimination based on trace element concentration shows that both I- and S-type granitoids of the WGC contain concentration of Y, Nb and Rb consistent with a collisional/volcanic arc set up. It is concluded that the North Indian continental margin had an active collisional set up during the Palaeo-proterozoic.
Field, microstructural and anisotropy of magnetic susceptibility (AMS) data from the
Palaeozoic M... more Field, microstructural and anisotropy of magnetic susceptibility (AMS) data from the Palaeozoic Mandi-Karsog pluton in the Lesser Himalayan region reveal a concordant relationship between fabric of the Proterozoic host rock and the granite. The pluton displays a prominent arcuate shape on the geological map. The margin-parallel mesoscopic and magnetic fabrics of the granite and warping of the host rock fabric around the pluton indicate that this regional curvature is either synchronous or pre-dates the emplacement of the granite body. Mesoscopic fabric, magnetic fabric and microstructures indicate that the northern part of the pluton preserves its pre-Himalayan magmatic fabric while the central and southern part shows tectonic fabric related to the Tertiary Himalayan orogeny. The presence of NW–SE-trending aplitic veins within the granite indicates a post-emplacement stretching in the NE–SW direction. Shear-sense indicators in the mylonites along the margin of the pluton suggest top-to-the-SW shearing related to the Himalayan orogeny. Based on these observations, it is envisaged that the extension that gave rise to this rift-related magmatism had a NE–SW trend, that is, normal to the trend of the aplite veins. Subsequently, during the Himalayan orogeny, compression occurred along this same NE–SW orientation. These findings imply that the regional curvature present in the Himachal Lesser Himalaya is in fact a pre-Himalayan feature and the pluton has formed by filling a major pre-Himalayan arcuate extension fracture.
The trans-Himalayan Ladakh batholith is a result
of arc magmatism caused by the northward subduct... more The trans-Himalayan Ladakh batholith is a result of arc magmatism caused by the northward subduction of the Tethyan oceanic lithosphere below the edge of the Eurasian plate. The batholith dominantly consists of calc-alkaline I-type granitoids which are ferromagnetic in nature with the presence of magnetite as the principal carrier of magnetic susceptibility. The mesoscopic and magnetic fabric are concordant and generally vary from WNW–ESE to ENE– WSW for different intrusions of ferromagnetic granites in different parts of the batholith. Strike of magnetic fabric is roughly parallel with the regional trend of the Ladakh batholith in the present study area and is orthogonal to the direction of India-Eurasia collision. In Khardungla and Changla section, the magnetic fabric is distributed in a sigmoidal manner. It is inferred that this sigmoidal pattern is caused by shearing due to transpression induced by oblique convergence between the two plates. U–Pb zircon geochronology of a rhyolite from the southern parts of the batholith gives a crystallization age of 71.7 ± 0.6 Ma, coeval with*68 Ma magmatism in the northern parts of the batholith. The central part of the batholith is characterized by S-type two-mica granites, which gives much younger age of magmatism at 35.5 ± 0.5 Ma. The magnetic fabric of these two-mica granites is at a high angle to the regional trend of the batholith. It is proposed that these two-mica granites were emplaced well after the cessation of subduction and arc magmatism, along fractures that developed perpendicular to the regional strike of the batholith due to shearing.
The present day South Tibetan Detachment
(STD) of Higher Himalaya is a system of low-angle normal... more The present day South Tibetan Detachment (STD) of Higher Himalaya is a system of low-angle normal faults. In the Himachal High Himalaya, the STD hanging wall is characterized by the presence of S-type per-aluminous Paleozoic (*475 Ma) granite called the Kinnaur Kailash Granite (KKG). This granite is later intruded by Cenozoic leucogranites (*18 Ma) in vicinity of the STD zone. In this work, microstructures, anisotropy of magnetic susceptibility (AMS), and U–Pb geochronology were carried out on the KKG and the leucogranites with an aim to (a) understand the conditions of fabric development and (b) decipher the tectonic relationship between deformation along the STD and the evolution of these granites. Microstructural features and magnetic anisotropy indicate that the granites are intensely deformed in vicinity of the STD and preserve their emplacement-related fabric in the interior parts. It is inferred that close to the STD zone, fabrics of both the KKG and the leucogranite are tectonic and are modified by the Cenozoic (*20 Ma) right-lateral slip and extensional tectonics. Magnetic fabric in the interior parts of the KKG is related to its emplacement indicating that original fabric was preserved. U–Pb geochronology of zircons from two samples of the KKG yields crystallization age of 477.6 ± 3.4 and 472 ± 4 Ma. The leucogranite gives a crystallization age of 18.5 ± 0.6 Ma. Zircons from the KKG also reveal signatures of a deformation event (20.6 ± 2.3 Ma) at its rim. It is inferred that deformation of the external rim of the KKG and crystallization of the leucogranites are synchronous and triggered by ductile deformation along the STD.
The present study demonstrates how the Paleo-Proterozoic Wangtu Gneissic Complex (WGC) of the
Les... more The present study demonstrates how the Paleo-Proterozoic Wangtu Gneissic Complex (WGC) of the Lesser Himalayan Crystalline sequence experienced superposed folding and doming prior to its exhuma- tion, with the help of integrated field, microstructural, magnetic fabric anisotropy and geochronological studies. The WGC forms the basement of the Lesser Himalaya and is bounded by Vaikrita Thrust (VT) to the northeast and Munsiari Thrust (MT) to the southwest. The regional structure consists of upright large scale early folds (D1) trending NW–SE. The mesoscopic fabric is related to axial plane foliation of the D1 folds and, to a lesser extent, late D2 folds. The axis of maximum compression for D1 and D2 folds are mutually orthogonal. The D1 folds have formed simultaneously with the major Himalayan thrusts whereas the D2 folds have developed during a later deformation event. The magnetic lineation at the hangingwall of the VT is sub-horizontal indicating stretching along the strike of the thrust. In the interior parts of the WGC, the magnetic fabric is of two types: (i) magnetic lineation demarks the intersection of mesoscopic and magnetic foliation indicating superposed deformation and (ii) scattered distribution of magnetic lineations due to D2 folding on initially curved and non-cylindrical D1 surface. 40 Ar–39 Ar dating of biotite from one site from the core of WGC gives an age of 9.3 ± 0.3 (2a) Ma. It is inferred that the dom- ing of the WGC took place at ∼9 Ma and, instead of large scale thrusting, it is characterized by superposed folding and strike-parallel stretching along the VT zone. It is suggested that the effect of superposed folding and ductile deformation of the Himalayan basement rocks has to be taken into account before cross-section balancing or any estimation of crustal shortening is attempted.
In the eastern part of Ladakh, the right-lateral Karakoram Fault Zone (KFZ) bifurcates into two s... more In the eastern part of Ladakh, the right-lateral Karakoram Fault Zone (KFZ) bifurcates into two strands called the Pangong Strand and the Tangtse Strand. These two strands bound a region called the Pangong Transpression Zone (PTZ), which consists of migmatitic dioritic gneiss, calc-silicates and the Durbuk Pluton; a pluton of two-mica leucogranite. Outcrop scale observations suggest pervasive migration of leucogranitic melt through the existing tectonic structures of the gneiss. Magnetic fabric from both the tectonized and undeformed parts of the Durbuk Pluton show parallelism with the tectonic fabric of the host gneiss, which, along with pervasive melt migration, indicates syn-kinematic relationship between deformation along the KFZ, leucogranite magmatism and emplacement of the Durbuk Pluton. UePb geochronology of zircons from the dioritic gneiss yields a crystallization age of 63.6 1.5 Ma and also shows younger zircon growth down to w13 Ma, which suggest arc magmatism at w65 Ma followed by partial melting and leucogranite magmatism in the KFZ till w13 Ma. One two-mica leucogranite sample from the Durbuk Pluton gives a crystallization age of 22.7 0.5 Ma. As the Durbuk Pluton is syn-tectonic with deformation along KFZ, it is inferred that the KFZ initiated at least w23 Ma ago.
Since granites do not preserve easily mappable foliations, lineations, and strain markers, determ... more Since granites do not preserve easily mappable foliations, lineations, and strain markers, determining the degree of shape preferred orientation (SPO) in them is challenging. The aim of this paper is to present a methodology for determining variation in the degree of SPO in granites and to test the feasibility of correlating the same with regional strain. The case of the Godhra Granite located in the southern parts of Aravalli Mountain Belt (India) is taken as an example. Degree of SPO is determined using two different techniques (a) anisotropy of magnetic susceptibility (AMS) and (b) strength of mineral lineation determined by calculating the concentration parameter (k) of von Mises distribution by digital image analysis of biotite (kbi) and feldspar (kf) in thin sections prepared parallel to the magnetic foliation plane. SPO data obtained using the above two techniques from 20 samples distributed across the entire granite are analysed. Samples with a higher k also have a stronger magnetic fabric (magnetic lineation, L and degree of magnetic anisotropy P0). Also, the degree of SPO is greater for granites from the southern parts as compared to the northern parts. It is argued that this variation in degree of SPO cannot be attributed to rheological differences or strain resulting from difference in aspect ratios. Regional strain is inferred as the dominant factor. Since the emplacement of the Godhra Granite is synchronous with the tectonic rejuvenation along the Central Indian Tectonic Zone (CITZ) that lies to its south, the higher degree of SPO in the southern parts is attributed to its proximity to the CITZ. Whilst caution needs to be exercised to directly apply magnetic data as a measure of strain-intensity on a regional scale, it can be a useful guide to select samples for detailed SPO analysis using some alternative technique, especially for granites and other magmatic rocks that are devoid of mesoscopic strain markers.
Anisotropy of magnetic susceptibility data for a
ferromagnetic granite (Godhra Granite, NW
India)... more Anisotropy of magnetic susceptibility data for a ferromagnetic granite (Godhra Granite, NW India) are presented and it is shown that the degree of magnetic anisotropy (P9) is not controlled by the mean susceptibility (Km). Analyses carried out across a high-strain zone lying between granite and adjacent gneiss show that P9 values are highest in samples that lie close to the contact and decrease away from it. Based on these results it is concluded that if P9 is not controlled by Km, then the former can be used to gauge strain-intensity variations in ferromagnetic granites.
Magnetomineralogical, petrographic and whole-rock
geochemical studies on the Palaeo-proterozoic W... more Magnetomineralogical, petrographic and whole-rock geochemical studies on the Palaeo-proterozoic Wangtu Gneissic Complex (WGC) of the Himachal Lesser Himalaya have been carried out to understand the tectonic setting of the northern Indian continental margin during the Palaeo-proterozoic. Petrography and magnetomineralogy suggest that, although the WGC is dominantly composed of S-type/two-mica granitoids having low magnetic susceptibility (< 500 × 10–6 SI units), part of the complex consists of hornblende– magnetite and biotite–magnetite-bearing I-type granitoids having susceptibility greater than 500 × 10–6 SI units. Comparison of magnetic susceptibility with major element concentration reveals that the high susceptibility (> 500 × 10–6 SI units) granites are low in silica content and enriched in ferro-magnesian content. Tectonic discrimination based on trace element concentration shows that both I- and S-type granitoids of the WGC contain concentration of Y, Nb and Rb consistent with a collisional/volcanic arc set up. It is concluded that the North Indian continental margin had an active collisional set up during the Palaeo-proterozoic.
Field, microstructural and anisotropy of magnetic susceptibility (AMS) data from the
Palaeozoic M... more Field, microstructural and anisotropy of magnetic susceptibility (AMS) data from the Palaeozoic Mandi-Karsog pluton in the Lesser Himalayan region reveal a concordant relationship between fabric of the Proterozoic host rock and the granite. The pluton displays a prominent arcuate shape on the geological map. The margin-parallel mesoscopic and magnetic fabrics of the granite and warping of the host rock fabric around the pluton indicate that this regional curvature is either synchronous or pre-dates the emplacement of the granite body. Mesoscopic fabric, magnetic fabric and microstructures indicate that the northern part of the pluton preserves its pre-Himalayan magmatic fabric while the central and southern part shows tectonic fabric related to the Tertiary Himalayan orogeny. The presence of NW–SE-trending aplitic veins within the granite indicates a post-emplacement stretching in the NE–SW direction. Shear-sense indicators in the mylonites along the margin of the pluton suggest top-to-the-SW shearing related to the Himalayan orogeny. Based on these observations, it is envisaged that the extension that gave rise to this rift-related magmatism had a NE–SW trend, that is, normal to the trend of the aplite veins. Subsequently, during the Himalayan orogeny, compression occurred along this same NE–SW orientation. These findings imply that the regional curvature present in the Himachal Lesser Himalaya is in fact a pre-Himalayan feature and the pluton has formed by filling a major pre-Himalayan arcuate extension fracture.
The trans-Himalayan Ladakh batholith is a result
of arc magmatism caused by the northward subduct... more The trans-Himalayan Ladakh batholith is a result of arc magmatism caused by the northward subduction of the Tethyan oceanic lithosphere below the edge of the Eurasian plate. The batholith dominantly consists of calc-alkaline I-type granitoids which are ferromagnetic in nature with the presence of magnetite as the principal carrier of magnetic susceptibility. The mesoscopic and magnetic fabric are concordant and generally vary from WNW–ESE to ENE– WSW for different intrusions of ferromagnetic granites in different parts of the batholith. Strike of magnetic fabric is roughly parallel with the regional trend of the Ladakh batholith in the present study area and is orthogonal to the direction of India-Eurasia collision. In Khardungla and Changla section, the magnetic fabric is distributed in a sigmoidal manner. It is inferred that this sigmoidal pattern is caused by shearing due to transpression induced by oblique convergence between the two plates. U–Pb zircon geochronology of a rhyolite from the southern parts of the batholith gives a crystallization age of 71.7 ± 0.6 Ma, coeval with*68 Ma magmatism in the northern parts of the batholith. The central part of the batholith is characterized by S-type two-mica granites, which gives much younger age of magmatism at 35.5 ± 0.5 Ma. The magnetic fabric of these two-mica granites is at a high angle to the regional trend of the batholith. It is proposed that these two-mica granites were emplaced well after the cessation of subduction and arc magmatism, along fractures that developed perpendicular to the regional strike of the batholith due to shearing.
The present day South Tibetan Detachment
(STD) of Higher Himalaya is a system of low-angle normal... more The present day South Tibetan Detachment (STD) of Higher Himalaya is a system of low-angle normal faults. In the Himachal High Himalaya, the STD hanging wall is characterized by the presence of S-type per-aluminous Paleozoic (*475 Ma) granite called the Kinnaur Kailash Granite (KKG). This granite is later intruded by Cenozoic leucogranites (*18 Ma) in vicinity of the STD zone. In this work, microstructures, anisotropy of magnetic susceptibility (AMS), and U–Pb geochronology were carried out on the KKG and the leucogranites with an aim to (a) understand the conditions of fabric development and (b) decipher the tectonic relationship between deformation along the STD and the evolution of these granites. Microstructural features and magnetic anisotropy indicate that the granites are intensely deformed in vicinity of the STD and preserve their emplacement-related fabric in the interior parts. It is inferred that close to the STD zone, fabrics of both the KKG and the leucogranite are tectonic and are modified by the Cenozoic (*20 Ma) right-lateral slip and extensional tectonics. Magnetic fabric in the interior parts of the KKG is related to its emplacement indicating that original fabric was preserved. U–Pb geochronology of zircons from two samples of the KKG yields crystallization age of 477.6 ± 3.4 and 472 ± 4 Ma. The leucogranite gives a crystallization age of 18.5 ± 0.6 Ma. Zircons from the KKG also reveal signatures of a deformation event (20.6 ± 2.3 Ma) at its rim. It is inferred that deformation of the external rim of the KKG and crystallization of the leucogranites are synchronous and triggered by ductile deformation along the STD.
The present study demonstrates how the Paleo-Proterozoic Wangtu Gneissic Complex (WGC) of the
Les... more The present study demonstrates how the Paleo-Proterozoic Wangtu Gneissic Complex (WGC) of the Lesser Himalayan Crystalline sequence experienced superposed folding and doming prior to its exhuma- tion, with the help of integrated field, microstructural, magnetic fabric anisotropy and geochronological studies. The WGC forms the basement of the Lesser Himalaya and is bounded by Vaikrita Thrust (VT) to the northeast and Munsiari Thrust (MT) to the southwest. The regional structure consists of upright large scale early folds (D1) trending NW–SE. The mesoscopic fabric is related to axial plane foliation of the D1 folds and, to a lesser extent, late D2 folds. The axis of maximum compression for D1 and D2 folds are mutually orthogonal. The D1 folds have formed simultaneously with the major Himalayan thrusts whereas the D2 folds have developed during a later deformation event. The magnetic lineation at the hangingwall of the VT is sub-horizontal indicating stretching along the strike of the thrust. In the interior parts of the WGC, the magnetic fabric is of two types: (i) magnetic lineation demarks the intersection of mesoscopic and magnetic foliation indicating superposed deformation and (ii) scattered distribution of magnetic lineations due to D2 folding on initially curved and non-cylindrical D1 surface. 40 Ar–39 Ar dating of biotite from one site from the core of WGC gives an age of 9.3 ± 0.3 (2a) Ma. It is inferred that the dom- ing of the WGC took place at ∼9 Ma and, instead of large scale thrusting, it is characterized by superposed folding and strike-parallel stretching along the VT zone. It is suggested that the effect of superposed folding and ductile deformation of the Himalayan basement rocks has to be taken into account before cross-section balancing or any estimation of crustal shortening is attempted.
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called the Pangong Strand and the Tangtse Strand. These two strands bound a region called the Pangong
Transpression Zone (PTZ), which consists of migmatitic dioritic gneiss, calc-silicates and the Durbuk
Pluton; a pluton of two-mica leucogranite. Outcrop scale observations suggest pervasive migration of
leucogranitic melt through the existing tectonic structures of the gneiss. Magnetic fabric from both the
tectonized and undeformed parts of the Durbuk Pluton show parallelism with the tectonic fabric of the
host gneiss, which, along with pervasive melt migration, indicates syn-kinematic relationship between
deformation along the KFZ, leucogranite magmatism and emplacement of the Durbuk Pluton. UePb
geochronology of zircons from the dioritic gneiss yields a crystallization age of 63.6 1.5 Ma and also
shows younger zircon growth down to w13 Ma, which suggest arc magmatism at w65 Ma followed by
partial melting and leucogranite magmatism in the KFZ till w13 Ma. One two-mica leucogranite sample
from the Durbuk Pluton gives a crystallization age of 22.7 0.5 Ma. As the Durbuk Pluton is syn-tectonic
with deformation along KFZ, it is inferred that the KFZ initiated at least w23 Ma ago.
(SPO) in them is challenging. The aim of this paper is to present a methodology for determining variation in the degree of SPO in granites and to
test the feasibility of correlating the same with regional strain. The case of the Godhra Granite located in the southern parts of Aravalli Mountain
Belt (India) is taken as an example. Degree of SPO is determined using two different techniques (a) anisotropy of magnetic susceptibility (AMS)
and (b) strength of mineral lineation determined by calculating the concentration parameter (k) of von Mises distribution by digital image analysis
of biotite (kbi) and feldspar (kf) in thin sections prepared parallel to the magnetic foliation plane. SPO data obtained using the above two techniques
from 20 samples distributed across the entire granite are analysed. Samples with a higher k also have a stronger magnetic fabric (magnetic lineation,
L and degree of magnetic anisotropy P0). Also, the degree of SPO is greater for granites from the southern parts as compared to the northern
parts. It is argued that this variation in degree of SPO cannot be attributed to rheological differences or strain resulting from difference in aspect
ratios. Regional strain is inferred as the dominant factor. Since the emplacement of the Godhra Granite is synchronous with the tectonic rejuvenation
along the Central Indian Tectonic Zone (CITZ) that lies to its south, the higher degree of SPO in the southern parts is attributed to its proximity
to the CITZ. Whilst caution needs to be exercised to directly apply magnetic data as a measure of strain-intensity on a regional scale, it can
be a useful guide to select samples for detailed SPO analysis using some alternative technique, especially for granites and other magmatic rocks
that are devoid of mesoscopic strain markers.
ferromagnetic granite (Godhra Granite, NW
India) are presented and it is shown that the
degree of magnetic anisotropy (P9) is not controlled
by the mean susceptibility (Km). Analyses carried out
across a high-strain zone lying between granite and adjacent
gneiss show that P9 values are highest in samples that lie close
to the contact and decrease away from it. Based on these
results it is concluded that if P9 is not controlled by Km, then
the former can be used to gauge strain-intensity variations in
ferromagnetic granites.
geochemical studies on the Palaeo-proterozoic Wangtu
Gneissic Complex (WGC) of the Himachal Lesser
Himalaya have been carried out to understand the tectonic
setting of the northern Indian continental margin
during the Palaeo-proterozoic. Petrography and magnetomineralogy
suggest that, although the WGC is
dominantly composed of S-type/two-mica granitoids
having low magnetic susceptibility (< 500 × 10–6 SI
units), part of the complex consists of hornblende–
magnetite and biotite–magnetite-bearing I-type granitoids
having susceptibility greater than 500 × 10–6 SI
units. Comparison of magnetic susceptibility with major
element concentration reveals that the high susceptibility
(> 500 × 10–6 SI units) granites are low in silica content
and enriched in ferro-magnesian content. Tectonic
discrimination based on trace element concentration
shows that both I- and S-type granitoids of the WGC
contain concentration of Y, Nb and Rb consistent with a
collisional/volcanic arc set up. It is concluded that the
North Indian continental margin had an active collisional
set up during the Palaeo-proterozoic.
Palaeozoic Mandi-Karsog pluton in the Lesser Himalayan region reveal a concordant relationship
between fabric of the Proterozoic host rock and the granite. The pluton displays a prominent
arcuate shape on the geological map. The margin-parallel mesoscopic and magnetic fabrics of the
granite and warping of the host rock fabric around the pluton indicate that this regional curvature is
either synchronous or pre-dates the emplacement of the granite body. Mesoscopic fabric, magnetic
fabric and microstructures indicate that the northern part of the pluton preserves its pre-Himalayan
magmatic fabric while the central and southern part shows tectonic fabric related to the Tertiary
Himalayan orogeny. The presence of NW–SE-trending aplitic veins within the granite indicates a
post-emplacement stretching in the NE–SW direction. Shear-sense indicators in the mylonites along
the margin of the pluton suggest top-to-the-SW shearing related to the Himalayan orogeny. Based on
these observations, it is envisaged that the extension that gave rise to this rift-related magmatism had
a NE–SW trend, that is, normal to the trend of the aplite veins. Subsequently, during the Himalayan
orogeny, compression occurred along this same NE–SW orientation. These findings imply that the
regional curvature present in the Himachal Lesser Himalaya is in fact a pre-Himalayan feature and the
pluton has formed by filling a major pre-Himalayan arcuate extension fracture.
of arc magmatism caused by the northward subduction of the
Tethyan oceanic lithosphere below the edge of the Eurasian
plate. The batholith dominantly consists of calc-alkaline
I-type granitoids which are ferromagnetic in nature with the
presence of magnetite as the principal carrier of magnetic
susceptibility. The mesoscopic and magnetic fabric are
concordant and generally vary from WNW–ESE to ENE–
WSW for different intrusions of ferromagnetic granites in
different parts of the batholith. Strike of magnetic fabric is
roughly parallel with the regional trend of the Ladakh
batholith in the present study area and is orthogonal to the
direction of India-Eurasia collision. In Khardungla and
Changla section, the magnetic fabric is distributed in a sigmoidal
manner. It is inferred that this sigmoidal pattern is
caused by shearing due to transpression induced by oblique
convergence between the two plates. U–Pb zircon geochronology
of a rhyolite from the southern parts of the
batholith gives a crystallization age of 71.7 ± 0.6 Ma,
coeval with*68 Ma magmatism in the northern parts of the
batholith. The central part of the batholith is characterized by
S-type two-mica granites, which gives much younger age of
magmatism at 35.5 ± 0.5 Ma. The magnetic fabric of these
two-mica granites is at a high angle to the regional trend of
the batholith. It is proposed that these two-mica granites
were emplaced well after the cessation of subduction and arc
magmatism, along fractures that developed perpendicular to
the regional strike of the batholith due to shearing.
(STD) of Higher Himalaya is a system of low-angle normal
faults. In the Himachal High Himalaya, the STD hanging
wall is characterized by the presence of S-type per-aluminous
Paleozoic (*475 Ma) granite called the Kinnaur
Kailash Granite (KKG). This granite is later intruded by
Cenozoic leucogranites (*18 Ma) in vicinity of the STD
zone. In this work, microstructures, anisotropy of magnetic
susceptibility (AMS), and U–Pb geochronology were carried
out on the KKG and the leucogranites with an aim to
(a) understand the conditions of fabric development and
(b) decipher the tectonic relationship between deformation
along the STD and the evolution of these granites.
Microstructural features and magnetic anisotropy indicate
that the granites are intensely deformed in vicinity of the
STD and preserve their emplacement-related fabric in the
interior parts. It is inferred that close to the STD zone,
fabrics of both the KKG and the leucogranite are tectonic
and are modified by the Cenozoic (*20 Ma) right-lateral
slip and extensional tectonics. Magnetic fabric in the
interior parts of the KKG is related to its emplacement
indicating that original fabric was preserved. U–Pb geochronology
of zircons from two samples of the KKG yields
crystallization age of 477.6 ± 3.4 and 472 ± 4 Ma. The
leucogranite gives a crystallization age of 18.5 ± 0.6 Ma.
Zircons from the KKG also reveal signatures of a
deformation event (20.6 ± 2.3 Ma) at its rim. It is inferred
that deformation of the external rim of the KKG and
crystallization of the leucogranites are synchronous and
triggered by ductile deformation along the STD.
Lesser Himalayan Crystalline sequence experienced superposed folding and doming prior to its
exhuma- tion, with the help of integrated field, microstructural, magnetic fabric anisotropy and
geochronological studies. The WGC forms the basement of the Lesser Himalaya and is bounded by
Vaikrita Thrust (VT) to the northeast and Munsiari Thrust (MT) to the southwest. The regional
structure consists of upright large scale early folds (D1) trending NW–SE. The mesoscopic fabric is
related to axial plane foliation of the D1 folds and, to a lesser extent, late D2 folds. The axis
of maximum compression for D1 and D2 folds are mutually orthogonal. The D1 folds have formed
simultaneously with the major Himalayan thrusts whereas the D2 folds have developed during a later
deformation event. The magnetic lineation at the hangingwall of the VT is sub-horizontal indicating
stretching along the strike of the thrust. In the interior parts of the WGC, the magnetic fabric is
of two types: (i) magnetic lineation demarks the intersection of mesoscopic and magnetic foliation
indicating superposed deformation and (ii) scattered distribution of magnetic lineations due to D2
folding on initially curved and non-cylindrical D1 surface. 40 Ar–39 Ar dating of biotite from one
site from the core of WGC gives an age of 9.3 ± 0.3 (2a) Ma. It is inferred that the dom- ing of
the WGC took place at ∼9 Ma and, instead of large scale thrusting, it is characterized by
superposed folding and strike-parallel stretching along the VT zone. It is suggested that the
effect of superposed folding and ductile deformation of the Himalayan basement rocks has to be
taken into account before
cross-section balancing or any estimation of crustal shortening is attempted.
called the Pangong Strand and the Tangtse Strand. These two strands bound a region called the Pangong
Transpression Zone (PTZ), which consists of migmatitic dioritic gneiss, calc-silicates and the Durbuk
Pluton; a pluton of two-mica leucogranite. Outcrop scale observations suggest pervasive migration of
leucogranitic melt through the existing tectonic structures of the gneiss. Magnetic fabric from both the
tectonized and undeformed parts of the Durbuk Pluton show parallelism with the tectonic fabric of the
host gneiss, which, along with pervasive melt migration, indicates syn-kinematic relationship between
deformation along the KFZ, leucogranite magmatism and emplacement of the Durbuk Pluton. UePb
geochronology of zircons from the dioritic gneiss yields a crystallization age of 63.6 1.5 Ma and also
shows younger zircon growth down to w13 Ma, which suggest arc magmatism at w65 Ma followed by
partial melting and leucogranite magmatism in the KFZ till w13 Ma. One two-mica leucogranite sample
from the Durbuk Pluton gives a crystallization age of 22.7 0.5 Ma. As the Durbuk Pluton is syn-tectonic
with deformation along KFZ, it is inferred that the KFZ initiated at least w23 Ma ago.
(SPO) in them is challenging. The aim of this paper is to present a methodology for determining variation in the degree of SPO in granites and to
test the feasibility of correlating the same with regional strain. The case of the Godhra Granite located in the southern parts of Aravalli Mountain
Belt (India) is taken as an example. Degree of SPO is determined using two different techniques (a) anisotropy of magnetic susceptibility (AMS)
and (b) strength of mineral lineation determined by calculating the concentration parameter (k) of von Mises distribution by digital image analysis
of biotite (kbi) and feldspar (kf) in thin sections prepared parallel to the magnetic foliation plane. SPO data obtained using the above two techniques
from 20 samples distributed across the entire granite are analysed. Samples with a higher k also have a stronger magnetic fabric (magnetic lineation,
L and degree of magnetic anisotropy P0). Also, the degree of SPO is greater for granites from the southern parts as compared to the northern
parts. It is argued that this variation in degree of SPO cannot be attributed to rheological differences or strain resulting from difference in aspect
ratios. Regional strain is inferred as the dominant factor. Since the emplacement of the Godhra Granite is synchronous with the tectonic rejuvenation
along the Central Indian Tectonic Zone (CITZ) that lies to its south, the higher degree of SPO in the southern parts is attributed to its proximity
to the CITZ. Whilst caution needs to be exercised to directly apply magnetic data as a measure of strain-intensity on a regional scale, it can
be a useful guide to select samples for detailed SPO analysis using some alternative technique, especially for granites and other magmatic rocks
that are devoid of mesoscopic strain markers.
ferromagnetic granite (Godhra Granite, NW
India) are presented and it is shown that the
degree of magnetic anisotropy (P9) is not controlled
by the mean susceptibility (Km). Analyses carried out
across a high-strain zone lying between granite and adjacent
gneiss show that P9 values are highest in samples that lie close
to the contact and decrease away from it. Based on these
results it is concluded that if P9 is not controlled by Km, then
the former can be used to gauge strain-intensity variations in
ferromagnetic granites.
geochemical studies on the Palaeo-proterozoic Wangtu
Gneissic Complex (WGC) of the Himachal Lesser
Himalaya have been carried out to understand the tectonic
setting of the northern Indian continental margin
during the Palaeo-proterozoic. Petrography and magnetomineralogy
suggest that, although the WGC is
dominantly composed of S-type/two-mica granitoids
having low magnetic susceptibility (< 500 × 10–6 SI
units), part of the complex consists of hornblende–
magnetite and biotite–magnetite-bearing I-type granitoids
having susceptibility greater than 500 × 10–6 SI
units. Comparison of magnetic susceptibility with major
element concentration reveals that the high susceptibility
(> 500 × 10–6 SI units) granites are low in silica content
and enriched in ferro-magnesian content. Tectonic
discrimination based on trace element concentration
shows that both I- and S-type granitoids of the WGC
contain concentration of Y, Nb and Rb consistent with a
collisional/volcanic arc set up. It is concluded that the
North Indian continental margin had an active collisional
set up during the Palaeo-proterozoic.
Palaeozoic Mandi-Karsog pluton in the Lesser Himalayan region reveal a concordant relationship
between fabric of the Proterozoic host rock and the granite. The pluton displays a prominent
arcuate shape on the geological map. The margin-parallel mesoscopic and magnetic fabrics of the
granite and warping of the host rock fabric around the pluton indicate that this regional curvature is
either synchronous or pre-dates the emplacement of the granite body. Mesoscopic fabric, magnetic
fabric and microstructures indicate that the northern part of the pluton preserves its pre-Himalayan
magmatic fabric while the central and southern part shows tectonic fabric related to the Tertiary
Himalayan orogeny. The presence of NW–SE-trending aplitic veins within the granite indicates a
post-emplacement stretching in the NE–SW direction. Shear-sense indicators in the mylonites along
the margin of the pluton suggest top-to-the-SW shearing related to the Himalayan orogeny. Based on
these observations, it is envisaged that the extension that gave rise to this rift-related magmatism had
a NE–SW trend, that is, normal to the trend of the aplite veins. Subsequently, during the Himalayan
orogeny, compression occurred along this same NE–SW orientation. These findings imply that the
regional curvature present in the Himachal Lesser Himalaya is in fact a pre-Himalayan feature and the
pluton has formed by filling a major pre-Himalayan arcuate extension fracture.
of arc magmatism caused by the northward subduction of the
Tethyan oceanic lithosphere below the edge of the Eurasian
plate. The batholith dominantly consists of calc-alkaline
I-type granitoids which are ferromagnetic in nature with the
presence of magnetite as the principal carrier of magnetic
susceptibility. The mesoscopic and magnetic fabric are
concordant and generally vary from WNW–ESE to ENE–
WSW for different intrusions of ferromagnetic granites in
different parts of the batholith. Strike of magnetic fabric is
roughly parallel with the regional trend of the Ladakh
batholith in the present study area and is orthogonal to the
direction of India-Eurasia collision. In Khardungla and
Changla section, the magnetic fabric is distributed in a sigmoidal
manner. It is inferred that this sigmoidal pattern is
caused by shearing due to transpression induced by oblique
convergence between the two plates. U–Pb zircon geochronology
of a rhyolite from the southern parts of the
batholith gives a crystallization age of 71.7 ± 0.6 Ma,
coeval with*68 Ma magmatism in the northern parts of the
batholith. The central part of the batholith is characterized by
S-type two-mica granites, which gives much younger age of
magmatism at 35.5 ± 0.5 Ma. The magnetic fabric of these
two-mica granites is at a high angle to the regional trend of
the batholith. It is proposed that these two-mica granites
were emplaced well after the cessation of subduction and arc
magmatism, along fractures that developed perpendicular to
the regional strike of the batholith due to shearing.
(STD) of Higher Himalaya is a system of low-angle normal
faults. In the Himachal High Himalaya, the STD hanging
wall is characterized by the presence of S-type per-aluminous
Paleozoic (*475 Ma) granite called the Kinnaur
Kailash Granite (KKG). This granite is later intruded by
Cenozoic leucogranites (*18 Ma) in vicinity of the STD
zone. In this work, microstructures, anisotropy of magnetic
susceptibility (AMS), and U–Pb geochronology were carried
out on the KKG and the leucogranites with an aim to
(a) understand the conditions of fabric development and
(b) decipher the tectonic relationship between deformation
along the STD and the evolution of these granites.
Microstructural features and magnetic anisotropy indicate
that the granites are intensely deformed in vicinity of the
STD and preserve their emplacement-related fabric in the
interior parts. It is inferred that close to the STD zone,
fabrics of both the KKG and the leucogranite are tectonic
and are modified by the Cenozoic (*20 Ma) right-lateral
slip and extensional tectonics. Magnetic fabric in the
interior parts of the KKG is related to its emplacement
indicating that original fabric was preserved. U–Pb geochronology
of zircons from two samples of the KKG yields
crystallization age of 477.6 ± 3.4 and 472 ± 4 Ma. The
leucogranite gives a crystallization age of 18.5 ± 0.6 Ma.
Zircons from the KKG also reveal signatures of a
deformation event (20.6 ± 2.3 Ma) at its rim. It is inferred
that deformation of the external rim of the KKG and
crystallization of the leucogranites are synchronous and
triggered by ductile deformation along the STD.
Lesser Himalayan Crystalline sequence experienced superposed folding and doming prior to its
exhuma- tion, with the help of integrated field, microstructural, magnetic fabric anisotropy and
geochronological studies. The WGC forms the basement of the Lesser Himalaya and is bounded by
Vaikrita Thrust (VT) to the northeast and Munsiari Thrust (MT) to the southwest. The regional
structure consists of upright large scale early folds (D1) trending NW–SE. The mesoscopic fabric is
related to axial plane foliation of the D1 folds and, to a lesser extent, late D2 folds. The axis
of maximum compression for D1 and D2 folds are mutually orthogonal. The D1 folds have formed
simultaneously with the major Himalayan thrusts whereas the D2 folds have developed during a later
deformation event. The magnetic lineation at the hangingwall of the VT is sub-horizontal indicating
stretching along the strike of the thrust. In the interior parts of the WGC, the magnetic fabric is
of two types: (i) magnetic lineation demarks the intersection of mesoscopic and magnetic foliation
indicating superposed deformation and (ii) scattered distribution of magnetic lineations due to D2
folding on initially curved and non-cylindrical D1 surface. 40 Ar–39 Ar dating of biotite from one
site from the core of WGC gives an age of 9.3 ± 0.3 (2a) Ma. It is inferred that the dom- ing of
the WGC took place at ∼9 Ma and, instead of large scale thrusting, it is characterized by
superposed folding and strike-parallel stretching along the VT zone. It is suggested that the
effect of superposed folding and ductile deformation of the Himalayan basement rocks has to be
taken into account before
cross-section balancing or any estimation of crustal shortening is attempted.