Granulite Terrain Research Articles

Imprints of diverse mantle deformational episodes in the Cauvery Suture Zone, South India

This study attempts to understand the upper mantle deformation pattern beneath the northern part of Southern Granulite Terrain (SGT), located to the south of Archean Dharwar craton in south India. The study region comprises the tectonic blocks – Nilgiri, Biligiri Rangan, Shevaroy and Madras blocks – separated from the northern part of Madurai block by the Cauvery Suture Zone (CSZ), associated with subduction–accretion–collision tectonics. Shear wave splitting observed on high quality XK(K)S (SKS, SKKS, PKS) waveforms from 119 teleseismic events recorded by 19 broadband seismic stations, mostly within the CSZ and the northern part of Madurai block, is utilized in this study. The 122 splitting measurements obtained, show an average delay time of 0.8s between the fast and slow waves. Strong lateral variations in the fast polarization azimuths (FPA) are observed, with the splitting parameters at the majority of stations showing a dependence on back azimuth. Importantly, excluding the Madras block, a poor correlation between the FPA and the present day absolute plate motion (APM) direction is observed. This dominance of non-APM strain reflects complex anisotropy preserved in the lithosphere. By plotting the fast polarization trends based on the ray piercing points at a depth of 150km, three distinct splitting patterns associated with the (i) Biligiri Rangan block and the eastern part of the Nilgiri block, (ii) eastern part of the CSZ and the northern Madurai block (NMB), and (iii) Madras block and its vicinity within the CSZ, are observed. The strong lateral heterogeneity in the deformation of the continental lithosphere might be either related with the shear zones that are associated with frozen magmatic intrusions, two episodes of ocean closure and the collision that followed, plate motion related strain or a combination of these. Similar deformation patterns in the northern part of Madurai block might be related to remnants of subducted oceanic lithosphere.

Archean tectonics and crustal evolution of the Biligiri Rangan Block, southern India

The Southern Granulite Terrain in India is a collage of crustal blocks ranging in age from Archean to Neoproterozoic. This study investigate the tectonic evolution of one of the northernmost block – the Biligiri Rangan Block (BRB) through a multidisciplinary approach involving field investigation, petrographic studies, LA-ICPMS zircon U–Pb geochronology, Hf isotopic analyses, metamorphic P–T phase diagram computations, and crustal thickness modeling. The garnet bearing quartzofeldspathic gneiss from the central BRB preserve Mesoarchean magmatic zircons with ages between 3207 and 2806Ma and positive ɛHf value (+2.7) which possibly indicates vestiges of a Mesoarchean primitive continental crust. The occurrence of quartzite-iron formation intercalation as well as ultramafic lenses along the western boundary of the BRB is interpreted to indicate that the Kollegal structural lineament is a possible paleo-suture. Phase diagram computation of a metagabbro from the southwestern periphery of the Kollegal suture zone reveals high-pressure (∼18.5kbar) and medium-temperature (∼840°C) metamorphism, likely during eastward subduction of the Western Dharwar oceanic crust beneath the Mesoarchean BRB. In the model presented here, slab subduction, melting and underplating processes generated arc magmatism and subsequent charnockitization within the BRB between ca. 2650Ma and ca. 2498Ma. These results thus reveal Meso- to Neoarchean tectonic evolution of the BRB. The spatial variation of crustal thickness, derived from flexure inversion technique, provides additional constraints on the tectonic linkage of the BRB with its surrounding terrains. In conjunction with published data, the Moyar and the Kollegal suture zones are considered to mark the trace of ocean closure along which the Nilgiri and Biligiri Rangan Blocks accreted on to the Western Dharwar Craton.

The Study of Major Element Geochemistry of Migmatites in and Around Melur Region, Madurai District, Tamil Nadu, India

The study investigating the exposed Archean lower continental crust of the Southern Granulite Terrain, India, shield important constraints on the nature and evolution of the deep crust, including the formation and exhumation of Granulites. The source for the high temperature granulite metamorphism for the southern granulite terrain may be attributed to high temperature carbonatite and alkaline intrusives in an extensional setting which followed an initial crustal thickening. The Southern Granulite Terrain (SGT) in India is one of the largest exposed Precambrian deep continental crustal sections in the world consisting of multiply deformed Archean and Neoproterozoic high grade metamorphic and magmatic rock.

Phase separation of ore forming fluid related to gold mineralization in Wynad Gold Field, Southern Granulite Terrain, India: Evidences from fluid inclusion studies

Fluid inclusion studies were carried out on auriferous quartz veins of Wynad Gold Field, Southern Granulite Terrain of India. Three types of primary fluid inclusions have been observed; Type-I: H2O–CO2 inclusions, Type-II: CO2 inclusions and Type-III: aqueous inclusions. The Type-I and Type-II inclusions are more abundant than Type-III inclusions. The coexistence of Type-I and Type-II inclusions are common within quartz grains in most of the samples studied. Variation in phase ratio and broad range of total homogenization temperature of Type-I and Type-III inclusions (i.e. 194°C to 300°C and 189°C to 282°C, respectively) indicate the entrapment of heterogeneous fluid in inclusions. This heterogeneity could be due to phase separation of original low saline H2O–CO2 ore fluid in response to drop in pressure and temperature. Gold along with other constituents could have precipitated in response to phase separation of the ore fluid.

Genetic aspects of gold mineralization in the Southern Granulite Terrain, India

Gold mineralization in Southern Granulite Terrain (SGT) of India has close spatial relationship with the shear zones (Moyar–Bhavani) present in Cauvery Suture Zone. Gold is found to be associated with primary quartz veins, placers and laterites. The gold prospects in SGT can be broadly grouped into three provinces i) Wynad-Nilgiri, ii) Malappuram and iii) Attappadi. The auriferous quartz veins are within the deformed biotite/hornblende bearing gneisses and amphibolite. Wall rock alteration is conspicuous around the mineralized veins and gives an assemblage of muscovite–calcite–ankerite–chlorite–biotite–pyrite related to fluid–rock interaction at the time of vein formation. Fluid inclusion studies of vein quartz gives an idea of the nature of the ore forming fluids, the fluid involved in gold mineralization is of low saline and aqueous-carbonic in composition and quite similar to the orogenic lode gold deposits reported world-wide. Micro-thermometric data indicates fluid immiscibility (phase separation) during trapping of fluid inclusions and this must have played an important role in gold deposition. Geochronological studies of mineral separates from Wynad-Nilgiri province using Rb–Sr and Sm–Nd isochron methods of the auriferous quartz veins gave an age of approximately 450Ma for the vein formation. The present studies on SGT gold mineralization indicate 1. During the Pan-African orogeny, extensive fluid influx from mantle and metamorphism extracted gold from a mafic source and were focused along major structural discontinuities of Moyar–Bhavani Shear Zone, 2. The aqueous–carbonic ore fluid interacted with rocks of the upper crust and triggered a set of metasomatic changes responsible for the dissolved components such as Ca, Si and Fe and finally precipitating in the veins and 3. The mineralizing fluid with dissolved gold in sulphide complex got destabilized due to fluid immiscibility and wall rock alteration leading to the deposition of gold with associated sulphide minerals in the vein system.

Morphotectonics of a small river basin in the South Indian granulite terrain: An assessment through spatially derived geomorphic indices

ABSTRACTNeotectonic activity in Mahe (Mayyazhi) river basin and its implication in landform development and stream characteristics were investigated with the help of the digital elevation model-derived geomorphic indices. The analysis is based on the commonly used geomorphic indices such as hypsometric curve and integral, drainage basin asymmetry, transverse topographic symmetric factor (T), stream length (SL) gradient index and longitudinal profiles of the stream. Hypsometric integrals indicate that the basin has reached the base level and lesser amount of material remains for erosion and transportation. The basin is asymmetric and has a south-south-east directional tilt, but with a probable spin caused due to the presence of major strike slip fracture, the mouth portion of the Mahe river has been tilted towards north-north-west. The SL index with sudden and non-uniform variations supports the finding and indicates the relative tectonic activity and its influence over the river networks in the area. Longitudinal profile of the stream also reveals gradational changes in the profile with the presence of knick points. Cross comparison of the analysed geomorphic indices points towards neotectonic activity in the basin, which modified the river basin to the present morphology and is reflected in the characteristics of the river and the basin as a whole.

Chapter 1 Precambrian basins of India: stratigraphic and tectonic context

The Indian shield represents a vast repository of the Precambrian geological record. The Precambrian sedimentary basins were developed on four major Archaean nuclei (Dharwar, Bastar, Singhbhum and Aravalli–Bundelkhand; Fig. 1.1) and are comparable to those of Australia, South Africa, Canada and Brazil in scale and importance for global studies of Precambrian crustal evolution. Some of the Indian basinal successions, as elsewhere, contain valuable economic resources. Notwithstanding their global relevance, many aspects of the Indian Precambrian basins and even entire basins are not well known to international readers. Although specialist papers, a book (Naqvi & Rogers 1987) and special publications (e.g. Mazumder & Saha 2012) have been written by several authors on specific aspects of the Precambrian geology of India, a comprehensive account of the time–space distribution of the Precambrian supracrustal successions is lacking; in particular, the interrelation between tectonics and sedimentation, and the similarities and dissimilarities of their evolutionary histories with other comparable successions in other parts of the world are yet to be addressed in any detail. The purpose of the present memoir is to fill in this knowledge gap. In addition, the memoir will assess the potential mineral resources of Indian Proterozoic basins and integrate piecemeal information to provide a comprehensive picture of Indian cratonic evolution during the Precambrian. Fig. 1.1. Sketch map showing four Archaean nuclei in India. Note Proterozoic sedimentary basins (dark grey patches) and the large Deccan basaltic province. SGT, Southern Granulite Terrain; NFB, Nallamalai fold belt; CITZ, Central Indian Tectonic Zone; PG, Pranhita Godavari Valley; Ab, Abujhmar Basin; Am, Ampani Basin; Bh, Bhima Basin; Ch, Chattisgarh Basin; Idv, Indravati Basin; Kal, Kaladgi Basin; Suk, Sukma Basin; Ban, Bangalore; Cd, Cuddapah; Hyd, Hyderabad; K, Kurnool; Kh, Khariar; Nel, Nellore; Ong, Ongole; Vij, Vijaywada (after Saha & Mazumder 2012). The content of the memoir has been divided …

Upper-mantle anisotropy beneath the south Indian Shield: Influenced by ancient and recent Earth processes

ABSTRACT We obtained shear-wave splitting parameters from core-refracted phases like SKS, SKKS, and PKS at 75 digital broadband seismic stations almost uniformly spread over the south Indian Shield representing varied geological terrains, including the western Dharwar craton, the eastern Dharwar craton, the Southern granulite terrain, and the continental margins along the west and east coasts. A majority of the sta-tions over the Dharwar craton show delay times of 1–1.6 s, indicative of a 150–200-km-thick anisotropic layer correlating with the lithospheric root, while segments like the Pan-African Southern granulite terrain have delay times of 0.5–0.7 s, suggesting an internally deformed and thin anisotropic layer, possibly due to recent plate-tectonic and geodynamic processes. The average direction of anisotropy is generally ~N30°E, correlating with the present-day plate motion, with local deviations where the direction of anisotropy correlates with the orienta-tion of the regional shear zones. Stations close to the continental margin show large time delays (up to 2 s) with the fast axis parallel to the rift axis. Further, we infer a layered anisotropic lithosphere in the south Indian Shield as revealed in 90° periodicity of the two anisotropy parameters (fast direction and delay time). The upper lithosphere represents the depleted Archean mantle, while the lower lithosphere could be transformed to more fertile mantle due to subsequent deformation. This study suggests that the observed anisotropy over the south Indian Shield is the result of complex interplay between the architecture of an Archean craton and its subsequent deformation in different geological domains due to deep Earth processes.

Mineralization in Granitic intrusive Of Kanigiri area in Andhra Pradesh: A root cause for Fluoride Dominance in Ground Water – A Case Study

The Kanigiri-Podili tract of Nellore and Prakasam districts of Andhra Pradesh were known for abnormal fluoride content and interesting mineralogy. Kanigiri town in Topo-Sheet No.57M/11 of Survey of India is surrounded by granitic intrusive. Alkaline magmatism was widespread in the granulite terrain of southern peninsular India during the proterozoic. The forty seven alkaline to mildly alkaline plutons and carbonatite complexes of varying dimensions and shapes cover a total area of approximately 450sq.Km and define an alkaline belt along the east coast of India. The rocks of the area are holocrystalline-hypidiomorphic texture and composed mainly of orthoclase, microcline, albite, and zircon as chief accessory minerals fluorite, topaz, apatite are the other accessosories as served in the rock. The present work focuses on the origin of fluoride in waters around Kanigiri area. In general the dissociation of granitic rocks exposed to prolonged sun and runoff results apatites in to water. This in turn enriches the Fluoride content. But here the mechanism is different. Endemic fluorosis is prevalent amongst the habitants of the high fluorine content in ground water. Streaks and Lenses of fluorite are common feathers in these granites. The earlier workers attributed the origin to the replacement of the existing rocks by “granitic juice” having a high volatile content. Small patches of Sodalite-Cancrinite-Fluorite-Nepheline bearing syenites hosted within the fluorite bearing grey granite near Podili. Detailed water sampling around Kanigiri and Podili has been made. They are analyzed to their complete major-ion analysis.

Electromagnetic signatures of collision zones in India

Indian subcontinent has experienced intense tectonic activity within the continent in the form of subduction, continent-continent collision, subduction-collision-accretion tectonics. Deep electrical sig� natures of intense tectonic activity are presented for three different regions: Himalayan region, Central Indian Tectonic zone and Southern Granulite terrain. Two long traverses at each region are discussed along with other geophysical data. The geoelectric sections across the Himalayan region have shown a clear north� ward dipping signature of the anomalous conductive features at upper to midcrustal depths. This model gave a clear evidence for the collision and subduction processes. The profiles across the Central Indian tectonic zone constituting major eastwest trending faults and mobile belts provided the evidence for the presence of mantle derived fluids at midcrustal depths and also ga ve a clear evidence for the collision processes between the Bundelkhand craton and the Dharwar craton. The collision-accretion tectonic process is observed in Southern Granulite terrain of south India. Evidence for the exhumation of mantle derived fluids to the mid� crustal depths has been observed along the Vattalakundu-Kanyakumari profile, while the subduction-colli� sion-accretion processes have been observed along Kolattur-Palani geotransect. In all the three tectonically active regions, the deep electrical structure mapped from magnetotellurics, gave a clear evidence for the pres� ence of anomalous conductive structures that can be related to active tectonic regime that has paved a way for better understanding of the evolution of the deep crust. In this paper all the earlier works in three regions with emphasis on tectonics are briefly reviewed.

Melt production, redistribution and accumulation in mid-crustal source rocks, with implications for crustal-scale melt transfer

Abstract Ascent of granitic melt initiates under suprasolidus conditions in the mid- to lower crust before continuing through subsolidus rocks to higher crustal levels. Whereas migration of melt in suprasolidus rocks can occur in pervasive net-like structures and involve relatively small melt volumes, ascent through the subsolidus crust requires more focused, dyke-like structures and larger volumes to prevent freezing. Migmatites in the Aus granulite terrain, southern Namibia, preserve evidence that large-scale melt redistribution and accumulation occurred in the near-source region under suprasolidus conditions. Melt that was mainly produced in metapelitic rocks utilised pervasive small-scale leucosome networks to migrate to areas surrounding pre-tectonic granite sheets. These areas are dominated by metapsammitic rocks, and abundant and voluminous leucogranite sheets attest to melt accumulation and residence occurring over a protracted period while the area was undergoing anatexis. However, the leucogranites have an anhydrous mineralogy and the surrounding rocks only preserve evidence for limited, high-temperature retrogression, consistent with substantial melt loss from the accumulation structures. We speculate that melt batches leaving the accumulation sites are likely to have been large, allowing for substantially more efficient ascent through subsolidus crust. Our results suggest that a degree of near-source melt accumulation is likely to occur during the early stages of meltmigration, and that this can significantly enhance the effectiveness of subsequent melt ascent.

The mafic–ultramafic complex of Aniyapuram, Cauvery Suture Zone, southern India: Petrological and geochemical constraints for Neoarchean suprasubduction zone tectonics

Several Precambrian mafic–ultramafic complexes occur along the Cauvery Suture Zone (CSZ) in Southern Granulite Terrain, India. Their origin, magmatic evolution and relationship with the associated high-grade rocks have not been resolved. The Aniyapuram Mafic–Ultramafic Complex (AMUC), the focus of the present study in southern part of the CSZ, is dominantly composed of peridotites, pyroxenites, gabbros, metagabbros/mafic granulites, hornblendites, amphibolites, plagiogranites, felsic granulites and ferruginous cherts. The rock types in the AMUC are structurally emplaced within hornblende gneiss (TTG) basement rocks and are highly deformed. The geochemical signature of the amphibolites indicates tholeiitic affinity for the protolith with magma generation in island arc-setting. N-MORB normalized pattern of the amphibolites show depletion in HFS-elements (P, Zr, Sm, Ti, and Y) and enrichment of LIL-elements (Rb, Ba, Th, Sr) with negative Nb anomalies suggesting involvement of subduction component in the depleted mantle source and formation in a supra-subduction zone tectonic setting. Our new results when correlated with the available age data suggest that the lithological association of AMUC represent the remnants of the Neoarchean oceanic lithosphere.

3D Lithosphere density structure of southern Indian shield from joint inversion of gravity, geoid and topography data

We present the 3D crustal and lithospheric structure and crustal average density distribution of southern Indian shield (south of 18°N), Sri Lanka and adjoining oceans. The model is based on the assumption of local isostatic equilibrium and is derived from joint inversion of free air gravity and geoid anomalies and topography data. The derived crustal thickness of 10–25km in the oceanic region increases to 34–35km along the coast. A crustal thickness of 34–38km is obtained beneath the Eastern Dharwar Craton and 36–45km beneath the Western Dharwar Craton and the Southern Granulite Terrain. Sri Lanka has a thinner crust of 30–35km. The lithosphere–asthenosphere boundary is located at depths of 70–120km under oceanic regions and ∼150–180km below the Dharwar Craton and the Northern block of Southern Granulite Terrain. A notably thinned lithosphere of ∼130km near Bangalore in the Eastern Dharwar Craton, ∼140km beneath the Southern block of Southern Granulite Terrain and ∼130km in Sri Lanka is observed. The thickness of the lithosphere (∼130km) near Bangalore is inferred as the frozen in signature of a small fossil mantle plume and/or tectono-compositional effect of a rifted margin and a suture. Considerable stretching and/or convective removal of pristine lithosphere in the Southern block of Southern Granulite Terrain and adjoining Sri Lanka, before disappearing completely in the Archaean Northern block of Southern Granulite Terrain and Dharwar Craton, is suggested.

Seismic imaging of crust beneath the Dharwar Craton, India, from ambient noise and teleseismic receiver function modelling

We use cross-correlation of continuous 18 months (2009 February to 2010 August) ambient noise data recorded over 35 broad-band seismographs in the Archean Dharwar Craton and the adjoining granulite terrain to generate Rayleigh-wave group velocity maps in the period 5–28 s. This is supplemented with longer period data (40–70 s) from earthquake source. Combined group velocity measurement was inverted jointly with the teleseismic receiver functions obtained at 50 stations (includes 15 stations operated during 1998–2002) to produce shear velocity image of the crust. The velocity image reveals thinner crust (34–38 km) in the late Archean (∼2.7 Ga) Eastern Dharwar Craton (EDC), while all other terrains (mid-Archean and Proterozoic) have crustal thickness from 40 to over 50 km. The mid-Archean (3.36 Ga) greenstone belt of the Western Dharwar Craton (WDC) has the thickest crust (∼50 km). The average crustal Vs beneath the EDC is ∼3.70–3.78 km s−1 as compared to 3.80–3.95 km s−1 beneath the WDC. We observe significant lateral variation in the thickness of lower crust (Vs ∼ 3.8–4.2 km s−1): ∼10–15 km in the EDC compared to ∼20–30 km in the WDC. The lowermost part of the crust (Vs ≥ 4.0 km s−1) is thin (<5 km) beneath the EDC in contrast to more thickness (10–27 km) beneath the WDC. Our analysis suggests intermediate composition for the crust beneath the EDC similar to those for other cratons. In contrast, the mid-Archean exposed WDC crust has more mafic composition and exceptional thickness—a scenario at variance with the global observations. We interpret this thick mafic crust to represent undeformed geological segment of 3.36 Ga. The EDC with a nearly flat Moho, felsic to intermediate composition of crust and thin basal layer may represent a reworked terrain during the late-Archean.

Preserved and modified mid-Archean crustal blocks in Dharwar craton: Seismological evidence

We report significant lateral variability in shear wave velocity and Moho depth in the Archean crust, beneath the Dharwar craton, using earthquake waveform data recorded over 50 broadband seismographs. The craton is a continuously exposed Archean continental fragment divided into the west Dharwar craton (WDC) of age 2.7–3.36Ga, and the east Dharwar craton (EDC) of age, dominantly, 2.5Ga. The craton progressively transitions into the Southern Granulite Terrain (SGT) with age of metamorphism around 2.6Ga.The inversion and modeling of receiver function data reveal significant variation of Moho depth, viz., 38–54km in the WDC, and 40–46km in the SGT and 32–38km in the EDC. The average shear wave velocity (Vs) of crust beneath the WDC is ∼3.85km/s as compared to ∼3.6km/s in the EDC. We infer highly variable thickness (16–30km) of mafic cumulate (Vs≥4.0km/s and Vp≥7.0km/s) beneath the WDC, in contrast with a thin one (<5km) beneath the late Archean EDC. The 3.36Ga greenstone belt in the WDC has maximum basal layer thickness of ∼30km. These results suggest the intermediate-mafic composition and exceptional thickness of crust beneath the WDC (>50km) as compared to felsic to intermediate composition for the EDC crust with almost flat Moho (down to ∼38km). These results suggest preserved mafic crustal root beneath the middle Archean terrain in the WDC that has remained inert since then. On the other hand, felsic to intermediate composition of crust with a nearly flat Moho beneath the late Archean EDC could be a consequence of regional delamination of lower crust. The preserved distinct Moho topography across the Archean terrains suggests it as compositional boundary. Considering the surface exposure of 15–20km crust, based on P–T condition, in the granulite segment of the WDC, we speculate a Himalaya-like crustal thickness (50–70km) beneath the middle Archean crust pointing toward a plate tectonic-like scenario at ∼3.0Ga.

Three‐dimensional lithospheric electrical structure of Southern Granulite Terrain, India and its tectonic implications

AbstractThe crustal as well as the upper mantle lithospheric electrical structure of the Southern Granulite Terrain (SGT) is evaluated, using the magnetotelluric (MT) data from two parallel traverses: one is an ~ 500 km long N‐S trending traverse across SGT and another a 200 km long traverse. Data space Occam 3‐D inversion was used to invert the MT data. The electrical characterization of lithospheric structure in SGT shows basically a highly resistive (several thousands of Ohm meters) upper crustal layer overlying a moderately resistive (a few hundred Ohm meters) lower crustal layer which in turn is underlain by the upper mantle lithosphere whose resistivity shows significant changes along the traverse. The highly resistive upper crustal layer is interspersed with four major conductive features with three of them cutting across the crustal column, bringing out a well‐defined crustal block structure in SGT with individual highly resistive blocks showing correspondence to the geologically demarcated Salem, Madurai, and Trivandrum blocks. The 3‐D model also brought out a well‐defined major crustal conductor located in the northern half of the Madurai block. The electrical characteristics of this south dipping conductor and its close spatial correlation with two of the major structural elements, viz., Karur‐Oddanchatram‐Kodaikanal Shear Zone and Karur‐Kamban‐Painavu‐Trichur Shear Zone, suggest that this conductive feature is closely linked to the subduction‐collision tectonic processes in the SGT, and it is inferred that the Archean Dharwar craton/neoproterozoic SGT terrain boundary lies south of the Palghat‐Cauvery shear zone. The results also showed that the Achankovil shear zone is characterized by a well‐defined north dipping conductive feature. The resistive block adjoining this conductor on the southern side, representing the Trivandrum block, is shown to be downthrown along this north dipping crustal conductor relative to the Madurai block, suggesting a northward movement of Trivandrum block colliding against the Madurai block. The lithospheric upper mantle electrical structure of the SGT up to a depth of 100 km may be broadly divided into two distinctly different segments, viz., northern and southern segments. The northern lithospheric segment, over a major part, is characterized by a thick resistive upper mantle, while the southern one is characterized by a dominantly conductive medium suggesting a relatively thinned lithosphere in the southern segment.

Magnetism of the lower crust: Observations from the Chipman Domain, Athabasca Granulite Terrain, northern Canada

Magnetic properties of lower crustal rocks produce anomalies seen in satellite, aeromagnetic, and ground studies, and are assumed to be responsible for observed long wave-length anomalies (LWA) of +/− 20 nT. The soon to be launched SWARM satellites will provide extensive data on the magnetization of the lower to middle crust. In anticipation of this event we are investigating magnetic properties in a superbly exposed section of lower crust in northern Saskatchewan. The Athabasca Granulite Terrain (AGT) is a complex region of felsic and mafic lower crustal rocks, part of the Snowbird Tectonic zone, stretching NE–SW across the Canadian Shield. The AGT is composed of a sequence of rocks identified as lower crustal in origin by their high pressure (> 1.0 GPa) and high temperature (~ 800 °C) metamorphism, dated at 2.6 Ga and 1.9 Ga, with uplift and exhumation at 1.85–1.80 Ga. The AGT is characterized by low (negative) aeromagnetic anomalies with distinct large positive anomalies in the southern and central regions. The Chipman Domain, on the east side, consists of tonalites, mafic granulites, and granite, intruded by the Chipman dike swarm at ~ 1.9 Ga, where anomalies cut across mapped lithologic boundaries. Susceptibility measurements from both field and lab readings range over several orders of magnitude, from 1 × 10 − 5 to 3 × 10 − 1 SI, with higher values related to both mafic granulite and some tonalite samples. Remanence values also show considerable variability, from 0.1 mA/m to 90 A/m, with the weakest magnetization found in the Chipman dikes and the Fehr granite. Forty samples out of 89 have Koenigsberger ratios greater than 1, but low initial remanence limits its influence on anomalies. Hysteresis and low temperature measurements identify magnetite as the predominant iron oxide. This section of lower crustal rocks has paramagnetic granites and dikes, with ferromagnetic mafic granulites and bimodal tonalites, defined by geographic location. • Distinct aeromagnetic anomalies are seen over the Athabasca granulite terrain. • Paramagnetic Fehr granite and Chipman dikes are unable to produce anomalies. • Bimodal Chipman tonalite shows both paramagnetic and ferromagnetic behavior. • Mafic granulites have high susceptibility and remanence but only minor occurrences.

The South India Precambrian crust and shallow lithospheric mantle: Initial results from the India Deep Earth Imaging Experiment (INDEX)

We present here the most comprehensive study of the thickness and composition ( $\textit{Vp}/\textit{Vs}$ ratio) of the South India Precambrian crust and the nature of shallower mantle inferred from analysis of teleseismic receiver functions from 70 broad-band seismic stations operated as a part of the India Deep Earth Imaging Experiment (INDEX). South India could be broadly divided into regions with thin crust (32–38 km) and thick crust (38–54 km). Thin crust domains include the East Dharwar Craton (EDC), Cuddapah basin and Madurai/Kerala Khondalite Block. The thicker crust domain includes the Western Dharwar Craton (WDC) and northern part of Southern Granulite Terrain. The WDC shows progressive increase in thickness from 38 km in north to 46–54 km in south, compared to an almost flat Moho beneath the EDC. Compositionally, most of the crustal domains are felsic to intermediate ( $\textit{Vp}/\textit{Vs}$ ~ 1.69–1.75) except the mid Archean block in the southern WDC where it is mafic ( $\textit{Vp}/\textit{Vs}$ > 1.81). Considering erosion depth in the WDC, we argue for Himalaya like ~70 km thick crust beneath it during the Archean. Variation in crustal thickness does not have a first-order influence on regional topography in South India and suggests significant role for the crustal composition. We also present evidence of mid-lithospheric low velocity at ~85–100 km beneath South India.

Strain distribution within a km-scale, mid-crustal shear zone: The Kuckaus Mylonite Zone, Namibia

The subvertical Kuckaus Mylonite Zone (KMZ) is a km-wide, crustal-scale, Proterozoic, dextral strike-slip shear zone in the Aus granulite terrain, SW Namibia. The KMZ was active under retrograde, amphibolite to greenschist facies conditions, and deformed felsic (and minor mafic) gneisses which had previously experienced granulite facies metamorphism during the Namaqua Orogeny. Lenses of pre- to syn-tectonic leucogranite bodies are also deformed in the shear zone. Pre-KMZ deformation (D1) is preserved as moderately dipping gneissic foliations and tightly folded migmatitic layering. Shear strain within the KMZ is heterogeneous, and the shear zone comprises anastomosing high strain ultramylonite zones wrapping around less deformed to nearly undeformed lozenges. Strain is localized along the edge of leucogranites and between gneissic lozenges preserving D1 migmatitic foliations. Strain localization appears controlled by pre-existing foliations, grain size, and compositional anisotropy between leucogranite and granulite. The local presence of retrograde minerals indicate that fluid infiltration occurred in places, but most ultramylonite in the KMZ is free of retrograde minerals. In particular, rock composition and D1 fabric heterogeneity are highlighted as major contributors to the strain distribution in time and space, with deformation localization along planes of rheological contrast and along pre-existing foliations. Therefore, the spatial distribution of strain in crustal-scale ductile shear zones may be highly dependent on lithology and the orientation of pre-existing fabric elements. In addition, foliation development and grain size reduction in high strain zones further localizes strain during progressive shear, maintaining the anastomosing shear zone network established by the pre-existing heterogeneity.

Long wavelength gravity anomalies over India: Crustal and lithospheric structures and its flexure

Long wavelength gravity anomalies over India were obtained from terrestrial gravity data through two independent methods: (i) wavelength filtering and (ii) removing crustal effects. The gravity fields due to the lithospheric mantle obtained from two methods were quite comparable. The long wavelength gravity anomalies were interpreted in terms of variations in the depth of the lithosphere–asthenosphere boundary (LAB) and the Moho with appropriate densities, that are constrained from seismic results at certain points. Modeling of the long wavelength gravity anomaly along a N–S profile (77°E) suggest that the thickness of the lithosphere for a density contrast of 0.05g/cm3 with the asthenosphere is maximum of ∼190km along the Himalayan front that reduces to ∼155km under the southern part of the Ganga and the Vindhyan basins increasing to ∼175km south of the Satpura Mobile belt, reducing to ∼155–140km under the Eastern Dharwar craton (EDC) and from there consistently decreasing south wards to ∼120km under the southernmost part of India, known as Southern Granulite Terrain (SGT).The crustal model clearly shows three distinct terrains of different bulk densities, and thicknesses, north of the SMB under the Ganga and the Vindhyan basins, and south of it the Eastern Dharwar Craton (EDC) and the Southern Granulite Terrain (SGT) of bulk densities 2.87, 2.90 and 2.96g/cm3, respectively. It is confirmed from the exposed rock types as the SGT is composed of high bulk density lower crustal rocks and mafic/ultramafic intrusives while the EDC represent typical granite/gneisses rocks and the basement under the Vindhyan and Ganga basins towards the north are composed of Bundelkhand granite massif of the lower density. The crustal thickness along this profile varies from ∼37–38km under the EDC, increasing to ∼40–45km under the SGT and ∼40–42km under the northern part of the Ganga basin with a bulge up to ∼36km under its southern part. Reduced lithospheric and crustal thicknesses under the Vindhyan and the Ganga basins are attributed to the lithospheric flexure of the Indian plate due to Himalaya. Crustal bulge due to lithospheric flexure is well reflected in isostatic Moho based on flexural model of average effective elastic thickness of ∼40km. Lithospheric flexure causes high heat flow that is aided by large crustal scale fault system of mobile belts and their extensions northwards in this section, which may be responsible for lower crustal bulk density in the northern part. A low density and high thermal regime in north India north of the SMB compared to south India, however does not conform to the high S-wave velocity in the northern part and thus it is attributed to changes in composition between the northern and the southern parts indicating a reworked lithosphere. Some of the long wavelength gravity anomalies along the east and the west coasts of India are attributed to the intrusives that caused the breakup of India from Antarctica, and Africa, Madagascar and Seychelles along the east and the west coasts of India, respectively.