Achankovil Shear Zone Research Articles

Estimation of Moho Depth Beneath Southern Indian Shield by Inverting Gravity Anomalies Constrained by Seismic Data

AbstractThis study presents a high‐resolution 3D Moho structure beneath southern India and its surrounding regions from observed gravity anomalies. The global gravity disturbance model (XGM2019e) with a grid resolution of 0.1° is considered for this study. The extended Bott's inversion algorithm and Gauss‐Fast Fourier Transform based forward model are adopted to invert for the Moho undulations beneath the Indian peninsula. The inversion algorithm is tested for a synthetic model having a predefined density contrast and mean Moho depth. The robustness of this inversion algorithm is further tested for noise‐incorporated gravity data. The control points are required for estimating two hyper‐parameters, viz. density contrast, and reference depth, which play a crucial role in the precise estimation of Moho depth. In real case study, the inverted Moho depth of Southern India and its surrounding regions by seismic constraint receiver function‐driven control points show a very complex architecture of Moho topography. The observed average crustal thickness in the study region is 35.35 km, corroborating with the previously reported Moho depths. The maximum crustal thickness is 53.04 km beneath the southern part of Archean Western Dharwar Craton and west of Salem block, around 44–47 km Moho depth is observed at the south of Salem block into Madurai block till Achankovil Shear Zone, which suggests the possible continuation of the Achaean crust of Palghat‐Cauvery Shear Zone System. The lowest crustal thickness values are observed along the eastern margin of the Cuddapah basin, which overlaps with the Proterozoic Krishna basin of the Eastern Ghats Mobile belt.

Crustal Density and Susceptibility Structure beneath Achankovil Shear Zone, India

Abstract The Southern Granulite Terrain (SGT) is a large tract of exposed Archean continental crust, divided into the Madurai Block (MB), Trivandrum Block (TB), and Nagercoil Block (NB). These crustal domains are linked with the NW-SE trending Achankovil Shear Zone (AKSZ). We combine gravity and magnetic data with previously published ground observations and geochronological data to re-evaluate the crustal architecture, evolution of the AKSZ, and possible extension of AKSZ into Madagascar. Analyses indicate that the long wavelength trends of the magnetic anomalies originate at ~20 km depth of different SGT blocks. These observations are corroborated with the gravity as well as computed gravity gradient anomalies. The presence of khondalite outcrops in Trivandrum Block implies that high magnetization crust is the main source of positive magnetic anomalies. Such magnetic anomalies advocate that SGT preserves the remanent of Archean crustal blocks in South India, a part due to variation in thermal and geochemical processes. The AKSZ, TB, and MB exhibit contrasting magnetic crustal signatures. The joint modeling results reveal a three-layered crustal configuration with varying Moho ranging from 41 to 34 km in NE to SW, respectively. It is also noted that AKSZ is a narrow and deep structure near to the Western Ghats Escarpment while it is wide and shallow in the far-east, which implies that the evolution of the Western Ghats is a late geological event.

Structure and chronology across the Achankovil terrain boundary shear zone system (South India), and its Madagascar connection in the Gondwanaland

In South India, the WNW-striking ~ 130-km-long Achankovil Shear Zone (AKSZ) separating the Trivandrum Block and the Madurai Block is speculated to continue within southern Madagascar in the East Gondwanaland. Two aspects continue to be debated, whether the AKSZ is a terrain boundary shear zone, and if so, when the accretion occurred. We present, for the first time, a detailed field-based analysis of contrasting mesoscale structures and deformation kinematics in the granulite facies rocks across the AKSZ. The mineralogical segregation layering (S2T) in the Trivandrum Block is sub-horizontal to gently inclined and describes F3T regional-scale open asymmetric WNW-trending sub-horizontal folds. By contrast, the S2M granulite facies layering in the Madurai Block describes regional-scale steeply plunging F3M folds. Both blocks share the NNW/NNE-trending sub-parallel sinistral and dextral S3 shears of the AKSZ. Deformation microstructures and garnet-bearing melt-hosted syn-S3 extensional shears attest to the persistence of high temperature during D2–D3 deformations. Monazite chemical dates and existing Pb–Pb zircon dates are overwhelmingly Pan-African in and across the AKSZ, but the Paleoproterozoic (2.1–1.8 Ga) dates in the Trivandrum Block, equivalent to those reported in Anosyan and Androyan domains of southern Madagascar, are lacking in the Madurai Block. By contrast, the Mid Neoproterozoic (800–750 Ma) Pb–Pb zircon dates in the Madurai Block, equivalent to the Antananarivo domain (Central Madagascar), are lacking in the Trivandrum Block. The structural–chronological contrasts between the two blocks suggest the AKSZ to be a Pan-African terrain boundary shear zone system that is continuous with the Ranotsara shear zone in Madagascar. [248]

Thick-skinned tectonics of the Achankovil Shear Zone, southern India, inferred from new deep seismic reflection image: Constraints on the East African Orogen

The East Gondwana was formed by the assembly of continental fragments, namely India, Madagascar, Sri Lanka, and Antarctica, with the evolution of 2000 km long East African Orogen around 550 Ma. Lack of knowledge on the subsurface structure of the orogen led to a wide range of evolutionary models. The present study attempts to resolve various models’ ambiguity by presenting new results of the first deep seismic reflection study across the Achankovil Shear Zone, Southern Granulite Terrain, India that was evolved during the East African Orogen. The seismic study is also the first of its kind among the east Gondwana fragments. The Common reflection surface stack images of the present study reveal hitherto unknown details along with a unique south-dipping reflection fabric extending from the surface to upper mantle depth. We interpret this fabric is formed due to subduction-accretion of Madurai and Trivandrum blocks during the East African Orogen and represents a suture zone. Two deep-seated shear zones, the Achankovil and Tenmalai were also developed at both ends of collision fabric with strike-slip characteristics. Orogenic collapse and post-orogenic extension are inferred from the subhorizontal lower-crustal reflection fabric, which is interpreted as an equilibrated younger Moho. The present study provided an opportunity to understand the formation and exhumation of granulites, which act as a window on the inaccessible lower continental crust. Gravity modelling is performed along the seismic profile. Integration of present results with the existing geological and geophysical data provided basic constraints on the structure and tectonics of east Gondwana fragments and the East African Orogen.

Signatures of late Quaternary land-sea interactions and landform dynamics along southern Kerala coast, SW India

The Kerala coast displays a spectrum of varied geomorphic features that are evolved as a function of Late Quaternary land-sea interaction. The southern Kerala coast, south of Achankovil Shear Zone (ASZ) has been characterised by cliffed coast with entrenched estuarine basins, pocket beaches, promontories, older and younger strand plains, etc., that are evolved as a result of the transgressive-regressive phases to which the coast has been subjected during the late Quaternary Period. In spite of having varied geomorphic features, lacuna in comprehending the evolutionary phases of the coastal landform features is the primary impediment that bolstered us to unravel the Late Quaternary land-sea interactions and landform changes of the coast between Thottapally and Vizhinjam. Moreover, the last two centuries are known to have been witnessing many natural as well as human induced environmental perturbations in the area.The present study attempts to reconstruct the late Quaternary landform dynamics based on the satellite data while the last century changes were extracted from the Digital Shoreline Analysis System (DSAS). The evidences based on geomorphic variations suggest that the present study area hosts two distinct paleo-coastlines – 1) coincides with the eastern boundary of the older strand plains (accreted during Late Pleistocene) and, 2) marks the boundary between older and younger strand plains (accreted during Early–Middle Holocene). A sector-wise analysis reveals that the northern half of the study area is characterised by many coast perpendicular estuarine basins with well-developed Bay Head Deltas in its fluvial end, and Flood Tide Islands near the estuarine mouth. On the contrary, the southern half is characterised by pocket beaches and coast parallel backwater bodies developed during the regressive phases of the sea. A high-resolution study of the shoreline changes during the period, 1920–2018 reveals that the younger strandlines are vulnerable to severe coastal erosion and shoreline retreat at many places compared to coastal accretion. The study stresses the imminent need for continuous monitoring and implementation of site-specific mitigation measures for the conservation and management of this coast known for its outstanding natural beauty and strategic beach placer deposits.

Neotectonic evidences associated with Achankovil shear zone using morphometric analysis and field investigations

Several studies have considered Achankovil shear zone as NW–SE trending Precambrian crustal scale structure. Two major faults, namely Thenmala and Thenmala south faults, are identified associated with this shear zone. The present study identified NW–SE trending segmented lineaments in these zones. Major drainages and rock units surrounding these faults show NW–SE trend. The present study applied both conventional and recent geomorphic parameters to identify anomalies related to these structures. Morphometric results suggest that the area between Thenmala fault and Thenmala south fault, especially the central part, exhibits anomalies supporting active tectonism. Field investigation carried out in this zone identified continuities of NW–SE trending faults as brittle deformation in the southeastern part of the study area. Faults observed in the well sections along these lineaments show multiple slip planes and gouge formations. Morphometric results and field evidences suggest reactivation of these NW–SE structures associated with the present stress regime of peninsular India. Moreover, several instances of historic and recent earthquakes reported from this area. Considering the general trend of the seismic source zone reported in the peninsular India the NW–SE trending faults can generate a magnitude > 5.0. In view of this peak ground acceleration is calculated for a magnitude of 5.5 as the maximum credible earthquake that can be generated by these two faults. Frequent earthquakes within short duration around these faults indicate that the above-mentioned structures may be tectonically active and may generate a ground acceleration between 2.6 and 2.8 in the nearby cities.

Hydrogeomorphological observations from Thenmala and Thenmala south fault, India

Achankovil shear zone is a North West-South East (NW-SE) oriented Precambrian structure of South India. Present work identified evidences of two faults (trending NW-SE) in the southern most end of this shear zone. They are Thenmala fault and Thenmala south Fault. This study is an attempt to understand the structural control of these NW-SE structures over the drainage development of the study area, based on geomorphological evidences and field data. Morphometric analysis of the drainage basins clearly indicate that basins located near NW-SE structures shows more anomalies. This anomalies in the vicinity of lineaments and faults indicate their influence over the drainage development. Observed brittle faults evidences with numerous slip planes from the south eastern end of the study area also support this argument. Morphometric evidences in association with brittle fault observations suggest that Achankovil shear zone may be week and reacting to the current stress regime.

Mineral Chemistry of Mafic Clotsin Putteti Alkaline Suite, Kanyakumari District, Tamil Nadu, India

Putteti is a well-known alkaline suite south of Achankovil Shear Zone in Southern Granulite Terrain (SGT). This intrusive is quite famous for the ubiquitous zircon megacrysts, and various sulphides and oxides. Syenite and pyroxenite, with textural and compositional diversity, form the major intrusive phases of this suite. They occur as alternate magmatic layers where pyroxenes form cumulates. Clots or segregates of pyroxenes are alsonoticedin the syenite. The amount of pyroxene in syenite varies considerably. Earlier workers explained this phenomenon as due to liquid immiscibility of a single magma. The temperature, composition and rheology of the original melt play a major role in deciphering the texture and mineralogy of the resultant rocks. Mineral chemistry helps to infer the thermal history of the members of the suite and the sequence of crystallization. Here we present thechemical data of pyroxenes in syenite and hybrid rock that help to establish the interrelationships of the members of the suite and their evolution.

Detrital zircon and igneous protolith ages of high-grade metamorphic rocks in the Highland and Wanni Complexes, Sri Lanka: Their geochronological correlation with southern India and East Antarctica

The high-grade metamorphic rocks of Sri Lanka place valuable constraints on the assembly of central parts of the Gondwana supercontinent. They are subdivided into the Wanni Complex (WC), Highland Complex (HC) and Vijayan Complex (VC), but their correlation with neighbouring Gondwana terranes is hindered by a poor understanding of the contact between the HC and WC. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U–Pb dating of remnant zircon cores from 45 high-grade metamorphic rocks in Sri Lanka reveals two domains with different age characteristics that correlate with the HC and WC and which help constrain the location of the boundary between them. The HC is dominated by detrital zircon ages of ca. 3500–1500 Ma from garnet–biotite gneiss, garnet–cordierite–biotite gneiss, some samples of garnet–orthopyroxene–biotite gneiss and siliceous gneiss (interpreted as paragneisses) and igneous protolith ages of ca. 2000–1800 Ma from garnet–hornblende–biotite gneiss, other samples of garnet–orthopyroxene–biotite gneiss, garnet–two-pyroxene granulite, two-pyroxene granulite and charnockite (interpreted as orthogneisses). In contrast, the WC is dominated by detrital zircon ages of ca. 1100–700 Ma from paragneisses and igneous protolith ages of ca. 1100–800 Ma from orthogneisses. This clearly suggests the HC and WC have different origins, but some of our results and previous data indicate their spatial distribution does not correspond exactly to the unit boundary proposed in earlier studies using Nd model ages. Detrital zircon and igneous protolith ages in the HC suggest that sedimentary protoliths were eroded from local 2000–1800 Ma igneous rocks and an older Paleoproterozoic to Archean craton. In contrast, the WC sedimentary protoliths were mainly eroded from local late Mesoproterozoic to Neoproterozoic igneous rocks with very minor components from an older 2500–1500 Ma craton, and in the case of the WC precursor sediments there was possibly additional detritus derived from early to middle Neoproterozoic metamorphic rocks. The relic zircon core ages in the HC are comparable with those of the Trivandrum Block and Nagercoil Block of southern India. In contrast, those ages in the WC match the Achankovil Shear Zone and Southern Madurai Block of southern India. These comparisons are also supported by Th/U ratios of detrital zircon cores from paragneisses (Th/U ratios of >0.10 for the former and not only >0.10 but also ≤0.10 for the latter). Comparisons with the Lützow-Holm Complex of East Antarctica indicate that the geochronological characteristics of the HC and WC broadly match those of the Skallen Group, and the Ongul and Okuiwa Groups, respectively.

Implications of shearing on drainage network development in Achankovil Shear Zone, South India: insights from DEM-based geomorphic indices and longitudinal profile analysis

The Achankovil Shear Zone (AKSZ) in the Southern Granulite Terrain separates the Trivandrum block from the Madurai block. Various geomorphic indices and longitudinal profiles of the river systems in the AKSZ, viz., Achankovil river basin (ARB) and Kallada river basin (KRB), were derived from SRTM DEM to decipher the influence of shearing and deformation on the regional drainage evolution. Although hypsometric analysis of the basins implies old stage of geomorphic evolution, horizontal shifts in the channel plan form are restricted (except in the Tertiary sediments), suggesting the structural controls over the drainage organization, which are also supported by the high topographic sinuosity. The transverse topographic symmetry (T) vectors indicate a southwesterly migration for the upstream channel segments of both ARB and KRB, while the northwesterly migration of the downstream courses can be correlated with the dextral shearing of the AKSZ. Even though the shear zone is considered to be the block boundary between the charnockite of Madurai and khondalite of Trivandrum blocks, the moderate to low profile concavity (θ) values are probably the result of suppressing the effect of the block–boundary interactions by shearing and denudation. The study proposes a model for evolution of drainage network in the AKSZ, where the mainstream of the basins was initially developed along NE–SW direction, and later the upstream and midstream segments were reoriented to the NW–SE trend as a result of intense shearing. Overall, the present study emphasizes the significance of geomorphic indices and longitudinal profile analysis to understand the role of shearing and deformation on drainage evolution in transcrustal shear zones.

Nomenclatural notes on the status of the name Chlorophytum clivorum (Asparagaceae)

Chlorophytum heynei Baker (1876: 322) was described based on a collection of Rottler from India in 1816. This species is characterized by elliptic-obovate with pseudopetiolate (petiole 5–8 cm channeled), scape shorter than the leaves, absence of sterile bracts; greenish flowers, anthers shorter than the filaments and papillose filaments (Robi et al. 2015). Most of the recent literature (Sasidharan & Sivarajan 1996, Sivarajan & Mathew 1997, Sasidharan 2013) misidentified this species with C. palghatense K.M.P.Kumar & Adsul (Prabhu et al. 2014: 282), but Robi et al. (2015: 78) resolved the taxonomy of these names. Unaware of this work, plants of Chlorophytum (Ker Gawler 1808: 1071) were described as C. clivorum Mathew & George (2015: 379), based on collections in the mountain forests of Achankovil (part of Achankovil Shear Zone), in Kollam district of Kerala. The critical analysis of the type specimens and protologues associated with both names shows that both the names C. heynei and C. clivorum apply to the same biological unit. Hence, C. heynei holds nomenclatural priority (McNeill et al. 2012, Art. 11.4).

Ariopsis peltata var. brevifolia (Araceae) from Achankovil Shear Zone region of Southern Western Ghats, India

In the bycatch of a scientific expedition in the Achankovil Shear Zone part of Southern Western Ghats, a variant of Ariopsis peltata was collected. It differs from the typical species mainly by the miniature leaf size, dome shaped male zone and by two rings of punctured cavities in synandria. This specimen is described and illustrated here as Ariopsis peltata var. brevifolia.

Consequences of sea level and climate changes on the morphodynamics of a tropical coastal lagoon during Holocene: An evolutionary model

Vembanad Lagoon is the largest backwater system in the southwestern coast of India, and it is one of the wetlands of international importance, as defined by the Ramsar Convention. The coastal area encompassing the Vembanad Lagoon comprises a spectrum of barrier islands, ridges and swales, lagoons, and flood plains. The lagoon and its adjoining coastal lands are known for economically viable deposits of ilmenite rich beach placers, lime shells, and high purity silica sands. The sedimentological, palynological, and geochronological studies reveal that the sea level and climate changes have brought significant modifications in the geomorphic settings of the system. The depositional regimes and the evolutionary phases showed marked differences in the northern and southern reaches of the Lagoon. The geomorphological settings, occurrence of a suite of major heavy minerals (sillimanite + opaque) and predominance of marine elements in the palynological assemblage in the southern reach of the lagoon are indicative of barrier spit development, which is further influenced by reworked Neogene sediments with characteristic sillimanite–opaque heavy mineral suite of khondalitic affinity. The northern half evolved during the transgressive–regressive events of Early–Middle Holocene from a sheltered coastal water body surrounded by thick coastal vegetation. The opaque–inosilicate dominant heavy mineral suite in the northern sector indicates contribution of sediments from charnokitic source rocks in the area north of the Achankovil Shear Zone. The proposed four stage evolutionary model of the Vembanad Lagoon indicates that it evolved from an embayment of the Arabian Sea by the progradation and development of a barrier spit during transgressive–regressive cycles under heavy rainfall events of the Holocene.

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.

Geometry and kinematics of the late Proterozoic Angavo Shear Zone, Central Madagascar: Implications for Gondwana Assembly

This paper documents the 20 to 60km wide N–S trending Angavo Shear Zone (ASZ) in central Madagascar and its tectonic implications by examining its structural styles, kinematics and geometry. Our study indicates that the ASZ is characterized by at least two ductile Late Proterozoic deformation events (D1 and D2) followed by a brittle neotectonic deformation (D3). The early D1 event produced a regionally extensive S1 foliation, stretching/flattening mineral lineation L1 and symmetrical structural fabrics such as recumbent and isoclinal intra-folial folds (F1), implying a flattening deformation. D1 deformational fabrics are locally overprinted by D2 structures. D2 is characterized by a penetrative S2 foliation, shallow south plunging L2 lineation, asymmetric and sheath folds (F2) consistent with a right lateral sense of movement exhibited by delta- and sigma-type porphyroclast systems and asymmetric boudinage fabrics. D2 represents a non-coaxial flow regime formed in a dextral west over east shear zone during a partitioned transpression in response to east–west-directed compression during the assembly of Gondwana. A close resemblance with the Achankovil shear zone in India is noticed; however the continuation of the ASZ in Africa is uncertain.

Deep electrical signatures across the Achankovil shear zone, Southern Granulite Terrain inferred from magnetotellurics

Two parallel magnetotelluric (MT) traverses across the Achankovil shear zone (AKSZ) in southern India using a wide band (1 kHz–1 ks) data acquisition system, reveal the regional geo-electric average strike direction of N 40°W, consistent with the geological strike direction of the AKSZ. The derived model shows distinct high electrical resistivity (> 1000 Ω m) for the upper crust below the Madurai Granulite Block (MGB) with a gentle dip towards south and a northerly dip below the Kerala Khondalite Block (KKB). The lower crust is resistive below both MGB and KKB while it is moderately conductive (< 500 Ω m) below AKSZ. Our study shows that the AKSZ is a tectonic divide between the MGB to the north and KKB to the south. The results together with an integration of seismic, gravity and heat flow data suggest a high dense and moderately conductive mantle material brought up to the mid–lower crust, and a thermally eroded crust with a flower structure at depth, in conformity with the model of AKSZ as the trace of a collisional suture.

Temporal evolution of the Angavo and related shear zones in Gondwana: Constraints from LA-MC-ICP-MS U–Pb zircon ages of granitoids and gneiss from central Madagascar

Granitoids of Neoproterozoic age are well exposed in the Precambrian basement rocks of Madagascar, in the vicinity of and within the Neoproterozoic dextral Angavo shear zone (ASZ). Pre-kinematic granitoids, a syn-kinematic granite, and a granitic gneiss have been dated using Laser Ablation Multicollector Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICP-MS). One pre-kinematic granitoid yields a weighted mean 206Pb/ 238U igneous zircon crystallization age of 823.6 ± 6 Ma, and a second a 756.6 ± 9 Ma igneous crystallization age and a 563 ± 11 Ma metamorphic age. A syn-kinematic granite gives a weighted mean 206Pb/ 238U zircon age of 549.8 ± 4 Ma, which points to the time of movement in the ASZ. A granitic gneiss yields a weighted mean 206Pb/ 238U protolith age of 732 ± 13 Ma and records a 558.8 ± 4 Ma metamorphic age. Accordingly, the ASZ was active and associated with the high-temperature metamorphism of the Angavo-Nondiana belt during the Ediacaran. The late Proterozoic ages of igneous intrusions recorded by the granites show that pre-tectonic granites were sheared in the ASZ, and recorded granulite–amphibolite facies metamorphic events. These U–Pb data suggest a link between the ASZ and the Achankovil shear zone in India, which were adjacent in several proposed Gondwana reconstructions.

The Suruli shear zone and regional scale folding pattern in Madurai block of Southern Granulite Terrain, south India

Through the application of remote sensing techniques followed by field checks, the exact extension and nature of Suruli shear zone in Madurai block of southern granulite terrain (SGT) in south India is brought out for the first time in this work. The dominant rock type exposed in this area is charnockite intruded by granites. The Suruli ductile shear zone extends from just west of Kadaiyanallur in the south to Ganguvarpatti in the north over a length of 150 km. Between Kadaiyanallur and Kambam, the shear zone extends roughly in N-S direction. From Kambam, it swerves towards NE and then towards ENE near Ganguvarpatti. The strongly developed transposed foliation and mylonite foliation within the shear zone dip towards east only and so the eastern block (Varushanad hills) is the hanging wall and the western block (Cardamom hills) is the footwall of the shear zone. In the eastern block, three distinct phases of regional scale folding (F1, F2 and F3) are recognized. In complete contrast, the western block recorded only the last phase (F3) regional scale folding. As the more deformed eastern block (older terrain) moved over the relatively less deformed western block (younger terrain) along the Suruli shear zone, it is proposed that this shear zone is a thrust or reverse fault, probably of Proterozoic age. As there are evidences for decreasing displacement from north to south (i.e., from Ganguvarpatti to Kadaiyanallur), the Suruli shear zone could be a rotational thrust or reverse fault with the pivot located close to Kadaiyanallur. As the pivot is located near Achankovil shear zone which trends WNW-ESE (dip towards SSW), the Suruli shear zone could be splaying (branching) out from Achankovil shear zone. In a nutshell, the Suruli shear zone could be a splay, rotational thrust or reverse fault.

LREE-rich hibonite in ultrapotassic rocks in southern India

We describe a rare terrestrial occurrence of hibonite (CaAl 12TiO 19) from the ultrapotassic rocks which occur as veinlets in graphite-bearing spinellite in the Achankovil Shear Zone (ACSZ), southern India. The ultrapotassic rocks are characterized by an unusual mineral assemblage, kalsilite + leucite + corundum + spinel + perovskite + apatite + hibonite ± phlogopite, along with mantle-derived graphite and Fe-sulfides. Three types of hibonite were identified in terms of chemical zoning as observed in back scattered images: (i) unzoned hibonite, (ii) zoned hibonite with brighter core and dark rim, and (iii) zoned hibonite with brighter core, a less bright intermediate zone and a dark outer rim corresponding to the variations in the distribution of LREEs (light rare earth elements), Ca, Al and Ti in the hibonite structure. The core regions of hibonite are highly enriched in LREEs and Sr, and depleted in Ca, Ti and Al, compared to the rim areas. Chondrite-normalized REE distribution patterns for hibonite exhibit extreme LREE enrichment (La N/Yb N = 763 to 374,494) with a negative Eu anomaly. Apatite displays MREE-enriched patterns. Partitioning of LREEs in hibonite and apatite crystallized from the melts is chiefly determined by their preference for hibonite > apatite. We propose a model that considers the formation of the LREE-rich Ti-depleted hibonite from ultrapotassic silica-undersaturated melts. We envisage that the melts were generated by low degree partial melting of a phlogopite-bearing peridotite that was fertilized in LREEs. The ultrapotassic rock that occurs as veins along the ACSZ represents a melt influx associated with deep-seated extension after the final amalgamation of the Gondwana supercontinent.

Spinel + quartz assemblage in granulites from the Achankovil Shear Zone, southern India: Implications for ultrahigh-temperature metamorphism

We report the finding of equilibrium spinel + quartz assemblage enclosed within garnet in garnet–orthopyroxene–cordierite granulites from Pakkandom within the Achankovil Shear Zone, a region which is considered as the trace of an accretionary suture in recent tectonic models on southern India. The spinel + quartz bearing granulites are composed of poikiloblastic garnet and subidioblastic orthopyroxene in the matrix of quartz, plagioclase, biotite, cordierite, and Fe–Ti oxides. Garnet contains numerous inclusions of sillimanite and biotite as well as spinel and quartz. The spinel in direct contact with quartz has moderate X Mg (= Mg/(Fe 2+ + Mg) = 0.44–0.47), and is Zn and Fe 3+ poor ( X Zn = Zn/(Fe 2+ + Mg + Zn) = 0.027–0.036, Fe 3+/(Fe 2+ + Fe 3+) = 0.12–0.17). Spinel is also present in the matrix surrounded by magnetite, but the matrix spinel contains more Zn( X Zn = 0.067–0.072) and does not show any contact relationship with quartz. Such Zn- and Fe 3+-poor spinel in direct contact with quartz has been regarded as a diagnostic evidence of ultrahigh-temperature (UHT) metamorphism. The high-temperature stability of the spinel + quartz is also supported by the results of geothermobarometric calculation of garnet–orthopyroxene assemblages that provides robust evidence for peak UHT metamorphism at 920–980 °C and 8–10 kbar, which was further confirmed by Al-in-Opx and magnetite–ilmenite geothermometers (900–950 °C and ∼1000 °C, respectively). The peak UHT event was followed by decompression down to 4.0–4.2 kbar and 640–670 °C toward the stability of cordierite along a clockwise P–T path. Similar spinel + quartz assemblage enclosed in poikiloblastic garnet has also been reported from the Palghat–Cauvery Shear Zone system, the trace of a major suture zone within the Gondwana amalgam with evidence for prograde high-pressure ( P up to 20 kbar) metamorphism followed by UHT event. The fine-grained spinel + quartz may thus indicate decompression from higher pressure conditions, confirming the recent attributes of the Achankovil Shear Zone being a collisional suture with evidence for subduction–exhumation processes which accompanied the final collision and amalgamation of the Gondwana supercontinent during the latest Neoproterozoic.