The NW–SE-trending Nagavali–Vamsadhara Shear Zone (NVSZ) in the central Eastern Ghats Mobile Belt (EGMB) is a discrete transverse structure that cuts across the regional EGMB fabric along India’s east coast and is interpreted as a major crustal discontinuity with counterparts in East Antarctica. We use both regional and detailed gravity and magnetic datasets (including approximately 1000 newly collected data points) to understand the scale and subsurface crustal architecture beneath the NVSZ and surrounding areas. We have modelled the crust across the NVSZ using joint gravity and magnetic modelling along three selected profiles to better understand the subsurface crustal architecture below the NVSZ. The jointly interpreted gravity and magnetic maps reveal that: (1) the NVSZ is characterised by subdued magnetic signatures, while gravity anomalies closely align with the exposed lithological units; (2) The relatively subdued magnetic response of the Nagavali Shear Zone (NSZ) and Vamsadhara Shear Zone (VSZ) contrasts other shear zones within the EGMB, which exhibit distinct magnetic anomalies, indicating fundamental differences in their magnetic properties and/or structural evolution; (3) The geophysical anomaly patterns suggest that the younger granitoid gneisses overprint the pre-existing charnockite and khondalite rocks; and (4) The NSZ and VSZ are interpreted as mid-crustal faults, while the lower crust beneath them behaves as a coherent single layer in the modelled profiles. Their geometry, together with the arcuate charnockite belts flanking granitoid gneiss, indicates development under an extensional regime that bent these belts, with subsequent repeated reactivation across multiple supercontinent cycles.

The Vindhyan basin is one of the major Proterozoic basins in the Indian shield bounded by Central Indian Tectonic Zone (CITZ) in the south and Bundelkhand craton in the north having thick sedimentary succession. Interaction of basin with CITZ and its imprints on regional crustal structure is important to understand its evolution. In this study, a detailed interpretation of gravity and aeromagnetic anomaly maps with constraints from multi-channel seismic data is carried out in the Son valley part of the Vindhyan basin to identify its structural links with the surrounding tectonic domains. The 3-D gravity inversion along with 2-D gravity and magnetic modelling helped to delineate the crustal architecture and Moho geometry which reveal that the Moho below the basin varies between 38-46 km with the deepest Moho in southern part along its boundary. The crustal models in the study area further reveal i) higher density and magnetization for lower Vindhyan sediments because of intrusive rocks/volcano clastic sediments, ii) the eastern part of basin is devoid of crustal underplating iii) basement high, complex rift tectonics and basin tilting in Damoh region, iv) undulating crustal layers intervened by deep crustal faults in the southern part of the basin and the CITZ region, and v) the Bundelkhand Tectonic Zone and Madaura Shear Zone in the craton are characterized by deep crustal faults. In this paper, we propose a geodynamic evolutionary model of the Vindhyan basin which reveals opening of the basin subsequent to the formation of CITZ during similar to 2000-2200 Ma. The basin sediments were deposited in multiple stages during 1700-1000 Ma followed by the basin closure during 1000-750 Ma. Hence, it is inferred that the post tectonic activity of the CITZ affected the Son Narmada fault zone and facilitated the opening of Vindhyan basin during the Paleoproterozoic period.

In the passive margin of southeast Australia, a mosaic of tectonic structures of the Otway Basin records the protracted Cretaceous to Eocene break-up evolution of Australia and Antarctica. Here, we use an innovative approach that combines Euler deconvolution and DBSCAN clustering of global magnetic data and drill-hole-constrained interpretations of deep 2D seismic traverse to image deep-rooted, pre-rifting basement crustal structures now covered by passive margin basins. The method is used to identify the complex network of Early Paleozoic faults that were reactivated and transformed into major basing-bounding listric faults during Cretaceous rifting. Major faults identified include the Bambra, Avoca, Yarramyljup, and Moyston faults. The Yarramyljup and Moyston faults segmented the Cretaceous Otway Basin, demonstrating how basement lithospheric heterogeneities can influence basin development. Our analysis also redefines the northwest margin of the Proterozoic VanDieland microcontinent. This microcontinent acted as a rigid crustal block during the Cretaceous extension and influenced the geometries of the passive margin basin depocenters. These insights transform our understanding of the crustal architecture and structural inheritance in the tectonic evolution of southeast Australia and establish a template for imagining deep crustal structures elsewhere.

Pranhita-Godavari (PG) Basin is an inter-cratonic failed rift basin between the Dharwar and Bastar cratons in the Southern Indian Shield. Previous studies reveal an unusually thick crust beneath it with high surface heat flow, a characteristic uncommon in inter-cratonic failed rift basins. To understand this uncommon phenomena, we carry out the aeromagnetic and Bouguer gravity data interpretation of the PG Basin constrained by seismic and seismological data. The Bouguer gravity anomaly highs on either side of the basin reflect crustal extension of the Karimnagar Granulite Belt (KGB) and Bhopalpatnam Granulite Belt (BGB). The Moho depth map, derived from constrained 3-D gravity inversion, shows depth variations between 35 and 47 km in the region. Additionally, the estimated Curie depths from aeromagnetic anomaly data vary between 22 and 30 km, with shallower depths (similar to 22-26 km) beneath the basin, and reconcile with the observed higher heat flow in the region. The crustal model reveals: i) a normal crustal thickness below the PG Basin; ii) a high-density lower crustal body (similar to 3.05 gm/cc) within the basin; iii) high-density, upper-mid crustal bodies interpreted as granulitic rocks (KGB and BGB) exhibit increased magnetisation at the margins of the PG basin. Both the KGB and BGB thicken with depth beneath the PG Basin before rapidly thinning-resembling boudinage beneath the basin depocenter. Both KGB and BGB represent once continuous package of granulite rocks. The thinning of granulite package is related to crustal thinning associated with rift formation and the development of PG Basin.

The Southern Indian Shield comprises the Bastar and Dharwar cratons, the Southern Granulite Terrain, the Eastern Ghat Mobile Belt, and the Pranahita Godavari and Cuddapah rift basins. Multiple amalgamations and large igneous provinces related to magmatism during the Archean to Proterozoic times significantly modified the lithosphere in this region, making it difficult to infer the structure and geological history. This necessitated the delineation of lithosphere structure through geophysical data. In this study, 2D joint potential field modelling constrained by seismic and seismological data is performed to delineate the lithospheric structure below the Southern Indian Shield region. The constrained potential field models reveal: i) Cuddapah Basin, Southern Granulite Terrain, and Eastern Ghats Mobile Belt regions are characterized by higher density crust, ii) a mid-crustal low-density layer below Dharwar and Bastar cratons, iii) deeper Moho below Pranahita Godavari and Cuddapah basins, iv) a comparatively thin lithosphere below Dharwar Craton; and v) relatively deeper lithosphere beneath Southern Granulite Terrain-Dharwar Craton boundary region. The high-density, thicker crust and shallow lithosphere beneath the Cuddapah Basin and the Eastern Ghat Mobile Belt have resulted from large igneous province events at ca 2200 Ma and 1800 Ma, followed by lithospheric metasomatism at ca 1000 Ma. The shallow Lithosphere-Asthenosphere Boundary beneath Pranahita Godavari graben reflects Proterozoic rifting and further reactivation during Gondwana breakup. The deeper Lithosphere-Asthenosphere Boundary beneath the Southern Granulite Terrain-Dharwar Craton boundary is attributed to orogenesis caused by subduction events between 780 Ma and 550 Ma. The distinct crustal structure, magnetisation, and shallow Lithosphere-Asthenosphere Boundary in the Dharwar Craton region are inferred to have resulted from amalgamation events during the Archean, overprinted by Proterozoic large igneous province events at ca 2200 Ma and 1800 Ma.

The Precambrian terrains of the Eastern Indian Shield (EIS) comprise of Bundelkhand, Singhbhum, and Bastar cratons with intervening Proterozoic mobile belts such as Central Indian Tectonic Zone, Eastern Ghat Mobile Belt, Singhbhum Mobile Belt and Chotanagpur Granite Gneissic Complex. This region is also characterised by the presence of Proterozoic Mahanadi Rift, Chhattisgarh and Vindhyan Basins with significant coverage of Indo-Gangetic Plain sediments in northern part. In this study, we present the results of a seismically well-constrained 2-D multi-scale geopotential modelling to delineate lithosphere structure across different Precambrian terrains of the EIS. The joint interpretation of the potential field data reveals that i) mobile belts are bounded by the deep crustal faults with denser crust, ii) presence of thick underplated crust below Singhbhum craton, Singhbhum Mobile Belt, Chotanagpur Granite Gneissic Complex and the surrounding rift basin, iii) localised Moho upwarp at a depth of ~36-37 km below the Proterozoic basins, iv) the Lithosphere-Asthenosphere Boundary (LAB) varying between 90-200 km below the EIS region. The distinct crustal structure along with relatively deeper LAB (130-200 km) below the mobile belts suggests the Proterozoic amalgamation and lithosphere reworking. Below the Singhbhum craton, LAB is observed at a depth of ~145-155 km, which is comparatively thinner with respect to other cratonic areas elsewhere. The observed crustal underplating and thinner LAB below the Singhbhum craton indicate the lithosphere erosion and magmatic upwelling caused by the major Paleo-Mesoproterozoic and early- Cretaceous Large Igneous Province (LIP) events.

Previous geophysical investigations of the western continental margin of India (WCMI) confirm the two-phase breakup history of the margin with the first breakup taking place between India and Madagascar that created the Mascarene Basin in the Late Cretaceous and the second breakup event in Early Paleocene with Seychelles separating from India. Despite numerous geoscientific studies along the WCMI, the opening of the Laccadive basin, situated along the southern part of the margin, remains poorly constrained. In this study, we evaluate the multi-channel seismic reflection and gravity anomalies at the margin to identify the early rift signatures in conjunction with the magnetic anomaly identifications in the Mascarene Basin. The analysis led to the identification of two trends of extensional structures, a NNW–SSE-oriented structure over the Laccadive Ridge north of Tellicherry Arch, interpreted to result from ENE–WSW extension, and a SSW–NNE-oriented structure in the Laccadive basin region towards the south, interpreted to result from NW–SE extension. Previous plate reconstruction models of the Mascarene Basin using marine magnetic lineations suggest that the ENE–WSW extension observed over the Laccadive Ridge could be related to the India–Madagascar separation. We associate the pattern of sediment deposition and the presence of a Paleocene trap volcanics, linked with the NW–SE grabens observed in the Laccadive basin region, to the extension between the Laccadive Ridge and the western coast of India after the separation of Madagascar from India. We further propose that the anticlockwise rotation of India and the passage of the Réunion plume have facilitated the opening of the Laccadive basin.

We present a novel approach that determines the location and dip of geologic structures by clustering Euler deconvolution depth solutions using Density-Based Spatial Clustering Applications with Noise (DBSCAN). This method and workflow rely on the association of changes in the location and relationships between Euler depth clusters and cluster boundaries with changes in rock susceptibility. We applied our method to global magnetic and high-resolution aeromagnetic datasets over Phanerozoic-Precambrian zone-bounding faults in west and central Victoria. The architecture of these structures at different scales from this imaging technique is comparable to interpreted 2D seismic reflection data. The results from the global magnetic data resolved the architecture of these structures below 5 km, while the aeromagnetic data used were limited to structural information of faults above 2 km depth. Therefore, this method shows the structural relationship of the west-dipping Avoca Fault that soles into the east-dipping Moyston Fault at a depth of similar to 22 km in central Victoria and at a shallower depth of similar to 15 km southward beneath the Quaternary basaltic rocks of the Newer Volcanic Province. In the vicinity of the Heathcote Zone, the method resolves the location, dip, and overprinting relationship between faults and extrusive rocks, such as the relationship between the Heathcote and Mount William Faults and the granitic Cobaw Batholith. We show how combining magnetic data at various scales can track faults from the near-surface to deeper roots while avoiding possible over-interpretation. We demonstrate how to optimise the DBSCAN parameters and a sensitivity analysis of how to determine clusters and cluster boundaries that are geologically relevant in the absence of geological constraints. Our technique provides an effective and rapid tool for imaging structures and can supplement complex and expensive imaging techniques to resolve the architecture of structures in complex geologic terrains.

The Precambrian terranes of the Eastern Indian Shield (EIS) comprise the Bundelkhand, Singhbhum, and Bastar cratons with intervening Proterozoic mobile belts such as Central Indian Tectonic Zone, Eastern Ghats Mobile Belt, Singhbhum Mobile Belt and Chotanagpur Granite Gneissic Complex; and the Proterozoic Mahanadi Rift, Chhattisgarh and Vindhyan Basins, with significant coverage of Indo-Gangetic Plain sediments in the northern part. This study presents the results of a seismically well-constrained 2-D multi-scale potential field modelling to delineate the lithosphere structure across different Precambrian terranes of the EIS. The joint interpretation of the potential field data reveals that (i) the mobile belts are bounded by the deep crustal faults with denser crust, (ii) presence of thick underplated crust below Singhbhum Craton, Singhbhum Mobile Belt, Chotanagpur Granite Gneissic Complex and the surrounding rift basin, (iii) localised Moho upwarp at a depth of 36–37 km below the Proterozoic basins, and (iv) the Lithosphere-Asthenosphere Boundary (LAB) varying between 90 and 200 km below the EIS region. The distinct crustal structure and deeper LAB (130–200 km) below the mobile belts suggest the Proterozoic amalgamation and lithosphere reworking. Below the Singhbhum Craton, the LAB is observed at a depth of 145–155 km, which is comparatively thinner than other cratonic areas elsewhere. The observed crustal underplating and the thinner LAB below the Singhbhum Craton indicate that the lithospheric erosion and magmatic upwelling was caused by the major Paleo-Mesoproterozoic and Early-Cretaceous large igneous province events.

This review aims to bridge the knowledge gap between geological and geophysical communities by elucidating the interpretation of aeromagnetic data. Aeromagnetic surveys measure the Earth's magnetic field variations and provide critical insights into subsurface geology, including basins, stratigraphy, igneous rocks and structural geology. The magnetic properties of rocks make these datasets valuable for identifying anomalies associated with various rock types and their magnetic responses. However, interpreting aeromagnetic data is complex due to the diverse geological processes that influence the formation and distribution of magnetic minerals, which must then be correlated with geological phenomena and features. Despite improved data accessibility and processing, many geoscientists still find interpreting aeromagnetic data challenging, resulting in a shortage of skilled expertise for research and industry applications. Accurate interpretation necessitates a thorough understanding of data collection and processing, recognising both the insights and limitations of the methods used and understanding how data resolution impacts the scale of interpretable geological features. This review is intended to assist those grappling with these challenges and to aid the geophysical community in interpreting complex geological features.Data treatment is explained with a focus on the reasons for specific processing methods rather than their mathematical foundations. Emphasis is placed on rock properties and their influence on aeromagnetic data expressions. The aeromagnetic expressions of common geological elements, including sedimentary, igneous, and metamorphic rocks, and their structures, such as stratigraphy and structural geometries related to folding and faulting, are explored. The discussion covers how these responses arise and how to identify them. Our explanations aim to bolster confidence in data interpretation for geologists new to aeromagnetic data and geophysicists who may not regularly interpret geological information from such data.Finally, we present strategies and pitfalls for interpreting aeromagnetic data, discuss automated interpretation methods, and offer practical guidance to improve interpretation skills and outcomes.

Seafloor spreading along the Carlsberg and Central Indian ridges has steered the tectonic evolution of the western Indian Ocean. These spreading ridges display variations in spreading rate, segmentation, and morphological characteristics, providing clues to the long-term evolution of the oceanic lithosphere in this region. To assess the influence of two notable off-axis thermal sources, the Réunion plume and the Indian Ocean Diffuse Boundary Zone, on factors such as rigidity and seafloor subsidence along these ridges, we computed the effective elastic thickness (Te), residual geoid-age slopes, and residual depth anomalies (RDA) of the region using gravity and geoid data. The results reveal a weaker lithosphere at the northern Central Indian Ridge (Te: 8.5–8.9 km) compared to the neighboring segments of the southern Central Indian Ridge (Te: 10.5–12.7 km) and the Carlsberg Ridge (Te: 10.5–14.7 km). Residual geoid and RDA variations suggest asymmetric seafloor spreading and subsidence along the entire ridge system. The asymmetric subsidence across the Central Indian Ridge is largely due to upper mantle contamination from the Réunion plume, while across the Carlsberg Ridge, it may be linked to its complex tectonic history. The rigidity and seafloor spreading patterns along the northern Central Indian Ridge are notably affected by thermal perturbations from the regional heat flow anomaly of the ongoing diffuse deformation zone. Moreover, the Te and segmentation patterns roughly correlate along the ridge system, suggesting a causal relationship between the two or the presence of underlying factors such as regional thermal structure influencing both.

Narmada-Son Lineament (NSL) is a prominent geo-tectonic feature located in central India, characterised by complex geological structures and evolutionary history. In this study, we present a detailed joint interpretation of the aeromagnetic and gravity anomaly maps to delineate the crustal structure in order to address the thermo-tectonic activity below the NSL region. The ENE-WSW linear trend observed in the aeromagnetic anomaly map correlates well with the major tectonic elements of the NSL and the Vindhyan basin. The low-pass filtered regional gravity anomaly map and corresponding residual anomaly map revealed several crustal-scale structural features in the study area. The Curie depth map of the region prepared from the magnetic anomaly map shows the variation in depth between 24 and 48 km. The corresponding heat flow map correlates well with the trends in surface heat flow data. The gravity-derived Moho depth map reveals that the values vary between 30 and 59 km in the NSL region with deeper Moho below the Vindhyan basin. Further, the crustal models obtained from the constrained potential field modelling for two long transects across the NSL in the present study area revealed i) the absence of crustal underplating in the eastern part of NSL below Hirapur-Mandla region, ii) the presence of mid-crustal horst like structure below NSL, iii) undulating Moho with higher density crustal rocks within the NSL compared to the surrounding areas. The modelled crustal structure and the analysis of Curie depth as well as heat flow maps of the study area, reveal that the NSL region was affected by repeated tectonic activities since the Proterozoic times, and the present-day tectonics of this belt is greatly influenced by the reactivation of pre-existing faults along the NSL.

<p>The Northwest Indian shield (NWIS) comprises of Archean Bundelkhand, Marwar and Dharwar cratons, Proterozoic mobile belts of Aravalli Delhi fold belts (ADFB) and Central Indian tectonic zone (CITZ), and the basins such as Vindhyan (VB), Cambay (CR) and the Kutch (KR). The major area of the NWIS is covered by the Cretaceous Deccan Volcanic Province (DVP) that makes it difficult to assess the lithosphere structure in this region. Here we present the seismically constrained multi-scale geopotential field interpretation of  gravity, magnetic and geoid across the major Precambrian terrains of NWIS to delineate the lithosphere structure and further to understand the evolution of these terrains. The Bouguer gravity anomaly map shows overall high gravity values except the Bundelkhand and Dharwar cratonic parts over the NWIS region. The subsurface extension of the Precambrian  terrains of the NWIS are indicated by the distinct anomaly signatures in regional gravity anomaly map. The residual gravity anomaly map is able to delineate the shallow source bodies and boundaries between various terranes that correlat well with the surface geological expressions. The constrained geopotential modelling carried out along SW-NE trending profile across the region reveals that the Moho and  Lithosphre Asthenosphere Boundary (LAB) below the DVP and CR is relatively shallow as compared to the ADFB. It has also been noticed that a high density layer at the base of the lower crust, represents the presence of  underplated crust. The shallower lithosphere structure observed below the CR region might indicate the Cretaceous reworking. The imprints of the Deccan magmatism through intrusive bodies and the modelled structure below NWIS have implications on the lithosphere evolution in the region. </p>

East Antarctica along with Greater India played a vital role in the accretion and breakup of the Indo-Antarctic landmasses during the supercontinents Nuna, Rodinia and Gondwana. Without geophysical potential field methods, interpreting the architecture of the ice-covered geological provinces of Antarctica is impossible. We present here a crustal element map of East Antarctica between Enderby Land and Princess Elizabeth Land (Indo-Antarctica tectonic element) using aerogeophysical data interpretation. The data reveal distinct anastomosing geophysical provinces that correlate with sparse geological data. Our crustal element map shows the Oygarden Province and the Northern and Southern Rayner provinces are arcuate belts that wrap around the Archean Napier Province. These provinces represent the remnants of an accretionary tectonic margin, which evolved between ca 1300 Ma and 900 Ma. The arcuate geometry of these Meso- to Neoproterozoic provinces formed during the collision with the Napier Province, which represents a microcontinent. This collision triggered widespread extension and ultra-high temperature metamorphism in the Northern and Southern Rayner provinces. The southernmost provinces include the Fisher Province, Lambert Province and a transition zone. The provinces are truncated by a suture zone with the Archean Ruker Province, following north-dipping subduction during the Meso- to Neoproterozoic. Our interpretation provides a template upon which to correlate geological provinces with the terranes on the conjugate eastern Indian margin.

The Indian shield region comprises of several Archean cratons, Proterozoic mobile belts, Proterozoic-Phanerozoic rift basins, and volcanic provinces and has been significantly modified by various tectono-thermal events and the supercontinental cycles. These overprinting processes and evolution can be understood, to some extent by characterizing the lithosphere structure. In this study, we carried out seismically constrained 3-D nonlinear gravity inversion using tesseroids in the spherical coordinate system to delineate the Moho geometry for the Indian shield. Further, the supervised machine learning approach using Deep Neural Network (DNN) is implemented on the geopotential data to obtain Lithosphere-Asthenosphere Boundary (LAB) across the Indian shield. The inversion results show that the Moho depth varies between 33 and 60 km below the Indian shield with a mean difference of - -0.3 km and a standard deviation of -4.9 km relative to the Moho from receiver function data. For the DNN model, the estimated LAB depth values range between 95 and 280 km below the Indian shield. The model gave rise to the coefficient of determination R2 of 0.93, Mean Absolute Error of -14 km, and Root Mean Square Error of -22 km between the observed and predicted LAB depths. Apart from the general agreement of Moho and LAB depth estimates with previous seismological studies, the present study provides better resolved information of lithosphere structure across the Indian shield. The modelled lithosphere structure therefore will be useful to integrate with the geochronological data in order to to understand the Proterozoic tectonic evolution of the Indian Shield.

The Laccadive-Chagos Ridge (LCR) is a prominent aseismic ridge in the Indian Ocean.The origin and nature of the crust beneath the LCR have been debated.Based on Ar-Ar geochronology of the volcanic basement rocks from ODP wells 713 and 715 from Chagos and Maldives ridges, a hotspot trail model was proposed for the genesis of the LCR.On the other hand, based on geophysical studies, the LCR has been inferred as a continental sliver/hyperextended continental crust which has undergone heavy underplating and Reunion hotspot volcanism.Even though these two different hypotheses differ on the genesis and nature of the LCR, both agree that the ridge experienced extensive Reunion hotspot trail volcanism, making the crust's nature challenging to decipher.We report Ar-Ar ages of two rhyolite rock samples from a well drilled on the Padua bank located on the northern extent of the Laccadive ridge.These acidic rocks are at a depth of ~1700m and ~2300m and underlie the Tertiary sediments forming the basement in this well.The previously determined K-Ar geochronology ages are 60.2 Ma and 102 Ma.The precise Ar-Ar dates of these rocks determined in the present study are 76.2 ± 0.4 Ma and 77.5 ± 0.5 Ma (2σ) respectively.These acidic rocks (78-76 Ma) coupled with onshore acidic rocks of the St-Mary's island (87-84 Ma) and Ezhimala (95-93 Ma) suggest continuous extension between the India and Laccadive ridge after the Indo-Madagascar continental breakup (88 ma).Contrary to the earlier hypothesis, the acidic rocks of Laccadive Ridge are older than the Reunion Plume trail volcanism (62-60 Ma).The geophysical and geochronological evidence suggests that the Laccadive ridge is continental in nature, and the volcanism predates the Reunion plume volcanism.These evidences suggest that the Laccadive Ridge is a continental thinned crust that has undergone rift-related volcanism rather than a hotspot trail.

The Red Sea provides an opportunity to study the processes during the transition from continental rifting to early-stage seafloor spreading during ocean initiation. We delineate variations of lithospheric architecture and the nature of extension along the Red Sea region through joint interpretation of gravity and geoid anomalies and gravity-topography transfer functions. We use lithospheric-scale models to compare stretching factors with upper mantle gravity anomaly, residual mantle Bouguer anomaly, and effective elastic thickness. Based on our observations, the Red Sea is divided into four segments; each having distinct lithospheric characteristics and stretching styles. These are: (i) southernmost Red Sea and Danakil having regionally weak and stretched lithosphere, (ii) southern Red Sea with fully developed seafloor spreading and asymmetric lithospheric architecture, (iii) central Red Sea having discontinuous magma accretion with newly formed seafloor spreading, and (iv) northern Red sea with a stronger lithosphere and limited stretching revealing a stage of continental rifting. In these segments, lithospheric stretching correlates with regions of weak lithosphere, including a regime of sublithospheric plume channel beneath the southern Red Sea. The Zabargad fracture zone between the central and northern segments is revealed as a major lithosphere-scale boundary that may act as a barrier to the propagation of seafloor spreading into the northern Red Sea. The weak and highly stretched lithosphere in this region may indicate the onset of a new spreading cell. Our results conclude that the evolution of the Red Sea is more complex than the previously suggested kinematic models of simple "unzipping" and illustrate that several extensional styles can exist within different segments during the initial stages of ocean formation.

V. S. Gokul 1,2,3., K. M. Sreejith4., G. Srinivasa Rao 5., M. Radhakrishna 1*., P. G. Betts 21 Department of Earth Sciences, Indian Institute of Technology Bombay, Mumbai 400 076, India.2 School of Earth, Atmosphere and Environment, Monash University, Clayton, Victoria 3800, Australia.3 IITB-Monash Research Academy, Indian Institute of Technology Bombay, Mumbai 400 076, India.4 Geosciences Division, Space Applications Centre, Ahmedabad 380 015, India.5 Department of Applied Geophysics, Indian Institute of Technology (Indian School of Mines) Dhanbad, Dhanbad 826 004, India (*Corresponding author:gokul.vs@monash.edu)AbstractThe Indian Ocean has the largest geoid anomaly, known as the Indian Ocean Geoid Low (IOGL). This long wavelength geoid depression has a magnitude of negative 106 m, and is centered south of India. The nature and depth of the sources causing this characteristic low are poorly constrained and has been the subject of debate. In this abstract, we focus on understanding the density contributions to the geoid low from the crust and upper mantle using joint analysis of geoid and gravity data along with published tomographic models in the region. Decomposition of geoid anomalies in the spectral domain indicate that mass anomalies below the upper mantle (> 700 km) contribute to 90% of the total geoid anomaly. In order to compute the upper mantle contribution to the IOGL, we used the Moho geometry and the crustal density structure from the 3-D gravity inversion, and the SL2013sv tomography model for the upper mantle density structure. The presence of density sources, which was not resolved in the modeling within the sub-lithospheric mantle is confirmed upon comparing the crustal and upper mantle (up to 700 km) geoid response below the IOGL with n=10 residual geoid anomaly. Integrated gravity-geoid 2-D modeling of the geometries of the anomalous sources located at the base of LAB and at a depth of 320-340 km, respectively, confirms that the contribution of density structures up to 700 km explains only the ten percent of the IOGL which matches well with the spectral decomposition results. This suggests that the lower mantle sources, such as paleo-subducted slabs or plume sources from the core-mantle boundary significantly contributes to the IOGL.

The breakup of Gondwanaland led to the creation of many rift basins, of which the Kutch basin is one. Previous geochronological studies of the Kutch onshore rocks have established multiple episodes of magmatism ranging from 124-60 Ma. The wells drilled on the Kutch offshore basin also encountered magmatic rocks at various depths, but their temporal relationship is not constrained. The present study reports the Ar-Ar ages of 5 igneous rocks from the Kutch offshore wells. As determined by petrographical and geochemical analysis, these samples comprise two basalts(b), two dolerites(d), and a rhyolite(r). The plateau ages of the samples are 80.5 ± 0.5(b), 81.4 ± 0.5(r), 100.3 ± 0.6(b), 72.6 ± 0.4(d), and 67.1 ± 0.6(d) (errors quoted at 2σ level). These ages establish magmatism offshore from 100 to 67 Ma. There are several levels where magmatic rocks occur in these wells. Dolerite stringers in Early Cretaceous to middle Jurassic sedimentary rocks have been reported from a few wells. The geochronology data from the Kutch onshore and adjoining areas in Rajasthan show a magmatic record from 190-60 Ma. There is a possibility that some magmatic rocks in the Kutch offshore basin encountered in different wells may also record the older magmatism and events from the break-up of Gondwana to Seychelles, thereby unfolding the tectono-magmatic history of this region.