Laxmi Ridge Research Articles

Réunion plume associated flood basalt volcanism on the northwestern continental margin of India and related tectonics

Late Cretaceous Deccan flood basalt province in central-western India was formed by the Réunion plume and continues to attract global attention due to its enormous volume and volcanic eruption straddling the K-T boundary. Furthermore, despite having a profound impact on crustal architecture of the Northwestern Continental Margin of India (NWCMI), very little information about the flood basalt extension in the NWCMI is reported. Therefore, it is necessary to document areal extent and volume of the flood basalt on the NWCMI. In this study, bathymetry, gravity, magnetic and seismic data are integrated to delineate the spatial extent and volume of offshore flood basalt. Crustal architecture, and the role of Réunion plume in evolution of the NWCMI are also discussed. According to the findings of this study, Réunion plume associated flood basalt has carpeted the entire NWCMI up to the Laxmi Ridge. It is also found in the Laxmi Basin, regardless of its crustal nature (continental/oceanic), which is still debated. The results revealed that the flood basalt occurs at depth of 500-7500 m and its thickness varies from few meters to 3900 m. The study also discovered several magmatic intrusions into the crust, which altered initial crustal configuration of the NWCMI. The study revealed an extrusive area of ∼5.37 × 105 km2 and a volume of ∼6.59 × 105 km3 of lava flow in the NWCMI. The estimated offshore surface area covered with the Réunion plume associated flood basalt is 7% larger than the Deccan trap-covered surface area on the adjacent onshore region.

Lithospheric structure and geodynamical evolution of the Laxmi Ridge: new inferences from satellite gravity studies

Lithospheric structure and geodynamical evolution of the Laxmi Ridge: new inferences from satellite gravity studies

The Secondary Minerals from the Pillow Basalt of Salsette-Mumbai, Deccan Volcanic Province, India

Secondary minerals occur within the tholeiitic basalts of Salsette Island in the greater Mumbai region, as well as in other localities in the Deccan Volcanic Province (DVP). However, the secondary minerals of Salsette Island show remarkable differences with respect to their mineral speciation and precipitation sequence, which are both due to their unique geological environment. The greater Mumbai region is built up by the Salsette subgroup, which represents the youngest sequence of the DVP. It formed subsequently to the main phase of DVP activity in Danian time (62.5 to 61.5 Ma), in the course of the India–Laxmi Ridge–Seychelles breakup. The main part of the Salsette subgroup consists of tholeiitic basaltic flows with pillows, pillow breccia, and hyaloclastite, which formed in contact with brackish and fresh water in a lagoonal environment. In some places, intertrappeans are represented by fossiliferous shallow water sediments. On the top, trachytic and rhyolitic subaqueous volcaniclastics occur, and some dioritic bodies have intruded nearby. Due to differing fluid rock interactions, several distinctly different secondary minerals developed in the void spaces of the hyaloclastite breccia of the interpillow matrix and in the pillow cavities. The highly permeable hyaloclastite breccia formed an open system, where pronounced precipitation occurred in the early phase and at higher temperatures. In contrast, the pillow cavities were a temporally closed system and contained, for example, more low-temperature zeolites. The genesis of the secondary minerals can be summarized as follows: During initial cooling of the volcanic rocks at about 62 Ma, the first mineralization sequence developed with chlorite, laumontite I, quartz, and calcite I. Ongoing magmatic activity caused reheating and the main phase of precipitation at prehnite–pumpellyite facies conditions. During generally decreasing temperatures, in the range of 270–180 °C, babingtonite, laumontite II, prehnite, julgoldite, yugawaralite, calcite II, ilvaite, pumpellyite, and gryolite developed. The fluid contained SiO2 + Al2O3 + FeO + MgO + CaO, and minor MnO and Na2O, and was predominately mineralized by the decomposition of basaltic glass. Further temperature decreases caused zeolite facies conditions and precipitation of okenite I, scolecite, heulandite, stilbite, and finally chabazite I, in the temperature range of 180 °C to less than 100 °C. As FeO, MgO, and MnO were then absent, an interaction of the fluid with plagioclase is indicated. According to Rb-Sr and K-Ar ages on apophyllite-K, a third phase of precipitation with apophyllite-K, okenite II, and chabazite II occurred in the late Eocene to early Oligocene (30–40 Ma). The new hydrothermal fluid additionally contained K2O, and temperatures of 50–100 °C can be expected.

Does the Laxmi Ridge continue towards the Laccadive Ridge? New insights from an integrated geophysical study

The Laxmi and Laccadive ridges are two major aseismic ridges in the NW Indian Ocean. Bathymetry, gravity and magnetic data and their derivatives are analyzed and modelled to assess interrelationship between both the ridges as well as to establish their crustal structure, nature and isostatic compensation. Boundaries of the Laxmi and Laccadive ridges are delineated by tilt derivative with constraints from available seismic-sections. Integrated gravity and magnetic models reveal that both the ridges have almost similar crustal layers and are underplated all along its length. Crustal models also depict that both the ridges are carpeted with flood basalt and heavily intruded. 3D coherence between mantle Bouguer anomaly and residual bathymetry reveals that the elastic plate thickness and subsurface to surface load ratio for both the ridges vary from 3 to 4 km and 0.7 to 0.8, respectively. Several characteristic similarities viz. crustal structure and nature, elastic plate thickness, magmatism have been observed for both the ridges. Seismic sections near the junction of the ridges suggest that the basement high corresponding to the Laxmi Ridge is further continued towards the Laccadive Ridge. In addition, the northwestern part of the Laccadive Ridge has NW-SE structural lineaments similar to that of the NW-SE segment of the Laxmi Ridge. Based on results of the present and previous studies, we infer that (i) the Laxmi Ridge is extended southeastward towards the Laccadive Ridge, (ii) both the ridges are continental slivers which are underplated as well as intruded and carpeted by volcanics, and (iii) both the ridges are locally compensated.

Integrating heavy-mineral, geochemical and biomarker analyses of Plio-Pleistocene sandy and silty turbidites: a novel approach for provenance studies (Indus Fan, IODP Expedition 355)

AbstractA multidisciplinary mineralogical, geochemical and biomarker study of Indus Fan sediments cored during International Ocean Discovery Program (IODP) Expedition 355 to the Laxmi Basin was carried out to define the different compositional signatures of sand, silt and clay. Upper Pliocene – lower Pleistocene turbidites from sites U1456 and U1457 were selected as the best candidates for this study. The integrated dataset presented here was obtained by coupling traditional and innovative bulk-sediment and single-mineral techniques on the same samples. Turbiditic deposits mostly consist of medium to fine silt, including rich and diverse heavy-mineral assemblages. Such a fine grain size forced us to push the limits of high-resolution quantitative heavy-mineral analysis down to as low as 5 μm. Heavy-mineral analysis allowed us to establish a Himalayan origin of the detritus in the studied turbidites. Heavy-mineral concentrations are higher in channel-fill than in overbank deposits. Mineralogical and geochemical data concur in revealing that fast-settling ultradense minerals such as zircon are preferentially concentrated in channel-fill deposits, whereas the top of overbank deposits are notably enriched with slow-settling platy phyllosilicates. Biomarker analysis represents a most suitable complementary technique that is able to investigate the provenance signature of the finer sediment fraction, largely consisting of clay. This technique allowed us to identify a largely terrigenous origin of organic matter at Site U1456 and an open marine origin at Site U1457. The latter site lies closer to the Laxmi Ridge, where thermal maturity increases with depth to reach the early oil window (127°C at c. 320 m below the seafloor).

Crustal underplating and effective elastic plate thickness of the Laxmi Ridge, northern Arabian Sea

Laxmi Ridge, a prominent basement high feature with negative gravity anomaly in the northern Arabian Sea, is generally considered as stretched continental crust. However, recently it is proposed that the ridge could be an extinct spreading ridge. Ship-borne and satellite-altimetry derived bathymetry, gravity and geoid data over the Laxmi Ridge are analysed and modelled to determine effective elastic plate thickness (Te) and crustal structure beneath the ridge. The estimation of Te and compensation mechanism of the ridge are examined based on 3D coherence and admittance analysis considering surface and subsurface loads which involve computation of coherence and admittance between Mantle Bouguer Anomaly (MBA) and sediment corrected bathymetry. The best fit model reveals that Te is about 3–4 km and amplitude of subsurface load at Moho is approximately equal to the surface load. Low value of Te implies elastically weak lithospheric plate which can be attributed to either weakening of lithosphere by re-heating and underplating caused by the then nearby Réunion Hotspot or volcanic emplacement on the young oceanic crust. Further, all along the length of the Laxmi Ridge, the MBA is negative and its magnitude reaches up to 115 mGal, which indicates either presence of thickened crust or less dense mantle beneath the ridge. 2D crustal models suggest that underplating material is prevalent all along the Laxmi Ridge. High amplitude (>400 nT) magnetic anomaly over the southern part of the ridge reveals several lineations in WNW-ESE direction suggesting magma outpouring along week zones. Geoid to topographic ratio over the ridge is negative (−1.98 m/km), possibly indicating shallow subsurface load (<20 km) with small wavelength (<200 km). Based on present study and available geophysical results, we infer that the Laxmi Ridge is a stretched continental sliver which is underplated, intruded by volcanics and isostatically compensated by Airy Model of Compensation.

Structure, mechanical properties and evolution of the lithosphere below the northwest continental margin of India

The continental breakup history at the northwest continental margin of India remained conjectural due to lack of clearly discernable magnetic anomaly identifications and the presence of several enigmatic structural/basement features whose structure was partly obscured by the Late Cretaceous Deccan magmatic event. In this study, a detailed analysis of the existing seismic and seismological data covering both onshore and offshore areas of the northwest Indian margin along with 3-D/2-D constrained potential field (gravity, magnetic and geoid) modeling has been carried out. The crustal structure and lithosphere–asthenosphere boundary (LAB) delineated across the margin provided valuable insights on the mechanism of continental extension. An analysis of the residual geoid anomaly (degree-10) map and the modeled LAB below Deccan volcanic province (DVP) revealed significant variation in upper mantle characteristics between the northwest (NW) and south central (SC) parts of DVP having thinner lithosphere in the NW part. The depth to LAB ranges 80–130 km at the margin with gradual thinning towards the western offshore having sharp gradient in the south (SC part of DVP) and gentle gradient in the north (NW part of DVP). The Moho configuration obtained from seismically constrained 3-D gravity inversion reveals that Moho depths vary 34–42 km below DVP and gradually thins to 16–20 km in the western offshore. The effective elastic thickness (Te) map computed through 3-D flexural modeling indicates that the Te values are in general lower in the region and range 12–25 km. Such lower Te values could be ascribed to the combined effect of the lithosphere stretching during Gondwana fragmentation in the Mesozoic and subsequent thermal influence of the Reunion plume. Based on the crustal stretching factors (β), Te estimates and the modeled lithosphere geometry at the margin in this study, we propose that the lithosphere below Laxmi–Gop basin region (β > 3.0) had undergone continuous stretching since India–Madagascar rifting (~ 88 Ma) /much prior to this event. However, this continuous stretching did not lead to breakup. Due to syn-rift cooling, the developed necking zone (brittle–ductile deformation) got ceased and led to the development of a new necking zone between Seychelles and Laxmi Ridge. Subsequent stretching between Seychelles and the Laxmi Ridge contemporaneous with the Deccan flood basalts eruption led to the seafloor spreading in the Western Basin (anomaly C28n). Thus, the Laxmi Ridge became a continental sliver.

Subsidence around oceanic ridges along passive margins: NE Arabian Sea

Abstract The northern part of the western continental margin of India formed due to the separation of the Seychelles from India at c. 63 Ma. This produced offshore tectonic elements such as the Gop Rift, the Saurashtra Volcanic Platform (SVP) and the Laxmi Ridge, as well as numerous seamounts, e.g. the Raman and Panikkar seamounts. The Laxmi Ridge and the Laxmi Basin have been studied using high-resolution 2D reflection seismic data and well data. Patch and pinnacle carbonate reefs, indicating shallow waters, are common in the north, whereas large, isolated platforms are usually noted in the south. Palaeo-depth estimates are made from well biostratigraphy. Subsidence studies of the SVP suggest that the burial history is consistent with the anomalously hot Réunion plume. We have performed a subsidence analysis south of the SVP on the Laxmi Ridge and Laxmi Basin. The sediment-unloaded basement depths, estimated using using flexural isostasy with effective elastic thicknesses of 10–40 km have been found to be 2000–4000 m in areas where carbonates exist. These carbonates indicate &lt;200 m bathymetry at c. 65 Ma, and the subsidence discrepancy is thus due to thermal cooling or anomalous heating due to the Deccan plume. Patch and pinnacle reefs in the north suggests that either the rise in sea-level or the rate of subsidence of the basement were fast. The presence of large platforms in the south indicates otherwise. This is possibly due to a greater influence from the Indus Fan sediments towards the north. In addition, the Laxmi Ridge is a spreading centre that remained emergent near or above sea-level due to plume support, which was also greater in the south due to proximity to the plume. When the plume support discontinued, the ridge subsided quickly to present-day depths, which matches the subsidence expected for 60–70 Myr old oceanic crust. Supplementary material: A table is available at https://doi.org/10.6084/m9.figshare.c.3470751

Identification of new deep sea sinuous channels in the eastern Arabian Sea.

Deep sea channel systems are recognized in most submarine fans worldwide as well as in the geological record. The Indus Fan is the second largest modern submarine fan, having a well-developed active canyon and deep sea channel system. Previous studies from the upper Indus Fan have reported several active channel systems. In the present study, deep sea channel systems were identified within the middle Indus Fan using high resolution multibeam bathymetric data. Prominent morphological features within the survey block include the Raman Seamount and Laxmi Ridge. The origin of the newly discovered channels in the middle fan has been inferred using medium resolution satellite bathymetry data. Interpretation of new data shows that the highly sinuous deep sea channel systems also extend to the east of Laxmi Ridge, as well as to the west of Laxmi Ridge, as previously reported. A decrease in sinuosity southward can be attributed to the morphological constraints imposed by the elevated features. These findings have significance in determining the pathways for active sediment transport systems, as well as their source characterization. The geometry suggests a series of punctuated avulsion events leading to the present array of disconnected channels. Such channels have affected the Laxmi Basin since the Pliocene and are responsible for reworking older fan sediments, resulting in loss of the original erosional signature supplied from the river mouth. This implies that distal fan sediments have experienced significant signal shredding and may not represent the erosion and weathering conditions within the onshore basin at the time of sedimentation.

Giant fossil mass wasting off the coast of West India: The Nataraja submarine slide

We use two-dimensional pre-stack depth migrated seismic reflection profiles and seafloor bathymetry to describe the continental margin structure and a massive mass-transport deposit off the west coast of India. This giant slide runs from the Gujurat–Saurashtra margin to the Laxmi Basin. It is over 330 km long, a maximum of 190 km wide and its run-out basal gradient is 1.2°. We name this giant mass wasting deposit the Nataraja Submarine Slide. This slide covers 49±16×103 km2 and represents a volume of 19×103±4×103 km3, making it the second by volume of any passive margin landslide/mass-transport deposit. Seismic facies analysis allows the internal structure of the mass-transport deposit to be described as far as the toe. This slide has been able to circumvent massive seamounts, thus highlighting the capacity of the flow and its potential energy during emplacement in a funnel between the slope of the Western Indian passive margin and the Laxmi Ridge. Stratigraphically, the emplacement of the Nataraja Slide predates the main pulse of sedimentation during the late Miocene–Recent associated with the Indus Fan but follows rapid sedimentation across S and SE Asia during the Early–Middle Miocene. The margin architecture at the head of this slide is associated with a gravity-controlled fold and thrust belt that may have caused slope steepening and triggering of the slide.

Re-examination of geophysical data off Northwest India: Implications to the Late Cretaceous plate tectonics between India and Africa

Abstract The Gop and Laxmi Basins lying off Northwest India have been assigned ambiguous crustal types and evolution mechanisms. The Chagos–Laccadive Ridge (CLR) complex lying along the southwest coast of India has been attributed to different evolutionary processes. Late Cretaceous seafloor spreading between India and Africa formed the Mascarene Basin, and the plate reconstruction models depict unequal crustal accretion in this basin. Re-interpretation of magnetic data in the Gop and Laxmi Basins suggests that the underlying oceanic crust was accreted contemporaneously from 79 Ma at slow half spreading rates (0.6 to 1.5 cm/yr) separating the Seychelles–Laxmi Ridge complex from India. The spreading ridge became extinct at 71 Ma in the central region between 19–20°N and 65.5–67°E, and at 68.7 Ma in the Gop Basin. Extinction progressed southwards with time until 64.1 Ma at ~ 14.5°N in the Laxmi Basin. This spreading probably limited the seafloor spreading in the northern Mascarene Basin, while normal to fast spreading continued in the south. The fracture zone FZ2 acted as a major boundary between the northern and southern Mascarene Basin, and may have also restricted the southward propagation of the Laxmi Basin spreading ridge. Plate reconstruction models generated in this study accommodate several microplates between Madagascar and India, and trace their movements over time within the India–Africa spreading corridor. Furthermore, these models suggest the formation of the Seychelles–Laxmi Ridge microplate by 79 Ma in a complex three plate system. This microplate may have existed up to 64.1 Ma and experienced an anticlockwise rotation of about 50°. Under the influence of the Deccan hotspot activity, the Laxmi Basin spreading ridge jumped southwestward splitting the Seychelles–Laxmi Ridge complex and initiating the Carlsberg Ridge. The presence of about 160 km of oceanic crust in the Gop and Laxmi Basins partially reduces the space problem faced in the earlier plate reconstruction models.

A relook into the crustal architecture of Laxmi Ridge, northeastern Arabian Sea from geopotential data

In this study, we undertake analysis of ship-borne gravity-magnetic and satellite-derived free-air gravity (FAG) data to derive the crustal structure of Laxmi Ridge and adjacent areas. 2D and 3D crustal modelling suggests that the high resolution FAG low associated with the ridge is due to underplating and that it is of continental nature. From Energy Spectral Analysis, five-depth horizons representing interface between different layers are demarcated that match those derived from 2D models. Magnetic sources from EMAG2 data, various filtered maps and absence of underplating in the EW section suggest that the EW and NW–SE segment of the Laxmi Ridge is divided by the Girnar fracture zone and probably associated with different stages of evolution. From the derived inclination parameters, we infer that the region to the north of Laxmi Ridge, between Laxmi and Gop Basins, is composed of volcanic/basaltic flows having Deccan affinity, which might have been emplaced in an already existing crust. The calculated inclination parameters derived from the best fit 2D model suggests that the rifting in the Gop Basin preceded the emplacement of the volcanics in the region between Laxmi and Gop Basins. The emplacement of volcanic/basaltic flows may be associated with the passage of India over the Reunion hotspot.

Active Channel Systems in the Middle Indus Fan: Results from High-Resolution Bathymetry Surveys

Multibeam swath bathymetry survey was carried out in the middle Indus fan region in the eastern Arabian Sea. Using high-resolution bathymetry data, major morphological features such as the Raman seamount and the Laxmi ridge have been mapped. This study also reveals the presence of sinuous channel systems, continuing towards the distal fan. Though there are several reports on the presence of channels in different regions of the Indus fan, we report here the presence of active channels to the east of the Laxmi ridge. The total length of all channels along the channel axis is about 915 km. The individual spreads of the channels vary from 189.8 to 1980.5 m. Most of the channels are shallow with the average depth measuring about 60 m. The longest channel is about 256.3 km long, 702 m wide and about 57 m deep. The channels observed are similar to the land-based fluvial channels. The channels identified are highly sinuous in nature, their meanders and cut-off meanders are similar to the characteristics of fluvial channels. In general, average channel course in the study area is more than twice the straight course.

Repeat ridge jumps and microcontinent separation: insights from NE Arabian Sea

Microcontinents separate due to ridge jumps associate either asymmetric sea floor spreading or plume–ridge interactions. India separated from Seychelles at ∼64 Ma by asymmetric sea floor spreading initially when the spreading centre in the Mascarene Basin jumped towards the Indian sub-continent between magnetic chrons C29 and C28. The subsequent tectonics is difficult to comprehend since Laxmi Ridge-another microcontinent-formed during the later phase. Most of the studies considered the Laxmi Ridge as a sliver. Others considered it to be oceanic crust. High resolution, deep (∼25 km) seismic data reveals that (i) the ridge possesses > 15 km deep sea-ward dipping reflector (SDR) packages; (ii) normal faulted rift valleys devoid of syn-rift sedimentary packages; and (iii) axial magma chambers 5–7 km beneath the ridge top. Additionally, from 2D forward gravity models we deduce that the ridge most possibly comprises of high density (oceanic) crust. We conclude the Laxmi Ridge to be indeed composed of oceanic crust and a fossil spreading centre. We thus identified the ridge jumps and their relation to the Seychelles microcontinent separation.Previous numerical models suggest that the time required for a ridge jump is controlled by magmatic heating, spreading rate at the ridge, and plate ages. For repeated ridge jumps, the additional factor is the dynamic relation between the plume and lithosphere in terms of melt transfer and heating. We find that the medium spreading rates and high magmatic heating due to the Réunion plume and young plates favoured rapid and repeated ridge jumps towards the plume.

Cretaceous to present kinematics of the Indian, African and Seychelles plates

An iterative inverse model of seafloor spreading data from the Mascarene and Madagascar basins and the flanks of the Carlsberg Ridge describes a continuous history of Indian–African Plate divergence since 84 Ma. Visual-fit modelling of conjugate magnetic anomaly data from near the Seychelles platform and Laxmi Ridge documents rapid rotation of a Seychelles Plate about a nearby Euler pole in Palaeocene times. As the Euler pole migrated during this rotation, the Amirante Trench on the western side of the plate accommodated first convergence and later divergence with the African Plate. The unusual present-day morphology of the Amirante Trench and neighbouring Amirante Banks can be related to crustal thickening by thrusting and folding during the convergent phase and the subsequent development of a spreading centre with a median valley during the divergent phase. The model fits FZ trends in the north Arabian and east Somali basins, suggesting that they formed in India–Africa Plate divergence. Seafloor fabric in and between the basins shows that they initially hosted a segmented spreading ridge that accommodated slow plate divergence until 71–69 Ma, and that upon arrival of the Deccan–Réunion plume and an increase to faster plate divergence rates in the period 69–65 Ma, segments of the ridge lengthened and propagated. Ridge propagation into the Indian continental margin led first to the formation of the Laxmi Basin, which accompanied extensive volcanism onshore at the Deccan Traps and offshore at the Saurashtra High and Somnath Ridge. A second propagation episode initiated the ancestral Carlsberg Ridge at which Seychelles–India and India–Africa Plate motions were accommodated. With the completion of this propagation, the plate boundaries in the Mascarene Basin were abandoned. Seafloor spreading between this time and the present has been accommodated solely at the Carlsberg Ridge.

Seychelles alkaline suite records the culmination of Deccan Traps continental flood volcanism

Silhouette and North Islands in the Seychelles represent an alkaline plutonic–volcanic complex, dated at 63 to 63.5Ma by U–Pb zircon and 40Ar/39Ar methods. This magmatism coincides with the final stages of the cataclysmic Deccan Traps continental flood volcanism in India (67 to 63Ma), and thus a causal link has been suggested. Recent reconstructions have placed the Seychelles islands adjacent to the Laxmi Ridge and at the western margin of the Réunion mantle plume at the time of formation of the complex. Here we present geochemical evidence in support of the notion that the Seychelles alkaline magmatism was initiated by the peripheral activity of the Réunion mantle plume and is thus part of the Deccan magmatic event. Positive εNd (0.59 to 3.76) and εHf (0.82 to 6.79) and initial Sr of 0.703507 to 0.705643 at 65Ma indicate derivation of the Seychelles alkaline magmas from a Réunion-like mantle source with an additional minor enriched component, suggesting entrainment of sub-continental lithospheric mantle. The similarity in trace element composition between the Seychelles suite and Deccan alkaline felsic and mafic rocks provides additional evidence for a common mantle source for the Seychelles and Deccan magmatism. Furthermore, we demonstrate the role of fractional crystallisation in the evolution of the alkaline suite. Modelling using major elements suggests that fractional crystallisation and varying degrees of accumulation of olivine, plagioclase, ilmenite, clinopyroxene, alkali feldspar and apatite can describe the spectrum of rock types, from gabbro, through syenite, to granite.

Three dimensional lithospheric structure of the western continental margin of India constrained from gravity modelling: implication for tectonic evolution

This paper describes a 3-D lithospheric density model of the Western Continental Margin of India (WCMI) based on forward modelling of gravity data derived from satellite altimetry over the ocean and surface measurements on the Indian peninsula. The model covers the north-eastern Arabian Sea and the western part of the Indian Peninsula and incorporates constraints from a wide variety of geophysical and geological information. Salient features of the density model include: (1) the Moho depth varying from 13 km below the oceanic crust to 46 km below the continental interior; (2) the lithosphere–asthenosphere boundary (LAB) located at depths between 70 km in the southwestern corner (under oceanic crust) and about 165 km below the continental region; (3) thickening of the crust under the Chagos–Laccadive and Laxmi Ridges and (4) a revised definition of the continent–ocean boundary. The 3-D density structure of the region enables us to propose an evolutionary model of the WCMI that revisits earlier views of passive rifting. The first stage of continental-scale rifting of Madagascar from India at about 90 Ma is marked by relatively small amounts of magmatism. A second episode of rifting and large-scale magmatism was possibly initiated around 70 Ma with the opening of the Gop Rift. Subsequently at around 68 Ma, the drifting away of the Seychelles and formation of the Laxmi Ridge was a consequence of the down-faulting of the northern margin. During this second episode of rifting, the northern part of the WCMI witnessed massive volcanism attributed to interaction with the Reunion hotspot at around 65 Ma. Subsequent stretching of the transitional crust between about 65 and 62 Ma formed the Laxmi Basin, the southward extension of the failed Gop Rift. As the interaction between plume and lithosphere continued, the Chagos–Laccadive Ridge was emplaced on the edge of the nascent oceanic crust/rifted continental margin in the south as the Indian Plate was moving northwards.

Thin oceanic crust and flood basalts: India‐Seychelles breakup

Recent seismic experiments showed that separation of India from the Seychelles occurred in two phases of rifting. The first brief phase of rifting between India and the Laxmi Ridge formed the Gop Rift, which is characterized by thick oceanic crust and underplating of the adjacent continental margins. The age of the Gop Rift is uncertain, initiation of seafloor spreading being some time between 71 and 66 Ma. This was then followed by rifting and seafloor spreading between the Laxmi Ridge and the Seychelles, the onset of which is well dated by magnetic anomalies at 63.4 Ma and characterized by thin oceanic crust. Both of these rift events occurred within 1000 km of the center of the Deccan flood basalts, which formed at 65 ± 1 Ma. To constrain the age of the Gop Rift and to explore the reasons for the change in crustal structure between the Gop Rift and Seychelles‐Laxmi Ridge margins, we employ a geodynamic model of rift evolution in which melt volumes, seismic velocity, and rare earth element (REE) chemistry of the melt are estimated. We explore the consequences of different thermal structures, hydration, and depletion on the melt production during the India‐Seychelles breakup to understand the reasons behind the thin oceanic crust observed. Magmatism at the Gop Rift is consistent with a model in which the seafloor spreading began at 71 Ma, ca. 6 Myr prior to the Deccan. The opening occurred above a hot mantle layer (temperature of 200°C, thickness of 50 km) that we interpret as incubated Deccan material, which had spread laterally beneath the lithosphere. This scenario is consistent with observed lower crustal seismic velocities of 7.4 km s−1 and 12 km igneous crustal thickness. The model indicates that when the seafloor spreading migrated to the Seychelles‐Laxmi Ridge at 63 Ma, the thermal anomaly was reduced significantly but not sufficient to explain the observed reduction in breakup magmatism. From observations here of 5.2 km oceanic crust, lower crustal seismic velocities of 6.9 km s−1 and a flat REE profile, we infer that breakup occurred in a region of mantle that became depleted by prior extension related to the Gop Rift.

Magmatic underplating of crust beneath the Laccadive Island, NW Indian Ocean

SUMMARY We investigate the crust and uppermost mantle velocity structure beneath the Laccadive Island through the inversion of teleseismic receiver functions following the neighbourhood algorithm. The velocity structure suggests that the 16-km thick-oceanic crust is underplated by 8-km thick high-velocity layer (Vs ∼ 4.25–4.4 km s −1 ; Vp ∼ 7.35–7.6 km s −1 ). The uppermost mantle shear velocity is ∼4.6 km s −1 (Vp ∼ 8.18 km s −1 ). The Moho at 24 km is possibly formed due to underplating of mantle-derived picritic melt at the base of oceanic crust produced during the interaction of the Indian plate with the Reunion hotspot. Continuity of crustal-underplated material beneath the Laccadive could be traced to the Laxmi Ridge in the north and to the Reunion Island in the south, suggesting this to be possible trace of the Reunion hotspot.

Early oceanic opening off Western India–Pakistan margin: The Gop Basin revisited

The Deccan Traps, one of the best known examples of rapid flood basalt, are considered as marking of the inception of a mantle plume on the Indian continental lithosphere. Their emplacement may be associated with the continental break-up of India and the Seychelles block and later formation of a new spreading centre, the Carlsberg Ridge, while spreading progressively ceased in the Mascarene Basin. Whether rifting, continental break-up, and seafloor spreading predated or were the consequence of the Deccan Traps emplacement is still a matter of debate. This issue is further complicated by the presence of a continental sliver, the Laxmi Ridge, and large basins lying landward of the Laxmi Ridge, such as the Laxmi and Gop basins, where nature of the crust is still ambiguous. The present study attempts to decipher the tectonic setting and the imprints of plume–ridge interaction in the Gop Basin, where the crust has been interpreted as either volcanic-intruded thinned continental crust or oceanic crust formed by a now-extinct spreading centre. Based on interpretation of an updated compilation of marine geophysical data, the present study supports the oceanic nature of the crust underlying the Gop Basin and proposes the Palitana Ridge as the extinct spreading centre in this region. The prominent but short sequence of fairly linear magnetic anomalies in the Gop Basin does not allow a unique identification; it can be reasonably explained either as A31r–A25r (∼ 69.3–56.4 Ma) or as A29r–A25r (∼ 64.8–56.4 Ma) sequence. The variations of the spreading rates assumed by both these models suggest that spreading in the Gop Basin significantly slowed around 65 Ma, contemporaneous with the magmatic outburst of the Reunion plume on the adjacent western Indian mainland. Subsequently, the Gop Basin spreading centre was waning whereas a new spreading centre was developing further south, close to the (relatively) southward migrating plume. In this last stage, the Gop Basin spreading centre was associated with an abundant magmatism, probably supplied from the plume region.