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The geoid is an equipotential surface that broadly mimics the mean sea level. The difference between the geoid and the reference spheroid at any location is referred to as a geoid anomaly. The geoid ‘highs’ (positive) or ‘lows’ (negative) are primarily associated with mass anomalies, thereby could offer important information about compositional and thermal properties in the Earth's interior. The maximum geoidal surplus (+85 m) is observed to the east of New Guinea whereas the largest deficit (−106 m) is observed in the Indian Ocean south of Sri Lanka – commonly known as the Indian Ocean geoid low (IOGL). On a global geoid map, the IOGL anomaly covers an extensive circular area spanning >2000 km in diameter (Fig. 1). Several different hypotheses have been put forth to explain this enigmatic anomaly. These include effects of isostatically uncompensated crust (Ihnen and Whitcomb, 1983), depression in the core-mantle boundary (Negi et al., 1987), slab graveyards in the mantle (Spasojevic et al., 2010), anomalous variations in the mantle transition zone (Reiss et al., 2017; Rao et al., 2020) and presence of a very low-velocity material arising from the African large low shear velocity province (LLSVP) or simply known as the African superplume (Ghosh et al., 2017). Most of these hypotheses rely upon either very sparse seismological observations, numerical modelling or remote sensing data. Global seismic tomographic models provide first-order information about the Earth's interior (Simmons et al., 2010, Simmons et al., 2012, Simmons et al., 2015). However, the uneven distribution of seismological networks has stymied production of high-resolution sub-surface images. In search of concrete causative mechanisms behind the IOGL anomaly, deep seismological observations from the Indian Ocean have been awaited for a long time. Between 2015 and 2020, the Ministry of Earth Sciences (MoES) India deployed a focused linear broadband passive seismological array comprising 17 ocean bottom seismometers (OBS) for two successive seasons comprising 14 months each (Fig. 1). These OBS stations thus continuously recorded local and teleseismic events for >28 months (Pandey, 2017). Besides, some recent studies also carried out active OBS deployments in this region to evaluate crustal and upper mantle structures (Pandey et al., 2022; Ningthoujam et al., 2022; Altenbernd-Lang et al., 2022). This special issue was conceived to present a compilation of new field observations as well as numerical modelling studies to infer potential mass anomalies within the crust and mantle beneath the IOGL region. A collection of nine papers presented in this volume explore the role of causative sources at varying depths to explain the IOGL anomaly. In summary, scientific contributions in this special issue suggest minimal crustal contributions towards the spectacular IOGL anomaly. On the other hand, new seismological studies suggest that the IOGL anomaly can be reasonably explained by a combination of positive mass anomalies in the lower mantle and/or negative mass anomalies in the upper mantle. Varied outcomes further stress upon the need to carry out more long-term seismological observations in order to image precise mantle structure beneath the IOGL region.

Recently, Pandey et al. proposed relict subduction initiation occurred along a passive margin in the northwest Indian Ocean, however, Clift et al. questioned their evidence for subduction initiation, suggesting that simpler rifting-related processes could more simply explain the available data. Here, Pandey et al. reply to Clift et al.’s comment, and argue that geochemical and isotope data for Laxmi basin lavas distinctly imply relict subduction initiation.

Physical review materials 5(11), 113607 (2021). doi:10.1103/PhysRevMaterials.5.113607, The thermodynamic stability of the premartensite (PM) phase has been a topic of extensive investigation in shape memory alloys as it affects the main martensite phase transition and the related physical properties. In general, the PM phase is stable over a rather narrow temperature-composition range. We present here evidence for chemical pressure induced suppression of the main martensite transition and stabilization of the PM phase over a very wide temperature range from 300 to ���5K in a magnetic shape memory alloy (MSMA), Ni$_{50}$Mn$_{34}$In$_{16}$, using magnetic susceptibility, synchrotron x-ray powder diffraction (SXRPD) studies, and first-principles calculations. The ac-susceptibility studies show a highly skewed and smeared peak around 300 K without any further transition up to the lowest temperature of our measurement (5 K) for ���5% Al substitution. The temperature evolution of the SXRPD patterns confirms the appearance of the PM phase related satellite peaks at T���300K without any splitting of the main austenite (220) peak showing preserved cubic symmetry. This is in marked contrast to the temperature evolution of the SXRPD patterns of the martensite phase of the Al free as well as ���3% Al substituted compositions where the austenite (220) peak shows a clear splitting due to Bain distortion signalling symmetry breaking transition. Our theoretical calculations support the experimental findings and reveal that the substitution at the In site by a smaller size atom, like Al, can stabilize the PM phase with preserved cubic symmetry. Our results demonstrate that Al-substituted Ni-Mn-In MSMAs provide an ideal platform for investigating the physics of various phenomena related to the PM state., Published by APS, College Park, MD

Interpretation of new multichannel seismic reflection data from the Andaman Forearc Basin (AFB) in the northern Indian Ocean is presented here. The high-quality multichannel seismic data from the Andaman Forearc region enable us to examine the seismic characters and to demarcate seismic sequences bounded by distinct unconformities. Ages of marked seismic horizons have been calibrated with available litholog data from nearby industry boreholes. Seismic interpretation of new data shows that the AFB is filled with ~ 4.5-s-two way travel time (TWT) thick Neogene to Recent sediments. The entire basin assemblage exhibits two distinct major sequences pertaining to the Neogene and Quaternary times. A large part of the basin is filled with intermittent mass transport deposits (MTD). We infer that the episodic uplift of the Invisible Bank, protuberance of the outerarc and regular deformation through reactivation of preexisting normal faults since the Pleistocene could be attributed as causal mechanisms for the MTDs. Strong bottom simulating reflectors are identified in the Late Miocene and younger sediments of the outerarc and AFB at a depth of ~ 0.6 s TWT and correspond to the presence of gas hydrates in this region. Our interpretations have significant implications for geodynamic as well as resource exploration in the AFB.

Made available in DSpace on 2020-12-10T17:04:36Z (GMT). No. of bitstreams: 0 Previous issue date: 2020-01-01 Charles T. McCord Jr Chair in Petroleum Geology at Louisiana State University Ministry of Earth Sciences, Government of India National Centre for Polar and Ocean Research (NCPOR) National Research Foundation of Korea CSIR-National Institute of Oceanography, India A giant mass-transport complex was recently discovered in the eastern Arabian Sea, exceeding in volume all but one other known complex on passive margins worldwide. The complex, named the Nataraja Slide, was drilled by International Ocean Discovery Program (IODP) Expedition 355 in two locations where it is similar to 300 m (Site U1456) and similar to 200 m thick (Site U1457). The top of this mass-transport complex is defined by the presence of both reworked microfossil assemblages and deformation structures, such as folding and faulting. The deposit consists of two main phases of mass wasting, each consisting of smaller pulses, with generally fining-upward cycles, all emplaced just prior to 10.8 Ma based on biostratigraphy. The base of the deposit at each site is composed largely of matrix-supported carbonate breccia that is interpreted as the product of debris-flows. In the first phase, these breccias alternate with well-sorted calcar-enites deposited from a high-energy current, coherent limestone blocks that are derived directly from the Indian continental margin, and a few clastic mudstone beds. In the second phase, at the top of the deposit, muddy turbidites dominate and become increasingly more siliciclastic. At Site U1456, where both phases are seen, a 20-m section of hemipelagic mudstone is present, overlain by a similar to 40-m-thick section of calcarenite and slumped interbedded mud and siltstone. Bulk sediment geochemistry, heavy-mineral analysis, clay mineralogy, isotope geochemistry, and detrital zircon U-Pb ages constrain the provenance of the clastic, muddy material to being reworked, Indus-derived sediment, with input from western Indian rivers (e.g., Narmada and Tapti rivers), and some material from the Deccan Traps. The carbonate blocks found within the breccias are shallow-water limestones from the outer western Indian continental shelf, which was oversteepened from enhanced clastic sediment delivery during the mid-Miocene. The final emplacement of the material was likely related to seismicity as there are modern intraplate earthquakes close to the source of the slide. Although we hypothesize that this area is at low risk for future mass wasting events, it should be noted that other oversteepened continental margins around the world could be at risk for mass failure as large as the Nataraja Slide. Louisiana State Univ, Dept Geol & Geophys, E253 Howe Russell Kniffen Geosci Complex, Baton Rouge, LA 70803 USA Texas A&M Univ, Int Ocean Discovery Program, 1000 Discovery Dr, College Stn, TX 77845 USA Woods Hole Oceanog Inst, Dept Geol & Geophys, Woods Hole, MA 02543 USA UCL, Dept Earth Sci, Gower St, London WC1E 6BT, England Univ Toulouse 3 Paul Sabatier, Geosci Environm Toulouse, 14 Ave Edouard Belin, F-31400 Toulouse, France GeoSep Serv, 1521 Pine Cone Rd, Moscow, ID 83843 USA Univ Queensland, Sch Earth & Environm Sci, St Lucia, Qld 4072, Australia Natl Ctr Polar & Ocean Res, Vasco Da Gama 403804, Goa, India Univ Milano Bicocca, Dept Earth & Environm Sci, Piazza Sci 4, I-20126 Milan, Italy Purdue Univ, Dept Earth Atmospher & Planetary Sci, 550 Stadium Mall Dr, W Lafayette, IN 47907 USA Univ Birmingham, Sch Geog Earth & Environm Sci, Birmingham B15 2TT, W Midlands, England Macquarie Univ, Fac Sci & Engn, Dept Earth & Planetary Sci, N Ryde, NSW 2109, Australia Ohio State Univ, Sch Earth Sci, 275 Mendenhall Lab,125 South Oval Mall, Columbus, OH 43210 USA Birbal Sahni Inst Palaeosci, 53 Univ Rd, Lucknow 226007, Uttar Pradesh, India Univ Bremen, MARUM, Leobener Str, D-28359 Bremen, Germany Kochi Univ, Dept Nat Environm Sci, 2-5-1 Akebono Cho, Kochi 7808520, Japan Pusan Natl Univ, Div Earth Environm Syst, Busan 60973, South Korea Wadia Inst Himalayan Geol, 33 GMS Rd, Dehra Dun 248001, Uttrakhand, India Natl Inst Ocean Technol, Marine Biotechnol Dept, Velacheiy Tambaram Main Rd, Chennai 600100, Tamil Nadu, India Columbia Univ, Ctr Climate Syst Res, 2880 Broadway, New York, NY 10025 USA Nanjing Univ, Sch Geog & Oceanog Sci, 163 Xianlin Ave, Nanjing 210023, Jiangsu, Peoples R China Oregon State Univ, Coll Earth Ocean & Atmospher Sci, 104 CEOAS Adm Bldg, Corvallis, OR 97331 USA Natl Ctr Earth Sci Studies, Geosci Div, Aakkulam Trivandrum 695031, India Natl Inst Oceanog, Geol Oceanog Div, Panaji 403004, Goa, India Oil & Nat Gas Commiss, 11 High,Bandra Sion Link Rd, Mumbai 400017, Maharashtra, India Univ Estadual Paulista, Inst Geociencias & Ciencias Exatas, 1515 Ave 24-A, BR-13506900 Rio Claro, SP, Brazil Kumaun Univ, Dept Geol, Naini Tal 263002, India Banaras Hindu Univ, Dept Geol, Varanasi 221005, Uttar Pradesh, India Xiamen Univ, Dept Geol Oceanog, Xiamen 361102, Fujian, Peoples R China Xiamen Univ, State Key Lab Marine Environm Sci, Xiamen 361102, Fujian, Peoples R China Hokkaido Univ, Grad Sch Environm Sci, Kita Ku, N10W5, Sapporo, Hokkaido 0600810, Japan Scripps Inst Oceanog, 9500 Gilman Dr, La Jolla, CA 92093 USA Chinese Acad Sci, Inst Oceanol, Key Lab Marine Geol & Environm, 7 Nanhai Rd, Qingdao 266071, Shandong, Peoples R China Univ Estadual Paulista, Inst Geociencias & Ciencias Exatas, 1515 Ave 24-A, BR-13506900 Rio Claro, SP, Brazil National Centre for Polar and Ocean Research (NCPOR): 90/2018 National Research Foundation of Korea: 2016R1A2B4008256 National Research Foundation of Korea: 2019R1A2C1007701

The tectonic evolution of Laxmi basin, presently located along western Indian passive margin, remains debated. Prevailing geodynamic models of Laxmi basin include two mutually competing hypotheses, culminating in either a hyper-stretched continental crust or an oceanic crust overlying an extinct spreading centre. The longstanding conundrum surrounding its precise crustal affinity precludes a complete understanding of the early opening of the Indian Ocean. Here, we present distinct geochemical and geophysical imprints from the igneous crust in Laxmi basin obtained through International Ocean Discovery Program Expedition 355. The geochemical and isotopic signatures of the Laxmi basin crust exhibit uncanny similarities with forearc tectonic settings. Our observations imply a relict subduction initiation event occurred in the Laxmi basin in the Late Cretaceous-Early Cenozoic that marks a significant Cenozoic plate reorganisation record in the northwest Indian Ocean. New findings therefore warrant re-evaluation of the Gondwana breakup to account for the nascent subduction in the northwest Indian Ocean., The tectonic evolution of the Laxmi basin, in the northwest Indian Ocean, remains controversial. Here, the authors present new geochemical and seismic data which indicate the Laxmi basin formed by a subduction initiation event during the break-up of Gondwana.

Made available in DSpace on 2022-04-28T19:05:30Z (GMT). No. of bitstreams: 0 Previous issue date: 2017-02-21 In the Arabian Sea, South Asian monsoon (SAM)-induced high surface water productivity coupled with poor ventilation of intermediate water results in strong denitrification within the oxygen minimum zone (OMZ). Despite the significance of denitrification in the Arabian Sea, we have no long-term record of its evolution spanning the past several million years. Here, we present the first record of denitrification evolution since Late Miocene (∼10.2 Ma) in the Eastern Arabian Sea, where the SAM generates moderate surface water productivity, based on the samples retrieved during the International Ocean Discovery Program (IODP) Expedition 355. We find that (i) the SAM was persistently weaker from ∼10.2 to 3.1 Ma; it did not intensify at ∼8 Ma in contrast to a few previous studies, (ii) on tectonic timescale, both the SAM and the East Asian Monsoon (EAM) varied synchronously, (iii) the first evidence of denitrification and productivity/SAM intensification was at ∼3.2-2.8 Ma that coincided with Mid-Pliocene Warm Period (MPWP), and (iv) the modern strength of the OMZ where denitrification is a permanent feature was attained at ∼1.0 Ma. National Centre for Antarctic and Ocean Research Department of Oceanography Pusan National University Department of Geology and Geophysics Louisiana State University, E253 Howe-Russell-Kniffen, Geoscience Complex International Ocean Discovery Program Texas A&M University, 1000 Discovery Drive College Department of Earth and Environmental Sciences University of Milano Bicocca, Piazza della Scienza 4 School of Geography Earth and Environmental Sciences University of Birmingham, Edgbaston Department of Earth and Planetary Sciences Faculty of Science and Engineering Macquarie University Level 2, The Australian Hearing Hub, 16 University Avenue Department of Earth and Environmental Sciences University of Texas at Arlington, Geosciences Building, 500 Yates Street Manipal Centre for Natural Sciences Manipal University, Dr. T.M.A. Pai Planetarium Building MARUM University of Bremen, Leobener Strasse Center for Advanced Marine Core Research/Natural Science Cluster Kochi University, 2-5-1 Akebono-cho Wadia Institute of Himalayan Geology, 33 GMS Road Marine Biotechnology Department National Institute of Ocean Technology, Velachery-Tambaram Main Road Department of Earth Sciences University of Southern California, 3651 Trousdale Parkway School of Geographical and Oceanographical Sciences Nanjing University, 163 Xianlin Avenue College of Earth Ocean and Atmospheric Sciences Oregon State University, 104 CEOAS Administration Building Geosciences Division National Centre for Earth Science Studies University College London, Gower Street Geological Oceanography Division National Institute of Oceanography ONGC 11 High, Bandra-Sion Link Road Instituto de Geociências e Ciências Exatas Universidade Estadual Paulista, 1515 Avenida 24-A Department of Geology Kumaun University Department of Geology Banaras Hindu University Department of Geological Oceanography College of Ocean and Earth Sciences Xiamen University, Xiping Building, Xiang'an South Road Graduate School of Environmental Science Hokkaido University, N10W5, Kita-ku Scripps Institution of Oceanography, 9500 Gilman Drive Key Laboratory of Marine Geology and Environment Institute of Oceanology Chinese Academy of Sciences, 7 Nanhai Road Laboratoire Géosciences Paris-Sud (GEOPS UMR8148-CNRS) Université de Paris-Sud (Orsay), Bâtiment 504 Instituto de Geociências e Ciências Exatas Universidade Estadual Paulista, 1515 Avenida 24-A

The Néel temperature (T$_N$) in conventional antiferromagnets (AFM) decreases with decreasing particle size. In contrast, we present here evidence for an exotic facet of multiferroicity, whereby one can raise the T$_{N}$ by more than 200 K by decreasing the particle size. We illustrate this by taking the example of a tetragonal composition with x = 0.5 in the solid solutions of (1-x)BiFeO$_{3}$-xPbTiO$_{3}$(BF-xPT). We attribute it to the increase in the strength of the superexchange interaction via a decrease in the ferroelectric distortion in nanocrystalline powder of BF-0.5PT. The BF-0.5PT nanoparticles also exhibit exchange bias effect due to AFM core-FM shell type magnetic nanostructure.

This paper deals with the structural Raman-active and the disorder-induced TO and LO Raman modes in Sr1−xCaxTiO3 (SCT) ceramics. Spectra were recorded at ambient pressure in a wide range of temperatures from 8 K up to ≈500 K. The chemical composition (x) was chosen so as to investigate the phase boundaries which have been previously determined in the x-T phase diagram. An eigenvector analysis is given for all the known phases. The Raman data confirm the occurrence of a double-tilt system of Imma space group lying between the tetragonal I4/mcm and the orthorhombic Pbnm structures for x ≥ 0.06. Higher-order transformation of the antiferroelectric phase (0.20 ≤ x ≤ 0.40) at low temperature is also suggested. The main interesting disorder-induced effects originate from the temperature behaviour of the hard polar modes TO2 and TO4, which appear in the Raman spectra of the paraelectric domain. This behaviour allows one to discuss the precursor range of ferroelectricity in the SCT system in terms of ferroelectric- or antiferroelectric-type dipolar interactions. Copyright © 2005 John Wiley & Sons, Ltd.

Results of X-ray powder diffraction, neutron powder diffraction, electron microdiffraction, and convergent beam electron diffraction (CBED) studies are presented to show that the space group of (Sr 1− x Ca x )TiO 3 (SCT) in the composition range 0.35 x ≤0.55 cannot be Cmcm . The correct space groups are Pbcm for 0.35 x ≤0.40 and Pbnm for 0.40 x ≤0.55. The analysis of powder XRD data reveals that the structure of SCT is noncubic for x ≥0.06. It is shown that four different types of orthorhombic phases appear in the SCT system with increasing Ca 2+ content.

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.