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The Visakhapatnam Bay (VB) basin is a passive margin rift basin located at the non-basinal segment of the Eastern Continental Margin of India (ECMI) and formed during the rift–drift events associated with the breakup of eastern Gondwanaland. In the present study, integrated analysis of potential field and multi-channel seismic reflection (MCS) data were carried out to understand the rift tectonics, crustal configuration and onshore–offshore structural continuity across this basin. The study revealed the following: (i) crustal models derived through joint gravity-magnetic modelling show limited stretching with 36–40 thick crust below the Eastern Ghat Mobile Belt (EGMB) thinning down to 16–20 km at the Ocean Continent Transition (OCT), (ii) extension of Charnockitic basement associated with the Eastern Ghat Mobile Belt (EGMB) into the offshore region. Comparison of the crustal configuration across the VB basin with that across the adjacent thick sedimentary area of the Krishna–Godavari shows that upper crustal configuration is significantly different in the VB area. The observed limited rift related structuration (horst-graben morphology), the mapped high angle break-away fault with large offset in the seismic data and narrow width (70–90 km) of extended crustal domain in VB basin suggests that this segment acts as transfer zone between Krishna–Godavari and Mahanadi rift zones. Further, long curve-linear trend of magnetic anomalies associated with Pudimadaka Lineament (PKL) demarcate the VB basin from the adjacent Krishna–Godavari rift zone. [ABSTRACT FROM AUTHOR]

Imprints of Marion hotspot magmatism representing the final stages of rifting between India and Madagascar from the Gondwana assembly occur scattered in several localities of southern India. In this study, we report evidence for rift‐related Santonian magmatism from alkali gabbro in the Madurai Block and present mineralogical, petrological, zircon U–Pb geochronological, and whole‐rock geochemical data on the Mundanmudi gabbroic complex. The rock shows mesocumulate textures with plagioclase laths forming the triple junctions and porphyritic texture with hornblende and clinopyroxene as phenocrysts. Geochemically, the gabbros are characterized by moderate to high SiO2 (50–58 wt.%), alkalies (4.14–7.00 wt.%), and high TiO2 (1.71–4.22 wt.%), with calc‐alkaline to high‐K alkaline affinity. Chondrite‐normalized REE patterns are highly fractionated with LREE enrichment relative to HREE and absence of Eu anomalies. The primitive mantle normalized trace element patterns have LILE enrichment relative to HFSE, exhibiting negative anomalies at Nb, Zr, and Ti. The HFSE relationships indicate a within‐plate tectonic setting for the genesis of these gabbros. The major element tectonic discrimination plots suggest MORB‐OFB affinity which is also supported by the Nb–Th relationship endorsing the divergent tectonic processes responsible for the generation of these gabbros. The zircon U–Pb mean age of ca. 85 Ma represents the magmatic emplacement age within a rift‐related tectonic setting during the Santonian (late Cretaceous). Based on the LREE and HREE relationship, it is inferred that the parental magma might have been derived at garnet lherzolite depths above the Marion plume, possibly correlatable with the large igneous province generated at this time in Madagascar. Our study provides new evidence for the final phase of the magmatism at ca. 85 Ma related to India–Madagascar rifting. [ABSTRACT FROM AUTHOR]

Middle Mesoproterozoic rift-related magmatisms of mantle plume origin are of global interest in understanding the breakup of the Columbia supercontinent. Here we investigate the age-equivalent basement rocks along the western marginal areas of the northwestern Gyeonggi Massif in the central-western Korean Peninsula. The sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon ages of the late Middle Mesoproterozoic mafic dykes and monzonitic intrusions from the Dongman Island and the early Late Mesoproterozoic syenites from the Kimpo area show 1259 ± 12 Ma, 1222 ± 9 Ma, and 1197 ± 5 Ma, 1194 ± 6 Ma, respectively, suggesting multiple magmatic pulses. The late Middle Mesoproterozoic monzonitic intrusions and early Late Mesoproterozoic syenites are characterized by A 1 -type granite affinity, formed in continental rifts. The Middle Mesoproterozoic mafic dykes show characteristics of transitional composition between tholeiitic-alkaline basalts to alkaline basalt having geochemical affinity of OIB (WPB) derived from enriched mantle sources, indicating a mantle plume origin. The data presented in this work, together with previous lithological, geochemical, and geochronological information on the magmatic suites from the western margin of the Gyeonggi Massif provide important insight into a Mesoproterozoic continental margin rift setting at ca. 1259–1194 Ma, and are broadly coeval with the Mesoproterozoic hotspot track identified in the Laurentia, North China Craton and Baltica, providing the imprints of the Columbia disruption in Northeast Asia. [ABSTRACT FROM AUTHOR]

Potassic and ultrapotassic magmatism from deep lithospheric sources in intra-cratonic settings can be the signal of subsequent voluminous mafic magmatism and the formation of Large Igneous Provinces (LIPs) triggered by mantle plumes. Here we report for the first time, precise zircon U–Pb age data from a suite of lamproites in the Bastar Craton of central India that mark the onset of Paleoproterozoic rifting and culminating in the formation of extensive mafic dyke swarms as the bar codes of one of the major LIP events during the Precambrian evolution of the Indian shield. The lamproites from the Nuapada field occur as dismembered dykes and are composed of phenocrysts and microphenocrysts of altered olivine together with microphenocrysts of phlogopite and magnetite within a groundmass of chlorite and calcite with accessory rutile, apatite and zircon. The rocks compositionally correspond to olivine phlogopite lamproite and phlogopite lamproite. Geochemical features of the lamproites correlate with their counterparts in Peninsular India and other similar suites elsewhere in the world related to rift settings, and also indicate OIB-like magma source. The associated syenite shows subduction-related features, possibly generated in a post-collisional setting. Magmatic zircon grains with high Th/U ratios in the syenite from the Nuapada lamproite form a coherent group with an upper intercept age of 2473 ± 8 Ma representing the timing of emplacement of the magma. Zircon grains in three lamproite samples yield four distinct age groups at ca. 2.4 Ga, 2.2 Ga, 2.0 Ga and 0.8 Ga. The 2.4 Ga group corresponds to xenocrysts entrained from the syenite whereas the 2.2 Ga group is considered to represent the timing of emplacement of the lamproites. The ca. 2.0 Ga zircon grains correlate with the major thermal imprint associated with mafic magmatism and dyke emplacement in southern Bastar and the adjacent Dharwar Cratons. A few young zircon grains in the syenite and lamproites show a range of early to middle Neoproterozoic ages from 879 to 651 Ma corresponding to younger thermal event(s) as also represented by granitic veins cutting across these rocks and extensive silicification. Zircon Lu–Hf isotope data suggest magma derivation from a refertilized Paleo-Mesoarchean sub-continental lithospheric mantle, or OIB-type sources. The differences in Hf-isotope composition among the zircon grains from different age groups indicate that the mantle sources of the lamproite are heterogeneous at the regional scale. A combination of the features from geochemical and zircon Hf isotope data is consistent with asthenosphere-lithosphere interaction during the lamproite magma evolution. The timing of lamproite emplacement in central India correlates with the global 2.2 Ga record of LIPs. We link the origin of the related mantle plume to the recycling of subducted slabs associated with the prolonged subduction-accretion history prior to the Neoarchean cratonization, as well as the thermal blanket effect of the Earth's oldest supercontinent. Pulsating plumes and continued rifting generated voluminous dyke swarms across the Bastar and Dharwar Cratons, forming part of a major global rifting and LIP event. [ABSTRACT FROM AUTHOR]

Alkaline and potassic igneous rocks, although minor components of the continental crust, provide important constraints on magma processes and tectonic settings. Here we report petrology, mineral chemistry, whole rock geochemistry, zircon U–Pb and Lu–Hf data on shonkinites from the Salem Block in the Southern Granulite Terrane of India. The shonkinite is composed mainly of porphyritic clinopyroxene, K-feldspar and olivine, and intrudes into the surrounding ultramafic rocks composed of wehrlite (clinopyroxene and olivine) and dunite (mostly olivine with minor spinel). The geochemical data on shonkinites from Salem show alkaline and ultrapotassic features with marked enrichment in incompatible elements, and high K 2 O and K 2 O/Na 2 O ratios suggesting magma derivation from metasomatized lithospheric mantle. Their HFSE depletion relative to LILE compositions, coupled with positive Nb anomalies suggests continental rift setting. The zircon grains in shonkinite show typical magmatic crystallization features and their LA-ICP-MS U–Pb data yield weighted mean age of 818 ± 6.3 Ma (MSWD = 0.32). The zircon ε Hf values range between −11.1 and −12.7, consistent with magma derivation from an enriched mantle, with depleted mantle model ages ( T DM ) in the range of between 2406 and 2513 Ma, indicating the involvement of Neoarchean–Paleoproterozoic subduction-related components. Together with similar-aged alkaline plutons with comparable geochemical features, we envisage a major alkaline magmatic event associated with rifting during Cryogenian in the northern part of the Southern Granulite Terrane. [ABSTRACT FROM AUTHOR]

Abstract: The Krishna–Godavari basin is a rifted passive margin basin that developed orthogonally to the NW–SE trending Pranhita–Godavari graben along the central Eastern Continental Margin of India in response to the continental rifting process and the subsequent seafloor spreading between India and eastern Antarctica during the early Cretaceous period. The 3-D gravity interpretation of both the onshore and offshore sections of the basin integrated with detailed seismic reflection and refraction data provided significant new information about the crustal architecture and the early breakup history of the basin. The gravity-derived crustal models indicate that the crust at the eastern Indian shield margin is 39–41 km thick and thins to as much as 20–23 km at the Ocean Continent Transition (OCT) in the offshore. There is a significant variation in the nature of crust and the configuration along the margin between the three crustal domains encompassing the basin: the Cuddapah basin, Pranhita–Godavari Graben, and the Eastern Ghat Mobile Belt that indicates the lateral segmentation of the margin. A zone of high density (3.0 g/cm3) crustal material at the OCT separates the pure continental and oceanic crusts on either side. The Moho is shallowest along this zone and brought the upper mantle notably close to the surface, which indicates that it could be comprised of Proto-Oceanic Crustal rocks. Furthermore, the present study highlights a major basement structural high in the deep offshore area of the margin. The geophysical signatures over this structure suggest that it is a crustal scale feature that is characterized by intrusive volcanic rocks, comprised of low density crust, and lies close to the OCT towards offshore. Based on the inferred tectonic reconstruction, we believe that this structural high could be a continental fragment that was left behind during the process of breakup between India and the Elan Bank. [Copyright &y& Elsevier]

Summary: This study aims to investigate groundwater recharge and flow patterns in tectonically active rift systems, exemplified by a case study in the Main Ethiopian Rift. The chosen approach includes the investigation of hydrochemical parameters and environmental isotopes (3H, δ2H, δ18O, δ13C-DIC, 14C-DIC, 87Sr/86Sr). Apparent groundwater ages were determined by radiocarbon dating after correction of 14C-DIC using a modified δ13C-mixing model and further validation using geochemical modelling with NETPATH. Hydrochemical and isotopic data indicate an evolutionary trend existing from the escarpments towards the Rift floor. Groundwater evolves from tritium-containing and hence recently recharged Ca–HCO3-type water on the escarpments to tritium-free Na–HCO3 groundwater dominating deep Rift floor aquifers. Correspondingly, rising pH and values coupled with increasingly enriched δ13C signatures point to hydrochemical evolution of DIC and beginning dilution of the carbon isotope signature by other carbon sources, related to a diffuse influx of mantle CO2 into the groundwater system. Especially thermal groundwater sampled near the most recent fault zones in the Fantale/Beseka region displays clear influence of mantle CO2 and increased water–rock interaction, indicated by a shift in δ13C and 87Sr/86Sr signatures. The calculation of apparent groundwater ages revealed an age increase of deep groundwater from the escarpments to the Rift floor, complying with hydrochemical evolution. Within the Rift, samples show a relatively uniform distribution of apparent 14C ages of ∼1800 to ∼2800years, with the expected down-gradient aging trend lacking, contradicting the predominant intra-rift groundwater flow described in existing transect-based models of groundwater flow. By combining hydrochemical and new isotopic data with knowledge of the structural geology of the Rift, we improve the existing groundwater flow model and propose a new conceptual model by identifying flow paths both transversal and longitudinal to the main Rift axis, the latter being strongly controlled by faulted and tilted blocks on the escarpment steps. The connection between groundwater flow and fault direction make this model applicable to other active rift systems with similar structural settings. [Copyright &y& Elsevier]

Abstract: The North China Craton (NCC) has figured prominently in recent reconstructions on the Paleoproterozoic supercontinent Columbia. Here we report abundant carbonatitic and mafic dykes from around the giant Bayan Obo rare earth element deposit in the northern margin of the NCC, and present geochemical and isotope geochronological data. The carbonatite (1354±59Ma) and mafic dykes (1227±60Ma) have comparable whole rock Sm–Nd isochron ages and Sr–Nd isotope compositions, suggesting a common source characteristic. Their geochemical characters including major and trace elements as well as REE patterns also attest to a common tectonic environment of magma generation and emplacement within a continental margin rift. The extensive mafic and carbonatitic magmatisms are associated with an extensional event that resulted in the formation of the Bayan Obo rift in the northern margin of the NCC, which we correlate with the final stages of fragmentation of the Columbia supercontinent amalgam. [ABSTRACT FROM AUTHOR]

Abstract: We present new brittle tectonic data constraining the onset of formation of the eastern passive margin of the Levant basin (Eastern Mediterranean basin) in Lebanon. From the identification of syn-tectonic growth faults, we infer an extensional tectonic regime starting in the Early Cretaceous and ceasing during the Cenomanian. The related stress field had a NNE–SSW direction of extension. It produced WSW–ENE to WNW–ESE normal faults with offsets as large as several hundred meters. Late Jurassic volcanic activity preceded this rifting event and continued until the late Aptian. Thickness and facies variations of the Lower Cretaceous sequence indicate that this rifting event led to the development of an E–W basin in Lebanon. This basin deepens westward, with a possible offshore continuation. The significant obliquity between the ∼NE–SW Early Mesozoic faults in southeastern corner of the Levant basin and ∼E–W Early Cretaceous faults recognized in Lebanon indicates that the mechanisms driving the development of the Eastern Mediterranean basin drastically changed during the Mesozoic. [Copyright &y& Elsevier]

The Sakoli Mobile Belt comprises bimodal volcanic rocks that include metabasalt, rhyolite, tuffs, and epiclastic rocks with metapelites, quartzite, arkose, conglomerate, and banded iron formation (BIF). Mafic volcanic rocks are tholeiitic to quartz-tholeiitic with normative quartz and hypersthene. SiO2 shows a large compositional gap between the basic and acidic volcanics, depicting their bimodal nature. Both the volcanics have distinct geochemical trends but display some similarity in terms of enriched light rare earth element–large ion lithophile element characteristics with positive anomalies for U, Pb, and Th and distinct negative anomalies for Nb, P, and Ti. These characteristics are typical of continental rift volcanism. Both the volcanic rocks show strong negative Sr and Eu anomalies indicating fractionation of plagioclases and K-feldspars, respectively. The high Fe/Mg ratios for the basic rocks indicate their evolved nature. Whole rock Sm–Nd isochrons for the acidic volcanic rocks indicate an age of crystallization for these volcanic rocks at about 1675 ± 180 Ma (initial 143Nd/144Nd = 0.51017 ± 0.00017, mean square weighted deviate [MSWD] = 1.6). The εNdt ( t = 2000 Ma) varies between −0.19 and +2.22 for the basic volcanic rock and between −2.85 and −4.29 for the acidic volcanic rocks. Depleted mantle model ages vary from 2000 to 2275 Ma for the basic and from 2426 to 2777 Ma for the acidic volcanic rocks, respectively. These model ages indicate that protoliths for the acidic volcanic rocks probably had a much longer crustal residence time. Predominantly basaltic magma erupted during the deposition of the Dhabetekri Formation and part of it pooled at crustal or shallower subcrustal levels that probably triggered partial melting to generate the acidic magma. The influence of basic magma on the genesis of acidic magma is indicated by the higher Ni and Cr abundance at the observed silica levels of the acidic magma. A subsequent pulse of basic magma, which became crustally contaminated, erupted as minor component along with the dominantly acidic volcanics during the deposition of the Bhiwapur Formation. [ABSTRACT FROM AUTHOR]

Abstract: The global context of glaciation in the Neoproterozoic has been hypothesized to result from the massive reorganization of the Earth''s land and ocean systems due to breakup and dispersal of the supercontinent Rodinia at about 750 Ma. This hypothesis gains support from unusually light O isotope records of hydrothermally altered rocks in a rift tectonic zone at that time, which is indicative of interaction with meteoric water of low mean annual temperature, and thus tectonic driving of cold paleoclimate at the time of hydrothermal alteration. Very negative δ 18O values of −14.4 to −10.0‰ are found for garnet from Neoproterozoic granite in South China, which are the lightest O isotope record so far reported for minerals from igneous rocks. Negative δD values of −129 to −109‰ are obtained for garnet, magnetite and zircon. Thus high-T meteoric-hydrothermal alteration occurred during magma emplacement. SHRIMP U–Pb dating for magmatic and hydrothermal zircons yields two groups of ages at 782±3 Ma and 748±3 Ma, respectively, responsible for granite crystallization and hydrothermal alteration. The garnet is in O isotope disequilibrium with zircon, indicating differential effects of subsolidus hydrothermal alteration on the minerals of different O diffusion rates. Occurrence of the unusually negative δ 18O granite at mid-low paleolatitudes provides a geochemical proxy for a cold paleoclimate at 748±3 Ma, possibly a continental glaciation corresponding to the Kaigas iceage. It suggests a tectonic link to the climatic effect of uplifted rift flanks due to the Rodinia breakup at about 750 Ma, and thus the ice–fire interaction by syn-rift magmatism of low δ 18O imprints in association with the mantle event of asthenospheric upwelling. Hence the tectonic driving is evident for regional glaciations in supercontinental rift basins. [Copyright &y& Elsevier]

Late Quaternary structure and kinematics at the southern termination of the Taupo Rift are investigated by means of active fault mapping and estimates of fault displacement and extension rates. Active faults in the southern Taupo Rift are normal in sense and include three major structures: the NNE-trending Mt Ruapehu Graben; the E-W to ESE-WNW-trending Ohakune-Raetihi fault set; and the NE-trending Karioi fault set. Displacements of young geomorphic features, fault plane exposures in roadcuts, and trenches across faults show that all faults are active contemporaneously. The Mt Ruapehu Graben is the southern extension of the modern (i.e., <26 ka) Taupo Rift (or Taupo Fault Belt, TFB), and has developed as a result of backarc extension related to the Hikurangi subduction margin. Geologic extension rate for the Mt Ruapehu Graben is estimated here at 2.3 ± 1.2 mm/yr. The other two structures strike almost perpendicular to the Taupo Rift terminating the Mt Ruapehu Graben to the south. The co-existence of three normal fault sets in mutual crosscutting relations and with no significant strike-slip on any of them may be related to local reorientation of the minimum principal stress axis (σ3) or to a stress tensor where |σ3| ≈ |σ2|. Complicated strains at the southern termination of the Taupo Rift are related to block rotations in the Hikurangi subduction margin. [ABSTRACT FROM AUTHOR]

Remote sensing data and revised seismic reflection profiles provide new insights about the origin of inverted deformation within Miocene-Recent basins of the Turkana rift (northern Kenya) in the eastern branch of the East African rift system. Contractional structures are dominated by weakly inverted sets of fault blocks within <3.7 Myr old synrift series. Most of reverse extensional faults involve components of oblique-slip, whereas associated hanging wall folds are characterized by large wavelength upright folding. The area of basin inversion is restricted to a 40 × 100 km elongated zone overlying a first-order N140°E trending fault zone in the basement, referred to as the N'Doto transverse fault zone (NTFZ). In the proposed kinematic model, inversion tectonics is assigned to permutation of principal stress axes (σ1/σ2) in addition to the clockwise rotation of extension (from nearly N90°E to N130°E) during Pliocene. The transition from pure extension (Miocene) to a wrench faulting regime (Pliocene) first results in the development of T-type fault networks within a dextrally reactivated shear zone (NTFZ). Inversion tectonics occurred later (<3.7 Ma) in response to a still rotated (∼20°) shortening axis (σ1) oriented N40°E that caused the oblique compression of earlier (NS to N20°E) extensional structures within the NTFZ. The origin of basin inversion and strain concentration in the Turkana rift is thus directly linked to a crustal weakness zone, transverse to the rift axis, and involving steep prerift anisotropies. [ABSTRACT FROM AUTHOR]

New data on the evolution of rift basins is presented after analysing the Late Jurassic stratigraphy of the Central Lusitanian Basin (west Iberia). Well, outcrop and regional 2D seismic reflection profiles are used to investigate the differences in stratigraphic signature between diapir- and fault-bounded sub-basins. During the Late Jurassic syn-rift phase, surface rupturing in fault-bounded sub-basins resulted in the formation of tectonic scarps from which footwall-derived gravity flows were sourced. In contrast, the diapir-bounded Bombarral-Alcobac¸a sub-basin evolved as a distal bowl-shaped depocentre with an axis located up to 10 km away from its basin margins. Low-gradient marginal slopes developed in the Bombarral-Alcobac¸a sub-basin during the Late Jurassic rifting, while growing salt pillows limited the vertical propagation of basement normal faults. Differences in tectonic evolution, basin physiography and sediment input are the main factors responsible for the distinct sedimentary evolutions recorded in the study area: (1) transverse footwall-derived sediment fans, predominant in fault-bounded regions, give place to axial southwards-prograding fluvial to shallow-marine units in the diapir-bounded sub-basins; (2) growing salt pillows, absent in the fault-bounded sub-basins, formed barriers to and limited the development of transverse drainage systems. [Copyright &y& Elsevier]

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One of the World's premier field geologists, Kristján Sæmundsson led immense geological mapping programs and authored or co-authored nearly all geological maps of Iceland during the past half century, including the first modern bedrock and tectonic maps of the whole country. These monumental achievements collectively yield the most inclusive view of an extensional plate boundary anywhere on Earth. When Kristján began his work in 1961, the relation of Iceland to sea-floor spreading was not clear, and plate tectonics had not yet been invented. Kristján resolved key obstacles by demonstrating that the active rifting zones in Iceland had shifted over time and were linked by complex transforms to the mid-ocean spreading ridge, thus making the concept of sea-floor spreading in Iceland acceptable to those previously skeptical. Further, his insights and vast geological and tectonic knowledge on both high- and low-temperature geothermal areas in Iceland yielded a major increase in knowledge of geothermal systems, and probably no one has contributed more than he to Icelandic energy development. Kristján's legacy is comprised by his numerous superb maps on a variety of scales, the high quality papers he produced, the impactful ideas generated that were internationally diffused, and the generations of colleagues and younger people he inspired, mentored, or otherwise positively influenced with his knowledge and generous attitude. • Kristján Sæmundsson led immense geological mapping programs for Iceland during the past half century. • He was a pioneer in relating Iceland to sea-floor spreading and plate tectonics. • his insights yielded a major increase in knowledge of geothermal systems and contributed hugely to Icelandic energy development. [ABSTRACT FROM AUTHOR]