Deccan Traps Research Articles

Structure and emplacement of the Coastal Deccan tholeiitic dyke swarm in Goa, on the western Indian rifted margin

Mafic dyke swarms, when controlled by a regional stress field, are emplaced perpendicular to the minimum principal compressive stress direction. However, mafic dyke swarms may also exploit pre-existing structural fabric in their host rocks. Distinguishing between the two scenarios is important for correct geodynamic understanding of magmatic events. Continental flood basalt provinces such as the Deccan Traps of India represent huge fissure eruptions produced by swarms of mafic dykes. The Coastal dyke swarm of the Deccan, along the western Indian rifted margin, is one of its three major dyke swarms. We present a field study of the Goa dykes, part of the Coastal swarm. The dykes are typical dolerites and basalts exposed in Proterozoic-age basement; lava flows which these dykes may have once fed have been stripped away by erosion, though exposed in the Western Ghats escarpment ~ 100 km to the east. The Goa dyke swarm contains 60 dykes, some of which form larger multiple-injection dykes. The dykes are short compared to typical (kilometers or tens of kilometers long) dykes in flood basalt provinces such as the Deccan or Iceland, and range in length from ~ 15 to ~ 220 m and in thickness from 3 cm to 20 m. Most dykes are vertical and the others dip steeply in various directions. Their structural attributes and internal features (such as vesicular zones and columnar jointing) provide clues to the processes of magma flow and solidification in the dykes. Field measurements yield an average crustal dilation of 8% due to dyke emplacement. Calculations of magmatic overpressures and magma chamber depths generally yield unrealistic values owing to the low aspect ratios of the dykes. Based on comparisons between the dyke trends and the host rock joint trends, we consider that a few of the Goa dykes were emplaced along pre-existing fractures, but the orientations of the great majority of the dykes reflect contemporaneous stress fields. Based on crosscutting relationships, we infer that the regional minimum horizontal compressive stress (σ3) changed over time from a ~E-W orientation to a ~NE-SW orientation and finally to a ~N-S direction.

Large-scale mass wasting on the Miocene continental margin of western India

Abstract 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 ∼300 m (Site U1456) and ∼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 calcarenites 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 ∼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.

40Ar/39Ar dating of tholeiitic flows and dykes of Elephanta Island, Panvel flexure zone, western Deccan Traps: A five-million-year record of magmatism preceding India-Laxmi Ridge-Seychelles breakup

Elephanta Island, located near Mumbai in the western Deccan Traps, is composed of subaerially erupted tholeiitic lava flows and dykes, with no evolved or pyroclastic rocks. In contrast, Deccan volcanism in Mumbai includes subaerial and subaqueous lava flows, intrusions, and pyroclastics, of highly varied composition. The Mumbai and Elephanta sequences, being in the Panvel flexure zone along the western Indian rifted continental margin, dip prominently seaward. The bulk of the Mumbai sequence stands dated at 62.5 Ma, contemporaneous with continental breakup between the Seychelles and Laxmi Ridge-India, voluminous offshore magmatism, and Panvel flexure formation. We have obtained whole-rock 40Ar/39Ar ages of 66–65 Ma on six Elephanta tholeiites (representing two lava flows and four dykes) and a picrite dyke. An ankaramite dyke yields a whole-rock 40Ar/39Ar age of 61.6 ± 0.4 Ma (2σ). These ages imply that the bulk of the Elephanta sequence was formed rapidly, well before continental breakup, and is contemporaneous with (and potentially a part of) the main Deccan sequence in the Western Ghats escarpment to the east. The younger age of the ankaramite dyke shows that the total duration of Elephanta magmatism was not <3.5 Myr and possibly as much as 6 Myr. Interestingly, the age of the Elephanta ankaramite dyke overlaps with that of the Powai ankaramite flow in Mumbai, suggesting the dyke to be a potential feeder of that flow.

Chemical erosion rates in the upper Blue Nile Basin and related atmospheric CO2 consumption

Silicate weathering of basaltic rocks constitutes a non-negligible sink of atmospheric CO2 but the role it plays in the regulation of past and future global climate is still matter of debate. In this study, silicate weathering rates for various sub-basins of the Ethiopian Traps, emplaced 30 million years ago, and the corresponding atmospheric CO2 consumption rates are evaluated. For this, major and trace elements were measured in the dissolved phases and in the sedimentary particles carried and deposited by the main rivers flowing through this steep region. Lithium isotopes and major elements were also measured in the extracted clay fractions in order to infer complementary information on weathering processes in this region. Clay δ7Li values correlate positively with Mg/Ti ratios, and are best explained by varying ratios of leaching versus clay formation rate.Although located in a region annually submitted to monsoon, average silicate weathering rate (16.1 tons/km2/year) and CO2 consumption rate (0.65 × 1012 mol/year) are estimated to be low when compared to other basaltic regions such as the Deccan Traps, and volcanically active islands of the tropical zone. This is surprising since the concentrations of Total Dissolved Solids of the Ethiopian rivers are among the highest ones. With a 2D rainfall model that takes into account the detailed topography of the region, annual occurrence of the Monsoon, and monitoring station data, we show that runoff intensity is a key parameter that explains this difference. We determine that, at present, the weathering of the Ethiopian Traps plays a negligible role in the carbon cycle. However, simple calculations, which integrate recent knowledge on African climate variations and on weathering controls, illustrate that during the African Humid Period (14–8 kyr), a significant increase of Monsoon precipitation may have resulted in much higher weathering rates and related CO2 consumption (0.91–1.5 × 1012 mol/year). This study therefore evidences the potential importance of this region in the past, and the need to quantify more precisely the variations of the monsoon intensity and its impact on tropical watersheds for reconstructing past CO2 levels.

Syn- to post-rift fault evolution in a failed rift: a reflection seismic study in central Cambay Basin (Gujarat), India

The Barmer and Cambay basins (India) formed as rifts during the Cretaceous to Paleocene, prior to Seychelles–India separation during Late Cretaceous. Both of the rifts failed subsequently and the Seychelles separated from India along the current Saurashtra–Mumbai continental margin. We study the fault kinematics, i.e., fault growth and linkages in the Cambay–Tarapur Block and comment on the fault evolution from syn- to post-rift stages. We use fault plane topography, throw-length and throw-depth plots to understand the kinematics of the fault planes. Throw-length plots show the along-strike linkage of faults. The throw-depth plots are used to understand the down and up dip linkage of the fault planes. Faults originating at the Deccan Traps level and faults confined only to post-rift progressed as single fault planes along-strike throughout their deformation history. Faults initiating within the syn-rift started as multiple along-strike segments and joined together as they grew. Along dip, faults can be classified as (i) faults originating at Deccan Traps and reaching post-rift, (ii) faults originating within syn-rift sediments and reaching post-rift, (iii) faults confined only within syn-rift and (iv) faults confined only within post rift. Some of the syn-rift faults also show spatially and temporally limited compression, indicating their possible oblique-slip nature. The faults were normal-slip or oblique-slip during syn-rift, and thereafter they propagated as normal dip-slip faults, as analysed from fault plane topography showing strike perpendicular ridges. Such ridges may indicate only the last movement on the fault plane. Due to the presence of thick shale horizons in the syn-rift, many faults show soft-linked segments along dip. Hydrocarbon occurrences in the shallow (1200–1600 m) Miocene sands can possibly be attributed to the normal faults controlling fluid flow into the reservoirs. (282 words).

Seismic imaging of Deccan-related lava flows at the K-T boundary, deepwater west India

Continuous improvements in geophysical technology have enabled seismic imaging of Mesozoic strata under the K-T transition boundary — a thick lava blanket spread along the continental margin of India's west coast. The new images reveal features of the subbasalt geology of this large offshore region. The relationship of the offshore lava flows with the equally vast lava flows of the onshore Deccan Traps adds a parameter of comparison to the whole Mesozoic stratigraphy of the south, west, and northwest coasts of India. The lava flows are part of the deepwater margin that embraces several basins, the largest being the southernmost underexplored Kerala-Konkan Basin. Newly acquired and processed regional seismic data sets were integrated with new data sets of potential field data to better uncover the Mesozoic stratigraphy. The 3D seismic data, acquired with long spreads and broadband processing through time and depth migration, enabled imaging of the complex structure of layers under basalt. India detached from Madagascar during Cretaceous time; the northward path to its present position was affected by Reunion hotspot activity that melted the west coast near the K-T transition time. The volcanic lithology observed at the K-T boundary is related to episodic emplacement of intrusive dykes and extrusive sills. A possible cause-effect event in that span of time is the Chicxulub meteorite impact that may have increased the volume of Deccan lava flows. The deepwater lava volumes estimated from interpretation of seismic data from the offshore region at the end of Cretaceous time are comparable to, or perhaps much greater than, the volumes estimated for the precisely age-dated Deccan Traps basalts exposed onshore. This suggests an enormous outpouring of flood basalts (onshore and offshore) at K-T time that was comparable to other major flood basalt events, such as the Siberian traps, and associated with mass extinctions.

A High‐Fidelity Benthic Stable Isotope Record of Late Cretaceous–Early Eocene Climate Change and Carbon‐Cycling

AbstractThe Late Cretaceous–Early Paleogene is the most recent period in Earth history that experienced sustained global greenhouse warmth on multimillion year timescales. Yet, knowledge of ambient climate conditions and the complex interplay between various forcing mechanisms are still poorly constrained. Here we present a 14.75 million‐year‐long, high‐resolution, orbitally tuned record of paired climate change and carbon‐cycling for this enigmatic period (~67–52 Ma), which we compare to an up‐to‐date compilation of atmospheric pCO2 records. Our climate and carbon‐cycling records, which are the highest resolution stratigraphically complete records to be constructed from a single marine site in the Atlantic Ocean, feature all major transient warming events (termed “hyperthermals”) known from this time period. We identify eccentricity as the dominant pacemaker of climate and the carbon cycle throughout the Late Maastrichtian to Early Eocene, through the modulation of precession. On average, changes in the carbon cycle lagged changes in climate by ~23,000 years at the long eccentricity (405,000‐year) band, and by ~3,000–4,500 years at the short eccentricity (100,000‐year) band, suggesting that light carbon was released as a positive feedback to warming induced by orbital forcing. Our new record places all known hyperthermals of the Late Maastrichtian–Early Eocene into temporal context with regards to evolving ambient climate of the time. We constrain potential carbon cycle influences of Large Igneous Province volcanism associated with the Deccan Traps and North Atlantic Igneous Province, as well as the sensitivity of climate and the carbon‐cycle to the 2.4 million‐year‐long eccentricity cycle.

Petrology and geochemistry of dolerite and lamprophyre sills in Mesozoic successions of Khanozai–Muslim Bagh area, northwestern Pakistan

In Khanozai–Muslim Bagh area, NW Pakistan, the sills are intruded into mainly Triassic–Jurassic successions of Indian platform sediments. They are petrographically identified as dolerite and lamprophyre. The dolerite is a normal dolerite and olivine dolerite and is altered compared with lamprophyre. Lamprophyre is classified as sannaite, camptonite, minette, and damtjernite. The geochemical signature of dolerite, olivine dolerite, and lamprophyre suggests that these rocks belong to alkali series by classification and may be alkaline in nature. Normal mid-oceanic ridge basalts (NMORB)–normalized plots of dolerite, olivine dolerite, and lamprophyre show higher enrichment of large ion lithophile elements (LILEs) relative to high field strength elements (HFSEs) and marked positive anomalies on Nb which confirm their origin from an enriched mantle source. The ocean island basalt (OIB)–normalized plots of these rocks exhibit patterns almost alike to those of OIB suggesting a source similar to OIB. The tectono-magmatic discrimination plots of dolerite, olivine dolerite, and lamprophyres plot them in the OIB field and indicate that they are alkaline rocks in nature. The petrogenesis and tectonic setting of these rocks suggest that they are OIB in nature and may represent a Late Cretaceous magmatic activity that erupted as hotspot fluid through the crust of Indian Plate during the Late Cretaceous. It is much similar to the other hotspot-related rocks that intruded into the Parh Group and Bela ophiolite melange. Much like the magmatism of Deccan Traps and the Chagos-Laccadive Ridge, these rocks may be the melt of a hotspot possibly Reunion that intruded into the Indian Plate margin when it had passed over it during the Late Cretaceous.

Understanding basement fracture architecture in Padra Field, South Cambay Basin, India, through full-azimuth imaging

Detection and characterization of fractures in reservoirs is of great importance for maximizing hydrocarbon productivity and recovery efficiency. Fractures play an important role in the producibility of unconventional reservoirs such as basement reservoirs. Basement reservoirs are typically found within metamorphic and igneous rock underlying a sedimentary basin, where faulting and tectonic uplift has led to creation of a fracture network. For fracture characterization, integration of information from seismic and nonseismic data such as cores and/or formation microimaging (FMI) logs is essential. Various seismic attributes such as coherency and curvature that are derived from reflection seismic data have been used for more than a decade to detect faults and fractures. In advanced seismic fracture detection technology, automatic fault extraction (AFE) from diffraction seismic data (discontinuity volume) more effectively detects finer scale features in seismic data. We demonstrate the utility of this methodology with an application to seismic data from the Padra Field, South Cambay Basin, India, where the basaltic Deccan Trap forms the basement, and hydrocarbons are produced from basement fractures. Diffraction imaging was applied during processing of the full-azimuth 3D-3C seismic data that cover this field. Using wavefield decomposition in the subsurface local angle domain, separate reflection (specular) and diffraction data volumes were produced. The high-resolution specular stack data imaged a prominent reflector well below the trap top, which is not visible in conventional seismic reflection data. Diffraction stack data also provided higher resolution fault definition and enhanced imaging of spatially consistent geological discontinuities. Subsequent application of the AFE technique to diffraction-imaged data yielded sharp and crisp definition of faults and fractures. We also performed velocity variation with azimuth analysis of 3D angle-azimuth reflection gathers to generate a fracture orientation map. Both sets of results were validated by fractures detected in FMI logs from wells in the field.

Curtain Grouting, a Tool Used for Stopping the Seepage from an Existing Dam

Hidkal dam, also known as Raja Lakhamagouda dam, was constructed in the year 1974 across the river Ghataprabha in Krishna River Basin. In this dam, there are two dykes—Dyke I and Dyke II, each of length 5227 and 506 m, respectively. After impounding the dam, it was observed that there is seepage at a distance 3 km downstream of Dyke I. To overcome the situation, seismic refraction studies in the upstream of Dyke I were conducted by CWPRS, Pune, for delineating the stretch of the seepage zone. After identifying the seepage zone, trial boreholes of Nx size were made to determine the permeability of the strata for designing effective treatment measures. Recovery of material from the trial boreholes was examined to identify the rock type met with in the foundation along with the degree of weathering. The rock extracted is Deccan Trap Basalt with various degrees of weathering ranging from highly weathered to disintegrated variety including the presence of quartzite. The permeability values vary between 26 and 498 Lugeons. So to make the strata impermeable, curtain grouting was carried out in three rows, 1.5 m apart from each other from Ch. 4650 to 5330 m near the upstream heel through Nx size borehole. The spacing of the boreholes is 3 m c/c, and boreholes are staggered and up to depth of 20 m are suggested to be carried out. During the grouting process, CWPRS engineers inspected the process of grouting from bottom of borehole to the surface. After the completion of grouting, post-permeability tests were conducted and it was seen that the permeability values had drastically reduced. The data obtained during investigations were used in the preparation of a mathematical model (FEM). After the seepage analysis of the model, it was observed that there was drastic reduction in the seepage.

Petrophysical variations within the basaltic lava flows from Tural-Rajawadi hot springs, Western India and their bearing on the viability of low-enthalpy geothermal systems

The Tural-Rajawadi hot springs (Temp. 30-62 °C) are a part of the West Coast Geothermal Province of India, located at the foothills of the Western Ghats in Maharashtra state. The hot springs manifest as several small pools/puddles along lineaments within the valley that exposes five (5) surficial lava flows belonging to the Cretaceous Deccan Traps. In addition, six (6) distinct lava flows were identified based on their internal structure, textural variations, vesiculation and presence of glassy rinds in the 203m deep exploratory borehole drilled at Rajawadi. These lavas are of pahoehoe sheet flow and rubbly pahoehoe morphologies. Temperature of water increased from 51 at the top to62 °C at the bottom. XRD studies of vesicle and joint fillings within the lava flows reveal peculiar assemblage of calcite, nontronite, clinopitolite, stilbite, apophyllite, cristobalite, etc. indicating mineralisation related to circulating hot geothermal fluids. The detailed petrophysical properties like textural diversity, crystal size distribution, specific gravity (2.766 to 2.999), resistivity (295.5 to 1790Ωm at 50Hz) and thermal conductivity were measured and their variations within two (2) thick subsurface lava flows is presented. These variations can be attributed to the crystallisation, cooling history, post emplacement hydrothermal alteration and mineralisation the lava flows have undergone. Understanding these variations is essential for assessing viability and commercial feasibility in using and exploiting low-enthalpy geothermal systems.

Two timelines for extinction

Mass Extinction![Figure][1] The Deccan Traps in India were a source of large-scale volcanic activity that affected the climate 66 million years ago. IMAGE: GERTA KELLER The Cretaceous-Paleogene extinction that wiped out the nonavian dinosaurs 66 million years ago was correlated with two extreme events: The Chicxulub impact occurred at roughly the same time that massive amounts of lava were erupting from the Deccan Traps (see the Perspective by Burgess). Sprain et al. used argon-argon dating of the volcanic ash from the Deccan Traps to argue that a steady eruption of the flood basalts mostly occurred after the Chicxulub impact. Schoene et al. used uranium-lead dating of zircons from ash beds and concluded that four large magmatic pulses occurred during the flood basalt eruption, the first of which preceded the Chicxulub impact. Whatever the correct ordering of events, better constraints on the timing and rates of the eruption will help elucidate how volcanic gas influenced climate. Science , this issue p. [866][2], p. [862][3]; see also p. [815][4] [1]: pending:yes [2]: /lookup/doi/10.1126/science.aav1446 [3]: /lookup/doi/10.1126/science.aau2422 [4]: /lookup/doi/10.1126/science.aaw0473

U-Pb constraints on pulsed eruption of the Deccan Traps across the end-Cretaceous mass extinction.

Temporal correlation between some continental flood basalt eruptions and mass extinctions has been proposed to indicate causality, with eruptive volatile release driving environmental degradation and extinction. We tested this model for the Deccan Traps flood basalt province, which, along with the Chicxulub bolide impact, is implicated in the Cretaceous-Paleogene (K-Pg) extinction approximately 66 million years ago. We estimated Deccan eruption rates with uranium-lead (U-Pb) zircon geochronology and resolved four high-volume eruptive periods. According to this model, maximum eruption rates occurred before and after the K-Pg extinction, with one such pulse initiating tens of thousands of years prior to both the bolide impact and extinction. These findings support extinction models that incorporate both catastrophic events as drivers of environmental deterioration associated with the K-Pg extinction and its aftermath.

The eruptive tempo of Deccan volcanism in relation to the Cretaceous-Paleogene boundary.

Late Cretaceous records of environmental change suggest that Deccan Traps (DT) volcanism contributed to the Cretaceous-Paleogene boundary (KPB) ecosystem crisis. However, testing this hypothesis requires identification of the KPB in the DT. We constrain the location of the KPB with high-precision argon-40/argon-39 data to be coincident with changes in the magmatic plumbing system. We also found that the DT did not erupt in three discrete large pulses and that >90% of DT volume erupted in <1 million years, with ~75% emplaced post-KPB. Late Cretaceous records of climate change coincide temporally with the eruption of the smallest DT phases, suggesting that either the release of climate-modifying gases is not directly related to eruptive volume or DT volcanism was not the source of Late Cretaceous climate change.

Probabilistic groundwater recharge zonation in hard rock terrain using geospatial techniques in Veniar watershed, South India

‘Water’ always has been a key resource for humankind but over the period with increasing population and rapid urbanization, water resource is under immense pressure. Groundwater is the prime source of drinking water and needs proper monitoring in order to have an optimum utilization strategy owing to the current scenario of over-exploitation. Being the part of Deccan trap region, the basin is hot and semi-arid, experiencing moderately dry climate accompanied by episodic drought and erratic showers therefore, the surface water of the basin is ephemeral. In the present study, the probabilistic recharge zones have been delineated by dint of various thematic layers viz. geological setting, geomorphic features, land use, slope gradient, infiltration number, soil physiognomies, curvature and lineament through multi-criteria decision analysis technique in the GIS domain. This has further divided into four categories, viz., very poor, poor, good, and very good zones. Thus, this paper is an attempt to demarcate probabilistic groundwater zones in the Vaniar watershed region using geospatial techniques for better planning and management of groundwater resources.

Elevated Barium concentrations in rain water from east-coast of India: role of regional lithology

Alkaline earth metals act as dominating acid-neutralizing species in atmosphere and hence, regulate the rain water chemistry significantly. In this contribution, concentrations of these metals (Mg, Ca, Sr and Ba) and other major ions in rain water samples, collected during south-west monsoon of year 2017, from a coastal location (Berhampur) in eastern part of India have been analyzed to trace their provenances and controlling factors. The chemical compositions of rain water reveal oceanic and continental supply of Mg and Sr to the site, whereas Ca and Ba are pre-dominantly supplied through continental sources. The dominancy of continental fluxes at this coastal site is mainly due to particulate fluxes from regional lithologies and favorable wind pattern for long-range transport from south-western/western directions. An inverse model involving chemical mass balance between rain water composition and its possible sources have been adopted in this study to quantify the source(s) contributions. These model results show that the continental Mg is mainly derived from long-range transport of mafic minerals from Deccan Traps (40 ± 21%) with sub-ordinate contribution (15 ± 6%) from regional lithologies. On average, about 70% of rain water Ca at Berhampur is derived from carbonates, whereas most of the Ba (~95%) is supplied from regional silicates (charnockites and khondalites). Owing to faster dissolution kinetics of these silicates with higher Ba content, the silicates contribute most of the rain water Ba concentration over this region. The median Ba content (29 nM) at this location is systematically higher than available literature Ba data for rain water worldwide (1-22 nM). The observed higher concentrations of Ba, a micronutrient, in rain water emphasize important role of regional lithology in the biogeochemical cycling of nutrients over the region via wet deposition.

Magnetotelluric investigations in the southern end of the Cambay basin (near coast), Gujarat, India

Cambay basin is one of the major onshore oil-bearing sedimentary basins of India situated at its western margin in Gujarat state. The region is seismically active and experiences three devastating earthquakes in the past. The region comes under the seismic zone IV and V and necessitates the extensive geophysical studies in for subsurface characteristics and identification of hidden faults in the region for seismic hazard estimation. A magnetotelluric (MT) survey has carried out in the southwestern part of the Cambay basin to identify the sedimentary thickness, the depth of Deccan trap and for identification of West Cambay margin that passes through the region. The MT data, at 13 stations, in the broad frequency range is acquired along an east-west profile with an inter-station spacing of 2–3 km. After processing and analysis of the data, transfer functions in the period band of 0.001–300 s are used for two-dimensional joint inversion. The resultant structure from the 2-D modeling infers the sedimentary thickness of ~0.5 km. A high conductive zone dipping towards east observed in the eastern part of the profile might be due to the West Cambay fault. Basement fracturing at a depth of 2.5–3.0 km at the central portion of the profile might act as a heat exchanger between the deeper geothermal source and nearby hot water spring.

Mineralogy, geochemistry and40Ar–39Ar geochronology of the Barda and Alech complexes, Saurashtra, northwestern Deccan Traps: early silicic magmas derived by flood basalt fractionation

AbstractMost continental flood basalt (CFB) provinces of the world contain silicic (granitic and rhyolitic) rocks, which are of significant petrogenetic interest. These rocks can form by advanced fractional crystallization of basaltic magmas, crustal assimilation with fractional crystallization, partial melting of hydrothermally altered basaltic lava flows or intrusions, anatexis of old basement crust, or hybridization between basaltic and crustal melts. In the Deccan Traps CFB province of India, the Barda and Alech Hills, dominated by granophyre and rhyolite, respectively, form the largest silicic complexes. We present petrographic, mineral chemical, and whole-rock geochemical (major and trace element and Sr–Nd isotopic) data on rocks of both complexes, along with40Ar–39Ar ages of 69.5–68.5 Ma on three Barda granophyres. Whereas silicic magmatism in the Deccan Traps typically postdates flood basalt eruptions, the Barda granophyre intrusions (and the Deccan basalt flows they intrude) significantly pre-date (by 3–4 My) the intense 66–65 Ma flood basalt phase forming the bulk of the province. A tholeiitic dyke cutting the Barda granophyres contains quartzite xenoliths, the first being reported from Saurashtra and probably representing Precambrian basement crust. However, geochemical–isotopic data show little involvement of ancient basement crust in the genesis of the Barda–Alech silicic rocks. We conclude that these rocks formed by advanced (70–75 %), nearly-closed system fractional crystallization of basaltic magmas in crustal magma chambers. The sheer size of each complex (tens of kilometres in diameter) indicates a very large mafic magma chamber, and a wide, pronounced, circular-shaped gravity high and magnetic anomaly mapped over these complexes is arguably the geophysical signature of this solidified magma chamber. The Barda and Alech complexes are important for understanding CFB-associated silicic magmatism, and anorogenic, intraplate silicic magmatism in general.

Mesozoic C-cycle perturbations and climate: evidence for increased resilience of the Cretaceous biosphere to greenhouse pulses

The Mesozoic C-isotope record traces the history of the global carbon cycle. Major perturbations of the carbon cycle triggered by extraordinary volcanic activity are recorded in negative spikes coupled with positive C-isotope excursions. Prominent examples are the C-isotope anomaly events at the Permian–Triassic boundary and at the Triassic–Jurassic boundary, in the Toarcian or in the Aptian. While the major volcanic pulses at the Permian–Triassic and Triassic–Jurassic boundaries are considered as the main trigger of mass extinctions, carbon cycle perturbations in the Jurassic and Cretaceous were not accompanied by comparable extreme changes in marine or terrestrial biota. The data suggest either that changes in degassing of large igneous provinces explain the difference in the biota’s response to perturbation or (and) that the resilience of the Mesozoic biosphere to volcanic pulses changed through time. It is hypothesized that two factors contributed to the increased resilience of the biosphere in the Late Mesozoic: (1) starting in the Middle Jurassic, pelagic carbonate developed into an important sink of CO2in the long-term carbon cycle, contributing to increased resilience of the carbon cycle to perturbations, and (2) increasing fragmentation of Pangea resulted in the establishment of a transequatorial current system coupled with equatorial upwelling. This circulation pattern was intensified during greenhouse pulses. Increased marine productivity and widespread basinal anoxia favoured burial of organic carbon. Increased resilience of the Cretaceous biosphere against volcanic activity may explain why Deccan Trap volcanism was no longer sufficient as a trigger of a mass extinction at the Cretaceous–Paleogene boundary.

The Late Cretaceous-Paleogene Deccan Traps: a Potential Global Heritage Stone Province from India

The Late Cretaceous/Palaeogene Deccan Traps, spread over half a million square kilometres in west-central parts of the Indian Peninsula, comprises predominantly basalts and subordinate felsic rocks (rhyolite, trachyte, tinguaites and spilites). The use of Deccan basalts and trachyte in the range of Indian architectonic heritage, from antiquity to current times, is quite fascinating. The abundance and ease of workability of Deccan Trap rocks render them the most preferred building material in all the architectonic heritage structures that document various cultural and ethnic habitations in the west central India. The Buddhist, Hindu and Jain temples of Ajanta, Ellora and Elephanta are the oldest in-situ basalt cave temples, dating back to second century. A contemporary iconic architectonic heritage ‘The Gateway of India’ personifies a majestic edifice built in buff coloured trachyte. In the present times, the Deccan basalts are being quarried for a variety of purposes such as masonry stone for buildings, road metal and as raw material for corrosion-resistant basaltic pipes and basalt wool that are also in great demand internationally. The trachyte quarries are almost extinct and relict trachyte outcrops are preserved in some places in and around Mumbai. The Deccan Trap rocks basalt and trachyte bear testimony to India’s cultural and architectonic heritage, are therefore, the most suitable candidates for according the status of Global Heritage Stone Resource (GHSR) as they fulfil all the requisite criteria specified by the IUGS subcommission on Hertaiage stones referred to as the Heritage Stone Subcommission (HSS). It is worthwhile to mention that both basalt and trachyte of the Deccan Traps are in geographic and geological proximity with each other and this qualifies Deccan Trap province to be recognised as a Global Heritage Stone Province (GHSP) from this subcontinent in consonance with the criteria for GHSP mentioned in the terms of reference given by HSS.