Western Deccan Volcanic Province Research Articles

Estimation of Geometrical Spreading Factor and Coda-Wave Attenuation Characteristics for the Saurashtra Horst in Western Deccan Volcanic Province, Gujarat, India

ABSTRACT In the present study, we have estimated the coda Q (QC) and the geometrical spreading factor (gamma) for the Saurashtra region that is located in the western part of the Deccan Volcanic Province, Gujarat, India. The waveform data from the seismic network operated by the Institute of Seismological Research, Gandhinagar, are used for the analysis. We have used records of a total of 221 local earthquakes with epicentral distances less than 70 km from the seismic stations. The moment magnitude range of the earthquakes is 2.5–5.3, and the depth range is 1–22 km. The QC has been estimated at five central frequencies (1.5, 3, 6, 12, and 20 Hz) using 20, 30, and 40 s window lengths for vertical, north–south, and east–west components, respectively. The obtained QC values show that the attenuation in the region has a strong dependence on frequency and points toward a heterogeneous medium. The geometrical spreading factors are also estimated for two cases, that is, with and without Q at the same central frequencies and window lengths. The average geometrical spreading factors with Q are 1.37, 1.48, 1.40, 1.08, and 1.04, at central frequencies 1.5, 3, 6, 12, and 20 Hz, respectively, whereas it is 1.59, 1.53, 1.47, 1.19, and 1.14, respectively, in the absence of Q. The estimated values also indicate that the medium is heterogeneous in the Saurashtra region. The estimated region-specific coda Q and the geometrical spreading factor may help in the modeling of strong ground motions and assessment of the seismic hazard for the Saurashtra region.

Source parameters and scaling relations for small earthquakes in the Saurashtra Horst of Western Deccan Volcanic Province, India and its seismotectonic implications

Source parameters and scaling relations for small earthquakes in the Saurashtra Horst of Western Deccan Volcanic Province, India and its seismotectonic implications

Source parameters and scaling relations for small earthquakes in mainland Gujarat region of Western Deccan Volcanic Province, India

The return period of large earthquakes in intraplate regions is more in comparison to the interplate regions. Also, the intraplate region is less instrumented and hence, these regions are devoid of acceleration time histories of large and great earthquakes. The mainland Gujarat region is one of such intraplate regions for which acceleration time histories are not available, however, is vulnerable for earthquakes of M ∼ 6.0. Simulation is performed in these scenarios and the prerequisite is the knowledge of source parameters and scaling laws. We analyzed the spectra of the P- and S-waves recorded by the stations of Gujarat Seismic Network for the mainland Gujarat region of western Deccan Volcanic Province, India. Additionally, we developed scaling laws based on the obtained source parameters. We analyze 177 digital records from 43 local earthquakes during the period 2007–2016 with a magnitude of Ml = 2.5–4.3, which are located in a distance range of 10–250 km, using Brune’s seismic source model (Brune 1970, 1971). According to our study, the calculated stress drop values do not vary considerably with seismic moment values and are generally between 1 and 100 bar for most of the events. Therefore, the earthquakes in this region adhere to the law of self-similarity. As widely observed for conventional earthquakes, we find an inverse relationship between seismic moment and corner frequency for these events. Based on a linear regression analysis between the assessed seismic moment (M 0) and corner frequency (fc), we developed the scaling relation M 0 f c 3 = 2.29 × 1015. Scaling relations developed here are in agreement with those developed for the nearby Kachchh region and various other seismically active areas around the world. Use of regional scaling is recommended instead of global scaling laws for better estimation of seismic hazard. The source parameters and scaling laws presented here can be used for a variety of applications in mainland Gujarat, including the assessment of seismic hazards and the development of earthquake scenarios.

Unified stratigraphy of Western Deccan Volcanic Province: A GPB perspective

Unified stratigraphy of Western Deccan Volcanic Province: A GPB perspective

Estimation of Coda-Q for Mainland Gujarat region of Western Deccan Volcanic Province, India

Estimation of Coda-Q for Mainland Gujarat region of Western Deccan Volcanic Province, India

Multi-tiered, disrupted crust of a sheet lava flow from the Diveghat Formation of Deccan Traps: Implications on emplacement mechanisms

The crust is a vesicular layer that caps the compact core of sheet lava flows. We describe for the first time, a crust composed of multiple layers (each distinguished by a chilled glassy rind) from the Diveghat Formation in the western Deccan Volcanic Province. The multiple layers of crust developed over a single compact core of a single sheet lava flow, are shown to have been sequentially deformed in multiple phases. This is interpreted to have resulted from the endogenous emplacement of lava in successive pulses (rather than as a continuous stream) during the extrusion of the sheet lava flow. This model has several implications on the mechanism of emplacement of sheet flows in continental flood basalt provinces.

Rajgad GPB: A megaporphyritic flow field, Western Deccan Volcanic Province, India

We describe the distribution and characters of a megaporphyritic basalt flow field that was arguably the earliest described ‘giant phenocryst basalt’ (GPB) from the Deccan Volcanic Province (DVP). It is a marker horizon exposed in a >30,000 km2 area below the Mahabaleshwar Formation in the western DVP. Its presence, distribution and stratigraphic importance as a regional marker horizon are enumerated. Available geochronological and paleomagnetic data suggest that the stratigraphic position of the Rajgad GPB coincides with polarity reversal in Chron 29 recorded from the basaltic lava sequence of the Western DVP.

Textural Variation in the Crustal Xenoliths in Basic Dykes, Western Deccan Volcanic Province, Maharashtra, India:Evidence of Partial Melting And Pyrometamorphism

In flood basalt provinces that cover Precambrian terrains, crustal xenoliths are rare. But when present in lava flows and in intrusives, they provide direct information about the basement composition and upper lithospheric crust. Although dykes hosting xenoliths are not common in Deccan Volcanic Province, there are a few known occurrences of mantle and/or crustal xenoliths in some dykes from Maharashtra and Madhya Pradesh, India. The NE–SW dykes from Western Deccan Volcanic Province of upland Maharashtra host crustal xenoliths exhibiting textural variation. Petrographic studies of these xenoliths were not dealt with in detail earlier. We document here, a detailed petrographic work, signifying the importance of pyrometamorphic changes in xenoliths, to understand the composition and explain the textural variation. Presence of elongated quartz ribbons, neocrystallization, incorporation of basaltic melt into mineral cracks, complex twinning in feldspars, exsolved Fe-oxide and silica melt inclusions represent high temperature effects. Major oxide data is suggestive of peraluminous nature of the xenoliths while high temperature minerals tridymite and mullite observed from X-ray diffractograms indicate pyrometamorphic effects. The original rock is quite completely transformed by various processes like partial melting, incipient hybridization and pyrometamorphism, leaving very little mineralogical and textural traces that give evidence of the original protolith. These processes explain the textural variation observed in these upper crustal xenoliths, which could be termed as buchites or buchite rocks.

Weathering characteristics of interflow volcanic boles from Mandla lobe, Eastern Deccan volcanic province

The Deccan volcanism straddling the Cretaceous–Paleogene boundary resulted into one of the most extensive continental flood basalts in the Western and Central Peninsular India. The interflow sedimentary units (boles) often cited as paleosols are the rare archives of paleoenvironmental conditions existed during successive lava flows. A debate on the origin of boles as true paleosols exists. Therefore, in order to trace out regional extent of such conditions (pedogenic/non-pedogenic) in the context of their paleoenvironmental significance during the Deccan volcanism, we studied here mineral magnetic characteristics of 36 color differentiated bole beds from the Mandla lobe, in the eastern Deccan volcanic province (EDVP) classified into four types (gray, yellow, brown and red) based on their Munsell notations. The gray, yellow and brown boles show dominantly ferrimagnetic composition (chiefly magnetite and titanomagnetite), while red boles characterize mixed ferri- and antiferromagnetic (mainly hematite) mineralogy. The granulometric (domain) parameters suggest larger single domain (SD) and the multidomain (MD) ferrimagnets in the majority of cases, whereas superparamagnetic (SP) particles (with frequency dependent susceptibility χfd~2.6 to 20.3×10−8m3kg−1) are limited to the few red and brown boles only. The significant presence of primary parent magnetic minerals (i.e. coarse SD–MD titanomagnetite and magnetite) in the majority of boles depict incipient to moderate weathering owing to limited time intervals during successive eruptions. The combination of hematite and SP particles present only in the few red and brown boles depicts baking effect rather than pedogenesis. The comparison of magnetic properties of boles from EDVP and western Deccan volcanic province (WDVP) shows significant differences in the concentration of SP ferrimagnetic particles and hematite attributed to variable weathering intensity, duration and heating related alterations from the upper flows. Both EDVP and WDVP boles indicate similar first order paleoenvironmental conditions without any major pedogenesis that can be regionally correlated.

Lava Channel of Khedrai Dam, Northeast of Nasik in Western Deccan Volcanic Province: Detailed Morphology and Evidences of Channel Reactivation

Abstract The 120 m-wide Khedrai Dam lava channel is one of the widest recorded in the Deccan Volcanic Province. Like many recent ones, this channel also has a pair of linear marginal levees made of breccia and confines the channel-fill lava. It is resting over a pahoehoe flow (substrate) of basalt studded with plagioclase megacrysts, also known as the Giant Plagioclase Basalt (GPB). The channel-fill lava, which makes the lining at the edges and basal portion of the channel, is a comb-layered basalt (Cl-B) characterized by alternating layers rich in curved, branching augite crystals and laths of plagioclase – a texture indicative of high order of supercooling and rapid cooling rates. The dominant and centrally filling lava of this channel is a plagioclase-phyric basalt (Pp-B) that has a wide variation in phenocryst number from megacryst-poor to megacryst-rich. Occurrence of centimetre to decimetre size angular fragments of Cl-B within Pp-B fill indicates mechanical dislodgements of channel-lining by flowing lava. This also suggests that the channel was reactivated before it finally lost its hydraulic gradient and solidified.

´A´ā lava flows in the Deccan Volcanic Province, India, and their significance for the nature of continental flood basalt eruptions

Newly identified ´a´ā lava flows outcrop intermittently over an area of ~110 km2 in the western Deccan Volcanic Province (DVP), India. They occur in the upper Thakurvadi Formation in the region south of Sangamner. The flows, one of which is compound, are 15–25 m thick, and exhibit well-developed basal and flow-top breccias. The lavas have microcrystalline groundmasses and are porphyritic or glomerocrystic and contain phenocrysts of olivine, clinopyroxene or plagioclase feldspar. They are chemically similar to compound pāhoehoe flows at a similar stratigraphic horizon along the Western Ghats. Petrographic and geochemical differences between ´a´ā flows at widely spaced outcrops at the same stratigraphic horizon suggest that they are the product of several eruptions, potentially from different sources. Their presence in the DVP could suggest relative proximity to vents. This discovery is significant because ´a´ā lavas are generally scarce in large continental flood basalt provinces, which typically consist of numerous inflated compound pāhoehoe lobes and sheet lobes. Their scarcity is intriguing, and may relate to either their occurrence only in poorly preserved or exposed proximal areas or to the flat plateau-like topography of flood basalt provinces that may inhibit channelization and ´a´ā formation, or both. In this context, the ´a´ā flow fields described here are inferred to be the products of eruptions that produced unusually high-effusion-rate lavas compared to typical flood basalt eruptions. Whether these phases were transitional to lower intensity, sustained eruptions that fed extensive low effusion rate pāhoehoe flow fields remains unclear.

Geology and Geochemistry of the Sangamner Mafic Dike Swarm, Western Deccan Volcanic Province, India: Implications for Regional Stratigraphy

Abstract Numerous large, NE‐SW‐ to E‐W‐trending mafic dikes outcrop around Sangamner in the western Deccan Volcanic Province. This area is part of a broader region postulated to be a shieldlike feature and a major eruption center. A combination of field, geochemical, and isotopic (Sr and Nd) characteristics is used here to understand the relationship of this dike swarm with the associated lava flows and their position in the established Deccan stratigraphy. Many dikes are compositionally similar to the Khandala and Poladpur formations belonging to the Lonavala and Wai subgroups, respectively, while one dike is similar to the Ambenali Formation. One dike has a composition distinct from all other dikes in this area as well as from most stratigraphic units, although there are many similarities in composition with the Bushe Formation as well as the Boyhare Member of the Khandala Formation. While several dikes are geochemically similar to specific flows/members within certain formations, their isotopic composi...

Assessment of the groundwater potential and quality in Bhatsa and Kalu river basins of Thane district, western Deccan Volcanic Province of India

Scarcity of groundwater necessitates the present study to evaluate groundwater potential and quality for designing suitable water management plans in the Bhatsa and Kalu river basins in the Thane district in the western Deccan volcanic province of India. A Geographical Information System platform is used to integrate and spatially analyse multiparametric data comprised of satellite, topographical, geological and hydrogeological information to generate several thematic maps, including groundwater potential zonation map. This study reveals that 70% of the area has medium to low groundwater potential, while only 10% has high potential. The static and dynamic groundwater potentials are estimated to be 10.7 and 4.8% of the annual rainfall. The groundwater quality in terms of hardness, total dissolved solids, salinity and chloride is suitable for domestic and irrigational purposes. A database is developed for sustainable water management program for the region and areas where suitable water conservation techniques need to be adopted are identified.

A GIS based hydrogeomorphic approach for identification of site-specific artificial-recharge techniques in the Deccan Volcanic Province

The Deccan Volcanic Province (DVP) of India, as a whole, faces a severe shortage of water despite receiving a high annual rainfall, this is primarily due to excess runoff and lack of water conservation practices. In this study, an attempt is made to identify zones favourable for the application and adaptation of site-specific artificial-recharge techniques for augmentation of groundwater through a Geographical Information System (GIS) based hydrogeomorphic approach in the Bhatsa and Kalu river basins of Thane district, in western DVP. The criteria adopted for the GIS analysis were based on the hydrogeomorphological characteristics of both basins extracted from the IRS-1C LISS-III data supported by information on drainage pattern, DEM derived slope, lineament density, drainage density, and groundwater condition. The integrated study helps design a suitable groundwater management plan for a basaltic terrain.

Possible lava tube system in a hummocky lava flow at Daund, western Deccan Volcanic Province, India

A hummocky flow characterised by the presence of toes, lobes, tumuli and possible lava tube system is exposed near Daund, western Deccan Volcanic Province, India. The lava tube system is exposed as several exhumed outcrops and is composed of complex branching and discontinuous segments. The roof of the lava tube has collapsed but original lava tube walls and fragments of the tube roof are seen at numerous places along the tube. At some places the tube walls exhibit a single layer of lava lining, whereas, at other places it shows an additional layer characterised by smooth surface and polygonal cracks. The presence of a branching and meandering lava tube system in the Daund flow, which represents the terminal parts of Thakurwadi Formation, shows that the hummocky flow developed at a low local volumetric flow rate. This tube system developed in the thinner parts of the flow sequence; and tumuli developed in areas where the tube clogged temporarily in the sluggish flow.

Platinum-group elements in Deccan mafic rocks: a comparison of suites differentiated by Ir content

A suite of 18 Deccan mafic rocks including 14 basalts and four dolerite dykes from seven different localities in the Maharashtra region of the western Deccan volcanic province were analyzed for Ir, Pt, Pd and Au by radiochemical neutron activation analysis (Ir, Pd and Au) and lead fire assay (Pt, Pd and Au) with ICP–MS finish. To provide additional petrogenetic insight, major elements, a suite of trace elements including rare earth elements (REEs), and Sr and Nd isotopic compositions were also determined. Based on the distribution of Ir values, two subsets of samples were distinguished. One includes 14 samples with average Ir concentration of 0.024 ppb, and the other, a much smaller group of four samples, averages 0.21 ppb Ir or some 10 times the average concentration of the larger group. These two subsets are differentiated by other compositional properties in addition to noble metal contents. These include lower REE, higher transition element (Ni, Cr) and S contents, large negative Nb–Ta anomalies in mantle normalized profiles, more radiogenic Sr and very negative ε Nd isotopic compositions in the Ir-rich group. In addition, Pd, Pt and Au averages are lower in the Ir-rich group. These geochemical features are interpreted to reflect processes of magma generation in a mantle environment and magma contamination in a crustal or lithospheric mantle setting. The critical mantle variable is percentage partial melting of source peridotite. Relatively higher percentage partial melting is required to generate the Ir-rich magmas. Based mainly on Sr and Nd isotopic evidence and Nb–Ta abundances, the Ir-rich group is considered more strongly modified by crustal and/or lithospheric mantle assimilation than the main group of melts. Fractional crystallization appears to have increased Pd, Au and Cu contents of the main group of magmas relative to the Ir-rich magmas, and is probably a significant factor distinguishing these Deccan rocks as having higher concentrations of these elements than other continental flood basalts (CFBs).

Seismological constraints on the structure and composition of western Deccan Volcanic Province from converted phases

Receiver function analysis using teleseismic data from one broadband and nine short period stations, was carried out in a 50 km x 100 km area, near Mumbai (Bombay), encompassing the coastal plains of the rifted margin of the western Deccan Volcanic Province (DVP) of India. This region is characterized by a thick (36–41 km) shield like crust which is at variance with earlier inferences in this part of DVP. The crustal composition as inferred from the Poisson's ratio of 0.26 ± 0.01 is felsic to intermediate in nature. The continental crust exhibits structural controls similar to the offshore shelf region but does not exhibit any typical seismic signatures of magmatic underplating nor appears to be perturbed by the Deccan volcanism.

Slabby pahoehoe from the western Deccan Volcanic Province: evidence for incipient pahoehoe–aa transitions

The pahoehoe–aa transition for a flow exposed near Bodshil village from the western part of the Deccan Volcanic Province (DVP) is reported for the first time. The ∼1-km-long Bodshil flow issued as a small sheet from a pre-existing lobe. Near the source, the crust is characterised by numerous squeeze-ups. A number of gaping fractures, parallel to sub-parallel to the flow direction, are exposed on the surface in the medial portion of the flow. About 800 m away, the flow completely transforms to slabby pahoehoe. The terminal portion of the flow is characterised by concentrations of slabs, blocks and lava balls. The size and concentrations of the slabs and lava balls appear to increase along the length of the flow. Petrographic studies reveal a dominant hypohyaline texture. The flow core is coarse and is characterised by plagioclase set in a glassy matrix. The presence of clinopyroxene in addition to plagioclase and glass distinguishes the crust and interslab crust from the core. On the basis of mineralogy, a temperature range of 1146±15°C to 1169±15°C is inferred for the Bodshil flow. Increased vesicle deformation across the transition is discernible and an average D-value of <0.4 indicates moderate strain rates during emplacement. In light of the morphology and petrography, the cooling history and the mode of emplacement of the Bodshil flow is discussed. The flow originated as a small toe at the leading edge of a pahoehoe flow, and grew into a sheet by the mechanism of inflation. Continuous inflation caused the brittle crust to uplift and produce a network of inflation clefts that were subsequently occupied by squeeze-ups. Temporary stagnation of the flow due to cessation of lava supply or storage allowed the crust to grow and thicken. Renewed movement of the stored and cooled lava to the flow front at a fairly high volumetric rate was responsible for the initial disruption of the crust. High rates of crustal disruption induced higher rates of degassing and cooling, which resulted in rapid crystallisation of the fluid core. Increase in crystallinity lead to the onset of yield strength, and it is envisaged that at least the terminal parts of the flow behaved as a Bingham fluid. The Bodshil flow is unique to the DVP because it is the first to record slabby pahoehoe and provide evidence for the incipient transformation of basaltic lava from pahoehoe to aa.

Tumuli and associated features from the western Deccan Volcanic Province, India

Whale-back-shaped uplifts called "tumuli" are common in the pahoehoe flows of the western Deccan Volcanic Province (DVP). Although they usually occur in hummocky flows, they are also associated with thicker sheet lobes. They have been subjected to a detailed morphometric and petrographic study for the first time. The tumuli are characterised by positive relief and "lava-inflation clefts" occupied by squeeze-ups. They display elongate as well as equant forms; some are constituted of a single flow lobe, whereas others display multiple flow lobes. Some tumuli appear to have developed along anastomosing tube systems. The detailed study of one of the tumuli reveals considerable petrographic and textural variations among the constituent flow units. Some of these, such as the enrichment of phenocrysts in squeeze-ups and breakouts, could be related to the emplacement dynamics of the tumulus. All the observed tumuli display much evidence of inflation or endogenous growth. Field observations and measurements reveal that the tumuli and associated pahoehoe features display a close similarity with their Hawaiian counterparts. This is a very significant observation since it points out to a similarity in nature and style of eruptions in Hawaii and at least in the western part of the DVP. This has an important bearing on determining the short, medium and long-term effusion rates in the Deccan; however, any concrete inference will have to await systematic volcanological studies of the lava features in the DVP.

Geochemical and petrological relations in some deccan basalts, Western Maharashtra, India

Abstract Petrographic and geochemical data are given for some basaltic rocks from the Koynaghat, Ambaghat and Panvel sections of the western Deccan volcanic province. This study confirms geochemical features established earlier for the Deccan basalts but brings out minor additional characters. Mineralogical and major-element compositions of the basaltic flows from the Koyna and Panvel sections indicate tholeiitic affinity; the Ambaghat flows exhibit a slight affinity towards alkali basalt. Rare earth element (REE) distribution patterns and trace-element abundances suggest minor fractionation of olivine and plagioclase during the evolution of the flows. The general similarity of chemical and mineralogical features over a wide area and the lack of conspicuous inter-element relationships suggest that the flows reflect the combined effects of partial melting, minor mineral fractionation and selective crustal contamination.