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1.
The results of remanent magnetic studies on eight of the nine Deccan Trap flows in the vicinity of Sagar (23°56′ N: 78°38′ E) are presented. It is found that the lower four flows in the sequence are of ‘reversed’ magnetic polarity. Of the upper four flows, the top and the bottom ones show ‘intermediate’ directions while the two flows sandwiched between these are ‘normal’. These results suggest a transitional stage between the polarity inversion of the geomagnetic field from ‘reversed’ to ‘normal’ during the eruption of these Deccan Trap flows. The remanent magnetic directions of these ‘reversed’ and ‘normal’ flows show fairly shallow inclinations and are comparable to the remanent magnetic directions of the Pavagarh basalts.  相似文献   

2.
The subsurface information gathered during exploration for oil and gas in the Cambay basin shows it as a deep graben with 5 km or more of Tertiary and Quaternary sediments resting on the Deccan Trap floor. The Trap floor of this graben extends from Lat. 24° N to about Lat. 19° N and possibly further south. The basin is divisible into separate morphotectonic blocks as a result of block differentiation in the Trap basement, reflected in the structural attitudes of the overlying sediments. This differentiation is believed to have originated in the Paleocene. The dominant structural grain of the area to south of the Narmada river is ENE-WSW with block faulting in the Traps along the older Satpura trend. North of the Narmada river, the trend is longitudinal upto the Meshwa river while further north the trends veer to a NNW-SSE alignment. These latter trends, in the greater part of the Cambay basin, were impressed early during its subsidence and are the result of reactivation along the old Dharwarian trends in post-Delhi times. Maximum thickness of the Traps penetrated so far is near Mechsana and Cambay where more than 1000 meters thickness has been drilled through. The drilling and gravity-magnetic evidence shows the thickness of Deccan Traps in this trough to be of the order of 2.5 km and points to the possibility of active subsidence of Cambay basin, concomitant with the outpouring of the basaltic lavas. The age of the Traps in the Cambay basin, as evidenced by the available data, is Upper Crealaceous. The influence of the structural grain of the basaltic floor on the overlying sedimentary sequence is evidenced during all the stages in the evolution of the Cambay Tertiary basin. Conglomerates, wackes and reddish brown clays of exclusive Trap derivation predominate in the sedimentary section in the initial stage of the basin evolution during Paleocene. General absence of well developed terrigenous reservoirs on a regional scale in the Paleogene section is due to predominance of Trap terrain as the provenance of clastic detritus, contributing essentially argillaceous matter.  相似文献   

3.
Recent investigations on the Deccan Traps reveal many new findings of alkaline rocks more commonly occurring as minor intrusions than as lava flows. In comparison to the vast extent of the Deccan Traps, the alkaline rocks are negligible in their volume and are confined to tectonic belts in parts of Western India. The rocks exhibit no systematic variation in their petrographical and chemical characters thereby suggesting that they were not derived from a primary alkali olivine-basalt magma. The possibility of derivation of alkaline magma locally along the rift zones is proposed. Some of the alkaline rocks are shown to have been formed due to the effective role of volatiles in bringing dissociation of feldspar in certain cases, and alkali metasomatism in others. The syntexis of the pre-Deccan Trap carbonate rocks along the Narmada rift zone is also responsible for some occurrences.  相似文献   

4.
In the attempt to study the buried Deccan Trap layers in the Cambay Basin, the ground magnetic surveys have not been very useful as the data combine the effect due to the crystalline basement and the Trap thickness. In some parts of the basin, some reflections in the seismograms obtained in the course of seismic surveys, could be correlated to the Trap surface. These can be tied with wells drilled in the basin upto the Traps. The synthesis of the gravity and seismic data has enabled us to prepare a map of the Trap surface in the Cambay basin. The depth of the Trap surlace increases from about 2000 m in the northern part of the basin to about 600 m in its deepest part near Broach. The Trap surface rises gradually south of Narbada in an average direction of SE with depths running from 2500 m to 500 m. The interpretation of the gravity anomalies, assuming their cause to be the variations in the thickness of the Trap, has enabled the determination of the average thickness of the Traps in the basin. The maximum thickness of the Trap is in the central part of the basin and is estimated to be about 2.4 km. The Traps appear to gradually taper towards the flanks of the basin.  相似文献   

5.
A greenhouse warming caused by increased emissions of carbon dioxide from the Deccan Traps volcanism has been suggested as the cause of the terminal Cretaceous extinctions on land and in the sea. We estimate total eruptive and noneruptive CO2 output by the Deccan eruptions (from 6 to 20 x 10(16) moles) over a period of several hundred thousand years based on best estimates of the CO2 weight fraction of the original basalts and basaltic melts, the fraction of CO2 degassed, and the volume of the Deccan Traps eruptions. Results of a model designed to estimate the effects of increased CO2 on climate and ocean chemistry suggest that increases in atmospheric pCO2 due to Deccan Traps CO2 emissions would have been less than 75 ppm, leading to a predicted global warming of less than 1 degree C over several hundred thousand years. We conclude that the direct climate effects of CO2 emissions from the Deccan eruptions would have been too weak to be an important factor in the end-Cretaceous mass extinctions.  相似文献   

6.
Deccan Traps are the most extensive geological formations of Deccan Peninsula with the exception of only the metamorphic and igneous complex of Archaean age. Based on their mode of emplacement, geomorphic setting and hydrogeological behaviour over an area of about 5,000 sq. km the authors have classified the Deccan Traps of western Maharashtra into 3 groups, namely, (1) The Deccan Traps of Dhulia district, characterised by numerous dolerite dykes, (2) Areally extensive trap flows of Sholapur and Osmanabad districts resulting from slow and quiescent type of flood eruption occupyng the gently undulating terrain, and (3) the traps of Kolaba, Thana and Bombay-Poona regions characterised by intertrappean sediments, dolerite dykes and volcanic ash beds, indicative of violent outbursts resulting in the Sahyadri geomorphologic unit. The groundwater possibilities in the three groups are to a great extent governed by the nature and constitution of the individual flows. The massive traps with their fracture porosities, the vesicular traps with their minutely interconnected and partly filled vesicles and the intertrappen sediments with their primary porosities play a decisive role in determining the groundwater possibilities in them. In Dhulia district the dolerite dykes to a great extent control the movement of groundwater, and success or otherwise of the well field area depends very much upon its location with reference to adjacent dykes. Areally extensive thick vesicular traps with their gentle dips towards east, in Sholapur district, have to be explored for possible artesian conditions in the downdip directions of the trappean units to be tapped. In the case of Poona, Thana and Kolaba districts, exploratory drilling based on geophysical data (to delineate the nature and extent of water bearing horizons) has to be resorted to. It is, therefore, imperative to sub-divide at this stage Taylor’s Single Unit of Deccan Trap Groundwater Province into 3 Sub-Provinces, based on geomorphological, geological and geohydrological setting in the region of western Maharashtra of the present investigation.  相似文献   

7.
2-D shallow velocity structure is derived by travel-time inversion of the first arrival seismic refraction and wide-angle reflection data along the E–W trending Narayanpur–Nandurbar and N–S Kothar–Sakri profiles, located in the Narmada–Tapti region of the Deccan syneclise. Deccan volcanic (Trap) rocks are exposed along the two profiles. Inversion of seismic data reveals two layered velocity structures above the basement along the two profiles. The first layer with a P-wave velocity of 5.15–5.25 km s?1 and thickness varying from 0.7–1.5 km represents the Deccan Trap formation along the Narayanpur–Nandurbar profile. The Trap layer velocity ranges from 4.5 to 5.20 km s?1 and the thickness varies from 0.95 to 1.5 km along the Kothar–Sakri profile. The second layer represents the low velocity Mesozoic sediments with a P-wave velocity of 3.5 km s?1 and thickness ranging from about 0.70 to 1.6 km and 0.55 to 1.1 km along the E–W and N–S profiles, respectively. Presence of a low-velocity zone (LVZ) below the volcanic rocks in the study area is inferred from the travel-time ‘skip’ and amplitude decay of the first arrival refraction data together with the prominent wide-angle reflection phase immediately after the first arrivals from the Deccan Trap formation. The basement with a P-wave velocity of 5.8–6.05 km s?1 lies at a depth ranging from 1.5 to 2.45 km along the profiles. The velocity models of the profiles are similar to each other at the intersection point. The results indicate the existence of a Mesozoic basin in the Narmada–Tapti region of the Deccan syneclise.  相似文献   

8.
The acid rocks of the Deccan Traps including microgranites, felsites, rhyolites and related rocks are contined to tectonically weak zones in Western India. The significance of their distribution is discussed and the opinions expressed on the genesis of the rocks are critically reviewed. In their formation, fractional crystallization of tholeiitie basalt magma, which is supposed to be the parental one, was aided in certain localities by the melting of the sialic crust and assimilation or partial melting of the pre-Deccan Trap rocks. Evidence for the latter is found in the gradational contacts of the acid rocks with the earlier rocks, their association with cruptive centres or faulted zones and absence of any definite trend of variation in petrographical and chemical characters. Some of the rhyolitic rocks are also formed due to hydrothermal alteration of sedimentaries along the Narmada Valley.  相似文献   

9.
Summary 38 oriented samples of Deccan Traps have been collected from the neighbourhood of Chincholi, Mysore State, India. The Natural Remanent Magnetisation of these rocks has been studied using an astatic magnetometer. It has been found that these rocks are magnetically reversed, the mean magnetic direction being N154°E in declination and 61° down in inclination. Thermoremanance studies conducted on four specimens showed that two specimens with weak NRM and a high secondary magnetisation have Curie temperatures around 560°C for the NRM and exhibited partial reversal of TRM at room temperature, while two specimens with high NRM and with little secondary magnetisation have Curie temperatures much lower than 560°C for the NRM.  相似文献   

10.
Recent studies of the Sylhet Traps (? Jurassic) and the overlying Cretaceous-Tertiary sedimentary cover in the southern part of the Khasi Hills, Shillong Plateau in Assam have led to a reconstruction of the tectonic history of the area since Jurassic times; a clear picture regarding the nature of volcanism has also emerged. The history begins with effusion of tholeiitic basalts, apparently through E-W fissures developed in the peneplaned crystalline basement. One of these fractures became a fault (the Raibah fault) along which the northern non-volcanic block moved up relative to the southern block experiencing volcanism. The fault was active during and after the volcanism till Upper Cretaceous times. The sequence of eruption was as follows: (1) tholeiitic basalts, (2) minor alkali basalts (nepheline tephrite), (3) tholeiitic basalts, (4) localised explosive effusion of minor rhyolites and acid tuffs, and (5) tholeiitic basalts. Neither feeder dykes nor volcanic vents have been noted in the Sylhet Traps. There are no agglomerates among the basic flows; the fragmental rocks are actually flow breccias. The formation of the various structures such as flow breccias, layering and flow folds in many of the basalt flows are thought to have been controlled by the angle of slope and the rate of flow. Thus, the Sylhet Trap flood basalts are characterised by quiet effusion through linear fissures. The effusion was followed by a dyke phase, intruding also along E-W fractures, expecially in the monoclinally bent southern portion; the subsequent tectonic history of the area is also characterised by relative uplift and downsinking of different basement blocks. It is concluded that in the Shillong Plateau uparching of the basement led to fracturing, effusion of basalts apparently along some zones of fissuring along which differential vertical movement of basement blocks was taking place. In the light of the foregoing conclusions, available data on the tectonics of the Rajmahal and the Deccan Traps are examined; both these flood basalt provinces have suffered broadly similar tectonic histories as the Sylhet Traps. The various features of flood basalts, viz., large extent, huge thickness, subaerial nature, a post-volcanic dyke phase are interpreted as a consequence of fusion of the Upper Mantle, development of tensional fractures eruptions apparently along fractures between adjoining basement blocks undergoing differential uplift.  相似文献   

11.
Many of the world's flood basalt provinces form elevated plateaux at the margins of continents, although in most cases their present large elevation is not the result of mountain building processes. Several explanations have recently been put forward to explain such occurrences of epeirogeny. The Deccan Trap basalt province forms one such elevated plateau, and results are presented here showing how the epeirogenic uplift in this region, combined with crustal subsidence probably associated with the rifting of the Indian continental margin, has affected the structure of the basalt sequence. Trace element analytical data are used for samples from numerous vertical sections through the Deccan Traps lava series along and around the Western Ghats ridge in India. The results reinforce the previously defined stratigraphy of the Mahabaleshwar area, and extend it over a region covering some 36 000 km2, reaching as far south as Belgaum and the Trap/basement contact. These results show that the lava pile is not flat lying, but forms a very low amplitude anticlinal fold structure plunging southwards by up to 0.3 ° over most of the area, although in the south there is evidence of a reversal of this plunge. The fold is interpreted as being the result of two tilting processes: (1) westward tilting near the coast, due to the foundering of the passive continental margin, and (2) epeirogenic uplift along the whole west coast of India producing the observed topography and the peninsula-wide drainage patterns, and also the easterly component of dip. Variations in the magnitude of the latter effect along the western continental margin may also be important in generating the plunge of the fold, although the possibility of some component of depositional dip may also be important. This latter possibility can be modelled using a simple computer program. The results of this modelling show that a migrating linear volcanic edifice fits the observations best.  相似文献   

12.
The Barda igncous complex of western Saurashtra (Gujarat) consists of a group of volcanic and subvolcanic igneous rocks, viz., pitchstone, rhyolite, felsite, granophyre and dioritic rocks. They are generally grouped under the Deccan Trap series and have been emplaced through the Deccan Trap basalt flows. Using different varieties of clinopyroxenes as characteristic phases several units of felsites and granophyres have been distinguished in the southeastern part of the Barda hills. The augites of the adjacent Deccan Trap basalt flows range between Ca33Mg61Fe6 and Ca41Mg44Fe15. (All compositions of minerals are derived from optical data). The clinopyroxene of a monzodiorite is Ca41Mg42.5Fe16.5. In the felsites and granophyres the clinopyroxenes range from salite (Ca46Mg31Fe23) through ferrosalite (Ca46Mg20Fe34) and ferroaugite (Ca43Mg17Fe46) to hedenbergite (Ca46Mg9Fe45). Beyond that a peralkaline trend is indicated by common occurrence of aegirine-augite granophyre in which the clinopyroxene is alkalic with 20 percent of NaFe3 Si2O6 (acmite) molecule. Several bodies of acmite-riebeckitebearing granophyres have been found; they also indicate a high degree of oxidation. A fayalitic olivine-bearing granophyre is of interest as it shows the strongest iron enrichment with 13 weight percent FeO + Fe2O3; its mineralogy indicates a very low oxygen fugacity of 1013 atmospheres during crystallization.  相似文献   

13.
A study of the geology of the Dhar Forest, the Pachmarhi plateau and the area around Bari in Central India has led to the conclusion that the pre-Deccan Trap topography which was completely covered by the lava flows and is being exposed with spectacular clarity by the process of exhumation, had much the same relief as the present land surface. The geomorphological studies of the Vindhyan and Pachmarhi plateau suggest a characteristic rise and increasing separation of different planation surfaces towards the edges of the Narmada rift valley and indicate upwarping movements in several distinct stages. In considering the possible causes of the upwarping movements special significance is attached to epeirogenic movements probably representing the various stages of Himalayan orogeny. The succession of events in Peninsular India suggests that these upwarping movements were caused by rising magma which led to the fissure eruptions of the Deccan Trap lavas; which presumably took place during a period of tension in the upwarped area. The problem of the origin of the Narmada rift structure is discussed and evidence is adduced to show that the final sinking of the crest of the upwarped area has caused the out-pouring of the Deccan Trap lavas. The individual lava flow with their typical field and microscopic characteristics maintaining their interflow differences have been traced over long distances. These studies have led to the correlation of the flows between the measured sections. Further, as regards the cause of the higher elevation of the base of the basal flow in Katangi (1950), the possibility of a post-Deccan Trap upwarping movement is briefly considered.  相似文献   

14.
The paleomagnetism of 22 flows which range in composition from olivine basalts to rhyolite and in elevation from 450 ft. to 2680 ft., from Mount Pavagarh, situated in Gujrat, India, has been studied. The igneous activity represented by these flows belongs to the Deccan Trap Plateau Basalt Series. Sixteen flows ranging in elevation from 900 to 2680 ft. showed normal magnetization with upward inclination, consistent with the position of India in the southern hemisphere during the period of their eruption. Four flows from elevation 450 ft. to 730 ft. showed intermediate as well as discordant directions. The paleomagnetic results obtained from these flows have been correlated with those of Deccan traps from other areas. The amount of paleosecular variation represented by these flows has been estimated and compared with that from similar vertical sequences of traps studied from other areas. It has been found that the magnitude of paleosecular variation varied during the period of Deccan trap activity. An estimate of continental drift has been made from mean pole position of the several vertical sequences of traps. This indicates that a drift of the Indian landmass of the order of 24° took place during the entire period represented by the Deccan trap activity. The results of paleomagnetic studies of other sedimentary and igneous formations of Cretaceous age are reviewed in the light of recent results on Deccan traps.  相似文献   

15.
While mapping parts of the Shahdol district, the author for the first time noticed a dolerite dyke at Tikwa and a basalt sill at Chirha intrusive into the Gondwanas (Supra-Barakars). Though the dolerite dyke shows no noteworthy features in the field, the sill encloses xenoliths of Supra-Barakar sandstone in its basal portion. The present paper gives an account of the mode of occurrence, petrography and age relationship of these intrusions. The presence of abundant xenocrysts of quartz in the basalt sill leads the author to term it a quartz-xenocryst-basalt. The occurrence of Deccan Trap flows in the close proximity of the intrusions suggest that they may be related to the period of Deccan Trap activity.  相似文献   

16.
The Deccan Trap basalts have long been considered to be products of fissure eruptions and the dykes intruding them have been supposed to represent the fissures of eruption. However, the question of the mode of eruption of Deccan Trap lavas seems to need more careful consideration in view of the features brought to light by detailed field work in Western Maharashtra. Detailed studies of dykes suggest that majority of the dykes could not have acted as feeders as previously supposed. When examined in detail the basalt flows have more often been found to have only a limited lateral extent and are not always quite horizontal. Thin irregular flows with ropy surfaces, dipping in different directions and piled up into a chaotic mass are frequently met with, indicating eruption from local vents of the central type. Volcanic vents are found at a number of widely separated localities. All this suggests that many lavas are products of central type of volcanicity. However, central type of volcanoes would be inadequate to account for the vast amounts of lavas, and as the known dykes are not likely to have acted as feeders, and dykes still remain to be reported from large portions of the Deccan Trap area, the question of how the lavas came to the surface largely remains unanswered. Extensive beds of volcanic breccia traceable over a few miles and upto 50 feet thick are met with. As such extensive beds are more likely to be associated with fissure cruptions the question arises whether these fissure eruptions were accompanied by considerable explosive activity.  相似文献   

17.
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18.
Volcanic rocks occupy considerable regions in the western portion of India, attaining a maximum thickness of 7000′ near Igatpuri. These rocks are essentially basaltic in nature and are generally referred to as plateau basalts. An attempt has been made in this paper to present some results of geological and geophysical investigations carried out in the Deccan Traps. Three areas (Ajanta - Long. 75″41′ -75° 45′ E, Lat. 20° 32′ - 20° 35′ 15″ N, 18 sq. miles in area; Ellora -Long. 75″ 11′ - 75° 16′ E, Lat. 20° 1′ - 20° 9′ N, 80 sq. miles in area; and Chincholi - Long. 77° 22′ - 77° 30′ E, Lat. 17° 22′ -17° 30′ N, 50 sq. miles in area) have been chosen for this study because of their geological setting. A large number of field specimens have been collected for petrographic study. This is supplemented by examination of microsections and chemical analyses of a few traps. In the Chincholi area where the trap overlies the granites, limestones seem to intervene in between trap and granites. With a view to estimate the possible thickness of the limestone beds, the distribution of intensity of magnetic field in a portion of the area has been studied with a magnetometer. Magnetic susceptibilities in case of few specimens have also been studied. Elastic constants of Deccan Traps have been determined for fifty specimens, employing the Wedge Method. These are further correlated with textural features and porosity values. Such an integrated geological and geophysical investigation on Deccan Traps is bound to reveal some interesting results.  相似文献   

19.
Geomagnetic induction anomalies identified in India during the last fifteen years are discussed with emphasis on geology. The effects on both short period (SSCs and bays) and long period (Sq and Dst) geomagnetic variations are summarized. The induction effects observed in the southern tip of peninsular India are highly complex due to the coastal effect, to crustal and upper mantle conductivity anomalies between India and Sri Lanka and to the daytime equatorial electrojet. Further complications arise from the existence of a conductive step structure along the coastline at the Moho boundary and a graben in the Palk Strait.The reversed coast effect in the Z-variation identified at Alibag situated on the Deccan Traps is accounted for by the remanent magnetization of the Deccan lavas, their thickness and the underlying conductive structures.Induced currents have been found to be channelled through conductive structures beneath the Aravallis along its strike following a path transverse to the Himalaya. An asthenospheric upwelling beneath the Aravallis has also been noted. Conductive step structures have also been invoked on the southern and the northern flanks of the Pamir-Himalaya with east-west current channelling at the Moho boundary.  相似文献   

20.
The palaeomagnetism of a vertical sequence of Deccan traps ranging in elevation from 1750 to 2900 ft. along the eastern margins of the Deccan trap exposures near Jabalpur, has been studied. Eleven different flows were sampled at approximately 100 ft. elevation interval. Nine flows at elevations between 2150 and 2900 ft. showed normal polarization with up dip. The mean direction for these siving unit weight to each flow was,D=343°E andI=28°(up). Two flows at elevations between 1740 and 2000 ft. showed intermediate directions with down dip. It appears that the latter corresponds to a zone of actual field reversal which has been encountered at several locations on the territory of the Deccan traps. The mean directions of the nine upper flows give a reliable estimate of the palaeosecular variation of the geomagnetic field during the period of normal polarity. The mean directions obtained from vertical sequences of traps studied from different localities are compared for estimating the amount of continental drift that might have taken place during the period of main Deccan trap activity. Correlation of magnetic directions of flows from various localities indicates that the geomagnetic field reversal at about 2000 ft. elevation can be traced over a large territory of the traps.  相似文献   

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