Palaeomagnetic investigations on the Deccan Traps from Chincholi,Mysore State,India

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.     

Palaeogeography,geomorphological setting and groundwater possibilities in the Deccan Traps of Western Maharashtra

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.     

Deccan Trap and the geologic framework of the Cambay basin

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.     

Deccan Trap volcanism

The Deccan Traps, now occupying an area of 200,000 sq. miles, must originally have been more wide-spread. Their thickness in Western India reaches 6000 ft. They have been encountered at depths of over 1500 ft. in Kathiawar and Sind (Pakistan), and have been faulted down to a depth of over 6000 ft. in the Cambay area. They are composed of numerous flows whose thickness varies from a few ft. to 200 ft. The flows are often compact in the lower portions and vesicular in the upper portions. Over most of the area (east of the Western Ghats) the rock is a tholeiitic basalt (50 to 51.5 % silica) whose pyroxene is intermediate in composition between pigeonite and diopside, and whose CIPW norm generally shows the presence of some quartz. In the Bombay Kathiawar region there are numerous eruptive Centres associated with a large range of differentiated types comprising both very basic and acid types. The study of the analyses of the various types indicates the presence of both the alkali-olivine basalt as well as the Calc-alkali lines of differentiation, which has brought up the question of the nature of the primary magma. It is noted that while the greater part of the area shows tholeiitic rock, olivine basalts and connected types appear in the more western areas, perhaps as a result of the local tectonic conditions. Recent geophysical data point to the presence of an « oceanic basalt » layer all around the earth both in oceanic and continental crust, while a less dense (presumably tholeiitic) layer overlies it (below the sial) in the continental segments. The « oceanic basalt » should therefore be taken as representing the primary magma, and tholeiite as a major type derived from it in the continental crust.     

Carbon dioxide emissions from Deccan volcanism and a K/T boundary greenhouse effect

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.     

Mantle-derived spinel lherzolite xenoliths from the deccan volcanic province (India): Implications for the thermal structure of the lithosphere underlying the deccan traps

Petrography, mineral and host-rock chemistry of the mantle-derived spinel lherzolite xenoliths in the nephelinites of Kutch, a late variant of the Deccan Traps province, India, are reported. The xenoliths register equilibration temperatures between 920° ± 70°C and 1060° ± 50°C and an ascent time between 19 and 168 days. Representing a palaeogeotherm of minimum heat flow between 60 and 70 mW/m², the xenoliths possibly came from the oceanic lithosphere under the Deccan field, following the Cretaceous-Eocene fragmentation of Gondwanaland.     

Palaeomagnetism and the Deccan Trap volcanism

The results of palacomagnetic studies made on the Deccan Traps by various workers are reviewed in the light of the recent palaeomagnetic data on these rocks and the general geological information. It is suggested that: (a) the earlier altitude-polarity classification of the Deccan Traps, suggesting that the flows below the general elevation of 2000±200 feet above mean sea level are of reversed magnetic polarity while those above this horizon are normal, is not without exceptions; (b) the geomagnetic field reversed its polarity several times during the eruption of these lavas; (c) the Deccan Trap eruptions probably consisted of several phases of volcanicity over a protracted period; and (d) the phases of Deccan Trap volcanism, the phases of Himalayan upheaval, and the northward drift of the Indian landmass were rather concrescent events.     

Application of low-field hysteresis and memory-phenomenon studies in determining the stability of magnetization of Deccan Trap basalts from Mount Girnar and Mount Pavagarh regions,India

Two techniques, namely alternating-field demagnetization and thermal demagnetization, are widely being used for determining the stability of magnetization of a rock specimen. Recently a faster and simpler technique known as low-field hysteresis loop and memory-phenomenon test has been developed for determining the stability of magnetization directions of igneous rocks. In this paper the results of this new technique, after applying it to about 1000 specimens obtained from 250 oriented rock samples collected from 42 sites of Deccan Trap basalts from Mount Girnar (21°30′N; 70°30′E) and Mount Pavagarh (22°30′N; 73°30′E), India, are presented.The agreement between the mean natural remanent magnetization directions determined by this procedure and those computed after alternating-field demagnetization has been found to be very good. All the specimens from Mount Girnar and 94% of Mount Pavagarh specimens showed a stable line without any memory in a field ≈ 10 Oe. This indicates that the rocks from these two localities are highly stable and are most suitable ones for the determination of a precise palaeomagnetic direction.     

Dykes around Dadiapada,broach district,Gujarat

The basaltic lava flows of the Deccan Traps in the Dadiapada area are traversed by abundant dykes trending mostly in ENE to WSW direction. The density of the dykes distribution is 2 to 3 per mile. They vary in size from a few feet to 300 feet in width and some can be traced for many miles. Some of the dolerites contain quartz in considerable amounts and it is significant to find micropegmatite texture in them indicating their tholetitic nature.     

Petrology of the picrite-basalt flows in the Igatpuri area,Nasik district,Maharashtra

In the course of studying the Deccan Trap Hows around Igatpuri (latitudes 19°38′ and 19°45′: longitudes 73°30′ and 73°42′), picrite-basalts, not hitherto reported from this area, have been found occurring associated with basaltic flows. Thirty-eight flows, of a total thickness of 2200 feet, have been delimited. Of these, 8 flows may be termed picrite-basalts with a thickness varying from 25 to 75 feet. A feature of these basic flows is the abundance of olivine phenocrysts, with a complete absence of pyroxene phenocrysts in two of the flows. Two flows may be termed oceanite, two ankaramite, while four flows have phenocrysts of olivine, pyroxene and lelspar of An 65–70 %. In the oceanite flows the olivine phenocrysts constitute 20 to 30 per cent of the rock. They are mostly fresh, but some are altered to iddingsite. As regards the basaltic flows, about half have scanty olivine phenocrysts, the remainder being devoid of olivine. Eight are coarse enough to be termed olivine-dolerites. In the picrite-basalts the pyroxene phenocrysts have an optic axial angle of from 55° to 60°. In the basaltic flows the angle varies from 45° to 52°. The olivine phenocrysts of the picrite-basalts are highly magnesian, whereas those of the basalts are more ferruginous, as determined by optical methods. West’s view that the origin of these picrite-basalts is due to differentiation by crystal settling followed by freezing and extrusion, seems to be supported by this study.     

Geochemistry of Los Humeros Caldera,Puebla, Mexico

Geochemistry of Pliocene to recent volcanic rocks from Los Humeros caldera (19°30′ N – 19°50′ N and 97°15° W – 97°35′ W) in East-Central mexico is described. The volcanic rocks from this area seem to represent both alkali and high-alumina basalt series, or both calcalkaline and high-K calc-alkaline sequences. The available bulk-chemical analyses (23 this study and 18 from unpublished literature) show that the entire sequence of rocks from basalts to rhyolites are present in this area. Different degrees of partial melting of the source region followed by extensive shallow-level crystal differentiation seem to have taken place before most volcanic eruptions. These processes are perhaps the most important mechanisms for magma genesis in Los Humeros caldera. Geophysical studies in this area are not sufficient and more detailed geophysical surveys and a better geological interpretation are needed in order to delimit the underlying magma chamber.     

Petrology of the dyke rocks of the western portions of Rajpipla hills,Broach district,Gujarat, India

The paper embodies the field, petrographic and petrochemical studies of the dykes occuring within the Deccan basalts, in the Western portions of Rajpipla hills. Major and minor dykes with different trends occur in the area varying in thickness from 2′ to 75′ and traceable lengthwise from few to several miles. The density of the dyke distribution is two per mile. The composition of the minor dykes ranges from teschenite to trachyte with dominant basaltic types and they seem to be coeval with the flows of the area. The major dolerite dykes are found to be post-lava. Both alkali-olivine basalt and tholeiitic types occur. The former phase preceeds the latter and includes the minor alkaline dykes. A differentiation trend based on new chemical analyses is proposed.     

GEOELECTRICAL INVESTIGATIONS IN EAST AFGHANISTAN*

During the years from 1965 until 1967, a German/Afghan team carried out a number of geoelectrical resistivity measurements according to the “Four-Point-Method” within the framework of Technical Assistance to Afghanistan. On two selected objects of investigation, i.e. Kabul (69°10 E, 34°30′N) and Zarka?an (67°40′E, 32° 54.5′N), the possibilities and limits of the resistivity method in the exploration of useful groundwater resources, of the geological structure, and in the treatment of problems pertaining to mineral deposits in Afghanistan are demonstrated. In connection with geological mapping these geophysical investigations provided certain conceptions on the structure and the stratigraphy of the intramontane basins in the region south of the Hindukush mountains. Field interpretations are checked by boreholes and by computed model graphs.     

Electrical imaging of Narmada–Son Lineament Zone,Central India from magnetotellurics

The Narmada–Son Lineament (NSL) Zone is the second most important tectonic feature after Himalayas, in the Indian geology. Magnetotelluric (MT) studies were carried out in the NSL zone along a 130 km long NNE-SSW trending profile. The area of investigation extends from Edlabad (20°46′16″; 75°59′05″) in the South to Khandwa (21°53′51″; 76°18′05″) in the North. The data shows in general the validity of a two-dimensional (2D) approach. Besides providing details on the shallow crustal section, the 2D modeling results resolved four high conductive zones extending from the middle to deep crust, spatially coinciding with the major structural features in the area namely the Gavligarh, Tapti, Barwani-Sukta and Narmada South faults. The model for the shallow section has brought out a moderately resistive layer (30–150 Ω m) representing the exposed Deccan trap layer, overlying a conductive layer (10–30 Ω m) inferred to be the subtrappean Gondwana sediments, the latter resting on a high resistive basement/upper crust. The Deccan trap thickness varies from around a few hundred meters to as much as 1.5 km along the traverse. A subtrappean sedimentary basin like feature is delineated in the northern half of the traverse where a sudden thickening of subtrappean sediments amounting to as much as 2 km is noticed. The high resistive upper crust is relatively thick towards the southern end and tends to become thinner towards the middle and northern part of the traverse. The lower crustal segment is conductive over a major part of the profile. Considering the generally enhanced heat flow values in the NSL region, coupled with characteristic gravity highs and enhanced seismic velocities coinciding with the mid to lower crustal conductors delineated from MT, presence of zones of high density mafic bodies/intrusives with fluids, presumably associated with magmatic underplating of the crust in the zone of major tectonic faults in NSL region are inferred.     

Remanent magnetism of the Deccan Trap flows around Sagar,Madhya Pradesh,India

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.     

Geophysical evidence of the Eastern Marginal Fault of the Cheb Basin (Czech Republic)

The western part of the Bohemian Massif hosts an intersection of two regional fault zones, the SW-NE trending Oh?e/Eger Graben and the NNW-SSE trending Mariánské Lázně Fault, which has been reactivated several times in the geological history and controlled the formation of the Tertiary Cheb Basin. The broader area of the Cheb Basin is also related to permanent seismic activity of M_L 3+ earthquake swarms. The Eastern Marginal Fault of the Cheb Basin (northern segment of the Mariánské Lázně Fault) separates the basin sediments and underlying granites in the SW from the Kru?né Hory/Erzgebirge Mts. crystalline unit in the NE. We describe a detailed geophysical survey targeted to locating the Eastern Marginal Fault and determining its geometry in the depth. The survey was conducted at the Kopanina site near the Nový Kostel focal zone, which shows the strongest seismic activity of the whole Western Bohemia earthquake swarm region. Complex geophysical survey included gravimetry, electrical resistivity tomography, audiomagnetotellurics and seismic refraction. We found that the rocks within the Eastern Marginal Fault show low resistivity, low seismic velocity and density, which indicates their deep fracturing, weathering and higher water content. The dip of the fault in shallow depths is about 60° towards SW. At greater depths, the slope turns to subvertical with dip angle of about 80°. Results of geoelectrical methods show blocky fabric of the Cheb Basin and deep weathering of the granite bedrock, which is consistent with geologic models based on borehole surveys.     

Geology and volcanology of the Edd-Bahar Assoli area (Ethiopia)

The paper presents geological and petrological data on one of the alkaline ranges developed along the borders of the Afar depression (Ethiopia). These alkaline ranges occur in a position transversal to the dominant NNW trend of the spreading zones of northern and central Afar which are characterized by magmas of tholeiitic affinity. The Edd-Bahar Assoli volcanic range consists of broad fields of basic lavas and numerous spatter cones outcropping in the area extending between 13°25′ and 13°75′ lat. N and 41°38′ and 42°15′ long. E. The mineralogical assemblage and the chemical data point to an alkaline nature for this range consisting mainly of alkali olivine basalts and basalts tending to hawaiites, the most evolved terms being largely subordinate. Petrologic differences between the Assab, Edd-Bahar Assoli and Erta Ale ranges are shown. The Edd-Bahar Assoli alkaline volcanism would be related to tectonic patterns trending both from NNW-SSE to N-S and from NE-SW to E-W. The supposed similarity with the transverse structure of the equatorial Atlantic ocean would thus not completely be ascertained in this zone. In Afar, the coexistence of an axial volcanism of tholeiitic affinity with an alkaline volcanism at the margin can better be explained by models based upon the upper mantle temperature distribution in a zone under oceanization.     

40Ar/39Ar dating on volcanic rocks of the Deccan Traps,India

⁴⁰Ar/³⁹Ar dating results on seven volcanic rocks from four areas of the Deccan Traps, India, suggest that volcanic activity more than 70 Ma ago might have occurred at least in limited areas.In the Igat Puri area, the uppermost flow shows an⁴⁰Ar/³⁹Ar age of 63 Ma, whereas a lower flow has an age of around 82–84 Ma.⁴⁰Ar/³⁹Ar ages of samples from the Bombay area also seem to favor the occurrence of volcanic activity more than 70 Ma ago. One rhyolite dyke from the Osam Hill in the Girnar Hill area shows a well-defined plateau age of 68 Ma, whereas two tholeiitic basalts from the Mahabaleshwar area indicate a total⁴⁰Ar/³⁹Ar age of around 63–64 Ma, though they show the effect of secondary disturbance in the age spectra.The volcanic activity(ies) more than 70 Ma ago may correspond to precursory one(s) for the main volcanic activity around 65 Ma ago in the Deccan Traps.     

Age determination of paleotsunami sediments around Lombok Island,Indonesia, and identification of their possible tsunamigenic earthquakes

Age determination of paleotsunami sediment from Lombok Island, Indonesia, and surrounding area has been carried out using the 210 Pb method in BATAN Jakarta. The basic theory of this method assumes that weathering of sediments, including paleotsunami sediments, will result in 210 Pb enrichment. The principle of this method is to calculate 210 Pb contents accumulation in a particular sedimentation interval from the surface to the deeper buried sediments. The results are then converted into age or depositional time in years ago unit. The dating results from the paleotsunami sediments of the Gawah Pudak(S8°46’2.91’’, E115°56’34.23’’) and Gili Trawangan areas(S8°21’1.38’’, E116°2’36.6’’) indicate the Gawah Pudak sediments were deposited 37 years ago(c. in 1977)and 22 years ago(c. in 1992). Three paleotsunami sediments from Gili Trawangan were deposited 149 years ago(c. in 1865), 117 years ago(c. in 1897) and 42 years ago(c. in 1972). These results are then compared to the available Indonesian earthquake catalogue data. This study reveals that paleotsunami sediments around Lombok Islands, from older to younger, were caused by the 1857 earthquake(epicentre in Bali Sea; M7; S8°00’09.45’’,E115°29’56.41’’), 1897 earthquake(epicentre in Flores Sea;M5.5; S6°47’59.62’’, E120°48’03.5’’ or Sulu Sea earthquake; M8.5; 70 km NW of Basilan Island), the 1975 earthquake(Nusa Tenggara; S10°6’16.61’’, E123°48’09.39’’), 1977 earthquake(in Waingapu, Sumba; M8.0;S11°5’39.34’’, E118°27’50.86’’) and the 1992 earthquake(Flores; M7.8; S8°28’52.11’’, E121°53’44.3’’).     

Observations of the F3-layer at equatorial region during 2005

The existence of the F3-layer has been observed at Brazilian equatorial stations. This paper reports on a 1-year observations at Parit Raja, Malaysia (Lat. 1°52′N, Long. 103°48′E). The greatest number of appearances of the F3-layer is around local noon while the least is after local dawn. Its occurrence is more pronounced during winter and the equinoxes. The mean height of reflection for the layer is about 600 km reaching a maximum of about 900 km during the winter of 2004/2005. It is seen that the critical frequency of the F2-layer decreases with the appearance of the F3-layer.