• Volume 103, Issue 3

      September 1994,   pages  353-437

    • Surface sensible heat flux over a deep moist convective region and its interplay with synoptic and mesoscale features

      R Pradhan U K De P K Sen

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      The surface parameters are being evaluated using a methodology which considers the vertical temperature structure of MONTBLEX ’90. For this 30 metre micrometeorological tower data are utilised. In this process, a concept of isolated layers has been introduced. The parameters have been evaluated following two successive iterative processes to give a consistent value both in the flux-profile relation and the related similarity relation.

      The heat flux obtained using the present methodology, is then utilised to examine its interplay with the synoptic as well as mesoscale features. The existence of such an interplay is observed.

    • The interannual variability of mid-latitude meridional circulation and its teleconnection with Indian monsoon activity

      J Chattopadhyay R Bhatla

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      The statistical relationship between the summer monsoon rainfall over all India, northwest India and peninsular India, onset dates of monsoon and the index of mid latitude, (35° to 70°N) meridional circulation at 500 hPa level over different sectors and hemisphere based on 19 years (1971–1989) data, have been examined. The results indicate that (i) the summer monsoon rainfalls over all India, northwest India and peninsular India show a significant inverse relationship with the strength of meridional index during previous January over sector 45°W to 90°E. (ii) The summer monsoon rainfalls over all India and peninsular India show a significant inverse relationship with the strength of meridional index during previous December over sector 90°E to 160°E, (iii) The summer monsoon rainfall over northwest India shows a significant direct relationship with the meridional index during previous May over sector 160°E to 45°W.

      Significant negative relationships are also observed between the meridional circulation indices of previous October (sector 3 and 4), previous December (sectors 1, 3 and 4), previous winter season (sector 3 and 4) and the onset dates of summer monsoon over India. The meridional circulation index thus can have some possible use for long range forecasting of monsoon rainfall over all India, northwest India and peninsular India, as well as the onset dates of monsoon.

    • Geomagnetic depth sounding over the Singhbhum and the surrounding regions of eastern India

      A N Hanchinal P B V Subba Rao Nandini Nagarajan D R K Rao B P Singh

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      Magnetovariational studies have been carried out in Singhbhum and surrounding regions during 1987 and 1989. Three deep-seated linear conductors have been identified. One of them is located to the north of Ranchi, Bokaro and Purulia extending in E-W direction coinciding with high heat flow region and Gondwana sediments. The trend of anomaly at Ranchi and Purulia at longer periods suggests a conductivity anomaly due to the mafic and ultramafic intrusions, considered to be responsible for the uplift of Chhotanagpur plateau. The second conductor is associated with the basin margin fault that separates the Singhbhum craton and Chhotanagpur plateau from the West Bengal basin. This conductive zone appears to extend further south and join the high heat flow region of Attri-Tarabalo. This conductor could be isolated only after eliminating the coast effect from the observed induction vectors. The third conductive zone follows the trend of Mahanadi valley located south of the Sukinda thrust. Conductive anomaly associated with the Sukinda and Singhbhum thrust zones could not be resolved due to the interference from neighbouring conductive structures. These two thrusts may not be very deep-seated structures. The Singhbhum granite batholith is found to be highly resistive and seems to extend to greater depths.

    • Minimum norm inversion of observed ground elevation changes for slips on the causative fault during the 1905 Kangra earthquake

      V K Gahalaut P K Gupta R Chander V K Gaur

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      We estimate the distribution of slip in the dip section of the causative fault for the 1905 Kangra earthquake by applying the minimum norm inversion technique to differences in pre- and post-earthquake levelling data collected along the Saharanpur-Dehradun-Mussoorie highway. For this purpose it is assumed that the causative fault of the 1905 Kangra earthquake was planar with a dip of 5° in the northeast direction and that it had a depth of 6 km at the southern limit of the Outer Himalaya in Dehradun region. The reliably estimated maximum slip on the fault is 7.5 m under the local northern limit of the Outer Himalaya. Using the inverted slip distribution we estimate that the maximum permanent horizontal and vertical displacements at the surface due to the Kangra earthquake were about 4 m and 1.5m respectively. The maximum transient displacements at the surface should have exceeded these permanent displacements. These estimates of maximum slip on the causative fault and the resultant maximum permanent and transient displacements at the surface during the Kangra earthquake may be taken tentatively as being representative of the great Himalayan earthquakes.

    • Evaluating U-Pb zircon data using a 3-dimensional concordia

      M Wiedenbeck

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      When assessing zircon U-Pb data, Wendt’s (1984; 1989) 3-dimensional projection for calculating concordia intercept ages has a fundamental advantage over other methods: the best-fit plane in three dimensions defines a sample’s age without requiring any advance knowledge about the isotopic composition of the non-radiogenic Pb. However, until now the general validity of this approach has never been investigated using data sets measured on terrestrial samples.

      Best-fit plane calculations were made for three terrestrial zircon samples. The t1 and t2 concordia intercept ages of these samples were found to be statistically equivalent to the ages calculated by other means. However, the 3-dimensional calculations gave detectable differences in ages and precision estimates as compared to the mean207Pb/206Pb and line regression techniques; such differences could be important at moderate to high precision level. It was also found that the 3-dimensional concordia provides useful information for discerning which analyses should or should not be included within the final data set.

    • Rb-Sr age of the Sivamalai alkaline complex, Tamil Nadu

      T V Subba Rao B L Narayana K Gopalan

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      The Sivamalai alkaline complex comprises ferro-, pyroxene-hornblende- and nepheline-syenites. Field relations show that the nepheline syenites followed the emplacement of non-feldspathoidal syenites. Mineralogical data on the syenite suite have been reviewed. The Sivamalai alkaline rocks are not strongly enriched in rare-earth elements like most miaskites. Rb-Sr isotopic analyses of a suite of six samples from the various members of the complex define an isochron corresponding to an age of 623 ±21 Ma () and initial Sr ratio of 0.70376 ±14 (2σ). This is consistent with a model of fractional crystallization of a parent magma derived from an upper mantle source with apparently no isotopic evidence for more than one magma source for the complex. The Sivamalai alkaline complex represents a Pan-African alkaline magmatic event in the southern granulite terrane of Peninsular India.

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