Volume 97, Issue 1
July 1988, pages 1-116
pp 1-10 July 1988
Chemial analyses of twelve amphiboles from the area around Terakanambi are presented. Results indicate that they are ferro-hornblende and ferro-pargasitic hornblende types in banded iron formations; magnesio hornblende in ultramafic rocks and edenite; and ferroan pargasite and ferroan pargasitic hornblende types in calcamphibolites. Titanium content in the amphiboles of the present study is relatively low compared to results from similar zones elsewhere. The lower titanium content of the amphiboles may be attributed to either bulk chemical composition or to low oxygen fugacity. Mg/Fe ratios vary considerably and it is mainly controlled by host rock composition. The plots of calcic amphiboles on (lOONa/Ca + Na)/(100 Al/Si + Al) and Aliv/Alvi diagrams indicate that they are of medium to low pressure type.
pp 11-20 July 1988
Theoretical expressions for the surface displacement and shear stress caused by a long strike-slip dislocation in an elastic layer overlying an elastic half-space are derived and the correspondence principle is used to obtain the quasi-static response when the half-space is Maxwell-viscoelastic. Variation of the surface displacement and shear stress with horizontal distance is studied for various times and vertical extents of the fault. It is seen that the quasi-static response differs significantly from the corresponding elastic response.
pp 21-34 July 1988
The short-term variability observed in the near surface meteorological parameters and in the vertical thermal structure of the upper layers of the northern Bay of Bengal during a weak monsoonal regime is examined with the aid of time series measurements. The variability of the mixed layer depth is interpreted in terms of forced mixing caused by the surface wind stress and free mixing by buoyancy flux, Ekman pumping controlled by the curl of the surface wind stress, convergence associated with a clockwise gyral circulation and stratification caused by freshwater discharges from rivers. The daily-averaged current vectors in the upper layers indicate the presence of clockwise gyral circulation in the polygon area.
pp 35-47 July 1988
Particulate organic carbon (POC) of 161 water samples collected from 8 depths (surface to 1000 m) at 21 stations was measured. The POC concentrations ranged from 154 to 554 ¼g per litre at the surface and decreased in the upper 300 m water column. At greater depths (> 300 m), POC concentrations increased and were similar (145 to 542 ¼g1−1) to those observed at surface. Deep water POC maximum was embedded within the oxygen minimum layer and was also associated with high phosphate-phosphorus. The POC contents increased, whereas oxygen decreased as the distance away from the shore increased. Phytoplankton biomass was a major source of POC. The observed pattern of POC is discussed with respect to some physicochemical and biological factors.
pp 49-52 July 1988
Aperiodicity in the time series of Indian summer rainfall for 116 years is analysed using the phase space approach. The question whether a low-dimensional strange attractor is associated with the chaotic behaviour of the monsoon system is investigated. It is found that a strange attractor of dimensionality around 5·1 exists and the system has 12 relevant degrees of freedom.
pp 53-62 July 1988
Monthly-mean winds and currents have been used to identify the driving mechanisms of seasonal coastal circulation in the North Indian Ocean. The main conclusions are: (i) the surface circulation off Arabia is typical of a wind-driven system with similar patterns of longshore current and wind stress; (ii) circulation off the west coast of India is consistent with the dynamics of a wind-driven eastern boundary current only during the southwest monsoon. During the northeast monsoon it is possible that the influence of the interior flow is important. (iii) There are at least three mechanisms that influence the surface circulation off the east coast of India: wind-stress, influence of fresh-water run off and contribution of the interior flow. It is difficult at present to assess the relative importance of these three processes.
pp 63-79 July 1988
Generally average rainfall over meteorological subdivisions is used for assessment of the variability of monsoon rainfall. It is shown here that variations of seasonal rainfall over the meteorological subdivisions of interior Karnataka are not coherent. A methodology for delineating coherent rainfall zones is developed in this paper and applied to derive such zones for the State of Karnataka.
pp 81-85 July 1988
A critical study on the type of the magnetic grains, both in composition and domain state, in rocks of different ages has been carried out. One simple, fast and non-destructive test, which can provide useful information on the nature of the magnetic grains in freshly collected samples, seems to be the ratio of susceptibilities at 77 and 300 K. This ratio, termed relative susceptibility, ranges from 0·1, for samples containing 70% ulvospinel bearing titanomagnetite (TM70) to 1·50 for cation deficient magnetite bearing ones. The results indicate that the value of 0·1 for TM70 is not greatly affected even if some amount of TM80, which is nonmagnetic at 300 K, is present in a rock sample. However, the coercive force at 77 K will increase considerably for such a sample. The effects of mixed compositions and domain states of magnetic grains on the overall behaviour of basalts are discussed.
pp 87-105 July 1988
In this paper, hydroclimatic fluctuations of the Upper Narmada catchment (upto Narmadasagar damsite) have been studied by examining the time series (1901–80) of (i) 1-to 10-day annual extreme rainfall; (ii) seasonal total rainfall between May and October; (iii) the precipitation concentration index (PCI); (iv) a modified version of PCI(MPCI); and (v) parameters of the periods contributing specified percentages of rainfall to annual total. Most of these parameters followed the normal distribution and did not show any significant long-term trend. However, some dominant long period oscillations have been noticed in extreme rainfall, seasonal rainfall, PCI and MPCI series. Influence of break-monsoon days over India during July and August on the rainfall activities of the Upper Narmada catchment has also been investigated and salient findings discussed.
pp 107-114 July 1988
Palaeomagnetic, geochemical and geochronological studies have been conducted on a set of dolerite dykes intruding the Peninsular gneisses near Huliyurdurga town, Karnataka, as a reconnaissance survey indicated a Cretaceous age for them. The dykes are mainly tholeiitic in composition with their 87Sr/86Sr ratios tightly clustered around 0·7045. Their palaeomagnetic data (Dm=329°,Im=−55°) and the corresponding palaeopole coordinates (λp = 34°S,Lp=108°E) are strikingly close to those of the Deccan Traps to the north. Whole rock K-Ar ages of these dykes ranging between 69 and 84 Ma are also similar to the range of K-Ar ages of the Deccan basalts. The chemical, palaeomagnetic and temporal coherence between the dykes and the Deccan basalts indicate that they may indeed be tectonically related events.
pp 115-116 July 1988