Interaction of surface erosion and sequential thrust progression: Implications on exhumation processes

This paper investigates the evolution of thrust wedges with concomitant surface erosion, and its bearing on the exhumation processes in orogenic belts. We performed sandbox experiments, simulating syn-orogenic erosion on forelandward sloping surfaces (∼4°). Experiments show that the erosion process has a significant control on the progression of frontal thrusts. In case of no-erosion condition, wedges with high basal friction develop frontal thrusts with strongly increasing spacing. In contrast, for the same basal friction the thrusts show uniform spacing as the wedge development involves concomitant surface erosion. On the other hand, the erosion promotes reactivation of hinterland thrusts in wedges with low basal friction. We show that erosion-assisted thrust reactivation is the principal mechanism for exhumation of deeper level materials in orogens. Efficiency of this mechanism is largely controlled by basal friction. The exhumation of deeper level materials is limited, and occurs within a narrow, sub-vertical zone in the extreme hinterland when the basal friction is high (μ_b = 0.46). In contrast, the process is quite effective in wedges with low basal friction (μ_b =0.36), resulting in exhumation along gently dipping foreland-vergent thrusts as well as along thrusts, subsequently rotated into steep attitude. The zone of exhumation also shifts in the foreland direction in the course of horizontal movement. Consequently, deeper level materials cover a large area of the elevated part of the wedge.     

Investigations of June 7, 1988 earthquake of magnitude 4.5 near Idukki Dam in Southern India

A mildly damaging earthquake of magnitude 4.5 and intensity VI occurred 20 km east of the Idukki reservoir, Kerala in southern India. With a network of 5 seismic stations, the aftershocks which continued for 3 1/2 months were monitored. The hypocentral parameters, b value,M ₁/M ₀ ratio indicate that this earthquake sequence does not qualify to be categorized as induced. The trend of the aftershocks, composite fault plane solution and local tectonics point towards reactivation of a NW-SE fault along the Kallar river. The existence of such a fault is also supported by gravity studies.     

An electromagnetic prospecting instrument for subsurface conducting zones

Summary A portable electromagnetic prospecting unit comprising a transmitter, receiver and resolver, designed and constructed to operate with a vertical setup of coils at a single frequency of 1250 cps, measures the components of the magnetic field at the receiver station resolved in phase and at quadrature with field near the transmitter. This may be used to locate vertical or steeply dipping conductors down to a depth of about 0.6 times the maximum workable transmitter receiver separation of 120 meters, the accuracy of measurements of the resolved components being better than 2%. The variation of resolved components over a buried conductor indicates its position and depth.This paper is published by the kind permission of the Director General, Geological Survey of India.     

A statistical analysis of exploration geochemical data for uranium

This paper describes a statistical analysis of reconnaissance exploration geochemical data for uranium. Three sets of data were analyzed, as they related to: (a) Charlotte-Winston-Salem and (b) Charlotte two-degree NTMS quadrangles of the south-eastern U.S.A. The coefficient of variation for uranium in each of the three sets of data was less than unity and hence no transformation of the original variable was needed. These data were subjected to correlogram analysis. A first-order Markovian model of the type: Y₀ exp (-a |p|) was fit by the least-squares method to serial correlation coefficients of these data using the method proposed by Deming (1948). The model was tested by computing the variance-volume relationship for assumed individual blocks of unit length. The noise in the input (record) was eliminated by the application of an optimum bilateral exponential smoothing technique developed by Agterberg. A comparison of spectral density estimates obtained by a maximum entropy method employing Yule-Walker equations and the Burg algorithm was also made. The prediction error coefficients needed to determine the order of the autoregressive process and hence the spectral densities were determined in both cases for the three sets of data.On leave from National Geophysical Research Institute, Hyderabad-7, India.     

The modified pseudo-depth section as a tool in resistivity and IP prospecting — A case history

Following the depths of investigation of different electrode arrays, given byRoy andApparao (1971) andRoy (1972), a modified pseudo-depth section was suggested for any array byApparao andSarma (1981) as a tool in resistivity and IP prospecting. The tool was used for the interpretation of IP and resistivity anomalies obtained in a virgin area—Jonnagiri village, Pattikonda Taluq, Kurnool district (A.P.) India. Two of the four bore holes recommended encountered sulphide mineralization, while the other two met with white altered ashy material at the position of the maximum anomaly contour in the depth sections. The drilling results prove the utility of the modified pseudosections in the interpretation of the field data.     

Fluvial process and morphology of the Brahmaputra River in Assam, India

The Brahmaputra River finds its origin in the Chema Yundung glacier of Tibet and flows through India and Bangladesh. The slope of the river decreases suddenly in front of the Himalayas and results in the deposition of sediment and a braided channel pattern. It flows through Assam, India, along a valley comprising its own Recent alluvium. In Assam the basin receives 300 cm mean annual rainfall, 66–85% of which occurs in the monsoon period from June through September. Mean annual discharge at Pandu for 1955–1990 is 16,682.24 m³ s^{− 1}. Average monthly discharge is highest in July (19%) and lowest in February (2%). Most hydrographs exhibit multiple flood peaks occurring at different times from June to September. The mean annual suspended sediment load is 402 million tons and average monthly sediment discharge is highest in June (19.05%) and lowest in January (1.02%). The bed load at Pandu was found to be 5–15% of the total load of the river. Three kinds of major geomorphic units are found in the basin. The river bed of the Brahmaputra shows four topographic levels, with increasing height and vegetation. The single first order primary channels of this braided river split into two or more smaller second order channels separated by bars and islands. The second order channels are of three kinds. The maximum length and width of the bars in the area under study are 18.43 km and 6.17 km, respectively. The Brahmaputra channel is characterised by mid-channel bars, side bars, tributary mouth bars and unit bars. The geometry of meandering tributary rivers shows that the relationship between meander wavelength and bend radius is most linear. The Brahmaputra had been undergoing overall aggradation by about 16 cm during 1971 to 1979. The channel of the Brahmaputra River has been migrating because of channel widening and avulsion. The meandering tributaries change because of neck cut-off and progressive shifting at the meander bends. The braiding index of the Brahmaputra has been increasing from 6.11 in 1912–1928 to 8.33 in 1996. During the twentieth century, the total amount of bank area lost from erosion was 868 km². Maximum rate of shift of the north bank to south resulting in erosion was 227.5 m/year and maximum rate of shift of the south bank to north resulting in accretion was 331.56 m/year. Shear failure of upper bank and liquefaction of clayey-silt materials are two main causes of bank erosion.     

Water quality assessment of Gautami — Godavari mangrove estuarine ecosystem of Andhra Pradesh, India during September 2001

Some chemical and biological parameters were analysed at sixteen stations in the mangrove ecosystem, of the neighbouring Gautami-Godavari (GG) river estuary and Kakinada (KKD) bay to understand the present status of water quality and the impact of external terrigenous inputs during southwest (SW) monsoon in the study areas. High concentrations of nutrients in the mangrove ecosystem compared to the bay and estuarine ecosystems reveal the importance of this zone as a source of nutrients to the adjacent coastal ecosystems. Low Si:N:P (29:4:1) ratios in these ecosystems are due to the enrichment of these nutrients through external anthropogenic inputs even after the utilization by phytoplankton in the biological cycle. The mean Chl b/Chla and Chl c/Chla ratios and high phaeopigments (Pp) concentrations compared to Chlb and high ratios of Chl a/Pp suggests the possibility of the potential growth of phytoplankton populations in lower light intensity and low turbulent areas of these mangrove ecosystems.     

A Godunov-Type Scheme for Atmospheric Flows on Unstructured Grids: Scalar Transport

This is the first paper in a two-part series on the implementation of Godunov-type schemes on unstructured grids for atmospheric flow simulations. Construction of a high-resolution flow solver for the scalar transport equation is described in detail. Higher-order accuracy in space is achieved via a MUSCL-type gradient reconstruction after van Leer and the monotonicity of solution is enforced by slope limiters. Accuracy in time is maintained by implementing a multi-stage explicit Runge-Kutta time-marching algorithm. The scheme is conservative and exhibits minimal numerical dispersion and diffusion. Five different benchmark test cases are simulated for the validation of the numerical scheme.     

The influence of Indian Ocean Dipole (IOD) on biogeochemistry of carbon in the Arabian Sea during 1997–1998

Data on ocean color chlorophylla (Chl a) obtained using Sea-viewing Wide Field of view Sensor (SeaWiFS), sea surface temperature (SST) by Advanced Very High Resolution Radiometer (AVHRR), and sea surface height (SSH) by TOPEX/POSEIDON were analyzed to examine the influence of Indian Ocean Dipole (IOD) on the physical and biogeochemical processes with special reference to phytoplankton primary production and air-sea fluxes of carbon dioxide in the Arabian Sea. Positive SST anomalies (SSTA) were found in the Arabian Sea (0.4 to 1.8°C) with higher values in the southwestern Arabian Sea that decreased towards north. The SSH anomalies (SSHA) and turbulent kinetic energy anomalies (TKEA) suggest decreased mixing during the IOD compared to the normal period. Chlorophylla displayed significant negative correlations with SSTA and SSHA in the Arabian Sea. Consistently, Chla showed negative anomalies (low Chl a) during the IOD period which could be due to reduced inputs of nutrients. The photic zone integrated primary production decreased by 30% during the IOD period compared to the normal whereas pCO₂ levels were higher (by 10–20μatm). However, sea to air fluxes were lower by 10% during the IOD period due to prevailing weaker winds. Primary production seems to be the key process controlling the surface pCO₂ levels in the Arabian Sea. In future, the influence of IOD on ecosystem structure, export production and bacterial respiration rates are to be probed throughin situ time-series observations.