Standardization of gravity and Bouguer anomalies in India

Abstract

About 150,000 gravity measurements made in India during the past two decades have been based on various floating datums. In this study an attempt is made to bring all this data to a standard datum by reoccupying some 300 of the old stations and tying them to the standard calibration range in India established by Manghnani and Woollard in 1963. A correlation analysis of the differences between the old and the new sets of gravimeter values indicated a simple regression of the differences in the absolute gravity values, roughly corresponding to the areas and the years of field survey parties. For the standardization of the old values to the national datum of Dehra Dun (979.0640 gals), the Survey of India stations required two corrections: (1) a constant and (2) a term linearly dependent on their absolute values. The Oil and Natural Gas Commission values required only a constant correction term for the base datum. Simple Bouguer anomalies were computed for about 4,000 of the standardized values together with about 1,000 new gravity stations covering formerly blank areas. The anomaly map does not show consistent correlation with surficial geology but reflects the major tectonic features well. Low and broad anomalies in areas of high density rocks, such as the Deccan Plateau basalts, the Cuddapah basin, etc., point to a dominance of the effects of deeper mass variations in the crust. Regional anomalies were therefore obtained by filtering out the components due to the near-surface and smaller features. Using a three-dimensional anomaly computation procedure, a structural map of the 'M' surface was derived assuming a sea level crustal thickness of 32 km and a density contrast of 0.4 gm/cc at the Moho boundary. Two large regions of low gravity are seen in the regional anomalies. One is a semi-circular low with a minimum of -120 mgal, centered over the Mangalore coast, and practically covers all of the triangular peninsula of India. The other lies in the region of northern India where there are steep gradients towards the Himalaya Mountains. The gradients increase from about 60 mgal/100 km over the deeper part of the Indo-Gangetic basin to about 140 mgal/100 km in the Punjab and Kashmir foothills of the Himalaya Mountains. A zone of relative high gravity values covers central India and roughly follows the northern boundary of the peninsula. The Moho surface shows general depression under the peninsula with a maximum depth of about 42 km under the Mangalore coast. It rises in the central India to form a NW-SE trending upwarp along Those axis the 'M' surface lies at a depth of about 30 km further west under Rajasthan. West of the Ganges delta the 'M' surface lies at a depth of about 34 km. North of this upwarp the Moho surface dips down again to depths of about 40 kill under the Himalayan foothills and lies at a depth of about 50 km and more under Kashmir. This interpretation of the regional gravity field appears to be substantiated by regional uplift over the area of postulated anti-clinal uplift of the 'M' surface as delineated by the Satpura Mountains, the Mahadeo Hills, the Maikal Range and the Kaimur Hills trending east-west across central India and the Aravalli Range which appears to lie on a north-south trending spur on the main crustal anticline in western India. The deepening of the 'M' surface as the Himalayan Mountains are approached is to be expected for a major orogenic feature of this type of Alpine age. The only question concerning the interpretation pertains to marked negative gravity values in southern India which may be related at least in part to a major negative mass inhomogeneity in the upper mantle. In the absence of seismic crustal measurements, this uncertainty can not be resolved at this time.