Geological factors in gravity interpretation illustrated by evidence from India and Burma

Abstract

Summary The interpretation of gravity data has suffered from lack of collaboration between geodesists and geologists, and few general discussions of the subject have made adequate allowance for the effects of relatively light sedimentary rocks in diminishing the observed values of gravity. In an extensive gravity survey of Burma and north-eastern India gravity anomalies (that is, the difference between the observed and computed values) were determined at over 6000 stations, and an endeavour was made to resolve the anomalies into the portions due to known geological causes and the residual (regional) portions due to deep-seated variations in density. Although the need for applying to gravity data a correction for known geology was emphasized by David White in 1924 and has been discussed by several subsequent writers, so far no attempt seems to have been made to carry out the necessary computations for an area in which sediments are thick. The calculations are laborious, but a graphical method supplied by Colonel (now Brigadier) E. A. Glennie, of the Geodetic Branch of the Survey of India, proved very useful. Difficulties were encountered in determining appropriate densities applicable to sediments buried at great depths, a branch of geology which has received little attention. Sections through representative gravity stations were drawn from geological maps, and by the graphical method it was possible to estimate the gravity effect attributable to known or inferred geological structures. This was subtracted from the anomaly to obtain a geologically corrected or residual anomaly Some of the geological corrections are of the same order as many of the anomalies, but in general their application does not lessen the anomalies, and the results confirm the view of de Graaff Hunter and Glennie that it is incorrect " to quote India as a region supporting the hypothesis of Hay ford isostasy ". The residual anomalies show a close relation to the main tectonic lines. A pronounced gravity minimum follows the lines,of maximum uplift in the hills separating Assam and Bengal from Burma and a gravity maximum almost coincides with the nearly parallel volcanic line of Burma. Both these lines can be correlated with the even larger anomalies described by Vening Meinesz in the Netherlands East Indies. A gravity minimum follows the general trend of the eastern Himalayas. The Bengal and Irrawaddy deltas (like the neighbouring deltas of the Indus, Mekong, and Red River or Tongking) are areas of excess gravity. All these major regional anomalies are attributable to variations in the depths to the basic and ultra-basic crustal zones. The broad regularity of the gravity features is interrupted by several local anomalies, of which the most important coincides with a belt of thrusting and may be due to hitherto unsuspected igneous intrusions.