Constraints on the structure of the Himalaya from an analysis of gravity anomalies and a flexural model of the lithosphere


  • Hélène Lyon-Caen,

  • Peter Molnar


The intracontinental subduction of India beneath the Himalaya presents several similarities to that occurring at island arcs. We study one of those similarities by analyzing gravity anomalies across the Himalaya assuming that the topography is supported by the Indian elastic plate, flexed under the weight of both the overthrust mountains and the sediments in the Ganga Basin. We first examine in detail the effects of each of the following parameters on the configuration of the elastic plate and on the gravity anomalies: the flexural rigidity, the position of the northern end of the elastic plate (the amount of underthrusting of such a plate beneath the range), and the density contrasts between the crust and mantle and between the sediments and the crust. A plate with a constant flexural rigidity of about 0.7×1025 N m (between 0.2 and 2.0×1025 N m) allows a good fit to the data from the Lesser Himalaya and the Ganga Basin. Such a plate, however, cannot underthrust the entire Himalaya. Instead, the gravity anomalies show that the Moho steepens from only about 3° beneath the Lesser Himalaya to about 15° beneath the Greater Himalaya. This implies a smaller flexural rigidity beneath the Greater Himalaya (0.1 to 1.0×1023 N m) than beneath the Ganga Basin and the Lesser Himalaya. Even with a thin, weak plate beneath the Greater Himalaya, the weight of the mountains depresses the plate too much unless an additional force or moment is applied to the plate. The application of a bending moment/unit length to the end of the plate of about 0.6×1018 N m is adequate to elevate the Indian plate and to bring the calculated gravity anomalies in agreement with those observed. Both, the smaller flexural rigidity and the bending moment can be understood if we assume that part or all of the Indian crust has been detached from the lower lithosphere that underthrusts the Greater Himalaya. We study the tectonic implications of these results by means of a series of idealized balanced cross sections, from the collision to the present, that reproduce several important features of the geology of the Himalaya and predict an amount of eroded material comparable to that in the Ganga Basin and the Bay of Bengal. These cross sections include high-grade metamorphic rocks near the Main Central Thrust and a steeper dip of it there than in the Lesser Himalaya. They predict rapid uplift only in the Greater Himalaya and at the foot of the Lesser Himalaya.