Gravity anomalies, crustal structure and thermo-mechanical support of the Himalaya of Central Nepal

Abstract

Summary We use two gravity profiles that we measured across Central Nepal, in conjunction with existing data, to constrain the mechanical behaviour and the petrological structure of the lithosphere in the Himalayan collision zone. The data show (1) overcompensation of the foreland and undercompensation of the Higher Himalaya, as expected from the flexural support of the range; (2) a steep gravity gradient of the order of 1.3 mgal km−1 beneath the Higher Himalaya, suggesting a locally steeper Moho; and (3) a 10 km wide hinge in southern Tibet. We compare these data with a 2‐D mechanical model in which the Indian lithosphere is flexed down by the advancing front of the range and sedimentation in the foreland. The model assumes brittle Coulomb failure and non‐linear ductile flow that depends on local temperature, which is computed from a steady‐state thermal model. The computed Moho fits seismological constraints and is consistent with the main trends in the observed Bouguer anomaly. It predicts an equivalent elastic thickness of 40–50 km in the foreland. The flexural rigidity decreases northwards due to thermal and flexural weakening, resulting in a steeper Moho dip beneath the high range. Residuals at short wavelengths (over distances of 20–30 km) are interpreted in terms of (1) sediment compaction in the foreland (Δρ = 150 kg m−3 between the Lower and Middle Siwaliks); (2) the contact between the Tertiary molasse and the meta‐sediments of the Lesser Himalaya at the MBT (Δρ = 220 kg m−3); and (3) the Palung granite intrusion in the Lesser Himalaya (Δρ = 80 kg m−3). Finally, if petrological transformations expected from the local (P, T) are assumed, a gravity signature of the order of 250 mgal is predicted north of the Lesser Himalaya, essentially due to eclogitization of the lower crust, which is inconsistent with the gravity data. We conclude that eclogitization of the Indian crust does not take place as expected from a steady‐state local equilibrium assumption. We show, however, that eclogitization might actually occur beneath southern Tibet, where it could explain the hinge observed in the gravity data. We suspect that these eclogites are subducted with the Indian lithosphere.

Crustal structure of the central Tibetan plateau and geological interpretation

“…• N-29.5 • N, where the Moho depth jumps from ∼45 km to ∼75 km, which is consistent with an increase in Moho depth under the northern Himalaya predicted by gravity measurements (Lyon-Caen and Molnar, 1985;Cattin et al, 2001) and a receiver function study ). However, the amplitudes in our images are different from those of Nábělek.…”

Flexural bending of the Zagros foreland basin

“…It thus seems that neither the 'crème-brûlée' model of the continental crust nor the 'Jelly sandwich' model are relevant; because as on the one hand, a strong coupling between the crust and mantle and some strength in the upper mantle are needed, and, on the other hand, T e exceeds the crustal thickness, which requires adequate strength in the upper mantle. For comparison, the elastic thickness of the lithosphere beneath foreland basin of the Himalaya, which is of Proterozoic to Archean lithospheric age, is estimated to 60-80 km (Lyon-Caen & Molnar 1983;Cattin et al 2001;Hetenyi et al 2006;Mooney 2010). The modelling of the gravity data suggests that the equivalent elastic thickness beneath the Zagros range is probably significantly lower than beneath the foreland (we get a best fitting value of ca.…”

“…A northward gradual decrease in T e is expected because of the bending-induced reduction of the elastic cores, and the presumably higher crustal temperatures within the range resulted from crustal thickening and vertical heat advection (e.g. Burov & Diament 1995;Cattin et al 2001;Hetenyi et al 2007). …”

Insights into the Great <i>Mw</i> 7.9 Nepal Earthquake of 25 April 2015

“…Seismic experiments show that the thickness of the crust increases to 70-80 km towards the north near southern Tibet 25,38 , and the Tibetan Plateau exerts a large force against the Indian plate 19,38 . This complies with the isostatic adjustment of this elevated landmass [7][8][9][10][11]19,39,40 . The epicentral parameters of these 611 events were compiled from the catalogue of the Indian Society of Earthquake Technology (ISET) 41 , International Seismological Centre (ISC) and US Geological Survey (USGS).…”

“…Segment-specific seismic activities 1,2 , rotational underthrusting and concomitant uneven southward migration of the Asian crust 3,4 , along-strike wide variation of the Indian plate obliquity (Figure 1), and occurrence of seismicity in the mantle-lithosphere of the Indian plate 5 clearly account for lateral changes in the dynamics/kinematics of the Himalaya. Although several studies involving gravity modelling were carried out for Nepal-Sikkim Himalaya [6][7][8][9][10][11][12][13] , the present study analyses the Bouguer gravity anomaly along a strike-orthogonal profile passing through the epicentre of the 7.9 magnitude Nepal earthquake for a detailed understanding of the spatial distribution of its aftershocks and other great shocks in this part of the Himalaya ( Figure 2). The geometries of different layers in the descending Indian plate and the southward converging Asian crust ( Figure 1) were initialized by other studies for minimizing the non-uniqueness in the modelling using Bouguer gravity anomaly data along the profile.…”