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

Cattin, R. and Martelet, G. and Henry, P. and Avouac, J. P. and Diament, M. and Shakya, T. R. (2001) Gravity anomalies, crustal structure and thermo-mechanical support of the Himalaya of Central Nepal. Geophysical Journal International, 147 (2). pp. 381-392. ISSN 0956-540X. doi:10.1046/j.0956-540x.2001.01541.x.

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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.

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Avouac, J. P.0000-0002-3060-8442
Additional Information:© 2001 RAS. Accepted 2001 June 8. Received 2001 January 6; in original form 2000 February 4. Article first published online: 20 Dec 2001. This paper has greatly benefited from thoughtful reviews by S. L. Klemperer and E. B. Burov, who raised several important questions and contributed significantly to the clarity of the paper. This study was funded by the IDYL program (CNRS /INSU) and by CEA. Mr Pandey, Tandukar and Bashyal of the Nepalese Department of Mines and Geology are thanked for their help in organizing the field survey. We are also grateful to Mr Timilsina and Chaudhary from PEPP and Mr Nilgobinda from the Nepalese Geodetic Survey, for their help in the field.
Group:Seismological Laboratory, Division of Geological and Planetary Sciences
Funding AgencyGrant Number
Centre National de la Recherche Scientifique (CNRS)UNSPECIFIED
Institut National des Sciences de l'Univers (INSU)UNSPECIFIED
Commissariat à l’Energie Atomique (CEA)UNSPECIFIED
Subject Keywords:eclogitization, gravity anomaly, Himalaya, layered rheology, mechanical modelling, thermal modelling
Issue or Number:2
Record Number:CaltechAUTHORS:20120910-084809471
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Official Citation:Cattin, R., Martelet, G., Henry, P., Avouac, J. P., Diament, M. and Shakya, T. R. (2001), Gravity anomalies, crustal structure and thermo-mechanical support of the Himalaya of Central Nepal. Geophysical Journal International, 147: 381–392. doi: 10.1046/j.0956-540x.2001.01541.x
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:33953
Deposited By: Ruth Sustaita
Deposited On:10 Sep 2012 16:17
Last Modified:09 Nov 2021 23:05

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