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Where did rotational shortening occur in the Himalayas? - Inferences from palaeomagnetic remagnetisations |
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| Authors: | E Schill C CrouzetP Gautam VK SinghE Appel |
| Institution: | a Institut für Geologie, Universität Tübingen, Sigwartstraße 10, 72076 Tübingen, Germany b Institut für Geophysik, ETH Zürich, Hönggerberg HPP O14, 8093 Zürich, Switzerland c Central Department of Geology, Tribhuvan University, Kirtipur, Kathmandu, Nepal d Department of Geology, Bundelkhand University, Jhansi 284128 U.P., India |
| Abstract: | In metacarbonates of the Lesser (LH) and Tethyan (TH) Himalayas of Kumaon/Garhwal (N-India) characteristic remanent magnetisations carried by pyrrhotite (unblocking temperatures: 250-330°C) and magnetite (demagnetising spectra: 15-50 mT) have been identified. Negative fold tests indicate remanence acquisition after the main folding phase, which is of short-wavelength character and occurs during the early orogenese of the Himalayas. A thermal or thermochemical origin of magnetisation is likely and the age of remanence acquisition is indicated to be about 40 Ma by 40K/39Ar cooling and 40Ar/39Ar crystallisation ages. In the Kumaon LH a long-wavelength tilting is indicated by a distribution of the remanence directions along a small-circle in N-S direction. Steepening of the remanence directions in the TH related to ramping on the Main Central Thrust (MCT) was not observed, in contrast to other related studies. In the Alaknanda valley of LH a 38±8 Ma age of remanence acquisition is supported by comparison of observed inclinations to the apparent polar wander path of India. Clockwise rotation of 20.3±11.7° (LH/Alaknanda valley) and 11.3±8.5° (TH) with respect to the Indian plate is observed, indicating that there is no significant evidence for rotational shortening along the MCT since about 40 Ma. Our results suggest that most of rotational underthrusting and oroclinal bending has not been accommodated by the MCT, but by the main thrusts south of it. The latest Miocene/Pliocene age of the Main Boundary Thrust indicates that oroclinal bending is a late-orogenic process. |
| Keywords: | paleomagnetism Himalayas rotation crustal shortening secondary magnetization remanent magnetization |
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