The kinematic evolution of the Nepalese Himalaya interpreted from Nd isotopes

Neodymium (Nd) isotopes from the Himalayan fold-thrust belt and its associated foreland basin deposits are useful for distinguishing between Himalayan tectonostratigraphic zones and revealing the erosional unroofing history as controlled by the kinematic development of the orogen. Neodymium. isotopic data from the Himalayan fold-thrust belt in Nepal (n = 35) reveal that the Lesser Himalayan zone consistently has a more negative epsilon (Nd)(0) value than the Greater and Tibetan Himalayan zones. Our data show the average epsilon (Nd)(0) value in the Lesser Himalayan zone is -21.5, whereas the Greater and Tibetan Himalayan zones have an average epsilon (Nd)(0) value of -16. These consistently distinct values throughout Nepal enable the use of Nd isotopes as a technique for distinguishing between Lesser Himalayan zone and Greater Himalayan zone rock. The less negative epsilon (Nd)(0) values of the Greater Himalayan rocks support the idea that the Greater Himalayan zone is not Indian basement, but rather a terrane that accreted onto India during Early Paleozoic time. Neodymium isotopic data from Eocene through Pliocene foreland basin deposits (n=34) show that sediment provenance has been dominated by Greater and Tibetan Himalayan detritus across Nepal. The epsilon (Nd)(T) values in the synorogenic rocks in western and central Nepal generally show an up-section shift toward more negative values and record the progressive unrooting of the different tectonostratigraphic zones. At similar to 10 Ma in Khutia Khola and similar to 11 Ma in Surai Khola, a shift in epsilon (Nd)(T) values from -16 to -18 marks the erosional breaching of a large duplex in the northern part of the Lesser Himalayan zone. This shift is not seen in eastern Nepal, where the epsilon (Nd)(T) values remain close to -16 throughout Miocene time because there has been less erosional unroofing in this region. (C) 2001 Elsevier Science B.V. All rights reserved.