Isotopic mapping major Himalayan structures
Abstract
Defining major structures and units in mountain belts is crucial to orogenic studies. In polymetamorphic terrains, such as the Himalaya, major thrust zones may not be unambiguously distinguished on the basis of structural analysis alone, requiring integrated isotopic studies that provenance sedimentary source regions. Examples of isotopic mapping include the use of both Nd model age data (Ahmad et al., 2000) and detrital zircon ages (Parrish et al., 1996) that have successfully discriminated between clastic sediments from the Lesser and the Higher Himalaya units (LHS and HHCS respectively) and therefore identify the Main Central Thrust Zone (MCTZ) in the Central Himalaya.. Pelites and psammites were collected on a transect through the Western Himalaya along the Sutlej Valley, Himachal Pradesh, for a combined study of bulk rock (Nd-Sr using isotope dilution on TIMS) and zircons (U-Th-Pb using laser ablation and isotope dilution PIMMS) to assess the validity of model age discrimination. The section ranges from very low grade Haimanta formation pelites underlying the Tethyan Sedimentary Series, through the Vaikrita Group (HHCS), the Wangtu Gneiss Complex and the Rampur Window, to Chail Group, Shimla Slates and Krol Belt rocks south of Shimla. Throughout the Sutlej section Nd model ages confirm the distinction between the HHCS and the (Inner) LHS, and we are testing the hypothesis that Sr isotopes show contrasts between the HHCS and Tethyan sediments. The data imply that the low-grade Outer Lesser Himalaya may be correlated with the High Himalaya, despite the contrast in metamorphic grade, carrying significant implications for the architecture of the orogen. We are assessing the regional significance of these findings and determining the along-strike continuity of major thrust zones from a similar transect through the Eastern Himalaya (Bhutan). References Ahmad, T.et al., 2000, Isotopic constraints on the structural relationships between the Lesser Himalayan Series and the High Himalayan Crystalline Series, Garhwal Himalaya, Geological Society of America Bulletin, 112, 467-477. Parrish, R. R. &Hodges, K. V., 1996, Isotopic constraints on the age and provenance of the Lesser and Greater Himalayan sequences, Nepalese Himalaya, Geological Society of America Bulletin, 108, 904-911.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA......451R