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Evolution and late stage deformation of the Himalayan metamorphic core, Kanchenjunga region, eastern Nepal

University of British Columbia

Understanding the recent history of the Himalayan orogen not only helps elucidates near-surface convergence accommodation processes, but also provides constraints for geometric modification of earlier midcrustal structures. New ⁴⁰Ar/³⁹Ar and fission track (FT) data from the former Himalayan metamorphic core exposed in the Kanchenjunga region of eastern Nepal help constrain the evolution and low temperature uplift history of this portion of the orogen. Within the Lesser Himalayan Sequence (LHS), new apatite FT dates, combined with existing apatite and zircon FT dates from the region, define general younging trends towards the north - up structural section - of ~2.9 to 1.3 Ma and ~6.2 to 4.6 Ma respectively. There appears to be a significant jump in apatite FT dates from 1.3 Ma to 2.4 Ma that is coincident with an abrupt change in existing muscovite ⁴⁰Ar/³⁹Ar ages from the Proterozoic to the Cenozoic. This break in ages is consistent with the mapped location of the Main Central thrust (MCT) fault in the area. In structurally lower rocks in the Greater Himalayan Sequence (GHS), north of the MCT, trends in both muscovite ⁴⁰Ar/³⁹Ar and apatite FT continue to decrease to the north. These trends are interpreted to be consistent with the exhumation and uplift of these rocks associated with the growth of a duplex system within the LHS developed through underplating. Cooling rates across the mapped area indicate fast cooling in the GHS in early to mid Miocene, coupled with very slow cooling in the LHS. In the late Miocene to Pleistocene, cooling rates slow down in the GHS and increase in the LHS, such that they are similar. This is consistent with development of late-stage duplexing within the LHS at this time and the coupled exhumation of the GHS. Biotite ⁴⁰Ar/³⁹Ar dates may indicate a complex history across the study area. Some biotite dates (~24-16 Ma) are older than nearby ²³²Th-²⁰⁸Pb monazite melt crystallization dates (~18-16 Ma). Previous studies have attributed similar old biotite dates to excess argon. It is possible, however, the old biotite dates indicate crystallization along the retrograde path prior to final melt crystallization.

Text

2014-12-17

Attribution-NonCommercial-NoDerivs 2.5 Canada

http://hdl.handle.net/2429/51563

Environmental Sciences

University of British Columbia

UBCO

http://creativecommons.org/licenses/by-nc-nd/2.5/ca/