Thermotectonic history of the Chiplakot Crystalline Belt in the Lesser Himalaya, Kumaon, India: Constraints from apatite fission-track thermochronology

摘要

Fission-track ages and confined track length distribution of apatite samples separated from the Chiplakot Crystalline Belt (CCB) of the Lesser Himalayan Crystalline (LHC) zone, located to the south of the Main Central Thrust (MCT)/Munsiari Thrust (MT) in Kumaon, India, have been determined. Ages from the CCB along the Kali and Darma valleys fall in two distinct groups. In the northern part of the CCB, the ages range from 9.8 ± 0.6 to 7.6 ± 0.6 Ma with a weighted mean of 9.6 ± 0.1 Ma, while in the southern part the ages vary from 17.9 ± 0.9 to 12.9 ± 1.1 Ma with a weighted mean of 14.1 ± 0.1 Ma. The bimodal distribution of track lengths indicates that the ages are mixed ages, rather than simple cooling ages. The apatite fission-track (AFT) ages and already published structural data of the CCB suggest a complex erosional, denudation history within the upper 3-4 km of the crust of the CCB. The ages further indicate that the CCB was thrust into place earlier than the Middle Miocene i.e. at the time of development of the MCT. Since, then these rocks have remained within the upper 3 km of crust and were affected by only moderate to slow erosion and exhumation. These results have important implications for the tectonic evolution of the LHC zone to the south of the MCT/MT. The exhumation of the LHC zone in different parts of the Himalaya was not uniform. In the Kumaon Himalaya, it was not controlled, as in the Himachal Himalaya, by any major tectonic event, since it was thrust over the Lesser Himalayan Meta-sedimentary (LHMS) zone, and underwent moderate to slow erosion and exhumation.