Detrital muscovite thermochronology in two drainage basins in western Bhutan

Abstract

New reconnaissance data for a poorly characterized area in the western Bhutan Himalaya show distinction between the 40Ar/39Ar cooling age distributions of detrital minerals in the two river catchments of the Punatsang chu and the Wang chu. Muscovites from five samples of Wang chu river sands yield ages (corresponding roughly to the time of bedrock cooling through a temperature of ca.350°C) between 9.37± 0.08 Ma and 13.98 ± 0.08 Ma. The majority of ages are less than 13 Ma, and the data for all samples have a unimodal distribution with an average age of ca. 11.4 ± 1.5 Ma. In contrast, muscovites from 14 Punatsang chu samples yield ages with nearly the same total range but with a distinctively multi-modal distribution. Two modes predominate in the Punatsang chu data: one at ca. 11.4 Ma, which is statistically indistinguishable from the single mode of the Wang chu data, and one at ca. 14.5 Ma. The full explanation for this distribution must await further bedrock and detrital dating studies, but one hypothesis is that the out-of-sequence Kakhtang thrust fault system - which is thought to transect the upper reaches of the river catchments - has juxtaposed two bedrock terrains with different cooling histories.

(cont.) A second possibility is that the younger mode of cooling ages is related to uplift of the footwall of the Chomolhari fault system, which includes the major bounding structures of the Yadong-Gulu rift northwest of the catchments. Exactly why the older mode of ages is not found in the Wang chu dataset is unclear. If the first of the above hypotheses is correct, the lack of an older mode in the Wang chu dataset may mean that the muscovites in the Wang chu fluvial sediments were derived exclusively from the Kakhtang thrust system hanging wall. If the second hypothesis is correct, the muscovites may have been derived exclusively from the Chomolhari fault system footwall. However, since we do not know the actual distribution of muscovites in the Wang chu catchment and we do not know that modern erosion is uniform in the catchment, it is also possible that the older mode is simply missing as an artifact not-uniform sampling. Again, more studies are needed to evaluate these alternative explanations.