Detrital geochronology of the Gondwana sequence of the Lesser Himalaya of the Nepal Himalaya

Abstract

Detrital age relationship between major lithounits of the orogeny and foreland basin strata can elucidate orogenic events. However, analyses of some selected samples of a certain time interval from a sedimentary basin can lead to misjudgement. We present detrital zircon U–Pb age analyses of some older sedimentary strata of the Tansen Group, a renowned basin for the explanation of the Himalayan Orogeny. Sandstone and tuff samples of the Permian and Cretaceous Periods from the Gondwana sequence of the Lesser Himalaya of eastern Nepal and western Nepal were subjected to U–Pb age analyses, and the results were compared with available younger strata of the same basin along with the major lithotectonic units of the Himalaya. The detrital zircon age population of different units enables us to propose a new stratigraphic framework for the eastern Nepal Gondwana sequence and correlate them with the well‐established Gondwana sequence of the western Nepal Lesser Himalaya. This study reveals that the source region for the Gondwana sequence (Permian to Cretaceous) and post‐Gondwana sequence (Eocene to Miocene) is alternating and casts doubt upon the famous implications for the India and Asia collision and metamorphic core exhumation made from the basin analysis of these sedimentary basins of the southern part of the Himalayan Thrust Fold Belt (HTFB). Furthermore, we argue that these basins cannot be used for the reconstruction of collision and exhumation history of the Himalaya as being used. Alternatively, the results are more consistent with the possibilities of recent unconventional hypotheses of sediment contribution of the Greater Himalaya to the Permian strata of the Lesser Himalaya and the Cretaceous rifting of the Tibetan–Himalayan Microcontinent followed by the accretion of the microcontinent before collision of India and Asia.