CONTINUOUS AND EPISODIC EXHUMATION OF THE CENTRAL HIMALAYAS?

The record of orogenic cooling and exhumation is preserved in foreland basins adjacent to convergent mountain belts. The Miocene to Pliocene Siwalik Group encompasses the Neogene foreland basin deposits of the Himalayas, but so far the long-term exhumational history of the central Himalayas has been poorly constrained. For that reason we analyzed detrital zircon of 24 samples collected from modern river sediment and sandstone of the Siwaliks in western and central Nepal. The zircons were dated with the fission-track method in order to estimate long-term exhumation rates of the central Himalayas by using the lag-time concept (cooling age – depositional age). Our data indicates three main age groups, which are found consistently in our samples. The oldest age group, with peak ages between 80-120 Ma, demonstrates recycling of zircon with pre-Himalayan cooling ages. These zircons were likely derived from Tethyan Himalaya sedimentary rocks or non-reset parts of the Lesser Himalayas. The second age group reflects a static peak at 15-18 Ma, which is observed in each sample. This peak could be related to a fast exhumation and cooling event associated with possible slab break-off around 18 Ma and/or with episodic movement along major thrust-faults, such as the Main Central Thrust. Sources for theses zircons include reset parts of the Lesser Himalayas and possibly the Miocene Leucogranites in the High Himalayas. The youngest age group displays a moving peak with a relatively constant lag-time of 4 m.y. This peak first appears around 11 Ma and is evidence of continuous exhumation in the central Himalayas. Currently we are analyzing the same single zircons with the U-Pb method, because U-Pb dating helps us with provenance discrimination between Lesser Himalayan and Greater Himalayan sources or with identifying zircon derived from Miocene Leucogranites.