Muscovite 40Ar/39Ar ages help reveal the Neogene tectonic evolution of the southern Annapurna Range, central Nepal
We present new muscovite 40Ar/39Ar ages from thirteen Greater Himalayan rocks and one Lesser Himalayan rock collected from four north-trending transects across the southern Annapurna Range. Combining the new data with previously published ages leads to the following new insight into the tectonic development of the southern Annapurna. Muscovite cooling ages from Greater Himalayan rocks are c. 16–10 Ma in the western Annapurna and c. 6–2 Ma in the eastern Annapurna, revealing a decrease of 4–14 Ma from west to east. Similarly, the muscovite cooling age from one Lesser Himalayan rock in the west is c. 7 Ma and ages from several samples in the east are c. 5–2 Ma, indicating a decline of 2–5 Ma towards the east. Earlier cooling in the western Annapurna can be explained by along-strike differences in the geometry of the frontal ramp on the underlying thrust that carries these Greater and Lesser Himalayan rocks and/or by a NE-striking fault that cut these rocks. In Greater Himalayan rocks from the Modi river valley, one sample yielded muscovite 40Ar/39Ar ages of 18.0±0.7 and 16.2±0.5 Ma for grain sizes of approximately 750 and 200 µm, respectively. In contrast, a sample collected 200 m structurally lower produced ages of 12.6±0.2 and 9.9±0.1 Ma for these two grain sizes. The north-dipping Bhanuwa fault has been proposed between these samples, with different authors arguing for normal or thrust-sense motion. Our newfound pattern of an older muscovite 40Ar/39Ar age pair in the hanging wall supports arguments for the existence of the Bhanuwa fault and suggests normal sense motion.