Muscovite 40Ar/39Ar ages help reveal the Neogene tectonic evolution of the southern Annapurna Range, central Nepal
Published:January 01, 2015
Aaron J. Martin, Peter Copeland, Jeffrey A. Benowitz, 2015. "Muscovite 40Ar/39Ar ages help reveal the Neogene tectonic evolution of the southern Annapurna Range, central Nepal", Tectonics of the Himalaya, S. Mukherjee, R. Carosi, P. A. van der Beek, B. K. Mukherjee, D. M. Robinson
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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.
Analytical methods, thin section observations, data tables, and plots of argon data for each sample are available at http://www.geolsoc.org.uk/SUP18774
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Tectonics of the Himalaya
The Himalayan mountain belt, which developed during the India–Asia collision starting about 55 Ma ago, is a dramatically active orogen and it is regarded as the classic collisional orogen. It is characterized by an impressively continuous 2500 km of tectonic units, thrusts and normal faults, as well as large volumes of high-grade metamorphic rocks and granites exposed at the surface. This constitutes an invaluable field laboratory, where amazing crustal sections can be observed directly in very deep gorges. It is possible to unravel the tectonic and metamorphic evolution of litho-units, to observe the mechanisms of exhumation of deep-seated rocks and the propagation of the deformation. Himalayan tectonics has been the target of many studies from numerous international researchers over the years. In the last 15 years there has been an explosion of data and theories from both geological and geophysical perspectives.
This book presents the results of integrated multidisciplinary studies, including geology, petrology, magmatism, geochemistry, geochronology and geophysics, of the structures and processes affecting the continental lithosphere. These processes and their spatial and temporal evolution have major consequences on the geometry and kinematics of the India–Eurasia collision zone.