Muscovite ⁴⁰ Ar/ ³⁹ Ar ages help reveal the Neogene tectonic evolution of the southern Annapurna Range, central Nepal

Abstract

We present new muscovite 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar ages of 18.0 + 0.7 and 16.2 + 0.5 Ma for grain sizes of approximately 750 and 200 mm, 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 northdipping 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 40 Ar/ 39 Ar age pair in the hanging wall supports arguments for the existence of the Bhanuwa fault and suggests normal sense motion.Supplementary material: Analytical methods, thin section observations, data tables, and plots of argon data for each sample are available at

Protracted thrusting followed by late rapid cooling of the Greater Himalayan Sequence, Annapurna Himalaya, Central Nepal: Insights from titanite petrochronology

“…Timing of MCT cooling varies non‐systematically along strike, with the latest and most rapid cooling at Marsyandi. High‐ T data from titanite and monazite (Catlos et al., ; Kohn et al., ; Corrie & Kohn, ; Kohn & Corrie, ; this study); intermediate temperature data from muscovite 40 Ar/ 39 Ar (Herman et al., ; Huntington & Hodges, ; Martin et al., ); low‐ T data from fission tracks; and U–Th/He (Blythe et al., ; Herman et al., ; Nadin & Martin, ; Robert et al., ). Timing of STDS movement from Murphy and Harrison (), Sachan et al.…”

“…Most intra‐GHS cut Formation I metapelites within 10 km of the MCT (e.g. Carosi et al., ; Kohn et al., ; Montomoli et al., ), for example, both the Sinuwa and Bhanuwa thrusts in the Modi Khola valley, ~50 km west of the study area (Corrie & Kohn, ; Martin, Ganguly, & DeCelles, ; Martin et al., ). In contrast, the CSZ uniquely lies ~20 km above the MCT in the orthogneiss of Formation III (Figure ).…”

“…Problems with the 40 Ar/ 39 Ar system in hornblende and biotite have long been recognized in the Himalaya (e.g. Copeland et al., ; Herman et al., ; Martin, Copeland, & Benowitz, ), and are not considered in later interpretations. However, we do consider muscovite cooling ages.…”

Tracking the Growth of the Himalayan Fold‐and‐Thrust Belt From Lower Miocene Foreland Basin Strata: Dumri Formation, Western Nepal

“…Extensive 40 Ar/ 39 Ar dating of muscovite from Himalayan source terranes has also been used to reconstruct the thermal evolution of the Himalayan fold‐thrust belt. 40 Ar/ 39 Ar dating of muscovite from the Greater Himalayan sequence typically yields ages spanning the middle to late Miocene (Bollinger et al, ; Catlos et al, ; Coleman & Hodges, ; Copeland et al, ; Copeland et al, ; Crouzet et al, ; DeCelles et al, , ; Edwards, ; Godin et al, ; Huntington & Hodges, ; Martin et al, ; Warren et al, ; Vannay & Hodges, ;Yin et al, ). In the Kathmandu klippe in central Nepal, 40 Ar/ 39 Ar muscovite ages increase southward from circa 5 to circa 20 Ma (Herman et al, ).…”

Tectonics of the Himalaya: an introduction

“…The authors consider the observed metamorphic discontinuities within the Greater Himalayan Crystallines to be second-order features not significantly affecting the extrusion of hot channel material. Martin et al (2014) date Greater and Lesser Himalaya rocks from the southern part of the Annapurna Range (central Nepal) and attribute the young Ar-Ar cooling ages to a frontal ramp and/or motion on a cross-cutting, northeast-trending fault. They infer the presence of a shear zone (the Bhanuwa fault) with a normal sense of motion.…”