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 <em>c.</em> 16–10 Ma in the western Annapurna and <em>c.</em> 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 <em>c.</em> 7 Ma and ages from several samples in the east are <em>c.</em> 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.我们报道了采自安纳普尔纳山脉（Annapurna Range）南部4条北向剖面的13件大喜喜马拉雅岩系（Greater Himalayan rocks）样品与1件小喜马拉雅岩系（Lesser Himalayan rock）样品的全新白云母⁴⁰Ar/³⁹Ar（40Ar/39Ar）定年结果。将本次新获得的数据与已发表的年代学数据相结合，我们对安纳普尔纳山脉南部的构造演化提出了新的认识。 大喜喜马拉雅岩系的白云母冷却年龄在安纳普尔纳西部约为16~10 Ma，东部约为6~2 Ma，显示出从西向东年龄降低4~14 Ma的空间梯度。同样，西部1件小喜马拉雅岩系样品的白云母冷却年龄约为7 Ma，东部多件样品的年龄约为5~2 Ma，表明从西向东年龄降低2~5 Ma的趋势。 安纳普尔纳西部更早的冷却事件，可通过承载大喜、小喜马拉雅岩系的下伏逆冲断层（underlying thrust）前缘斜坡的走向几何形态差异，或是切割这些岩系的北东向断裂（NE-striking fault）来解释。 在莫迪河谷（Modi river valley）的大喜喜马拉雅岩系样品中，1件分别对应粒径约750 μm与200 μm的分样，获得了18.0±0.7 Ma与16.2±0.5 Ma的白云母⁴⁰Ar/³⁹Ar定年结果。与之相对，在构造位置低200 m的另一件样品中，对应两个粒径的分样年龄分别为12.6±0.2 Ma与9.9±0.1 Ma。 此前有学者在这两件样品之间提出存在北倾的巴努瓦断层（Bhanuwa Fault），并就其运动性质为正断层还是逆冲断层展开了争论。我们本次发现的上盘（hanging wall）白云母⁴⁰Ar/³⁹Ar年龄更老的分布模式，支持了巴努瓦断层的存在，并指示其运动性质为正断层。