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.

本研究报道了采自安纳普尔纳山脉（Annapurna Range）南部四条南北向剖面的13件大喜马拉雅岩系（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（muscovite）氩-40/氩-39（40Ar/39Ar）测年数据。将本次新获得的测年数据与已发表的年代学资料相结合，我们对安纳普尔纳山脉南部的构造演化获得了全新的认识：大喜马拉雅岩系样品的白云母冷却年龄在安纳普尔纳山脉西部约为16~10 Ma，东部约为6~2 Ma，显示自西向东年龄降低了4~14 Ma；类似地，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，表明自西向东年龄降低了2~5 Ma。安纳普尔纳山脉西部更早的冷却事件，可通过两种机制解释：一是承载大喜马拉雅与小喜马拉雅岩系的下伏逆冲断层（thrust）前缘斜坡沿走向的几何形态差异，二是切割这些岩系的北东向断层活动。在采自莫迪河谷（Modi river valley）的大喜马拉雅岩系样品中，1件粒径分别约750 μm与200 μm的分馏样品，分别获得了18.0±0.7 Ma与16.2±0.5 Ma的白云母氩-40/氩-39测年结果；与之形成鲜明对比的是，在构造位置低200 m处采集的另一件样品，上述两种粒径分馏的测年结果分别为12.6±0.2 Ma与9.9±0.1 Ma。已有学者在这两件样品之间提出了北倾的巴努瓦断层（Bhanuwa fault），并对其运动性质存在正断层或逆冲断层两种学术争议。我们新发现的上盘（hanging wall）白云母氩-40/氩-39年龄偏老的分布模式，既支持了巴努瓦断层的存在，也指示其运动性质为正断运动。

搜集汇总

背景与挑战

背景概述

该数据集提供了尼泊尔中部安纳普尔纳山脉南段14个岩石样本的白云母40Ar/39Ar年龄数据，用于研究新近纪构造演化。关键发现显示冷却年龄从西向东减少，揭示了区域构造差异，并支持Bhanuwa断层的存在和运动性质，数据集以CSV格式提供原始数据，便于地质分析。