Nd isotopic data reveal the material and tectonic nature of the Main Central Thrust zone in Nepal Himalaya
Nd isotopic data reveal the material and tectonic nature of the Main Central Thrust zone in Nepal Himalaya
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DOI：
10.1016/j.tecto.2007.11.051
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发表时间：
2008-04
期刊：
影响因子：
2.9
通讯作者：
T. Imayama;K. Arita
中科院分区：
文献类型：
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作者：
T. Imayama;K. Arita
主中央逆冲断层（MCT）是高喜马拉雅结晶（HHC）和小喜马拉雅变质沉积（LHS）之间的构造-变质边界，在第三纪重新活化，但早古生代已经形成为碰撞边界。为了调查的MCT的性质，我们分析了全岩Nd同位素比值的岩石从MCT和周边地区的Taplejung-Ilam地区的尼泊尔远东，安纳普尔纳-Galyang地区的尼泊尔中部，和Maikot-Barekot地区的尼泊尔西部。我们将MCT区定义为介于上MCT（UMCT）和下MCT（LMCT）之间的韧脆性剪切带。MCT带的原岩可能对喜马拉雅山的构造演化提供关键约束。LHS在岩石地层学上分为上、下两个单元。在Taplejung-Ilam地区，不同的岩性单元及其εNd（0）值如下：HHC（−10.0至−18.1）、MCT带（−18.5至−26.2）、LHS上部单元（−17.2）和LHS下部单元（−22.0至−26.9）。除了伊拉姆推覆体的南部前缘外，整个UMCT的εNd（0）值存在明显的缺口。在Annapurna-Galyang和Maikot-Barekot地区，不同的岩性单元及其εNd（0）值如下：HHC（−13.9至−17.7），MCT区（−23.8至−26.2，异常值为−12.4），LHS上部单元（−15.6至−26.8）和LHS下部单元（−24.9至−26.8）。这些同位素数据清楚地区分了LHS下部单元和HHC。将这些数据与先前发表的数据相结合，HHC中的最低εNd（0）值为−19.9。我们把εNd（0）值低于-20.0的岩石看作LHS。相比之下，大于-19.9的岩石并不总是HHC，其中一些可能属于LHS，因为HHC和LHS之间的Nd同位素比值重叠。MCT带的大多数岩石的Nd同位素比值与LHS的相似，但与HHC的差异很大。UMCT附近εNd（0）值的空间分布模式表明，在第三纪UMCT活动期间，HHC和LHS没有实质性的结构混合。εNd（0）值空间分布的不连续性在整个喜马拉雅山脉沿UMCT沿着是横向连续的。这些事实支持了UMCT最初是HHC和LHS之间的物质边界的理论，表明MCT带主要是在第三纪HHC逆冲推覆期间承担滑动面的作用而发展的。
The Main Central Thrust (MCT) is a tectono-metamorphic boundary between the Higher Himalayan crystallines (HHC) and Lesser Himalayan metasediments (LHS), reactivated in the Tertiary, but which had already formed as a collisional boundary in the Early Paleozoic. To investigate the nature of the MCT, we analyzed whole-rock Nd isotopic ratios of rocks from the MCT and surrounding zones in the Taplejung–Ilam area of far-eastern Nepal, Annapurna–Galyang area of central Nepal, and Maikot–Barekot area of western Nepal. We define the MCT zone as a ductile–brittle shear zone between the upper MCT (UMCT) and lower MCT (LMCT). The protoliths of the MCT zone may provide critical constraints on the tectonic evolution of the Himalaya. The LHS is lithostratigraphically divided into the upper and lower units. In the Taplejung–Ilam area, different lithologic units and their εNd(0) values are as follows; HHC (−10.0 to −18.1), MCT zone (−18.5 to −26.2), upper LHS unit (−17.2), and lower LHS unit (−22.0 to −26.9). There is a distinct gap in the εNd(0) values across the UMCT except for the southern frontal edge of the Ilam nappe. In the Annapurna–Galyang and Maikot–Barekot areas, different lithologic units and their εNd(0) values are as follows; HHC (−13.9 to −17.7), MCT zone (−23.8 to −26.2 except for an outlier of −12.4), upper LHS unit (−15.6 to −26.8), and lower LHS unit (−24.9 to −26.8). These isotopic data clearly distinguish the lower LHS unit from the HHC. Combining these data with the previously published data, the lowest εNd(0) value in the HHC is −19.9. We regard rocks with εNd(0) values below −20.0 as the LHS. In contrast, rocks with those above −19.9 are not always the HHC, and some parts of them may belong to the LHS due to the overlapping Nd isotopic ratio between the HHC and LHS. Most rocks of the MCT zone have Nd isotopic ratios similar to those of the LHS, but very different from those of the HHC. The spatial patterns in the distribution of εNd(0) value around the UMCT suggest no substantial structural mixing of the HHC and LHS during the UMCT activities in the Tertiary. A discontinuity in the spatial distribution of εNd(0) values is laterally continuous along the UMCT throughout the Himalayas. These facts support the theory that the UMCT was originally a material boundary between the HHC and LHS, suggesting the MCT zone was mainly developed with undertaking a role of sliding planes during overthrusting of the HHC in the Tertiary.