Formation mechanism and uplift process of the Kathmandu nappe in the Nepal Himalaya : a geochronological approach

本项目旨在通过同位素年龄测定，阐明尼泊尔中部加德满都推覆体的形成机制和隆升过程。加德满都推覆体的南边界可以在野外找到。野外观测和实验室工作否定了加德满都推覆的双重推覆理论。Rb-Sr等时线年龄测定表明，推覆体南缘主中央逆冲（MCT）带的糜棱岩片麻岩年龄为晚元古代，与北根带MCT带的糜棱岩片麻岩年龄一致。由于缺乏独居石，MCT带千里石石榴石中独居石的U-Th年龄测定未能成功，但一些年龄测定结果显示，MCT带中部的中新世年龄为22-13 Ma，南缘的晚上新世年龄为4-2 Ma。这表明切切推覆体的逆序逆冲活动时间小于13ma，大于4ma 4。加德满都推覆体锆石和磷灰石的FT年龄从南部约9 Ma向北至北部2-1.5 Ma，表明推覆体的隆升是由深部斜坡构造和斜坡的展向冲断作用以及印度中地壳的北楔作用引起的。推覆体北部Gosainkund山脉的锆石FT年龄分布表明，山体南侧和北侧的年龄与样品高度的关系对比强烈，表明南侧岩石在约2.6 Ma之前经历了锆石的闭合温度，此后北侧隆升速度更快。

The project aimed to elucidate the formational mechanism and uplift process of the Kathmandu nappe in central Nepal on the basis of isotopic age dating, and obtained the following results.1. The southern limit of the Kathmandu Nappe can be traced in the field. The field observations and laboratory works denied the double-nappe idea for the Kathmandu nappe.2. The Rb-Sr isochron age determination showed that the mylonitic augen gneisses in the Main Central Thrust (MCT) zone from the southern margin of the Nappe is late Proterozoic in age which is consistent with those found in the MCT zone in the northern root zone.3. The U-Th age dating for monazites included in garnet of phyllites in the MCT zone was not carried successfully due to lack of monazite, but some age determinations yielded the Miooene ages of 22-13 Ma for the middle part of the MCT zone and the late Pliocene ages of 4-2 Ma for the southern marginal part. This suggests the out-of-sequence thrust cutting the Nappe to be active younger than ca. 13 Ma and older than 4 Ma.4. FT age datings of zircons and apatites from the Kathmandu Nappe are younging northwards from ca.9 Ma in the southern part to 2-1.5 Ma in the northern part, suggesting the uplift of the Nappe to be caused by the ramp structure in the depth and the splay thrust from the ramp and by the northward wedging of the Indian middle crust.5. Zircon FT age distribution across the Gosainkund Mountains in the northern part of the Nappe presents that the relationship of the ages and sample altitudes on the southern and northern sides of the mountain is in strong contrast, suggesting that the rocks of the southern side passed through the closure temperature of zircon before ca. 2.6 Ma and then the northern side was uplifted more rapidly.