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

该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，取得了以下成果.加德满都推覆体的南部界限可以在野外找到。野外观测和室内研究否定了加德满都推覆体的双推覆体观点。Rb-Sr等时线年龄测定表明，推覆体南缘主中央冲断带中的糜棱岩眼球状片麻岩的时代为晚元古代，与北方根部主中央冲断带中的糜棱岩眼球状片麻岩的时代一致.由于缺少独居石，对MCT带千枚岩石榴石中的独居石进行U-Th定年未能成功，但对MCT带中部的中新世和南缘的上新世分别获得了22-13 Ma和4-2 Ma的年龄。这表明切割推覆体的逆冲断层的活动时间比约晚。13 Ma和大于4 Ma。加德满都推覆体的锆石和磷灰石FT年龄由南部的约9 Ma向北推移至北方的2 ~ 1.5 Ma，表明推覆体的隆升是由深部的斜坡构造、斜坡的展布逆冲以及印度中地壳向北的楔入作用造成的。推覆体北方戈萨肯贡山地区锆石FT年龄分布表明，南北两侧的年龄与样品海拔高度的关系存在强烈的反差，表明南侧岩石在约2000年前就已通过锆石闭合温度。2.6然后北方一侧上升得更快。

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.