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

该项目以同位素测年为基础，旨在阐明尼泊尔中部加德满都推覆构造的形成机制和隆升过程，取得了以下成果。加德满都推覆带的南界可以在野外追踪到。野外观察和实验室工作否认了加德满都凹陷的双推覆构造。Rb-Sr等时线测年结果表明，推覆体南缘主干中央冲断带中的糜棱岩金片麻岩的年龄为晚元古代，与北部根带中央冲断带中发现的年龄一致。由于缺乏独居石，MCT带千层岩石榴石中的独居石U-Th年龄测定不成功，但MCT带中部的中新世年龄为22~13 Ma，南缘的上新世晚期为4~2 Ma。这表明切割推覆体的无序逆冲作用早于约13 Ma，大于4 Ma。加德满都推覆体的锆石和磷灰石的FT年龄从南部的约9 Ma向北延伸到北部的2-1.5 Ma，这表明推覆体的抬升是由于深部的斜坡构造和斜坡向北的展布推覆以及印度中地壳的向北楔入造成的。推覆体北部戈桑昆德山脉的锆石FT年龄分布与南北两侧样品高度的关系形成强烈对比，表明南侧岩石在2.6 Ma左右通过了锆石的闭合温度，之后北侧抬升得更快。

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.