喜马拉雅及其邻区的重力异常、岩石圈构造与板块动力模型

GRAVITY ANOMALIES, LITHOSPHERIC STRUCTURE AND PLATE TECTONIC DYNAMICS IN THE HIMALAYAS AND NEIGHBOURING AREAS

本文按统一比例尺编制了印度-青藏地区1°×1°重力异常图和地形高程图,并用滑动平均方法得到了本区5°×5°重力异常图。用地改后的1°×1°重力异常,采用组合体模型人一机联作选择法,计算了横跨印度-青藏-蒙古长达4680km的岩石圈剖面,还给出了一个楔形体重力正演公式。基本结果有:(1)MBT、MCT的倾角为10°±5°,ITS、NS、KS的倾角为75°±5°;(2)地壳滑脱面的深度在青藏之下约20km,向高喜马拉雅、MCT、MBT抬升至15km;(3)青藏高原南、北边缘均为岩石圈结构的斜坡带,界面倾角由上向下而增大。在大、小喜马拉雅之下,壳内界面(Ⅰ、Ⅱ)的倾角约12°,Moho倾角为18°,岩石圈底面倾角约36°。在祁连山带所有界面倾角都小于喜马拉雅带,其中壳内界面倾角仅约1°,Moho倾角约2°,岩石圈底面倾角约12°;(4)岩石圈厚度由印度、蒙古向高喜马拉雅和祁连山带逐渐增加,与青藏岩石圈的边缘上翘形成主动俯冲和相对逆冲势态。印度岩石圈厚度(或上地幔顶部低密层埋深)不超过50km,蒙古高原(南)厚约70km,到高喜马拉雅和祁连山下分别增加至145和122km,青藏中心地带(怒江两侧)岩石圈厚135km,向南,北边缘各减小到120和90～102km,在高喜马拉雅和祁连山下面形成25和10km的断差;(5)

This paper deals with the lithospheric structure and plate dynamical models in the Himalayas and neighbouring areas including India, Tibet and Mongolia, using completed Bouguer gravity anomalies and the finite element method. The maps of 1°× 1° Bouguer gravity anomalies, topographic contours and 5°× 5° Bourger gravity anomalies have been presented based on unified scales, respectively. A 4680 km long lithospheric transect across India-Himalayas-Tibet-Mongolia has been carried out using the optimization method by computer versus manually controlled composed models, based on the completed Bouguergravity anomalies after terrain correction, and a forward gravity formula has been deduced for a wedge-shaped geometric body. The basic results can be outlined as:1) The dip angles of MBT and MCT are indicated as 10°±5°, and those of ITS, NS and KS as 75°±5°.2) The depth of crustal detachment planes is calculated as 20 km under the Tibetan plateau, and is uplifted to 15 km towards the High Himalayas, MCT and MBT.3) There are two dip slope zones of the lithospheric structure on both the South and North borders of the Tibetan platcau, in which the dip angles of density interfaces increase downwards from above. Under the High and Lesser Himalayas, they are about 12° for interfaces Ⅰ and Ⅱ in the crust, 18°for the Moho discontinuity, 36° for the bottom of the lithosphere. They are, however, less than those for the Himalayas zone, under the Qilian Mountain tectonic zone, where the dip angles show only about 1° for the interfaces in the crust, 2° for Moho, and 12° for the bottom of the lithosphere.4) The lithospberic thicknesses, or the depths of the Lower density layer in the top upper mantle, increase gradually from India to the High Himalayas and from Mongolia to the Qilian Mountain zone, respectively, indicating the configurations of an actively underthrusting and passively overthrusting under both border slopes of the Tibetan lithosphere. The values of the thicknesses, or the depths, appear to be about 50 km for India, and 70 km for the Mongolian plateau (south), and then greatly increased to 145 km for the High Himalayas, and 122 km for the Qilian Mountain zone. In the central zone, on both sides of the Nujiang River, of the Tibetan plateau, this value reaches 135 km, and then decreases to 120 km or 90 to 102 km on the south and north border, individually bounded by the High Himalayas and the Qilian Mountain. This case makes a throw of 25 km of the lithosphere under the High Himalayas and of about 10 km under the Qilian Mountain.5) There is a relatively thin lower density layer of 3.22g/cm^3 between the 5 km thick layer of higher density (3.28g/cm^3) beneath Moho, and the asthenosphere with low density of 3.10 g/cm^3, less than the 3.16 g/cm^3 in the corresponding layers under India and Mongolia. This lower density constitution under the Tibetan plateau is distinctly different from both its side blocks of the lithosphere, and makes a significant contribution to the great magnitude of negative gravity anomalies in the area.The finite element analysis indicates two horizontal shear zones of the lithosphere at depths of 20 km and 90 km under the Tibetan plateau, with ι_(max)-10 × 10~7 pa and 20 to 25 × 10~7 Pa, respectively. They have all been uplifted with north direction of dip under the High Himalayas, MCT, and MBT. The maximum principal stresses, σ_1, also display anomalous gradient variations from-70 × 10~7 to 30 × 10~7 Pa, implying a stress concentration under these zones. The facts provided here can be considered as mechanical evidence for thrusting of MCT and MBT, and for step-gliding of the Tibetan lithosphere along the south border under the High Himalayas.All results of this paper support such a mechanism of underthrusting that the Indian lithosphere has underthrusted partly into the High Himalayas, accompanied with the step-gliding of rocks from the lower part of the softened Tibetan lithosphere along the underthrusting plate margins. Meanwhile, the Indian upper crust has been underthrusting the H