青藏高原—北印度洋地区的均衡大地水准面异常和岩石圈构造

ISOSTATIC GEOID ANOMALIES AND LITHOSPHERIC STRUCTURE OF THE QINGHAI-XIZANG PLATEAU—NORTH INDIAN OCEAN REGION

本文研究了青藏—北印度洋地区的均衡大地水准面异常和岩石圈构造特点.讨论了印度,青藏碰撞的俯冲机制及青藏高原隆起的成因问题。由Airy和Pratt两种均衡模型能得到可以比较的大地水准面异量级,在大陆区域更相接近.用180阶减20阶大地水准面同异常反演出的地壳密度分布.呈南北向正负交替和东西展布的波状特征.表明与岩石圈物质水平运动的挤压—松弛(或拉伸)作用有关.在此水平运动背景上,Airy型补偿原理可被重新描述为正、负垂直密度矩相抵销.岩石圈厚度由洋向陆从60km左右增加到青藏高原中部约110—130km。局部达140km.喜马拉雅带是深部构造和地球物理异常的转换过渡带、其碰撞机制显俯冲—逆掩型.

This paper deals with the isostatic geoid anomalies and lithospheric structure of the Qinghai-Xizang plateau-Northern Indian Ocean region, and also the subduction mechanism of the Indian plate underthrusting beneath the Himalayan collision zone and the uplift cause of the Qinghai-Xizang plateau. The classical Airy and Pratt models give geoid anomalies of comparable magnitude which are closer to each other in the continental area than in the ocean. The crustal density distribution, derived from the geoid difference between the spherical harmonics of degree 180 and 20, displays wave-like features of latitudinally alternating and longitudinally extending positive/negative variations, which are related to the stress-compressing/relaxing (or extending) of the lithospheric material in horizontal movement. With this background of the horizontal movement, the compensation principle of the Airy isostatic model can be redescribed as a compensation of positive and negative vertical dipole moments of local density distribution. The lithosphere thickens continentalwards from 60km±in the Indian Ocean to 110-130 km, locally reaching 140 km, in the central part of the Qinghai-Xizang plateau. The Himalayan collision zone is a transitional zone of deep structure and geophysical anomalies between the Indian and the Tibetan plates, and the collision mechanism belongs to a type of subdueting (northwards)/thrusting (southwards) action.