喜马拉雅中东部镜像均衡重力异常的形成研究

Formation of the mirrored symmetry of isostatic gravity anomalies in central and eastern Himalayas

青藏高原南缘处于重力不均衡状态，由北向南可依次分为高原近重力均衡区、喜马拉雅山正均衡异常区和山前盆地负均衡异常区，正、负异常呈现壮观的镜像分布。本文选取喜马拉雅中东部的均衡重力异常数据，结合地貌高程、地壳厚度、降雨量、冰川及山前沉积等的分布状况，探讨地貌分异与均衡重力异常分布的相互关系。由上述资料获得3条跨越喜马拉雅山的综合剖面，结果显示喜马拉雅中东部正均衡重力异常的分布与冰川、河流等代表的地表剥蚀作用存在明显的空间耦合关系，而与降雨量无直接联系；山前盆地负均衡重力异常与沉积厚度的分布也存在很好的耦合。利用数值模型计算得到了喜马拉雅地区的均衡调整时间域在1Ma左右的时间尺度内。通过与地貌响应时间域相对比，以及对地表剥蚀厚度的估计，认为山脉地区的正均衡异常主要由地壳厚度补偿不足引起（侧重Airy假说），而山前盆地的负均衡异常主要由低密度沉积层的分布引起（侧重Pratt假说）；由于地貌响应时间快于均衡调整时间，在大约5～2Ma以来，地壳的均衡调整始终延迟于山脉的持续剥蚀和山前的持续沉积，使得岩石圈朝着“反均衡”方向演变，最终形成了喜马拉雅现今壮观的镜像均衡重力异常分布。

The isostatic gravity anomalies（IGA） on the southern margin of the Tibetan Plateau can be divided in to the quasi-isostasy zone of the plateau, positive IGA zone of the Himalayas and negative IGA zone of the foreland basins, which two forms the spectacular mirrored symmetry on gravity anomaly maps. This paper chooses the IGA data in central and eastern Himalayas and its foreland basins, together with some fundamental geological data on the relief, crustal thickness, precipitation, glacier and sedimentary thickness, to discuss and understand the relationship between the formation of the isostatic gravity anomalies and morphology differentiation. Profile analysis shows that the positive isostatic anomalies in the Himalayas are clearly coupled with regions that are strongly denuded by glacier and rivers, but have no direct relationship with the distribution of precipitation. Also, the negative isostatic anomalies in the foreland basins are clearly coupled with the Cenozoic sedimentary thicknesses, especially with the thicknesses deposited since mid-Miocene. Estimation of the denudation thicknesses indicates that the positive isostatic anomalies in the Himalayas are mainly caused by the under- compensated crust beneath the mountains （the Airy model）, while the negative isostatic anomalies are mainly caused by the distribution of the Cenozoic low density deposition （the Pratt model）. Numerical calculation is then carried out to get the isostatic adjustment time which is on the time scale about 1 Ma for the Himalayas. The comparison result with the landscape response time indicates that just because the morphology evolution is faster than isostatic adjustment, the crust＇s adjustment couldn＇t response the continuous denudation in the ranges and the continuous deposition in the basins to form a ＂mountain root＂, which result in the ＂anti-isostasy＂ movement of the lithosphere to form the spectacular distribution of the isostatic gravity anomalies since about 2.5 Ma on the southern margin of the Tibetan Plateau.