基于最小二乘配置在椭球面上解算喜马拉雅地区GPS应变分布

GPS strain field calculation of Himalayan area using least square collocation in ellipsoidal surface

随着空间大地测量技术不断发展,GPS观测的地壳水平形变速度场精度也在不断提高,更加严密的GPS应变分析模型将有助于促进更高精度的地壳运动模型的构建.大地线长度与对应球面弧长之间的差异与纬度、经度变化均有关,并且与纬度变化影响最为显著,纬度越低,相应的椭球面效应约显著.本文在最小二乘配置模型的基础上进一步研究并推导了基于椭球坐标系的GPS应变分析模型,通过该模型进一步计算了青藏高原南部喜马拉雅构造带及阿萨姆构造结地区现今GPS应变分布.最大、最小主应变的显示喜马拉雅山脉中部的南北向压缩变形最强,西部次之,东部最弱.在印度板块的俯冲推挤作用下,喜马拉雅构造带内部地壳的变形过程并不统一.本文研究发现雅鲁藏布江缝合带与亚东—古鲁断裂带是该区域地壳水平形变的两条重要分界构造,雅鲁藏布江缝合带南部、亚东—古鲁断裂西侧的条带状地区可能是青藏南部吸收来自印度板块俯冲挤压作用的主要区域,最大剪应变分布及面膨胀值分布均表明亚东—古鲁断裂带是喜马拉雅构造带东西向拉伸变形过程中的一条重要的分界构造.沿喜马拉雅构造带分布的地壳剧烈变形区域集中分布在断裂以西,向东跨过该断裂的GPS应变场大幅减弱.青藏高原东南缘以阿萨姆构造结为中心的顺时针旋转变形存在旋转内、外圈层速度不一致现象,旋转速率由内向外逐渐增强.

With the continuous development of space geodesy technology,GPS observations＇ accuracy of crustal deformation has been increased. A more completed GPS model will help to promote the construction of a more sophisticated model of crustal movement and reveal more accurate geological phenomenons. The difference between length of a geodetic line and spherical arc is both associated with changes in latitude and longitude. Besides,the influence of the latitude variation is more obvious than longitudes,the lower the latitude is,the corresponding ellipsoid effect more significantly. A GPS strain model of ellipsoidal coordinate system based on the least square collocation method was deduced in this study. The GPS strain field of Himalayan and Assam area in the southern Tibet was calculated according to the method we deduced in this study. Max and min strain rate distribution shows that the northern-southern crustal deformation strength is not uniform across the central Himalayas. The central part of the Himalaya mountain belt suffers the most serious deformation,then the western part and the area east of Yadong-Guru fault zone. Such characteristics of local crustal deformation in Himalayas indicate that under the subduction of the India plate,the lithosphere of the study area deforms not uniformly.The surface inflation rate and maximum shear strain distribution reveal that both the Yaluzangbu suture belt and Yadong-Guru fault zone are two remarkable tectonic boundary in the study area. A banded region south of Yaluzangbu suture belt and west of YadongGuru fault zone might be the main region which absorbs the subduction of Indian plate. Yadong-Guru fault zone is another remarkable tectonic boundary belt in the process of E-W tensile deformation of Tibet according to distributions both of the surface inflation rate and the maximum shear strain rate. The local severe deformation zone along the Himalaya is concentrated in the west area of Yadong-Guru fault and there is an obvious attenuation of GPS strain rate when it eastward through the fault zone. According to this study,the rates of rotational deformation in the Assam area change regularly,it gradually increases from the geometrical center of the Assam to its outer layers.