摘要
由于缺乏足够的青藏高原内部观测的地震数据,难以对地壳上地幔构造进行准确的地震体波三维层析成像。在中国西部设有412个地方地震台站,2005～2008年观测仪器全部实现了数字化,为解决地震体波层析成像的观测数据问题提供了新的可能性。经过收集和整理这些地方地震台2009～2015年的数字化观测数据,得到了通过青藏高原的足够地震射线和对应的走时数据,处理后在北京和美国进行了三维层析成像计算,取得了一致的结果。利用地方地震台站的数据开展青藏高原地壳地震波速度的三维层析成像研究,得到1°×1°×20 km网格的纵波三维速度结构图像,揭示了青藏高原地壳上地幔波速结构特征。三维层析成像研究结果表明,青藏高原岩石圈地幔的速度变化很不连续,与软流圈没有连续的分界面。青藏高原软流圈内局部的高速和低速异常密集分布,与克拉通地区均匀分布的模式不同。这种软流圈内异常密集分布的模式,是新生代以来特有的大规模能量交换和物质运动的反映。成像结果发现在高原软流圈底部有一个清晰和稳定的高波速异常,它是现今特提斯洋板块的反映,纬度在28°～40°N之间。根据印度板块和特提斯洋板块的推进运动速度的估计,可以把现今特提斯洋板块构造模型沿时间轴反推回50 Ma和100 Ma以前,得到反映特提斯洋板块演化和青藏高原上地幔地质作用过程的模式。此模式说明,在50 Ma前特提斯洋板块已经俯冲到青藏高原软流圈,然后慢慢地下沉到软流圈底部。根据这个新生代青藏高原上地幔地质作用过程的反推模式,对不同时期特提斯洋板块和印度板块陆—陆俯冲前沿进行了定量的定位。这个作用过程的模式对板块构造学说以往的认识有一个重要突破:不认为所有的大洋板块都会俯冲到上地幔底部,宽度较小、俯冲速度较快的特提斯洋板块可能
Objectives: In order to perform 3D seismic body-wave tomography,one needs seismic data observed on the Qinghai-Tibet Plateau. there are 412 local seismic stations that have digitized data after 2008, providing possibility for the seismic tomography. This paper presents results of 3D P-wave tomographic images of the asthenosphere under the Qinghai-Xizang Plateau for studying continental dynamics.Methods: After combining and processing the local seismic data,We select more than 260000 rays and their travel-time for the 3D seismic inversions,obtaining 3D velocity images of scale of 1°×1°×20 km.Results: Our 3D P-wave tomography results show that patterns of the velocity disturbance change greatly with depth,indicating the upper mantle velocity perturbation and structure characteristics. Different from craton areas,the asthenosphere under the Qinghai-Xizang Plateau contain many local anomalies of both high and low velocities,reflecting magnificent energy and mass motion during collision between the Eurasia-India plates. A very clear high-velocity anomaly lies on the 410 km discontinuity on the asthenosphere between 28°~ 40° N,indicating the today’s position of the Tethys Ocean Plate. Based on the north-wade movement velocity of the India plate during 60~ 10 Ma,we can look back upon past position of the Tethys Ocean Plate,therefore reconstructing the plate movement pattern around the Qinghai-Xizang Plateau. The results show that the Tethys Ocean Plate reached the asthenosphere under the Qinghai-Xizang Plateau about 50 Ma, and then slowly sink to bottom of the asthenosphere,causing upwelling of the asthenosphere and big-scale volcanism in the north part of the plateau.Conclusions: The results indicate that not all of ocean plates subducted into the bottom of the upper mantle,some of them might subducted into the bottom of the asthenosphere and stayed there for many million years.
作者
杨文采
瞿辰
任浩然
黄联捷
胥颐
于常青
刘晓宇
YANG Wencai;QU Chen;REN Haoran;HUANG Lianjie;XU Yi;YU Changqing;LIU Xiaoyu(Zhejiang University, Hangzhou, 310027, China;Institute of Geology,Chinese Academy of Geological Sciences, Beijing, 100037, China;Los Alamos National Laboratory, Los Alamos,NM 87545,USA;ChinaUniversity of Geosciences, Beijing, 100083, China)
出处
《地质论评》
CAS
CSCD
北大核心
2019年第3期521-532,共12页
Geological Review
基金
中国地质调查局资助的基础研究项目(编号:#12120113093800)的成果~~
