松潘—甘孜地块地壳和上地幔顶部现今变形模式数值模拟研究  

Numerical simulation of the present deformation pattern in the crust and top of the upper Mantle in the Songpan-Garzêblock

作　　者：万永魁 刘峡[2,3] 刘瑞丰 张扬[2] 沈小七 郑智江[2] WAN YongKui;LIU Xia;LIU RuiFeng;ZHANG Yang;SHEN XiaoQi;ZHENG ZhiJiang(Institute of Geophysics,China Earthquake Administration,Beijing 100081,China;The first Monitoring and Application Center,China Earthquake Administration,Tianjin 300180,China;Institute of Geology,China Earthquake Administration,Beijing 100029,China)

机构地区：[1]中国地震局地球物理研究所,北京100081 [2]中国地震局第一监测中心,天津300180 [3]中国地震局地质研究所,北京100029

出　　处：《地球物理学报》2022年第7期2484-2502,共19页Chinese Journal of Geophysics

基　　金：国家自然科学基金(41472180);国家重点研发计划专题(2018YFC150330506)共同资助。

摘　　要：松潘—甘孜地块至四川盆地是整个青藏高原地形梯度最大的地区,自西向东30~50 km范围内存在约4 km的显著地形高差,这可能与松潘—甘孜地块埋深20~25 km处的低阻低速层、地壳至上地幔顶部水平向运动速率随深度增加以及地幔对流拖曳力均有关联.为获得松潘—甘孜地块地壳和上地幔顶部现今变形模式,定量评价低阻低速层、中下地壳和上地幔顶部变形对川西高原隆升的影响.本文依据跨龙门山断裂带探测剖面,构建了长300 km、宽104 km的二维有限元接触模型,以1991—2016年GPS观测结果为初始约束,考虑岩石在长时间载荷蠕变作用的前提下,通过改变低阻低速层蠕变参数、模型西边界和底边界加载条件,共计开展了11项数值模拟实验.将模拟结果与综合多学科、不同时间尺度获得的地表隆升速率进行对比,各项模拟结果横向对比,对松潘—甘孜地块地壳至上地幔顶部现今变形及物质运移模式展开讨论,主要结论如下:(1)松潘—甘孜地块长期稳定、更为客观的最大隆升速率可能为1.4~1.5 mm·a^(-1);(2)低阻低速层可以作为中下地壳和上地幔物质运移的一个滑移面,构成上地壳与中下地壳的“解耦”带,促进松潘—甘孜块体整体隆升,但影响有限,不足以形成如此规模的强烈的地表抬升;(3)同时考虑低阻低速层作用及模型西边界加载速率随深度线性增加1.8倍的前提下,模拟的地表隆升速率与综合多学科、不同时间尺度获得的隆升速率最为吻合,这一动力条件下,中下地壳和上地幔物质受四川盆地强烈阻挡,速率快速减小分布于松潘—甘孜地块内部(模型50~150 km范围),松潘—甘孜地块东缘和龙门山断裂带速率减小已不显著,故地表隆升中心位于川西高原而非龙门山断裂带;(4)青藏高原东缘物质汇聚到一定程度后,会迫使部分物质向两侧运动,实现物质运移动态平衡.The largest topographic gradient area in the Tibetan Plateau is located at the region from the Songpan-Garzêblock to the Sichuan basin.A significant topographic difference of~4 km exists within the range of 30~50 km in east-west direction,which may be related to the soft layer at a depth of 20~25 km in the Songpan-Garzêblock,velocity increase of horizontal movement along with depth in the crust and the top of upper mantle and the dragging on account of the mantle convention.Based on the cross-Longmenshan fault detection profile,a two-dimensional finite element contact model in 300 km length and 104 km width is constructed,attempting to acquire the present deformation pattern of the crust and the top of upper mantle in the Songpan-Garzêblock,and quantitatively evaluate the influence of soft layer,middle-lower crust and the top of upper mantle deformation on plateau uplift.Taken the observed GPS results from 1991 to 2016 as an initial constraint and long-term load creep effect of the rock taken into consideration simultaneously,a total of 11 sets of numerical simulation tests are carried out by changing the loading conditions correlated with the west boundary,bottom boundary and soft layer.Though comparing simulation results with surface uplift rates obtained by multiple disciplines and different time scales,as well as contrasting all the results of simulations to each other,this study discusses the present deformation pattern of the crust and the upper mantle,and the model of material migration in the Songpan-Garzêblock.The main conclusions are as follows:(1)The long-term stable maximum uplift rate may be 1.4~1.5 mm·a^(-1)in the Songpan-Garzêblock.(2)Soft layer can be used as a slip surface for the material migration of the middle-lower crust and upper mantle,and forms a“decoupling”zone between the upper crust and the middle-lower crust.Soft layer promotes the vertical uplift of the Songpan-Garzêblock in a limited extend,but fails to produce such a strong surface lift.(3)Taken the affection of soft laye

关 键 词：松潘—甘孜地块 变形模式 数值模拟 接触单元 

分 类 号：P541[天文地球—构造地质学]