华北盆地滑脱构造的地震学证据

SEISMIC EVIDENCE OF DETACHMENT IN NORTH CHINA BASIN

在华北盆地中西部的临城一巨鹿深地震反射剖面上，ＣＤＰ叠加剖面在双程走时２．５-４．５ｓ部分显示出一系列连续性好、能量强的低角度反射事件，在剖面上显示的延伸距离达４０ｋｍ．这些反射事件解释为滑脱断层，它自西向东倾角变缓，发育在深度为ｓｋｉｎ（剖面西端）至１０ｋｍ（东端）的结晶基岩中．在剖面的浅部显示出两个相似的单边断陷盆地，其主断裂以铲形归并到这一滑脱构造．上盘在伸展过程中向东滑移，拉张和重力滑动作用可能是形成这些断裂的主要原因．剖面东部的中下地壳内众多的反射事件具有叠层状的特征，并遭受强烈的变形，可能表示地幔物质上涌侵入至地壳内．岩浆侵入在地壳上部形成附加的伸展应力场，同时使下地壳增热，粘度下降，某些矿物发生脱水作用，脱出的水上移并储存于中地壳内．伸展应力场及热和水的作用促进滑脱构造的形成．邢台地震的震源深度分布表明，这一地区脆韧过渡带的深度为１０-２５ｋｍ，滑脱面为过渡带的上界面．

On the Lincheng-Julu deep seismic reflection profile in west-central North China basin,a series of low-angle reflection events endowed with good continuity and strong energy are illustrated with two-way traveling time(TWT) from 2.5s to4.5s in the CDP stacking profile,and it stretches to 40k.These reflection eventsare interpreted as a detachment,whose dip decreases from west to east,and develope in the crystalline basement from 5km deep at the western end of the profile to 10km deep in the eastern part.Two similar one-sided faulted basins are shown in the shallow part of the profile.The major boundary faults of the basins merge into the detachment.The upper plate slipped eastward during the extension,the pullapart and gravity slip are possibly the main causes to generate the shallow faults.Alot of reflection events of mid-lower crust,having borne strong deformation.are characterized as lamination in the eastern part of the profile,which may indicate that mantle material upwell into the crust.Magma intrusion generates an additional extensional stress field in upper crust,and makes lower crust hot and its viscosity lower,resulting in the dehydration of certain minerals.The dehydrated water transports upward and deposits in the middle crust.This extensional stress fieldand the compound action of the thermal and water impels the decoupling of upper and lower crust and the formation of the detachment.The focal depth distributionof Xingtai earthquakes shows that the retion of brittle-ductile transition in the study area is the depth 10-25km,and the detachment is the upper boundary of the transition.