摘要
莫霍面是地壳与地幔的分界面,是由地壳步入地幔的一个边界层与重力补偿面,是壳幔物质与能量交换的动力边界,记录了作为岩石圈重要组成部分的地壳的演化历史,对地壳与地幔的形成演化以及深层动力过程的研究均有着重要作用。对于莫霍面的探测研究,多借助于地质学及地球物理的方法来揭露,在这些方法当中,地震探测方法分辨率较高,是人们探测地壳地幔结构的主要手段。我国自上个世纪50年代就开始了针对莫霍面的深地震探测,并利用积累的资料编制了中国大陆莫霍面图和东亚莫霍面深度图等,揭示了中国大陆及邻区的深部构造格局。近年来,随着深地震探测资料,特别是深地震反射剖面的快速积累,在全球及我国大陆一些莫霍面变化的更细致的特征被揭露出来。全球研究表明这些变化记录了地壳演化。总结和学习,以及深入研究这些新资料,为我们深入研究大陆地壳的变形及其地球动力学奠定了基础。本文首先系统总结回顾了用于深部探测的地震学方法和国内外莫霍面的研究现状,并对中国大陆已有及新采集的深部地震探测成果进行了收集整理分析:对其中84条深地震测深剖面莫霍面深度沿线按照20-30 km间隔进行采样,获得了2298个深地震测深莫霍面深度采样点;收集了687个宽频地震台站的莫霍面深度值,以及包括9条未公开发表的40余条深地震反射剖面,在此基础上,绘制了中国大陆莫霍面深度图,以此资料为基础,结合其他地质与地球物理资料研究了中国大陆莫霍面深度和变化与地壳变形关系,及其反应的地球动力学过程。本文研究表明:1中国大陆莫霍面构造格架与中国大地构造地貌有强烈的对应关系,呈现较为明显的两横两竖的“视井状”特征,其中两横为天山-阴山-燕山带和西昆仑-阿尔金-祁连-秦岭-大别山带,两竖为大兴安岭-太行山-武陵山重力梯度带和贺兰山-六盘山-龙门山和小江断裂带。两横一般表现为莫霍面向造山带下加深,而两竖都是莫霍面陡变带。另外,郯庐断裂带虽然在不同区段莫霍面形态表现不一,但莫霍面总体呈上隆状态,尤其在渤海湾-下辽河-依兰伊通地堑内强烈上隆,对整个中国大陆东部的地壳变形起到了重要作用。2通过新的地震探测资料,深入研究了莫霍面深度变化的细致特征,获得了一些新发现和新认识:1)四川盆地内部西部和东部的莫霍面显现出较大的差别,川西莫霍面较为平整,往东即将进入华蓥山山前断裂带时,莫霍面加深和错断,说明四川盆地并非一个完整统一的刚性陆块,川西块体刚性更强一些,在青藏高原造山带向东挤压过程中可能起到了应力传递的作用,造成了川东莫霍面的加深和错断。2)在青藏高原造山带内部,班公湖-怒江缝合带两侧存在明显的莫霍面错断现象,说明该带新生代以来再次活化,并发生了侧向滑移。同时,羌塘地体莫霍面相对较浅,下地壳并呈层状过渡带形式出现,可能暗示了印度同欧亚两大板块的俯冲碰撞前缘位于羌塘地体内部,从而导致下地壳拆沉及地幔岩浆上涌,并形成了新的较浅的莫霍面。3)青藏高原东北缘松潘-甘孜地块及西秦岭造山带下莫霍面较平,说明在西秦岭造山带下曾经历了强烈的伸展作用,同时下地壳存在倾斜叠置现象,不支持青藏高东北缘可能存在管道流的猜想。4)银川地堑沉降中心下莫霍面没有上隆,反而加深;同时,在银川地堑东缘,黄河断裂带是一个岩石圈尺度的剪切走滑断裂带,切断地壳,并控制了贺兰山山前断裂带,表明了银川地堑是新生代以来受印度同欧亚板块碰撞影响而形成的被动裂谷,并具有拉分盆地特征。5)依兰伊通断裂带莫霍面明显上隆,显示郯庐断裂带北段整个处于一条北东向的伸展减薄带内,说明沿着该带发育的油气盆地受控于同一伸展构造。6)燕山造山带和大别造山带下具有弱山根,没有完全被减薄展平。3莫霍面记录了盆地、造山带及断裂带等不同属性构造单元的地壳演化:渤海湾盆地等主动裂谷下莫霍面强烈上隆,被动裂谷下莫霍面存在加深、展平和上隆等不同现象,如银川地堑、贝加尔地堑和莱茵地堑;造山带的生长发育可宏观分为明显山根,弱山根和无山根三个阶段,每一个阶段可以简单对应于台湾造山带、大别造山带和西秦岭造山带。大型走滑断裂带和古老缝合带内主断裂带下一般都存在莫霍面错断,是壳幔物质相互交换的场所,以调节地壳伸展或缩短。4按照中国大陆莫霍面格架,结合其他地质及地球物理资料,可以把中国大陆分为三个动力学体系:西部体系、中部体系和东部体系。其中西部体系是指贺兰山-龙门山南北带以西,受新生代以来印度同欧亚两大板块碰撞影响,主要表现为青藏高原周缘挤压、地壳增厚,腹地伸展、地壳减薄,以及天山造山带复活等;中部体系夹持于南北带和大兴安岭-太行山-武陵山梯度带之间,地壳相对稳定,莫霍面变化相对较小,处于挤压向伸展转换的过渡带,是东西两大构造域的缓冲地带;东部体系则指梯度带以东地区,主要受濒太平洋构造域运动影响,强烈的伸展减薄作用改造了整个地区的莫霍面深度及特征。华北-东北地区的地壳减薄模式可能同美国内华达山-盆岭省相似,地幔岩浆上涌底侵华北及松辽盆地下地壳,造成盆地与两侧大兴安岭-太行山隆起和-小兴安岭-辽东-鲁西隆起带下地壳及岩石圈地幔发生拆沉,造成了地壳减薄。新生代以后,欧亚板块东缘的华南大陆向菲律宾板块下俯冲,形成了台湾中央造山带下巨厚的山根,可能同中生代两者的俯冲方向相反。5通过地震震区开展的深地震反射剖面的对比,发现在这些震区下方都发育有超壳断裂带,可能控制了深部孕震构造。
Moho, the boundary between crust and mantle, is a gravity compensation area and a dynamic boundary for exchange of material and energy. Thus, it records the evolution of the crust, as the main part of the lithosphere, and is important for studying the formation and deep dynamic process of crust and mantle. Geophysical methods are mostly used to conduct the detection study of the Moho. Among these methods, the seismic probing methods are of high resolution, and therefore main techniques to probing the structure of the crust and mantle.Since 1950s, the scientists have began to implement the deep seismic probings of the Moho, and plotted the map of Moho depth of China and East Asia, which revealed the deep structure of China and adjacent regions. With the quike accumulation of the deep seismic probings, especially the deep seismic profiles, there are more and more subtle Moho viaration has been detected. The global Moho researchs imply the Moho viaration record the crust evolution. From the review and deep research of these new profiles, we can research the crustal deformation and geodynamics in-depth.This thesis systematically reviews the seismic methods for deep probing and the current studies on Moho at home and abroad; synthesizes the published and newly obtained deep probing data in the continent in China. Among them, Moho depth along 84 deep sesmic sounding (DSS) profiles has been sampled per 20-30 km,2298 DSS sample points were obtained; Moho depth of 687 broadband seismic stations and over 40 deep seismic profiles including 9 unpublished have been collected. Then, we ploted the Moho depth map of the continent in China(1:5000000), furthermore, Combined with other geological and geophysical evidences, we discussed the basic character of the Moho depth, and the corresponding relationship between the crustal deformation and the Moho depth and variation, finally we discussed the geodynamic implications revealed Moho character.The study of this thesis suggests:1 Moho framework of the continent in China is strongly correlated with China's tectonics and topography, expressed in two horizontal and two vertical striples. Two horizontals striples are Tianshan-Yinshan-Yanshan orogeny and Altyn-Qilian-Dabie range, while two vertical ones are Daxing' anling-Taihangshan-Wulingshan gravity gradient zone and Helanshan-Liupanshan-Longmenshan range. Under the two horizontals, the Moho is generally deepened towards range, while the two verticals both represent sharp change zone of Moho. Besides, under the Tan-Lu fault belts, especially Bohai Bay, Liaohe basin and Yilan-Yitong garben, the Moho generally uplifts and plays an important role in the crustal deformation in eastern China.2 From the comparition and research of deep seismic profiles obtained newly, we discovered some new phenomenon, and reach new conclusions as follows:a The Moho in eastern and western Sichuan Basin is discrepant. It is somewhat flat in the western part while deeper and offset exist in the eastern part. Such discrepancy may imply that the Sichuan basin is not a homogeneous rigid block. The western part may be rather rigid and is a stress transfer area under the compression of the Tibet orogeny, which further caused the Moho deepening and offset.b In the heartland of Tibet, ca.4 s Moho reflection difference crossed the Bangonghu-Nujiang suture belt. In the Qiangtang block (QB), the Moho is relatively shallower and the lower crust has layered transition zone, which indicate the frontier of the subduction between Indian and Eurasian continent may be located beneath the QB, then resulted in lower crustal delamination, magma upwelling, and formed a new and shallower Moho.c The Moho is relatively shallower and flat under the Songpan-Ganzi block and western Qinlin orogeny in northeastern Qinghai-Tibet plateau, indicating the western Qinlin orogeny had undergone a strong extensional event; the incline duplication reflection in the lower crust disprove the channel flow in northeastern Qinghai-Tibet plateau.d There is no Moho uplift under the subsidence center of The Yinchuan graben, but Moho depression, and Huanghe fault in the eastern margin of Yinchuan graben intersects the whole crust and controls Helan piedomont fault to the west. These evidence indicate that Yinchuan graben is probably a passive rift formed by the collision of Indian and Eurasian continent and also has pull-apart basin characteristics.e Under the Fangzheng rift in the Yilan-Yitong fault belt, the Moho significantly uplift, indicating the northern segment of Tan-Lu fault belt is located in a NE-direction thinning zone, and the oil and gas basin in this zone is controlled by the same extensional tectonics.f The Yanshan Orogeny and Dabie Orogeny have weak roots, which reflect no complete thinning and flattening.3 The Moho records the crustal evolution of tectonic units with different attributes, such as basin, orogeny, fault etc. The Moho usually uplift under the positive rift basin, for instance in the Bohai bay, but shows different Moho undulation under passive rift, for instance lifted Moho under Laiyin rift, flat Moho under Baikall rift, and depressed Moho under Yinchuan rift. The development of orogen in plate margin possibly experiences three stages "giant root-weak root-no root", each stage can be simply corresponded to Taiwan orogeny, Dabie orogeny, Qinlin Orogeny. Moho offset often exists under large-scale strike-slip faults and suture zones, which accommodate the crustal shortening or stretching.4 According to the Moho framework of China as well as other geological evidences, the continent in China could be divided into three geodynamic domains:western domain, middle domain and eastern domain. The western one lies to the west of the Helanshan-Longmengshan range and has been affected by the collision of Indian and Eurasian paltes. The main character is crustal thickening in the marigin of Tibet plateau, crustal thinning in the interior of the plateau and revival of Tianshan orogeny. The middle zone lies between Helanshan-Longmenshan zone and Daxinanlin-Taihangshan-Wulingshan gradient zone. This zone has stable crust and minor Moho variation relatively, represent a buffer zone between western and eastern zones and a transfer zone from compression to extension. The eastern zone is located to the east of the gradient zone, mainly affected by the peri-Pacific tectonic domain. The strong extensional and thinning event have changed the Moho depth and character in the entire eastern zone. Specifically, the crustal thinning in north China and northeastern China is possibly resemble to the thinning model in Sierra Neveda and basin-range province:the magma underplated the lower crust in North China and Songliao Basin and resulted in the delimination of the lower crust and lithosphere mantle of the Daxinanlin-Taihangshan and Xiaoxinanlin-Liaodong-Luxi uplift; South China continent, as the southeastern part of the Eurasian plate, is subducting beneath the Philippines plate, and caused the formation of Taiwan orogenic belt and its giant root, this character maybe imply a shift of subduction direction from Mesozoic to Cenozoic age.5 through comparison of deep seismic profile in the earthquake zones, ultracrust fault are confirmed exsited, which maybe control the earthquake-preparing structure.