Moho Depth And Variation Of The Continent In China And Its Geodynamic Implications

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sample points were obtained; Moho depth of 687 broadband seismic stations and over 40 deep seismic profiles including 9 unpublished have been collected.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.