郯庐断裂带中南段的岩石圈精细结构

郯庐断裂带是中国东部规模最大的构造活动带,有着复杂的形成演化历史,对中国东部的区域构造、岩浆活动、矿产资源的形成和分布以及现代地震活动都有重要控制作用.2010年在郯庐断裂带中南段的江苏宿迁市附近,采用深地震反射探测方法对郯庐断裂带及其两侧地块的岩石圈结构进行了解剖.结果表明,该区莫霍面和岩石圈底界均向西倾,其中,地壳厚度约为31~36km,岩石圈厚度约为75~86km,且岩石圈厚度在郯庐断裂带下方出现突变.郯庐断裂带在剖面上表现为由多条主干断裂组成的花状构造,其内部发育有断陷盆地和挤压褶皱,具有伸展、挤压和走滑并存的构造形迹,暗示郯庐断裂带的形成和演化经历了多期复杂的构造活动.这一断裂带错断了近地表沉积层,向下切割莫霍面和岩石圈地幔,属岩石圈尺度的深大断裂构造系统.软流圈高温高压热物质沿断裂带的上涌、岩浆底侵或热侵蚀作用造成岩石圈出现拉张伸展和岩石圈减薄,并可能使岩石圈组构及其物质成分发生改变.本项研究结果不但可进一步加深对郯庐断裂带深、浅部结构的认识,而且还可为分析研究华北克拉通东部的深部过程和浅部构造响应提供资料约束.

Fine lithosphere structure beneath the middle-southern segment of the Tan-Lu fault zone

The Tan-Lu Fault Zone (TLFZ) is the largest active tectonic belt in the eastern China. To study the fine lithospheric structure, faults' tectonic patterns and deep-shallow tectonic relations of TLFZ, we constructed the 100 km-long deep seismic reflection profiling in the middle-southern segment of TLFZ (near Suqian City in Jiangsu Province) in 2010, and then got fine lithospheric structure images and faults' deep-shallow tectonic features of TLFZ and its both sides.#br#For deep seismic reflection data acquisition, a receiver interval of 25 m, 720 receiving channels and 60-fold spread geometry were used. To get reflection wave information of interfaces from different depths, we apply large-small combination shots in seismic wave explosion, among which the small shot spacing is 150 m, the drill hole depth is 25~30 m and the dynamite is 24 kg. The large shot spacing is 750 m, the drill hole depth is 50 m and the dynamite is 200~300 kg.#br#According to the complex seismic wave field in the raw data, we lay more considerations on keeping effective reflector and improving the signal/noise ratio of data during the data processing. Methods mainly include surface consistent amplitude compensation, tomographic static correction, time-varying band-pass filter and 2-D filter, surface consistent deconvolution, velocity analysis, several iterations of the residual static correction and post-stack eliminating noise. While calculating the average velocity distribution of the lithosphere along the profile, we refer to the velocity structure information of deep seismic wide-angle reflection/refraction profile in the research area.#br#The research results show that along the profile the crustal thickness is about 31~36 km and the lithospheric thickness is about 75~86 km, where crustal and lithospheric thickness there is a mutation under the TLFZ fault zone. The TLFZ cuts the whole crust and the lithosphere mantle, and there are the different tectonic features in the crust and lithospheric mantle. In the crust, the TLFZ is a large "flower-shaped" structure by consisting of multiple major-fault. while in the lithospheric mantle, the TLFZ shows as the upper mantle tectonic zone being composed of two steep dip-angle faults, which is wide above and narrow below with the width being about 15 km at the bottom of the crust, while being about 6~8 km in the lithospheric mantle. In addition, two deep crustal faults are also found on the west margin of the Sulu orogenic belt on the eastern side of TLFZ, which cut the crustal structure beneath the layer of about 4.5 km and combine into a fault system with the TLFZ at the bottom of crust, about the depth of 30 km.#br#As the boundary tectonic zone between the North China and Yangtze blocks, the TLFZ is with complicated formation and evolution processes. Inside the zone there are a series of compression folds, flower-shaped structures, fault basin and its boundary normal faults which indicate the zone has experienced a sophisticated formation and evolution processes like strike-slip, compression and stretch. From the shallow to the deep, this fault zone cuts near-surface sediment, upper crust and lower crust, the Moho interface and lithospheric mantle, of which transforms from brittle, brittle-ductile to ductile with increasing depth. Hot material from asthenosphere upwelling to lithosphere along the fault zone, magmatic under-plating or thermal erosion leads to the lithospheric extension and thinning, which probably causes changes in lithospheric structure and its substance composition. The results of this paper and other studies all indicate that the TLFZ is a large fault tectonic belt in the lithospheric scale and a mechanically weak zone, which plays an important role during the lithospheric destruction and thinning in the eastern of the North China Craton.