摘要
深反射地震被地学界公认为是探测岩石圈精细结构、板块俯冲、壳内推覆、壳/幔相互作用等深部地质问题最有效的技术手段之一。本文首先对两条深反射地震数据进行了认真的分析和处理,然后依据处理所得的两条深反射地震叠加剖面(CJ01和CJ02),同时结合区域构造演化规律,详细而全面地分析了剖面所揭示的结构及构造演化特征,为长江中下游(下扬子)地区的深部结构及动力学演化的认识提供了可靠而详实的来自深反射地震的证据。本文研究表明:1共法线角Kirchhoff叠前时间偏移方法可以扩大并聚焦地下面元的反射能量及获得地下波阻抗界面的倾角范围,是解决复杂构造区深反射地震偏移成像的关键,也是解释环节的所需。尽管使用该方法做了一些实验工作,也取得了明显的效果,但对本次实际数据的应用效果不是很好,笔者将在接下来的工作时间里继续完善这一很有潜力的偏移方法,以期实现一个能够实用的适应复杂构造区和低信噪比数据的新偏移方法。2信噪比低、地下结构复杂多变及静校正严重是深反射地震数据处理的主要问题,为了保证处理结果能够真实地反映地下地质结构特征,在资料处理过程中,采用了处理-解释-处理紧密结合的方式,即在取得每一步的成果后,结合对本区地质构造演化的认识,重新调整处理流程和参数;每一步主要处理步骤均先反复试验,取得效果后再进行实际应用,最终取得了令人满意的信噪比高且同相轴连续性好的叠加剖面。3通过对本次处理所得的两条深反射地震剖面的解释,取得的关于长江中下游及周缘地区的主要地质认识如下:(1)在南北向挤压应力场作用下,扬子板块以鳄鱼嘴式向华北板块之下俯冲,同时造成下扬子区中下地壳发生拆离。(2)联合剖面结果显示,在下扬子及江南造山带下部存在一巨型逆冲推覆构造体系,控制着该区的构造格架,并且应力场特征自北西向南东逐级增强。(3)通过本次深反射地震剖面大体圈闭了岩浆房在深部的展布样式,并详细阐明了壳/幔作用过程及地幔岩浆大面积侵入及造成其上地表展布大面积火成岩的原因。(4)密集式的叠瓦状壳/幔边界反射波组可能是华夏板块的俯冲板片残余或扬子板块在挤压应力场作用下发生错断的反映。(5)郯庐断裂带是一个碰撞期后而非同碰撞期的产物,且穿过了莫霍面,具走滑性;郯庐断裂带对大别,尤其是长江中下游地区的岩浆活动有明显的控制作用;郯庐断裂带对周缘构造格架影响有限。(6)长江断裂带及滁河断裂带的八字型左撇是存在的。
Deep seismic reflection is one if the most effective techniques in probing deep geological problems, such as fine structure of the lithosphere, plate subduction, thrusting nappe within crust, crust-mantle interaction, etc, which has been widely accepted by earth scientists. In this paper, Two lines of profile(CJ01and CJ02) was yield out after a careful analyzing and processing of deep seismic reflection data while take consideration of the rule of local region tectonic evolution. Based on that result, a detailed and comprehensive analysis of the structure and tectonic evolution of the cross-section was revealed, which provide reliable and detailed evidence in understanding the deep structure and dynamic evolution of the Yangtze River (Lower Yangtze) region. The study shows that:1Normal angle of Kirchhoff pre-stack time migration is the key to solve complex tectonic zone using deep reflection seismic migration imaging technology, which can expand and focuses the reflected energy of underground bin and get the inclination range of impedance interface underground, is also required in seismic interpretation procedure. Although some experimental work has been done and some significant results has been achieved in using this method, however, Application to those actual seismic data is not very satisfactory, I will continue to refine this promising migration methods in the next work term, in order to realize a new practical method which could to adapt to complex structure and apply to low SNR (signal to noise ratio) seismic data.2The main problems of deep seismic reflection data processing including: low signal to noise ratio of seismic data, complexity of underground structures and hard seismic statics corrections. In order to ensure that the results could truly reflected the geology structure underground, An tightly integrated way of processing-interpretation-reprocessing was used in the data processing procedure, that means every step of the obtained results would do a re-adjustment and re-pick of parameters combined with the local region geological tectonic evolution during processes of the seismic data; Each step of main processing was first tested, then practical application would be applied after credible results obtained, finally the satisfactory stacked profile was achieved with high SNR and good continuity of phase axis. 3Based on the interpretation of two deep seismic reflection profiles obtained by innovative approach mentioned above, the main geological understanding of the Yangtze River and related area around as follows:(1) The Yangtze plate is subducting beneath the North China plate in the way of crocodile-type under the action of north-south compressional stress filed, causing the middle and lower part of Yangtze crust detachment occurs.(2) Combined profiles showed that there is a huge overthrust tectonic system controls the structural framework of lower Yangtze and Jiangnan orogenic belt, and the characteristics of stress field progressively enhanced from northwest to south.(3) The distribution pattern of deep magma chamber is generally determine while a detailed description of the crust/mantle interaction processes and large-scale mantle magma intrusion is the reason which caused a large distribution of igneous rocks on the surface of studied area based on these deep seismic reflection profiles.(4) Intensive imbricated reflected wave group from crust/mantle boundary could be reflection of Chinese plate subducting slab remnants or Dislocation of Yangtze plate under the extrusion stress field.(5) Tan-Lu fault is a tectonic structure formed after the collision rather than duration of the collision. Which has a nature of strike-slip kind. This cult also clearly in control of the magmatic activity of Dabie area, especially in the Yangtze River region.it possibly plays a limited impact on tectonic framework around related area.(6) The inverted splay and left-handed character type is present in the Yangtze River fault zone and Chuhe fault zone.