Abstract:

The research area involved in this paper is the middle-southern segment of Liaocheng-Lankao fault zone(Lanliao fault zone)and its adjacent area. In order to study the fine crustal structure image and the tectonic features of the faults in this tectonic zone, we conducted a 70km-long deep seismic reflection profile along EW direction in Puyang City, Henan Province and got clearer lithospheric structure image along the profile.

As regards data acquisition, we applied the geometry with 30m group interval, 1 160 recording channels and more than 90 folds. Seismic wave exploding applies the 30kg shots of dynamite source with the hole depth of 40~50m. In addition, in order to ensure the signal-to-noise ratio of the deep reflector, explosive quantity of dynamite source is increased to 96kg every 1 000m interval. In data processing, the most important thing is to improve the signal-to-noise ratio. Data processing methods mainly include one-dimensional time-varying filtering combined with two-dimensional filtering, tomographic static correction, residual static correction, deconvolution, normal moveout correction(NMO), dip moveout correction, common mid-point(CMP)stack and post-stack denoising, post-stack migration, etc.

The section with high signal-to-noise ratio has been obtained. There are obvious characteristics of reflection wave groups in the crust, which reflects abundant information about geological structure. On this section, according to this study, the characteristics of deep and shallow structure and crustal reflection structures on both sides of the Lanliao fault zone are obviously different. The crust in this area is composed of brittle upper crust and ductile lower crust. There are rich reflective layers and clear tectonic framework in the upper crust. In the western area of Lanliao fault zone, there is a set of dense reflectors with strong energy, which reflects the sedimentary interface of different times since Mesozoic in the basin. The basement slope with gentle dip to the east is the bottom boundary of the “dustpan-shaped” sedimentary depression. The reflected wave of the crystalline basement presents a group of strong reflection wave groups with good continuity in the eastern area of Lanliao fault zone, which are parallel unconformities on the Ordovician strata of Paleozoic or older strata. There are some secondary faults in the hanging wall of Lanliao Fault, which together with the Lanliao fault zone control the tectonic framework of “dustpan-shaped” sedimentary depression, the Dongpu sag. The reflection structure of the lower crust is relatively simple. On the whole, it is mainly arc reflection with strong energy and short duration.

The depth of Moho surface beneath the central-southern Lanliao fault zone in this area is 31.7~34.8km, where the fault is characterized by a strong reflection band with piecewise continuous distribution in horizontal direction and a duration of about 0.3~0.8s in vertical direction. And it is a transition zone with a certain thickness after geological deformation, rather than a sharp first-order discontinuity, which is consistent with the research results of Li Songlin et al.(2011). This profile reveals 2 deep faults(F_D1 and F_D2)that offset the Moho surface, extend down to the top of the upper mantle and create conditions for the upwelling of hot materials from asthenosphere and the energy exchange in this area. It may also be the cause of arc reflection in the lower crust.

The deep seismic reflection profile shows that faults in the upper crust are well developed. Lanliao Fault is the largest boundary fault in this area, which controls the formation and evolution of the “dustpan-shaped” sedimentary depression and plays an important role in the filling of Paleogene strata in the sag. Pucheng Fault F_P1 and Weixi Fault F_P3 are developed in the hanging wall of Lanliao Fault, which are basement normal faults in the same direction as Lanliao Fault and control the structural framework of the depression. Pucheng Fault, Weixi Fault and Lanliao Fault constitute a domino fault system, which makes the basement of the depression incline to the SEE direction. In addition, a reverse secondary normal fault(Changyuan Fault F_P2)is developed in the hanging wall of Lanliao Fault, which intersects with Weixi Fault F_P3 at TWT 3.0s. These faults and Lanliao faults jointly control the basic structural pattern of the sedimentary sag.

The deep and shallow tectonic framework in this area is controlled by the shallow faults in the upper crust and the deep faults in the lower crust. Deep faults(F_D1 and F_D2)create conditions for the upwelling of hot materials from asthenosphere, while shallow faults play an important role in the formation and evolution of basin structures.

Key words: Dongpu Depression, deep seismic reflection profile, first-arrival tomography, crustal fine structure, deep and shallow tectonic features

摘要：

长70km、 近EW向穿过兰聊断裂带中南段及其邻区的深地震反射剖面, 揭示了该区地壳的精细结构和断裂的深、 浅构造特征, 对深入了解该区的深部地震构造环境、 探讨东濮凹陷的深部动力学过程及其构造演化都起着十分关键的作用。研究结果表明, 兰聊断裂带的深、 浅部构造特征及其两侧的地壳反射结构特征区别较大, 该区地壳由脆性的上地壳和韧性的下地壳组成。上地壳反射波组特征明显, 构造形态清晰可辨: 兰聊断裂以西存在大量向E倾斜的强反射同相轴, 代表了中生代以来不同时代的沉积界面, 其中一套起伏变化形态向E缓倾的基底斜坡是箕状沉积凹陷的底界; 兰聊断裂以东, 基底反射波表现为一组近水平的连续性较好的强反射波组, 平行不整合于古生界奥陶系或更古老的地层之上; 在兰聊断裂上盘还发育有同向及反向次级断裂, 共同控制了箕状沉积盆地东濮凹陷的构造格局。下地壳反射结构较为简单, 整体上以弧状反射为主, 其能量较强、 延续长度较短。莫霍面表现为横向上分段连续、 纵向上持续一定时间的强反射条带, 在兰聊断裂下方的莫霍面反射波组能量明显减弱, 与两侧反射特征截然不同。剖面揭示了2条错断莫霍面的深大断裂(F_D1 和F_D2), 向下延入上地幔顶部, 为本区上地幔软流物质的上涌与能量交换创造了条件, 也可能是下地壳弧状反射产生的原因。

关键词: 东濮凹陷, 深地震反射剖面, 初至波层析成像, 地壳精细结构, 深浅构造特征