南海海盆区莫霍面分布规律及其对深部钻探的意义
2022,96(8):2635-2646
摘要：钻遇莫霍面是人类一直以来的梦想。深海海底是地球上离莫霍面最近的地方，目前有研究推测南海是世界上莫霍面深度最浅的海域之一，但缺乏足够的直接证据。深反射地震探测可以直接揭示岩石圈的构造形态，是莫霍面探测的重要手段。本文基于长达15000 km的深反射多道地震剖面的解释、处理、制图和分析，结合前人的研究，形成了南海海盆区莫霍面反射特征和空间分布的初步认识。① 南海东部次海盆南部早期经历了较快速扩张，岩浆供应充足，受扩张停止后岩浆活动影响较小，基底平坦，地质构造相对简单，同时洋壳地震速度结构不存在异常，且有较强的广角莫霍面反射波和可识别的地幔顶部折射波，具备莫霍面钻探的基本条件。② 南海海盆不同区域的莫霍面反射强度存在较大差异。其中东部次海盆莫霍面反射最为强烈且清晰，西北次海盆次之，西南次海盆仅有零星出现的清晰莫霍面反射且可信度不高。③ 识别南海海盆区莫霍面地震反射长度超过3500 km，首次形成了海盆区深度域莫霍面地震反射空间分布图。与重力反演的莫霍面深度相比，利用深反射多道地震计算的莫霍面深度细节更为丰富，并且可以在垂向上清晰刻画莫霍面的结构。整体上，南海海盆区莫霍面地震反射强烈和可信度高的区域中，深度较浅的区域之一是东部次海盆南部，最浅处仅约9. 5 km，其中水深4. 01 km，洋壳厚度仅5. 54 km。综合判断，东部次海盆南部是南海重要的莫霍面钻探备选区，这对南海莫霍面钻探选址具有重要意义。
关键词：南海；深反射多道地震；莫霍面；洋壳地震速度模型；钻探选址
Distribution characteristics of MohoroviAcˇiAc＇ discontinuity in the South China Sea basin and suggestions for drilling preparation area
Abstract：Drilling into the MohoroviAcˇiAc＇ discontinuity (Moho) has always been a human dream. The deep seabed is the closest place to the Moho on Earth. At present, some studies speculate that the South China Sea is one of the shallowest part of the Moho in the world, but there is a lack of sufficient direct evidence.Deep reflection seismic detection can directly reveal the structural form of the lithosphere and is an important means of Moho detection.Based on interpretation, processing, mapping and analysis of the 15000 km deep reflection multi channel seismic profiles and existing research, this paper has formed a preliminary understanding of the reflection characteristics and spatial distribution of the Moho in the South China Sea basin (SCSB). (1) The southern part of the eastern sub basin experienced rapid expansion in the early stage, with sufficient magma supply and little influence of magmatic activity after the expansion stopped. The basement is flat, and the geological structure is relatively simple. At the same time, there is no abnormality in the seismic velocity structure of the oceanic crust, and there are strong wide angle Moho reflection waves and identifiable mantle top refracted waves, which meet the basic conditions for Moho drilling. (2) The intensity of Moho reflection in different regions of the SCSB varies greatly. Among them, the Moho surface reflection in the East sub basin is the strongest and clearest, followed by the Northwest sub basin, while the South west sub basin has only sporadic clear Moho reflections, and the reliability is not high. (3) This study identified the Moho seismic reflection over 3500 km in the SCSB, and formed the spatial distribution map of the Moho seismic reflection in the depth region of the basin for the first time. Compared with the Moho depth obtained by gravity inversion, the Moho depth calculated by deep reflection multi channel seismic is richer in details, and the structure of the Moho can be clearly depicted in vertical direction. On the whole, among the areas with strong Moho seismic reflection and high reliability in the SCSB, the southern part of the East sub basin is one of the areas with shallower Moho, with the shallowest area only about 9.5 km (among them, the water depth is 4. 01 km and the thickness of the ocean crust is only 5. 54 km). Based on comprehensive judgment, the southern part of the East sub basin is an important Moho drilling candidate area in the South China Sea, which is of great significance to the selection of the Moho drilling site.
Key words：South China Sea; multi channel seismic; Moho reflection; oceanic crust seismic velocity model; Moho drilling sites
收稿日期：2022-06-17
基金项目：本文为南方海洋科学与工程广东省实验室（广州）人才团队引进重大专项“南海深海盆区莫霍面地震反射空间分布研究”（编号 GML2019ZD0207）和中国地质调查局项目（编号 DD20201118）联合资助成果。
