海洋地质与环境重点实验室知识库

构造作用与岩浆活动是被动陆缘伸展过程中的两种重要机制，也是反映陆缘构造演化特征的关键指标。但是我们对两者的认识非常有限。对于大多数张裂陆缘来说，这两种作用在调节岩石圈伸展减薄过程中的相对重要性仍然是未知的。南海陆缘被认为是贫岩浆型陆缘，发育有大量的裂后岩浆活动。西北陆缘是南海伸展范围最大的区域，裂谷作用持续时间长，且受走滑断层、地幔柱物质的影响，具有独特的构造环境与广泛的岩浆活动，成为研究构造作用与岩浆活动之间关系的天然实验室。

本文基于多波束数据、钻井数据、多道地震数据、广角地震数据与重力数据，采用地震解释、多道地震与广角地震数据联合走时层析成像反演、基于速度结构的重力正演模拟及三维数据地震属性分析等方法对南海西北陆缘与西北次海盆两侧陆缘的构造特征与岩浆活动进行系统的研究。在研究结果的基础上对西北陆缘尤其是西沙海槽的伸展模式、西北次海盆洋陆转换的差异、岩浆活动的期次及其对沉积盆地中多边形断层和流体的影响等展开分析讨论，旨在完善对南海构造演化的系统性认知。

西沙海槽作为一个夭折裂谷可以为陆缘张裂初期的构造演化提供新的见解。西沙海槽初期断层呈分布式，随着伸展减薄作用的进行逐渐集中于海槽中部，且发育有切穿整个地壳的大型拆离断层。海槽中部上地幔发生蛇纹石化。这表明西沙海槽初始阶段为分布式的纯剪模式，后转变为单剪模式。上地壳厚度变化较小，下地壳由两侧向中心快速减薄甚至完全消失。多道地震中识别出显著的连续壳内深反射，本文认为其与下地壳的流动有关。地壳随着伸展减薄温度升高压力变小，下地壳可能发生侧向流动致使下地壳快速减薄。

西北次海盆两侧陆缘过渡带处发现小规模的下地壳高速层。中沙地块两侧Moho面及地壳厚度变化呈不对称特征。南侧地壳快速减薄过渡成为洋壳，其北侧地壳变化较缓。结合西沙海槽剖面结构及IODP钻探结果，本文认为由于蛇纹石化地幔的形成加速了陆缘的裂解，西北次海盆由陆缘伸展快速过渡到海底扩张，深部地幔在短时间内上涌并底侵在地壳底部形成了高速层。中沙地块与礼乐滩地块共轭，且同为刚性地块，所以两者之间的洋陆过渡在极其狭窄的空间内完成。中沙地块北侧与北部陆缘相接，所以洋陆过渡较为缓慢。

南海西北陆缘与南海其他地区类似发育有强烈的裂后期岩浆活动，本次研究揭示了635个岩浆体，包括408个火山、124个岩席、98个火成岩底辟、4处熔岩流和1处岩盖。岩浆活动的峰值出现在5.3 Ma左右的几个百万年内。岩浆体的分布与断层和地壳减薄密切相关。研究区与东北陆缘的岩浆活动存在差异。基于前人研究，本文认为地幔柱物质在23.8 Ma左右到达并停留在南海岩石圈基底。这些物质沿着断层上涌形成岩浆活动，并在随后的构造事件特别是红河断层反转的影响下增强。裂后岩浆活动加速陆缘岩石圈的冷却，导致构造沉降加速，同时水深快速增加。

中建南盆地的三维地震数据揭示出多边形断层和岩浆体的相互作用。本文识别出的岩浆体包括8个火山、50个岩席和1个火成岩底辟。多边形断层以线形、多边形和放射状为特征，广泛分布在中新统和第四系沉积物中。影响多边形断层分布的控制因素是岩性差异、温度和压力。火山和火成岩底辟通过差异压实作用促进了放射状断层的形成。由火山和上覆放射状断层形成的垂直通道调节了压差或排出相变产生的水，进而破坏了火山间形成多边形断层的必要条件。在盆地底部，岩浆沿着多边形断层平面侵入，形成倾斜板状、勺状的和碗状的岩席。此外多边形断层充当岩浆垂向运移的通道促进了浅层沉积物中岩席的形成。

南海西北陆缘在裂谷期的伸展过程中以构造作用为主导，可能经历由纯剪切向简单剪切模式的转变并发育蛇纹石化地幔，加速了陆缘的裂解。地块的性质也会对陆缘的结构特征产生影响。岩浆活动主要发育在裂后期，尤其是5.3 Ma左右。岩浆活动影响了盆地中的多边形断层、麻坑等与流体相关的构造，反过来岩浆活动也受多边形断层等构造的影响。

Tectonics and magmatism are two important mechanisms in the process of passive margin extension, and they are also key indicators of the the tectonic evolution. However, our understanding of them during this process is very limited. For most rifted margins, the relative importance of these two roles in lithospheric extension remains unknown. The margin of South China Sea is considered to be a magma-poor margin with numerous post-rift magmatism developed. The northwest margin is the region with the largest stretching extent in the South China Sea, with a long duration of rifting and influenced by strike-slip fault and mantle plume, and has a unique tectonic environment with extensive magmatism, which makes it a natural laboratory for the study of the relationship between tectonics and magmatism.

This study is based on multi-beam data, drilling data, multi-channel seismic data, wide-angle seismic data and gravity data, using seismic interpretation, joint travel-time tomography inversion of multi-channel seismic and wide-angle seismic data, gravity forward modelling based on velocity structure and seismic attribute extraction of 3D data to study the structural characteristics and magmatism of the northwest margin and northwest sub-basin of the South China Sea. Based on results, we analyze and discuss the extension model of the northwest margin, especially the Xisha Trough, the continent-ocean transition in the northwest sub-basin, the stages of magmatic activity and its impact on polygonal faults and fluids in sedimentary basins. It aims to improve the overall understanding of the evolution of the South China Sea.

As an aborted rift, the Xisha Trough can provide new insights into the tectonic evolution in the early stages of continental rifting. The faults in the Xisha Trough were distributed in the early stage, and gradually concentrated in the center as extension progressed, and large detachment faults developed that cut through the entire crust. This study found serpentinized mantle developed in the center of the trough. This shows that the Xisha Trough is in a pure shear mode in the initial stage, and then changes to a sample shear mode. The thickness of the upper crust changes little, and the lower crust thins rapidly from both sides to the center or even disappears completely. Many continuous deep reflections in crust were identified in multi-channel seismic data. As the crust thins and the temperature increases, the pressure decreases, and lateral flow of the lower crust, causing the lower crust to rapidly thin.

Small-scale lower crustal high-velocity layers are found in the continent-ocean transition zone of the northwest sub-basin. The Moho surface and crustal thickness changes on both sides of the Zhongsha bank are asymmetrical. The crust on the south side thinned rapidly and transitioned to oceanic crust, while the crust on the north side changed slowly. Combining the structure of the Xisha Trough and the IODP drilling results, this study thinks that the formation of the serpentinized mantle accelerating the break-up of margin, the northwest sub-basin rapidly transitioned from continental extension to seafloor spreading, and the upwelling of the deep mantle underplated at the bottom of crust leading to the high-velocity layer formed at the lower crust. The Zhongsha block is conjugate with the Liyue bank, and they are both rigid blocks, so the transition zone is extremely fast. The north side is connected to normal continental margin, so the transition is relatively slow.

In the northwest margin, strong post-rift magmatism developed. This study revealed 635 igneous rock bodies, including 408 volcanoes, 124 sills, 98 igneous diapirs, 4 lava flows and 1 laccolith. Magmatic activity peaked around 5.3 Ma. The distribution of igneous bodies is closely related to faulting and crustal thinning. There are differences with the magmatism at the northeast margin. Based on previous studies, this study thinks that the mantle plume material reached and stayed at the bottom of lithosphere of the South China Sea around 23.8 Ma. The upwelling of these materials along the fault resulted in magmatism, which was enhanced by subsequent tectonic events, particularly the Red River Fault reversal (RRF). Post-rift magmatism accelerates the cooling of lithosphere, resulting in accelerated subsidence and rapid deepening of water depth.

3D seismic data from the Zhongjiannan Basin reveal the interaction of polygonal faults and magmatism. The magmatic bodies identified include 8 volcanoes, 50 sills and 1 igneous diapir. Polygonal faults are characterized by linear, polygonal, and radial features and are widely distributed in Miocene and Quaternary sediments. The controlling factors affecting the distribution of polygonal faults are lithological differences, temperature and pressure. Volcanoes and igneous diapirs promote the formation of radial faults through differential compaction. Vertical paths formed by volcanoes and overlying radial faults regulate pressure differences or drain water produced by phase changes, thereby destroying the necessary conditions for the formation of polygonal faults. At the basin floor, magma intruded along polygonal fault planes, forming inclined sheets, spoon-shaped, and bowl-shaped sills. In addition, polygonal faults act as pathways for vertical magma migration and promote the formation of sills in shallow sediments.

Tectonics dominated the extension process of the northwest margin during the syn-rift stage, and it may have undergone a transition from pure shear to simple shear mode and serpentinized mantle was developed, which accelerated the break-up. The nature of the plate also has an impact on the structure of the margin. Magmatism was mainly developed during the post-rift phase, especially around 5.3 Ma. The magmatism affected the fluid-related structures such as polygonal faults and pockmarks in the basin, and in turn the magmatism was influenced by polygonal faults.

目 录

第1章 绪论........................................................................................ 1

1.1 选题背景与研究意义........................................................................................ 1

1.1.1 被动陆缘构造演化研究意义..................................................................... 1

1.1.2 南海西北陆缘研究的必要性..................................................................... 1

1.2 国内外研究现状................................................................................................ 3

1.2.1 被动陆缘的类型与定义............................................................................. 3

1.2.2 陆壳、洋壳与蛇纹石化地幔的性质......................................................... 4

1.2.3 陆缘伸展模式研究..................................................................................... 7

1.2.4 岩浆活动的研究......................................................................................... 8

1.2.5 南海西北陆缘伸展与岩浆活动研究现状............................................... 10

1.3 研究内容与技术路线...................................................................................... 10

1.3.1 浅部沉积与构造特征............................................................................... 10

1.3.2 深部构造特征........................................................................................... 10

1.3.3 岩浆活动的时空展布............................................................................... 11

1.3.4 技术路线................................................................................................... 11

1.4 主要工作量与创新点....................................................................................... 11

第2章 区域地质背景...................................................................... 13

2.1 南海地质背景.................................................................................................. 13

2.2 西北陆缘.......................................................................................................... 15

2.3 西沙海槽.......................................................................................................... 15

2.4 中建南盆地...................................................................................................... 16

第3章 西沙海槽地壳结构与伸展模式........................................... 17

3.1 研究数据.......................................................................................................... 17

3.1.1 多波束数据............................................................................................... 18

3.1.2 多道地震数据........................................................................................... 18

3.1.3 广角地震数据........................................................................................... 18

3.1.4 重力数据................................................................................................... 20

3.2 研究方法.......................................................................................................... 20

3.2.1 tomo2d的联合反演................................................................................... 20

3.2.2 不确定性分析........................................................................................... 22

3.2.3 重力正演................................................................................................... 22

3.3 研究结果.......................................................................................................... 23

3.3.1 地壳速度结构........................................................................................... 23

3.3.2 构造特征................................................................................................... 24

3.3.3 重力正演结果........................................................................................... 25

3.4 讨论.................................................................................................................. 26

3.4.1 地幔的蛇纹石化及其对伸展模式的指示............................................... 26

3.4.2 伸展过程中下地壳的变化....................................................................... 28

3.4.3 西沙海槽的演化及指示意义................................................................... 29

3.5 章节小结.......................................................................................................... 30

第4章 南海西北次海盆共轭陆缘地壳结构................................... 31

4.1 研究数据.......................................................................................................... 31

4.1.1 多道地震数据........................................................................................... 32

4.1.2 广角地震数据........................................................................................... 33

4.2 研究方法.......................................................................................................... 33

4.3 研究结果.......................................................................................................... 34

4.3.1 P波速度结构与构造特征......................................................................... 34

4.3.2 重力模拟................................................................................................... 37

4.4 讨论.................................................................................................................. 37

4.4.1 西北次海盆的洋陆过渡模式................................................................... 37

4.4.2 中沙地块两侧洋陆过渡的差异及成因................................................... 39

4.5 章节小结.......................................................................................................... 39

第5章 南海西北陆缘岩浆活动....................................................... 41

5.1 研究数据.......................................................................................................... 41

5.1.1 地震与多波束数据................................................................................... 42

5.1.2 地震层位的识别和划分........................................................................... 43

5.2 研究方法.......................................................................................................... 45

5.2.1 岩浆体的识别及年龄的判断................................................................... 45

5.2.2 岩浆体与其他地质体之间的差异........................................................... 47

5.3 研究结果.......................................................................................................... 48

5.3.1 岩浆体类型............................................................................................... 48

5.3.2 岩浆体的时空展布................................................................................... 54

5.4 讨论.................................................................................................................. 55

5.4.1 东北陆缘与西北陆缘岩浆活动差异....................................................... 55

5.4.2 岩浆活动的控制因素............................................................................... 56

5.4.3 岩浆活动的影响....................................................................................... 59

5.5 章节小结.......................................................................................................... 59

第6章 岩浆活动与多边形断层相互作用....................................... 61

6.1 数据与方法...................................................................................................... 61

6.1.1 地震与钻井数据....................................................................................... 61

6.1.2 岩浆体与多边形断层的识别................................................................... 62

6.2 研究结果.......................................................................................................... 64

6.2.1 岩浆体地震反射特征............................................................................... 64

6.2.2 多边形断层的特征与分布....................................................................... 69

6.2.3 岩浆活动与多边形断层的空间关系....................................................... 73

6.3 讨论.................................................................................................................. 74

6.3.1 中建南盆地岩浆活动时间....................................................................... 74

6.3.2 岩性差异、温度和压力控制着多边形断层的分布............................... 75

6.3.3 岩浆活动与多边形断层的相互作用....................................................... 76

6.4 章节小结.......................................................................................................... 77

第7章 结论与展望.......................................................................... 79

7.1 主要结论.......................................................................................................... 79

7.2 存在问题及下一步工作展望.......................................................................... 80

参考文献............................................................................................... 81

附录一 OBH-1996 IV各个台站地震记录...................................... 106

附录二 OBS2006-1各台站地震记录.............................................. 110

致 谢................................................................................................. 115

作者简历及攻读学位期间发表的学术论文与其他相关学术成果... 117