南海北部陆缘异常构造沉降形成机制与破裂阶段热状态的数值模拟

The anomalous tectonic subsidence feature of subsidence deficit during breakup stage, and then rapid post-breakup subsidence, has been widespread found in the Atlantic magma-poor continental margins. Studies also found the same anomalous subsidence feature on the northern continental margin of the South China Sea. To illuminate the formation mechanism behind the anomalous subsidence feature, and obtain the knowledge of the thermal status during breakup stage from surface anomalous response, this application will focus on the three regions of Baiyun sag and its southern slope, Taixinan basin and its closed slope and the eastern Qiongdongnan Basin-Xisha Trough. Firstly, we will obtain the present thermal status from heat flow, water depth, and seismic velocity and so on, and reveal the thermal history recorded by the IODP drill samples around the breakup unconformity, and calculate tectonic subsidence history. And then, under the constraints of the present thermal status, the revealed thermal history and the subsidence history, we will employ a thermo-mechanically coupled numerical model to illuminate the formation mechanism of the anomalous subsidence feature, obtain the thermal status during breakup stage and its temporal and spatial variation. At last, we will analyze the deformation evolution and breakup mode, discuss on the relation between the thermal status during breakup and the magmatic developing, high-velocity lower crustal layer, and present thermal status, and its role on the evolution and hydrocarbon resource potentials of the deep-water basins. This research is of theoretical significance and practical application, and will contribute to the research of the anomalous subsidence’s formation mechanism in the passive margin, and provide the important basis for answering the scientific issues related with the thermal status during the breakup of the South China Sea, and the deep-water hydrocarbon exploration.

大西洋贫岩浆型被动大陆边缘普遍存在破裂阶段构造沉降亏损、破裂后快速沉降的异常现象。研究发现南海北部陆缘也存在类似的异常沉降现象。为阐明这种异常构造沉降的形成机制，并从表层异常响应中提取破裂阶段深部热状态信息，申请课题拟聚焦白云凹陷及其南侧陆坡、台西南盆地及其邻近陆坡和琼东南盆地东部-西沙海槽，利用热流、水深、波速等资料约束的现今热状态、大洋钻孔破裂面附近样品的热史记录，和构造沉降史对动力学数值模拟结果进行约束，阐明异常沉降的成因机制，获取破裂阶段的热状态及其时空变化，分析陆缘的变形过程、破裂方式，探讨破裂阶段热状态与岩浆发育、地壳底部高速层、现今热状态之间关系，分析其对深水盆地演化与油气资源潜力的影响。该项研究具有理论意义和实际应用价值，必将为被动大陆边缘异常构造沉降的成因机制研究做出贡献，为回答与南海破裂阶段热状态相关的科学问题和深水油气勘探提供重要依据。

南海北部大陆边缘与大西洋伊比利亚等被动大陆边缘普遍存在破裂阶段构造沉降亏损、破裂后快速沉降的异常现象。为阐明这种异常构造沉降的成因，项目首先利用地热学、盆地分析、热年代学等方法获取南海南北部共轭陆缘的热流分布特征、现今热状态、构造沉降特征和新生代张裂前构造环境，然后借助热力学耦合动力学数值模型，分析构造异常沉降成因，取得了以下主要成果：①新获得南海陆缘和西北次海盆49个高质量的海底热流数据、热流分布图及4条典型剖面的现今地温场图；南海陆缘现今热状态普遍受到新近纪多期岩浆侵入影响，除此之外，南海东北部热流还受到流体活动、泥火山影响，白云凹陷还可能受到小尺度地幔对流影响，礼乐地块礁体发育区热状态还受到礁体与周围海水热交换的影响。② 南海南北部陆缘普遍具有岩石圈破裂前后构造沉降亏损、裂后快速补偿沉降的异常现象，但是不同区域沉降异常特征有所不同，琼东南盆地在张裂结束近10Ma后才发生快速的构造沉降，白云凹陷早渐新世破裂后，早中新世后发生快速构造沉降，南海东北部则在破裂后发生快速构造沉降，礼乐地块破裂及随后的漂移阶段，构造沉降都明显变缓，中中新世后才发生快速构造沉降。③南海北部陆缘新生代裂谷作用并非直接从晚中生代岛弧环境演化而来，白垩纪末还经历过一次小型的造山事件，该造山事件可能是在远程俯冲作用驱动下，卢克尼亚-南沙地块与南海北部陆缘发生的碰撞造山事件。④动力学数值模拟表明，初始岩石圈流变学结构对伸展构造变形模式、破裂方式、破裂热状态和构造沉降都有重要影响。地幔对流传热和动力支撑可以改变构造沉降史，如果早期构造沉降盈余，地幔对流会引发均衡抬升调整，如果早期沉降亏损，随着地幔对流活跃程度降低或者对流区迁移，将会发生快速补偿沉降。⑤南海陆缘构造异常沉降可能是小幅度深部热异常和地幔对流联合作用的结果，小幅度热异常可以有效抑制岩浆大量形成，又可以增加热浮力和地幔对流活跃程度，导致沉降变缓和亏损，地幔对流区迁移和活跃度降低则有利于发生快速沉降。这些成果不仅有助于认识南海陆缘热状态、张裂前构造背景和异常构造沉降特征及其形成机制，而且有助于理解不同伸展构造模式与构造沉降、地幔对流与构造沉降间的关系，获取的高质量热流数据对深水油气勘探和地球动力学研究有重要价值。项目部分成果已经发表7篇论文，其中SCI论文5篇。