南海扩张过程及海陆变迁:菲律宾巴拉望-民都洛陆块地层沉积记录

The surface uplift of the Tibetan Plateau and the onset of seafloor spreading of the South China Sea have had a remarkable influence on the tectonism, landforms, drainage systems and climate in this area during the Cenozoic. When and how, however, the changes happened remain debated. The spreading history of the South China Sea, which is still disputed, is mainly based on the comparison of magnetic anomalies lineaments. The southern South China Sea and its relevant research are of great importance for understanding seafloor spreading of the South China Sea and paleogeographical environment. However because of few seismic profiles and exploration wells we know a little about the tectonic and sedimentary evolution of the basins in the southern South China Sea. The relationship of the two conjugate continental margins are unclear and therefore the tectonic evolution history of the South China Sea is not well constrained. The Eocene-Miocene marine sequences in Palawan-Mindoro, Philippines were exposed by eastward subduction of the South China Sea oceanic lithosphere beneath the Philippine Sea Plate since the Middle Miocene and following arc-continent collision. Therefore, Palawan-Mindoro Island provides one of the best geological windows looking into the detailed geological information about the rifting event, stratigraphy, sedimentology, paleoclimate and paleogeography in the southern margin of the South China Sea. Petrographic and marine micropalentological analysis is applied in this study to the Eocene-Miocene sequences in the Palawan-Mindoro, Philippines to constrain the provenance and sedimentation. In addition, we also determine detrital zircon U-Pb geochronology and Hf isotopic values to test if they were derived from the SE China mainland or southwestern provenance (Indochina-Sunda Shelf and possibly northwestern Borneo). In particular we address：(1) the relationship between the basins in the conjugate continental margins and seafloor spreading of the South China Sea; (2) the provenance and its implication to paleogeographical evolution; (3) the Oligocene-Miocene regional transition of tectonic and landform framework.

南海南、北共轭陆缘盆地的对比研究是深入了解南海扩张过程的重要途径。但由于历史因素，目前我国对南海南缘盆地构造-沉积演化研究还非常薄弱，极大地限制了对南海扩张及海陆变迁等基础地质问题的整体认识。由于南海洋壳俯冲，巴拉望-民都洛陆块与菲律宾活动带碰撞造山，沉积于巴拉望-民都洛陆块的始新世-中新世海相层序得以完整出露，成为唯一可以直接对南海南缘盆地沉积层序进行详细研究的陆上窗口。本项目以海陆结合的研究思路，在对巴拉望-民都洛陆块的沉积层序、碎屑组成及生物化石等进行系统分析的基础上，对关键层位进行详细的地球化学、碎屑锆石U-Pb年龄谱及Hf同位素分析，建立巴拉望-民都洛新生代地层格架、示踪地层沉积物的来源及时空演化。通过与南海南、北缘盆地已有成果的对比，深入探讨：(1)南海扩张与南、北缘盆地演化的关系；(2)南海扩张前(始新世-早渐新世)古地理格局；(3)渐新世/中新世构造-沉积事件的整体响应。

南海东南缘的构造演化对于了解南海扩张过程及古地理变迁具有重要意义。但由于历史因素，目前对南海南缘盆地构造-沉积演化研究还非常薄弱。南海东南缘的巴拉望微陆块自早渐新世伴随着南海的扩张而从华南大陆裂离出来，并于中新世与卡加延海脊–菲律宾活动带发生弧陆碰撞，沉积于巴拉望-民都洛陆块的始新世-中新世海相层序得以完整出露，成为唯一可以直接对南海南缘盆地沉积层序进行详细研究的陆上窗口。 本项目以海陆结合的研究思路，在对巴拉望-民都洛陆块的沉积层序、碎屑组成及古生物化石等进行系统分析的基础上，对关键层位进行了详细的地球化学、碎屑锆石 U-Pb 年龄谱及 Hf 同位素分析。得到如下的主要成果和认识：.微体化石结果表明，南海东南部民都洛陆块破裂不整合的时代推测在早-中渐新世(33-28Ma)。通过与南海北缘盆地破裂不整合面对比，发现南海盆地破裂不整合面存在明显的穿时性，从东北向西南逐渐变年轻，对应南海海底扩张从东北向西南打开。巴拉望－民都洛陆块破裂不整合的时代接近珠江口盆地（~30 Ma）和 IODP U1435 站位（~34 Ma）的破裂不整合 ，说明巴拉望微陆块具有与珠江口盆地共轭的关系。南海经历了从早期北陆南海逐渐演变为现今北海南陆的过程，南海北缘早期存在一个向东开口的海湾，可能为古南海的一部分。伴随南海的扩张，海侵范围由东向西逐渐扩展，从一个狭窄的海湾，形成今日的形貌。.泥岩微量元素、Nd 同位素及碎屑锆石 U-Pb 定年结果均指示巴拉望-民都洛岛始新世-渐新世地层沉积物主要源区为华南沿海地区，中中新世以后为巴拉望-民都洛陆块的近源剥蚀，这与南海北缘盆地沉积存在明显的差异。台湾新生代地层砂岩碎屑锆石U-Pb年龄的结果显示，台湾新生代地层物源在晚渐新世-早中新世(~31-~25Ma)发生明显的改变。在~31Ma以前南海北缘盆地沉积物主要来自华南沿海地区，这与民都洛地层相似。在~31-25Ma期间，源区逐渐向内陆迁移，在~25Ma后，扬子地块物源逐渐增多。而在南海南缘，~25Ma后，南海南缘盆地仍以陆块内部中生代地层、花岗岩及火山岩为主要物质来源。对应于南海的扩张，南海洋盆阻挡扬子地块的物源向南输送。

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