四国海盆起源与沉积环境演化

四国海盆位于西太平洋最大的边缘海——菲律宾海的东北部，是太平洋板块向欧亚板块俯冲而形成的一个目前已不活动的弧后盆地，具有高的热流值和洋壳裂开历史，并且具有双翼不对称扩张模式。其磁条带具有“东西不对称、南北不对等”的特点，从而造成四国海盆“北宽南窄、东凸西平”的地形、地貌特征。四国海盆具有典型的弧后盆地沉积特征，主要包括半远洋沉积、火山碎屑沉积和少量的远洋沉积。在早中新世、中中新世、晚中新世、上新世、更新世盆地发育的不同阶段，都有一定的岩相组合特征，并反映了当时的沉积环境。中中新世在盆地的北部和东部，晚中新世在盆地的北部、东北部，上新世和更新世在盆地的北部、东北部和西北部均发育有碎屑沉积。由于中更新世南海海槽的形成及其对物源的阻隔作用，在盆地北部发育了远洋粘土和钙质生物沉积。中中新世，盆地的北部、东侧和西侧均有火山活动发育，晚中新世仅在东部岛弧区有火山活动；上新世仅盆地的北部有少量的火山活动，晚更新世火山活动在盆地北部发育。

THE ORIGIN AND SEDIMENTARY ENVIRONMENT EVOLUTION OF THE SHIKOKU BASIN

The Shikoku basin lies in the northwest of the Philippine Sea,one of the largest marginal basins in the world, and is a topographically and tectonically complex region dominated by subduction of the Pacific plate and the Indo-Australian plate. The Shikoku basin is an inactive back-arc basin with high heat flow and a history of rifting due to the seafloor spreading. The main evidence for the rifting comes from analysis of linear magnetic-anomaly patterns. The magnetic-anomaly patterns of the Shikoku basin have unsymmetrical characters in its east-west direction, which sustain Shih's Double-Limb Unsymmetrical Spreading Model. The lithofaces of the Shikoku basin are typical of marginal or back-arc basins, consisting predominantly of hemipelagic and pyroclast sediments with minor pelagic components. Thickness relationships and distribution of principal facies in each of the five time-equivalent intervals (early Miocene through Pleistocene) of the DSDP sites suggest the time-variable source areas for coalescing clastic wedges within the basin and indicate the timing of volcanism in neighboring volcanic arcs. Clastic sediments came principally from northern and eastern sources during the middle Miocene, from northern and northeastern sources during the late Miocene, and from northern,northeastern and northwestern sources during the Pliocene and early Pleistocene. The appearance of pelagic clay and calcareous biogenic sediments in the upper Pleistocene at site 297 probably marks the inception of the Nankai Trough in mid-Pleistocene time, restricting the previous rapid accumulation of hemipelagic muds in the northern Shikoku basin. Neogene sedimentation rates were generally lower in the Parece Vela basin than in the Shikoku basin. Major volcanic activity during the middle Miocene was in the east of these basins, and volcanism was also prominent in the west and north. During the late Miocene, minor volcanism was restricted to the eastern arc. Pliocene volcanism was active only in the northeast. During the late Pleistocene, there was active volcanism in the north.