Sedimentary and subsidence history of ODP Sites 135-840 and 135-841

Sedimentary sections recovered from the Tonga platform and forearc during Ocean Drilling Program Leg 135 provide a record of the sedimentary evolution of the active margin of the Indo-Australian Plate from late Eocene time to the Present. Facies analyses of the sediments, coupled with interpretations of downhole Formation MicroScanner logs, allow the complete sedimentary and subsidence history of each site to be reconstructed. After taking into account the water depths in which the sediments were deposited and their subsequent compaction, the forearc region of the Tofua Arc (Site 841) can be seen to have experienced an initial period of tectonic subsidence dating from 35.5 Ma. Subsidence has probably been gradual since that time, with possible phases of accelerated subsidence, starting at 16.2 and 10.0 Ma. The Tonga Platform (Site 840) records only the last 7.0 Ma of arc evolution. However, the increased accuracy of paleowater depth determinations possible with shallow-water platform sediments allows the resolution of a distinct increase in subsidence rates at 5.30 Ma. Thus, sedimentology and subsidence analyses show the existence of at least two, and possibly four, separate subsidence events in the forearc region.
Subsidence dating from 35.5 Ma is linked to rifting of the South Fiji Basin. Any subsidence dating from 16.2 Ma at Site 841 does not correlate with another known tectonic event and is perhaps linked to localized extensional faulting related to slab roll back during steady-state subduction. Subsidence from 10.0 Ma coincides with the breakup of the early Tertiary Vitiaz Arc because of the subduction polarity reversal in the New Hebrides and the subsequent readjustment of the plate boundary geometry. More recently, rapid subsidence and deposition of a upward-fining cycle from 5.30 Ma to the Present at Site 840 is thought to relate to rifting of the Lau Basin. Sedimentation is principally controlled by tectonic activity, with variations in eustatic sea level playing a significant, but subordinate role. Subduction of the Louisville Seamount Chain seems to have disrupted the forearc region locally, although it had only a modest effect on the subsidence history and sedimentation of the Tonga Platform as a whole.

大洋钻探计划（Ocean Drilling Program）第135航次从汤加台地（Tonga platform）与弧前区（forearc）获取的沉积剖面，完整记录了印度-澳大利亚板块（Indo-Australian Plate）活动边缘自始新世晚期（late Eocene）至今的沉积演化历程。通过对沉积物开展岩相分析（Facies analyses），并结合井下地层微成像测井（Formation MicroScanner）的解释结果，可重建各钻探站位的完整沉积与沉降历史。在考量沉积物沉积时的古水深及后续压实作用后，可见汤加弧（Tofua Arc）841站位（Site 841）的弧前区经历了始于35.5 Ma的初始构造沉降期。自此之后沉降过程大致呈渐进式，且可能在16.2 Ma与10.0 Ma出现加速沉降阶段。汤加台地840站位（Site 840）仅记录了过去7.0 Ma以来的弧演化过程，不过依托浅水台地沉积物可更精准地测定古水深这一优势，得以分辨出5.30 Ma时沉降速率出现显著升高的事件。综上，沉积学与沉降分析结果表明，弧前区至少存在两次、最多四次独立的沉降事件。
始于35.5 Ma的沉降作用与南斐济盆地（South Fiji Basin）的裂谷活动相关。841站位16.2 Ma以来的沉降并未与已知的其他构造事件对应，其成因可能与稳态俯冲（steady-state subduction）过程中板片回撤（slab roll back）引发的局部伸展断裂有关。10.0 Ma以来的沉降则与始新世维蒂亚弧（Vitiaz Arc）的裂解事件相吻合，该事件由新赫布里底（New Hebrides）地区的俯冲极性反转（subduction polarity reversal）及后续板块边界几何形态（plate boundary geometry）调整所触发。更近时期，840站位自5.30 Ma至今发生的快速沉降与一期向上变细沉积旋回，被认为与劳盆地（Lau Basin）的裂谷作用相关。沉积作用主要受构造活动控制，全球海平面变化（eustatic sea level）虽也起到重要但次要的作用。路易斯维尔海山链（Louisville Seamount Chain）的俯冲似乎对弧前区造成了局部扰动，但整体而言对汤加台地的沉降历史与沉积作用仅产生了温和影响。