Oceanic crustal structures and temporal variations of magmatic budget during seafloor spreading in the East Sub-basin of the South China Sea
Oceanic crustal structures and temporal variations of magmatic budget during seafloor spreading in the East Sub-basin of the South China Sea
复制标题
南海东部次盆地洋壳结构及海底扩张过程中岩浆收支的时间变化
DOI：
10.1016/j.margeo.2021.106475
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发表时间：
2021-04
期刊：
影响因子：
2.9
通讯作者：
Wang Yanlin
中科院分区：
文献类型：
--
作者：
Yu Junhui;Yan Pin;Qiu Yan;Matthias Delescluse;Huang Wenkai;Wang Yanlin
在贫岩浆大陆裂谷作用后，南海经历了约33 ~15 Ma的海底扩张，并在约25 Ma时发生了向南的洋脊跳跃，形成了东、西北和西南洋盆。然而，由于缺乏海洋地壳图像，对扩张过程中的岩浆过程知之甚少。在这里，我们提出了一个复合剖面，包括三个地震线，从北方大陆-海洋过渡（COT）的东部次盆地的灭绝扩张中心，以揭示地壳结构。在最北部的洋壳部分，第一洋壳厚度仅3.9- 4.5km，基底粗糙，断层众多，莫霍面反射体弥散而微弱，这意味着岩浆收支相对较低，构造伸展从贫岩浆大陆裂谷期延续到初始扩张期。从北方COT向海方向到发生跳跃事件的废弃山脊，基底变得光滑，断层减弱。莫霍面反射层趋于连续和强烈，火成岩地壳逐渐变厚（局部达8.4 km），表明在早期扩张阶段岩浆收支显著增加。然而，在从废弃山脊到绝灭扩张轴的中部，基底再次变得粗糙，地壳断层。地壳逐渐变薄，在绝灭扩张中心只有3.7公里，那里仍然可以看到一个深的中央谷，但严重改变了后扩张火山作用，这意味着减少岩浆预算和增加构造伸展在晚期扩张阶段。因此，东次海盆地壳结构变化反映了岩浆收支在扩张过程中由增到减的显著变化，峰值出现在洋脊跳跃附近。薄的构造化地壳和围绕灭绝扩张中心的深中谷进一步支持了在南海最后扩张阶段（结束于~15-16 Ma）期间缓慢的海底扩张，也表明海南地幔柱的岩浆贡献可能非常有限。
Following magma-poor continental rifting, the South China Sea (SCS) experienced seafloor spreading from ~33 Ma to ~15 Ma with an intervening southward ridge jump at ~25 Ma, generating the oceanic East, Northwest and Southwest Sub-basins. However, the magmatic processes during spreading remain poorly known owing to the lack of oceanic crustal images. Here we present a composite profile comprising three seismic lines that run from the northern continent-ocean transition (COT) to the extinct spreading center of the East Sub-basin to reveal the crustal structures. In the northernmost oceanic portion, the first oceanic crust featuring rough basement with numerous faults, and diffuse and weak Moho reflector, is only 3.9–4.5 km thick, implying relatively low magmatic budget and protracted tectonic extension continued from the magma-poor continental rifting to initial spreading. From the northern COT ocean-ward to the abandoned ridge where the jump event occurred, the basement becomes smooth with subdued faults. The Moho reflector tends to be continuous and strong, and the igneous crust gradually becomes thick (locally up to 8.4 km), suggesting considerably increasing magmatic budget during the early spreading stage. However, in the central portion spanning from the abandoned ridge to the extinct spreading axis, the basement becomes rough with crustal faults again. And the crust gradually becomes thin, to only 3.7 km at the extinct spreading center where a deep median valley is still visible, though severely altered by post-spreading volcanism, implying decreasing magmatic budget and increasing tectonic extension during the late spreading stage. Therefore, the crustal structure variations in the East Sub-basin reflects a prominent increasing to decreasing change of magmatic budget during spreading with the peak around the intervening ridge jump. Thin, tectonized crust and deep median valley around the extinct spreading center further support slow seafloor spreading during the last spreading stage of the SCS which ended at ~15–16 Ma, and also suggest that magma contribution from the Hainan mantle plume might be very limited if any.
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