华南南缘晚白垩世晚期的构造反转及其机制--来自广西玉林盆地的证据

Latest Cretaceous Structural Inversion in Southeast Part of the South China and its Mechanism: Evidence from the Yulin Basin, Guangxi

中国东部自白垩纪以来在区域伸展的构造背景下经历了多期挤压构造反转,其中华南地区晚白垩世晚期由伸展到挤压的构造反转直接证据较少。本文通过野外构造与锆石U-Pb定年分析,对华南南缘广西玉林盆地白垩纪‒古近纪的盆地演化进行了系统梳理,重点分析了盆地上白垩统与古近系之间角度不整合的构造特征、时限及变形机制。结果表明,玉林盆地形成于晚白垩世早期的伸展裂陷,发育上白垩统西垌组火山碎屑岩和罗文组河湖相沉积;晚白垩世晚期,研究区发生了NW-SE向挤压构造反转,形成了白垩系罗文组与古近系邕宁群之间的角度不整合。该结果显示华南南缘晚白垩世晚期经历了挤压构造反转,其时间与燕山运动最后一幕大致相近。关于这一构造反转的机制争议较大,本文认为该反转的力源可能来自华南以东或以南地区的构造事件:①同期存在古南海的扩张,其洋脊的推力或许是一种可能的机制;②如果不存在古南海,晚白垩世向西俯冲的古太平洋板块俯冲角度变小,与上覆板块的耦合度增强,也可能会驱使华南南缘发生构造反转。

The post-Cretaceous deformation of eastern China is dominated by crustal extension, accompanied by phases of compression. However, direct evidence for the latest Cretaceous tectonic inversion from extension remains elusive. To improve the understanding of the Mesozoic-Cenozoic evolution of South China, we carried out a systematic structural and zircon U-Pb study of the Late Cretaceous-Early Cenozoic Yulin Basin in Guangxi province, located in the southern margin of South China. This study focused on the structural characteristics, time limits, and deformation mechanism of the angular unconformity between the Upper Cretaceous and the Paleogene strata. The results showed that the Yulin Basin formed during the early Late Cretaceous rifting, developed pyroclastic rocks(the Xidong Formation) and fluvial-lacustrine siliciclastic deposits(the Luowen Formation) in the lower and the upper members of the Upper Cretaceous, respectively. The NW-SE compressional inversion occurred in the late Late Cretaceous, as shown by the angular unconformity between the Upper Cretaceous Luowen Formation and the Paleogene Yongning Group. This result attests to the late Late Cretaceous compressional structural inversion in the southern margin of South China. Such an inversion is synchronous with the last episode of the “Yanshan Movement” which is common in North China, even though their underlying geodynamic mechanisms may differ. The mechanism of the structural inversion in the southern margin of South China has been controversial. We suggest that the tectonic inversion is likely related to the influence from regions to the southeast. If there existed the late Late Cretaceous spreading of a proto-South China Sea, the ridge push might result in a coeval compressional stress regime in the southern part of South China. Otherwise, the deformation was possibly controlled by the paleo-Pacific subduction, which would have involved the subduction of the younger and hotter slab beneath the eastern margin of the South China Block. Buoyancy of the young and hot subducting plate would have reduce the subduction angle of the slab therefore enhanced the coupling between the underlying and overlying plates, and resulted in a compressional stress regime in the hinterland of the overlying southern part of the South China.