白垩纪以来东亚地貌演化与构造驱动:来自沉积盆地与构造变形的记录

Cretaceous-Cenozoic First-order Landscape Evolution of the East Asia and its Tectonic Drivers:A Synthesis of Sedimentary and Structural Records

白垩纪以来,东亚大陆构造的演变受东缘太平洋板块西向俯冲及南海打开与西缘新特提斯洋闭合及随后印度‒欧亚板块碰撞的双重控制,东亚大陆地形经历了“跷跷板”式的演变:白垩纪‒早新生代地形东高西低,与现今东倾地形相反;晚渐新世以来东倾的一级地貌格局逐渐形成。为了进一步完善该模型,本文报道了西江中‒上游流域内玉林、十万大山、南宁和百色盆地白垩纪‒新生代古流向研究结果,并综合了珠江口盆地碎屑物源和青藏高原东南缘构造、古高程与水系演化研究进展,获得以下认识:①白垩纪,西江中‒上游地区盆地物源主要源自盆地东侧(可能是云开大山),反映了东侧地形相对较高,与“跷跷板”模式所指出的中生代东高西低的地形一致。②古近纪,珠江口盆地沉积物主要源自沿海花岗岩体,西江中‒上游玉林与十万大山盆地物源仍然主要源自东侧,指示西江水系尚未贯通,东部沿海高地形仍然存在;结合该时期南宁和百色盆地物源来自东西两侧,青藏高原东南缘强烈压扭性变形和古高程研究所指示的地表抬升,认为古近纪东亚地形应是两侧高、中部低的“V”字型样式。③晚渐新世以来,珠江口盆地物源信号逐渐与现代珠江一致;在南宁盆地发现的新近纪河流相砂砾岩所指示的古流向与现今河流基本一致。这些证据说明珠江水系在晚渐新世以来逐渐形成,反映了沿海地区地形已被夷平。随着青藏高原东南缘的持续抬升,现今西高东低的东亚地形逐渐成型。我们发现东亚地形“跷跷板”式的演变过程中,在古近纪经历了“V”字型的过渡状态,为进一步刻画东亚地形演变历史提供了新证据。

The Cretaceous-Cenozoic deformation of the East Asia was dominated by multiple tectonic events,including the closure of the Neo-Tethys Ocean and subsequent Indo-Eurasia collision in the western margin,the westward Pacific subduction,and the subsequent opening of the South China Sea in the eastern margin.In the meantime,the landscape of the East Asia experienced magnificent change,which has been interpreted by a“seesaw”model.The model suggests that the Mesozoic-Paleogene topography was west-dipping with lowlands in the west(what are now the southeast Tibetan Plateau)and high mountains in the east continental margin,which was named as the East Asia coastal mountains.Neogene uplift of the Tibetan Plateau and opening of the East Asia marginal seas inverted the landscape to the modern east-tilting landform.Here we report new paleocurrent studies of the sedimentary basins(which are the Yulin,Shiwandashan,Nanning and Baise Basin from east to west)in the middle-upper reaches of the Xijiang River.Combining the results of detrital provenance studies of the Pearl River Mouth Basin,paleoaltimetric and tectonic studies of the southeast Tibetan Plateau,this work suggests that(1)during the Cretaceous,detritus of sedimentary basins in the middle-upper reaches of the Xijiang drainage mainly sourced from the east,indicating highlands in the east,consistent with the“seesaw”model.(2)During the Paleogene,detritus of the Pearl River Mouth Basin sourced mainly from the coastal granitic intrusions,whereas the detritus of the Yulin and Shiwandashan Basins continued to source from the east,indicating the Xijiang River has not formed.In addition,the Nanning and Baise Basins formed,with detritus sourced from both the west and the east.Further to the west,the southeast Tibetan Plateau has accommodated significant transpressional deformation and elevation gain.Such a synthesis of various pieces of information indicates a Paleogene V-shaped landscape,with highlands in both the western and eastern margins of the East Asia.(3)Since the late Oligocene,detrital signals became gradually similar to those of the modern Pearl River Mouth Basin.Further,newly mapped Neogene conglomerate along the Xijiang River shows paleocurrents similar to the modern flow direction.These lines of evidence indicate the late Oligocene formation of the Xijiang River,implying the coastal highlands may have been denudated to low elevations.With continued uplift of the Tibetan Plateau,the modern-like eastward dipping landscape of the East Asia has been shaped.This study is the very first to reveal a Paleogene V-shaped landscape for the East Asia,updating the understanding of the landscape evolution.