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

田云涛, 秦咏辉, 胡 杰, 张贵洪, 刘一珉, 潘黎黎,颜照坤, 张增杰, 孙习林, 张培震

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

田云涛1, 2, 秦咏辉1, 胡 杰3, 张贵洪1, 刘一珉1, 潘黎黎1,颜照坤4, 张增杰1, 孙习林1, 张培震1, 2

(1. 中山大学 地球科学与工程学院, 广东省地球动力作用与地质灾害重点实验室, 广东 珠海 519082; 2. 南方海洋科学与工程广东省实验室(珠海), 广东 珠海 519082; 3. 成都理工大学, 油气藏地质及开发工程国家重点实验室, 四川 成都 610059; 4. 东华理工大学, 核资源与环境国家重点实验室, 江西 南昌 330013)

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。

东亚; 青藏高原; 构造地貌; 沉积盆地; 古水系演化; 古地形重建

0 引 言

现代东亚地貌西高东低: 西部为高耸的青藏高原, 东部为丘陵与平原, 东缘为边缘海盆地, 这一格局主要受控于板块边界的构造作用, 包括西部晚中生代‒新生代新特提斯洋的消亡、随后印度‒欧亚板块间的陆陆碰撞及东部太平洋板块的俯冲(Yin and Harrison, 2000; 舒良树和王德滋, 2006; Wang et al., 2013; Ye et al., 2018)。

伴随着中‒新生代东亚周缘动态、活跃的构造作用, 东亚的地形也经历了多次重大转变。中‒新生代地形“跷跷板”式的演化模型被广为接受, 该模型认为, 与现今东倾地形相反, 中生代和古近纪东亚地形呈现出东部发育沿海山脉, 而西部地势较低的格局(陈丕基, 1979, 1997; 汪品先, 1998, 2005), 其地形发育主要受控于当时太平洋板块向西俯冲, 东亚东缘为安第斯型活动大陆边缘, 火山岛弧位于现今南海北部洋陆过渡带的位置(Xu et al., 2017; Cui et al., 2021)。随着新近纪青藏高原的抬升和东亚边缘海的拉开, 这一西倾的地形发生倒转, 最终形成现今西高东低的特征(陈丕基, 1979, 1997; 汪品先, 1998, 2005)。该地形演化模型的重要证据是: 新近纪之前东亚缺乏大型的东流水系(如黄河和长江)(李吉均等, 1996; 汪品先, 1998, 2005); 以及晚中生代‒古近纪东亚陆缘沿海山脉的屏障作用和热效应, 使同时期华南内陆处于干旱‒半干旱气候并广泛发育沙漠‒盐湖相沉积(陈丕基, 1979, 1997)。

对东亚地形演变的探究, 可具体归结至对沿海山脉是否存在和其时限, 以及青藏高原抬升历史的研究。关于沿海山脉, 其范围大致位于南海北部洋陆过渡带至沿海地区(图1a), 因此内陆地区白垩纪‒新生代沉积盆地的水系演化或可为探究沿海山脉的范围与演化提供依据。近年来国内外学者通过古水系、古高程、沉积盆地、构造分析和地表剥蚀等研究, 在珠江口盆地物源演变、珠江水系演化、云贵高原构造与古高程重建等领域取得了新的进展。本文通过对前人工作的整理与归纳, 并结合本次对西江中‒上游盆地水系演化方面的研究(图1b), 试图进一步完善对东亚地形演化的认识。

图a虚线范围内为白垩纪‒早新生代东亚东部沿海山脉可能的分布范围。

1 沿海山脉的分布与时限

1.1 珠江口盆地物源演变及古地形意义

本文总结了珠江口盆地钻孔和珠江水系的碎屑锆石数据(Shao et al., 2016, 2017; Xu et al., 2016b; Zhong et al., 2017; Cao et al., 2018; Wang et al., 2019), 并绘制了始新统以来的碎屑锆石年龄谱综合图。结果显示, 始新统碎屑锆石年龄谱存在一个～115 Ma的主峰; 下渐新统的年龄谱主峰为～150 Ma, 次峰为～450 Ma, 另有少量～1000 Ma、～1800 Ma、～2500 Ma的锆石年龄; 上渐新统年龄谱与下渐新统特征类似, 但是～1000 Ma、～1800 Ma、～2500 Ma的锆石年龄比例有一定程度增加, 但仍为次峰; 下、中中新统与现代珠江的沉积物具有类似的碎屑锆石年龄分布特征, 主要包括～145 Ma、～450 Ma、～1000 Ma、～1800 Ma、～2500 Ma的主峰(图2)。碎屑锆石年龄谱揭示渐新世以来2500～1000 Ma碎屑锆石逐渐增多。结合珠江水系基岩特征: 沿海地区主要为中生代的花岗岩基, 向西为加里东期中‒高级变质的云开大山, 再往西为扬子地块; 推测珠江口盆地始新世‒中新世沉积物碎屑锆石年龄谱的变化, 反映了珠江水系逐渐向西拓展。渐新统以来, 2500～1000 Ma的碎屑锆石逐渐增多的现象, 反映了现代珠江可能形成于这一时期。

地层与沉积相改自: Cao et al., 2018; 米立军等, 2019。珠江口盆地εNd值来源: BY7-1-1据Yan et al., 2018; ODP1148据Li et al., 2003。碎屑锆石年龄数据来源: S2-5(Zhong et al., 2017); C345(Xu et al., 2016b); H9-(1, 2)、X28-(4, 5, 6, 7)、X28-(8, 9)(Cao et al., 2018); L21-(1, 2)、L21-3、L21-(4, 5, 6)、L13-1、L13-(2, 3)、L18-1、L18-2、X28-1和X28-2(Shao et al., 2016); P27和P33-2(Wang et al., 2019); HZ-1、L21-(3, 4, 5, 6)和U1435-(1～9)(Shao et al., 2017)。

珠江口盆地沉积物全岩地球化学、Nd和Sr同位素等结果也支持碎屑锆石U-Pb年龄反映的古水系演变过程。例如, 珠江口盆地BY7-1-1与ODP1148钻孔Nd同位素曲线均呈现出两个显著的变化特征, 即27～25 MaNd值突降, 并在11 Ma左右出现陡增。这两次Nd值变化反映了珠江水系的形成和调整: 27～25 MaNd值突降并在后期持续降低, 反映了珠江水系不断向西江上游扩展, 因为该流域岩石Nd值较低, 约为−15～−7(Yan et al., 2018); 而11 Ma左右Nd值陡增, 可能反映了柳江和桂江支流的汇入, 由于这两个流域内岩石Nd值较高(约为−7～−5; Yan et al., 2018)。

珠江水系自古近纪向西不断的拓展, 反映了现今地形形成的过程。在沿海山脉假说的框架下, 指示受沿海山脉高地形阻挡(当时可能是分水岭), 古近纪珠江口盆地碎屑供给主要源自沿海山脉东侧; 晚渐新世以后, 珠江口盆地逐渐接受源自现今珠江中‒上游的碎屑供给, 揭示了沿海山脉逐渐被剥蚀夷平, 珠江水系逐渐向西拓展, 并袭夺了沿海山脉以西水系。

1.2 西江中‒上游盆地水系演化

本文选择西江中‒上游中‒新生代盆地(玉林盆地、十万大山盆地、南宁盆地和百色盆地)(图1), 利用野外古水流测量手段, 重建白垩纪以来西江中‒上游的水系格局, 为珠江水系及流域内地形的演变提供沉积学方面的约束。古水流方向主要依据砾岩中叠瓦状排列的砾石与斜层理(图3c～e): 地层校正水平后, 叠瓦状排列的砾石最大扁平面倾向即为古水流的上游方向, 而斜层理纹层倾斜的方向则指示古水流的下游方向(Potter and Pettijohn, 1977)。经野外多点位的观测, 获得白垩系、古近系和新近系古水流数据, 并绘制了古水流玫瑰花图(图3)。

(a) 玉林、十万大山、南宁、百色盆地分布与古水流测试结果; (b) 白垩纪、古近纪和新近纪古流向玫瑰花图, 其中白垩纪玉林盆地古流向图中黑色部分采自盆地东‒中部, 而灰色部分采自盆地西侧; (c) 十万大山盆地白垩系中的斜层理; (d～e) 南宁盆地新近系砾石层与叠瓦状排列的砾石。

西江中‒上游地区白垩纪的古水流记录主要分布在玉林盆地和十万大山盆地中(图3a)。玉林盆地白垩系中同时获得东南向和西北向的古水流信息, 其中东南向古水流仅分布在盆地西侧, 而西北向的古水流则广泛分布于盆地东侧与中部, 指示西北向水流和沉积物供给多于东南向水流。玉林盆地西北侧的十万大山盆地白垩系中, 同样发育以北向‒西北向为主的古水流, 揭示华南南部西江中段在白垩纪主体上受西北流向水系的控制, 指示这些盆地东侧地区地形相对较高, 与沿海山脉模型一致。

古近纪, 随着百色盆地和南宁盆地的形成, 四个研究盆地均记录了该区域的水系格局(图3b)。其中, 玉林盆地和十万大山盆地古近系中古水流为自东南向西北, 指示这些盆地东侧地区地形仍相对较高。前人研究表明, 南宁盆地和百色盆地古近纪时期为一连通的大型湖泊(国家地质总局宜昌地质矿产研究所, 1979), 此时该湖盆为来自盆地周边(包括东、西侧)的物源供应(陈元壮等, 2004)。这也反映了西江中游地区古近纪时发育双向水系, 自东向西和自西向东的水流在广西百色‒南宁湖盆汇集, 指示南宁和百色盆地所在区域地势相对较低, 东西两侧地形相对较高。

新近纪, 西江中‒上游水系格局可在南宁盆地中找到记录。在南宁盆地右江‒邕江附近多个点位发现一套新近纪砾石层(图3a), 该砾石层由黄色砾石组成, 夹砂岩透镜体(图3d), 层厚大于5 m, 最厚处大于20 m, 其角度不整合覆盖于灰白色渐新统粉砂岩‒泥岩之上。野外可见层中砾石磨圆度较好, 主要成分为石英岩(>90%), 指示沉积物搬运距离较远、水动力较强, 推测其可能为大型河流(古右江)的河道沉积。砾石层古水流测量结果显示(图3b), 西江中‒上游新近纪发育自西向东的大型河流, 与现今南宁盆地的水系格局相似, 指示此时现今地形雏形已基本形成。遗憾的是, 由于该套砾石层尚无定量的年龄限定, 其在珠江水系及沿海地形演化方面的意义尚待进一步挖掘。

综上, 西江中‒上游地区白垩纪‒古近纪发育自东向西的古水系, 流入玉林盆地和十万大山盆地。始新世, 珠江口盆地接受来自盆地西侧近缘水系的碎屑物质供给, 指示两个盆地之间存在分水岭, 因此沿海山脉可能持续到了始新世。新近纪, 珠江口盆地碎屑物源信息和南宁盆地砾石层所揭示古水流信息, 均说明与现代类似的珠江水系已经形成, 指示沿海山脉逐渐被剥蚀夷平。

另外, 古近纪之后百色‒南宁湖盆发育, 并接受东、西两侧汇入的水系与碎屑物质供给, 可能指示盆地西侧的青藏高原东南缘逐渐形成。

2 青藏高原东南缘构造与地形演化

现今青藏高原东南缘地貌特征明显不同于喜马拉雅式和龙门山式的陡变地貌, 其在～1500 km水平距离内, 平均海拔从青藏高原内部的～5 km向东南逐渐缓慢降低至～1 km(Clark et al., 2006; Liu-Zeng et al., 2008), 该向东下倾的地形可能在渐新世前后形成。

青藏高原东南缘被NW-SE走向哀牢山‒红河走滑断裂、NE-SW走向雅砻‒玉龙逆冲断裂和金河‒箐河逆冲断裂, 以及SN走向鲜水河‒小江走滑断裂所围限, 这些大型断裂晚始新世‒早中新世(主要集中在渐新世)时期经历了强烈的转换挤压和缩短作用变形(图4)。①哀牢山‒红河断裂在34～28 Ma以来经历了约700±200 km大规模的左旋走滑剪切作用(Leloup et al., 1995, 2001; Searle et al., 2010; Liu et al., 2019), 并随之于30～17 Ma 断裂上盘经历快速剥蚀(Leloup et al., 2001; Cao et al., 2011), 剥蚀量约为15～10 km(Wang et al., 2020c)。②雅砻‒玉龙断裂和金河‒箐河断裂表现为由北西向南东逆冲的推覆构造, 其逆冲变形导致了这两条断裂上盘分别于35～25 Ma和20～17 Ma遭受快速剥蚀过程(Zhang et al., 2016a; Cao et al., 2019; Zhu et al., 2021)(图4)。③渐新世, 鲜水河‒小江走滑断裂系受强烈挤压变形的影响, 于32～27 Ma发生了混合岩化作用, 安宁河断裂(鲜水河断裂向南的连续段)上盘在24～18 Ma发生了快速剥蚀过程(Wang et al., 2020a)(图4)。值得一提的是, 这些断裂在晚中新世也遭受了显著的构造变形(Wang et al., 2012a, 2020c; Tian et al., 2013; Zhang et al., 2017)。因此, 青藏高原东南缘大型走滑兼/或逆冲断裂发育时间大致在晚始新世‒渐新世。

青藏高原东南缘新生代盆地沉积物的古海拔重建结果指示, 晚始新世‒渐新世青藏高原东南缘抬升显著(图4)。始新世, 雅砻‒玉龙逆冲断裂以北始新世盆地(黎明盆地、兰坪盆地和剑川盆地)古海拔已达到2.7±0.3 km～3.3±0.5 km(Hoke et al., 2014; Li et al., 2015); 青藏高原东南缘的吕合盆地和岔科盆地整体古海拔比现今海拔低约1 km, 并呈现向东南方向逐渐降低的趋势(Hoke et al., 2014; Li et al., 2015; 唐茂云等, 2021)。这些结果表明, 青藏高原东南缘晚始新世时期地形整体表现为向东南倾斜的特征。但晚中新世, 青藏高原东南缘小龙潭盆地古海拔已达到现今的高度(Li et al., 2015)。

数据来源: (1) Leloup et al., 1995, 2001; (2) Li et al., 2020; (3) Shen et al., 2016; Cao et al., 2019; (4) Cao et al., 2020; (5) Liu-Zeng et al., 2018; (6) Wang et al., 2012a; (7) Li and Zhang, 2013; (8) Zhang et al., 2016a; (9) Wang et al., 2012b; (10) Zhu et al., 2021; (11) Wang et al., 2020a; (12) Li et al., 2015; (13) Hoke et al., 2014; (14) 唐茂云等, 2021; (15) Gourbet et al., 2017; (16) Su et al., 2019; (17) Xiong et al., 2020。

Gourbet et al. (2017)基于大陆效应(降雨的同位素组成随着远离海岸线而逐渐降低), 对剑川盆地的古海拔数据进行重新研究, 认为早期重建的盆地古海拔偏高了约1.5 km。但也有研究者认为青藏高原东南缘‒南海北缘地区当时处于热带, 仅存在弱的(甚至是不存在)大陆效应(Hoke, 2018)。还有研究者认为, Gourbet et al. (2017)重新分析所使用的基准值不能代表海平面(Ingalls et al., 2018)。尽管研究者对于本地区古海拔重建的具体结果还存在争议, 但他们都一致认为始新世以来, 青藏高原东南缘‒南海北缘地形始终保持向东南逐渐降低的趋势。

古水流重建结果也支持青藏高原东南缘地区大河东南流的格局在晚始新世已经形成, 并指示此时青藏高原东南缘的高海拔地貌已有雏形。剑川盆地沉积学和碎屑锆石U-Pb年代学等研究也显示, 该盆地始新世沉积物主要源自金沙江流域, 晚始新世长江石鼓第一湾形成, 随后金沙江不再流经剑川盆地(Zheng et al., 2020; Zhang et al., 2021b)。然而, 关于金沙江何时切穿三峡仍存在较大争议。长江下游南京地区晚新生代砂砾层碎屑锆石U-Pb年龄研究表明, 渐新世‒中新世之交一条发源于青藏高原的长江已经东流入海(Zheng et al., 2013); 但是, 碎屑白云母和钾长石Ar-Ar定年与Pb同位素物源研究却认为, 这套地层主要由近源的长江下游地区供给(Sun et al., 2021; Zhang et al., 2021a), 表明当时古长江流域范围与现代长江有很大区别。另有研究认为, 渐新世‒早中新世长江中‒上游水系可能汇入莺歌海盆地(Clift et al., 2006, 2020)。然而, 碎屑钾长石Pb同位素物源示踪显示, 晚始新世以来, 莺歌海盆地最主要的物源来自红河(Zhang et al., 2021b), 并且莺歌海盆地沉积物与青藏东南缘其他主要大河(如怒江、澜沧江、金沙江、雅砻江和岷江等)的物源信号差异较大, 表明这些河流晚始新世以来并未汇入此盆地。另一种可能, 长江中‒上游水系曾在始新世‒早中新世南流, 经思茅盆地, 最后汇入东南亚的呵叻盆地(Chen et al., 2017; Wang et al., 2020b)。虽然目前呵叻盆地已有少量沉积学和碎屑锆石U-Pb年龄物源示踪研究的报道(Cater, 1999; Wang et al., 2020b), 但详细的水系重组过程仍需进一步深入研究。

3 东亚南部构造与地貌演化及动力学机制

综合前述关于沿海山脉和青藏高原东南部的地形演化, 显示白垩纪东亚南部地形东高西低, 东部发育沿海山脉, 西部青藏高原东南缘尚未抬升(图5a; 古新世‒早渐新世东侧沿海山脉尚存, 西部青藏高原东南缘已经抬升(图5b), 区域上呈“V”字型地形特征; 晚渐新世以后, 东侧沿海山脉逐渐消失, 青藏高原东南缘持续抬升, 逐渐形成现今西高东低的地形(图5c)。

东亚东部一级地形的演变, 与东亚东、西陆缘主要构造事件在时间和动力学机制上相对应。白垩纪之前, 太平洋板块前进式俯冲导致东亚陆源发生强烈的挤压、地壳缩短、大量岩浆作用和地表的抬升, 形成安第斯型大陆边缘(Li and Li, 2007; Suo et al., 2019)。此后, 太平洋板块俯冲后撤使得东亚南部发生广泛的地壳伸展作用, 期间伴随着短暂的地壳缩短(Wang and Shu, 2012; Li et al., 2014), 在华南板块之上形成了一系列NE向的盆地和山岭, 类似于北美的盆岭省构造(Gilder et al., 1991; Wang and Shu, 2012)。与北美的情况相似, 沿海地区向内陆倾斜的地形在伸展背景下仍然得以保留, 并延续到古近纪(图5a)。因此, 晚渐新世之前, 沿海地区高地形与太平洋板块的俯冲以及晚中生代由前进式俯冲转为后撤撕裂(Zhou and Li, 2000; Dong et al., 2018; Guo et al., 2021)有关。

白垩纪, 青藏高原地区地壳缩短和地表隆升主要集中于羌塘和拉萨地块(Kapp and DeCelles, 2019)。新生代早期, 印度‒亚洲板块间的陆陆碰撞及随后的汇聚最终使得青藏高原及其邻近地区发生强烈的隆升(Kapp and DeCelles, 2019)。始新世以来, 青藏高原东南缘地壳缩短、侧向挤出导致地表持续抬升, 并形成了向东南方向逐渐降低的地势(Hoke et al., 2014; Li et al., 2015; Xiong et al., 2020), 构成了本研究所重建的古近纪“V”字形过渡地形的西支(图5b)。

图5 东亚一级地貌演化

晚新生代, 青藏高原东南缘持续发生垂向和侧向生长(Tapponnier, 2001; Clark et al., 2005; Tian et al., 2014); 而且受西太平洋俯冲后撤及其他动力机制影响, 日本海和南海等东亚边缘海在晚渐新世和中中新世拉开(Taylor and Hayes, 1983; Jolivet et al., 1994; Xu et al., 2016a)。东南沿海的地形随着地壳减薄与侵蚀基准面的降低逐渐下降。另外, 同期东亚季风的形成(Guo et al., 2002; Zhang et al., 2018), 导致沿海地区降雨量增大, 剥蚀加速, 地形降低。这些因素共同作用导致了沿海山脉的夷平, 进而形成了现今东倾的一级地形(图5c)。

4 结论与展望

通过对西江中‒上游玉林、十万大山、南宁和百色等白垩纪‒新生代盆地沉积学的研究, 以及对前人关于珠江口盆地沉积物物源与青藏高原东南缘构造与地形演化研究的综合, 支持并丰富了东亚大陆地形演变的“跷跷板”模型: 即中生代地形东高西低, 与现今东倾地形相反; 古近纪地形呈两侧高、中部低的“V”字型样式; 晚渐新世以来, 现今东倾的地形逐渐形成。该地形演变过程受控于东亚东、西陆缘的构造事件, 包括东缘太平洋板块西向俯冲及边缘海的打开, 西缘新特提斯洋闭合及随后印度‒欧亚板块陆陆碰撞和汇聚。现今对东亚东缘地形演化的研究多为定性的推测, 少有定量的古高程重建(Zhang et al., 2016b)。由于沿海山脉走向与东亚夏季风的风向(即水汽来源方向)近垂直, 其存在与否及时限对于东亚夏季风的形成可能具有较为重要的意义, 因此也值得关注。

致谢:感谢长沙市地震局陈东旭、田野、唐苑、屠艳艳、周启明在野外工作中提供的帮助, 以及中国科学院广州地球化学研究所郭锋研究员和两位匿名审稿专家提出的宝贵建议。

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

TIAN Yuntao1, 2, QIN Yonghui1, HU Jie3, ZHANG Guihong1, LIU Yimin1, PAN Lili1, YAN Zhaokun4, ZHANG Zengjie1, SUN Xilin1, ZHANG Peizhen1, 2

(1. Guangdong Provincial Key Laboratory of Geodynamics and Geohazards, School of Earth Sciences and Engineering, Sun Yat-sen University, Zhuhai 519082, Guangdong, China; 2. Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai 519082, Guangdong, China; 3. State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu 610059, Sichuan, China; 4. State Key Laboratory of Nuclear Resources and Environment, East China University of Technology, Nanchang 330013, Jiangxi, China)

TheCretaceous-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.

East Asia, Tibetan Plateau; tectonic geomorphology; sedimentary basin; paleo-drainage evolution; landscape evolution

2021-12-10;

2022-02-25

国家自然科学基金项目(U1701641、42172229)、南方海洋科学与工程广东省实验室(珠海)自主科研项目(SML2021SP315)和广东省引进人才创新创业团队(2016ZT06N331)联合资助。

田云涛(1984–), 男, 教授, 博士生导师, 主要从事构造地质与热年代学研究。E-mail: tianyuntao@mail.sysu.edu.cn

P542

A

1001-1552(2022)03-0471-012

10.16539/j.ddgzyckx.2022.03.005