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金沙江金江街段河流阶地年代及对河谷水系演化历史的启示
董铭1,, 苏怀1,, 史正涛1, 明庆忠2, 何回丽3
1. 云南师范大学旅游与地理科学学院,昆明 650500
2. 云南财经大学旅游文化产业研究院,昆明 650221
3. 中国科学院昆明植物研究所,昆明 650201

作者简介:董铭(1976-), 女, 吉林白山人, 硕士, 副教授, 主要从事第四纪环境演化与数字地貌研究。E-mail: dongm2001@sina.com

通讯作者:苏怀(1977-), 男, 云南保山人, 博士, 副教授, 主要从事河流地貌研究。E-mail: suhuai07@163.com
摘要

100多年来,关于金沙江独特水系格局的形成历史一直是地学界争论的重要话题之一。多数学者认为,现代金沙江水系是古长江袭夺古红河上游发展过来的。红河海底扇5.5 Ma泥沙供给中断被认为与这一袭夺事件有关。然而,长期以来人们一直没有找到与这一时代相匹配的地貌证据。最近在金沙江金江街段找到了多达8级的河流阶地序列,ESR测年结果显示这些阶地的形成年代为1.07 Ma、0.70 Ma、0.65 Ma、0.51 Ma、0.47 Ma、0.44 Ma、0.30 Ma和0.18 Ma,结合GPS高程测量数据,推算最近1.0 Ma以来的河谷平均下切速率为147 mm/ka。以填充河谷地形为主要手段的古地形恢复结果(基于DEM数据)显示,古长江袭夺古红河上游形成现代金沙江水系发生在这一区域内海拔2000 m左右的古地形面解体之后,依照河谷平均下切速率外推,古地形面解体时代为5.5 Ma,即现代金沙江水系形成于5.5 Ma之后。我们的研究结果与红河海底扇的资料形成一个相互呼应的证据链,为重建现代金沙江水系格局形成历史提供重要依据。

关键词: 金沙江; 金江街; 阶地; 水系演化;
The age of river terraces in the Jinjiangjie reach of the Jinsha River and its implications for valley and drainage evolution
DONG Ming1,, SU Huai1,, SHI Zhengtao1, MING Qingzhong2, HE Huili3
1. College of Tourism and Geography Sciences, Yunnan Normal University, Kunming 650500, China
2. Research Institute of Tourism and Culture Industry, Yunnan University of Finance and Economics, Kunming 650221, China
3. Kunming Institute of Botany, CAS, Kunming 650201, China
Abstract

The Jinsha River has attracted considerable attention for nearly a century due to its unusual drainage basin morphology. Most models describing its evolution suggest that the modern Jinsha River, draining the Tibetan Plateau margin, was once a tributary to a single, southward river system called "Paleo-Red River", which drained into the South China Sea and then its flow direction changed to east to join the Yangtze River due to river capture. The Red River submarine fan, considered to have been primarily fed by the Paleo-Red River system, suddenly disappeared at 5.5 Ma provides an important chronological constraint on this reorganization of drainage lines and reversal event. However, no geomorphic evidence has been found to agree with this hypothesized timeframe. Here, we present electron spin resonance (ESR) ages from eight terraces preserved in the Jinjiangjie reach of the Jinsha River together with their GPS altimetry data. Their ages from old to young are 1.07 Ma, 0.70 Ma, 0.65 Ma, 0.51 Ma, 0.47 Ma, 0.44 Ma, 0.30 Ma and 0.18 Ma, with a calculated average river incision rate of 147 mm/ka since 1.0 Ma. The paleo-topography, reconstructed by filling the deeply incised river gorges with digital elevation model (DEM) data, shows that the upper reach of the Paleo-Red River was captured by the Yangtze River and changed its flow direction eastward at the time of disruption of the 2000 m asl paleo-topographic surface in the Jinsha River drainage basin. The age of the paleo-topographic surface formation would be approximately 5.5 Ma using the average river incision rate extrapolation, suggesting that the present Jinsha River system was born after 5.5 Ma. This data support the chronological constraint from the Red River submarine fan, and hypothesized evolution of the Jinsha River.

Keyword: Jinsha River; Jinjiangjie reach; terrace; drainage evolution;
1 前言

金沙江河谷水系的演化历史是国内外地貌学和地质学界研究的热点问题之一[1,2,3,4,5],河流阶地是目前学术界公认的研究河谷水系演化历史的重要信息载体[6,7]。通过对河谷两侧阶地年代学和沉积学研究,人们可以确定现代河谷水系的形成时代。然而,目前在金沙江中下游河段(石鼓至宜宾)河流阶地的研究还比较薄弱。主要体现在,以往的研究多集中在测定低阶地的年龄,时代一般不老于晚更新世,而对高阶地,即形成于中更新世、甚至早更新世的阶地研究较少[8,9,10]。究其原因,主要是高阶地在深切河谷区不易保存、而在容易保存的宽阔段落又被巨厚的坡积物覆盖。近年来通过长期野外考察,研究团队在金沙江金江街段找到了多达8级以上的河流阶地,保存如此完整的阶地序列在整个金沙江中下游干流河段都属罕见的。为此,本文对这些阶地进行了系统的年代学研究,以期为金沙江中下游的河谷水系演化历史研究提供重要线索。

2 阶地序列及形成年代
2.1 阶地序列

金沙江金江街段位于金沙江中游(石鼓至攀枝花),北邻程海、南接宾川盆地,金沙江一级支流宾居河在此与之交汇(图1),是整个金沙江中游河谷谷底较为宽阔的地区。野外调查发现这一地区金沙江至少在河谷谷坡上发育8级以上阶地(图2),各级阶地由低到高描述如下:

图1 研究区及其各级阶地代表性剖面位置 Fig. 1 The location of the study area and representative sections of terraces

图2 金沙江金江街段阶地序列综合剖面 Fig. 2 The terrace sequences in the Jinjiangjie reach of the Jinsha River

T1堆积阶地。河床相砾石层可见厚度8 m以上,上覆2 m左右河漫滩相粉砂层。主要分布在沿江村、安坪村和寨子村等段落,差分GPS(cm级精度)测量阶地面(河漫滩相粉砂层顶)海拔高度在1202.12~1199.37 m,拔河高度约10 m。

T2在金沙江左岸表现为堆积阶地,主要分布在沿江村、寨子村等河谷宽阔的地段,差分GPS测量阶地面(河漫滩相粉砂层顶)海拔高度在1249.33~1231.25 m,拔河高度40~50 m。河床相砂砾石层可见厚度约10 m,上覆3 m左右含砾石透镜体河漫滩相粉砂,之上是10 m左右具有水平层理的亚黏土粉砂层,这套粉砂层在一些文献里被认为是堰塞湖沉积[11]。之上有薄层黄土覆盖。在金沙江右岸,该级阶地基座出露,基座为二叠系玄武岩。

T3基座阶地。基座为二叠系玄武岩,零星分布于金沙江两岸,河床相砂砾石层厚约2 m,之上是2 m左右河漫滩相粉砂层覆盖,差分GPS测量阶地面(河漫滩相粉砂层顶)海拔高度在1259.87~1254.55 m,拔河高度约70 m。

T4基座阶地。基座为二叠系玄武岩,在金沙江两岸都有分布。河床相砂砾石层厚2 m左右,之上河漫滩相粉砂层厚2~3 m。差分GPS测量阶地面海拔高度在1270.37~1270.97 m,拔河高度约80 m。

T5基座阶地。基座为二叠系玄武岩,仅零星分布于金沙江左岸,河床相砂砾石层厚1.5 m左右,上覆河漫滩相粉砂层厚约2.5 m,差分GPS测量阶地面(河漫滩相粉砂层顶)海拔高度在1274.97~1275.04 m左右,拔河高度约85 m。

T6基座阶地。基座为二叠系玄武岩,在金移村保存最为完整,河床相砂砾石层厚约3 m,上覆2 m左右的河漫滩粉砂,之上是厚达40 m含亚黏土的粉砂层,其底部依稀可见水平层理,结构与T2阶地上覆亚黏土粉砂层类似。差分GPS测量阶地砾石层顶面海拔1282.43~1282.55 m,拔河高度约92 m。

T7基座阶地。基座为二叠系玄武岩,右岸局部地区河床相砂砾石层下面有一层具水平层理粉砂层,河床相砂砾石层厚约2 m,顶部河漫滩相粉砂层厚约3 m,差分GPS测量阶地面(河漫滩相粉砂层顶)海拔高度在1310.11~1309.87 m,拔河高度120 m。

T8基座阶地。基座为二叠系玄武岩,在金沙江左岸局部段落河流相砂砾石层之下粉砂层下伏。河流相砂砾石层厚2~3 m。差分GPS测量阶地面(河漫滩相粉砂层顶)海拔1335.31~1334.93 m,拔河高度140 m。

野外调查发现,各级阶地河流相砾石成分主要为玄武岩、辉长岩、闪长岩、砂岩、粉砂岩和少量石灰岩,磨圆度为次圆,与现代金沙江河滩砾石一致。

2.2 阶地形成年代及河谷下切速率

目前能够测定更新世以来轨道尺度河流阶地年代的最可靠方法是黄河流域普遍采用的黄土—古土壤断代法[12,13],但是在金沙江中下游地区河流阶地上普遍缺乏黄土堆积,这种方法难以推广。近年来,人们尝试使用10Be、26Al等宇生核素测定金沙江流域的河流阶地暴露年龄和埋藏年龄[14,15]。尽管取得了一些重要进展,但还存在一些技术障碍。比如,对暴露年龄而言,由于很难准确估算出研究区的侵蚀状况,测年结果与真实值可能存在较大偏差[16],而测定埋藏年龄则需要厚达8.5 m以上的沉积物覆盖在测年样品上才能有效屏蔽宇宙射线的干扰[17],而在金沙江流域覆盖超过8 m以上沉积物河流阶地是比较少见的,多数阶地河流相砾石层加上上覆的盖层也只有2~3 m厚,很难满足测定要求。此外,宇生核素测年费用很高。因此,本文选用价格相对较低、对沉积物覆盖厚度要求不高、且在国内方便测试的电子自旋共振(ESR)测年技术来测定河流阶地的埋藏年龄。实践表明ESR测年技术仍然是目前河流阶地年代学研究的有效手段[18,19]。ESR测年技术最大的问题在于残留信号不能被阳光有效晒褪[20],最近笔者通过对现代河漫滩样品测试发现,在金沙江流域未晒褪的残留信号引起的误差仅有0.01 Ma左右(表1,河漫滩样品),对中更新世以前形成的阶地测年造成的误差(相对误差)还不算大。

表1 金沙江金江街段各级阶地的ESR年龄 Tab. 1 The ESR ages of terraces in the Jinjiangjie reach of the Jinsha River

ESR年代学样品采自各级阶地河床相砂砾石层上部的砂层,工作剖面多数为机械开挖的新鲜露头。采样点之上至少有2 m以上河漫滩相沉积物覆盖以确保样品自然埋藏后ESR信号不被日光晒褪。除T1和T3阶地外,其余阶地ESR测年材料为石英。T1和T3阶地测年材料为次生碳酸盐。与石英一样,碳酸盐晶胞内的[CO3]2-基团在射线轰击下也能形成空位,俘获自由电子变成顺磁中心。阶地中的次生碳酸盐ESR年龄代表河流下切,阶地形成,次生碳酸盐发育的年龄,也可以大致代表河流阶地的形成年龄。

样品制备与测量工作在成都理工大学应用核技术研究所进行,样品U、Th、K含量用CIT-3000F数字化全自动铀钍钾谱仪测定,经湿度修正计算得到年吸收量。古剂量则通过建立附加剂量曲线求出、附加剂量采用60Co照射,强度分别设为0、400 Gy、700 Gy、1000 Gy、1500 Gy、2000 Gy、3000 Gy、4000 Gy、6000 Gy。顺磁测定在德国ER-200D-SRC电子自旋共振仪中进行,微波频率为9.7652 GHz,功率0.21~0.30 mW,扫场范围3462.5~3550.0 G,中心场3500.0 G。选用E'信号进行拟合推算,附加剂量拟合曲线线性关系良好(图3),各级阶地年代的测试结果如表1所示。

图3 代表性样品附加剂量与ESR信号强度关系 Fig. 3 The relationship between additional dose and ESR signal intensity of the representative sample

推算河谷下切速率的河流阶地高程,主要采用精度到cm级的差分GPS测量,测点位置选在阶地河床相砂砾石层顶部。这里地层界线清晰便于识别,且与ESR年代学样品采集位置接近,有利于减小下切速率的计算误差。每级阶地的高程以5个平行测点的平均值代表。考虑到同级阶地砾石层顶面的高差变化范围都在1 m左右,推算河谷下切速率时,阶地高程值(单位:m)取整数值。初步推算发现,对于拔河高度超过100 m,时代超过1 Ma的阶地,1 m的高程误差造成的下切速率误差不到1%。

利用拟合手段,建立各级阶地年龄与高程的线性关系,结果表明,1 Ma以来金沙江金江街段河谷下切速率约为147 mm/ka(图4)。该下切速率是金沙江金江街段获取的首个跨度超过1 Ma年的有绝对测年数据标定的河谷平均下切速率。在金沙江流域,目前干流河谷的平均下切速率能由8级以上阶地年龄测算的也仅此一处。

图4 金沙江金江街段河流阶地高程与年代关系 Fig. 4 The relationship between height and age of river terraces in the Jinjiangjie reach of the Jinsha River

3 古水系格局重建

在金沙江中游地区,河谷谷肩至分水岭高度广泛发育着一系列宽阔平坦的古地形面,先前的研究认为,金沙江现代深切河谷的地形就是在这些平坦地形面解体之后形成的[21,22]。也就是说,河谷侵蚀是塑造现代金沙江中游地形的最主要过程,而古地形面上的侵蚀是微不足道或次要的[23]。显然,如果逆向思考,将河谷中侵蚀掉的物质填回,侵蚀前的古地形就可以大致恢复出来,进而就能重建当时的古水系格局。

结合野外地形调查,设定海拔1600 m、1800 m、2000 m及2200 m为古地形填充高度以重建相应的古水系格局。模型所用的地形数据为GDEM数据,空间分辨率为30 m×30 m,水系提取利用ArcGIS软件中的ArcSWAT模块实现。结果发现:填充海拔1800 m下的河谷,金沙江与周围水系格局与现代水系格局相差无几;而填充至海拔2000 m后,古水系格局开始发生明显变化,龙川江、普渡河等支流不再流向金沙江,而是汇入红河,在青藏高原东南面出现了一个流域面积比现在大得多的古红河水系。进一步将河谷填充至2200 m以上,南流的红河水系表现得更为明显(图5)。该研究结果表明,过去普遍认为的古长江袭夺古红河水系形成现代金沙江水系[21, 24]的假说在仅考虑区域地貌主要受河谷侵蚀塑造的层面上是成立的。

图5 填充至不同海拔高度河谷恢复的古水系场景 Fig. 5 Ancient drainage reconstruction scenarios created by filling the valley to different altitudes (a, b, c and d represent valleys filled to altitudes of 1600 m, 1800 m, 2000 m and 2200 m, respectively, to reconstruct ancient drainage)

根据本文测算的更新世以来的河谷平均下切速率外推,计算海拔2000 m地形面解体的时间,结果为5.5 Ma,即金沙江现代水系格局形成于5.5 Ma之后。值得注意的是在我们的古水系重建模型中,长江第一弯附近的河流即使在5.5 Ma前也未出现明显改道和袭夺迹象,表明即使长江第一弯真是袭夺弯,它的形成应该早于上新世。这与最近在长江第一弯利用宇宙成因核素测年推算的第一弯袭夺不晚于9 Ma的结论[25]是一致的。

需要强调的是本文的古水系格局重建模型是在没有考虑区域大断层活动影响下建立的。因为目前很多资料表明金沙江流域的大型断层发生大规模活动的时期主要在渐新世至中新世[26,27,28],而本文探讨的水系演化问题已经是中新世末到上新世初的事情了。

4 讨论

相比黄河流域,作为中国南方最大的河流长江的上游金沙江流域的河流阶地年代学研究是比较薄弱的。以往的研究多获取的是晚更新世以来的河流阶地年龄,而对于早、中更新世的阶地年龄涉及不多[8,9,10]。本文报道的金沙江金江街段河流阶地是迄今为止人们在金沙江干流获取的有年代标定的阶地级数最多的序列。这些有年代标定的阶地对了解区域地壳活动历史[28],验证青藏高原东南缘的大地构造学说[29]有重要意义。在先前的研究中,有学者以河流阶地高度估算过金沙江金江街段更新世以来的河谷下切速率,大概在450 mm/ka左右,并将此认为是更新世以来高原东南缘下地壳加速活动的证据[28]。然而这一估算缺少绝对测年数据支撑,而本文通过绝对测年获得的河谷下切速率仅有147 mm/ka,比先前的认识要小的多。

多年来,人们一直试图从河流阶地中直接找出金沙江水系袭夺演化的直接证据[3, 10, 30-32],但至今没有得到公认可信的结果。究其原因主要是河流阶地的证据与河口沉积学证据在时代上不相匹配。从公认的金沙江水系演化模型[21]来看,如果现代金沙江是古长江向西袭夺古红河水系的结果,那么在红河河口的沉积物(红河海底扇)就应该有所记录。红河海底扇在地质历史时期是发生过急剧萎缩事件,只不过时间在5.5 Ma[29]。然而,迄今为止在金沙江流域所报道过的河流阶地年龄很少有老于更新世的[3, 8-10],在金江街段获得的最老阶地年龄也只有1.07 Ma。因此,利用河流阶地很难直接记录古长江袭夺古红河历史。但是如果结合古地形分析,就可以利用河流阶地测年数据外推获得与红河海底扇记录年代相当的现代金沙江水系格局的形成年龄(5.5 Ma)。有必要指出的是,古水系的演化是一个连续变化的过程[1],本文的研究结果只是将具有现代流向的金沙江水系出现的历史推到5.5 Ma以前,并不意味之前金沙江水系就没有发生过变化。

5 结论

(1)金沙江金江街段至少发育8级以上阶地,各级阶地的ESR年龄分别为:1.07 Ma、0.70 Ma、0.65 Ma、0.51 Ma、0.47 Ma、0.44 Ma、0.30 Ma和0.18 Ma。以河流阶地年代推算的1.0 Ma以来河谷的平均下切速率为147 mm/ka。

(2)填充河谷恢复古地形结果显示,现代金沙江水系格局诞生于海拔2000 m地形面解体之后,按河谷下切速率外推时代为5.5 Ma。

The authors have declared that no competing interests exist.

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绪言云南西部地区的地形是十分复杂的,峡谷刻入高原,谷底与两旁山顶的高差往往在1,000米以上,尤其是金沙江横切大雪山部分,造成了举世闻名的虎跳涧峡谷,相对高差竟达3,000米左右。此外,矗立正高原而上的山岭,绝对高度超过4,000米以上者为数亦不少。例如丽江县玉龙山主峰的高度约达6,000米,山顶终年积雪,有近代冰川。
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[33] Wang Yingmin, Xu Qiang, Li Dong, et al.Late Miocene Red River submarine fan, northwestern South China Sea. Chinese Science Bulletin, 2011, 56(14): 1488-1494.
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