2021, Vol.41
2 甘肃省地质调查院, 甘肃 兰州 730000)
新生代印度与欧亚板块不断汇聚导致青藏高原快速隆起并持续扩展，成为全球最高的高原，对东亚地貌、构造和气候格局以及全球环境都产生了重要影响[1~5]。因此，有关高原隆升历史、扩展过程、动力机制、环境效应等问题依然是备受国际地学界关注的前沿课题[4, 6~8]。随着挤压应力由俯冲带进一步向青藏高原东北和东南边缘传递[4]，其外围发育了一系列中低山体，被普遍认为构成了高原最年轻的组成部分[9~11]。探讨它们的隆起历史对揭示高原扩展过程有重要意义[10~14]，吸引了国内外众多地球科学家的关注，是当前地学研究的热点问题之一。
目前，在青藏高原东北缘外围发育了一系列中低山体，如黑山、金塔南山、合黎山、榆木山等(图 1a)。已开展的低温热年代学、沉积学和构造地质学的研究显示[10~18]，自祁连山向东北方向，这些山体的隆起时代不但表现出逐渐变年轻的趋势，而且控制它们隆升的断层滑动速率也趋于减小[10~12, 15~18]；另外，山前盆地的沉积旋回演化也能够与高原的阶段性隆升很好地对比[13, 19~20]。这些变化特征揭示，青藏高原可能正在向东北方向逐步扩展[9~11]。在这些中低山体中，金塔南山与祁连山分别限定了酒东盆地的北部和南部边界，构成了盆山耦合的地貌体系[10, 12]。黑河阶地最新的宇生核素10 Be定年揭示，金塔南山可能隆起于230 ka[12]；然而，断层滑动速率的反推结果显示该山体可能相对较老，隆起于1.5 Ma[10]。不仅如此，学术界对金塔南山隆升机制的理解也存在不同观点，王金荣等[22]认为金塔南山隆升可能与阿尔金断裂的动力学过程有关，而郑文俊等[10, 14]将金塔南山隆升直接归因于青藏高原向东北缘的挤压。综合这些已有的研究成果，目前地学界对金塔南山隆升时代和机制的认识仍存在较大的意见分歧，究其原因除对山体隆起标志的认识不统一外，研究手段不同也是重要因素。一般认为，流经活动构造区的河流能够敏感地响应地面升降和气候变化而发生调整[23~25]，其变化过程是揭示区域地貌格局演化的重要途径之一[25~28]。其中，古河道的形成就是在自然因素(构造活动、气候变化等)或人为因素(拦河筑坝)的影响下，河流发生袭夺、改道等过程从而导致原河道被废弃[29]。剔除人为因素，在活动构造区进一步的研究认为它的发育主要受控于流域快速构造活动和气候环境的波动[30]。所以，重建河流的河道演化过程对揭示所经区域的地貌发育有重要意义[31~33]。
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图 1 北大河流域地质图 (a)北大河流域地貌、水系和地层分布图, 地层资料来源于参考文献[21], 插入图指示研究区在青藏高原的位置; (b)地形与构造横断面, 其具体位置对应图 1a中的白色线段A-A′, 该图指示文殊山和金塔南山的隆升分别受控于嘉峪关-文殊山断裂和金塔南山北缘断裂, 构造信息据参考文献[10~11] Fig. 1 Geological map over the Beida River catchment. (a)Geomorphology, drainage, and strata distribution across the Beida River catchment, the strata data is from the reference [21], the inset map displays the location of the study area on the Tibetan Plateau; (b)The topographical and tectonic section, its position is coincident with the white line(A-A′)within Fig. 1a. Which shows that the Wenshu Shan(Shan=Mountain in Chinese)and Jintanan Shan are uplifted by the Jiayuguan-Wenshushan fault and Jintanan Shan fault, respectively, the information correlated with these faults refers to the references [10~11] |
我国古河道研究主要集中在东部沿海和平原区[29~30, 34~40]，如华北平原黄河古河道[29]、莱州湾南岸平原浅埋古河道[30]、长江三角洲及下游荆江段古河道[34~35]等。其中，渤海和黄海系列钻孔研究发现末次冰盛期以来黄河和长江发育了多条埋藏古河道，其成因与气候波动和海平面升降密切相关[36~37]。在国际上，物源分析揭示尼罗河发育的一系列古河道主要受控于非洲板块持续向北移动[41]；然而对加拿大红河古河道的形成过程研究却显示，显著的气候波动才是河道废弃的主因[42]。以上国内外相关工作显示，古河道发育是河流响应构造活动和气候波动的结果[31~33, 43~45]。近年来对典型构造活动区的喜马拉雅山南麓系列河流古河道研究进一步印证了上述认识[32~33]。相比之下，目前对青藏高原东北缘的古河道研究却较为薄弱。
在活动构造区，干流与山体走向或者构造线平行的河流称之为纵向河[46]。起源于祁连山的北大河向北纵贯酒东盆地，其中游河段就沿近东西走向的金塔南山南麓发育了系列纵向古河道(图 2a)。因此，它们的地貌和沉积证据不仅记录了河流演化过程，也能够很好地揭示金塔南山的隆起过程，对理解青藏高原向东北方向的最新扩展有重要意义。因此，论文首先根据野外河道地貌和沉积的跟踪调查与连续差分测量，恢复北大河沿金塔南山南麓发育的东西向系列古河道空间展布；其次，通过河道内堆积的泥炭层进行加速器14C测年，建立这些古河道的年代序列；最后，重建北大河的河道迁移过程，并与古地震数据和气候记录对比，探讨亚轨道时间尺度气候波动和快速构造活动共同作用下的河流响应过程，从而揭示金塔南山全新世的隆升历史。
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图 2 研究区域图 (a)初步恢复的北大河3条古河道空间分布图, 其位置对应图 1a中的红色长方形, 它们在空间上近乎平行于近东西向延伸的金塔南山; (b)金塔南山南麓至现代北大河河床间残留的串珠状湖泊及干涸的河道分布图, 其位置与图 2a中红色长方形对应, 它们首尾相连在空间上整体呈3条带状分布, 其东西两端都最终与北大河现代河床汇合, 是该河流遗弃的古河道; (c)横跨北大河现代河道与金塔南山间的地形剖面, 其位置对应图 2a中线段B-B′, 3条古河道与北大河现代河床的边界被大致限定, 其主槽沉积之上直接堆积一套泥炭层 Fig. 2 Map of study area. (a)Spatial distribution of the three paleo-channels by the Beida River. Its position is coincident with the red rectangle within Fig. 1a, and the three paleo-channels are formed by the Beida River nearly parallel to the east-west extensional Jintanan Shan; (b)Spatial distribution of these residual local lakes and dry channels from the southern front of the Jintanan Shan to the current Beida River channel, and its location is coincident with the red rectangle within Fig. 2a. These lakes and dry channels can be connected with each other as three strips in the whole spatial sense, their both tips meet the current riverbed of the Beida River, thus presenting in shape three paleo-channels by this river; (c)Topographical section across the extent between the current Beida River channel and Jintanan Shan, its location is coincident with the line(B-B′) in Fig. 2a. The three paleo-channels and current riverbed of the Beida River are clearly displayed in which their main-channel-bed sediments are generally overlain by a peat layer directly |
在青藏高原东北缘外围，金塔南山呈近东西向隆起于黑山与合黎山之间，并与祁连山共同限定了酒东盆地北部和南部边界，在空间上勾勒出盆山耦合的地貌体系(图 1a)。酒东盆地是新生代的前陆盆地，其构造活动主要受控于玉门-北大河断裂、嘉峪关-文殊山断裂、黑山断裂和金塔南山北缘断裂[18, 47~49](图 1a和图 2a)。综合已有研究工作发现，晚更新世以来这些断裂无论自南向北还是自西向东，垂直错断速率都呈减缓趋势[10~11, 14]。这种分布特征说明青藏高原东北缘的地壳缩短量除被祁连山隆升部分吸收外，可能还被最外围一系列中低山体，如黑山、文殊山、金塔南山、合黎山的变形抬升接纳，进一步揭示它们可能代表了高原向东北扩展的最新和最前沿部分[10~11]。其中，金塔南山的隆升被认为主要受控于山体北麓呈东西向展布的金塔南山北缘断裂[10, 50](图 1b)，该活动断裂总长度达60 km，其地表破裂带主要由洪积扇上不连续分布的断层陡坎组成[51]。目前学术界除对它的活动性质认识有争议外，关于其垂直错断速率的研究结果也表现出较大差异[10, 50~51]。尽管如此，可以确定该断裂在全新世依然具有较强的活动性，并伴有多次古地震发生[52~53](图 2a)。
现今北大河起源于祁连山系的托赖山与托赖南山之间的冰川，流域面积约8847 km2，冰川融水补给量约占该河流总径流量的17.6 % [54]。北大河出祁连山后北流进入酒东盆地，在西端绕过文殊山后转向东流，沿北部边界的金塔南山南麓发育与山体走向近乎平行的河道。最后它在鸳鸯池水库径直转向北流，经鸳鸯峡谷通过金塔南山后进入金塔盆地(图 2a)，纵贯酒东盆地的盆山耦合地貌体系，因此能够敏感地响应该区构造活动与气候变化而发生系列调整[23~25]。由于来自祁连山的侵蚀物质被河流搬运进酒东盆地，因此盆地内堆积的第四纪沉积物(图 1a)以砂岩和砂砾岩为主[20, 55]。其地层从下往上依次划分为玉门砾岩、酒泉砾岩和戈壁砾岩[20, 55]。另外，受海拔高度影响，北大河流域不同区域气候有明显差异[56]，其上游祁连山流域属大陆性高山气候，山顶终年积雪，而下游盆地属大陆性荒漠气候，气候干燥[57]。因此，该河下游径流可能主要受上游流域的冰川进退和降水变化影响[58]。
2 研究方法 2.1 古河道划分古河道最重要的特征就是它伴生的河床相沉积，其底部为卵石或者粗砂层，向上过渡为砂层或粉砂层[59]；而在空间形态上，古河道一般以一系列串珠状的湖泊及暗色干涸的河道为特征[60]。因此，野外根据沉积和形态特征并结合遥感影像可以对古河道进行追踪，重建其空间分布格局。
野外地貌和沉积考察发现，一系列串珠状湖泊及干涸的河道在金塔南山南麓至现代北大河河道间呈有规律的条带状分布。它们首尾相连，在空间上共计可以清楚地划分成3组条带，且整体都表现出东西向展布的特征，与金塔南山山体近乎平行(图 2a)。另外，部分湖泊和干涸的河道还表现出明显的河曲和牛轭湖的形态特征，因此本文认为这些串珠状湖泊及干涸的河道构成了3条纵向古河道(图 2a和2b)。它们的东西两端都最终与北大河现代河床汇合，不仅如此，进一步沿这些古河道跟踪调查还发现，其宽度在100~250 m，高出北大河现代河漫滩至少1 m，此外，它们的宽深比也可与北大河现代河道对比(图 2c)。3条古河道的主槽沉积以砂砾堆积为主，砾石平均粒径对应的值均在-5左右[61]，与北大河现代河道的砾石大小相近，其中砾石岩性也能够与北大河现代河道的砾石岩性对比，主要由砂岩、碳酸盐岩、石英岩、玄武岩、花岗岩、砂砾岩、片岩、片麻岩、安山岩和闪长岩组成[61]。这些地貌和沉积特征指示这3条古河道是北大河的废弃河道。一旦被遗弃古河道部分段落即被局地湖泊占据，在主槽沉积之上开始堆积一套泥炭层(图 2c)。论文根据年代由老到新的原则，将北大河3条古河道依次命名为古河道1、2和3(图 2a和2b)。
在现今酒泉市北门的北大河左岸附近，早先开展的区域地貌研究认为北大河在该处发育了两级全新世河流阶地[62]。我们注意到不但这两级阶地面的高差极小，而且所谓的“阶地砾石层”实际上分布于横跨金塔南山南麓的北大河宽大河谷的谷底，而非谷坡上。因此，本文认为先前在酒泉市北门附近划分的这两级阶地可能对应于北大河的两条古河道。根据它们全新世的可能年代范围，论文采用加速器质谱14C(AMS14C)测年方法建立北大河3条古河道的年代序列。
2.2 样品采集与前处理北大河3条古河道的主槽沉积普遍被灰黑色的泥炭层直接覆盖，为AMS14C测年提供了材料条件。野外南北向横跨这些古河道开展采样点对比选择工作，发现了蒲家庄(对应古河道1)、陈家老庄子(对应古河道2)和怀茂村(对应古河道3)三处天然剖面(表 1)。它们均未受到后期扰动，发育显著的水平层理。拨开剖面表面暴露层后，实验样品采集于无现代生物作用扰动的泥炭层底部。3组样品在烘箱内脱水烘干后，分别移入镜下剔除其中的动植物残体，然后依序开展AMS14C测年的前处理工作。
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表 1 样品信息及年代 Table 1 Information of samples and chronology |
采用酸-碱-酸方法对采集于北大河3条古河道的14C样品进行前处理[63]。首先，将它们分别放入经稀硝酸和超纯水清洗过的烧杯中，并加入2 mol/L的盐酸搅拌使其与样品充分反应至无气泡析出，再加入超纯水清洗至中性，从而去除样品中的碳酸岩类。其次，加入0.5 %的NaOH进行60 ℃水浴碱洗，去除样品中渗入的富里酸和腐殖酸，待上层液体澄清后，将其倒掉，然后再用超纯水将样品反复洗至中性。再次，加入1 mol/L的稀盐酸溶液并在60 ℃水浴锅中加热2 h，然后搅拌让其与样品充分反应至无气泡析出后，再反复用超纯水洗至中性，去除空气中CO2的影响。最后，用处理过透气的锡纸包住试管口放入60 ℃烘箱烘干，并根据样品含碳量称取200~500 mg加入燃烧管中，同时加入半勺CuO和一根银丝，在真空中进行反应提取CO2，然后采用自动化石墨装置(AGE Ⅲ)制成石墨靶。
2.3 AMS14C测年3组样品的石墨靶测试工作在兰州大学西部环境教育部重点实验室的年代学实验室进行，石墨合成采用全自动石墨化装置(瑞士IONPLUS，AGE Ⅲ)，石墨测试在兰州大学MICADAS系统(瑞士IONPLUS，MICADAS 20)上完成。样品分别被引入加速器离子源内，其碳原子在高能状态下被离子化，然后对单个14C原子进行计数，完成样品与现代碳标准物质中14C/12 C的原子数之比R(14/ 12)的测试。数据处理采用BATS软件，采用国际标准OXII(Oxalic acid，NIST SRM 4900C)和本底(Phthalic anhydride，Sigma-Aldrich 320064)进行本底扣除和数据校正。基于Intcal 13校正曲线[64]，用OxCal v.4.3.2 [65]软件将14C年代校正为日历年，用“cal.a B.P.”表示，最终年代数据见表 1、图 2a和图 3所示。
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图 3 重建的全新世北大河沿金塔南山南麓发育的古河道迁移过程 (a)7471 cal.a B.P.前，北大河沿金塔南山南麓发育古河道1并以此为流路通过酒东盆地；(b)7434 cal.a B.P.前，北大河南移占据了古河道2；(c)5706 cal.a B.P.前，北大河北移至离金塔南山最近的古河道3；(d)5706 cal.a B.P.后，古河道3废弃，北大河迁移至现代河床，现代水系格局形成 Fig. 3 Reconstructed migration process of the Beida River channel along the southern front of the Jintanan Shan in Holocene. (a)The Beida River had created channel 1 along the southern front of the Jintanan Shan before 7471 cal.a B.P. and flowed through the Jiudong Basin via this channel; (b)The Beida River had migrated toward south and formed the channel 2 by 7434 cal.a B.P.; (c)The Beida River had moved toward north again and reached the channel 3 by 5706 cal.a B.P., which is the nearest to the Jintanan Shan in comparison with other paleo-channels; (d)After 5706 cal.a B.P., the paleo-channel 3 was abandoned, the Beida River flowed through the Jiudong Basin via the modern riverbed, the modern drainage system was formed |
根据野外地貌和沉积考察，在金塔南山南麓至北大河现代河床间初步划分出3条古河道，它们整体沿着山体呈东西向展布，两端都与北大河现代河床汇合；此外，这些古河道的形态和沉积特征也都能够与北大河现代河床对比。因此，本文认为这3条古河道是北大河改道致原河道被遗弃形成的。对它们部分段落的天然剖面结构特征观察发现，主槽沉积之上直接堆积泥炭层，说明这些古河道一旦被遗弃部分段落可能会被局地湖泊系统占据，从而使得生物活动产生的泥炭得以积累，因此古河道上泥炭层堆积的年代就大致代表了它的废弃年代。进一步从主槽沉积特征上可以发现，这3条古河道均未表现出明显的交错层理，而是在部分段落发育了大型的斜层理，其位置对应于典型河曲或牛轭湖[66]。根据这些沉积和形态特征，我们认为3条古河道可能是相互独立的曲流河。主槽沉积之上的泥炭层最底部年龄大致限定了古河道1至3分别在7471 cal.a B.P.、7434 cal.a B.P.和5706 cal.a B.P.左右被北大河遗弃。
根据上述年代序列，论文重建了全新世北大河沿金塔南山南麓的迁移过程(图 3)。7471 cal.a B.P.前，北大河出祁连山后绕过文殊山西端东流，沿着古河道1的流路呈东西向平行于金塔南山山体发育，最后在鸳鸯峡转向北流后出酒东盆地(图 3a)；至7471 cal.a B.P.，北大河已遗弃原古河道1向南移动，最终在7434 cal.a B.P.前，占据了古河道2并以此为流路通过酒东盆地(图 3b)；而至7434 cal.a B.P.，它又开始向北移动而遗弃了原古河道2，最终在5706 cal.a B.P.前，占据了距金塔南山最近的古河道3(图 3c)。此后北大河沿该山体南麓发育的河道可能依然不稳固，至5706 cal.a B.P.已遗弃古河道3并开始向南移，最终迁移到现在的位置而出酒东盆地(图 3d)。
4 讨论一般而言，河流作为一个系统能够敏感地响应内、外部因素的变化而发生调整[67~70]，这其中，河道的迁移就是河流系统发生调整的一种地貌表现形式[12~13, 26]。然而，无论外部因素(构造和气候)还是内部因素(河道粗糙度)的变化，最终都是通过影响河流功率与泥沙间的平衡关系而导致河流发生系列调整[71~72]。酒东盆地内第四纪沉积物以砂岩和砂砾岩为主，其最上部为戈壁砾岩[20, 55]，尤其在古河道发育的金塔南山南麓至北大河现代河床间的区域更是无显著岩性差异[61]，河道粗糙度近乎一致。野外沿3条古河道跟踪考察也未发现淤塞或河道决堤的相关沉积记录，因此，可以排除内部因素的变化导致的河道迁移。另外，酒东盆地受控于玉门-北大河断裂、嘉峪关-文殊山断裂、黑山断裂和金塔南山北缘断裂(图 1a和图 2a)，构造活动十分活跃[10~11, 14]，已开展的盆地钻孔研究显示，酒东盆地早在3 Ma就已开始抬升[73]，因此，北大河在抬升的背景下只能通过不断的下切或者改道来适应外部环境的变化[74]。综合以上证据，本文认为北大河发生河道迁移的原因可能来自于外部因素，即区域构造活动与气候变化。
河流系统能够敏感地响应构造活动和气候变化而发生调整的原因在于，它们能够通过改变河道比降、径流量和流域泥沙输送量从而打破河流功率与泥沙间的平衡关系[71~72]。先前在高原东北缘祁连山及其前陆盆地开展的一系列低温热年代学、沉积学及构造地质学的研究成果显示，构造活动从祁连山北麓向东北方向呈传递性[10, 12, 15, 20, 47, 75]。位于河西走廊以北的金塔南山，被认为是青藏高原向东北挤压扩展的最前缘，代表高原最新的组成部分[9~11]。它的隆起主要受控于金塔南山北缘断裂[10, 50~53]，该断裂现今主要以高角度逆冲的形式沿山体北麓延伸，驱使金塔南山挤压隆起，导致酒东盆地的地势高差加大[50]。金塔南山北缘断裂活动时，玉门-北大河断裂、嘉峪关-文殊山断裂也发生同期的活动[49, 76]，根据郑文俊[14]的研究，引起它们活动的挤压应力来自祁连山向东北方向的进一步扩展。其中金塔南山北缘断裂全新世以来垂直滑动速率达0.11 mm/a[10]，地震探槽的研究也显示该断裂在全新世以来持续活动并且引起地震频繁发生[52~53]，不仅如此，阶地变形研究进一步揭示金塔南山在该断裂的控制下晚更新世以来以褶皱的形式不断向盆地扩展生长[50]，最终，它以近东西走向的山体横列于北大河北流的流路上(图 2a)。那么全新世以来随着断裂的频繁活动，金塔南山不断向盆地扩展就可能导致北大河河道抬升，比降减小。为保证持续的流动，推测北大河背离逼近的金塔南山山体而向南寻找更低处的河道流动，最终发生了改道南移。通过对比河道迁移(图 4a)与该断裂发生强烈活动的时间(图 4d)，发现北大河沿金塔南山南麓发育的古河道向南迁移主要对应于该断裂的活动期，而面向山体北移主要发生在相对平静期。
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图 4
北大河沿金塔南山南麓河道的迁移过程与亚轨道时间尺度气候波动和构造活动对比
(a)重建的北大河沿金塔南山南麓河道迁移过程；(b)来源于祁连山敦德冰芯的全新世气候变化记录[77]; (c)格陵兰冰芯记录的全新世气候变化曲线[83]；(d)金塔南山北缘断裂全新世以来的主要古地震事件记录[52~53] Fig. 4 Correlation between the migration process of the Beida River channel along the southern front of the Jintanan Shan with climate fluctuations in suborbital time-scale and tectonic activities. (a)Reconstructed migration process of the Beida River channel along the southern front of the Jintanan Shan; (b)Climatic record from the Dunde ice core of the Qilian Shan in the Holocene period[77]; (c)Climatic history in Holocene obtained from the Greenland ice core[83]; (d)Paleo-seismic events occurred within Jintanan Shan fault since Holocene period[52~53] |
上述对比虽然可以将金塔南山北缘断裂的快速活动作为驱动北大河河道迁移的构造因素进行分析，但并不意味着完全排除气候因素。我们将北大河沿金塔南山南麓的迁移历史与河西走廊全新世气候记录[77~80]进一步对比分析发现，亚轨道尺度的气候快速波动也可能是导致河道迁移的重要因素(图 4b和4c)。≤ 7471 cal.a B.P.至＞7434 cal.a B.P.间，北大河背离金塔南山向南移动，不仅源于山体在此期间快速抬升，而且还因为区域气候在此时段趋于变冷。北大河上游发育冰川(图 1a)，融水约占径流量的17.6 % [54]。因此，变冷的气候不但引起植被退化，导致进入河道的泥沙增多，还会引起河流径流量降低、动力下降[79~81]。此时段，低能的北大河受北部金塔南山构造抬升影响，只能由古河道1向南移至地势相对较低的古河道2(图 3a和3b)，才能通过酒东盆地继续向北挺进。≤ 7434 cal.a B.P.至＞5706 cal.a B.P.间，酒东盆地不但构造活动表现出相对平静的特征，而且区域气候也显著向暖湿转变(图 4b)。这样北大河在此时段可能获得较高的动力使其流路能够面向山体移动，由古河道2向北迁移至古河道3(图 3b和3c)。5706 cal.a B.P.以来，虽然河西走廊的气候经历了多次冷干-暖湿的波动，但整体趋于变干[77~78, 82~83]。不仅如此，金塔南山北缘断裂活动加强，分别在6 ka B.P.左右，3.5±0.4 ka B.P.之后和0.325±0.026 ka B.P.之前发生古地震事件(图 4d)，其中前两次古地震均破裂全段[52~53]，这可能揭示金塔南山抬升加快。那么，在水动力下降和山体抬升共同影响的驱动下，北大河仅能背向金塔南山南移，由古河道3迁移至地势更低的现代河道方能通过酒东盆地继续北流(图 3c和3d)。
基于北大河河道迁移过程与短时间尺度构造和气候记录的对比分析，本文认为全新世以来在活动构造区河流改道可能主要受控于区域快速构造活动和亚轨道尺度气候波动。在东喜马拉雅山区，雅鲁藏布江偏离现在河道短期内向南流入横向河流苏班西里河也是受控于全新世气候波动和流域快速构造活动的共同作用[84]。此外，这一结论还与河套地区黄河古河道迁移过程的研究成果一致[85]。因此，在酒东盆地沿金塔南山南麓发育的3条纵向古河道是北大河响应山体快速构造活动和区域亚轨道尺度气候波动相互作用的结果。
5 结论金塔南山南麓至现代北大河河床间发育3条古河道，空间展布、宽深比和砾石岩性组成分析显示，它们是北大河废弃的古河道。通过对古河道上覆泥炭层的加速器质谱14C测年，建立北大河遗弃3条古河道的年代序列分别为7471 cal.a B.P.、7434 cal.a B.P.和5706 cal.a B.P.。按照年代由老到新的顺序将3条古河道命名为古河道1、古河道2和古河道3。在该年代序列的限定下，北大河沿金塔南山南麓的迁移过程被重建。通过与金塔南山北缘断裂的构造活动记录以及流域全新世气候变化进一步对比发现，北大河面向山体北移主要发生在气候变暖和构造相对平静期，而背向山体快速南移则主要对应于气候变冷和构造抬升期。因此本文认为，在构造活跃的地区，快速构造抬升和亚轨道尺度气候波动是驱动河流改道的主要原因。结合酒东盆地活动构造的研究成果，北大河河道的迁移过程可能反映了青藏高原中全新世以来向东北缘的扩展。
致谢: 感谢兰州大学西部环境教育部重点实验室何建华老师在AMS14C测年过程中的指导和帮助；感谢审稿专家和编辑部老师提出的建设性意见和建议，使文章最终得以发表。
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2 Geologic Survey Institute of Gansu Province, Lanzhou 730000, Gansu)
Abstract
The Jiudong Basin is restricted by the Qilian Shan(Shan=Mountain in Chinese), uplifted as the northeastern margin of the Tibetan Plateau, to the south, and by the Jintanan Shan to the north. The landscape across this basin is thus characterized by an alternation between basin and mountain. The Beida River deriving from the Qilian Shan debouches into this basin, and excavating a series of longitudinal channels in an east-west direction, along the southern front of the Jintanan Shan, until it turns northward and flows through the Jiudong Basin via the Yuanyang Gorge. Based on preliminary field investigation, a total of three abandoned channels were identified along the southern front of the Jintanan Shan. Moreover, they extend in an east-west direction, nearly parallel to the Jintanan Shan, and then connect with the current riverbed of the Beida River at their both tips. The sedimentary characteristics of the three abandoned channels can also be correlated with the Beida River. Therefore, their formation was probably attributed to the Beida River diversion. The study on the formation process of the three abandoned channels can provide excellent insight into the fluvial response to the climatic fluctuation and tectonic activity. In order to establish a chronological sequence for them, the AMS 14C dating has been employed to constrain the basal ages of the peat bed overlying directly the main-channel-bed sediment. Obtained result shows that the three channels were abandoned by the Beida River at 7471 cal.a B.P., 7434 cal.a B.P., and 5706 cal.a B.P., respectively. They are named here as paleo-channels 1, 2, and 3 according to above chronological sequence. Under the constrain by this chronological framework, the migration process of the Beida River channel along the southern front of the Jintanan Shan has been reconstructed. Further comparison with the paleo-seismic data along the Jintanan Shan fault and the climatic record in suborbital timescale from the ice core at the northeastern Tibetan Plateau, we found that the channel of the Beida River tended to move toward north near to the Jintanan Shan when the climate was warmed and the tectonic activity was in a relative quiet state, while its fast southward migration away from the mountain mainly occurred in a tectonic active period combined with climate cooling. The Beida River originates from the glaciers in the Qilian Shan, meltwater thus contributes most of its runoff. In the warming climate, the increasing runoff, combined with the decreasing sedimentary flux on account of increasing vegetation coverage, may lead to a high energy for the Beida River in hydrological force. Therefore, the relative quiet tectonics coupled with warming climate enables this river to move further toward north near to the Jintanan Shan. On the contrary, the Beida River has to turn back and even further southward migration away from this mountain in a tectonic vigorous and cold climatic period.Therefore, we concluded that the channel abandonment in the Jiudong Basin may be attributed to the fluvial response of the Beida River to the climate fluctuation in suborbital timescale and the rapid tectonic activity of the Jintanan Shan. Previous works on the active tectonics over the northeastern Tibetan Plateau have revealed that the growth and extension of the Jintanan Shan is controlled by the Jintanan Shan fault, whose activity may be linked with the northeastern extrusion of the Tibetan Plateau. Therefore, the migration process of the Beida River channel reconstructed in this paper may imply a further northeastward extrusion of the Tibetan Plateau in mid-Holocene.