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走廊南山河流纵剖面高海拔裂点的成因
陈苗,, 胡小飞,, 王维
兰州大学资源环境学院 西部环境教育部重点实验室,兰州 730000

作者简介:陈苗(1990-), 女, 安徽淮南人, 硕士研究生, 研究方向为第四纪地质学专业。E-mail: chenm15@lzu.edu.cn

通讯作者:胡小飞(1980-), 男, 河北张北人, 副教授, 硕士生导师, 研究方向河流过程与构造变形。E-mail: feixhu@lzu.edu.cn
摘要

河流水力侵蚀物理模型表明基岩河道纵剖面在均衡状态时表现为平滑上凹的形态,其特征反映了构造、基岩抗侵蚀能力和气候的作用;然而自然界河道纵剖面多呈现以裂点为特征的不均衡形态,不均衡的剖面形态以及裂点的研究同样可以对外力作用的变化起到很好的指示作用。位于北祁连的走廊南山高海拔河道纵剖面普遍呈现不均衡形式且发育海拔较高的裂点。通过对裂点成因分析发现,这些裂点并不主要受控于岩性、气候、构造等因素,而反映了冰川作用遗留地形与河流地形的分界。这一结果说明在对河道纵剖面高海拔裂点进行分析时要考虑到古冰川遗留地形也会对现代河道纵剖面产生重要影响,为进一步认识和理解造山带地貌演化以及控制因素提供了思路。

关键词: 河道纵剖面; 陡峭指数; 裂点; 走廊南山; 冰川作用;
The cause of high-altitude knickpoints on river longitudinal profiles along the Zoulang Nan Shan
CHEN Miao,, HU Xiaofei,, WANG Wei
Key Laboratory of Western China's Environmental Systems, College of Earth Environmental Sciences, Ministry of Education, Lanzhou University, Lanzhou 730000, China
Abstract

The stream-power incision model shows that a bedrock channel longitudinal profile is characterized by a smooth, concave-up shape at the steady state, and its characteristics reflect the influences from external forces, such as tectonics, climate, and rock resistance. However, most of the natural rivers present a transient state characterized by knickpoints on longitudinal profiles, which can also infer the influences from external forces. Widespread knickpoints at high altitudes on river longitudinal profiles along the Zoulang Nan Shan (mountain), which is a part of northern Qilian Mountains, provide a particular case for studies on the factor affecting the disequilibrium profile. The analysis of the knickpoints indicates that the formation of the knickpoint at high altitudes is not influenced by lithology, climate and/or tectonics. By comparing the plaeo-glaicial evidences, we proposed that the high-altitude knickpoint reflects the boundary between residual glacier valleys and fluvial channels. The result suggests that we should pay more attention to the inheritance landform by ancient glaciation when analyzing the knickpoint located at high altitudes. This study would greatly increase the knowledge about the geomorphic evolution on high mountain ranges along orogenic belts.

Keyword: river longitudinal profile; steepness index; knickpoint; Zoulang Nan Shan; glaciation;
1 引言

河流水力侵蚀物理模型[1,2]和实地测量研究[3,4]表明,活动造山带河流处于均衡状态时(steady state,河流高程保持恒定,下切速率与抬升速率相等),河道纵剖面表现为平滑上凹形态,河道坡度与汇水面积的对数关系为一条直线,其截距(陡峭指数,表征河道整体坡度的参数)的大小可以用来反映构造、基岩抗侵蚀能力、气候的特征[4,5,6,7]。然而,活动造山带河流多表现为不均衡的状态,河道纵剖面出现上凸河段(convexity)或裂点(knickpoint)[8,9]。不均衡河道纵剖面的形态经常被用来反映区域抬升速率的变化[6-7, 10-12]、侵蚀基准面的快速下降[13]或气候的变化[6, 14];这种形态特征指示了河流处于瞬时状态(transient state),裂点会随着时间而发生向上游的移动[15]。除了随时间变化的外界因素外,区域一些空间因素的变化也可能导致裂点的出现,如岩性的差异[16]、断裂的发 育[17]、抬升速率的上下游的差异[11]。在发育冰川的河谷上游,由于冰川与河流侵蚀效能的差异,常常导致冰川谷与河谷交界的附近出现纵剖面形态的转折(slope-break)[18,19,20,21],类似于河流纵剖面的裂点。这种由于侵蚀效能差异产生的裂点,对于我们分析山区地貌形态特征及其形成原因会产生重要的影响,尤其是在冰川退缩遗留的冰川谷地、现代发育河流河谷区域。而目前对于这种冰川谷地的河流纵剖面形态特征与非冰川区河流纵剖面形态的差异及其演化研究还非常薄弱。

祁连山位于青藏高原东北缘,平均海拔3000 m以上,最大高程超过5000 m(图1)。该区域第四纪以来构造活动强烈,河流普遍发生快速下切,并记录了山体的抬升过程与特征[22,23,24];同时,其高海拔特征也为冰川发育提供了条件,使其晚更新世以来经历了多次冰川作用[25,26,27,28,29,30,31]图1),这为本文研究冰川与河流侵蚀效能差异形成的纵剖面特征及探索祁连山地貌演化提供了理想的场所。本文拟对祁连山中段走廊南山现代河道纵剖面进行提取与分析,对比区域河流陡峭指数空间差异及裂点海拔分布情况,分析研究区域内高海拔裂点(> 3600 m)的形成原因;并结合该区域古冰川研究,探讨冰川作用对河道纵剖面形态特征的影响。

图1 研究区地形图(末次冰盛期ELA的分布源于施雅风等[25])
注:ELA,equilibrium line altitude,冰川物质平衡线高度;LGM,Last Glacial Maximum,末次冰盛期。
Fig. 1 Topographic map of the study area (distribution of the ELA during Last Glaciation Maximum from Shi et al[25])

2 研究区域

祁连山是印度板块与欧亚板块晚新生代以来持续挤压碰撞的产物,由一系列北西—南东向条状山脉组成[32],山势西高东低,平均海拔3000 m以上,与其北部河西走廊形成了2000~3000 m地形高差,海拔4000 m以上的山体占整个山系面积的30%左右[33]。祁连山北缘断裂是一条由一系列断裂组成的大致与祁连山平行的逆冲断裂,祁连山内部发育左旋走滑的海原断裂(图1),研究表明这些断裂第四纪以来活动强烈,对地貌演化起着重要的控制作用[22,23,24]

研究区的岩性组成比较复杂,主要由广泛分布的沉积岩、前寒武纪变质岩和早元古代变质岩及各种火山岩构成[34]。祁连山地处中国大陆腹地,远离海洋,是受季风影响的边缘区域,因而形成了大陆性干旱、半干旱气候[35],年平均降水量较少;受到地形影响,高海拔地区降水量较大,降水也由东南向西北递减。该区域发育了一系列内流水系,黑河就是其中重要的流域之一,是中国西北地区的第二大内陆河流,主要由数十条发育于祁连山山前和腹地的次级水系构成,大部分河流大致垂直于祁连山脉走向,跨越了不同的地质地貌和构造单元。青藏高原的多次隆升为祁连山冰川发育造就了地形优势。“昆黄运动”可能使祁连山抬升进入冰冻圈,从而在40多万年前(MIS12)发生了最早的冰川作用,之后祁连山又普遍经历了倒数第二次冰期、末次冰期与全新世的冰 期[25,26,27,28,29,30,31],并在祁连山留下了较为丰富的冰川侵蚀和堆积地貌,与河流和坡面地形共同构成祁连山现代地貌(图1)。

野外考察发现位于祁连山的这些河道大多为基岩河道或基岩—冲积混合型河道,便于进行河流水力侵蚀物理模型的分析[22]。结合走廊南山特殊的构造背景及冰川方面研究,为分析河道纵剖面形态特征提供了较为理想的场所。

3 研究方法
3.1 理论基础

对于自然状态下的侵蚀河道,河道纵剖面可以表现为河道坡度(S)与上游流域面积(A)之间的幂函数关系[3-4, 10, 36]

S = k s A - θ (1)

式中:ks为河道陡峭指数,反映河道纵剖面的整体陡峭程度;θ为河道纵剖面的凹度系数,反映河道纵剖面的上凹程度(范围:0.4~0.7)[8]。同时,在均衡状态下,利用河流水力侵蚀模型[5]

E = K A m S n (2)

可以得到与经验观察类似幂函数关系:

S e = ( U / K ) 1 / n A - m / n (3)

式中:E为河流侵蚀速率;Se为均衡河道坡度;U为岩石抬升速率;K为有量纲的侵蚀系数,其值主要反映岩性、气候条件和河流负载等因素的作用[5, 12];mn为正常数。对比公式(1)与(3),均衡状态时,则有ks = (U/K)1/n,θ = -m/n。为了对不同流域面积的河流进行对比,可以用一个定值θref(一般用0.45)来对陡峭指数进行标准化,得到标准化的陡峭指数ksn[37]

S = k sn A - θref (4)

式中:均衡状态的河道纵剖面表现为平滑上凹形态,可以用统一的河流陡峭指数(ksn)表示其陡峭度。在瞬时状态时,河道纵剖面则被裂点(knickpoint,河流纵剖面上坡度的突变点)分成不同陡峭指数的河段。这种结合陡峭指数的河道纵剖面的分析方法在对区域构造活动研究得到较多应用[38,39,40,41,42]

裂点作为非均衡状态剖面的证据,一般被用于指示构造或气候沿河道纵剖面的变化,此时裂点将河道分成陡峭指数不同的两部分,并以动力波的形式向河源后退[9, 43]。研究发现产生于同一次基准面下降(或构造活动)的裂点以与上游面积成正比的后退速率,但以相同的垂直速率移动,则使流域内不同河流的裂点处于同一海拔[43,44]。同时,由于基岩岩性差异形成的裂点通常被认为是固定的,但也会将河道分成陡峭指数差异的上下两部分[37, 44-45]

3.2 河流陡峭指数和纵剖面的提取

利用ArcGIS系统,结合MATLAB脚本程序,从分辨率30 m的DEM(http://www.opentopography.org/)中提取河道高程和流域面积等参数。虽然提取精度可能受到DEM数据分辨率的限制,但一些研究表明30 m的DEM数据可以捕捉到河道形态重要的特 征[6, 10],为进一步降低由数据产生的精度问题,用250 m的移动窗口对河道进行平滑,每隔12.192 m的垂直距离进行河道比降的计算,最后根据公式(4)(为了直观性,一般用对数形式:logS = -θref logA+logksn),设置θref = 0.45进行拟合就可以得到标准化的河道陡峭指数ksn[10, 46]。计算河道陡峭指数时,河流最上游选择崩积河道大致与基岩河道过渡部分,取上游临界面积Acr为105~106 m2。最后根据ksn发生变化部位,结合河流纵剖面特征确定裂点位置[6],并基于提取的河流纵剖面,利用公式(3)拟合得到裂点上下河段的凹度系数(θ)。

4 结果

按照上述步骤,本文对位于祁连山中段走廊南山南北两侧的河流,主要包括黑河干流北岸、梨园河、马营河、摆浪河进行标准化的河道陡峭指数的提取与分析(图1)。为了方便对比,将提取的陡峭指数按照大小进行分级,得到研究区域标准化的陡峭指数(ksn)分布(图2)。

图2 标准化的河道陡峭指数(ksn)分布 Fig. 2 Map of normalized channel steepness indexes (ksn)

图2结果表明,研究区ksn值分布范围较大,以裂点为界将河道分成了ksn值存在明显差异的两段或多段,即大多数支流纵剖面呈不均衡的形态特征[9]。同时,在走廊南山南北两侧分别选取两条典型河道对河流纵剖面形态进行直观显示(图3),拟合河道纵剖面与实际河道纵剖面拟合效果较好,且两条河道纵剖面形态都显示海拔较高的裂点之上的河道剖面较为平缓,而裂点之下河道剖面较陡;位于走廊南山北侧的河道19,存在海拔较低的裂点,但低海拔裂点上下两段陡峭指数值差别不大,值都较大,并与海拔较高的裂点之上河段较低的陡峭指数值存在明显区别(图3b),可能说明较高海拔裂点与海拔较低裂点成因不同。

图3 两条典型河道的纵剖面图(河道3和19位置参考图2) Fig. 3 Two typical river longitudinal profiles (3 and 19), whose locations are shown on Fig. 2

虽然图2表现这些裂点的高度在2300~4300 m都有分布,但其中一个明显的特征是多数河流出现海拔较高的裂点,这些裂点的高程多大于3600 m,这些高海拔裂点的集中出现可能反映了同期或类似的外界条件变化[37, 43-45]。对这些高海拔的裂点高程、上下河段的ksn以及θ进行统计分析发现:高海拔裂点在3600~4300 m高程都有分布,且高程与上游流域面积大致存在递减关系(图4);对比高海拔裂点上下河段ksn的数值,发现高海拔裂点之上的河道ksn普遍较小,且分布范围较小,大多属于50~100之间(图2、图5a),而高海拔裂点之下的河段ksn普遍高于裂点之上的数值,且分布范围较大,位于100~350之间;河道凹度系数θ值分布范围也较大,但高海拔裂点上下河段的θ值没有明显差别,多位于0.1~0.7之间,整体在0.45附近(图5b)。

图4 高海拔裂点上游流域面积与裂点海拔对比关系
注:图中编号的裂点表示摆浪河上游流域的河道,参考图2,编号61河流上游对应摆浪河14号冰川,摆浪河现代冰川和古冰川范围参考Zhou等[29]的研究。
Fig. 4 The relationship between the drainage area of the upper knickpiont and the elevation of the high altitude knickpiont. Knickpionts in the drainage basin of the Bailang River are marked by numbers (refer to Fig. 2, No.61 stream is correspon-ding to Glacier No. 14), and the extents for the modern glacier and the ancient glacier are referenced to Zhou et al[29]

图5 研究裂点上下游河段(a)标准化的陡峭指数分布与(b)凹度系数分布 Fig. 5 Values of (A) normalized channel steepness indexes (ksn) and (B) concavity indexes (θ) derived from the upper and lower reaches of the high altitude knickpoints, respectively

5 讨论

根据裂点形成的可能原因,本文从岩性、气候、构造与冰川作用几个方面进行高海拔裂点成因的讨论与分析。

5.1 岩性

将区域裂点分布与岩性图进行叠加(图6),可以发现大部分高海拔裂点没有位于地层界线附近(距地层分界线> 1 km)[43],在岩性均一的地层单元内,也有高海拔裂点分布。另外,对比不同岩性内ksn,发现其数值没有明显差别(图6),说明岩性对高海拔裂点之上ksn影响不大。在梨园河与黑河分水岭两侧,存在个别裂点大致位于地层界线附近(距地层分界线< 1 km),然而,分析这些裂点上下游河段陡峭指数(ksn)发现,这些裂点上游河段主要是抗侵蚀能力较强花岗岩、奥陶纪变质岩等岩类(图6),却具有较低的陡峭指数,而裂点下游河段流经松散的第四纪沉积物、石炭纪沉积岩等,抗侵蚀能力较弱,这与抗侵蚀能力越强的岩石具有越大的陡峭指数[14]的理论相反。因此,岩性分布可能对裂点的发育与演化产生影响[44],但在本研究区岩性差异不是产生这些高海拔裂点的主要原因。

图6 研究区岩性分布 Fig. 6 Geological map of the study area

5.2 气候

气候主要是通过大气降水改变径流量来影响河流的侵蚀能力,从而影响河道纵剖面的形态[7, 47]。前文提到研究区域属于大陆性干旱、半干旱气候区[35],年平均降水量较少;受到地形影响,高海拔地区降水量较大,降水由东南向西北递减。胡小飞等[22]在研究祁连山北翼流域河道陡峭指数时明确指出该区域降水量较低,且变化范围不大,对河流纵剖面的影响较小,在本研究区裂点与降水变化也没有直接的对比关系,因此降水变化也不是形成高海拔裂点的主要原因。

5.3 构造

研究区域穿过不同的构造单元,且研究表明这些断裂系统第四纪以来活动强烈[22,23,24]。从陡峭指数空间分布可以发现,高海拔裂点下游河段陡峭指数存在较大变化(图5a),这可能反映了由断裂活动导致的抬升速率的空间差异或抬升阶段的时间差异[22]。而裂点上游河段的陡峭指数的空间变化并不明显,与裂点下游陡峭指数的变化没有对应关系,可能说明裂点形成与下游的构造活动关系不大。另外,由于产生于同一次构造抬升(或基准面降低)的裂点应该处于同一海拔[43,44],而图4中裂点海拔在3600~4300 m之间,高程范围波动较大,与裂点后退模型[43,44]不符。这些特征说明构造活动也不是形成这些高海拔裂点的主导因素。

5.4 冰川作用

首先,从裂点海拔与上游流域面积近似反比的关系可以看出,这种变化符合冰川研究中冰川流域面积越大,冰川末端向下游伸展越低的理论[19],虽然在裂点上游现代冰川已经退缩到较高的高度(图4),但这些现代冰川前端谷地均为古冰川作用形成的“U”型谷,且可以向下平缓延伸到海拔3800 m或以下的高度。例如,摆浪河14号和16号冰川汇合后形成新冰期的冰碛垄,并沿14号谷东侧延伸到海拔3800 m的高度[29],裂点出现的位置与新冰期冰川作用范围有较好的对应关系(图4)。另外,从裂点上下游的谷地地形特征分析(图7)可以发现,高海拔裂点上游河段横断面从形态上为“U”型谷、谷地较宽广,裂点下游为典型的“V”型谷、谷地较狭窄,符合冰川与河流侵蚀形成的谷地形态差异[48,49,50]。最后,将提取的摆浪河纵剖面与Zhou等[28]对摆浪河的上游古冰川的研究进行对比(图4,图8),发现摆浪河海拔相对更高(~4200 m)的裂点对应现代冰川末端,相对低的裂点位置对应新冰期冰川或末次冰盛期末端(~3800~4000 m)附近[25]。对比高海拔裂点位置与末次冰期范围,发现大部分高海拔裂点大致位于现代冰川与末次冰盛期范围之内(图8)。

图7 两条典型河道的裂点上下河段横断面特征(河流编号见图2) Fig. 7 Cross-sections of the upper and lower reaches along two typical rivers, and locations of rivers are shown in Fig. 2

图8 (a)研究区裂点位置与末次冰期冰川分布范围; (b)摆浪河上游典型河流纵剖面
注:ELA的分布源于施雅风等[25];摆浪河14号冰川新冰期和现代冰川末端参考Zhou等[29]的研究,河流编号见图2,其中61河流上游对应14号冰川。
Fig. 8 (a) Comparison between knickpoints' location and the ELA of the Late Glacial Maximum (the ELA data is from Shi et al[25]); (b) Typical longitudinal river profiles in the drainage basin of the Bailang River (The glacier data for the modern and the Neoglacial period are referenced to Zhou et al[29]. Location of rivers are shown in Fig. 2, and No.61 stream is corresponding to Glacier No.14)

通过以上分析,本文认为走廊南山河道纵剖面上高海拔的裂点出现的主要原因是冰川作用。在现代冰川不发育的古冰川谷地,较近冰期的冰川侵蚀与堆积形成的地形与下游河流作用形成的地形以裂点的形式截然分开。这里简单利用裂点后退模型(v = KAm),并借鉴其他研究的参量设置(K = 5.0×10-7,m = 0.5)[13, 43-44],估算了目前裂点后退速率分布0.3~2.4 mm/a,平均后退速率~1 mm/a,末次冰盛期(距今~2万年)冰川作用产生的裂点至今也只可能后退~6~48 m,平均后退距离~20 m,只占支流河长很小的部分,说明目前裂点后退很慢,较大程度的保留了冰川侵蚀与堆积的地形。另外考虑到冰川退缩后在谷地遗留大量冰碛物,也会对该河段河流侵蚀起到抑制作用,所以新近冰期作用遗留的地形在河道纵剖面上仍然较明显的表现出来[19]。可能由于岩性分布的复杂性,或末次冰期以来的多次冰期作用[25,26,27,28,29,30,31],在走廊南山南侧一些区域,高海拔裂点位置与末次冰期作用范围对应关系较差,表明要进一步理解古冰川作用对河流纵剖面形态的影响,不仅需要详细的区域冰川研究资料,还需要对岩性、构造等因素进一步分析。

因此,走廊南山高海拔裂点的出现主要反映了冰川作用遗留地形与河流作用地形的分界,与普遍意义上的裂点发育(构造过程、侵蚀基准面下降或岩性差异)有较大的区别。研究结果指示在进行河道纵剖面分析时,对于海拔较高的裂点的分析需要谨慎对待,古冰川作用也可能是形成这些不均衡河道的重要原因之一。

6 结论

本文利用ArcGIS系统,结合MATLAB脚本程序,从分辨率30 m的DEM提取了走廊南山南北两侧河道纵剖面,对河道陡峭指数(ksn)和高海拔裂点进行了提取与分析。

结果表明研究区普遍出现高海拔裂点(> 3600 m),将河流纵剖面分成上游平缓、下游陡峭的两段河段;高海拔裂点上游河段的陡峭指数较低,并呈现较小的变化。经分析发现,区域岩性分布与气候因素不是产生这些高海拔裂点的主要原因;较低海拔裂点可能反映了构造抬升速率的时间或空间差异,但这些高海拔裂点可能由构造活动之外的其他因素作用形成。多种证据表明走廊南山高海拔裂点主要反映了冰川作用遗留地形与河流作用地形的分界。指示在对河道纵剖面上海拔较高裂点进行分析时,要考虑到古冰川作用也会对河道纵剖面产生重要的影响,但要进一步理解冰川对河流纵剖面形态的作用程度,不仅需要详细的区域冰川研究资料,还需要对区域岩性、构造等因素进行深入的研究。

致谢:感谢王杰老师和曹泊老师对文章中冰川部分提出的宝贵建议以及两位审稿专家和编辑部老师提出的宝贵意见。

The authors have declared that no competing interests exist.

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<p>青藏高原东北缘循化-贵德地区晚新生代构造活动强烈,同时晚新生代黄河的发育和演化也提供了研究高原构造隆升扩展以及黄河侵蚀下切过程的理想场所。黄河水系通过调整河流纵剖面的形态在垂向上响应不同的构造隆升以及河床岩性的变化。利用数字高程模型数据,提取黄河水系河流纵剖面形态,利用基岩河道河流水力侵蚀模型(stream-power incision model)为依托,尝试揭示晚新生代以来强烈构造活动在黄河水系河流纵剖面形态特征上的表现。分析发现,在循化-贵德地区岩性分布均匀的新生代盆地内部,黄河水系纵剖面在河流水力侵蚀模型坡度-面积双对数图解中大部分表现为上凸特征,即基岩河道的河床随时间的变化为正值,指示了基岩的隆升速率大于河流的下切侵蚀速率。在河流流经盆地内部不同岩性地区,特别是前新生界地层区,河流纵剖面也表现为一定的上凸特征,表明了新生界与前新生界地层岩性差异对河流纵剖面的控制作用。综合来看,青藏高原东北缘循化-贵德地区黄河水系河流纵剖面形态特征是晚新生代强烈构造活动和岩性差异共同控制的结果,揭示了循化-贵德地区造山系统地形发育仍然处于前均衡或接近均衡状态,同时也从侧面表明青藏高原东北缘地区是整个高原隆升扩展的最新和最前缘部位的时空格局特征。</p>
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[1] Glacial occupation of alpine valleys results in a distinct signature in the long-valley profile, including steepening of the profile in the headwaters, flattening at lower elevations, and a step in the profile at the convergence of headwater tributaries. We present analytic results for glacial erosion patterns by making the following assumptions: (1) the initial profile is linear, (2) the width of the valley is uniform, (3) the annual mass balance varies linearly with elevation, (4) the glacier at any time is quasi-steady, (5) the erosion rate is proportional to ice discharge per unit valley width, and (6) glacial erosion rates far exceed fluvial erosion rates. A steady glacier under these conditions would erode a parabolic divot in the longitudinal valley profile, with its maximum depth coinciding with the down-valley position of the equilibrium line altitude (ELA). The calculated flattening of the valley floor down valley of the ELA and the steepening of it up valley captures the essence of the glacial signature. When a reasonable probability distribution of ELAs is allowed, the predicted erosion peaks at 30 40% of the down-valley distance to the glacial limit, and the pattern merges smoothly with the steeper fluvial profile downstream of the glacial limit. Profiles of mass balance that are capped at a maximum value produce shorter glaciers and a slight asymmetry in the expected erosion pattern. Only when we mimic the tributaries of glacial headwaters by specifying a valley width distribution do we obtain the upper step observed in many valley profiles.
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[1] The response of glaciated landscapes to rapid rock uplift, driven by tectonic convergence, is an important, often neglected, aspect of proposed interactions between plate tectonic processes and climate change. Rivers typically respond to more rapid rock uplift in part through increasing channel gradients. In contrast, the 090008glacial buzzsaw090009 hypothesis suggests that glaciers can erode as quickly as the fastest rock uplift rates (609000910 mm/yr) without any increase in mean elevations. However, it has not been established how this is achieved. We examined moving window maps, swath and longitudinal profiles, hillslope relief, and hypsometry for glacierized and formerly glacierized basins in areas of spatially variable rock uplift rate in the Southern Alps, New Zealand, and around Nanga Parbat, Pakistan, to determine whether glaciers have a specific response to rapid rock uplift. The response of these glaciated landscapes to rapid rock uplift (609000910 mm/yr) comprises (1) modest steepening of the longitudinal profiles in smaller glaciated basins, (2) maintenance of shallow downvalley slopes in larger glaciated basins (&gt;09080430 km2, Southern Alps; &gt;090804100 km2, Nanga Parbat), (3) development of tall headwalls, and (4) steepening of the basin as a whole, dominated by hillslope lengthening. Around Nanga Parbat, headwalls several kilometers high constitute &gt;50% of the basin relief. At rapid rock uplift rates, although glaciers can incise the valley floor swiftly, they cannot prevent headwalls from reaching exceptional heights. The associated increase in mean distance between cirque heads (i.e., a decrease in drainage density) causes regional mean elevation to rise with increasing rock uplift rate. However, this is much less than the changes in elevation expected in unglaciated ranges.
DOI:10.1029/2006JF000667      [本文引用:1]
[21] Brardinoni F, Hassan M A.Glacially-induced organization of channel-reach morphology in mountain streams. Journal of Geophysical Research, 2007, 112: F03013. doi: 10.1029/ 2006JF000741.
[1] We examine the spatial distribution of channel-reach morphologies in formerly glaciated mountain drainage basins of coastal British Columbia, Canada. Using field- and geographic information systems-derived data, we show that the local channel slope and the degree of colluvial-alluvial coupling imposed by the glacial valley morphology dictate the spatial organization of channel types. In particular, the complex, glacially induced channel long profile produces characteristic sequences of channel reaches that depart from the downstream succession (colluvial/boulder-cascade/step-pool/rapids/riffle-pool) distinctive of simple unglaciated mountain streams. Typically, the presence of one hanging valley in the river long profile produces and separates two full successions of channel types: a headmost one characterized by an ephemeral/seasonal hydrologic regime and a downstream one, where water runoff is perennial. We document that channel types are well separated in plots of slope versus shear stress, area versus shear stress, and slope versus relative roughness. In agreement with these outcomes, multivariate discriminant analyses coupled with principal component analysis of 98 study reaches yield a highly successful channel-type classification when slope, shear stress, and relative roughness are considered. Success rates, depending on whether or not boulder-cascade reaches are pooled together with step-pools, are 88% and 75%, respectively. Previous work in unglaciated settings has suggested that mountain channels have distinct bed morphology states that vary primarily with slope; our study reveals that even in formerly glaciated valleys, where slope is largely inherited from glacial times, these distinct bed states exist and vary (mostly) with slope, adding considerable strength to this empirical knowledge.
DOI:10.1029/2006JF000741      [本文引用:1]
[22] Hu Xiaofei, Pan Baotian, Kirby E, et al.Spatial differences in rock uplift rates inferred from channel steepness indices along the northern flank of the Qilian Mountain, northeast Tibetan Plateau. Chinese Science Bulletin, 2010, 23: 2329-2338.
[本文引用:6]
[胡小飞, 潘保田, Kirby E, . 河道陡峭指数所反映的祁连山北翼抬升速率的东西差异. 科学通报, 2010, 23: 2329-2338.]
[23] Liu Xingwang, Yuan Daoyang, Zheng Wenjun, et al.Research on Late Quaternary slip rates of the Fodongmiao-Hongyazi Fault at the north margin of Qilianshan Mountains. Chinese Journal of Geology, 2012, 47(1): 51-61.
[本文引用:3]
[刘兴旺, 袁道阳, 郑文俊, . 祁连山北缘佛洞庙—红崖子断裂晚第四纪滑动速率研究. 地质科学, 2012, 47(1): 51-61.]
佛洞庙&mdash;红崖子断裂是发育于祁连山北缘中段河西走廊南缘的一条重要的块体边界断裂,总体走向北西西,长约110 km。该断裂为一条全新世活动的逆&mdash;左旋走滑断裂,也是1609年红崖堡7 1/4 级地震的发震断裂。断裂活动形成了一系列陡坎、断层崖以及冲沟和阶地左旋等断错地貌。我们通过详细的野外考察,选择典型断错地貌进行大比例尺差分GPS测量,结合所获相应地貌面的年代数据,得到该断裂晚第四纪平均垂直滑动速率为(0.61&plusmn;0.28)mm/a,水平滑动速率为(1.27&plusmn;0.58)mm/a,其结果与相邻断裂相吻合。
[24] Hetzel R., Tao M, Stokes S, et al. Late Pleistocene/Holocene slip rate of the Zhangye thrust (Qilian Shan, China) and implications for the active growth of the northeastern Tibetan Plateau. Tectonics, 2004, 23: TC6006. doi: 10.1029/2004TC001653.
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[25] Shi Yafeng, Zhao Jingdong, Wang Jie, et al.New Understanding of Quaternary Glaciations in China. Shanghai: Shanghai Popular Science Press, 2011.
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[施雅风, 赵井东, 王杰, . 中国第四纪冰川新论. 上海: 上海科学普及出版社, 2011.]
[26] Liu Zechun.Quaternary glaciations in Qilian Mountains. Journal of Nanjing University, 1963, 2: 33-48.
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[刘泽纯. 祁连山的第四纪与冰川作用. 南京大学学报, 1963, 2: 33-53.]
[27] Guo Pengfei.Study on Quaternary glaciations in the middle section of Qilian Mountains. Glaciology and Geocryology, 1984, 6(1): 6-15.
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[28] Zhao Jingdong, Zhou Shangzhe, Cui Jianxin et al. ESR chronology of Bailanghe valley and new understanding of Qilianshan Mountain's Quaternary glaciation. Journal of Mountain Science, 2001, 19(6): 481-488.
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[赵井东, 周尚哲, 崔建新, . 摆浪河流域的ESR年代学与祁连山第四纪冰期新认识. 山地学报, 2001, 19(6): 481-488.]
[29] Zhou Shangzhe, Li Jijun, Zhang Shiqiang. Quaternary glaciation of the Bailang River Valley, Qilian Shan. Quaternary International, 2002, 97/98: 103-110.
The Qilian Shan, on the Northeast margin of the Qinghai–Tibetan Plateau, is weakly influenced by the Asian monsoon. Until recently, the Quaternary glacial geology of this region has been poorly understood. This paper describes a sequence of Quaternary glacial deposits in the upper reaches of the Bailang River. Using electron spin resonance (ESR), thermoluminescence (TL) and radiocarbon dating, tills, loess, buried soils and landforms were dated. The oldest till was dated 65463 ka BP by ESR. A younger till and its outwash terrace were dated to 65135 and 130 ka BP. The loess on this outwash terrace was dated to 141.7±11.4 ka BP at its base and 43.7±3.5 ka BP in its central part by TL dating. A buried soil on a younger till was dated at 6920±78 ka BP using 14C. The glacial landforms and these dating results show that glacial advances occurred during the Little Ice Age, the Neoglacial, MIS 2–4, MIS 6, and MIS 12. Glaciation during MIS 12 implies that the Qilian Mountains were rising coevally with the Qinghai–Tibetan Plateau, and were probably at a sufficient elevation for glaciation since at least 463 ka BP.
DOI:10.1016/S1040-6182(02)00055-1      [本文引用:8]
[30] Owee L A, Spencer J Q, MA H, et al.Timing of Late Quaternary glaciation along the southwestern slopes of the Qilian Shan. Boreas, 2003, 32: 281-291.
Moraines along the southwestern slopes of the Qilian Shan were dated using cosmogenic radionuclide (CRN) surface exposure techniques to help define the timing of glaciation in northernmost Tibet. The CRN data show glaciers extending 5–10 km beyond their present positions during the global Last Glacial Maximum (LGM) and probably maintained at their maximum extent until the Lateglacial. These data help support the view that glaciers throughout Tibet and the Himalaya were maintained at or near their maximum LGM extent until the Lateglacial. An optically stimulated luminescence date of 11.8 ± 1.0 ka on silt that caps a latero-frontal moraine shows that glaciers had retreated significantly by the end of the Pleistocene and that loess was beginning to form in this region in response to the changing climate during and after the Younger Dryas Stade.
DOI:10.1111/j.1502-3885.2003.tb01083.x      [本文引用:3]
[31] Yang Wei, Zhou Shangzhe, Narama C, et al.The stage-division and environmental significance of moraine in Qiyi Glacier, Qilian Shan. Journal of Lanzhou University (Natural Science), 2006, 42(6): 12-15.
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[杨威, 周尚哲, Narama C, . 祁连山中段新冰期以来的冰川序列及环境变化. 兰州大学学报(自然科学报), 2006, 42(6): 12-15.]
通过对七一冰川及其附近冰碛垅形态、植被覆盖、岩石种类及Schmidt hammer反弹R值、剖面内部组成的观测、对比和分析,认为新冰期时冰川扩张到海拔4km处,较现代冰川末端降低约300m,冰川前进约2km左右.小冰期的三列冰碛垅形成时间间隔基本相同,规模依次减小,表明小冰期的三次冷期中以15世纪最冷,持续时间最长,而17世纪次之,19世纪持续时间最短.
[32] Yuan Daoyang, Zhang Peizhen, Liu Baichi, et al.Geometrical imagery and tectonic transformation of Late Quaternary active tectonics in northeastern margin of Qinghai-Xizang Plateau. Acta Geologica Sinica, 2004, 78(2): 270-278.
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[袁道阳, 张培震, 刘百篪, . 青藏高原东北缘晚第四纪活动构造的几何图像与构造转换. 地质学报, 2004, 78(2): 270-278.]
青藏高原东北缘所在的柴达木-祁连活动地块内的晚第四纪活动构造可以划分为主边界左旋剪切构造带、块内次级右旋剪切构造带和挤压会聚构造带等三种基本类型。在区域北东向构造应力作用下,块体发生了NE向的挤压缩短、顺时针方向的旋转和向SEE方向的挤出等构造变形。在块体内部形成了挤压推覆构造、次级剪切构造、剪切压扁构造和弧形挤出构造等四种典型的构造转换方式,实现不同方向与不同性质活动构造之间的转换平衡。
[33] Wang Zongtai, Liu Chaohai.Development conditions and distribution features and regional divisions of present glaciers in Qilian Mountains. Acta Geographica Sinica, 1983, 38(2): 141-153.
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[34] Chengdu Institute of Geology and Mineral Resources, CAGS. The Geology Map of Tibet Plateau and Adjacent Areas. Beijing: Geological Publishing House, 2007: 3-41.
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[中国地质科学院成都地质矿产研究所. 青藏高原及邻区地质图(1∶1500000). 北京: 地质出版社, 2007: 3-41.]
[35] Yan Yeqing, Hu Yajie.Analyse on temporal and spatial distribution characteristic of rainfall in Gansu Province. Gansu Science and Technology, 2011, 27(1): 63-66.
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[闫业庆, 胡雅杰. 甘肃降水量时空分布特征浅析. 甘肃科技, 2011, 27(1): 63-66.]
对甘肃省11个代表性站点1951~2005年降水量系列的分 析,发现甘肃省降水量变化呈现出明显的带状分布,且降水量变化具有显著的地域性和时间性.利用距平及累积距平法对甘肃省降水量进行分析,发现在近50年中 西北地区降水量较小,但有逐渐增加的趋势,增加幅度较小;南部地区降水量丰富,但有逐渐减小的趋势,降幅较为明显;中部黄土高原地区降水量基本持续在一个 相对稳定的水平,且降水量枯、丰水年具有明显的周期变化特征.在甘肃省内部进行局部的南水北调,将对减缓甘肃省北部的用水压力产生很大的作用,使水资源利 用更加完善.
[36] Sklar L, Dietrich W E.River longitudinal profiles and bedrock incision models: Stream power and the influence of sediment supply//Tinkler K J, Wohl E E. Rivers Over Rock: Fluvial Processes in Bedrock Channels. Geophysical Monograph Series. Washington DC: AGU Press, 1998, 107: 237-260.
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[37] Wobus C W, Whipple K X, Kirby E, et al.Tectonics from topography: Procedure, promise, and pitfalls. Geological Society of America Bulletin, 2006, 398: 55-74.
Abstract content prepared wholly by U.S. government employees within scope of their employment. Individual scientists are hereby granted permission, without fees or further requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in other subsequent works and to make unlimited copies for noncommercial use in classrooms to further education and science. For any other use, contact Copyright Permissions, GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA, fax 303-357-1073, editing@geosociety.org. GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect official positions of the Society. ABSTRACT Empirical observations from fluvial systems across the globe reveal a consistent power-law scaling between channel slope and contributing drainage area. Theoretical arguments for both detachment-and transport-limited erosion regimes suggest that rock uplift rate should exert first-order control on this scaling. Here we describe in detail a method for exploiting this relationship, in which topographic indices of lon-gitudinal profile shape and character are derived from digital topographic data. The stream profile data can then be used to delineate breaks in scaling that may be asso-ciated with tectonic boundaries. The description of the method is followed by three case studies from varied tectonic settings. The case studies illustrate the power of stream profile analysis in delineating spatial patterns of, and in some cases, temporal changes in, rock uplift rate. Owing to an incomplete understanding of river response to rock uplift, the method remains primarily a qualitative tool for neotectonic investi-gations; we conclude with a discussion of research needs that must be met before we can extract quantitative information about tectonics directly from topography.
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[44] Berlin M M, Anderson R. Modeling of knickpoint retreat on the Roan Plateau, western Colorado. Journal of Geophysical Research, 2007, 112: F03S06. doi: 10.1029/2006JF000553.
[1] The Roan Plateau in western Colorado constitutes a natural experiment for studying landscape response to a drop in base level. Late Cenozoic incision of the upper Colorado River led to elevational isolation of the Plateau and initiation of a wave of incision into its southern edge. Knickpoints (oversteepened reaches that contain waterfalls 60 110 m in height) mark the upstream extent of this headward propagating wave. That this incision has occurred in a laterally extensive, well-stratified, and essentially flat-lying bedrock and in an area with relatively uniform climate, implies that it should serve as a good test of existing knickpoint propagation models. We predict the locations of knickpoints by using a stream power-based celerity model, in which knickpoint recession rate is a power function of drainage area and is proportional to rock susceptibility to erosion. Models of the Parachute and Roan drainages (17 and 16 knickpoints, respectively) show expected rapid initial knickpoint propagation rates, which decline as drainage area decreases stepwise at tributary junctions. The modeled positions of knickpoints match well with observed features, using a single combination of parameters to model retreat in both drainage basins. We compare our celerity model results with past studies and explore how longitudinal profile analysis may be used to derive independently the exponent on drainage area in the celerity model.
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Simple physical arguments, analogue experiments and numerical experiments all suggest that the internal dynamics of actively deforming collisional mountain ranges are influenced by climate. However, obtaining definitive field evidence of a significant impact of climate on mountain building has proved challenging. Spatial correlations between intense precipitation or glaciation and zones of rapi...
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