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1.
ABSTRACT

This study investigated the late Quaternary climate and environmental characteristics of two tributary valleys (Xingmu and Depu Valleys) in the Parlung Zangbo Valley, southeastern Tibetan Plateau. Optically stimulated luminescence (OSL) samples collected from moraines at the mouth of Xingmu Valley produce a wide age range from 13.9 ka to 76 ka. The ages measured from the lenticular sand are consistent with the relative geomorphic sequence of the landforms. Lenticular sand layers below the moraine were dated to 37.9 ka and 44.7 ka, indicating that fluvial processes were likely dominant in the valley during Marine Isotope Stage (MIS) 3. The outer moraine ridges at the valley mouth were formed during 13.9 ka and 26.5 ka, corresponding to MIS2. At Depu Valley, OSL samples from two sets of lateral and terminal moraines close to the modern glacier, provide ages from 1.4 ka to 29.2 ka. The paleosol layer widely developed during 2.6 cal ka BP and 8.7 cal ka BP in the study area, reflecting a relatively warm condition during the mid-Holocene.  相似文献   

2.
新疆阿尔泰山东段冰碛物光释光测年研究   总被引：3，自引：0，他引：3  
贾彬彬  周亚利  赵军 《地理学报》2018,73(5):957-972
冰川是塑造地表形态最积极的外营力之一,对冰川地貌的年代学研究是重建古冰川发展史的关键,也是研究气候变化的重要途径。冰碛物是冰川作用的直接产物,代表过去发生的冰川事件,对冰碛物进行准确测年能够为重建古冰川的进退、理解区域古气候变化提供年代学支撑。本文在新疆阿尔泰山东段采集了8个冰碛物样品以进行光释光测年,利用单片再生剂量法对90~125 μm的石英颗粒进行等效剂量的测定。通过等效剂量值频率分布特征及De（t）坪区图分析得出大部分冰碛物的光释光信号晒退不彻底,所以利用一阶动力学公式对持续激发的光释光信号晒退曲线（CW-OSL）进行多组分拟合拆分,得到快速、中速、慢速3种组分,依据分离出的快速组分确定等效剂量值。研究结果显示,距今32 ka以来阿尔泰山东段区域在MIS3阶段、MIS2阶段、8.2 ka左右、全新世大暖期及新冰期等5个时段有冰川发育,冰川发育与气候变化密切相关。  相似文献   

3.
Susquehanna River terraces are used to establish time lines along a 150 km reach of the river, from the Lower Piedmont to the edge of the Appalachian Plateau. This is achieved by generating soil chronosequences at two locations — Marietta, PA, in the Lower Piedmont, and Muncy, PA, near the glacial border on the boundary between the Valley and Ridge province and the Appalachian Plateau. These sites preserve the most complete record of fluvial incision on the Susquehanna River with flights of seven Quaternary terraces ranging in elevation from 3 m to 51 m above the modern river.Soil characteristics used to develop the soil chronosequences include complexity of horizonization, thickness of B horizon, clay content of B horizon, soil color, CBD extractable Fe, Al, and Mn, total extractable Fe, and clay mineralogy. Terrace age constraints are based on soil development, correlation to regional glacial stratigraphy, correlation to dated fluvial and glaciofluvial deposits, and by paleomagnetic analysis of sediments. Terrace ages at the Muncy site range from modern (< 150 ybp) to Middle Middle through Early Middle Pleistocene (∼ 300 ka to ∼ 770 ka). Marietta has terrace ages ranging from modern (< 150 ybp) to Early Pleistocene through Late Pliocene (∼ 770 ka to ∼ 2400 ka).  相似文献   

4.
青藏高原东部当子沟末次冰期冰川演化光释光测年   总被引：1，自引：0，他引：1  
青藏高原东部横断山脉沙鲁里山北支雀儿山北麓的当子沟, 保留了4 组末次冰期冰碛垄, 每组冰碛垄由若干道小冰碛垄组成, 是末次冰期多次冰川波动的理想地貌证据。为恢复该地末次冰期冰川演化历史, 从这4 组冰碛垄中采集了7 个光释光测年样品进行年代测定。等效剂量采用单片再生法(SAR)和标准生长曲线法(SGC)进行测试。年代结果显示:第1~3 组冰碛垄形成于22.4-16.5 ka BP, 属深海氧同位素2 阶段(MIS-2), 大致与全球末次盛冰期(LGM)相当。第4 组冰碛垄形成于MIS-3。MIS-3 冰川规模大于MIS-2。第1~3 组9 道次一级小冰碛垄表明, 约在22.4-16.5 ka BP期间当子沟冰川经历了9 次小波动。冰川在稍早于22.4 ka BP达到MIS-2 的最大范围, 此后规模在波动中逐渐萎缩, 可能是冰川对MIS-2 后期太阳辐射增强、气温回升的响应。最里侧冰碛垄形成于16.5 kaBP, 可能标志着该地冰消期的开始。此后, 当子沟冰川大幅快速退缩。冰川融水在最里侧冰碛垄里侧低洼谷地汇集并被阻塞形成当子错。  相似文献   

5.
摆浪河流域的ESR年代学与祁连山第四纪冰期新认识   总被引：5，自引：1，他引：4  
摆浪河发源于走廊南山北坡,是黑河的一条支流。在河源区保存有六条完整的冰碛和较为完整的阶地序列。应用ESR技术对采自该处较老的第三、第五、第六套冰碛和冰水成因的主阶地上的砾石层与上覆黄土进行了测年研究,测试结果分别为13.4ka、135.3ka、462.9ka、130.1ka、130.2ka。根据测试的结果并结合地貌地层学得出：3800m以上“U”型谷中的第一、第二套冰碛为全新世中小冰期、新冰期沉积的冰碛;第三套冰碛物是末次冰期晚期沉积的,与深海氧同位素2阶段相对应;第五套冰碛物是与深海氧同位素6阶段相对应的冰期沉积的;第六套冰碛物是与深海氧同位素12阶段相对应冰期沉积的;由此推断介于第三套与第五套冰碛之间的第四套冰碛是末次冰期早期沉积的,即可与深海氧同位素4阶段相对应。主阶地与第五套冰碛是同一时期形成的,这也与地貌地层学的推断相一致,从此处完整的冰川沉积序列还可以得出：在更新世中,祁连山地区至少存在三次冰期,经历四次冰川作用。祁连山部分地段或是整体至少在463ka前就已经抬升到当时冰期气候相耦合的高度。ESR可以用于河流、冰川沉积物的测年。  相似文献   

6.
螺髻山地处青藏高原东南缘,是确切存在第四纪古冰川遗迹的典型山地之一,该区冰川地貌演化对于研究环境变化具有重要的科学意义。螺髻山东坡清水沟保存两套古冰川槽谷,分别为上槽谷和下槽谷,其中下槽谷保存完整,而上槽谷在3450~3600 m的阴坡部分出现缺失。采用野外地貌调查与模型分析相结合的方法,对冰川槽谷地貌进行分析,结果表明:清水沟槽谷的抛物线模型中,|A|值在1.3101~15.2064 之间变动,B 值变化于0.9695~3.2965 之间,且随着海拔由高到低,都存在着先变小后变大的规律,A、B值同时反映出在海拔3450~3600 m处冰川槽谷的演化不符合常态。分析认为岩性差异和河流溯源侵蚀是影响上槽谷形态的主要原因。对保存在清水沟上下槽谷内的高、低侧碛进行ESR年代测定,结果显示:高侧碛形成于58-84 ka BP左右的末次冰期早期,对应深海氧同位素4 阶段(MIS4);低侧碛形成于13-17 ka BP,属于于全球末次冰盛期晚期的产物。两次冰川作用分别塑造出两套冰川槽谷,即在末次冰期早期冰川作用形成上槽谷,末次冰期晚期形成下槽谷。  相似文献   

7.
Moraine morphology is a valuable indicator of climate change. The glacial deposits of ten valleys were selected in the Parlung Zangbo River Basin, southeastern Tibetan Plateau, to study the glacial characteristics of the Last Glaciation and the climate change processes as revealed by these moraines. Investigation revealed that a huge moraine ridge was formed by ancient glacier in the Marine Isotope Stage 2 （MIS2）, and this main moraine ridge indicates the longest sustained and stable climate. There are at least two smaller moraine ridges that are external extensions of or located at the bottom of the main moraine ridge, indicating that the climate of the glacial stage before MIS2 was severer but the duration was relatively shorter. This distribution may reflect the climate of MIS4 or MIS3b. The glacial valleys show multi-channel, small-scale moraine ridges between the contemporary glacial tongue and the main moraine ridge. Some of these multi-channel mo- raine ridges might be recessional moraine, indicating the significant glacial advance during the Younger Dryas or the Heinrich event. The moraine ridges of the Neoglaciation and the Little Ice Age are near the ends of the contemporary glaciers. Using high-precision system dating, we can fairly well reconstruct the pattern of climate change by studying the shape, extent, and scale characteristics of glacial deposits in southeastern Tibet. This is valuable research to understand the relationship between regional and global climate change.  相似文献   

8.
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9.
有关塔克拉玛干沙漠演化的年代学数据尚少,本文在地貌认知较少的克里雅河下游沙漠深处的圆沙和喀拉墩一带16处剖面获取了18件河流沉积光释光（OSL）年龄。新测得的年龄数据分布于3.4—44.1 ka。全新世样品多分布于圆沙与喀拉墩干三角洲及其之间的地区,年龄集中在11 ka、8—9 ka、5—6.5 ka、4.6 ka与3.4—3.7 ka。深海氧同位素3阶段（MIS3）样品位置在克里雅河与和田河之间,分布在约38—47 ka前后。末次冰期冰盛期（LGM）末样品分布在通往圆沙古河道的西侧,年龄在14.5 ka前后。根据沉积物样品及年代学数据认为：克里雅河下游曾在MIS3阶段末、LGM末与全新世多次发生洪泛;数次的洪泛事件对克里雅河下游地貌发育产生重要影响;克里雅河下游干三角洲及现代河流西侧的流动沙丘主要形成于全新世,与前人“发育于汉唐以来”的观点不完全一致。克里雅河下游河流沉积发生时间与塔里木盆地周缘山脉冰进冰消有一定对应,暗示气候变化可能间接影响着塔克拉玛干沙漠沙丘的发育。  相似文献   

10.
The Kanas River originates on the southern slope of Youyi Peak, the largest center of modern glaciers in Altai Mountains, China. Three sets of moraines and associated glacial sediments are well preserved near the Kanas Lake outlet, recording a complex history and landscape evolution during the Last Glacial. Dating the moraines allows the temporal and spatial glacier shift and climate during the Last Glacial to be determined, and then constrains when and how the Kanas Lake basin was formed. Dating of the glacial tills was undertaken by utilizing the optically stimulated luminescence (OSL) method. Results date four samples from the three sets of moraines to 28.0, 34.4, 38.1, and 49.9 ka and one sample from outwash sediment to 6.8 ka. The Kanas Lake basin is a downfaulted basin and was eroded by glacier before 28.0 ka, and the glacial moraines blocked the glacier-melt water after the glacier retreat, which made the present-day Kanas Lake eventually form at least before 6.8 ka BP. In Altai Mountains, the glacier advance was more extensive in Marine Isotope Stage (MIS) 3 than MIS 2, probably because the mid-latitude westerlies shifted northward and/or intensified during the MIS 3, resulting in a more positive glacier mass balance. Nevertheless, the Siberian High dominated the Altai Mountains in MIS 2, resulting in a relative decrease in precipitation.  相似文献   

11.
天山阿特奥依纳克河流域冰川沉积序列   总被引：1，自引：0，他引：1  
阿特奥依纳克河位于我国天山的最西段,最大现代冰川作用中心托木尔峰的南麓。在第四纪冰期与间冰期的气候旋回中,该处留下了形态较为完整的6套冰川沉积。应用ESR测年技术 (辅以OSL测年技术) 对冰碛物及其相应的冰水沉积物进行了定年,测得6套冰碛年龄分别为7.3±0.8ka BP (OSL,冰水沙);12.3±1.2ka BP (OSL) 与15~29ka BP;46~54ka BP;56~65ka BP;155.8±15.6ka BP与234.8±23.5ka BP;453.0±45.3ka BP,测年结果表明它们分别形成于新冰期、海洋同位素阶段(MIS)2、3b、4、6、12。第三套冰碛测年结果表明该处MIS3b冰进规模较大,其规模基本上与末次盛冰期 (MIS2) 的规模相当。此处最老冰碛测年结果与我国中段天山乌鲁木齐河源高望峰冰碛的测年结果 (459.7±46ka BP与477.1ka BP) 遥相呼应,老冰碛的年龄显示我国天山西段与中段至少于MIS12进入了冰冻圈,开始发育冰川。  相似文献   

12.
According to the glacial landforms and deposits with the optically stimulated luminescence (OSL) dating results, two glacial stages of the last glacial cycle (LGC) and Late Glacial were identified. The Late Glacial stage (Meteorological Station glacier advance) took place about 11 ka (11.3±1.2 ka), and the last glacial maximum (LGM), named Black Wind Mouth glacier advance, occurred at 20 ka (20.0±2.1 ka). Based on the Ohmura’s formula in which there is a relationship between summer (JJA) atmospheric temperature (T) and the annual precipitation (P) at ELA, the present theoretical equilibrium line altitude (ELAt) in Changbai Mountains was 3380±100 m. Six methods of accumulation-area ratio (AAR), maximum elevation of lateral moraines (MELM), toe-to headwall altitude ratios (THAR), the terminal to summit altitudinal (TSAM), the altitude of cirque floor (CF), and the terminal to average elevation of the catchment area (Hofer) were used for calculation of the former ELAs in different stages. These methods provided the ELA for a range of 2250–2383 m with an average value of 2320±20 m during the LGM, which is 200 m higher than the value of previous investigation. The snowlines during the Late Glacial are 2490 m on northern slope, and 2440 m on western slope. The results show that the snowline on northern slope is 50 m higher than that on western slope during the Late Glacial, and the average snowline is 2465m. The ΔELA values were more than 1000 m during the LGM, and about 920 m lower than now during the Late Glacial stage respectively. Compared with Taiwanese and Japanese mountains in East Asia during the LGM, the effect of the uplift on ELA in Changbai Mountains during the glaciations (i.e. 20 m uplift in the LGM and 11 m in the Late Glacial) is not obvious. Foundation: National Natural Science Foundation of China, No.40571016 Author: Zhang Wei (1969–), Ph.D and Professor, specialized in Quaternary environment and climate geomorphology.  相似文献   

13.
Sedimentary deposits from the Smith Canyon dune field, south-central Columbia Basin, Washington, U.S.A. document climatically-influenced Late Pleistocene and Holocene aeolian and fluvial deposition in a region impacted by glacial outburst floods and tephra falls. The depositional history is summarized by five environmentally distinctive and climatically sensitive sedimentary units (temporal limits estimated): Unit 1 (c. 15·5–8 ka), pedogenically altered glacial outburst flood and minor aeolian silt and clay; Unit 2 (c. 8–6·9 ka), fluvial and minor aeolian sand; Unit 3 (c. 6·9–6·8 ka), flood-induced fluvial sand with gravel-sized tephra clasts; Unit 4 (c. 6·8–3·9 ka), aeolian dune sand; Unit 5 (c. 3·9 ka to present), pedogenically altered, stabilized dune sand. Estimated age ranges are based on stratigraphic position, tephrochronology, and correlation with temporally constrained strata from elsewhere in the region.  相似文献   

14.
三峡库区长江阶地冲积物的年代测定   总被引：7，自引：1，他引：6  
三峡库区的长江阶地是研究川江、峡江水系演变的重要证据。发育冲积物的阶地主要出现在库区西半部的川东和重庆的中低山、丘陵区,最多有6级。阶地研究中遇到的主要问题是难以确定阶地的形成时代。我们采用TL法和ESR法测量了三峡库区6个地点阶地的堆积年龄。测量数据显示,ESR法基本不适合测定三峡库区冲积物的年龄;尽管TL法本身存在缺陷,但测得的T1~T4的堆积年龄得到其他独立的测年结果的支持。阶地T1~T4的TL年龄依次为7~17 ka、28~46 ka、62~81 ka和103~105 ka。此外,根据年代地层对比的方法以及前人的古地磁测量结果,推测三峡库区第四级阶地的年龄大于101 ka,第五级阶地的年龄大于392 ka,而第六级阶地的年龄小于780 ka。从而初步建立了三峡库区阶地堆积的时间序列。  相似文献   

15.
The southern foreland basin of the Pyrenees (Ebro basin) is an exorheic drainage basin since Late Miocene times. Remnants of an early exorheic Ebro drainage system are not preserved, but morphology provides evidence for the Pliocene–Quaternary drainage development. The incision history of the Ebro system is denoted by (i) extensive, low gradient pedimentation surfaces which are associated with the denudation of the southern Pyrenean piedmont around the Pliocene–Quaternary transition and (ii) deeply entrenched Quaternary river valleys. Presumably since the Middle Pleistocene fluvial incision intensified involving the formation of extensive terrace staircase in the Ebro basin. Terrace exposure dating in major Ebro tributary rivers indicates climate‐triggered terrace formation in response to glacial–interglacial climate and glacier fluctuations in the Pyrenean headwaters. The overall (semi)parallel longitudinal terrace profiles argue for progressive base level lowering for the whole Ebro drainage network. The landscape evolution model, TISC, is used to evaluate climatic, tectonic and base level scenarios for terrace staircase formation in the Ebro drainage system. Model simulations are compared with morpho‐climatic, tectonic and chronologic data. Results show that climatic fluctuations cause terrace formation, but the incision magnitudes and convergent terrace profiles predicted by this climate model scenario are not consistent with the (semi)parallel terraces in the Ebro basin. A model including previous (late Pliocene) uplift of the lower Ebro basin results in rapid base‐level lowering and erosion along the drainage network, small late stage incision magnitudes and terrace convergence, which are not in agreement with observations. Instead, continuous Quaternary uplift of both the Pyrenees and the Ebro foreland basin triggers (semi)parallel terrace staircase formation in southern Pyrenean tributary rivers in consistency with the observed longitudinal terrace profiles and Middle–Late Pleistocene incision magnitudes. Forward model simulations indicate that the present Ebro drainage system is actively incising, providing further evidence for uplift.  相似文献   

16.
对贵州清水江上游马寨、翁东、三江、施洞沿江4个剖面的阶地特征、年代学结果进行了综合分析。发现以凯里断层为界,上游地区的马寨和翁东2个剖面的T2阶地形成时代约为51~57 ka B.P.,T1阶地的形成时代约为25 ka B.P.,下游地区的三江和施洞2个剖面的T2阶地形成时代约为122~102 ka B.P.,T1阶地的形成时代约为78 ka B.P.。选取各剖面的T2阶地的基座高度来计算了河流下切速率,发现上游地区2个剖面（马寨、翁东）的河流下切速率较接近,约为0.41~0.34 m/ka,明显高于下游地区的2个剖面（三江、施洞）的0.16~0.20 m/ka,表现为上游下切速率高,越往下游方向下切速率逐渐降低。这表明自晚更新世以来,清水江上游区域受到构造作用的影响而发生差异抬升,具体表现为西部构造抬升幅度大,阶地下切速率快;东部构造抬升幅度小,阶地下切速率慢。  相似文献   

17.
According to the glacial landforms and deposits with the optically stimulated luminescence (OSL) dating results, two glacial stages of the last glacial cycle (LGC) and Late Glacial were identified. The Late Glacial stage (Meteorological Station glacier advance) took place about 11 ka (11.3±1.2 ka), and the last glacial maximum (LGM), named Black Wind Mouth glacier advance, occurred at 20 ka (20.0±2.1 ka). Based on the Ohmura’s formula in which there is a relationship between summer (JJA) atmospheric temperature (T) and the annual precipitation (P) at ELA, the present theoretical equilibrium line altitude (ELAt) in Changbai Mountains was 3380±100 m. Six methods of accumulation–area ratio (AAR), maximum elevation of lateral moraines (MELM), toe–to headwall altitude ratios (THAR), the terminal to summit altitudinal (TSAM), the altitude of cirque floor (CF), and the terminal to average elevation of the catchment area (Hofer) were used for calculation of the former ELAs in different stages. These methods provided the ELA for a range of 2250–2383 m with an average value of 2320±20 m during the LGM, which is 200 m higher than the value of previous investigation. The snowlines during the Late Glacial are 2490 m on northern slope, and 2440 m on western slope. The results show that the snowline on northern slope is 50 m higher than that on western slope during the Late Glacial, and the average snowline is 2465m. The ELA △ values were more than 1000 m during the LGM, and about 920 m lower than now during the Late Glacial stage respectively. Compared with Taiwanese and Japanese mountains in East Asia during the LGM, the effect of the uplift on ELA in Changbai Mountains during the glaciations (i.e. 20 m uplift in the LGM and 11 m in the Late Glacial) is not obvious.  相似文献   

18.
Quaternary glacial stratigraphy and relative sea-level changes reveal at least two glacial expansions over the Chelyuskin Peninsula, bordering the Kara Sea at about 77°N in the Russian Arctic, as indicated from tills interbedded with marine sediments, exposed in stratigraphic superposition, and from raised-beach sequences mapped to altitudes of at least up to ca. 80 m a.s.l. Chronological control is provided by accelerator mass spectrometry 14C dating, electron-spin resonance and optically stimulated luminescence geochronology. Major glaciations, followed by deglaciation and marine inundation, occurred during marine oxygen isotope stages 6–5e (MIS 6–5e) and stages MIS 5d–5c. These glacial sediments overlie marine sediments of Pliocene age, which are draped by fluvial sediment of a pre-Saalian age, thereby forming palaeovalley/basin fills in the post-Cretaceous topography. Till fabrics and glacial tectonics record expansions of local ice caps exclusively, suggesting wet-based ice cap advance, followed by cold-based regional ice-sheet expansion. Local ice caps over highland sites along the perimeter of the shallow Kara Sea, including the Byrranga Mountains and the Severnaya Zemlya archipelago, appear to have repeatedly fostered initiation of a large Kara Sea ice sheet, with the exception of the Last Glacial Maximum (MIS 2), when Kara Sea ice neither impacted the Chelyuskin Peninsula nor Severnaya Zemlya, and barely touched the northern coastal areas of the Taymyr Peninsula.  相似文献   

19.
We present a new record from the Última Esperanza region (51°25’-52°25'S), southwestern Patagonia, to unravel the timing and structure of glacial fluctuations during the Last Glacial Termination (T1). This sector of southern South America represents the only windward-facing continental landmass in the Southern Hemisphere that intersects the core of the Southern Westerly Wind belt.Geomorphic, stratigraphic and geochronological evidence indicate the following stages during and since the Last Glacial Maximum (LGM): (i) deposition of prominent moraine complexes during at least two advances dated between ~ 39 and > 17.5 ka; (ii) development of an ice-dammed proglacial lake (glacial lake Puerto Consuelo) accompanying ice recession; (iii) active deposition of moraine complexes at intermediate positions followed by recession at ≥ 15.2 ka; (iv) lake level drop and subsequent stabilization between 15.2-12.8 ka; (v) a glacial readvance in glacial lake Puerto Consuelo between 14.8-12.8 ka; (vi) ice recession, stabilization, and lake level lowering between 12.8-10.3 ka; and (vii) glacial withdrawal and disappearance of glacial lake Puerto Consuelo prior to 10.3 ka. By comparing our results with the chronologies from neighboring regions we explore whether there was a consistent temporal/geographic pattern of glacial fluctuations during the LGM and T1, and examine their implications at regional, hemispheric, and global scales. The correspondence of these variations with key paleoclimate events recorded in the Southern and the Northern Hemispheres suggest a common forcing that, most likely, propagated through the atmosphere. Regional heterogeneities at millennial timescales probably reflect the influence of processes related to deep ocean circulation, and changes in the position/intensity of the Antarctic Polar Front and Southern Westerly Winds.  相似文献   

20.
Staircases of strath terraces and strongly incised valleys are the most typical landscape features of Portuguese rivers. This paper examines the incision achieved during the late Cenozoic in an area crossed by the Tejo river between the border with Spain and the small town of Gavião. In the more upstream reach of this area, the Tejo crosses the Ródão tectonic depression, where four levels of terraces are distinguished. During the late Cenozoic fluvial incision stage, the Ródão depression underwent less uplift than the adjacent areas along the river. This is reflected by the greater thicknesses and spatial extent of the terraces; terrace genesis was promoted by impoundment of alluvium behind a quartzitic ridge and the local presence of a soft substratum. Outside this tectonic depression, the Tejo has a narrow valley incised in the Hercynian basement, with some straight reaches that probably correspond to NE–SW and NNW–SSE faults, the terraces being nearly absent. Geomorphological evidence of tectonic displacements affecting the Ródão dissected terrace remnants is described. Geochronological dating of the two younger and lower terrace levels of this depression suggests a time-averaged incision rate for the Tejo in the Ródão area, of ca. 1.0 m/ka over the last 60 thousand years. A clear discrepancy exists between this rate and the 0.1 m/ka estimated for the longer period since the end of the Pliocene. Although episodes of valley incision may be conditioned by climate and base-level changes, they may also have been controlled by local factors such as movement of small fault-bounded blocks, lithology and structure. Regional crustal uplift is considered to be the main control of the episodes of valley incision identified for this large, long-lived river. A model is proposed in which successive regional uplift events—tectonic phases—essentially determined the long periods of rapid river downcutting that were punctuated by short periods of lateral erosion and later by some aggradation, producing strath terraces.  相似文献   

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