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21.
Dating deposits in caves formed by rivers may yield insight into rates and processes of bedrock incision. Three cave passages at different levels have developed in the walls of the Qianyou River valley in the Qinling mountains, south of the city of Xian, China. Twelve speleothem samples near the position of palaeowater tables in three cave passages are dated by the 230Th dating method. The results show that the river cut down at the rate of 0.23 ± 0.02 mm/yr from 358,000 ± 38,000 to 247,000 ± 28,000 yr ago, 0.19 ± 0.03 mm/yr from 247,000 ± 28,000 to 118,000 ± 19,000 yr ago, and 0.51 ± 0.08 mm/yr from 118,000 ± 19,000 yr ago until today. These fall very close to the glacial-interglacial transition following marine oxygen isotope stages 10, 8, and 6, respectively. The increase in downcutting rates during the interglacial period is consistent with warm, wet weather, increasing rates of erosion. This may lead us toward an underlying mechanism for modulating incision that is not in a direct response to the presence of headwater glaciers. 相似文献
22.
23,000 yr of vegetation history of the Upper Lerma, a tropical high-altitude basin in Central Mexico
Pollen analysis on a 9.54-m sediment core from lake Chignahuapan in the upper Lerma basin, the highest intermontane basin in Central Mexico (2570 m asl), documents vegetation and limnological changes over the past ∼23,000 14C yr. The core was drilled near the archaeological site of Santa Cruz Atizapán, a site with a long history of human occupation, abandoned at the end of the Epiclassic period (ca. 900 AD). Six radiocarbon AMS dates and two well-dated volcanic events, the Upper Toluca Pumice with an age of 11,600 14C yr B.P. and the Tres Cruces Tephra of 8500 14C yr B.P., provide the chronological framework for the lacustrine sequence. From ca. 23,000 14C yr B.P. to ca. 11,600 14C yr B.P. the plant communities were woodlands and grasslands based on the pollen data. The glacial advances MII-1 and MII-2 correlate with abundant non-arboreal pollen, mainly grasses, from ca. 21,000 to 16,000 14C yr B.P., and at ca. 12,600 14C yr B.P. During the late Pleistocene, lake Chignahuapan was a shallow freshwater lake with a phase of lower level between 19,000 and 16,000 14C yr B.P. After 10,000 14C yr B.P., tree cover in the area increased, and a more variable lake level is documented. Late Holocene (ca. 3100 14C yr B.P.) deforestation was concurrent with human population expansion at the beginning of the Formative period (1500 B.C.). Agriculture and manipulation of the lacustrine environment by human lakeshore populations appear at 1200 14C yr B.P. (550 A.D.) with the appearance of Zea mays pollen and abundant charcoal particles. 相似文献
23.
24.
鉴于保护地调查评价历史与目标,国家公园地质考察评价基准是地质学关于地球的“时间、空间、演化”知识的发现路径,既识别国家公园候选地的国家符合性地质事件,又识别国家公园自然性、独特性、优美性、多样性、整体性的地质机制,提供国家公园最重要的自然生态系统与自然景观保育管理的地质学支撑。浙江丽水百山祖国家公园符合性的地质事件是公园发现的4.0~4.2 Ga冥古宙陆核物质的锆石晶体、中生代岛弧陆盆火山酸性火山岩系列和从流纹岩山岭到花岗岩—变质岩谷底的“峰—岭—丘—谷(盆)”的亚热带滨海山地地貌结构,地貌结构发育演化是百山祖公园最重要的自然生态系统、区域性流域水源地、独特山水景观的形成基础。保护百祖山公园地貌结构和过程的完整性与真实性是确保其重要自然生态系统自然演化和生态服务产品持续供给的基础。 相似文献
25.
利用FNL及常规资料,对比分析了2010年2月22—24日(过程Ⅰ)和2015年12月10—13日(过程Ⅱ)天山北坡2次暴雪过程。结果表明,暴雪区上空θse锋区陡立和条件性对称不稳定及次级环流是形成暴雪的主要机制。不同点是:过程Ⅰ暴雪产生在西西伯利亚低涡底部强锋区上,南北支短波槽汇合的区域,冷高压为西北路径;过程Ⅱ是乌拉尔山大槽东移北收,冷高压为偏西路径;2次过程在温压的时间演变上有显著的区别。在高低空配置上也有明显的区别:过程Ⅰ 500 hPa以下为暖平流,以上为冷平流,低层为暖湿结构;过程Ⅱ 700 hPa以下为冷平流,700—600 hPa为暖平流,低层有湿冷空气锲入。过程Ⅰ暴雪区位于θse锋区上,锋区低层强,中高层弱;过程Ⅱ暴雪区位于θse锋区中后部,锋区低层弱,中高层强。水汽输送和输入量及比湿过程Ⅰ大于过程Ⅱ。 相似文献
26.
27.
James F Ni 《Journal of Earth System Science》1989,98(1):71-89
The Himalayan mountains are a product of the collision between India and Eurasia which began in the Eocene. In the early stage
of continental collision the development of a suture zone between two colliding plates took place. The continued convergence
is accommodated along the suture zone and in the back-arc region. Further convergence results in intracrustal megathrust within
the leading edge of the advancing Indian plate. In the Himalaya this stage is characterized by the intense uplift of the High
Himalaya, the development of the Tibetan Plateau and the breaking-up of the central and eastern Asian continent. Although
numerous models for the evolution of the Himalaya have been proposed, the available geological and geophysical data are consistent
with an underthrusting model in which the Indian continental lithosphere underthrusts beneath the Himalaya and southern Tibet.
Reflection profiles across the entire Himalaya and Tibet are needed to prove the existence of such underthrusting. Geodetic
surveys across the High Himalaya are needed to determine the present state of the MCT as well as the rate of uplift and shortening
within the Himalaya. Paleoseismicity studies are necessary to resolve the temporal and spatial patterns of major earthquake
faulting along the segmented Himalayan mountains. 相似文献
28.
Georgeta E. Ionescu 《Cretaceous Research》1993,14(6)
The stratigraphic and palaeogeomorphologic conditions of bauxite formation in the North Apuseni mountains are related to the geotectonic evolution of western Romania during the Late Jurassic and Early Cretaceous. Bauxitization, accumulation and preservation of the bauxites were directly associated with karst development in the area under study. The petrographic and mineralogic nature of the bauxite deposits reveal superimposed aliagenetic, catagenic and, locally, metamorphic features. A comparative analysis of the principal chemical components of the different bauxite-bearing zones of the North Apuseni mountains are used to show similar development. 相似文献
29.
Ultra-high pressure metamorphic rocks in the Dabie-Su-Lu region, China: Their formation and exhumation 总被引:17,自引:0,他引:17
Cong Bolin Wang Qingchen Zhai Mingguo Zhang Ruyuan Zhao Zhongyan Ye Kai 《Island Arc》1994,3(3):135-150
Abstract Based on petrological, structural, geological and geochronological research, the authors summarize the progress of ultra-high pressure (UHP) metamorphic rock study since 1989 by Chinese geoscientists and foreign geoscientists in the Dabie-Su-Lu region. The authors introduce and discuss a two-stage exhumation process for the UHP metamorphic rocks that have various lithologies; eclogite, ultramafics, jadeitic quartzite, gneiss, schist and marble. The metamorphic history of UHP metamorphic rocks is divided into three stages, that is, the pre-eclogite stage, coesite eclogite stage, and retrograde stage. Prior to UHP metamorphism, the ultramafics had a high temperature environment assemblage of mantle and others had blueschist facies assemblages. The granulite facies assemblages, which have recorded a temperature increase event with decompression, have developed locally in the Weihai basaltic rocks. Isotopic ages show a long range from > 700 Ma to 200 Ma. The diversity in protoliths of UHP metamorphic rocks may be related to the variation of isotopic ages older than 400 Ma. The Sm-Nd dating of ~ 220 Ma could reflect the initial exhumation stage after the peak UHP metamorphism in relation to the collision between the Sino-Korean and Yangtze blocks and subsequent events. Petrological and structural evidence imply a two-stage exhumation process. During the initial exhumation, the UHP metamorphic rocks were sheared and squeezed up in a high P/T regime. In the second exhumation stage the UHP metamorphic rocks were uplifted and eventually exposed with middle crustal rocks. 相似文献
30.
华蓥山地区二叠纪生物礁顶部钙结壳的岩石学特征及成因探讨 总被引:3,自引:0,他引:3
华蓥山地区上二叠统长兴阶生物礁顶部普遍存在一套数米至十余米厚的钙结壳.含有钙结壳中常见的渗滤豆石、钙质结核、蜂窝状构造、根模、钙化细菌丝体、花瓣构造、微型钟乳石等典型组构,并伴有变形构造、溶蚀构造和角砾化等现象。这套钙结壳的发现对研究华南二、三叠纪之交的古气候,古地理和沉积发育史具有重要意义,本文主要描述钙结壳的岩石学特征,并简要讨论了本地区钙结壳的成因。 相似文献