基准面变化与层序地层——以塔里木盆地陆相地层为例

以海平面变化 为基出的层序地层学理论在研制陆相盆地中遇到了困难,地层基准面变化 在解释地层层序成因和地层层序发挥了重要作用。     

塔里木盆地三大遗址群的兴衰与环境变化

塔里木盆地中的三大遗址群都经历了公元前的兴起、1～3世纪的兴盛和发展,以及7世纪的废弃。它们的兴衰变化具有一定的同步性。这意味着导致水资源条件变化的原因是共同的,即气候变化,而不是人为因素。     

中国小冰期的气候

在建立了近百年中国10个区的年平均气温序列的基础上,利用史料、冰芯δ18O及树木年轮,重建了各区近400～1000年的10年平均气温序列。分析表明,近千年来中国可能有5次冷期分别出现于1100’s～1150’s,1300’s～1390’s,1450’s～1510’s,1560’s～1690’s及1790’S～1890’S。第4次冷期主要在中国大陆中部最明显,而第5次冷期在中国南部较强。新疆、东北及闽台气温变化与中国其它部分特别是南部及西南有一定差异。如果把最后两次冷期作为中国小冰期的两次主要寒冷阶段,其30年平均气温可能比20世纪中期中国最暖的30年(1920’S～1940’S)低0．6～0．11℃。     

火山活动与构造气候旋回

火山活动是一种重要的地质现象,它的发生总是与构造运动相关联,构造运动有强有弱,持续时间有长有短,具有旋目性,火山活动同样也具有旋回性。大规模的火山喷发往往会对气候变化产生影响,而影响的程度取决于火山所处的位置和喷发的性质及程度。强烈的火山喷发将造成局部地区乃至全球气温下降,由此人们推测火山喷发多发生在冰期,然而统计资料表明,绝大多数火山喷发发生在间冰期,此时的构造运动也比较活跃,而在冰期时很少有火山喷发。至于火山喷发后在短时间内造成的降温与长时间的冰期不能相提并论。火山活动有旋回性,它影响的气候变化也具备有旋回性,这是构造气候旋回的一种表现形式。     

气候变化对新疆玛纳斯河流域水文水资源的影响

考虑积雪和降水不均等特点,对流域进行分带处理,提出和建立了包含积雪融雪结构的水文评价模型。利用该模型求得未来不同气候变化情况下的月均流量过程,分析气候变化对玛纳斯河流域水文水资源的影响。结果表明:若气温升高2℃,降水减少20%,则夏季径流减少52.59%,冬季径流减少1.77%,年均径流减少46.87%。     

走滑拉分断块对沉积的控制作用——准噶尔盆地东部侏罗系中下统沉积主控因素新认识

通过在南缘露头剖面相构型识别,北部地震剖面解释的沉积体侧向迁移特征,建立了走滑拉分构筑的可容空间变化型式。此沉积对构造的响应模式,不仅可以很好的解释拼合砂体的分布,古流向偏转等问题,而且对于预测盆内沉积体走向也是一个很有用的工具。     

遥感技术在全球环境变化研究中的作用

具有快速、多波段、周期性、大面积覆盖等观测能力的空间遥感技术,是全球环境变化研究中不可替代的重要手段。随着遥感技术在国际上的蓬勃发展,遥感技术在我国全球环境变化研究中也开始得到了广泛的应用。简述了遥感技术在我国土地覆盖、森林与草场、海洋调查与灾害监测方面的应用情况以及所取得的成果。这些研究为遥感技术在全球环境变化研究中的深入应用奠定了基础。     

探讨气候变化的新热点:大气气溶胶的气候效应

气溶胶的气候效应是近年来研究的热点问题。针对这一问题已开展过许多观测研究和模式研究。但是由于气溶胶问题的复杂性以及对气溶胶的观测历史短,至今尚缺少全球范围的气溶胶系统观测资料,所以目前气溶胶的辐射强迫仍是个很不确定的量。这直接影响对全球气候变化的预测。目前国内外正在加强这方面的研究工作,并且有了一些初步的结果。概述了近期国内外对气溶胶的气候效应的研究状况及结果,介绍了今后国际上将要在这一领域开展的研究工作。     

COULOMB STRESS CHANGE ON ACTIVE FAULTS IN SICHUAN-YUNNAN REGION AND ITS IMPLICATIONS FOR SEISMIC HAZARD

Coulomb stress change on active faults is critical for seismic hazard analysis and has been widely used at home and abroad. The Sichuan-Yunnan region is one of the most tectonically and seismically active regions in Mainland China, considering some highly-populated cities and the historical earthquake records in this region, stress evolution and seismic hazard on these active faults capture much attention. From the physical principal, the occurrence of earthquakes will not only cause stress drop and strain energy release on the seismogenic faults, but also transfer stress to the surrounding faults, hence alter the shear and normal stress on the surrounding faults that may delay, hasten or even trigger subsequent earthquakes. Previously, most studies focus on the coseismic Coulomb stress change according to the elastic dislocation model. However, the gradually plentiful observation data attest to the importance of postseismic viscoelastic relaxation effect during the analysis of seismic interactions, stress evolution along faults and the cumulative effect on the longer time scale of the surrounding fault zone. In this paper, in order to assess the seismic hazard in Sichuan-Yunnan region, based on the elastic dislocation theory and the stratified viscoelastic model, we employ the PSGRN/PSCMP program to calculate the cumulative Coulomb stress change on the main boundary faults and in inner blocks in this region, by combining the influence of coseismic dislocations of the M≥7.0 historical strong earthquakes since the Yongsheng M7.8 earthquake in 1515 in Sichuan-Yunnan region and M≥8.0 events in the neighboring area, and the postseismic viscoelastic relaxation effect of the lower crust and upper mantle. The results show that the Coulomb stress change increases significantly in the south section of the Xianshuihe Fault, the Anninghe Fault, the northern section of the Xiaojiang Fault, the southern section of the Longmen Shan Fault, the intersection of the Chuxiong-Jianshui Fault and the Xiaojiang Fault, and the Shawan section of the Litang Fault, in which the cumulative Coulomb stress change exceeds 0.1MPa. The assuming different friction coefficient has little effect on the stress change, as for the strike-slip dominated faults, the shear stress change is much larger than the normal stress change, and the shear stress change is the main factor controlling the Coulomb stress change on the fault plane. Meanwhile, we compare the Coulomb stress change in the 10km and 15km depths, and find that for most faults, the results are slightly different. Additionally, based on the existing focal mechanism solutions, we add the focal mechanism solutions of the 5 675 small-medium earthquakes(2.5≤M≤4.9)in Sichuan-Yunnan region from January 2009 to July 2019, and invert the directions of the three principal stresses and the stress shape factor in 0.1°×0.1° grid points; by combining the grid search method, we compare the inverted stress tensors with that from the actual seismic data, and further obtain the optimal stress tensors. Then, we project the stress tensors on the two inverted nodal planes separately, and select the maximum Coulomb stress change to represent the stress change at the node. The results show that the cumulative Coulomb stress change increase in the triple-junction of Sichuan-Yunnan-Tibet region is also significant, and the stress change exceeds 0.1MPa. Comprehensive analysis of the Coulomb stress change, seismic gaps and seismicity parameters suggest that more attention should be paid to the Anninghe Fault, the northern section of the Xiaojiang Fault, the south section of the Xianshuihe Fault, the southern section of the Longmen Shan Fault and the triple-junction of the Sichuan-Yunnan-Tibet region. These results provide a basis for future seismic hazard analysis in the Sichuan-Yunnan region.     

Holocene profile changes along a California coastal stream

Walker Creek in Marin County, California is a coastal stream draining to Tomales Bay, which lies in the San Andreas Rift Zone. Its valley contains an alluvial fill with a basal gravel dated at 5000 years BP. In upstream parts of the watershed, channels are incised arroyo-like in the fill leaving the valley floor standing as a high terrace averaging 5·5 m (18 ft) high. Below this terrace is an inner terrace of historic age that stands 2·4 m (8 ft) above the streambed. The stratigraphy and morphology of this valley are seen in others nearby, and indicate that in the last half of Holocene time in this region a single episode of valley alluviation was followed by two episodes of valley cutting. The second episode of valley cutting is occurring in the present time. During the last 60 years the flow has become seasonal, the stream has incised 1·5 m (5 ft) below the inner terrace in upstream reaches, aggraded 1·2 m (4 ft) in downstream reaches, and extended its estuary. Incision upstream has begun to re-expose the bedrock valley floor and is associated with aggradation downstream that has caused the flood plain to overtop both terraces. This has decreased the stream's gradient. Using a stream that is currently effecting major changes in its valley and channel morphology, two aspects of hydraulic adjustment in fluvial systems are examined. The changes in the average slope of the longitudinal profile are small but measureable. Profile concavity has not changed measurably. The various profiles that have existed in Holocene time show that stream gradient can be, but is not necessarily, slightly adjusted during valley filling and cutting. Flow measurements at a high discharge show that the channel has begun to assume the hydraulic geometry of an ephemeral channel. Adjustments of depth, velocity, and roughness appear to be hydraulic adjustments in response to changing watershed conditions.