大理州2005年初夏干旱成因探析

大理州2005年初夏出现了严重干旱气象事件.从500pHa高空环流特征、500pHa距平场特征、西太平洋副高特征量及北太平洋中低纬海温距平场、OLR距平场对这次干旱的成因进行了分析,分析结果表明：2005年初夏（4月1日～6月10日）降水异常偏少气温明显偏高是发生干旱的主要原因,高空大气环流异常、西太平洋副高持续偏强偏西、冷空气活动偏北是导致干旱的直接原因;孟加拉湾南部4月OLR场呈明显正距平,抑制低值系统的发展和东北移影响云南,是影响2005年大理初夏干旱重要原因;北太平洋中低纬海温特征与大理州初夏干旱有一定的遥相关性.     

荒漠植被红砂（Reaumurta soongorica）水热响应的年轮学研究

 红砂是黄土高原荒漠植被带主要建群植物种之一，在植被恢复重建和生态建设中具有重要意义。本文以皋兰县40 a天然植被封育区为例，运用树木年轮学方法，对不同坡向红砂与当地水热条件变化的时空响应及分布格局进行了研究。研究结果表明，红砂具有明显的年轮特征，可以进行树木年轮方面的研究。虽然不同坡向水分条件和植被状况有很大差异，但红砂径向生长对区域水热变化具有非常一致的响应模式：同生长季降水呈正相关关系，尤以7月降水量最为显著；同生长季气温呈负相关关系，以6月最为显著。根据不同坡向红砂年龄分布格局分析结果，并考虑到水土流失的防治效果，黄土高原西部荒漠植被带的封育期限不宜少于10 a。     

库姆塔格沙漠"羽毛状沙丘"形态的示量特征

通过卫星照片判读和实地观测,库姆塔格沙漠的“羽毛状沙丘”由两种风沙地貌构成：东北-西南走向的新月形“沙垄”为“羽轴”；垄间分布的波状微起伏的“大沙波”为“羽枝”,“大沙波”与沙垄的夹角为75°～103°。二者组合成类似“羽毛”的风沙地貌。新月形“沙垄”由单个新月形沙丘的前一沙丘的迎风坡与后一沙丘东翼相连构成“沙垄”。组成“沙垄”的新月形沙丘的两翼平均长37．5m,翼间距30～66m,沙丘高3～19m；在所观测区域内,沙丘沙的分选性由南向北变差。新月形“沙垄”长为3～22km,垄间距为1～3km,其问分布浅色和暗色相间的波状微地貌,暗色部分平均宽24．3m,浅色部分的平均宽11．6m。暗色部分表层沙粒的粒径有60%在1．00～0．25mm之间,以暗色矿物为主,而浅色部分表层沙粒的粒径的90%在1．00～0．25mm之间,分选性相对暗色部分较好,以石英等浅色矿物为主。暗色和浅色微地貌成对出现,相对高差约7cm。这种波状微地貌在库姆塔格沙漠中北部重复出现,类似于风沙地貌分类的沙波,暂称其为“大沙波”。     

近45 a来中国西北年极端高、低温的变化及对区域性增暖的响应

利用中国西北五省(区)1960-2004年129个台站逐日最低、最高温度资料,从中统计出年极端高温发生频次、年极端低温发生频次、年极端高温强度、年极端低温强度以及年极端高温和低温开始和结束日期,分析了它们近45 a来的变化情况。结果表明:近45 a来中国西北年极端高温发生频次的增加趋势是明显的,而年极端低温发生频次的减少趋势更显著;中国西北近45 a来年极端高温的强度在不断增强,而极端低温的强度在不断减弱;近45 a来中国西北年极端高温开始日期逐渐提前,结束日期逐渐推迟,而年极端低温的开始日期在逐渐推迟,结束日期在逐渐提前;年极端低温发生频次的减少对于西北区域增暖的响应比年极端高温发生频次增加更显著,而年极端低温强度的减弱要比年极端高温强度的增强对西北区域性增暖的响应偏弱,年极端高温发生频次的增加同年极端高温强度的增强对西北区域性增暖的响应程度基本相当。     

The impact of cover rock rheology on the style of folding in the Zagros fold-thrust belt

The present day morphology of the Zagros fold-thrust belt is dominated by magnificent exposures of NW–SE trending folds. These folds differ in their size and geometry and these differences are related mainly to the rheological profile of the cover rock. The cover rock succession of the Zagros consists of a sequence of competent and incompetent units which vary both along and across the belt. Field based study combined with the use of satellite images reveals that the thickness and facies distribution of the cover rock succession has a significant impact on the style of deformation. During the shortening linked to the current convergence of the Arabian and Iranian plates, the incompetent units act as detachment horizons which localise thrusting and which act as décollement above which detachment folds form. In addition, where these incompetent units are thick (e.g.> 1 km), they allow the deformation above and below them to become completely decoupled enabling disharmonic folding to occur. As a result the folds above and below the incompetent units in the central part of the Zagros Folded Belt, have significantly different geometries and wavelengths. As the Zagros folds host the majority of the hydrocarbon reserves in Iran and Iraq, an understanding of the processes that influence their geometry and spatial organization at different levels in the cover rock is crucial for the future exploration in the region.     

Variability in Early Amazonian Tharsis stress state based on wrinkle ridges and strike-slip faulting

We present evidence for a decrease in the magnitude of Tharsis-circumferential compressive stress during the Late Hesperian to the Middle Amazonian based on chronologic changes in the predominant style of faulting in southern Amazonis Planitia. Using high-resolution MOLA topography, we identify a population of strike-slip faults that exhibit Middle Amazonian-aged displacements of regional chrono-stratigraphic units. These strike-slip faults are adjacent to an older population of previously documented Late Hesperian-aged thrust faults (wrinkle ridges). Along-strike orientations of these thrust and strike-slip faults reveal the Tharsis-radial stress to be the area's most compressive remote principal stress and that this stress orientation and magnitude persisted throughout the Late Hesperian to the Middle Amazonian. We show that the change in the predominant style of faulting from thrust faulting to strike-slip faulting during this time requires a decrease of the Tharsis-circumferential compressive stress to a magnitude less than lithostatic load, with negligible change in stress orientation.     

The bimodal rift-related Juscelândia volcanosedimentary sequence in central Brazil: Mesoproterozoic extension and Neoproterozoic metamorphism

The Barro Alto Complex and Juscelândia volcanosedimentary sequence are exposed in the central part of the Neoproterozoic Brasília belt of central Brazil. The former is a large (approximately 150 km long), boomerang-shaped, mafic-ultramafic, layered complex formed by two different intrusions metamorphosed under granulite facies. These rocks are tectonically overlain by rocks of the Juscelândia volcanosedimentary sequence, represented mainly by biotite-gneiss and amphibolite, or amphibolite facies metamorphic equivalents of rhyolite and basalt, respectively. New SIMS U–Pb zircon data and Sm–Nd isochron data presented herein help clarify the igneous and metamorphic evolution of the Juscelândia volcanosedimentary sequence, as well as its relationship with the Barro Alto Complex. Zircon grains from two biotite gneisses were analyzed by SIMS (SHRIMP) and indicate Mesoproterozoic dates, approximately 1.28 Ga, interpreted as the time of bimodal volcanism in a tectonic setting transitional between a continental rift and an ocean basin. Metamorphism is constrained by Sm–Nd garnet-whole-rock isochrons for garnet amphibolite and pelitic schists of the Juscelândia sequence, as well as for clinopyroxene-garnet amphibolite and garnet granulite of the Barro Alto Complex, which give ages between 0.74 and 0.76 Ga, in agreement with SIMS dates for metamorphic zircon rims. These new data are significant, because they establish that a single metamorphic event affected both the Barro Alto Complex and the Juscelândia sequence. Based on these new data, we present a modified tectonic model for the Brasília belt.     

Early Jurassic rift structures associated with the Soapaga and Boyacá faults of the Eastern Cordillera, Colombia: Sedimentological inferences and regional implications

The NW-trending Bucaramanga fault links, at its southern termination, with the Soapaga and Boyacá faults, which by their NW trend define an ample horsetail structure. As a result of their Neogene reactivation as reverse faults, they bound fault-related anticlines that expose the sedimentary fill of two Early Jurassic rift basins. These sediments exhibit the wedge-like geometry of rift fills related to west-facing normal faults. Their structural setting was controlled further by segmentation of the bounding faults at approximately 10 km intervals, in which each segment is separated by a transverse basement high. Isopach contours and different facies associations suggest these transverse anticlines may have separated depocenters of their adjacent subbasins, which were shaped by a slightly different subsidence history and thereby decoupled. The basin fill of the relatively narrow basin associated with the Soapaga fault is dominated by fanglomeratic successions organized in two coarsening-upward cycles. In the larger basin linked to the Boyacá fault, the sedimentary fill consists of two coarsening-upward sequences that, when fully developed, vary from floodplain to alluvial fan deposits. These Early Jurassic rift fills temporally constrain the evolution of the Bucaramanga fault, which accommodated right-lateral displacement during the early Mesozoic rift event.     

Geologic correlation between the Neoproterozoic Sergipano belt (NE Brazil) and the Yaoundé belt (Cameroon, Africa)

The Yaoundé belt (Cameroon) and the Sergipano belt (NE Brazil) belonged to a major and continuous Neoproterozoic orogen at the northern margin of the ancient Congo-São Francisco craton. The Yaoundé belt comprises schists, quartzites, gneisses and migmatitic gneisses grouped into three domains; the low-grade Mbalmayo Group in south and the medium- to high-grade Yaoundé and Bafia Group in north. The Sergipano belt is divided into six domains, the three southernmost of which are mostly made up of clastic and chemical metasedimentary rocks whereas the others are more diverse with a migmatite–gneiss complex, and two metavolcanicplutonic complexes. In general, the two belts show structural vergence and decrease of metamorphic grade towards the craton; three main deformation phases are recognized in the Sergipano belt in contrast with two described in the Yaoundé belt. The minimum age of Pan-African-Brasiliano collision in the Sergipano belt is constrained at 628 ± 12 Ma on syn-collision granites, whereas in the Yaoundé belt collision took place between 620 and 610 Ma, i.e. the age of granulite facies metamorphism. Sm–Nd isotope geochemistry and U–Pb age dating indicate that most clastic metasedimentary rocks in both belts were derived from sources to the north and, to a lesser degree, from the cratons to the south.     

A new tectonic scenario for the Sanandaj–Sirjan Zone (Iran)

Recent geochemical studies of volcanic rocks forming part of the ophiolites within the Zagros and Naien-Baft orogen indicate that most of them were developed as supra-subduction ophiolites in intra-oceanic island arc environments. Intra-oceanic island arcs and ophiolites now forming the Naien-Baft zone were emplaced southwestward onto the northeastern margin of the South Sanandaj–Sirjan Zone, while those now in the High Zagros were emplaced southwestward onto the northern margin of Arabia. Thereafter, subduction continued on opposite sides of the remnant oceans. The floor of Neo-Tethys Ocean was subducted at a low angle beneath the entire Sanandaj–Sirjan Zone, and the floor of the Naien-Baft Ocean was subducted beneath the Central Iranian Micro-continent. The Naien-Baft Ocean extended into North-West Iran only temporarily. This failed ocean arm (between the Urumieh-Dokhtar Magmatic Assemblage and the main Zagros Thrust) was filled by thick Upper Triassic–Upper Jurassic sediments. The Naien-Baft Ocean finally closed in the Paleocene and Neo-Tethys closed in the Early to Middle Eocene. After Arabia was sutured to Iran, the Urumieh-Dokhtar Magmatic Assemblage recorded slab break-off in the Middle Eocene.