Greenhouse-gas versus aerosol forcing and African climate response

There are many indicators that human activity may change climate conditions all around the globe through emissions of greenhouse gases. In addition, aerosol particles are emitted from various natural and anthropogenic sources. One important source of aerosols arises from biomass burning, particularly in low latitudes where shifting cultivation and land degradation lead to enhanced aerosol burden. In this study the counteracting effects of greenhouse gases and aerosols on African climate are compared using climate model experiments with fully interactive aerosols from different sources. The consideration of aerosol emissions induces a remarkable decrease in short-wave solar irradiation near the surface, especially in winter and autumn in tropical West Africa and the Congo Basin where biomass burning is mainly prevailing. This directly leads to a modification of the surface energy budget with reduced sensible heat fluxes. As a consequence, temperature decreases, compensating the strong warming signal due to enhanced trace gas concentrations. While precipitation in tropical Africa is less sensitive to the greenhouse warming, it tends to decrease, if the effect of aerosols from biomass burning is taken into account. This is partly due to the local impact of enhanced aerosol burden and partly to modifications of the large-scale monsoon circulation in the lower troposphere, usually lagging behind the season with maximum aerosol emissions. In the model equilibrium experiments, the greenhouse gas impact on temperature stands out from internal variability at various time scales from daily to decadaland the same holds for precipitation under the additional aerosol forcing. Greenhouse gases and aerosols exhibit an opposite effect on daily temperature extremes, resulting in an compensation of the individual responses under the combined forcing. In terms of precipitation, daily extreme events tend to be reduced under aerosol forcing, particularly over the tropical Atlantic and the Congo basin. These results suggest that the simulation of the multiple aerosol effects from anthropogenic sources represents an important factor in tropical climate change, hence, requiring more attention in climate modelling attempts.     

松辽盆地西缘边界断裂带中北段尼尔基L型构造岩构造年代学及其构造意义

松辽盆地西缘与大兴安岭东缘的边界断裂带,又称嫩江-八里罕断裂带,位于东北地区中部,中亚造山带东段。断裂带呈NNE走向,沿内蒙古自治区、黑龙江省与吉林省的交界,一直向南延伸至河北省境内与平场-桑园大断裂相接,全长1200km。该断裂带传统认识上被认为是一条NNE向的大型正断层或拆离断层。目前对于断裂带性质的研究主要集中在其北段的嫩江断裂,但侧重区域地球物理方面,通过布格重力异常、航磁异常、深反射剖面研究确定了断裂带的走向、展布位置,及伸展拆离断层或大型低角度正断层的性质;其南段断裂带(红山-八里罕断裂)的研究,主要与喀喇沁隆起的形成时间、性质、演化阶段等方面相联系;然而对断裂带中段的嫩江-八里罕断裂的研究少有报道。近期,我们报导了嫩江-白令海断裂带岭下及白城地区发现的具有左旋走滑性质的韧性剪切带,并同断裂带南段的红山-八里罕断裂带楼子店地区进行系统的对比。通过对岭下地区韧性剪切带详细的野外调查、构造要素测量、显微构造研究、同构造变形岩石的有限应变分析、石英EBSD组构分析以及白云母40Ar/39Ar年代学研究,我们认为岭下韧性剪切带与楼子店韧性剪切带的变形特征一致,证实了红山-八里罕断裂带至少可以向北延伸至中段岭下地区,其性质同为左行走滑韧性剪切带,并具有相同的早期走滑时限(~130Ma)。本次,我们进一步对嫩江-白令海断裂带中北段尼尔基地区韧性剪切带进行研究,通过对带内韧性变形岩石构造要素系统测量和统计、显微构造观察、有限应变测量等方面的研究,表明其岩石类型属L型构造岩,具左行剪切特征。综合分析发现除构造岩的类型(岭下与楼子店地区发育S-L型构造岩),其余与岭下与楼子店地区韧性剪切带性质相同。另外,年代学表明L型花岗质岩石中发育的锆石,1)呈自形具有致密的韵律环带,且Th/U比值介于0.06至1.25之间,显示了岩浆成因特点;2)年龄大体分为两个区间258~403Ma(n=5)和207~174Ma(n=15,加权平均年龄为190.0±6.1Ma);3)较老年龄258~403Ma的锆石普遍具有"核-边"结构,证实了该区域存在多期古生代岩浆事件。207~174Ma的锆石中,两粒锆石呈现~190Ma核部年龄,反映了区域内曾经历了早侏罗世的一次强烈的岩浆侵入事件,此后尼尔基地区韧性剪切带形成于中侏罗世(~170Ma),而并非是前人认为的二叠纪。白云母激光40Ar/39Ar年代学显示了一个稳定的年龄坪为158.99±0.61Ma。通过显微构造的观测发现长石残斑与S-C组构指示了左行韧性剪切特征,其动态重结晶主要为膨凸式特点(BLG)。而石英颗粒边界相嵌现象明显,为亚晶粒旋转(SR)向颗粒边界迁移(GBM)重结晶过渡阶段。综合长石和石英的重结晶特征指示,其变质条件为高绿片岩相,变形温度在500℃左右,显然高于白云母的封闭温度(300~400℃),故此我们认为尼尔基左行韧性剪切带的快速隆升时间应为中晚侏罗世(158.99±0.61Ma)。结合最新发表的东北地区黑龙江蓝片岩(前人称之"黑龙江群")与郯庐断裂带北段两个分支(佳依断裂与敦密断裂)的年代学数据,以及西太平洋板块的俯冲速度和方向的相关证据,我们认为1)嫩江-八里罕断裂可能经历了三期变形演化阶段,包括中-晚侏罗世左行伸展走滑阶段(~160Ma),早白垩世中期左行走滑/斜滑剪切阶段(~130Ma)及早白垩世晚期的伸展阶段;2)两期走滑剪切阶段与郯庐断裂带及其北段两个分支具有相似性,其中早期~160Ma的走滑剪切事件相对于佳木斯地块与松嫩地块的拼合时间(~170Ma)晚近10Myr;3)松辽盆地形成早期受到中-晚侏罗世的左行走滑断裂的控制,该左行走滑断裂的成因应归功于侏罗纪时西太平洋板块向欧亚大陆下快速的斜向俯冲有关。     

Modèle de formation du gisement d'argent d'Imiter (Anti-Atlas oriental,Maroc). Nouveaux apports de l'analyse structurale et minéralogique

Based on a combined geometrical and mineralogical analysis, a three-stage model of formation of the mineralized veins of the giant Imiter silver deposit (Anti-Atlas, Morocco) is herein proposed. A first episode is characterized by the development of quartz, pink dolomite and Ag-rich minerals veins formed during a dextral transpressive event. The second episode is associated with a normal left-lateral motion that re-opens previous structures, filled by pink dolomite gangue. Alteration stages contribute to a local Ag enrichment. To cite this article: J. Tuduri et al., C. R. Geoscience 338 (2005).     

柴达木盆地北缘晚中生代—新生代构造应力场——来自构造节理分析的证据

柴达木盆地的构造演化,尤其是中、新生代的构造演化历史及其影响程度一直存在争议。在冷湖0号、冷湖4号、鄂博梁、结绿素等剖面开展了野外构造节理测量,应用地层恢复技术,分析了柴北缘晚中生代—新生代构造应力场的演化过程。结果表明：晚侏罗世之后,柴北缘主要受东西方向的挤压,这与区域上的研究结果一致;早白垩世北西—南东向的挤压应力可能再次影响了侏罗系;晚白垩世受南北挤压应力的影响,地层隆升遭受剥蚀;始新世下干柴沟期至上新世狮子沟期,柴北缘处于弱活动时期,接受稳定沉积;上新统狮子沟组沉积之后,由于阿尔金山的隆升,盆地北缘主体受北西—南东向应力的影响,涉及地层包括上干柴沟组、下油砂山组和上油砂山组;早、中更新世七个泉期之后,受南北向构造应力场的强烈影响,在狮子沟组、上油砂山组和下油砂山组中都有其分量,较老地层的地表露头中也有记录,这期构造运动对柴北缘油气运聚具有破坏和二次成藏的作用。因此,围绕柴北缘其他地质剖面开展更进一步的构造节理测量和分析工作,对油气资源勘探具有指导意义。