哀牢山-红河剪切带左行走滑作用起始时间约束

位于哀牢山-红河剪切带NW延伸方向上的点苍山变质杂岩体遭受强烈的左行走滑剪切变形、变质作用改造,岩石中保存了典型的高温矿物组合以及由它们构成的宏观和微观高温变形构造特征,其中糜棱岩中具有极其发育的长石矿物拉伸线理而形成典型的L与LS型构造岩是其一个明显的特征。本文对点苍山地区高温糜棱岩主要矿物开展了显微构造与矿物变形、变形机制及组构分析,并对于遭受高温糜棱岩化改造的一个花岗质岩体开展了SHRIMP锆石U-Pb定年分析。结果表明岩石中长石、角闪石、石英等主要矿物具有典型的达角闪岩相条件下的高温晶质塑性变形和动态生长特征,它们也为走滑剪切变形活动提供了充分的微观构造证据。对于点苍山高温糜棱岩化改造的眼球状或似斑状二长花岗岩的显微构造分析结果表明,这套花岗质岩石从走滑剪切前期岩浆的侵位之后经历了早期强烈的岩浆期后交代作用—亚岩浆流动—高温固态塑性剪切变形的递进演化过程。由此可见,岩浆的上升与就位受左行走滑剪切作用的制约,岩体又遭受了强烈剪切变形改造。同时对这套构造前期就位花岗质岩石中的锆石进行定年分析,获得33.88±0.32Ma的岩浆结晶年龄,为此,我们有充分的理由认为,在点苍山地区哀牢山-红河剪切带左行走滑剪切作用的起始时间至少应该为早渐新世30.88±0.32Ma。     

The impact of salt on the late Messinian to recent tectonostratigraphic evolution of the Cyprus subduction zone

Messinian evaporites of locally more than 3‐km thickness occupy the subduction zone between Cyprus and Eratosthenes Seamount. Based on a dense grid of seismic reflection profiles, we report on compressional salt tectonics and its impact on the Late Miocene to Quaternary structural evolution of the Cyprus subduction zone. Results show that evaporites have experienced significant post‐Messinian shortening along the plate boundary. Shortening has initiated allochthonous salt advance between Cyprus and Eratosthenes Seamount, representing an excellent example of salt which efficiently escapes subduction and accretion. Further east, between Eratosthenes Seamount and the Hecataeus Rise, evaporites were compressionally inflated without having advanced across post‐Messinian strata. Such differences in the magnitude of salt tectonic shortening may reflect a predominately north–south oriented post‐Messinian convergence direction, raising the possibility of a later coupling between the motion of Cyprus and Anatolia than previously thought. Along the area bordered by Cyprus and Eratosthenes Seamount a prominent step in the seafloor represents the northern boundary of a controversially debated semi‐circular depression. Coinciding with the southern edge of the salt sheet, this bathymetric feature is suggested to have formed as a consequence of compressional salt inflation and seamount‐directed salt advance. Topographic lows on top of highly deformed evaporites are locally filled by up to 700 m of late Messinian sediments. The uppermost 200 m of these sediments were drilled in the course of ODP Leg 160 and interpreted to represent Lago Mare‐type deposits (Robertson, Tectonophysics, 1998d, 298 , 63‐82). Lago Mare deposits are spatially restricted to the western part of the subduction zone, pinching out towards the east whereas presumably continuing into the Herodotus Basin further west. We suggest a sea level control on late Messinian Lago Mare sedimentation, facilitating sediment delivery into basinal areas whereas inhibiting Lago Mare deposition into the desiccated Levant Basin. Locally, early salt deformation is believed to have provided additional accommodation space for Lago Mare sedimentation, resulting in the presently observed minibasin‐like geometry.     

The origin of large varioles in flow-banded pillow lava from the Hooggenoeg Complex,Barberton Greenstone Belt,South Africa

Exceptionally well-preserved pillowed and massive phenocryst-free metabasaltic lava flows in the uppermost part of the Palaeoarchaean Hooggenoeg Complex of the Barberton Greenstone Belt exhibit both flow banding and large leucocratic varioles. The flow banding is defined by blebs and bands of pale and dark green metabasalt and was the result of mingling of two types of basalt (Robins et al. in Bull Volcanol 72:579–592, 2010a). Varioles occur exclusively in the dark chlorite-, MgO- and FeO-rich metabasalt. Varioles are absent in the outermost rinds of pillows and increase in both abundance and size towards the centres of pillows. In the central parts of some pillows, they impinge to form homogeneous pale patches, bands or almost homogenous cores. Individual varioles consist essentially of radially orientated or outwardly branching dendritic crystals of albite. Many varioles exhibit concentric zones and finer-grained rims. Some varioles seem to have grown around tiny vesicles and vesicles appear to have been trapped in others between a core and a finer-grained rim. The matrix surrounding the ocelli contains acicular pseudomorphs of actinolite and chlorite after chain-like, skeletal Ca-rich pyroxenes that are partly overgrown by the margins of varioles. Varioles are enriched in the chemical constituents of feldspar but contain concentrations of immobile TiO₂, Cr, Zr and REE that are similar to the host metabasalts. The shape, distribution, texture and composition of the varioles exclude liquid immiscibility and support an origin by spherulitic crystallisation of plagioclase from severely undercooled basalt melt and glass. Nucleation of plagioclase was strongly inhibited and took place on vesicles, on the bases of drainage cavities and along early fractures. Eruption in deep water and retention of relatively high concentrations of volatiles in the melt may be the principal cause of spherulitic crystallisation in the interiors of pillows rather than only in their margins as in younger submarine flows.     

Intra-plate deformation in west-central Europe

The central part of Europe north of the alpine orogenic belt is generally seen as a relatively stable area of the western tip of the Eurasian plate. Indeed, up to now, no geodetically significant motions have been detected although an active rift system running roughly in SSE–NNW direction along the Rhine valley could have some effect on the stability of this region. Presently, the increasing accuracy of geodetic point motions should allow the study of small motions at levels down to nearly 0.1 mm/yr. We start our investigation with a closer look at the ‘true’ accuracy and significance of GPS derived point velocities of permanent stations. We compare and discuss the different levels of formal errors obtained by the three analysis centers considered in this study (EPN, JPL and SOPAC) and present additional ways of assessing the accuracy using the redundancy offered by different independent analyses and multiple systems operating at one site. On the average, all results indicate that a one-sigma level of ±0.3 mm/yr can be seen as a conservative estimate for the horizontal accuracy of point motions in central Europe. On the basis of this assumption we find that at present, the actual velocity field as determined by different analysis groups and centers does not show any significant east–west extensional deformation. We do however see a prominent north–south compressional velocity gradient of about 1 mm/yr/1000 km (1 nanostrain/yr) which could be associated with the Alpine thrust in conjunction with a south-directed horizontal component of the Fennoscandian Glacial Isostatic Adjustment.     

Wintertime Cyclone Activity and Its Relation to Precipitation over China

The authors examined the variability in wintertime cyclone activity and storm tracks and their relation to precipitation over China for the period 1951–2006 using the observational data.Two apparent modes of variability were assumed for the cyclone activity and storm tracks.The first mode describes the oscillation in the strength of the storm tracks in East Asia,which significantly increased since the mid-1980s,whereas the second mode describes a seesaw oscillation in the storm track strength between the Central-Southeast China and northern East Asia.The storm tracks over the Central-Southeast China have increased since the late 1960s.The possible causes for the variation of the cyclone activity and storm tracks are also explored.It is shown that wintertime precipitation,which has increased since the mid-1980s,concentrates in Central-Southeast China.The enhancement may be caused by the first mode of variability of storm tracks,whereas the interannual variability of precipitation may be linked to the second mode of the storm track variability.     

Magmato-sedimentary Carboniferous to Jurassic evolution of the western Tauern window,Eastern Alps (constraints from U-Pb zircon dating and geochemistry)

New geochronological U-Pb (LA-ICP-MS) zircon data and geochemical analyses from the Variscan orthogneisses and metavolcanic rocks in the western Tauern window are presented and used to reconstruct the pre-Alpine evolution of this area. The late- and post-Variscan stage in the Tauern window was characterised by distinct magmatic pulses accompanied by the formation of volcano-sedimentary basins. The magmatic activity started in the Visean (335.4 ± 1.5 Ma) with the intrusion of a K-rich, durbachitic biotite-granite (protolith of the Ahorn gneiss). Following a period of exhumation and erosion, Westfalian–Stefanian volcanics were deposited (Grierkar meta-rhyodacite: 309.8 ± 1.5 Ma; Venntal meta-rhyolite: 304.0 ± 3.0 Ma). A renewed magmatic pulse occurred in the Early Permian, producing large volumes of tonalites and granodiorites (Tux meta-granodiorite: 292.1 ± 1.9 Ma). The youngest magmatism is characterised by pyroclastic and tuffitic deposits (Pfitsch meta-rhyolite: 280.5 ± 2.6 Ma; Schönach valley meta-andesite: 279.0 ± 4.8 Ma). This volcanism was probably related to crustal extensional faulting within an intra-continental graben and horst setting, asthenospheric upwelling and heat flow increase due to the onset of the Permian rifting. The Permo-Triassic peneplanation and subsidence is documented by shallow marine and evaporitic deposits. Probably in the Middle Jurassic times, the area was flooded and in the Late Jurassic the whole area was covered by limestones, representing post-rift sediments on the southern European continental margin.     

Age and origin of groundwater resources in the Ararat Valley,Armenia: a baseline study applying hydrogeochemistry and environmental tracers

Within the Ararat Valley (Armenia), a continuously growing water demand (for irrigation and fish farming) and a simultaneous decline in groundwater recharge (due to climate change) result in increasing stress on the local groundwater resources. This detrimental development is reflected by groundwater-level drops and an associated reduction of the area with artesian conditions in the valley centre. This situation calls for increasing efforts aimed at more sustainable water resources management. The aim of this baseline study was the collection of data that allows for study on the origin and age distribution of the Ararat Valley groundwater based on environmental tracers, namely stable (δ²H, δ¹⁸O) and radioactive (³⁵S, ³H) isotopes, as well as physical-chemical indicators. The results show that the Ararat Valley receives modern recharge, despite its (semi-)arid climate. While subannual groundwater residence times could be disproved (³⁵S), the detected ³H pattern suggests groundwater ages of several decades, with the oldest waters being recharged around 60 years ago. The differing groundwater ages are reflected by varying scatter of stable isotope and hydrochemical signatures. The presence of young groundwater (i.e., younger that the 1970s), some containing nitrate, indicates groundwater vulnerability and underscores the importance of increased efforts to achieve sustainable management of this natural resource. Since stable isotope signatures indicate the recharge areas to be located in the mountains surrounding the valley, these efforts must not be limited to the central part of the valley where most of the abstraction wells are located.