The Quaternary evolution of the Gulf of Corinth, central Greece: coupling between surface processes and flow in the lower continental crust

The Gulf of Corinth in central Greece is an active normal fault zone with particularly clear evidence of isostatic footwall uplift, constrained by Quaternary marine terraces, and hanging-wall subsidence and sedimentation. It is bounded to the south by a Pliocene to Early Pleistocene sedimentary basin, which is now eroding into the Gulf. Previous work has suggested that the relief across this region has increased dramatically since the Early Pleistocene, due to the isostatic response to increased rates of footwall erosion and hanging-wall sedimentation. It is indeed assumed here that incision accompanying the draw-down of global sea-level at 0.9 Ma, during the first major Pleistocene glaciation, initiated the erosion of the basin south of the Gulf of Corinth and so abruptly increased the sedimentation rate in the Gulf. The resulting transient thermal and isostatic response to these changes is modelled, with the subsiding depocentre and eroding sediment source coupled by flow in the lower continental crust. The subsequent enhancement of relief, involving an increase in bathymetry from near zero to 900 m and 500 m of uplift of the eroding land surface in the sediment source, is shown to be a direct consequence of this change. The model is sensitive to the effective viscosity of the lower crust, and can thus resolve this parameter by matching observations. A value of 6×10¹⁹ Pa s is indicated, suggesting a viscosity at the Moho no greater than 10¹⁸ Pa s. Similar transient topographic effects caused by increased rates of sedimentation and erosion are likely to be widespread within the geological record, suggesting that this coupling process involving flow in the weak lower crust may be of major geological and geomorphological importance.     

Mantle structure and rifting processes in the Baikal–Mongolia region: geophysical data and evidence from xenoliths in volcanic rocks

The origin of the Baikal rift zone (BRZ) has been debated between the advocates of passive and active rifting since the 1970s. A re-assessment of the relevant geological and geophysical data from Russian and international literature questions the concept of broad asthenospheric upwelling beneath the rift zone that has been the cornerstone of many “active rifting” models. Results of a large number of early and recent studies favour the role of far-field forces in the opening and development of the BRZ. This study emphasises the data obtained through studies of peridotite and pyroxenite xenoliths brought to the surface by alkali basaltic magmas in southern Siberia and central Mongolia. These xenoliths are direct samples of the upper mantle in the vicinity of the BRZ. Of particular importance are suites of garnet-bearing xenoliths that have been used to construct P–T- composition lithospheric cross-sections in the region for the depth range of 35–80 km.Xenolith studies have shown fundamental differences in the composition and thermal regime between the lithospheric mantle beneath the ancient Siberian platform (sampled by kimberlites) and beneath younger mobile belts south of the platform. The uppermost mantle in southern Siberia and central Mongolia is much hotter at similar levels than the mantle in the Siberian craton and also has significantly higher contents of ‘basaltic’ major elements (Ca, Al, Na) and iron, higher Fe/Si and Fe/Mg. The combination of the moderately high geothermal gradient and the fertile compositions in the off-cratonic mantle appears to be a determining factor controlling differences in sub-Moho seismic velocities relative to the Siberian craton. Chemical and isotopic compositions of the off-cratonic xenoliths indicate small-scale and regional mantle heterogeneities attributed to various partial melting and enrichment events, consistent with long-term evolution in the lithospheric mantle. Age estimates of mantle events based on Os–Sr–Nd isotopic data can be correlated with major regional stages of crustal formation and may indicate long-term crust–mantle coupling. The ratios of ^143/144Nd in many LREE-depleted xenoliths are higher than those in MORB or OIB source regions and are not consistent with a recent origin from asthenospheric mantle.Mantle xenoliths nearest to the rift basins (30–50 km south of southern Lake Baikal) show no unequivocal evidence for strong heating, unusual stress and deformation, solid state flow, magmatic activity or partial melting that could be indicative of an asthenospheric intrusion right below the Moho. Comparisons between xenoliths from older and younger volcanic rocks east of Lake Baikal, together with observations on phase transformations and mineral zoning in individual xenoliths, have indicated recent heating in portions of the lithospheric mantle that may be related to localised magmatic activity or small-scale ascent of deep mantle material. Overall, the petrographic, P–T, chemical and isotopic constraints from mantle xenoliths appear to be consistent with recent geophysical studies, which found no evidence for a large-scale asthenospheric upwarp beneath the rift, and lend support to passive rifting mechanism for the BRZ.     

Pseudotachylytes in the southern border fault of the Cenozoic intracontinental Teletsk basin (Altai, Russia)

The Cenozoic intracontinental Teletsk basin in the Central Asian Altai Mountains is composed of a complexly structured northern and a more simple southern sub-basin. These sub-basins formed in two distinct kinematic stages when first the NNW-striking Teletsk- and then the NE-striking West-Sayan shear zones became reactivated in the Cenozoic under dominant NS-oriented horizontal compression. Although the entire Teletsk basin strikes roughly NS, the southern sub-basin is parallel to the NNW-trending, amphibolite facies Teletsk ductile shear zone, while the northern sub-basin is NS-striking and flanked by differently structured, greenschist facies basement. Basement reactivation closely controlled the southern sub-basin formation, but this is less clear for the northern sub-basin. Contrasts between northern and southern basement and the exclusive occurrence of pseudotachylytes along the margins of the southern basin are explored for their contribution to the formation of the Teletsk basin with two distinct sub-basins.In the ductile shear fabric of the basement flanking the southern sub-basin, concordantly interleaved pseudotachylytes and isolated breccia lenses reflect local brittle deformation along the ductile fabric. The genetic link between breccia lenses and pseudotachylyte occurrences was defined by microstructural investigation. It allows to explore their possible development in a dextral strike–slip zone. These rocks occur in a large fault-bounded segment of the basement. The geometry of the structures in the segment is comparable with a dextral strike–slip sidewall-ripout structure along the Teletsk shear zone. Seismic slip related to pseudotachylytes is attributed to the sudden stress release on the NNW-striking Teletsk shear zone, when the latter became unconstrained by reactivation of the NE-trending West-Sayan fault zone at its northern boundary. The boundary of the sidewall-ripout structure was reactivated as a large listric fault in a later stage. The northern sub-basins roughly develop along an NS strike and are assumed to reflect reactivation of the ductile shear zone underneath the variably structured greenschist facies basement outcropping along the flanks of the sub-basin.     

The presence, characteristics and earthquake implications of the St. Lawrence fault zone within and near Lake Ontario (Canada–USA)

Questions persist concerning the earthquake potential of the populous and industrial Lake Ontario (Canada–USA) area. Pertinent to those questions is whether the major fault zone that extends along the St. Lawrence River valley, herein named the St. Lawrence fault zone, continues upstream along the St. Lawrence River valley at least as far as Lake Ontario or terminates near Cornwall (Ontario, Canada)–Massena (NY, USA). New geological studies uncovered paleotectonic bedrock faults that are parallel to, and lie within, the projection of that northeast-oriented fault zone between Cornwall and northeastern Lake Ontario, suggesting that the fault zone continues into Lake Ontario. The aforementioned bedrock faults range from meters to tens of kilometers in length and display kinematically incompatible displacements, implying that the fault zone was periodically reactivated in the study area. Beneath Lake Ontario the Hamilton–Presqu'ile fault lines up with the St. Lawrence fault zone and projects to the southwest where it coincides with the Dundas Valley (Ontario, Canada). The Dundas Valley extends landward from beneath the western end of the lake and is marked by a vertical stratigraphic displacement across its width. The alignment of the Hamilton–Presqu'ile fault with the St. Lawrence fault zone strongly suggests that the latter crosses the entire length of Lake Ontario and continues along the Dundas Valley.The Rochester Basin, an east–northeast-trending linear trough in the southeastern corner of Lake Ontario, lies along the southern part of the St. Lawrence fault zone. Submarine dives in May 1997 revealed inclined layers of glaciolacustrine clay along two different scarps within the basin. The inclined layers strike parallel to the long dimension of the basin, and dip about 20° to the north–northwest suggesting that they are the result of rigid-body rotation consequent upon post-glacial faulting. Those post-glacial faults are growth faults as demonstrated by the consistently greater thickness, unit-by-unit, of unconsolidated sediments on the downthrown (northwest) side of the faults relative to their counterparts on the upthrown (southeast) side. Underneath the western part of Lake Ontario is a monoclinal warp that displaces the glacial and post-glacial sediments, and the underlying bedrock–sediment interface. Because of the post-glacial growth faults and the monoclinal warp the St. Lawrence fault zone is inferred to be tectonically active beneath Lake Ontario. Furthermore, within the lake it crosses at least five major faults and fault zones and coexists with other neotectonic structures. Those attributes, combined with the large earthquakes associated with the St. Lawrence fault zone well to the northeast of Lake Ontario, suggest that the seismic risk in the area surrounding and including Lake Ontario is likely much greater than previously believed.     

Stability of earthquake moment tensor inversions: effect of the double-couple constraint

We show that spurious large non-double-couple components can be obtained in inversions for the full deviatoric moment tensor for shallow crustal earthquakes due to inaccurate Earth models. The traditional “best double-couple” solution does not in general provide an optimal estimate of a double-couple mechanism, and is only reliable when the non-double-couple component of the full deviatoric solution is small. The inverse problem for the moment tensors of the 1998 Antarctic Plate and 2000 Wharton Basin strike-slip earthquakes is shown in each case to have two well-fitting minima in the misfit function of pure double-couple solutions. Such pairs of solutions are most likely to exist for earthquakes which are close either to vertical strike-slip or to dip-slip on a fault plane dipping at 45°. It is shown theoretically that these pairs of solutions arise from the combination of the pure double-couple constraint and the instability of two elements of the moment tensor. No significant non-double-couple component is found for the shallow thrusting 1996 Biak, Indonesia earthquake.     

Agricultural land use in the coastal area of the Alexander von Humboldt National Park, Cuba and its implication for conservation and sustainability

In the Alexander von Humboldt National Park in eastern Cuba many endemic animals and plants are found in various different natural habitats, which are considered to be the most important ones for in-situ conservation in the entire Insular Caribbean. In some areas of the National Park agriculture is practised. Thus, the objective of this study was to document and analyse the different land use activities and their consequences for local resource management and conservation of biodiversity in two village areas. A particular question was: what has changed since the foundation of the National Park in 1996? As time series data for land use and aerial photographs were not available for this part of Cuba, a qualitative evaluation was carried out. For this, six different land use units were mapped in 2001 and additional information gathered for areas with special interest related to sustainable land use and resource conservation. Although most parts of the study area are influenced to various degrees by human impact, the different types of land use seem presently not to have a crucial or detrimental impact on the land resources of the Alexander von Humboldt National Park. However, exploitation of the natural resources in certain areas could be improved with different management options to reach sustainability as well as to meet the conservation objectives of the National Park. This includes reduced or abandoned agricultural use of steep slopes to reduce erosion risk as well as a facilitated regeneration of natural vegetation in many parts of the study area to be able to conserve the high valuable biodiversity of the Park. Environmental education seems to have played an important and successful role since the foundation of the Park in 1996. Since then, cropping on steep slopes as well as illegal logging and poaching could be reduced.     

Milan: The city of constant renewal

Traditional models of urban development are no longer adequate to describe current metropolitan transformations. These are now at the centre of a debate concerning management and administration. In Italy, delays in resolving problems of urban and metropolitan government, despite the legal framework provided by Law 142/90, have weighed heavily on the larger urban areas of the country: Rome, Naples, Milan, which have not been able to tackle the issue of metropolitan government. Recent legislation, while not providing a pre-defined institutional solution, allows separate administrative districts to collectively establish metropolitan institutions of `variable geometry'. The Milan urban area is not one city, but a system of mutually-dependent cities, linked to each other and the rest of the world by a transport network still requiring much investment. The vitality of its economic structure (especially its small firms) is held back by seriously inadequate infrastructure and low external economic efficiency. The provincial capital may boast `historic centrality' but the most interesting potential for development is to be found on the periphery and in the administrative districts immediately surrounding it, in the recovery of derelict industrial areas and dormitory towns established in the 1950s and 1960s, especially to the north. Recovery of derelict areas, green areas, and better transport links within the urban area and with the outside world are the key elements in the reorganization of `Greater Milan'. In this situation of rapid transformation the most appropriate political strategies involve negotiated planning. This revised version was published online in August 2006 with corrections to the Cover Date.     

华北地台北缘成矿带大青山绿岩型金矿地质特征

本文详细阐述了大青山地区绿岩型金矿的地质特征和找矿标志,指出金矿化带赋存在二道洼群大理岩与其它岩层的接触部位,同时受韧性剪切带控制,在大理岩褶曲部位或下层位有利于形成较富矿体。该类型金矿的发现,大大地拓展了在大青山地区的找矿空间。     

在"数字国土工程"中我区地学数据库建设的现状及发展趋势

近年来,在国土资源部的高度重视下,我区相继建立了一系列地学数据库。这些地学数据库的建立不仅为地矿行业提供了基础性数据,也加速了"数字国土工程"和国土资源信息化建设的步伐。随着信息技术的高速发展,利用地理信息系统(GIS)将各类地学数据有机地结合起来,形成地学数据应用与管理系统,最终通过网络服务于社会,必将是各类地学数据发展的必然趋势。     

川滇黔桂76种元素地球化学图编制中分析方法与分析质量研究(一) 不同实验室产生地球化学图的相似性——以Ag、Cs、Ga、Ge为例

编制中国76种元素地球化学图将使我们首次了解元素周期表上除氢、氧及惰性气体之外所有元素在中国大陆表层的分布,川滇黔桂76种元素地球化学图的编制是这一研究项目的试点。本文以不同实验室取得的Ag、Cs、Ga、Ge分析数据为例,探索、衡量不同实验室地球化学图相似性的方法,以优选实验室进行川滇黔桂4省区约4000件组合样76种元素的大规模分析工作。