Multiphase structural evolution of the western margin of the Girardot subbasin, Upper Magdalena Valley, Colombia

More than 1400 km of two-dimensional seismic data were used to understand the geometries and structural evolution along the western margin of the Girardot Basin in the Upper Magdalena Valley. Horizons are calibrated against 50 wells and surface geological data (450 km of traverses). At the surface, low-angle dipping Miocene strata cover the central and eastern margins. The western margin is dominated by a series of en echelon synclines that expose Cretaceous–Oligocene strata. Most synclines are NNE–NE trending, whereas bounding thrusts are mainly NS oriented. Syncline margins are associated mostly with west-verging fold belts. These thrusts started deformation as early as the Eocene but were moderately to strongly reactivated during the Andean phase. The Girardot Basin fill records at least four stratigraphic sequences limited by unconformities. Several periods of structural deformation and uplifting and subsidence have affected the area. An early Tertiary deformation event is truncated by an Eocene unconformity along the western margin of the Girardot Basin. An Early Oligocene–Early Miocene folding and faulting event underlies the Miocene unconformity along the northern and eastern margin of the Girardot Basin. Finally, the Late Miocene–Pliocene Andean deformation folds and erodes the strata along the margins of the basin against the Central and Eastern Cordilleras.     

Thermal evolution of the orogenic lower crust during exhumation within a thickened Moldanubian root of the Variscan belt of Central Europe

At the eastern margin of the Bohemian Massif (Variscan belt of Central Europe), large bodies of felsic granulite preserve mineral assemblages and structures developed during the early stages of exhumation of the orogenic lower continental crust within the Moldanubian orogenic root. The development of an early steep fabric is associated with east–west-oriented compression and vertical extrusion of the high-grade rocks into higher crustal levels. The high-pressure mineral assemblage Grt-Ky-Kfs-Pl-Qtz-Liq corresponds to metamorphic pressures of ∼18 kbar at ∼850 °C, which are minimum estimates, whereas crystallization of biotite occurred at 13 kbar and ∼790 °C during decompression with slight cooling. The late stages of the granulite exhumation were associated with lateral spreading of associated high-grade rocks over a middle crustal unit at ∼4 kbar and ∼700 °C, as estimated from accompanying cordierite-bearing gneisses. The internal structure of a contemporaneously intruded syenite is coherent with late structures developed in felsic granulites and surrounding gneisses, and the magma only locally explored the early subvertical fabric of the felsic granulite during emplacement. Consequently, the emplacement age of the syenite provides an independent constraint on the timing of the final stages of exhumation and allows calculation of exhumation and cooling rates, which for this part of the Variscan orogenic root are 2.9–3.5 mm yr⁻¹ and 7–9.4 °C Myr⁻¹, respectively. The final part of the temperature evolution shows very rapid cooling, which is interpreted as the result of juxtaposition of hot high-grade rocks with a cold upper-crustal lid.     

Hydrology and hydrogeology of Sakaryabaşı Karstic springs, Çifteler, Turkey

The Sakarya River is one of the largest rivers in Turkey and is fed mainly from Sakaryabaşı springs. The Sakaryabaşı springs are located in the Central Anatolia and issue from confined/semi-confined karst having a thermal component and therefore, having quite different hydrogeological characteristics as compared to the Taurus Karst region, a typical example of the Mediterranean type of karst. The karstic carbonate rocks that form the groundwater reservoir are overlain by a thick semi-pervious overburden of mainly clastics of Neogene age. Tectonics is the major factor controlling the occurrence of the karst springs in the area where topography is rather flat. This study aimed at explaining the occurrence and movement of the karst groundwater within the system by use of hydrogeological, chemical, and isotopic tools. Isotopic composition of the waters revealed that all waters in the region are of meteoric origin and the thermal component is due to deep circulation. The catchment area of the hydrogeological system extends to the south and groundwater movement is towards the outlets, which are in a depression along a major fault. The movement of the groundwater, based on analysis of remotely sensed images, is controlled mainly by structural elements.     

Seismic activity associated with the subducting motion of the Philippine Sea plate beneath the Kanto district, Japan

The Pacific plate and the Philippine Sea plate overlap and subduct underneath the Kanto region, central Japan, causing complex seismic activities in the upper mantle. In this research, we used a map selection tool with a graphic display to create a data set for earthquakes caused by the subducting motion of the Philippine Sea plate that are easily determined. As a result, we determined that there are at least four earthquake groups present in the upper mantle above the Pacific plate. Major seismic activity (Group 1) has been observed throughout the Kanto region and is considered to originate in the uppermost part of mantle in the subducted Philippine Sea plate, judging from the formation of the focal region and comparison with the 3D structure of seismic velocity. The focal mechanism of these earthquakes is characterized by the down-dip compression. A second earthquake layer characterized by down-dip extension (Group 2), below the earthquakes in this group, is also noted. The focal region for those earthquakes is considered to be located at the lower part of the slab mantle, and the Pacific plate located directly below is considered to influence the activity. Earthquakes located at the shallowest part (Group 3) form a few clusters distributed directly above the Group 1 focal region. Judging from the characteristics of later phases in these earthquakes and comparing against the 3D structure of seismic velocity, the focal regions for the earthquakes are considered to be located near the upper surface of the slab. Another earthquake group (Group 4) originates further below Group 2; it is difficult to consider these earthquakes within a single slab. The seismic activities representing the upper area of the Philippine Sea plate are Group 3. This paper proposes a slab geometry model that is substantially different from conventional models by strictly differentiating the groups.     

地缘经济关系测度与分析的理论方法探讨--以云南省为例

文章首先简要分析了地缘经济学的发展进程和研究内容，提出以保护国家利益增强综合国力为主要研究目的的地缘经济理论在一定程度上可以指导不同级别区域经济合作策略的制定。然后在探究了地缘经济关系涵义的前提下，设计了地缘经济关系的测度体系，并用该方法对云南省地缘经济关系进行了数量测度和类型判别，进而对该省地缘经济关系进行了分析和评价，对经济发展提出了建议。     

Precambrian fluvial deposits: Enigmatic palaeohydrological data from the c. 2–1.9 Ga Waterberg Group, South Africa

Precambrian fluvial systems, lacking the influence of rooted vegetation, probably were characterised by flashy surface runoff, low bank stability, broad channels with abundant bedload, and faster rates of channel migration; consequently, a braided fluvial style is generally accepted. Pre-vegetational braided river systems, active under highly variable palaeoclimatic conditions, may have been more widespread than are modern, ephemeral dry-land braided systems. Aeolian deflation of fine fluvial detritus does not appear to have been prevalent. With the onset of large cratons by the Neoarchaean–Palaeoproterozoic, very large, perennial braided river systems became typical. The c. 2.06–1.88 Ga Waterberg Group, preserved within a Main and a smaller Middelburg basin on the Kaapvaal craton, was deposited largely by alluvial/braided-fluvial and subordinate palaeo-desert environments, within fault-bounded, possibly pull-apart type depositories.

Palaeohydrological data obtained from earlier work in the Middelburg basin (Wilgerivier Formation) are compared to such data derived from the correlated Blouberg Formation, situated along the NE margin of the Main basin. Within the preserved Blouberg depository, palaeohydrological parameters estimated from clast size and cross-bed set thickness data, exhibit rational changes in their values, either in a down-palaeocurrent direction, or from inferred basin margin to palaeo-basin centre. In both the Wilgerivier and Blouberg Formations, calculated palaeoslope values (derived from two separate formulae) plot within the gap separating typical alluvial fan gradients from those which characterise rivers (cf. [Blair, T.C., McPherson, J.G., 1994. Alluvial fans and their natural distinction from rivers based on morphology, hydraulic processes, sedimentary processes, and facies assemblages. J. Sediment. Res. A64, 450–489.]). Although it may be argued that such data support possibly unique fluvial styles within the Precambrian, perhaps related to a combination of major global-scale tectono-thermal and atmospheric–palaeoclimatic events, a simpler explanation of these apparently enigmatic palaeoslope values may be pertinent. Of the two possible palaeohydrological formulae for calculating palaeoslope, one provides results close to typical fluvial gradients; the other formula relies on preserved channel-width data. We suggest that the latter will not be reliable due to problematic preservation of original channel-widths within an active braided fluvial system. We thus find no unequivocal support for a unique fluvial style for the Precambrian, beyond that generally accepted for that period and discussed briefly in the first paragraph.     

An approximate projection of availability of the fresh groundwater resources in the South West district of NCT Delhi, India: a case study

The future availability and sustainability of fresh groundwater resources in the South West district of the national capital territory (NCT) Delhi, India, have been projected. Due to a rapid decline in groundwater level and quality, the district has been required by the Government of India to regulate development of groundwater resources. Shallow groundwater is mostly saline and water resources in the area are limited. The methodology applied here involves microzonation of the district in terms of thickness of fresh groundwater and then quantification of present and future availability of freshwater in different freshwater zones, including tentative timescales. The calculation method has been aided by data on historic trends in water level at representative groundwater monitoring stations, located either in fresh groundwater zones or near to them. It is estimated that the presently available 481 million m³ of resources will be reduced to 374 million m³ by year 2007 and to 303 million m³ by the year 2012, and by the year 2022 the district will have only 176 million m³ of available fresh groundwater resources.     

Hydrogeology of thermal waters in Viterbo area, central Italy

A conceptual hydrogeological model of the Viterbo thermal area (central Italy) has been developed. Though numerous studies have been conducted on its geological, geochemical and geothermal features, there is no generalized picture defining the origin and yield of the hydrothermal system. These latter aspects have therefore become the objectives of this research, which is based on new hydrogeological and geochemical investigations. The geological setting results in the coexistence of overlapped interacting aquifers. The shallow volcanic aquifer, characterized by fresh waters, is fed from the area around the Cimini Mountains and is limited at its base by the semiconfining marly-calcareous-arenaceous complex and low-permeability clays. To the west of Viterbo, vertical upflows of thermal waters of the sulphate-chloride-alkaline-earth type with higher gas contents, are due to the locally uplifted carbonate reservoir, the reduced thickness of the semiconfining layer and the high local geothermal gradient. The hot waters (30–60°C) are the result of deep circulation within the carbonate rocks (0.5–1.8 km) and have the same recharge area as the volcanic aquifer. The upward flow in the Viterbo thermal area is at least 0.1 m³/s. This flow feeds springs and deep wells, also recharging the volcanic aquifer from below.     

New data on Sr-and C-isotopic chemostratigraphy of the Upper Riphean type section (Southern Urals)

New data on Sr-and C-isotopic systematics of carbonate rocks from the Upper Riphean stratotype (Karatau Group of the southern Urals) are obtained for several southwestern sections of the Bashkirian meganticlinorium, which have not been studied before. The results obtained supplement the Sr-and C-isotopic information for the group upper horizons thus detailing chemostratigraphic characterization of the entire succession. Limestone and dolostone samples used to analyze the Sr isotope composition satisfy strict geochemical criteria of the isotopic system retentivity and have been subjected to preliminary treatment in ammonium acetate to remove secondary carbonate phases. Data on 255 samples of carbonate rocks (171 studied for the first time) show that δ¹³C value varies in the Karatau Group succession from ?2.8 to +5.9 ‰ V-PDB with several in-phase excursions from the general trend in all the sections studied in the area 90 × 130 km. The δ¹³C variation trend demarcates several levels in the carbonate succession of the Karatau Group suitable for objectives of regional stratigraphy and for C-isotope chemostratigraphic subdivision of the Upper Riphean. The results of Sr isotopic analysis of 121 samples (51 unstudied before) from the Karatau Group imply that rocks in its lower part (the Katav Formation and basal horizon of the Inzer Formation) experienced considerable secondary alterations, while limestones and dolostones of the overlying interval of the group are frequently unaltered. In the “best” samples satisfying geochemical criteria of the isotopic system retentivity, the ⁸⁷Sr/⁸⁶Sr initial ratio increases from 0.70521–0.70532 in the lower Inzer deposits to 0.70611 in the upper Min’yar carbonates, decreasing to <0.70600 near the top of the latter. Above the regional hiatus separating the Min’yar and Uk formation, this ratio grows from 0.70533 to 0.70605–0.70609 in the limestone succession of the last formation.     

金沙江干热河谷区土壤侵蚀危险度评价——以云南元谋为例

利用空间主成分分析方法，选取降水、坡度、植被、土地利用、土壤类型、人口、高程等作为评价指标，建立空间主成分评价模型Pi=0．78X1＋0．06X2＋0．05X3＋0．04X4，将研究区分为无险型、轻险型、危险型、强险型和极险型五种类型，较为准确地得到了危险度综合评价结果。因此，空间主成分分析模型，在土壤侵蚀危险程度评价方面的应用较为成功，是一种科学有效的研究方法。