河西—祁连山东部地区北东向构造形变特征及其地震活动

近20年来河西-祁连山东部地区三条规模较大的北东向构造带在区域垂直形变图上反映明显。因受区域应力场强度的控制,其垂直形变表现出随时间波浪状演化式。区域垂直形变图像与莫霍面等深线相似,说明这几条北东向构造带的深部成因机制。它们对区内的中强地震起着很强的控制作用,与北西西及北北西向断裂构造共同构成了本区的控震构造网络。     

Chemical and boron isotopic compositions of tourmaline from the Paleoproterozoic Houxianyu borate deposit,NE China: Implications for the origin of borate deposit

The Houxianyu borate deposit in northeastern China is one of the largest boron sources of China, hosted mainly in the Paleoproterozoic meta-volcanic and sedimentary rocks (known as the Liaohe Group) that are characterized by high boron concentrations. The borate ore-body has intimate spatial relationship with the Mg-rich carbonates/silicates of the Group, with fine-grained gneisses (meta-felsic volcanic rocks) as main country rocks. The presence of abundant tourmalinites and tourmaline-rich quartz veins in the borate orebody provides an opportunity to study the origin of boron, the nature of ore-forming fluids, and possible mineralization mechanism. We report the chemical and boron isotopic compositions of tourmalines from the tourmaline-rich rocks in the borate deposit and from the tourmaline-bearing fine-grained gneisses.Tourmalines from the fine-grained gneisses are chemically homogeneous, showing relatively high Fe and Na and low Mg, with δ¹¹B values in a narrow range from +1.22‰ to +2.63‰. Tourmalines from the tourmaline-rich rocks, however, commonly show compositional zoning, with an irregular detrital core and a euhedral overgrowth, and have significantly higher Mg, REE (and more pronounced positive Eu anomalies), V (229–1852 ppm) and Sr (208–1191 ppm) than those from the fine-grained gneisses. They show varied B isotope values ranging from +4.51‰ to +12.43‰, which plot intermediate between those of the terrigenous sediments and arc rocks with low boron isotope values (as represented by the δ¹¹B = +1.22‰ to +2.63‰ of the fine-grained gneisses of this study) and those of marine carbonates and evaporates with high boron isotope values. In addition, the rim of the zoned tourmaline shows notably higher Mg, Ti, V, Sn, and Pb, and REE (particularly LREEs), but lower Fe, Co, Cr, Ni, Zn, Mn, and lower δ¹¹B values than the core. These data suggest that (1) the sources of boron of the borate ore-body are mainly the Paleoproterozoic meta-volcanic and sedimentary rocks, and (2) the ore-forming fluids should be the high temperature metamorphic fluids related to the amphibolite-facies metamorphism of the Paleoproterozoic foldbelt, which leach boron from the boron-rich meta-volcanic and sedimentary rocks of the Liaohe Group, and the boron-rich metamorphic fluids subsequently interacted with the marine Mg-rich carbonates and evaporates, forming borate deposit, the tourmaline overgrowth in the rim and the tourmaline-rich rocks.     

C–O–H–N fluids in quartz segregations from a major ductile shear zone: the Berzosa fault, Spanish Central System

Abstract The Berzosa fault is a major ductile shear zone, the Berzosa Shear Zone (BSZ), which separates the ‘Ollo de Sapo’anticline from the inner higher-grade crystalline axis of the Iberian Hercynian Belt. This shear zone is the site of abundant early kinematic quartz (± Al-silicates) segregations, rich in fluid inclusions. Host rocks are medium-grade staurolite schists and sillimanite gneisses. Fluid inclusions in selected quartz segregations across the Berzosa shear zone have been studied by microthermometric methods as well as, in some instances, by Raman analysis. The recorded fluid inclusion history begins at the end of an intense secondary recrystallization period during late-peak metamorphic conditions and lasts until late in the uplift history of the zone. Three types of inclusions have been found, which in a time sequence are: CO₂± H₂O; H₂O+salt (B-type); and, N₂+CH₄. Three types of B inclusion may be distinguished in turn, depending on whether they were trapped during an earlier dynamic-recovery phase (B₁-type), formed later as intergranular trails (B₂-type), or were trapped apparently along with N₂+CH₄ in clusions from a heterogeneous fluid (B₃-type). Considerations from isochores confirm that CO₂± H₂O inclusions were trapped during late-peak and high-T retrograde metamorphic conditions (in the range 650–500°C and 5–2 kbar), whilst N₂+CH₄ inclusions, along with the B₃-type of inclusions, formed at low-pressures (<1 kbar) and temperatures (± 300°C). B₂-type inclusions were trapped chronologically between these two in a period in which strong inverse lateral thermal gradients developed in the zone. Inferred P-T paths for the area are convex to the T-axis.     

Methodology for groundwater recharge assessment in carbonate aquifers: application to pilot sites in southern Spain

Aquifer recharge can be determined by conventional methods such as hydrodynamic or hydrologic balance calculations, or numerical, hydrochemical or isotopic models. Such methods are usually developed with respect to detrital aquifers and are then used on carbonate aquifers without taking into consideration their hydrogeological particularities. Moreover, such methods are not always easy to apply, sometimes requiring data that are not available. Neither do they enable determination of the spatial distribution of the recharge. For eight regions in southern Spain, the APLIS method has been used to estimate the mean annual recharge in carbonate aquifers, expressed as a percentage of precipitation, based on the variables altitude, slope, lithology, infiltration landform, and soil type. The aquifers are representative of a broad range of climatic and geologic conditions. Maps of the above variables have been drawn for each aquifer, using a geographic information system; thus they can be superimposed to obtain the mean value and spatial distribution of the recharge. The recharge values for the eight aquifers are similar to those previously calculated by conventional methods and confirmed by discharge values, which corroborates the validity of the method.     

Geological and environmental implications of the evaporite karst in Spain

In Spain, evaporite outcrops cover approximately 7% of the total area of the country. Most of the evaporitic formations are made up of Ca-sulfates (gypsum/anhydrite) or Ca-sulfates and halite. Certain Paleogene marine evaporites also contain K-Mg-chlorides, and some Tertiary continental formations bear substantial amounts of Na-sulfates in the subsurface (glauberite and thenardite). Mesozoic evaporitic formations commonly wedge out towards the ground surface, passing into condensed sequences and dissolution-collapse breccias. Some of these highly porous breccias constitute major regional aquifers. In several areas, interstratal karstification of the evaporites has given rise to gravitational deformations such as basin structures, monoclines, and collapse structures covering several square kilometers that record a cumulative subsidence in excess of 200 m (Teruel and Calatayud Grabens). A widespread consequence of evaporite dissolution processes in Spain is the hydrochemical degradation of surface waters. Some of the largest and most outstanding lake systems, from an environmental perspective, occur in karstic depressions developed in evaporitic formations (Fuente de Piedra, Gallocanta, Bujaraloz, and Bañolas lakes). Sinkhole activity is a major geohazard in several evaporite karst areas. The sinkhole risk has a particularly high impact in sectors where Tertiary evaporites are overlain by Quaternary alluvial aquifers (Calatayud, Zaragoza, and Madrid areas). Some of the detrimental effects of subsidence include severe damage to historical monuments (Calatayud), the demolition of a whole village (Puilatos), or the derailment of a freight train (Zaragoza area). The deepest gypsum caves are found in Triassic diapiric structures (El Sumidor Cave, 210 m deep), and the longest ones are developed in horizontally lying Neogene sequences (Sorbas caves, and Estremera maze cave). The Cardona diapir hosts salt caves up to 4,300 m long whose genesis is related to flooding of mine galleries caused by the interception of a phreatic conduit. The main anthropogenic impacts on the endokarstic systems are related to the disposal of wastewaters and the destruction of caves by quarrying. The fluvial valleys that cross Tertiary evaporitic outcrops commonly show peculiar geological characteristics related to dissolution-induced synsedimentary subsidence phenomena: (1) Thickened alluvium filling dissolution basins up to several tens of kilometers long and more than 100 m deep. The largest thickenings are found in areas where the bedrock contains halite and glauberite. (2) Superimposed alluvial units locally bounded by angular unconformities. (3) Abundant deformational structures and paleosinkholes related to the rockhead and/or interstratal karstification of the substratum. These fluvial valleys typically are flanked by a prominent gypsum escarpment. Rock-falls favored by the dissolutional enlargement of joints derived from these scarps are the type of mass movement which has caused the highest number of casualties in Spain.     

Application of the European water framework directive in a Western Mediterranean basin (Málaga,Spain)

The water framework directive (WFD) is applied within the Guadalhorce river basin, a Western Mediterranean basin in the Málaga province (South Spain). Criteria defining different surface and groundwater bodies are described. The basic hydrographic network is constituted of low-mountain and low-altitude Mediterranean mineralized rivers. Heavily modified surface water bodies correspond (1) to areas where dams regulate the main watercourses, (2) to areas downstream of reservoirs, where river flow is reduced, and (3) to the coastal sector of the river where artificial channelling has caused morphological variations. Groundwater bodies are related to carbonate and porous aquifers and, locally, to aquifers influenced by dissolution of evaporites. The main impacts to water bodies are irrigated lands and livestock farming. There are also point sources of pollution, such as wastewater, landfills, golf courses, industrial zones, quarries and petrol stations. In addition, groundwater is frequently pumped for human supply and irrigation. Qualitative status of groundwater bodies was done by chemical analysis of samples from a monitoring network and the quantitative status by examining variations in piezometric levels. Both revealed the existence of water bodies at risk of not meeting the environmental objectives of the WFD. The main indicators of pollution are nitrates related to agricultural activities, and total organic carbon (TOC), PO₄³⁻ and NH₄⁺ in relation to wastewater.     

Dolomite in caves: Recent dolomite formation in oxic, non-sulfate environments. Castañar Cave, Spain

Dolomite is a common mineral in the rock record but rare in recent superficial environments. Where it does occur, it is related to anoxic, sulfate-rich environments and microbial activity. The occurrence of some dolomite deposits in caves, however, indicates that dolomite formation is also possible in oxic, non-sulfate settings. Dolomite is forming at 17 °C and in oxic-vadose conditions on the host rocks and aragonite speleothems of the Castañar Cave, Cáceres, Spain. It appears as spheroids and dumbbells 50–300 μm in diameter that internally consist of micron-sized rhombic to rounded crystals. Initially this dolomite is Ca-rich, non-stoichiometric and poorly ordered. Mg-rich solutions allow the precipitation of metastable Mg-rich carbonates, such as huntite. This soon transforms into this Ca-rich dolomite, which later “ages” to form a more stoichiometric dolomite. These dolomites show similarities to those grown under anoxic, sulfate-rich conditions and their presence in caves provides a different setting that may contribute to the understanding of the “dolomite problem”, including their initial formation and later recrystallization processes.     

Sources of Mw 5+ earthquakes in northeastern Italy and western Slovenia: An updated view based on geological and seismological evidence

We present an overview of the seismogenic sources of northeastern Italy and western Slovenia, included in the last version of the Database of Individual Seismogenic Sources (DISS 3.0.2) and a new definition of the geometry of the Montello Source that will be included in the next release of the database. The seismogenic sources included in DISS are active faults capable of generating M_w > 5.5 earthquakes. We describe the method and the data used for their identification and characterization, discuss some implications for the seismic hazard and underline controversial points and open issues.In the Veneto–Friuli area (NE Italy), destructive earthquakes up to M_w 6.6 are generated by thrust faulting along N-dipping structures of the Eastern Southalpine Chain. Thrusting along the mountain front responds to about 2 mm/a of regional convergence, and it is associated with growing anticlines, tilted and uplifted Quaternary palaeolandsurfaces and forced drainage anomalies. In western Slovenia, dextral strike–slip faulting along the NW–SE trending structures of the Idrija Fault System dominates the seismic release. Activity and style of faulting are defined by recent earthquakes (e.g. the M_s 5.7, 1998 Bovec–Krn Mt. and the M_w 5.2, 2004 Kobarid earthquakes), while the related recent morphotectonic imprint is still a debated matter.We reinterpreted a large set of tectonic data and developed a segmentation model for the outermost Eastern Southalpine Chain thrust front. We also proposed the association of the four major shocks of the 1976 Friuli earthquake sequence with individual segments of three major thrust fronts. Although several sub-parallel active strike–slip strands exist in western Slovenia, we were able to positively identify only two segments of the Idrija Fault System. A comparison of the regional GPS velocity with long-term geological slip-rates of the seismogenic sources included in DISS shows that from a quarter to half of the deformation is absorbed along the external alignment of thrust faults in Veneto and western Friuli. The partitioning of the deformation in western Slovenia among the different strike–slip strands could not be quantified.     

Induced seismicity in the Castanhão reservoir, NE Brazil — Preliminary results

The Castanhão reservoir was built in the state of Ceará, a dry region in Northeastern Brazil, to regulate the flow of the Jaguaribe River, for irrigation, and for power generation. It is an earth-filled dam, 60 m high, with a water capacity of 4.5 × 10⁹ m³. The seismicity in the area has been monitored since 1998, with a few interruptions, using one analog or one digital station and, during a few periods, a three-station network. The first earthquakes likely to be induced events were detected in 2003, when the water level was about 20 m high. In early 2004 a very heavy rainfall season quickly filled the reservoir. Shortly after, an increase in the seismic activity occurred and many micro-earthquakes were recorded. We suggest that this activity resulted from an increase in pore pressure due to undrained response. Therefore, we may classify this cluster of micro-earthquakes as “initial seismicity.” We deployed a network with four analog stations in the area, following this activity, to determine the epicentral zone. At least three epicentral areas under the reservoir were detected. The spatio-temporal analysis of the available data revealed that the seismicity occurs in clusters and that these were activated at different periods. We identified four sets of faults (N–S-, E–W-, NW–SE-, and NE–SW-oriented), some of which moved in shallow crustal levels and as recently as the Quaternary (1.8 Ma). Under the present-day stress regime, the last two sets moved as strike-slip structures. We suggest a possible correlation between dormant faults and the observed induced seismicity.