Microstructural evidences for thermal runaway and deep palaeoseismic activity (Cabo Ortegal,NW Spain)

Abstract The occurrence of high‐pressure granulite facies tectonites with features at the outcrop and thin section scale, characteristic of both pseudotachylytes and ultramylonites, is reported in the Cabo Ortegal complex (NW Spain). We interpret that these rocks formed due to a seismic event at great depth (1.4 GPa) under high‐temperature conditions (740 °C). Shear heating and thermal runaway mechanism would have been responsible for the failure of the still hot and mechanically rigid granulites and provoked melt formation and the accumulation at dilatational jogs. Ambient high P and T conditions would have led to subsequent recrystallization of the previously formed melt accounting for the present weak planar fabric.     

Arshan palaeoseismic feature of the Tunka fault (Baikal rift zone,Russia)

The traditional concept of the rift development of flank depressions in the Baikal rift zone is now doubted in view of some indicators for compression deformations identified by the seismogeological and geodetic methods. Besides, the paleoseismological investigations revealed seismogenic strike-slips and reverse faults in the Tunka fault zone that is a major structure-controlling element of the Tunka rift depression. However, a detailed study of the upslope-facing scarp in the Arshan paleoseismogenic structure zone has shown that its formation might be due to rift mechanism of basin formation. Age estimation has been made for the previously unknown pre-historic earthquake whose epicentral area coincides with the western flank of the Arshan paleoseismogenic structure. Judging from previously determined ages of paleoearthquakes, the mean recurrence period for faulting events on the central Tunka fault is 2780–3440 years.     

Glaucony from the Cretaceous of the Sierra de Guadarrama (Central Spain) and its application in a sequence-stratigraphic context

Detailed sedimentological and geochemical analyses of Upper Cretaceous glaucony-bearing deposits from the middle portion of the Castro de Fuentidueña Formation, in Central Spain, enable identification of a multiphase history of glaucony accumulation. Despite its relatively high maturity (K₂O > 7%), glaucony from the transgressive subtidal sandstones has anomalously low concentration in the host rock (generally <10%), suggesting remobilization from a different source. The remarkable thickness of the glaucony-bearing horizons, concurrently with concentration of glaucony in laminae, small size, and high degree of roundness and sorting provide further evidence for an allochthonous origin of the green grains. In contrast, authigenic glaucony from the overlying offshore clays exhibits higher abundance (up to 45%) and lower maturity (K₂O < 7%) and is interpreted to have formed in situ. Allochthonous glaucony originally developed in more distal areas during a prolonged period of sediment starvation. The green grains were then swept away from their place of origin and concentrated into tidal bars and channels within the upper transgressive systems tract of the third-order depositional sequence. Maximum concentration of autochthonous glaucony is recorded at the transition from tidal-influenced to offshore deposits: this glaucony, which is relatively less evolved due to lower time available for maturation, is interpreted to reflect the turnaround from transgressive to highstand conditions, marking the condensed section of the depositional sequence. As postulated by previous sequence-stratigraphic models, this study documents that allochthonous glaucony can be widespread throughout the transgressive systems tract (TST), while the condensed section (CS) typically hosts autochthonous glaucony. Contrary to the existing literature, however, this study shows that glaucony from the TST may be even more mature than glaucony from the CS, if enough time for maturation during transgression is allowed before the ultimate concentration of the green grains. Identification of spatial and temporal characteristics of glaucony, thus, is critical to a reliable sequence-stratigraphic interpretation of the glaucony-bearing deposits.     

龙门山汶川-茂县断裂带的岩石磁学特征及其地震作用的指示意义

汶川-茂县断裂带是龙门山后山断裂,是松潘-甘孜褶皱带与龙门山断裂带之间的边界断层,然而,在2008年M_W 7.9级汶川大地震中并没有发生破裂。同时,汶川-茂县断裂带在汶川地震之前是否发生过历史大地震仍缺少确切的证据。这不仅制约着汶川地震发生机制的认识,而且还影响对龙门山形成演化过程的理解。因此,确定汶川-茂县断裂带的断裂作用环境对于认识龙门山断裂带的地震发生机制至关重要。断裂岩的岩石磁学可以有效地揭示断裂带的物理和化学环境。本文以汶川-茂县断裂带北部地表露头的断裂岩为研究对象,通过岩石磁学研究,并结合显微结构观察和地球化学分析,探讨汶川-茂县断裂带的断裂作用环境。断层泥和断层角砾岩最大磁化率值分别约为围岩的30倍和15倍,具有高磁化率值特征。断层泥的主要载磁矿物为磁铁矿、磁黄铁矿、针铁矿;断层角砾岩的主要载磁矿物为磁黄铁矿、针铁矿。断层泥的高磁化率值异常的主要原因是围岩中的含铁矿物在地震摩擦热和流体作用下新生成磁铁矿和磁黄铁矿。断层角砾岩的高磁化率值异常是围岩在含有大量硫元素的低温热液流体作用下生成了磁黄铁矿。大量针铁矿指示了震后期还原性的低温热液流体作用。断裂岩的高磁化率值异常和新生磁铁矿指示了汶川-茂县断裂带曾经发生了摩擦热温度>500℃的大地震活动,发震和震后期均为还原性的硫元素含量较高的低温热液流体环境。

     

Geochemistry of thermal springs,Alhama de Granada (southern Spain)

The waters of the thermal springs at Alhama de Granada vary in temperature between 27 and 45°C. Temporal changes in the composition of the principal spring (Baños Viejos) indicate that a small degree of mixing may occur between deep thermal waters and shallow groundwater. Slight compositional variations also occur between the various thermal springs in the study area. These spatial variations are due to the different local hydrodynamic conditions in the springs. Towards the north in less hydraulically transmissive rocks, cooling of the rising water is more noticeable, as are ion exchange and processes of SO₄ reduction. The chemical composition of the water is related to the dissolution of evaporites (SO₄ and Cl salts), carbonates and silicates, and to the possible existence of sources of S within the rock. Estimates of the mean residence times have been obtained based on ¹⁴C_DIC and T. The state of thermodynamic equilibrium at the spring discharge was calculated using the SOLMINEQ.88 program. The results indicate that all the samples are supersaturated with respect to quartz, chalcedony, cristobalite, calcite, aragonite and dolomite, and undersaturated with respect to gypsum, anhydrite and halite. The use of different geothermometers and modelling of saturation indices for quartz, albite and anhydrite indicate temperatures of about 110°C.     

Plagioclase porphyroblast growth and its relation to deformation in the Alhamilla unit (Sierra Alhamilla, Betic Cordilleras, SE Spain)

Abstract. Plagioclase porphyroblasts from silvergrey schists belonging to the Nevado Filabride Complex in the Sierra Alhamilla (Betic Zone, SE Spain) are interpreted as having been formed preand synkinematically with respect to the second phase of deformation. Different types of inclusion patterns represent 'snap-shots'(high growth-rate/strain-rate ratio features) of the formation of a diffentiated crenulation cleavage during this second phase of deformation, by the processes of kinking, crenulation and associated differentiation.
Regional considerations indicate an Alpine age for this tectono-metamorphic event, which can be explained by the'hot emplacement'of the higher Nevado Filabride units. The observed structural evolution is not consistent with a pre-Alpine polyphase deformation history.     

Geochemistry and petrogenesis of two-Pyroxene andesites from Sierra de Gata (SE Spain)

Summary The Miocene volcanic suite of Sierra de Gata (Betic Cordillera) is composed of dacites, rhyodacites and rhyolites, besides basaltic andesites and andesites. The latter are very fresh and are the least fractionated rock types; thus they have been investigated in order to put some constraints on the genetic interpretation of the volcanic sequence. These rocks are composed of plagioclase, orthopyroxene, clinopyroxene and magnetite both as phenocrysts and in the groundmass. Olivine and ilmenite occur occasionally. The andesites exhibit orogenic chemical features. They are characterized by very low Ni, Co and Cr contents and show negative Eu anomalies, which support fractionation of mafic phases and plagioclase at depth. The petrogenesis of the andesites, however, cannot be related simply to fractionation processes of a single mafic magma since the lack of good interelemental correlations, the scattering of Sr-isotope ratios and the isotope disequilibrium between the phases conflict with such a simple mechanism. A model which can better explain the petrogenesis of the analyzed andesites considers the intervention of different mafic melts, generated in a heterogeneous mantle and characterized by different chemical and isotopic compositions, which fractionated at depth and concomitantly interacted significantly with the continental crust.

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Geochemie und Petrogenese von Pyroxen-Andesiten der Sierra de Gata (SE Spanien)

Zusammenfassung Die miozäne Vulkanitabfolge der Sierra de Gata (Betische Kordillere) wird von Daciten, Rhyodaciten, Rhyoliten sowie basaltischen Andesiten and Andesiten aufgebaut, letztere sind in einem guten Erhaltungszustand and zeigen die geringste Fraktionierung. Sie wurden daher genauer untersucht, um Aussagen über die Genese dieser Vulkanite machen zu können.Phenokrysten und Grundmasse bestehen aus Plagioklas, Orthopyroxen, Klinopyroxen und Magnetit. Manchmal kommen Olivin und Ilmenit hinzu. Die Geochemie der Andesite folgt typisch orogenen Trends. Sie sind durch sehr niedrige Ni-, Co- und Cr-Gehalte und negative Eu-Anomalien charakterisiert; dies belegt eine Fraktionierung der mafischen Gemengteile und des Plagioklases in der Tiefe. Die petrogenetische Entwicklung der Andesite kann allerdings nicht einfach mit Fraktionierungsprozessen eines einzigen mafischen Magmas erklärt werden. Das Fehlen von Element-Korrelationen, das starke Streuen der Sr-Isotopenverhältnisse und das Isotopenungleichgewicht zwischen den einzelnen Mineralphasen stehen in Widerspruch zu einem derartigen einfachen Modell. Ein Modell, das die Petrogenese der untersuchten Andesite besser erklärt, zieht die Beteiligung verschiedener mafischen Schmelzen-sie bildeten sich in einem heterogenen Mantel und sind durch Unterschiede im Chemismus und ihrer Isotopenzusammensetzung charakterisiert—in Betracht. Dabei kam es zu Fraktionierung dieser Magmen in der Tiefe und zu beträchtlichen Reaktionen mit kontinentalem Krustenmaterial.

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With 5 Figures     

Land subsidence of Jakarta (Indonesia) and its relation with urban development

Jakarta is the capital city of Indonesia with a population of about 9.6 million people, inhabiting an area of about 660 square-km. In the last three decades, urban development of Jakarta has grown very rapidly in the sectors of industry, trade, transportation, real estate, and many others. This exponentially increased urban development introduces several environmental problems. Land subsidence is one of them. The resulted land subsidence will also then affect the urban development plan and process. It has been reported for many years that several places in Jakarta are subsiding at different rates. The leveling surveys, GPS survey methods, and InSAR measurements have been used to study land subsidence in Jakarta, over the period of 1982–2010. In general, it was found that the land subsidence exhibits spatial and temporal variations, with the rates of about 1–15 cm/year. A few locations can have the subsidence rates up to about 20–28 cm/year. There are four different types of land subsidence that can be expected to occur in the Jakarta basin, namely: subsidence due to groundwater extraction, subsidence induced by the load of constructions (i.e., settlement of high compressibility soil), subsidence caused by natural consolidation of alluvial soil, and tectonic subsidence. It was found that the spatial and temporal variations of land subsidence depend on the corresponding variations of groundwater extraction, coupled with the characteristics of sedimentary layers and building loads above it. In general, there is strong relation between land subsidence and urban development activities in Jakarta.     

High-pressure metamorphism in metabasites from the Betic Cordilleras (S.E. Spain) and its evolution during the Alpine orogeny

The Nevado-Filábride complex is the lowest tectonic unit of the Betic Zone sensu stricto (ss) of the Betic Cordilleras (S.E. Spain). The upper series of this complex consists of a metamorphosed sequence intruded by basic and ultrabasic igneous rocks. High-pressure metamorphism in the eclogite and blueschist facies is recorded in the metabasites, but this was partially obliterated by further successive metamorphic stages in the almandine-amphibolite and greenschist facies.Coronitic and granoblastic eclogites appear side by side in the large stocks of basic rocks. The coronitic eclogites originate from coarse-to medium-grained olivine gabbros, and the granoblastic eclogites from fine-grained basic rocks (dolerites and porphyritic basaltic rocks). Higher chemical mobility and rate of diffusion, as well as the availability of fluids during the eclogite facies metamorphism, are responsible for the greater degree of recrystallization found in the granoblastic eclogites. The availability of fluids during this metamorphic stage was controlled by the difference in the hydration of the protolith and by variable proximity to surrounding water-rich metasediments.The minerals in the eclogites are chemically homogeneous, suggesting that they are almost completely equilibrated, even in the coronitic eclogites. The estimated equilibrium P-T conditions were found to be the same (approximately 550° C at 12 kbar pressure) in both coronitic and granoblastic eclogites, and it has, therefore, been deduced that the coronitic eclogites do not represent the first and lower-grade step of a prograde metamorphism in which the granoblastic eclogites are the higher-grade step.No relationship was found between shearing and eclogite crystallization. Nevertheless, a first fabric/foliation developed in the later blueschist facies stage, and syntectonic growth of the minerals was detected in glaucophane-bearing rocks.The further metamorphic evolution of the metabasites from high-to intermediate-pressure conditions is documented by the formation of minerals belonging to albiteepidote and almandine-amphibolite facies assemblages. The application of the amphibole zonation model, in order to deduce the P-T path, does not give realistic values.High-pressure metamorphism is related to an early subduction event in the Betic Cordilleras, with a later more-or-less isothermal uplift to shallower levels.     

Latest Campanian and Maastrichtian Siderolitidae (larger benthic foraminifera) from the Pyrenees (S France and NE Spain)

The siderolitids from the uppermost Campanian and Maastrichtian deposits of the Pyrenees have been re-studied. This has revealed a high diversity and rapid replacement of taxa, confirming the group as a good tool for high resolution biostratigraphy. Two genera have been found in the uppermost Campanian–Maastrichtian interval in the Pyrenean deposits: Siderolites Lamarck, and Wannierina Robles-Salcedo. Siderolites, with canaliferous spines or denticulate periphery, is represented by four species replacing each other from the latest Campanian to Maastrichtian: Siderolites praecalcitrapoides (latest Campanian), S. pyrenaicus sp. nov. (early Maastrichtian), Siderolites calcitrapoides (late Maastrichtian) and Siderolites denticulatus (late Maastrichtian). Wannierina is characterised by well-developed keels and ramified marginal canals. Two species of Wannierina have been identified and they succeeded one another from latest Campanian to early Maastrichtian: Wannierina vilavellensis sp. nov. (latest Campanian) and Wannierina cataluniensis (early Maastrichtian). The species of the genus Siderolites inhabited shallow waters of tropical to subtropical platforms with moderate-to-high water-energy conditions and those of the genus Wannierina are typical of deep–water low-energy environments but still in the eutrophic zone.     

Dating early Holocene palaeoseismic event(s) in the Gulf of Bothnia, Baltic Sea

Hutri, K.-L., Heinsalu, A., Kotilainen, A. T. & Ojala, A. E. K. 2007 (January): Dating early Holocene palaeoseismic event(s) in the Gulf of Bothnia, Baltic Sea. Boreas , Vol. 36, pp. 56–64. Oslo. ISSN 0030–9483.
Deformation structures in submarine Holocene sediments caused by palaeoseismicity have recently been found in the Olkiluoto area, Gulf of Bothnia, Baltic Sea, within old fracture zones of bedrock. In this study, the palaeoseismic event(s) was dated and the palaeoenvironment was characterized using palaeomagnetic, biostratigraphical and lithostratigraphical methods, thereby enhancing the reliability of the chronology. The variations in the inclination and declination of the Olkiluoto sediment core showed very good correlation with the palaeosecular variations recorded in the annually laminated long lake sediment record from Lake Nautajarvi in central Finland. Combined litho-, bio- and palaeomagnetic stratigraphy revealed an age estimation of 10 650 to 10 200 cal. yr BP for the palaeoseismic event(s), which coincides with postglacial bedrock faulting in northern Fennoscandia.     

Hydrogeochemical effects of groundwater mining of the Sierra de Crevillente Aquifer (Alicante,Spain)

The groundwater mining of the Crevillente aquifer (southeastern Spain) has resulted in the progressive deterioration of water quality, with particularly significant increases in chloride, sulfate, and sodium. The possibility of a vertical hydrochemical zoning is deduced that would require examining the importance of the geometry and lithology (evaporitic materials) in the salinization process. The time of water-rock contact (residence time) and dilution by infiltration of rainwater also influences the hydrogeochemistry of the aquifer. The hydrochemical data are useful in defining the conceptual model of the aquifer, completely karstified with relative homogeneity.Project AMB92-0211 (CICYT)     

芦山地震发震构造及其与汶川地震关系讨论

芦山地震发生在巴彦喀拉块体与华南块体之间龙门山推覆构造带南段。野外考察表明,芦山地震在震中区没有形成具有构造地质意义的地震地表破裂带,仅在各山前陡坡地带出现平行于山麓陡坡的张性地裂缝、山地基岩崩塌、滑坡等边坡震动失稳现象和震动引起的砂土液化现象。重新定位的芦山地震余震分布、震源机制解和地表构造地质等分析表明,芦山地震的发震断层为一条现今尚未出露地表、其上断点仍埋藏在地下9 km以下地壳中的一条盲逆断层,走向212°,倾向NW,倾角38°±2°,上断点以上至地表的构造变形符合断层扩展背斜模型。根据汶川地震和芦山地震的余震空间分布、地震破裂过程、深浅构造关系等差异反映出它们是分别发生在龙门山推覆构造带中段和南段的两次独立地震破裂事件。     

Paleoseismological analysis of an intraplate extensional structure: the Concud fault (Iberian Chain,eastern Spain)

The Concud fault is a 13.5 km long, NW–SE striking normal fault at the eastern Iberian Chain. Its recent (Late Pleistocene) slip history is characterized from mapping and trench analysis and discussed in the context of the accretion/incision history of the Alfambra River. The fault has been active since Late Pliocene times, with slip rates ranging from 0.07 to 0.33 mm/year that are consistent with its present-day geomorphologic expression. The most likely empirical correlation suggests that the associated paleoseisms have potential magnitudes close to 6.8, coseismic displacements of 2.0 m, and recurrence intervals from 6.1 to 28.9 ka. At least six paleoseismic events have been identified between 113 and 32 ka. The first three events (U to W) involved displacement along the major fault plane. The last three events (X to Z) encompassed downthrow and hanging-wall synthetic bending prompting fissure opening. This change is accompanied by a decrease in slip rate (from 0.63 to 0.08–0.17 mm/year) and has been attributed to activation of a synthetic blind fault at the hanging wall. The average coseismic displacement (1.9–2.0 m) and recurrence period (6.7–7.9 ka) inferred from this paleoseismic succession are within the ranges predicted from empirical correlation. Such paleoseismic activity contrasts with the moderate present-day seismicity of the area (maximum instrumental Mb = 4.4), which can be explained by the long recurrence interval that characterizes intraplate regions.     

New discoveries of vertebrate remains from the Triassic of Riba de Santiuste,Guadalajara (Spain)

The palaeontological sites of Riba de Santiuste and Sienes (Riba de Santiuste area) are located in the province of Guadalajara, Spain. They include a stratigraphic interval in Muschelkalk facies belonging to the “Cuesta del Castillo Sandstones” Formation and “Royuela Dolostones, Marls and Mudstones” Formation. These sites include numerous fossil plants, direct vertebrate remains, and vertebrate swim traces. The vertebrate remains correspond to a multitude of anatomical elements of Sauropterygia (Nothosauroidea, Placodontia) and possible Archosauria (Rauisuchia) remains. The fossil material attributed to nothosaurs includes teeth, coracoids, a thoracic vertebra, some isolated vertebral centra, humerus, rib fragments, and some dorsal and caudal vertebrae. The remains attributed to placodonts correspond to fragments of skull, quadrate, teeth and osteoderms. Other undetermined sauropterygian remains, such as ulnas, fragments of long bones, fragments of ribs, and articular facets of ribs have been also recovered. Additionally, a fragment of mandible and an intervertebral disk of indeterminate reptiles whose size could be compatible with archosaurs are also described. These bones are exceptionally well-preserved because the fossilization processes have preserved the microstructure of the tissues. The sites also show vertebrate traces, with parallel scratch impressions interpreted as swim traces. The relative stratigraphic position and the palaeontological content of these sites suggest a Ladinian age (Middle Triassic). The interpretation of the sedimentary facies here described also suggests that the sites could correspond to detrital-carbonate mixed deposits of coastal intertidal to supratidal environments.     

Application of groundwater sustainability indicators to the carbonate aquifer of the Sierra de Becerrero (Southern Spain)

The objective of this study was to test the applicability of groundwater sustainability indicators defined by UNESCO, together with the International Academy of Environmental Sciences (IAES), the International Association of Hydrogeology (IAH) Group on Groundwater Indicators and the Geological Survey of Spain (IGME), to the aquifer scale. We selected four main indicators based on their relevance in the field of groundwater sustainability and because they proved to be the most reliable, based on the data collection and methodology utilized. These indicators were applied to a small—26 km² of permeable outcrops—carbonate aquifer situated in the province of Seville (southern Spain), which has semi-arid climate conditions (500 mm/year). The integral application of all these indicators in this particular groundwater body leads us to conclude that, at present, the aquifer is undergoing intensive use. Therefore, the exploitation of its water resources is surpassing the threshold of sustainability when both the quantity and the quality of the groundwater are taken into consideration. The continued increase in exploitation generates a descending trend in the evolution of the piezometric levels, a consequence of adaptation to the new hydrodynamic situation, and also results in exhaustion of the springs that drain the aquifer in undisturbed conditions. At the same time, there is a trend of increasing salinity in the groundwater and a risk of contamination by nitrate which, according to the EU Water Framework Directive and the Groundwater Daughter Directive (EU Official Journal of the European Communities L327, 2000; EU Official Journal of the European Communities L372/19, 2006), should be controlled and reduced. In the future, application of the methodology described here may prove useful for the evaluation of similar systems, either in southern Spain or in other countries with semi-arid climates.     

Asperity distribution of the 1964 Great Alaska earthquake and its relation to subsequent seismicity in the region

The 1964 Alaska earthquake was the second largest seismic events in the 20th century. The aim of this work is the use of surface deformation data to determine asperity and slip distributions on the fault plane of the Alaska earthquake: these distributions are calculated by a Monte Carlo method. To this aim, we decompose the fault plane in a large number of small square asperity units with a side of 25 km; this allows us to obtain plane surfaces with an irregular shape. In the first stage, each asperity unit is allowed to slip a constant amount or not to slip at all, providing the geometry of the dislocation surface that best reproduces the observed displacements. To this purpose, a large number of slip distributions have been tried by the use of the Monte Carlo method. The slip amplitude is the same for all the asperities and is equal to the average fault slip inferred from the seismic moment. In the second stage, we evaluate the slip distribution in the dislocation area determined by the Monte Carlo inversion: in this case, we allow unit cells to undergo different values of slip in order to refine the initial dislocation model. The results confirm the previous finding that the slip distribution of the great Alaska earthquake was essentially made of two dislocation areas with a higher slip, the Prince William Sound and the Kodiak asperities. Analysis of the post-1964 seismicity in the rupture region shows a strong correlation between the larger earthquakes (M_w≥6) and the distribution of locked asperities following the 1964 event, which can be considered as an independent test of the validity of the model. We do not find slip values higher than 25 m for any of the patches, and we determine two separate high-slip zones: one correspondent to the Prince William Sound asperity, and one (18 m slip) to the Kodiak asperity. The slip distribution connected with the 1964 shock appears to be consistent with the following seismicity in the region.