We use a fracture index distribution method of geostatistical modelling and prediction to characterize quantitatively the fracture system in two-dimensions (2D) in the Mina Ratones area, located in the Albalá Granitic Pluton (SW Iberian Massif). The fracture index (FI) is a quantitative estimate of the fracture density in discrete domains. To validate the results of geostatistical modeling a detailed structural map of the area was also made on a scale of 1:1000. The resulting grids, expressed as pixel-maps, describe the continuous value of the FI in 2D for the whole Mina Ratones area. Based on the modelled distribution of the FI and their correlation with mapped faults, we distinguish two structural domains in the studied area: elongated bands of fracture zones with high FI values and romboidal blocks located between them with low FI values. The separation between both domains is gradual. Though a threshold value of the FI that separate both structural domains is not clearly defined, the fracture zones generally present FI>1 and the individualized blocks FI<0.50. As a consequence, the obtained grid of the FI permits the quantitative structural classification of the granitic massif in 2D and understanding fault zone architecture in the Mina Ratones area. 相似文献
In the East Karkonosze complex (Karkonosze = Riesengebirge), which occurs at the northern margin of the Bohemian massif, rocks of the glaucophane-schist facies and transitions between the glaucophane-schist facies, greenschist facies and epidote-amphibolite facies are present. They belong to the Leszczyniec Volcanic Formation (LVF) of Cambrian/Ordovician age and to the mainly metasedimentary Czarnów Schist Formation (CSF) of Ordovician/Silurian age. Similar high-pressure, low-temperature rocks occur in the southern Karkonosze and in the Kaczawa Mountains within metavolcanic formations of approximately the same age. Petrographic and electron probe studies show complex relationships between minerals including chemical zoning. In the East Karkonosze three stages of metamorphism pre-dating contact metamorphism by late Variscan (lowermost Upper Carboniferous) granite intrusion were distinguished [stage 1: ocean floor, amphibolite facies (observed only in part of the LVF); stage 2: high-pressure, low-temperature, variably glaucophane-schist facies, high-pressure greenschist facies and epidote-amphibolite facies; stage 3: medium-pressure greenschist facies accompanied by strong deformations]. Glaucophane-schist facies rocks formed in stage 2 survived the later stages of metamorphism only in the southern part of East Karkonosze, i. e. in Lasocki Range and Rýchory. Using the Maruyama et al. (1986) geobarometer the glaucophane-bearing rocks formed at 6.5–7 Kb, those with crossite at 5–6 Kb and rocks with magnesioriebeckite/riebeckite at 4–5 Kb. Other estimates for glaucophane-bearing rocks give somewhat higher values of pressure, i. e. 7–12 Kb at temperatures between 300 and 530°C. The highest temperatures are recorded in the glaucophane- and garnet-bearing rocks. Stilpnomelane may occur in all of these rocks. The subduction/obduction episode responsible for this high-pressure, low-temperature metamorphism is considered to have taken place in the early Variscan, although no geochronology is yet available to confirm this. 相似文献
New hadrosaurid material is recorded from Fontllonga (Ager syncline, province of Lleida), in the Catalonian Pyrenees, comprising a dentary with part of the dental battery, recovered from Late Maastrichtian strata (Tremp Formation), close to the presumed Cretaceous/Tertiary boundary. This hadrosaurid is more derived than is Telmatosaurus transsylvanicus from the Haeg Basin (Romania), but lacks a number of features that diagnose the clade Hadrosaurinae plus Lambeosaurinae. It is attributed to the Euhadrosauria on the basis of the following synapomorphies: dentary teeth not recurved distally, narrow dentary crowns, and denticles not supported by subsidiary ridges. The phylogenetic relationships of the best-known iguanodontids and hadrosauroids are evaluated using mandibular and dentary tooth characters. On the basis of a cladistic analysis, the family of Hadrosauridae is diagnosed by more than 29 dentary tooth positions, parallel-sided vertical furrows formed by dentary alveoli, and absence of caudal secondary ridges. Within Hadrosauridae, successive sister-groups of Hadrosaurinae plus Lambeosaurinae are the Fontllonga taxon and Telmatosaurus. The clade Hadrosaurinae plus Lambeosaurinae is characterised mainly by a coronoid process inclined rostrally and by the absence of secondary ridges on dentary crowns. The Fontllonga find suggests the diversity of European hadrosaurids at the end of the Cretaceous to have been greater than previously thought. Moreover, the primitive character of European hadrosaurids as compared to western North American and Asian relatives may be explained in terms of geographical isolation during the Late Cretaceous. 相似文献
Sandstones and shales from the Upper Neoproterozoic (UN) succession in the Central Iberian Zone (CIZ) show parallel REE patterns and relatively restricted and similar ranges and average values of some element ratios such as Al2O3/TiO2,Ti/Nb, Eu/Eu*, (La/Yb)n, (Gd/Yb)n and Th/U. This remarkable geochemical homogeneity for related medium- and fine-grained rocks is unusual, and strongly suggests a recycled source area. However, the Lower Cambrian (LC) equivalent rocks are, in general terms, geochemically less mature, more heterogeneous and more fractionated. Their average REE patterns are practically coincident, probably as a consequence of REE redistribution related to the reworking of sediments during a stage of sea level fall in Lower Cambrian times. Ti and Zr abundances, chemical index of alteration (CIA) values and element ratios such as Al2O3/TiO2, K/Rb, Ti/Nb and Rb/Zr can be used for discriminating purposes between the UN and LC siliciclastic rocks. Detrital zircons from a UN and a LC sandstone display morphological differences. However, U-Pb data are discordant and cannot be interpreted in a straightforward manner. Nevertheless, age data are compatible with a model in which the continent of Gondwana would have supplied zircons, showing a bimodal age distribution, in variable proportions to the respective sandstone units. 相似文献
New U–Pb age-data from zircons separated from a Northland ophiolite gabbro yield a mean 206Pb/238U age of 31.6 ± 0.2 Ma, providing support for a recently determined 28.3 ± 0.2 Ma SHRIMP age of an associated plagiogranite and 29–26 Ma 40Ar/39Ar ages (n = 9) of basalts of the ophiolite. Elsewhere, Miocene arc-related calc-alkaline andesite dikes which intrude the ophiolitic rocks contain zircons which yield mean 206Pb/238U ages of 20.1 ± 0.2 and 19.8 ± 0.2 Ma. The ophiolite gabbro and the andesites both contain rare inherited zircons ranging from 122–104 Ma. The Early Cretaceous zircons in the arc andesites are interpreted as xenocrysts from the Mt. Camel basement terrane through which magmas of the Northland Miocene arc lavas erupted. The inherited zircons in the ophiolite gabbros suggest that a small fraction of this basement was introduced into the suboceanic mantle by subduction and mixed with mantle melts during ophiolite formation.
We postulate that the tholeiitic suite of the ophiolite represents the crustal segment of SSZ lithosphere (SSZL) generated in the southern South Fiji Basin (SFB) at a northeast-dipping subduction zone that was initiated at about 35 Ma. The subduction zone nucleated along a pre-existing transform boundary separating circa 45–20 Ma oceanic lithosphere to the north and west of the Northland Peninsula from nascent back arc basin lithosphere of the SFB. Construction of the SSZL propagated southward along the transform boundary as the SFB continued to unzip to the southeast. After subduction of a large portion of oceanic lithosphere by about 26 Ma and collision of the SSZL with New Zealand, compression between the Australian Plate and the Pacific Plate was taken up along a new southwest-dipping subduction zone behind the SSZL. Renewed volcanism began in the oceanic forearc at 25 Ma producing boninitic-like, SSZ and within-plate alkalic and calc-alkaline rocks. Rocks of these types temporally overlap ophiolite emplacement and subsequent Miocene continental arc construction. 相似文献
The Alleret maar (Massif Central, France) provides a long lacustrine sequence (40.6 m) attributed to the early Middle Pleistocene. Sediment, pollen and diatoms analysis of its upper part (AL2 core, 14.6 m) indicates two temperate phases marked by high lake levels, forest development and vegetation expansion. They are separated by a cold period during which lake level drops, coarse sediment input increases and steppic and xerophilous plants develop. Pollen data suggests that this sequence belongs to the upper part of the Cromerian complex. These results are in agreement with the 557 ± 3 ka (±12 ka, including all errors) 40Ar/39Ar age obtained from an interbedded tephra layer emitted by the Mont-Dore/Sancy strato-volcano and establish that this sequence probably covers the MIS 15 substages. 相似文献
The SUDETES 2003 wide-angle refraction/reflection experiment covered the area of the south-western Poland and the northern Bohemian Massif. The good quality data that were gathered combined with the data from previous experiments (POLONAISE'97, CELEBRATION 2000) allowed us to prepare a 3D seismic model of the crust and uppermost mantle for this area. We inverted travel times of both refracted and reflected P waves using the JIVE3D package. This allowed us to obtain a model of P-wave velocity distribution as well as the shape of major boundaries in the crust. We also present a detailed uncertainty analysis for both the boundary depths and the velocity field. In doing the uncertainty analysis we found an interesting, strong dependence between uncertainty and inversion scheme (order of used phases). We also compared the model with surface geology and found good correlation between velocity inhomogeneities in the uppermost crust (down to 2 km) and major geological units. The higher velocity lower crust (6.9–7.2 km/s) could result from remelting of the lower crust or magmatic underplating. 相似文献