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Focal mechanisms for three recent earthquakes in Finland are determined using P-wave polarities together with SV/P and SH/P phase amplitude ratios. The events occurred on May 11, 2000 in Toivakka, Central Finland (ML=2.4), on September 15, 2000 in Kuusamo, northeastern Finland (ML=3.5), and on May 2, 2001 in Kolari, western Finnish Lapland (ML=2.9).In order to obtain reliable estimates of the source parameters, one-dimensional crust and upper mantle velocity models are derived for the epicenter areas from deep-seismic sounding results. The starting models are modified by one-dimensional ray tracing using the earthquake observations. The events are relocated by employing P- and S-phase arrival times from the nearest seismic stations and the final velocity models. Synthetic waveforms, calculated with the reflectivity method, are used to further constrain and verify the source and structural parameters.The Toivakka earthquake indicates thrust- or reverse-faulting mechanism at a depth of 5 km. After comparison with aeromagnetic and topographic data we suggest the eastward dipping nodal plane (358°/42°) was the fault plane. The best-fitting fault plane solution of the Kolari earthquake suggests pure thrust-faulting at a depth of 5 km. The nodal plane striking 035°/30° correlates well with surface observations of the postglacial, possibly listric fault systems in the source area. The Kuusamo earthquake (focal depth 14 km) has a normal-faulting mechanism with the nodal planes trending 133°/47° or 284°/47°. Preference is given to the SW-dipping nodal plane, as it seems to coincide with topographic and magnetic lineament directions that have been active after the last ice age.The three earthquakes have occurred in old Precambrian faults and shear zones, which have been reactivated. The reactivated faults are favourably oriented in the local stress field. 相似文献
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Pollen records from 21 sites were used to reconstruct Holocene hydrological changes in northern Fennoscandia. The inferred dominance of maritime-type Betula-forests from c. 9600 BP onwards indicate an oceanic climate in the region during the early Holocene, with high amounts of precipitation and low seasonal contrasts. This interpretation is supported by oxygen isotope records obtained on lacustrine carbonates from Abisko, northern Sweden. Enrichment of 18O during the early Holocene suggests enhanced zonal circulation and a pronounced influence of Atlantic air-masses in northern Sweden. The pattern of Pinus expansion in the area was time-transgressive, ranging from 9200-8000 BP in the extreme north-east to 7900-5500 BP in the western and south-western parts of the region. The replacement of Betula-forests by Pinus-forests indicates a climatic change towards drier summers and increased seasonality. At Abisko, a close correlation between regional Pinus expansion and depletion of 18O in lacustrine carbonates suggests that the expansion of Pinus was triggered by a gradual attenuation of the influence of westerlies. Thus, in large-scale palaeohydrological terms, the early expansion of Pinus in the north-easternmost parts of Fennoscandia as compared to the successively later expansion in the west and south-west may reflect a gradually decreasing influence of Atlantic air-masses in northern Fennoscandia during the early and mid Holocene. 相似文献
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《International Geology Review》2012,54(5):509-546
The Eastern Segment abutting the Transscandinavian Igneous Belt (TIB) mostly consists of rocks with overlapping igneous ages. In the Eastern Segment west of Lake Vättern, granitoids of clear TIB affinity exhibit strong deformational fabrics. This article presents U–Pb zircon ages from 21 samples spanning the border zone between these deformed TIB rocks in the east, and more thoroughly reworked rocks in the west. Magmatic ages fall in the range 1710–1660 million years, irrespective of the degree of deformation, confirming the overlapping crystallization ages between deformed TIB rocks and orthogneisses of the Eastern Segment. A common history is further supported by leucocratic rocks of similar ages. Prolonged orogenic (magmatic) activity is suggested by continued growth of zircon at 1.66–1.60 Ga. Six of the weakly gneissic rocks show zircons with cathodoluminescence-dark patches and embayments, possibly partly replacing metamict parts of older magmatic crystals, with 207Pb/206Pb ages dominantly between 1460 and 1400 million years, whereas three of the gneisses have zircon rims with calculated ages of 1440–1430 million years. Leucosome formation took place at 1443 ± 9 and 1437 ± 6 Ma. The minimum age of SE–NW folds was determined by an undeformed 1383 ± 4 million years crosscutting aplitic dike. Sveconorwegian zircon growth was not found in any of the samples from the studied area. To our knowledge, 1.46–1.40 Ga metamorphism affecting the U–Pb zircon system has not previously been reported this far northeast in the Eastern Segment. We suggest that the E–W- to SE–NW-trending deformation fabrics in our field area were produced during the Hallandian–Danopolonian orogeny and escaped later, penetrative Sveconorwegian reworking. 相似文献
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Ca. 2.5–2.4 Ga Sumian magmatism is widespread in the Karelia and Kola cratons of Fennoscandia and probably represents at least two intermixed large igneous provinces (LIPs). It is distinct from other Paleoproterozoic LIPs (Jatulian 2.22–2.1 Ga and Ludicovian 2.06–1.96 Ga) elsewhere in the Fennoscandian Shield. A poorly understood portion of Sumian magmatism is the Vetreny Poyas (Windy Belt) subprovince, which covers ∼75,000 km2 in southeastern Fennoscandia. This subprovince consists of four genetically related complexes which developed at different levels in the crust: a volcanic complex (komatiitic basaltic lava flows on Golets, Levgora and Myandukha hills, and Victoria lava lake on Levgora hill), a subvolcanic complex (mafic–ultramafic sills and lopoliths including Ruiga, Kirichgora, Kozhozero and Undozero), plutonic complexes (Burakovsky and Vyzhiga) and a dyke complex (gabbronoritic Avdeyevo and Shala dykes and peridotitic Vinela and Koppalozero dykes). Similar patterns are present in other Sumian belts elsewhere in Karelia, for instance in southern Lapland and the Kola Peninsula. 相似文献
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Anne Kress Giles H.F. Young Matthias Saurer Neil J. Loader Rolf T.W. Siegwolf Danny McCarroll 《Chemical Geology》2009,268(1-2):52-57
Annually resolved and replicated tree-ring stable isotope series have the potential to reconstruct growing season environmental parameters over multi-millennial timescales. As this archive may require only minimal statistical detrending, it has the potential to preserve a large portion of low frequency climate signals. To date, many studies have utilised only the latewood portion of the tree ring, in an attempt to minimize carry-over effects from previous year reserves and maximise the annual nature of the climate signal preserved. However, the old trees from tree-line locations, necessary to build long chronologies, often display narrow ring-widths (< 0.5 mm), making accurate earlywood–latewood separation difficult and particular time consuming. The resulting samples may also be too small for efficient cellulose purification or multiple isotopic determinations. As photosynthates from the current year are predominantly used in conifer ring formation at marginal sites with short growing seasons, latewood separation may not be especially advantageous in determining a useful climate signal and therefore unnecessary where resources are limited. To test this hypothesis, Scots pine from Northern Norway and European larch from the Swiss Alps are used. Both sites are tree-line locations where growth is predominantly temperature limited. Tree rings were cut and extracted to cellulose for both the earlywood and latewood of each annual growth ring and stable carbon isotope ratios were measured. Our results demonstrate a very high common carbon isotope signal between earlywood and latewood in both species (rlarch = 0.68 and rpine = 0.79), which also show high correlations with summer temperature over the investigated period (AD 1980–2004 for larch and AD 1929–1978 for pine). High turnover rates and small reserve pools at these tree-line locations may account for these high common signals. These results suggest that for European tree-line conifers, the separation of earlywood from latewood is unnecessary to resolve an annual isotopic signal and make a reliable climate calibration. Using the whole ring may provide additional analytical advantages and consequently even improve climate calibrations. 相似文献
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Abhijit Gangopadhyay Jay Pulliam Mrinal K. Sen 《Geophysical Journal International》2007,170(3):1210-1226
We describe a waveform modelling technique and demonstrate its application to determine the crust- and upper-mantle velocity structure beneath Africa. Our technique uses a parallelized reflectivity method to compute synthetic seismograms and fits the observed waveforms by a global optimization technique based on a Very Fast Simulated Annealing (VFSA). We match the S , Sp, SsPmP and shear-coupled PL phases in seismograms of deep (200–800 km), moderate-to-large magnitude (5.5–7.0) earthquakes recorded teleseismically at permanent broad-band seismic stations in Africa. Using our technique we produce P - and S -wave velocity models of crust and upper mantle beneath Africa. Additionally, our use of the shear-coupled PL phase, wherever observed, improves the constraints for lower crust- and upper-mantle velocity structure beneath the corresponding seismic stations. Our technique retains the advantages of receiver function methods, uses a different part of the seismogram, is sensitive to both P - and S -wave velocities directly, and obtains helpful constraints in model parameters in the vicinity of the Moho. The resulting range of crustal thicknesses beneath Africa (21–46 km) indicates that the crust is thicker in south Africa, thinner in east Africa and intermediate in north and west Africa. Crustal P - (4.7–8 km s−1 ) and S -wave velocities (2.5–4.7 km s−1 ) obtained in this study show that in some parts of the models, these are slower in east Africa and faster in north, west and south Africa. Anomalous crustal low-velocity zones are also observed in the models for seismic stations in the cratonic regions of north, west and south Africa. Overall, the results of our study are consistent with earlier models and regional tectonics of Africa. 相似文献
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Laser ablation ICPMS U–Pb and Lu–Hf isotope data on granitic-granodioritic gneisses of the Precambrian Vråvatn complex in central Telemark, southern Norway, indicate that the magmatic protoliths crystallized at 1201 ± 9 Ma to 1219 ± 8 Ma, from magmas with juvenile or near-juvenile Hf isotopic composition (176Hf/177Hf = 0.2823 ± 11, epsilon-Hf > + 6). These data provide supporting evidence for the depleted mantle Hf-isotope evolution curve in a time period where juvenile igneous rocks are scarce on a global scale. They also identify a hitherto unknown event of mafic underplating in the region, and provide new and important limits on the crustal evolution of the SW part of the Fennoscandian Shield. This juvenile geochemical component in the deep crust may have contributed to the 1.0–0.92 Ga anorogenic magmatism in the region, which includes both A-type granite and a large anorthosite–mangerite–charnockite–granite intrusive complex. The gneisses of the Vråvatn complex were intruded by a granitic pluton with mafic enclaves and hybrid facies (the Vrådal granite) in that period. LAM-ICPMS U–Pb data from zircons from granitic and hybrid facies of the pluton indicates an intrusive age of 966 ± 4 Ma, and give a hint of ca. 1.46 Ga inheritance. The initial Hf isotopic composition of this granite (176Hf/177Hf = 0.28219 ± 13, epsilon-Hf = − 5 to + 6) overlaps with mixtures of pre-1.7 Ga crustal rocks and juvenile Sveconorwegian crust, lithospheric mantle and/or global depleted mantle. Contributions from ca. 1.2 Ga crustal underplate must be considered when modelling the petrogenesis of late Sveconorwegian anorogenic magmatism in the region. 相似文献
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