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Tallinn, Estonia

Vainu M.,Tallinn University | Terasmaa J.,Tallinn University | Haelm M.,Geological Survey of Estonia
Hydrology Research | Year: 2015

Groundwater dynamics affect lake water budgets, but its major factors and mechanisms still need clarification. This study evaluates the effects of surrounding groundwater flow on seepage direction and assesses factors that affect seepage flux in a closed-basin lake in northeastern Estonia - Lake Martiska. A piezometric map was used to determine directions of groundwater flow around the lake. Seepage meters were applied for measuring flux at 44 locations along eight transects in the lake in relation to water depth, distance from the shore, sediment type and thickness of organic sediment. Additionally nearshore ice-free areas were mapped in winter. Seepage patterns followed the estimated directions of groundwater flow in nearshore areas. Outseepage records showed the impacts of nearby groundwater-abstraction wells on groundwater flow. However, the within-lake seepage direction and flux differed from the expected at 6-15 m from the shore and water depth of 1-2 m. Seepage flux and physical factors of the lake were uncorrelated. Even with a 3.2 m thick layer of gyttja, seepage influx was 13 ml m-2 min-1; therefore thick lacustrine sediments do not necessarily prevent inseepage. The results suggest that a local confined aquifer around and underneath the lake may cause the observed inseepage pattern. © IWA Publishing 2015.

Suuroja K.,Geological Survey of Estonia | Suuroja S.,University of Tartu
Baltica | Year: 2010

The Neugrund marine impact structure is located on the southern coast of the entrance to the Gulf of Finland (59°20' N, 23°31' E), straight eastward of the Osmussaar Island (Odensholm, Swed.; Odin's Grave, Engl.). The structure is very well preserved and the only one with morphological units, visible and easily accessible for the researchers and skin-divers. The Neugrund is a complex meteorite crater about 20 km in diameter. In the centre of the structure emerges the inner crater with a two-ridged rim wall having approximately 7 km rim-to-rim diameter: an inner ridge of about 6 km and an outer ridge of about 8 km. The presence of a central peak (uplift) of about 5.5 km diameter in the deep part of the crater is not proven. A 4-5 km wide terrace or zone of dislocations surrounds the inner crater. The Neugrund impact structure formed in the Early Cambrian (ca. 535 Ma ago) as the result of impact of an asteroid about 1 km in diameter.

Mark-Kurik E.,Tallinn University of Technology | Poldvere A.,Geological Survey of Estonia
Estonian Journal of Earth Sciences | Year: 2012

An updated version of the Devonian stratigraphical chart of Estonia with comments is presented. Estonian regional stratigraphical units are correlated with the standard conodont zonation and miospore zonation, used in the western part of the East European Platform. The fossil fish zonations, largely accepted in the Main Devonian Field, are discussed. Differences in the position of series and stage boundaries and age determination of regional units in the Baltic area, Belarus and NW Russia are dealt with. Two key markers for the correlation of the Middle Devonian of the Baltic area and Scotland, based mainly on placoderms, are described. Special attention is paid to occurrences of inarticulate brachiopods and finds of rare articulate brachiopods in siliciclastic rocks of the Baltic area, indicating their marine origin.

White R.S.,University of Cambridge | Drew J.,University of Cambridge | Martens H.R.,University of Cambridge | Key J.,University of Cambridge | And 3 more authors.
Earth and Planetary Science Letters | Year: 2011

We have captured a remarkable sequence of microearthquakes showing progressive melt intrusion of a dyke moving upward from a sill at 18. km depth in the mid-crust of the northern volcanic rift zone in Iceland. Two-thirds of the earth's crust is created at mid-ocean rifts. Two-thirds of that crust is formed by intrusion and freezing before it erupts of molten rock generated within the underlying mantle. Here we show seismicity accompanying melt intrusion from 17.5 to 13.5. km depth along a dyke dipping at 50° in the mid-crust of the Icelandic rift zone. Although the crust at these depths is normally aseismic, high strain rates as melt intrudes generate microearthquakes up to magnitude 2.2. Moment tensor solutions show dominantly double-couple failure, with fault mechanisms sometimes flipping between normal and reverse faulting within minutes in the same location, but breaking along fault planes with the same orientations. We suggest several possible reasons for the flipping fault mechanisms: the breakage of solidified plugs of basalt within the dyke itself as more melt intrudes; intrusion along sub-parallel fractures or dykelet fingers into the local stress field created near the tip of a propagating dyke; or movement on small jogs or offsets between adjacent en echelon dykes. Although the faulting is caused ultimately by melt movement, there is no resolvable volumetric component in the moment tensor solutions. The inferred fault planes from microearthquakes align precisely with the overall plane of the dyke delineated by hypocentres. Melt injection occurs in bursts propagating at 2-3. m/min along channels c. 0.2. m thick, producing swarms of microearthquakes lasting several hours. Intervening quiescent periods last tens to hundreds of hours. © 2011 Elsevier B.V.

Key J.,University of Cambridge | White R.S.,University of Cambridge | Soosalu H.,University of Cambridge | Soosalu H.,Geological Survey of Estonia | Jakobsdottir S.S.,Icelandic Meteorological Office
Geophysical Research Letters | Year: 2011

Lower crustal earthquakes (12-25 km depth) have been detected since August 2005 in the Askja volcanic system along the north Iceland rift, in the normally ductile part of the crust. The earthquakes occur in three clusters, which have stable dimensions and locations through time and are interpreted as positions of repeated melt supply from the mantle to the lower crust. Seismic velocity Vp/Vs ratios are consistent with the presence of partial melt in the lower crust at Askja. The spatial separation of the clusters shows that there are multiple positions of melt injection within this one magmatic segment and all three positions are currently active. This pattern of melt supply is more like that observed on fast spreading ridges than slow spreading ridges and is probably a consequence of the increased melt production beneath Iceland compared to the rest of the Mid-Atlantic Ridge. However, the relative number of earthquakes in each cluster shows that two thirds of the melt is supplied to the central volcano Askja (i.e., segment center). During the last major rifting episode shallow lateral melt migration occurred from the magma chamber beneath the volcano. Therefore on long time scales melt supply is probably greater at the segment center, with melt redistribution in the upper crust, even though there are multiple points of lower crustal injection along the segment. Copyright 2011 by the American Geophysical Union.

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