Lithospheric and upper mantle structure beneath the western Bohemian Massif obtained from teleseismic P and S receiver functions [Elektronische Ressource] / Geoforschungszentrum Potsdam in der Helmholtz-Gemeinschaft. Barbara Heuer

freie_universitat_berlin - Heuer , Charles Barbara

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Publié par	freie_universitat_berlin
Publié le	01 janvier 2006
Nombre de lectures	41
Langue	English
Poids de l'ouvrage	8 Mo

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ISSN 1610-0956
Scientific Technical Report STR 06/12 GeoForschungsZentrum PotsdamScientific Technical Report STR 06/12 GeoForschungsZentrum PotsdamScientific Technical Report STR 06/12 GeoForschungsZentrum PotsdamScientific Technical Report STR 06/12 GeoForschungsZentrum Potsdam
Abstract
The Bohemian Massif is the largest coherent surface outcrop of the Variscan basement in
central Europe. The investigation area of this study, the western Bohemian Massif, is
situated at the junction of three Variscan structural units: the Saxothuringian in the north,
the Teplá-Barrandian and Moldanubian units in the south. The Palaeozoic suture between
the Saxothuringian and Teplá-Barrandian/Moldanubian units has been reactivated since the
Upper Cretaceous/Tertiary as part of the European Cenozoic Rift System. This led to the
evolution of the 300 km long and 50 km wide ENE-WSW trending Eger (Oh ře) Rift. The
western part of the Eger Rift is known for geophysical and geological phenomena such as
the occurrence of earthquake swarms, CO dominated free gas emanations of 2
subcontinental lithospheric mantle signature in mineral springs and mofettes,
Tertiary/Quaternary volcanism and neotectonic crustal movements.
To explain the observed phenomena, several possible scenarios have been suggested: a
small-scale mantle plume, lithospheric thinning beneath the Eger Rift, and presently
ongoing magmatic processes near the crust-mantle boundary, including magmatic
underplating.
This thesis focuses on the seismic structure of the lithosphere and upper mantle beneath
the western Eger Rift area with the aim of investigating deep-lying possible causes of the
phenomena observed at surface.
For the investigation, data of the international passive seismic experiment BOHEMA
carried out in 2002/2003 was used. The BOHEMA network consisted of 6 1 permanent and
84 temporary stations and was centred on the western Eger Rift. The resulting large data
set allowed a high resolution P and S receiver function study using P-to-S and S-to-P
converted waves, respectively, to map seismic discontinuities in the lithosphere and upper
mantle. Data from an earlier passive seismic experiment was additionally used to
complement the BOHEMA data set. The results of the analysis are described in this thesis
‘from top to bottom’.
A high resolution Moho depth map of the investigated area could be obtained from
more than 5000 P receiver function traces. It shows crustal thicknesses of 27 to 3 1 km in
the Saxothuringian unit, 30 to 33 km in the Teplá-Barrandian and 34 to 39 km in the
Moldanubian unit east of the Bavarian Shear Zone, which generally agrees with earlier
results from seismic studies. A dominant feature in the Moho depth map is an area of thin
crust of about 26 to 28 km beneath the western Eger Rift with irregular internal geometry.
This apparent Moho updoming was already observed with less resolution in a previous
receiver function study. It corresponds well with the area of CO degassing fields, the 2
region of earthquake swarm occurrence and the location of Quaternary volcanoes at
surface. The Moho depth values have an estimated uncertainty of ± 2 km.
Furthermore, the first map of average crustal v /v ratios is presented for the p s
investigated area. The mean values associated with structural units of the Variscan orogen
vary between 1.69 and 1.75. For individual locations the variations are larger (between
1.66 and 1.81).
Scientific Technical Report STR 06/12 I GeoForschungsZentrum Potsdam
In the area of Moho updoming and CO gas emanations, additional phases were 2
observed in the P receiver function data: a positive phase at about 6 s delay time, followed
by a strong negative phase at about 7 to 8 s. A mapping of the occurrence of these
additional phases showed that they form a coherent structure centred on the western Eger
Rift. The phases can be modelled by a discontinuity at 50 km suggested by results of
seismic reflection and refraction investigations and a velocity decrease at 65 km depth. The
velocity decrease might perhaps be explained by local asthenospheric updoming and/or a
confined body of partial melt.
S receiver functions were used to investigate the base of the lithosphere as a second,
independent method. If the velocity reduction observed at 8 to 14 s delay time is
interpreted as the lithosphere-asthenosphere transition, the data show lithospheric thickness
of 80 to 90 km beneath the Saxothuringian and the northern Teplá-Barrandian unit.
Towards the south, the thickness strongly increases in the Moldanubian unit to 115 to
135 km, which corresponds well with results of previous studies. The data of the transition
from the thinner Saxothuringian/Teplá-Barrandian lithosphere to the thicker Moldanubian
lithosphere show a doubling or broadening of the negative signal, which could point to
either an abrupt increase of lithospheric thickness, or a very steep slope, or possibly even a
structure from palaeosubduction within the lithosphere of this part of the investigated area.
However, asthenospheric updoming beneath the contact of the Saxothuringian and
Moldanubian units, centred beneath the western Eger Rift as suggested in P receiver
function data, cannot be stated from S receiver functions. Two scenarios are suggested to
explain the occurrence of the negative phase in the P and S receiver functions at different
depths: (1) The negative phases in the P and S receiver functions represent two distinct
velocity reductions. A thin low velocity layer is detected by P receiver functions in the
lithospheric mantle at approximately 65 km depth that cannot be resolved by S receiver
functions. The velocity reduction observed in S receiver function data might be interpreted
as the lithosphere-asthenosphere transition. (2) The negative phases in the P and S receiver
functions represent in principle the same negative velocity gradient (the lithosphere-
asthenosphere transition), but strongly influenced by the different frequency contents of the
P and S waves and by the possible nature of the transition from high to low velocities with
increasing depth. Both scenarios imply a thin region of strongly decreased seismic velocity
at about 65 km depth which might be associated with the occurrence of partial melt in this
depth range.
The P-to-S converted waves from the discontinuities of the mantle transition zone at
410 and 660 km depth show slightly increased delay times compared to the IASP91 global
reference model. However, the thickness of the mantle transition zone is not affected and
thus points to normal temperatures in the mantle transition zone and increased v /v ratio p s
somewhere in the upper mantle above the mantle transition zone. Furthermore, a coherent
converted phase is observed in P receiver functions that might be attributed to the
discontinuity at 520 km depth.

Scientific Technical Report STR 06/12 II GeoForschungsZentrum Potsdam
Zusammenfassung
Das Böhmische Massiv bildet das größte zusammenhängende Gebiet anstehenden
variskischen Grundgebirges in Mitteleuropa. Das Untersuchungsgebiet dieser Arbeit, das
westliche Böhmische Massiv, befindet sich an der Nahtstelle dreier variskischer
Struktureinheiten: dem Saxothuringikum im Norden, und dem Teplá-Barrandium und
Moldanubikum im Süden. Die paläozoische Sutur zwischen Saxothuringikum und Teplá-
Barrandium/Moldanubikum wurde durch das Europäische Känozoische Riftsystem seit der
Oberkreide/Tertiär reaktiviert. Dies führte zur Entstehung des 300 km langen und 50 km
breiten, ENE-WSW streichenden Eger (Oh ře) Rifts. Der westliche Teil des Eger Rifts ist
bekannt für geologische und geophysikalische Phänomene wie das Auftreten von
Erdbebenschwärmen, CO -dominierte Gasaustritte aus Mineralquellen und Mofetten mit 2
subkontinentaler lithosphärischer Mantelsignatur, tertiären/quartären Vulkanismus und
neotektonische Krustenbewegungen.
Um die beobachteten Phänomene zu erklären, wurden verschiedene Szenarien
vorgeschlagen: ein kleinräumiger Mantelplume, verringerte Lithosphärenmächtigkeit unter
dem Eger Rift und gegenwärtig aktive magmatische Prozesse nahe der Kruste-Mantel-
Grenze einschliesslich magmatischem underplating.
Die vorliegende Arbeit befasst sich mit der seismischen Struktur der Lithosphäre und
des oberen Erdmantels unter dem westlichen Eger Rift. Ziel ist die Untersuchung
möglicher tiefliegender Ursachen der an der Oberfläche beobachteten Phänomene.
Dafür wurden Daten des internationalen passiven seismischen Experiments BOHEMA
genutzt, welches 2002-2003 durchgeführt wurde. Das BOHEMA-Stationsnetz bestand aus
61 Permanent- und 84 Mobilstationen, die im und um das westliche Eger Rift zentriert
lagen. Der daraus hervorgehende große Datensatz erlaubte eine hochauflösende P und S
Receiver Function Analyse, um seismische Diskontinuitäten in der Lithosphäre und im
oberen Erdmantel mittels P-zu-S bzw. S-zu-P konvertierter Wellen zu kartieren. Zur
Ergänzung k