Control of paths of quaternary volcanic products in western Bohemian Massif by rejuvenated Variscan triple junction of ancient microplates |
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| Authors: | V Babu?ka J Plomerová |
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| Institution: | (1) Institute of Geophysics, Acad. Sci. Czech Republic, Boční II/1401, 141 31 Praha 4, Czech Republic |
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| Abstract: | Our objective is to look for deep paths of Cenozoic volcanism and migration routes of active mantle volatiles through the
lithosphere of the western Bohemian Massif. We show that the rejuvenated junction of three mantle domains, delimited by different
orientation of seismic anisotropy and belonging to originally separated microplates — the Saxothuringian (ST), Moldanubian
(MD) and Teplá-Barrandian (TB) — can provide the easiest upward routes of fluids through the deep lithosphere. Geographic
distribution of mantle-fluid escapes at the surface suggests fluid migration through the ductile lower crust and through partly
open faults in the rigid upper crust, which is locally detached and shifted from its lower part and from the mantle lithosphere.
Present-day escapes of mantle-derived helium and CO2 concentrate mainly in two tectonically different crust edifices — in the Cheb Basin (CHB) and in an allochtonous block called
the Mariánské Lázně Complex (MLC). Crystalline basement of the CHB developed above the Variscan ‘triple junction’ of the mantle
lithosphere domains. The basement was extended during the Cenozoic and dissected by systems of faults into small partly sunken
blocks. Thanks to buoyancy the mantle fluids migrate upwards along the lithosphere junction into the faulted basement of the
CHB. The highest CO2 flow and the highest 3He/4He ratios are observed at intersections of major normal faults and along the southern boundary of the Smrčiny (Fichtelgebirge)
granite Pluton. The fluid escapes are separated from the earthquake swarm epicentres. Routes of the fluids to the MLC are
longer and more complicated. Surface escapes tap the mantle fluids mainly from the Mariánské Lázně Fault (MLF) and from the
tectonic boundaries along which the MLC block of the TB lower crust was thrust over the ST complexes. Hypocentres of earthquake
swarms of the two major focal areas at Novy Kostel and Lazy, located mainly at depths of 6–13 km, reside either in granite
or in underlying gneiss, while the escapes of mantle fluids follow major faults or boundaries of crystalline units outside
the Smrčiny and Karlovy Vary granite Plutons. We suggest that primarily those parts of faults in the upper crust, which is
strengthened by granite magmatism and rigid enough to selectively accumulate stresses, are seismoactive. On the other hand,
other parts of the faults tapping ascending mantle volatiles are ‘lubricated’ by the fluids and secondary mineralogical changes,
and thus they cannot accumulate sufficient stresses to be released by earthquakes. A comparison of the most probable paths
of the mantle fluids with the space-time distribution of the Novy Kostel hypocentres does not seem to support the model of
the earthquake swarms triggered by pressurized fluids of mantle origin. |
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| Keywords: | western Bohemian Massif boundaries of lithosphere domains migration paths of active mantle fluids |
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