Download New geophysical evidence on magmatic emplacements in along the

New geophysical evidence on magmatic emplacements along the contact zones of major
units of the Bohemian Massif, Czech Republic
Miroslav Novotný1 and Veronika Štědrá2
- Institute of Geophysics CAS, Boční II/1401, 141 31 Praha 4, Czech Republic,
phone: +420 267 103 386, fax: +420 272 761 549, e-mail: [email protected]
2
- Czech Geological Survey, Klárov 3, 118 21 Praha 1, Czech Republic
1
Abstract
The joint interpretation of the 9HR reflection pattern and the CEL09 velocity model
of the upper and middle crust succeeded to trace voluminous magmatic intrusions ascending
along the contact zones between the Saxothuringian, Teplá-Barrandian and Moldanubian units
up to 3–4 km depths. The interpretation is supported by the gravity and magnetic field
modeling along the 9HR profile and petrophysical data available.
The regional deep reflection 9HR and the refraction CEL09 profiles intersect the main
regional units of Bohemian Massif approximately in NW-SE direction. In the NW part of the
section, the horizontal velocity gradient in the 1–15 km depth is massively vertically disturbed
in contrast to its “moldanubian” part to the SE from the CBSZ, where the pattern of velocity
gradient is rather homogeneous. The enhanced seismic amplitudes in the 9HR migrated time
section enabled us to localize the magmatic channels rooted at the mantle-crust boundary and
branched in the middle and upper crust. The P-wave velocities inside the magmatic bodies
exhibit typical velocity/density decreas during their ascent by gravity differentiation and also
by stoping of hosted rocks Characteristic diapiric tectonic features formed in the vicinity of
these penetrating magmatic bodies.. These structures appear in the enhanced 9HR reflectivity
pattern by means of following signatures: (i) the variable dips of deformed reflection
boundaries in the penetrated hosted complexes are steeply increasing upwards, (ii) strongly
reflectantchamber walls, i.e., their NW and SE traces in the profile section, related to the
particular chamber axis. As a rule, both walls were observed thanks to the prevailing SW-NE
strike of penetrated structures; (iii) weaker multiple interfaces of anatectic zones inside the
magmatic bodies are often detected thanks to the dynamite technology used; (iv) last but not
least, the kernels of magmatic chambers are clearly marked by vanishing reflectivity inside.
These signatures were detected across the whole crustal section in the ascending magmatic
columns rooted in the 30–40 km depths.
The most voluminous intrusions are observed as mantle upwelling under the Sokolov
basin in the West Bohemian seismo-active region, under the Mariánské Lázně Complex and
the adjacent West Bohemian shear zone. The further voluminous magmatic ascents were
found beneath the Central Bohemian shear zone and in the SE end of the 9HR profile under
the Prachatice granulite massif.
The P-wave velocity image revealed two buried pronounced velocity elevations within
the Teplá-Barrandian unit (TBU) at the 6100 ms-1 isovelocity, characteristic of magmatic
intrusions in this area. These elevations ascend up to the 4 km depth at the flanks whereas the
central part of the TBU is rooted at the 12 km depth, corresponding to the Teplá-Barrandian
basement The marginal elevations seem to reflect the massive near-isotropic bodies of
ultrabasics and metabasites in the Mariánské Lázně Complex (MLC) in the NW, and the
central basaltic volcanic belt in the SE. Both elevations produced the two major gravity highs
accompanied by several minor positive anomalies that result from narrow outcropping
basaltic belts.
A similar situation occurs near the Boskovice furrow developed at the contact of
Moldanubian and Moravian terranes that is, however, encountered only by the CEL09 profile.
A profound velocity depression at the 6100 ms-1 isovelocity reaches here the depth of 12 km.
Its SE flank involves a major P-wave velocity elevation interpreted as a high-density
magmatic body ascending up to the 4 km level and producing the most distinct gravity peak
within the profile. This magmatic structure again played an active role in the rifting process
and formation of the Carpathian foredeep, leading to the relative vertical ~10 km drop of
lower-density rocks in the Boskovice furrow..
Keywords Deep reflection profile 9HR, DRTG tomography, refraction profile CEL09,
magmatic emplacement, diapiric tectonics, contact zones in the Bohemian Massif, West
Bohemian and Central Bohemian shear zones, Boskovice furrow