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Pliocene and Pleistocene. This was paralleled by slow intermittent subsidence of the
southern part of the Rhine Graben, in which Quaternary sediments were deposited.
The Ruhr Valley graben started its subsidence during the Miocene to Recent
times and its major strike direction is parallel to the present northwest-directed
maximum horizontal compressional stress
field. The Rhenish shield progressively
domed upward during Miocene till Recent times. The area was affected by volcanic
activity, but the Quaternary volcanic activity shifted to the Eifel region located,
west of Rhine, where Laacher See volcano erupted 11,000 years ago. According to
Zigler precision survey indicates that the Rhenish shield straddling the Rhine river
was uplifted at present at a rate of 0.4
0.6 mm per year, whereas in the northern part
of the Rhine Graben, subsidence rates reached a maximum of 1 mm/year between
Karlsruhe and Frankfurt. The southern parts of the Rhine Graben was however,
stable.
Ziegler (1992) concluded that the asthenosphere
-
lithosphere boundary below
the thermally uplifted region of Massif Central (France), the southern Rhine Graben
and the Rhine
-
-
Leine
-
Ruhr Valley graben triple junction, domed up to the level of
50
-
60 km below the surface and laterally descended to a depth of 80
-
100 km.
According to Zeigler (1992), the refraction and re
ection seismic and gravity data
could be interpreted either in terms of a tensional failure of lithospheric extension
model (Fig.
14.2
, top) or in terms of mantle plume model (Fig.
14.2
, bottom), both
of which should develop crustal doming and the associated gravity anomalies.
According to the mantle plume model, the crustal doming was caused by litho-
spheric extension and thermal thinning of the subcrustal lithosphere over upwelling
mantle diapirs and plumes, involving small scale mantle convection. The other
model involved tensional failure and assumed that the melts ascended diapirically
from the base of the lithosphere to M-discontinuity, where they spread out laterally,
maintaining density equilibrium which resulted in the updoming of the overlying
crust.
Telesiesmic survey however, suggested that under the southern parts of Rhine
Graben, where a low density body was suspected in the upper mantle, no important
uplift of the asthenosphere/lithosphere boundary occurred. Presence of weak upper
mantle velocity anomalies, which were aligned with the graben axis, could be
related to the intrusion of the partial melts, during the middle and late Miocene
doming stage of the southern Rhine Graben. Geophysical data of the Rhenish
Shield Dome suggested an uplift of the asthenosphere/lithosphere boundary almost
to the crust/mantle boundary. Petrological data suggested that volcanism both
within the rift zone and on the
flanks were derived from a source lying at a depth of
70
100 km, corresponding to the base of lithosphere of the Rhine Graben area. This
was in accordance with geophysical con
-
guration of the southern Rhine Graben
region. Deep re
ection seismic data showed that thinning of the lower crust started
along the
Black Forest rift dome at a considerable distance
away from the zone of upper crustal extension into the Rhine Graben. The
re
flanks of the Vosges
-
ectivity of the lower crust increased under the
flanks towards the graben margin.
Below the Rhine Graben the re
ectivity below the lower crust decreased possibly as
a result of energy absorption of the sedimentary
field of the graben. The top of the
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