| As the Lower Rhine Graben (LGR),
the Upper Rhine Graben (URG) is a branch of the
Rhine graben system which belongs to the
reactivated Cenozoic rift in western Europe. The
interaction between the rift system and the
compressive foreland of the Alps (Jura belt) and
the direction of inherited tardi-hercynian
structures make the tectonic regime of this
domain more transtensive or transpressive than
the LRG. Relief of the rift borders is still
important and the Moho topography shows an
important rise (up to 24 km) under the Kaiserstul
region. Seismic (historical and instrumental)
activity is an evidence of present day tectonic
activity. The quaternary faults
and associated morphology along the flanks of the
graben, and the values ranging from 0.8 mm/yr to
2 mm/yr vertical rate of deformation obtained
from the comparison of levelings during the last
100 years (Fourniguet 1987, Mälzer et al., 1983,
Chorowitz et al.1989) combined with the seismic
activity are the most significant elements for
recent and present-day crustal deformation.
Looking at quaternary thickness map (fig), one could
conclude to subsidence rates in a range from 0.02
mm/y to 0.2 mm/y. The main subsiding zones
(Bartz, 1972) are near Manheim (350 m quaternary
thickness), between Strasbourg and Offenburg (150
m) and East of Colmar (>225 m). This last zone
is complicated by diapirism movements.
Considering the thickness of tertiary sediments,
similar values are obtained (Manheim area, 3300
since lower oligocene : 0,1 mm/y, this subsidence
axis goes to the south under Strasbourg city -
1500 m). South of Strasbourg, the subsidence axis
crosses the graben to border of the Vosges
massif. Rates determined by geodetic measurements
are one order of magnitude higher than geological
ones.
Tertiary and Quaternary thickness
maps indicate most favourables zones of highest
rate of subsidence for recent period.
Based on the main geological structures and
Cenozoic subsidence data, the Upper Rhine graben
can be divided into several tectonic units:
- Borders faults (Cenozoic fault system)
limiting Vosges massif to the West and
the Schwartzwald to the East. The Saverne
Area shows a large indentation of this
fault system (the "champs de
fracture de Saverne"). The graben is
assymetric; to the north, the subsiding
zone is located against the eastern fault
system (Karlsruhe-Manheim area), to the
south, the most subsiding zone is located
against the Vosges border faults
(Colmar-Thann area).
- The inner graben fault system with a
presupposed recent activity (from
geomorphological studies ; Vogt ).
Important diapyrism occur in the south of
this zone, certainly interfearing with
active tectonic.
- The Mulhouse-basel transition zone
(between transtensive tectonic to the
north and transpressive to the south)
wich is characterized by large ondulation
of topography and thin quaternary
deposits.
- The Jura (thrust) front organized in
multiple E-W active quaternary folds
(Meyer et al, 1994, Report IPSN-EOPGS 97,
Report GSC 97) separated by N020 transfer
left lateral ramps.
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