Upper Rhine Graben: quantitative aspects of rifting and syn-rift sedimentation with focus on the Palaeogene series in the southern part

Basin evolution in the Upper Rhine Graben (URG) was studied according to palecology,
sedimentology, cross-section balancing and numerical modelling, with focus on the genesis of
the Palaeogene series of the southern URG. Middle Eocene to Early Oligocene deposits
accumulated under restricted conditions during an under filled basin stage. Sedimentation was
controlled by differential tectonic subsidence and re-sedimentation of graben shoulder derived
clastics within the graben and involved various depositional settings. Evaporites and marls
were deposited in the depo-centre and a lacustrine to brackish facies developed in the
marginal parts of the basin, while alluvial fans formed along the border faults. During Middle
to Late Oligocene the URG was affected by supra-regional subsidence and connected to the
overfilled North Alpine Foreland Basin, as reflected by a major marine transgression. The rift
basin was converted into an over-filled open depositional system.
Palecology of Early Oligocene laminites suggest marine influenced to isolated lake settings
that experienced rapid fluctuations in salinity and lake level. Next to fluctuating Palaeo
humidity the depositional dynamics were controlled by the elevated rift shoulders forming a
barrier against external sediment/ water supply. Syn-rift subsidence was first order controlled
by the width of the graben compartments and can be explained with extensional strain
partitioning. Cross-section balancing shows that that the extension among the different rift
compartments is almost the same and amounts to about 5 km. Consequently high extensional
strain led to depo-centre formation in the narrow rift compartments, while relative low strain
and subsidence occurred in the broad rift segments. Rifting durated from the Middle Eocene
to the Early Miocene, renewed extension occurred during the Pliopleistocene. It occurred at
constant but very low strain rates (1.7*10-16s-1) and involved brittle crustal deformation on a
high viscous mantle. The necking level in the URG is located near the Moho. Under this
circumstanced crustal extension is entirely compensated by rift basin formation. This will lead
to strong static unloading equating the load of the replaced crust (Basin volume * crustal
density). Flexural Isostasy modelling shows the Recent pattern of shoulder uplift can be
explained with long term changes in crustal static loading due to URG rifting and Alpine
Orogeny and an elastic plate thickness of 15 km, however the real observed rock uplift is
higher than the modelled one. Eocene to Early Oligocene Rifting is likely to have occurred on
a wide Alpine subduction related flexural forebulge. Regional subsidence during the middle
Oligocene in the URG area might be explained with relaxation of this forebulge due to
mechanical slab failure occurring at the transition from syn-post collisional Alpine orogeny
stage. Renewed uplift that finalised Palaeogene sedimentary deposition and rifting however
corresponds to a change in stress field from extension to compression and caused Neogene
rise of the Vosges Black Forest Arc.