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

摘要

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