Thermo-Tectono-Stratigraphic Forward Modelling of the Upper Rhine Graben in reference to geometric balancing: Brittle crustal extension on a highly viscous mantle

Abstract

Four structural cross-sections through the central segment of the Upper Rhine Graben (URG) were balanced by means of Thermo-Tectono-Stratigraphic Forward Modelling (TTSF-Modelling). Results were compared to geometric retro-deformation of pre-rift reference horizons applying line length and area balancing methods. TTSF-Modelling with a deep necking level (> 20 km) and/or a high effective elastic thickness (Te ≥ 15 km) yielded extension values similar to those of geometric balancing, while modelling with shallower necking depths and/or lower Te yielded unrealistic high extension values. A best fit of geometric balancing, indicating 5 km rift orthogonal extension, was reached by TTSF-Modelling with a Te of 15 km and a ‘pre-rift’ necking depth of 29 km coinciding with the Moho discontinuity. This is compatible with (a) the geophysically mapped Moho that does not shallow significantly beneath the central segment of the URG and its shoulders, (b) seismicity indicating brittle–elastic deformation of the entire crust and a-seismic, ductile deformation of the lithospheric mantle, (c) compensation of crustal faults and shear zones in the crust–mantle transition zone. Modelled time-extension paths imply rifting during the Middle Eocene to Early Miocene, a Late Miocene post-rift stage and renewed rifting during the Pliocene to recent. Apparent northward migration of extension in time is an effect of uplift processes, which are not related to rifting. Correcting for these, the extension history for the four cross-sections becomes very similar, suggesting plane strain deformation and rifting at very low strain rates of about 1.7 × 10 −16 s −1 involving brittle–elastic deformation of the crust and ductile deformation of the highly viscous, high strength upper mantle that controls the position of the lithospheric necking level.