Thermo-Tectono-Stratigraphic Forward Modelling of the Upper Rhine Graben in reference to geometric balancing: Brittle crustal extension on a highly viscous mantle Sebastian Hinsken a, ⁎, Stefan M. Schmalholz b , Peter A. Ziegler a , Andreas Wetzel a a Geologisch-Paläontologisches Institut, Universität Basel, Switzerland b Institute of Geology and Palaeontology, University of Lausanne, Switzerland abstract article info Article history: Received 19 August 2010 Received in revised form 6 December 2010 Accepted 23 December 2010 Available online 4 January 2011 Keywords: Upper Rhine Graben Extensional basin restoration Cross-section balancing Thermo-Tectono-Stratigraphic Forward Modelling Necking level 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 (N 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. © 2011 Elsevier B.V. All rights reserved. 1. Introduction The Upper Rhine Graben (URG) forms part of the European Cenozoic Rift System, a chain of kinematically linked extensional basins that stretches from the Mediterranean to the North Sea (Prodehl et al., 1995; Ziegler, 1992, 1994). The URG has been intensely studied and explored for hydrocarbons, minerals and geothermal energy. Numerous wells and a dense grid of 2D industrial reflection- seismic lines and gravity surveys document its structural configuration and stratigraphy (e.g., 1970; Bartz, 1974, Doebl, 1967; Doebl and Teichmüller, 1979; Durst, 1991; Lutz and Cleintuar, 1999; Rotstein et al., 2006; Wirth, 1962). In addition, the crustal and lithospheric configuration and graben evolution were deciphered (e.g. Berger et al., 2005a,b; Dèzes et al., 2004; Rotstein et al., 2006; Ziegler et al., 2004). Nonetheless, there is an ongoing debate about the evolution of the URG (e.g. Bourgeois et al., 2007; Dèzes et al., 2004, 2005; Michon and Merle, 2005; Rotstein and Schaming, 2008; Ziegler, 1994). Most authors agree that the URG evolved by passive rifting in response to the build-up of far-field intraplate compressional stresses during the Alpine orogeny (Ziegler, 1994). A two-stage model, involving Paleogene more or less orthogonal extension and Neogene sinistral transtension was advocated by Illies and Greiner (1978), Buchner (1981), Michon and Merle (2000) and Dèzes et al. (2004). In contrast Schumacher (2002) proposed a five-stage model, while Behrmann et al. (2003) advanced a model of continuous sinistral transtension. Hinsken et al. (2007) showed for the southern part of the URG that its Paleogene evolution involved pure shear orthogonal rifting with a deep level of lithospheric necking. Although numerous efforts have been made to quantify crustal extension across the URG (Table 1). This study presents for the first time the results of detailed cross-sections balancing, including back stripping of the basin fill. Pre-rift reference horizons were restored to quantify extension and combined with back stripping of the syn-rift sediments. Then the basin was retro-deformed and time-extension paths determined (e.g. Buchanan and Nieuwland, 1996; Woodward et al., 1987). Alternatively, time-extension paths can be derived from subsidence analyses of the syn-rift sediment as basin volume is related to the amount of crustal stretching (Allen and Allen, 2005; McKenzie, 1978). To address involved non-linear processes such as changes in heat flow and isostatic compensation, advanced basin restoration was Tectonophysics 509 (2011) 1–13 ⁎ Corresponding author. Present address: Statoil Petroleum ASA, Grenseveien 32, 4035 Stavanger, Norway. Tel.: +47 958 72 225. E-mail address: sehi@statoil.com (S. Hinsken). 0040-1951/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2010.12.006 Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto