Centrifuge modelling of deformation of a multi-layered sequence ...

478 Int J Earth Sci (Geol Rundsch) (2012) 101:463–482significant viscosity contrasts exist between the basementand cover. In addition, it suggests that the absence of aweak detachment zone promotes infolding. The centralHearne domain may therefore represent a juvenile stage inthe evolution of cuspate infolding.In contrast to infolding of the cover and basement, somecover sequences that contain upright folds are separatedfrom a penetratively deformed ductile basement by ashallow-dipping décollement. For example, in the SongpanGarzê fold belt of eastern Tibet upright buckle folds of aPalaeozoic sedimentary succession are separated from apenetratively deformed and locally migmatized granitoidbasement across a basal décollement of high-temperature,mid-pressure metamorphosed Devonian-Silurian slates(Calassou 1994; Xu et al. 1992; Roger et al. 2004; Harrowfieldand Wilson 2005). Folds in the cover sequenceshow axial planes orthogonal to the basal décollementwhere sheath folds indicate penetrative high shear strainductile deformation that also locally effects the basement(Roger et al. 2004: Harrowfield and Wilson 2005). Harrowfieldand Wilson (2005) suggest that this represents thetransition from a pure-shear dominated cover sequence to asimple-shear dominated ductile substrate. Models 61 and54 preserve a remarkably planar detachment between thedetachment zone (PDMS) and the underlying ductilesubstrate beneath upright folds in the cover sequence,analogous to the proposed development of the Songpan-Garzê fold belt (Harrowfield and Wilson 2005). This suggeststhe presence of a weak viscous detachment zone ofsufficient thickness will promote a planar interface betweenthe basement and the overlying basal décollement andcover sequence.Detachment foldingDetachment folding has been extensively documentedwhen a competent sequence is folded over a less-viscousductile substrate (e.g. Costa and Vendville 2002). One ofthe best examples of detachment folding is found in theJura Mountains of Switzerland where a rigid basement isseparated from Mesozoic sediments by a Triassic evaporitelayer (Fig. 12a; Buxtorf 1916). Detachment folds have alsobeen documented in the Tian Shan region of the TibetanPlateau, where Indo-Asian convergence has deformedTertiary terrestrial sediments above a basal evaporativehorizon into box-like to upright isoclinal folds (Fig. 12b;Scharer et al. 2004). Structures akin to detachment folds ina layered sedimentary sequence develop when a layer ofPDMS is present at the interface in our models (Fig. 12c).Detachment folding has been divided by Mitra (2003) intoFig. 12 Comparison of models to natural examples. a Detachmentfolds from the Jura mountains (modified from Mitra 2003 afterBuxtorf 1916). b Detachment folds from Tian Shan, China (modifiedfrom Scharer et al. 2004). c Detachment folds across the ductileinterface of Model 61. d Close-up photographs of the layeredsequence from Model 61 showing the transition from a disharmonicbox fold to a ‘lift-off’ fold (Mitra 2003) e from stages A and B,respectively. f Lift-off fold of the Weissenstein anticline in the Juramountains modified from Mitra (2003) after Buxtorf (1916)123