Discrete element modelling of the influence of cover strength on basement-involved fault-propagation folding Stuart Hardy a,b, ⁎ , Emma Finch c,1 a ICREA (Institució Catalana de Recerca i Estudis Avançats), Spain b GGAC, Facultat de Geologia, Universitat de Barcelona, C/ Martí i Franquès s/n, 08028 Barcelona, Catalonia, Spain c Basin and Stratigraphic Studies Group, Department of Earth Sciences, University of Manchester, Manchester M13 9PL, U.K. Received 26 August 2004; received in revised form 12 January 2006; accepted 18 January 2006 Abstract A discrete element model is used to investigate the influence of sedimentary cover strength on the development of basement- involved fault-propagation folds. We find that uniformly weak cover best promotes the development of classical, trishear-like fault- related folds showing marked anticlinal thinning and synclinal thickening, with cover dips increasing downwards towards the fault tip. Uniformly strong cover results in more rounded fold forms with only minor hinge thickening/thinning and significant basement fault-propagation into the sedimentary cover. Heterogeneous, layered, cover sequences with marked differences in strength promote the development of more complex and variable fold forms, with a close juxtaposition of brittle and macroscopically ductile features, which diverge from the predictions of simple kinematic models. In these structures the upper layers are often poor indicators of deeper structure. In addition, we find that in layered cover sequences fault-propagation into the cover is a complex process and is strongly buffered by the weaker cover units. © 2006 Elsevier B.V. All rights reserved. Keywords: Folding; Faulting; Trishear; Modelling; Basement 1. Introduction Fault-propagation folds, and their associated blind faults, have recently been recognized as extremely important for their seismic hazard potential (e.g. Shaw and Shearer, 1999; Allmendinger and Shaw, 2000) and for their importance in controlling stratigraphic archi- tectures in sedimentary basins (e.g. Ford et al., 1997; Gawthorpe et al., 1997). They are also the location of many oil and gas traps (e.g. Mitra and Mount, 1998). Where a faulted, rigid basement is involved in the deformation, the folds are sometimes called “drape” or “forced” folds (Stearns, 1978; Fig. 1a,b) where the overall shape and trend are dominated by the forcing (basement) member below, in contrast to other fault- related folds which are the result of fault movement within the cover rocks. Here we refer to such structures simply as basement-involved fault-propagation folds. Evidence from well-exposed folds in the Laramide orogen (e.g. Erslev and Mayborn, 1997), the Bighorn Tectonophysics 415 (2006) 225 – 238 www.elsevier.com/locate/tecto ⁎ Corresponding author. ICREA (Institució Catalana de Recerca i Estudis Avançats), Universitat de Barcelona, C/ Martí i Franquès s/n, 08028 Barcelona, Catalonia, Spain. Fax: +34 934021340. E-mail addresses: stuart.hardy@icrea.es (S. Hardy), Emma.Finch@man.ac.uk (E. Finch). 1 Fax: +44 1612753947. 0040-1951/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2006.01.002