Normal faults in thrust sheets: pre-orogenic extension, post- orogenic extension, or both? Enrico Tavarnelli Centro di Geodinamica, Universita Á della Basilicata, Via Anzio, 85100 Potenza, Italy Received 28 January 1998; accepted 18 January 1999 Abstract In fold-and-thrust belts that experienced both pre-orogenic and post-orogenic extension, it may be dicult to establish whether observed normal faults pre-dated, post-dated, or were synchronous with thrusting. Geometrical structural patterns may be insucient to constrain the relative chronology of extensional and contractional deformations. The systematic use of kinematic criteria makes it possible to unequivocally de®ne the timing relationships of reverse and normal fault development, and hence to correctly unravel complex structural evolutions. Kinematic analysis in the southernmost Umbria±Marche Apennines of Italy, where both normal and thrust faults are present, revealed a history of repeated tectonic inversion, characterised by two distinct stages of extension separated by an episode of folding and thrusting. Structural overprinting relationships observed at thrust±normal fault intersections were useful for: (i) removing sequentially younger deformations; and hence (ii) separating and quantifying the eects of orogenic contraction from those of both pre-orogenic and post-orogenic extension. # 1999 Elsevier Science Ltd. All rights reserved. 1. Introduction Inversion tectonics applies to regions that experi- enced a switch in deformation regime from extension to compression (positive inversion) or from com- pression to extension (negative inversion: Williams et al., 1989). A common characteristic of inverted regions is the modi®cation of their previous architecture by newly formed structures (Butler, 1989; Hayward and Graham, 1989). The degree of modi®cation may vary greatly depending upon scale, strain rate and intensity of deformation. Should deformation occur under brittle to semi-brittle conditions, pre-existing fault sur- faces may either be truncated by, or reactivated as, younger faults (Welbon, 1988; Faccenna et al., 1995; McClay, 1995). Because of their relatively long history, orogenic belts commonly bear the signature of at least one epi- sode of positive inversion, from early passive-margin or continental rift extension to late collision (e.g. the French Western Alps: Gillcrist et al., 1987; the Spanish Pyrenees: Bond and McClay, 1995). Orogenic belts emplaced in passive-margins may also experience post- orogenic extension (e.g. the Northern Apennines: Carmignani et al., 1994; the North American Cordillera: Constenius, 1996), so that it is often di- cult to establish whether observed normal faults either pre-dated or post-dated the thrusts (O'Dea and Lister, 1995). Yet, the eects of pre-thrust extension must be separated from those of post-orogenic extension, because a correct de®nition of the age of normal faults relative to thrusts can provide important constraints for quantitative estimates in cross-section restoration. Reliable criteria are needed to enable us to do so. In multiply inverted settings, thrust sheets are often aected by hinterland-dipping normal faults, which terminate downwards against the thrust surfaces. In the absence of stratigraphic data, this geometrical pat- tern alone does not constrain the relative chronology of thrusting and normal faulting, as two distinct in- terpretations can be made: (i) thrust faults are older and are partly reactivated as low-angle normal faults (Fig. 1a); or (ii) thrust faults are younger, and propa- Journal of Structural Geology 21 (1999) 1011±1018 0191-8141/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0191-8141(99)00034-6 www.elsevier.nl/locate/jstrugeo E-mail address: tavarnelli@unibas.it (E. Tavarnelli)