Synchronous development of Type 2 and Type 3 fold interference patterns: evidence for recumbent sheath folds in the Allendale Area, Broken Hill, NSW, Australia C.J. Forbes * , P.G. Betts, G.S. Lister Australian Crustal Research Centre, School of Geosciences, Monash University, Melbourne, Australia Received 12 July 2002; received in revised form 14 March 2003; accepted 28 March 2003 Abstract Synchronous development of Type 2 and Type 3 fold interference patterns occur in the poly-deformed Broken Hill Inlier. The interference patterns have resulted from the superposition of recumbent F2 folds and , N – S-oriented upright F3 folds. The synchronous development of Type 2 and Type 3 fold interference patterns is attributed to variation in the hinge of F2 folds by as much as 908, suggesting the development of a regional-scale sheath-like fold geometry during D2. Overprinting relationships along the eastern limb of the Pap Synform suggests that it formed part of a recumbent fold hinge that was flattened during horizontal crustal shortening. This has resulted in the development of a modified Type 3 fold interference pattern. The results of this study show that the Type 2 and Type 3 fold interference patterns can develop during the same deformation event in an evolving orogen. q 2003 Elsevier Ltd. All rights reserved. Keywords: Fold interference patterns; Sheath fold; Broken Hill; Australia; Proterozoic; Orogen 1. Introduction The outcome of numerous studies of natural examples of fold interference patterns (e.g. Flin Flon, Manitoba, Canada; Stauffer and Mukherjee, 1971; Stauffer, 1988; Cantabrian Mountains, NW Spain; Julivert and Marcos, 1973), and numerical (e.g. Ghosh, 1970) and analogue (e.g. Watkinson, 1981; Ghosh et al., 1992, 1995; Grujic, 1993) models of fold superposition and the resultant geometries has been the recognition of four principle types of fold interference patterns. Ramsay (1962) and Ramsay and Huber (1987) classified the resultant geometries as Type 0 through to Type 3 interference patterns. The identification of fold interference patterns in the field provides significant insight into the shortening history and kinematic controls of deformation within studied terranes. The geometry of superimposed fold generations to produce fold interference patterns is well understood (e.g. Ramsay, 1962; Thiessen and Means, 1980; Ramsay and Huber, 1987). However, not all rocks deform in an ideal manner to produce ‘classic’ fold interference patterns, and map and outcrop patterns of fold interference patterns can easily become complex, as demonstrated by Thiessen (1986). Cross-sections through a fold interference pattern that result in the perfect representation of a Type 2 or Type 3 outcrop pattern are not always preserved. Thiessen (1986) showed that parallel cross-sections through refolded folds can produce a wide variety of patterns. Fold interference patterns that appear highly complex in outcrop can be described using simple refolding geometries, and vice versa; patterns that appear simple may be attributed to more complex refold histories. Therefore, it is important to attempt to observe the complete structure of an area and not make conclusions from observation of any one part of a structure. The Palaeoproterozoic Broken Hill Block, central western New South Wales, Australia (Fig. 1) has undergone a complex deformation history during the Olarian Orogeny (1.60 – 1.58 Ga; Page et al., 2000) and Delamerian Orogeny (c. 520 – 490 Ma; Harrison and McDougall, 1981). This has resulted in the development of various tectonic models for the evolution of this area. There has been an emphasis on 0191-8141/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0191-8141(03)00074-9 Journal of Structural Geology 26 (2004) 113–126 www.elsevier.com/locate/jsg * Corresponding author. Tel.: þ 61-3-9905-1127; fax: þ61-3-9905-5062. E-mail address: caroline@mail.earth.monash.edu.au (C.J. Forbes).