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Evolution of Chevron Folds by Profile Shape Changes: Comparison Between Multilayer Deformation Experiments and Folds of the Bendigo-Castlemaine Goldfields, Australia

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Evolution of Chevron Folds by Profile Shape Changes: Comparison Between Multilayer Deformation Experiments and Folds of the Bendigo-Castlemaine Goldfields, Australia TECTONOPHYSICS ELSEVIER Tectonophysics 258(1996)125-150 Evolution of chevron folds by profile shape changes: comparison between multilayer deformation experiments and folds of the Bendigo-Castlemaine goldfields, Australia T.J. Fowler, C.N. Winsor * ~ Geology Department, La Trobe Unit'ersity Bendigo, P.O. Box/99, Bendigo, Victoria 3550, Australia Received 3 February 1995; accepted 13 November 1995 Abstract The Bendigo-Castlemaine goldfields lie within the well-known chevron-folded Ordovician turbidites of Victoria, Australia. Detailed re-examination of surface and subsurface maps indicates that there are other common fold shapes (boxfolds and flat-topped folds with subsidiary hinges) which are enclosed within chevron folded layers and are traceable into them. Plasticine multilayer experiments were performed to examine the fold profile shape evolution of chevrons and associated folds. In the experiments chevrons evolved from sinusoidal folds or boxfolds. Sinusoidal folds became chevrons mainly via hinge sharpening, while boxfolds evolved into chevrons via hinge migration and fusion of the hinges. For boxfolds, hinge migration rates controlled rates of limb steepening versus median segment (i.e., the flat top of the boxfold) length reduction during bulk shortening. Periodic slowing or "jamming" of hinge migration led to stepwise) limb-dip increases, and buckling of median segments producing analogous fold styles to those seen in the Bendigo-Castlemaine folds. Limb steepening in a boxfolded multilayer must lead to dilations spanning the median segment and/or curving of boxfold axial planes. The latter dilations experience the same shape changes as their enclosing folded layers. In nature such dilation may be represented by bedding-parallel veins which are subsequently incorporated onto chevron limbs as a result of hinge migration. Thus bedding-parallel veins which are continuous over chevron hinges and are folded in the hinge zones need not be pre-folding or early-folding. 1. Introduction 1975; Honea and Johnson, 1976; Williams, 1980; Behzadi and Dubey, 1980). However, relatively few Extensive experimental work on the generation of studies have aimed to correlate observations of natu- chevron folds in viscous and elastic multilayers has ral chevron folds with experimental folds (Bayly, been reported (e.g. Ghosh, 1968; Cobbold et al., 1970; Cobbold et al., 1971; Chapple and Spang, 1971: Johnson and Ellen, 1974; Johnson and Honea, 1974; Cobbold, 1976; Johnson and Page, 1976; Dubey and Cobbold, 1977; Dubey, 1980; Stewart and Alvarez, 1991 ). The Bendigo and Castlemaine goldfields within Corresponding author. E-mail: [email protected]. the Bendigo-Ballarat Zone, Lachlan Fold Belt, Aus- latrobe.edu.au. I Present address: Mining Engineering, University of South tralia (Gray, 1988; Ramsay and Willman, 1988) (Fig. Australia, The Levels, South Australia 5095, Australia. 1) are well-known for the excellent chevron folds 0040-1951/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0040-1951(95)00191-3 126 7~J. Fowler, C.N. Winsor/Tectonophysics 258 (1996) 125-150 Bendigo-Ballarat Zone LEGEND Lower- Middle Ordoviciar I---I slates and meta-arenites Harcourt Granodiorite \ Anticlinal trace \ w' ==============================:": Steep reverse fault " '//~i"'::"::":L::":K'~" i:: , 2C 22Z2222D'D'2> 2:22; ~A Location of feature shown in Fig. 7A .,, o ~::i!iii sN I /? e "~1 ~o ^ ~/~,4/l~,J~ ~" / " ~~k,~ " v~/ I # ' - Be La 0 1 2 KM i i i E Y ;v '/ " <Ill /vTP Y v n~j.Vc h V T.J. Fowler, C.N. Winsor / Tectonophysics 258 (1996) 125 150 127 enclosed in them (e.g., the numerous examples illus- described and have implications for the debate on the trated in Hills, 1972). In these goldfields there is a syn- or pre-folding origin of bedding-parallel veins. wealth of detailed cross-sectional data, a legacy of the long history of gold mining (Dunn, 1896; Stone, 1937; Thomas, 1953). Recently, all surface expo- 2. Geological setting of the Bendigo-Castlemaine sures in these goldfields have been thoroughly chevron folds remapped (Willman, 1988; Willman and Wilkinson, 1992), providing a comprehensive database on the Folds of the Bendigo-Castlemaine region occur orientation and style variations of the regions' folds. in the Ordovician Castlemaine Supergroup (Cas and The aim of this contribution is to consider geo- VandenBerg, 1988), which consists of regularly al- metrical aspects of chevron folds in the Bendigo- ternating turbiditic quartz-rich meta-sandstone and Castlemaine region, with the assistance of plasticine dark slate beds. The folded sequence in the Bendigo multilayer models. In particular, the problem of the goldfields is at least 1450 m thick, and is represented fold profile shape evolution of chevrons is investi- by mainly amalgamated sandstones in its lower few gated. Fold shape changes during amplification have hundred metres, overlain by a mudstone-rich succes- a marked effect on a fold's strain history, with sion with some sandstone bed packages (Willman important implications for flexural-slip activity dur- and Wilkinson, 1992). In the area between Castle- ing folding -- a problem yet to be clarified (Tanner, maine and Chewton, the exposed sequence is 600 m 1989; Fitches et al., 1990) and for the origin of thick and is sandstone-rich in its lower parts, with minor structures associated with chevron folds, par- mudstones becoming more common stratigraphically ticularly bedding-parallel veins and hinge reefs, upwards. At both Bendigo and Castlemaine, the which are common hosts to gold mineralisation in thickness of individual sandstone beds ranges up to 2 these terrains (e.g., papers in Keppie et al., 1986). m (averaging 0.5 m), though amalgamated sandstone In the Bendigo-Castlemaine region, as elsewhere, units reach 20 m in aggregate thickness (Willman. the sequence of fold shapes passed through during 1988). Dark mudstones have thicknesses reaching 25 amplification is not obvious from the final chevron m (Willman, 1988), and average about 0.4 m. fold shape adopted. Some evidence for the earlier Details of the orientation and style of the chevrons shapes (preceding final chevron form) may be and associated folds of this region are given below. gleaned by examining the non-chevron shapes which The folding deformation event has been assigned to are enclosed within or are transitional into the the Middle Devonian Tabberabberan Orogeny chevrons. It is argued that at least some folds of the (VandenBerg, 1978: Gray, 1988; Willman, 1988) Bendigo-Castlemaine region involved alternative and is estimated to have involved an east-west shape change histories diverging from a common shortening of the sedimentary layers by at least 65c~ parent fold, viz. a boxfold. A simple geometrical (Talent and Thomas, 1973; VandenBerg, 1978). model is developed to calculate limb dips at each Shortening involved folding, reverse faulting and stage of shortening during the evolution of a boxfold major midcrustal detachments on eastward vergent to chevron fold. Some interesting parallels between listric thrusts (Gray and Willman, 1991a,b: Fergus- concordant auriferous quartz hinge reefs in the son et al., 1986: Gray et al., 1991b). Folding was Bendigo-Castlemaine region and experimental inter- accompanied by low temperature greenschist facies layer voids formed during fold shape changes are metamorphism and slaty cleavage formation. Fig. I. Map location of the Bendigo and Castlemaine goldfields, southeastern Australia. Locations of outcrops, detailed maps and vertical sections in Figs. 2 to 9 are shown. Fold axial traces are too numerous, therefore only a few anticlinal traces are shown. In the Bendigo goldfield they are: the Carshalton (C), New Chum (NC), Garden Gully (G), Hustler's (H) and Sawpit (SP) Anticlines. In the Castlemaine goldfield they are: the Devonshire (D), Eureka (E), West Wattle Gully (W), Specimen Hill (S) and Nimrod and Burn's Hill (N) Anticlines. Cross-sections X-X' is adapted from Gray and Will man (1991a), Y-Y' is adapted from Cox et al. (1991b). Ca = Castlemainian: Ch = Chewtonian, Be = Bendigonian; La = Lancefieldian (subdivisions of the Lower Ordovician). 128 TJ. Fowler, C.N. Winsor/ Tectonophysics 258 (1996) 125-150 Deformation and gold mineralization preceded the In both the Bendigo and Castlemaine areas fold intrusion of Late Devonian granitoids (e.g. the Har- wavelengths mainly lie in the range 200-300 m and court Granodiorite, Fig. 1, which separates the two fold amplitudes usually range between 300 and 400 goldfields). The timing of gold mineralization ap- m. Limb dips average 70 °. These data suggest typical pears to correspond with the later stages of deforma- fold arc lengths of 700-900 m. Most of the folds are tion, with gold being transported in metamorphic remarkably continuous along their hinges, individual fluids along fold- and fault-related permeable struc- folds being traceable for up to 25 km along trend tures (Ceplecha and Wall, 1976; Cox et al., 1986; (Willman, 1988). Mapped axial traces of folds are Cox et al., 1991a; Gray et al., 1991a), particularly in parallel and gently sinuous though gradual conver- fold hinge zones. gences and divergences are notable (Willman and Striated bedding slip-planes and bedding-parallel Wilkinson, 1992). In profile the axial planes are laminated quartz veins are developed on fold limbs continuous and gently to strongly sinuous to depths and are folded in hinge zones. Crack-seal growth of 1500 m or more (Stone, 1937). textures in the laminated veins have been described by Cox (1987) and Jessell et al. (1994). Quartz fibres 3.2. Details of the hinge zone of chevron folds which developed parallel to the slip vector, show variable orientation between and within veins (as Hinge zones (representing the area across the described by Tanner, 1989), but statistically are dis- hinge where curvature of layers is perceptible) are posed at nearly fight angles to the local fold axes generally less than 5 m wide and often less than 2 m (Fowler and Winsor, 1992). Individual thick lami- wide. Limb areas are characterised by usually planar nated veins have been inferred to have formed early uniformly dipping beds.
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