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 '/ "
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. Typically, limbs lack meso- during, or before folding (Jessell et al., 1994). scopic folds, except for minor centimetre-scale asymmetric folds developed in very thin sandstone layers embedded in mudstone layers. A spaced con- 3. Description of the macroscopic folds in the vergent fanning stripy cleavage is developed in sand- Bendigo-Castlemaine goldfields stones, particularly in the inner arc of fold hinges. This is overprinted in the sandstones by a weak grain 3.1. Orientation and dimensions alignment axial plane cleavage (Yang and Gray, 1994). A divergent finely-spaced slaty or crenulation cleavage occurs in the slates. The Bendigo folds generally trend N15W (Fig. 1), Dip isogon patterns in the fold hinge zones resem- have steeply dipping (> 75°E) axial planes and ble Classes IB and IC (Ramsay, 1967, p. 365) for plunge gently to the north or south (< 20 °) defining sandstone layers (with minor complications due to subtle domal structures especially in the centre of the primary bed thickness variations) (see Fig. 2A,B - field (Dunn, 1896; Stone, 1937; Willman, 1988). location Fig. 1). In the hinge zones, dip isogon Fold interlimb angles are 40 ° on average, i.e. signifi- patterns are approximately Class 2 or 3 (Fig. 2C - cantly smaller than the ideal 60 ° expected for mature location Fig. 1). Usually hinge zones consist of chevron folds (de Sitter, 1958; Ramsay, 1974). single hinge angular or rounded folds, the former The Castlemaine chevron folds trend close to more common in slate beds, the latter in sandstone north-south (Fig. 1), have upright to 80°W dipping beds (Wilkinson, 1988). There are occasional para- axial planes, plunge typically north at 0-25 ° (excep- sitic mesoscopic folds in thin sandstone units in the tionally reaching 50 ° plunges) and have fold inter- hinge zone. limb angles of 20-40 ° (Baragwanath, 1903; Cox et al., 199 lb). There is a progressive tightening of folds eastwards in both goldfields, with axial planes also 3.3. Boxfold and other non-chevron .fold s~les becoming less steeply inclined to the west in that direction (Cox et al., 1991b; Gray and Willman, Vertical cross-sections of the goldfields (Fig. 1, 1991a). and Fig. VIII-34 of Hills, 1972, p. 246) have repre- T.J. Fowler, C.N. Winsor / Tectonophysics 258 (I 996) 125 150 129
A 3O 0
o 1o ze S 20 10 30 35 25 ~0
, I 55 60
6° 0
65~ '~ v._._.--.~65
L
Fig. 2. Fold profiles from the Bendigo (A) and Castlemaine (B and C) goldfields showing constructed dip isogons (small numbers represent dip isogon in degrees). Locations are shown in Fig. 1. Scale bar in all cases is 30 cm long. (A) Folded sandstone layers. The lowest layer shows Class 1C dip isogon patterns. The upper two layers are affected by primary layer thickness variations but appear to approximate Class 2 dip isogons. (B) Folded sandstone layer showing Class IB dip isogon pattern. (C) Folded slate layer showing Class 2 (and Class 3) dip isogon patterns. sented the folds as chevrons with ideal similar style. gentle subsidiary folds, (2) boxfolds, (3) anticlines However, the simple chevrons shown in Fig. 1 must with subsidiary synclines (m-folds) or synclines with be diagrammatic, since in transverse mine sections subsidiary anticlines (w-folds), (4) folds with anoma- and surface exposures many of the individual folded lous limb curvatures, and (5) folds with strongly layers are seen to depart considerably from both sinuous axial planes. Examples of (1)-(5) are usu- chevron shape and similar style, as detailed below. ally enclosed within and traceable into regular A few of the non-chevron folds appear to be chevrons along their axial planes, and do not corre- bulbous hinge folds that accommodate unusually late with layers of anomalous thickness. The terms thick individual competent and incompetent layers used to describe the above non-chevron folds are (Ramsay and Huber, 1987; Wilkinson, 1988). Also presented in Fig. 3 and characteristics of examples of some fold shapes have been subsequently strongly the non-chevron fold hinge zones are described be- modified by late-stage limb thrusting (Chace, 1949; low. Willman, 1988). Such folds are not considered fur- An anticline with steeply dipping planar limbs ther. Apart from the latter, there are numerous exam- and a flat 100 m wide gently folded hinge zone is ples of (I) folds with broad flat tops, sometimes with shown in Fig. 4A (location Fig. 1) from the Castle- 130 T.J. Fowler, C.N. Winsor / Tectonophysics 258 (1996 125 150
A B
PROJECTED .------INTERLIMB -' ~-----~... ," ~,~.,..~ ANGLE , BOXFOLD • ' /""-"3", SUBSIDIARY FOLD AXIAL ,' ', ,' ~, INTERLIMB ANGLE PLANE ~ , , // /' /. . ',, \ ," MEDIAN ', / BOXFOLD / ._ ¢....--~- \ " SEGMENT"] ', ~ HINGE ,' - - " REFLEX L . ANGLE /L3./ ''\\ FOLD l t f / \\
LIMB DIP AXIAL PLANE DIP Fig. 3. Sketch of a boxfold (A) and a flat-topped fold with subsidiary folds (B), showing the terms used in this paper to describe various physical and angular elements of the non-chevron folds. maine railway-cutting. The subsidiary folds have Steeply dipping cleavage is well-developed through- interlimb angles of 105 ° or more and are approxi- out the broad hinge zone, but is weak or absent on mately cylindrical (Fig. 4B). The subsidiary folds the macroscopic fold limbs. plunge from 20 ° north to 35 ° south (Fig. 4C). The An anticlinal fold with subsidiary syncline (m- projected interlimb angle is estimated to be only 35 °. fold) is shown in Fig. 5A (location Fig. 1). The two
A
METRES o 20 40 80 so lOO I i I J I ~ I t I i I
B C
o POLE TO S O POLE TO S~ x FI AXIS a ]]'-AXIS (S o POLES)
Fig. 4. Broad flat-topped fold with open subsidiary folds from railway cutting east of Castlemaine (see Fig. I for location). (A) Sketch of fold profile from photographs. Bold lines are bedding planes, lighter steep lines are cleavages, F = fault. (B) and (C) Stereographic project ion data (Schmidt net) of beds (S0), axial plane slaty cleavages (S]) and fold axes (F]) collected from this fold. T.J. Fowler, C.N. Winsor / Tectonophysics 258 (1996) 125-150 131 subsidiary anticlines have interlimb angles of 60 ° angles (105-150 °) than the projected interlimb angle and 95 ° and the intervening syncline has a 70 ° (36°). The subsidiary folds are cylindrical, and coax- interlimb angle. The projected interlimb angle is ial with the line of intersection of the projected significantly smaller (43°). These subsidiary folds are planar fold limbs (Fig. 5D). Cleavage is well-devel- cylindrical and coaxial with the line of intersection oped in the hinge zone. of the projected planar fold limbs (Fig. 5B). A A boxfold is presented in Fig. 6 (location Fig. 1). synclinal fold with subsidiary anticline (w-fold) is The median segment (up to 55 m in width) shows presented in Fig. 5C (location Fig. 1). Again the gentle subsidiary folds with typical interlimb angle subsidiary folds have significantly larger interlimb 130 °. while the projected interlimb angle is 53 °. The
B
D
Fig. 5. (A) Anticline with subsidiary syncline (m-fold), from Forest Creek, Chewton (see Fig. 1 for location). (B) Schmidt net stereographic projection of structural data from (A). See Fig. 4 for symbols. (C) Syncline with very gentle subsidiary anticline (w-fold), from Expedition Pass (see Fig. 1 for location). (D) Schmidt net stereographic projection of structural data from (C). See Fig. 4 for symbols. In (A) and (C), bedding planes are outlined by white cord. Small black and white scale bar is 30 cm long. 132 TJ. Fowler, C.N. Winsor / Tectonophysics 258 (1996) 125-150
~32 I
H
~ 56
40 ~ 42 \
,~ 146 metres 51 "" ,~t ".. 0 10 20 30 40 50 I I I I i I •.., •,.. )/", 37
"~==~' "~ / ". \\
39 /
o //o O~T
:" SHAFT0 " ~
' 0,,, \
.. ~ eo / ', \ \ \ \ ° POLE TO SO \14 • POLE TO St ~ AXiS o ~-AXIS (So POLES) T.J. Fowler, C.N. Winsor/Tectonophysics 258 (1996) 125-150 133 subsidiary fold hinges plunge to the north at about Fig. 8A (at depth 410 m). A thick concordant quartz 40 ° and these folds are roughly cylindrical though reef wraps over the fold hinge at the 10 Level in Fig. there is a possibility of conical geometry (Fig. 6). 8B. The boxfold shape passes southwards into rounded Box folded layers passing downwards into m- and boxfolded layers (with median segment much folded layers (with concordant hinge reefs) and then reduced to 20 m) before passing into a chevron fold into chevron folded layers are shown in Fig. 9A from with an interlimb angle of 43 °. Along the boxfold the Confidence Extended Mine. Note the large hinge eastern hinge, layers may be traced northwards into reef associated with the first fault in this figure. The another chevron fold with an interlimb angle of 66 °. chevron tblded layers of the Garden Gully Anticline Surface structural maps, mine level plans and in Fig. 9B are overlain by flat-topped folded layers detailed mine sections have provided numerous ex- with subsidiary folds, and above these by more amples of non-chevron folds equivalent to those regular but open folds. This fold shape transition is observed in surface exposures. Examples of bifurcat- similar to that seen in Fig. 6. ing fold axial traces which are shown in Fig. 7 Thomas (1953, Fig. 5) shows an example of a (locations Fig. 1) are abstracted from Willman and "double saddle" reef bound by m-tblded layers Wilkinson (1992) and Willman (1995). The features from the New Chum Anticline, Bendigo. Cox et al. shown in Fig. 7 include: (1991b) briefly noted the existence of folds in the (1) an anticline-syncline pair branching from a Castlemaine goldfield, with broad anticlinal hinge major anticline (Fig. 7A) zones transitional along strike into boxfolds and (2) bifurcation and rejoining of the axial plane of chevrons and passing vertically into chevron shapes. an anticline or syncline (Fig. 7B, C, F). The process of axial plane bifurcation may repeat (Fig. 7D). These locally bifurcated folds have been referred to 4. Experimental plasticine multilayer folds as "double folds" (Herman, 1923). The fold axial bifurcations are mapped equivalents of the m- and Plasticine (modelling clay) multilayer deformation w-folds described above. experiments were performed to examine the stages (3) two en echelon anticlines connected by a of development of chevron folds. Multilayers were syncline (or synclines and a connecting anticline) composed of alternating layers of stiff white plas- (Fig. 7E). ticine ("Rainbow", manufactured by Newbound, Numerous mine cross-sections reveal non-chevron Australia) and soft grey or coloured plasticine fold shapes similar to those described from surface ("Plastiboy" by Micador, France). The stiff white exposures. Typical examples are shown in Figs. 8 plasticine had a viscosity of about 4 X 10 ~ Pa.s. and 9 (location Fig. 1). Chevron folded layers en- while the soft plasticine had a viscosity of 5 x 107 closing m-folded layers are shown in Fig. 8A,B, Pa. s (both measured over shortening strains up to from the South New Moon Mine and Hustler's Reef 20%, with a rate of about 10 4 s ~ at 18°C i.e. room Mine. Note that strong sinuosity of the axial plane is temperature). This gave a layer viscosity contrast of associated with hinge bifurcation in Fig. 8B and that about 8. The layers were rolled on wet glass with a in this figure the subsidiary anticlines have interlimb marble rolling pin to the minimum practical thick- angles exceeding 70 ° (and the intervening syncline ness, which was 0.8 mm on average. Multilayer has an interlimb angle of l l0°), while the projected sheets were initially 12 cm X 10 cm (to fit a press interlimb angle is 40 °. A flat hinge reef of vein of similar dimensions) and were composed of 11 to quartz has formed concordant to the folded layer in 31 individual layers, with lightly gear-oil-brushed
Fig. 6. Boxfolded sandstone layers (stippled) near the Daphne shaft, 3 km south of Chewton (see Fig. 1 for location). Dips and strikes of beds and plunge of fold axes are shown. The boxfold passes south into a chevron fold, and northwards its eastern hinge becomes a chevron fold identified as the West Wattle Gully Anticline. Stereonets (Schmidt) show structural data collected from this area. See Fig. 4 for symbols. See text for discussion. 134 T.J. Fowler, C.N. Winsor/Tectonophysics 258 (1996) 125-150
B C D N N o~ J / N
Confidence~.. Extended ~.
0.5K IOll O ii'LKennington E ~ Reservoir
0.5 KM ~ Exhibition 0 0,5
rgetic
.o
Castlemaine I 0
~ ~ X I " 1. 0 KM 0.5 Hustlers Chewton KM T.J. Fowler. C.N. Winsor/ Tectonophysics 258 (1996~ 125 150 135 interfaces. The multilayer stacks were embedded in 3 band (monoclinal fold) (Fig. l lE,F). Kink bands cm thick slabs of (stiff) putty-plasticine mixture. either transformed to asymmetric rounded folds The stiffness of the embedding material was in- which then became chevrons (by the same mecha- tended to decrease the tendency of these models to nism described above), or later transformed to box- develop anticlinorial structures. The multilayer was folds. Both of these transitions could sometimes be compressed parallel to the layers between two thick seen in different layers of the same fold (Fig. 1 IG,H). polished lubricated steel platterns. Layers were short- In the normal course of transformation from box- ened to 50-60% of their original length at rates of fold shape to chevron, the boxfold median segment approximately 10-4 s i, under the same conditions shortened via hinge migration, with simultaneous used to determine the viscosity of the plasticine (i.e., limb growth, and usually limb steepening (Fig. 12A- room temperature, 18°C). Minor extension (about D). When boxfold reflex angles (see terms in Fig. 3) 5%) occurred parallel to fold hinges during the de- decreased to a critical angle (120°). instead of hinge formation. Photographs were taken throughout each migration continuing to fusion, the short remnant experiment to record fold shape changes. Examples median segment arched upwards in a halt-wave- of the experiments which produced chevron folds are length rounded fold, which then sharpened to form a shown in Fig. 10. Note that numerous other fold chevron. Chevrons usually appeared in the core of features (e.g. cuspate and bulbous hinge folds (Fig. the folds by 20c~: shortening. These transitions are in 10A, B, C): m-folds and boxfolds (Fig. 10B); strong accord with experiments by Johnson and Honea axial plane sinuosity (Fig. 10A) are evident in these (1975), who recorded sinusoidal and "concentric" models. boxfolds to chevrons in multilayers where layer con- tacts possessed finite shear strength. 4. I. Development o/' cherron .fi~lds in the multilavers During sharpening of the arched hinge segment, the point of sharpening occurred in different layers at Chevron folds were generated from several differ- different locations in the hinge zone. This resulted in ent parent fold shapes (Figs. 11 and 12). When interlayer detachments at the hinge, sometimes pro- interlayers were well-brushed with oil (providing ducing asymmetric hinge voids (Fig. 12E-L). Such moderately easy slip on layers), rounded sinusoidal voids also formed if a boxfolded layer showed me- fold shapes developed by 6% shortening (Fig. dian segment arching earlier than adjacent layers. l lA,B). The cores of these folds transformed to and where limbs steepened without compensating chevrons by 18% shortening by sharpening of the change in the dip of the boxfold axial plane. Strongly hinge and straightening of the limbs. As the sharpen- sinuous axial planes also formed as a result of these ing occurred in different layers at different stages, variations in hinge sharpening history. arcuate hinge voids were generated between the chevron folded layer and the rounded layer below it 4.2. Limb and median segment chan~es during box- (Fig. 11C,D). fold transfi~rmation to chet:rons Experiments where the oil applied to the inter- faces was more sparing than for those described The progressive changes in limb and median seg- above (hence interlayer slip was more difficult), ment lengths and limb dips, for some folded layers in generated very low amplitude open rounded sinu- the multilayer experiments, were measured on pho- soidal folds by 5% shortening. As shortening contin- tographs (enlarged 200%) at various stages in the ued one of these folds usually amplified faster than evolution of chevrons from boxfolds. Two represen- the others and adopted either a roughly symmetrical tative examples are the 6th and 8th dark layers boxfold shape (Fig. 12A,B) or developed into a kink (counted downwards from the top of the multilayer)
Fig. 7. Fold axial plane traces from the Bendigo and Castlemaine 1:10,000 maps (Willman and Wilkinson, 1992: Willman, 1995), see Fig. 1 lk)r locations. (A) Branching anticline-syncline pair. (B), (C), (D) and (F). Bifurcating axial traces. (E) En echelon synclines with connecting anticline. Named squares are mine shafts. See text for discussion. 136 T.J. Fowler, C.N. Winsor/Tectonophysics 258 (1996) 125-150 in Fig. 12E-L. The measured quantities are plotted by limb dips of 50 °, after which steepening of limbs against bulk shortening (e%, measured as shown in occurs by pure shear strain. Fig. 18) in Fig. 13. Generally, between limb dips of The data presented in Fig. 13, indicate that: 9 and 60 °, the arc length of the box folds was found (1) Median segment length reduction appears to not to significantly change. Interlayer slip activity be roughly linearly related to bulk shortening e had ceased by the time limbs reached 50-60 ° dips (though the decrease is commonly stepwise). The (Fig. 12A-D). The experiments of Behzadi and process of hinge migration here is analogous to Dubey (1980) also confirm that slip apparently ceases mode 2 (mobile-hinge) growth of kink bands as
A B
Sth New Moon Mine ~x Hustler's Reef Mine __F Garden Gully Anticline ~ Hustler's Anticline 47, \ \
m 100
10
50
x
m i 100
.50
I
Fig. 8. Vertical approximately east-west cross-sections, looking north (Bendigo Goldfield--see Fig. 1 for locations). In both (A) and (B). the dark layers and lenses represent significant quartz hinge reefs. (A) Garden Gully Anticline at the South New Moon Mine. The folded layers pass from a chevron shape at depth of 520 m upwards into a broad hinge zone with flat hinge reef, then into an m-fold, then back into a chevron beneath the fault at about a depth of 320m. (B) Hustler's Anticline at Hustler's Reef Mine. Numbers along the main shaft represent the drive levels. The 10 and 15 Levels are at depths of 270 m and 402 m respectively below the sill of the shaft. Note the m-folded layer 50 m above the 15 Level, which passes upwards and downwards into chevron folded strata. (A) is from the South New Moon Transverse Section, Bendigo Office of the Geological Survey of Victoria, Victorian Department of Mines (undated). (B) is from Whitelaw (1914), plate XVIII). TJ. Fowler, C.N. Winsor / Tectonophysics 258 ~1996) 125-150 137 described by Weiss (1980). Hinge migration had all (5) asymmetric limb lengths and limb dips (Fig. but ceased by the attainment of 120 ° reflex angle. 13A,B), (2) Erratic jamming (i.e., interruption of hinge (6) stepwise increases in limb lengths (e.g., at migration) or slowing, and unjamming of either or -e = 7%,, 22%, 30% for the right limb. Fig. 13A: at both boxfold hinges occurs at several stages (perhaps -e = 12%, 27% for right and left limbs, Fig. 13B), due to unintended layer thickness variations). and stepwise increases in limb dip (e.g., at - e = 7%, (3) variable inclination of median hinge segments Fig. 13A). Both are compensation for non-uniform to the shortening direction (as a result of rotation of reduction in median segment length during continued the median segment around one jammed (or less shortening, mobile) hinge), (7) rare temporary limb dip decrease during pro- (4) stepwise decreases in median segment length gressive shortening (in the unusual circumstances (Fig. 13B) (as a result of non-uniform hinge migra- where hinge migration rates exceeded those neces- tion rates), sary to maintain limb dips during shortening).
A B
Confidence Extended Mine Garden Gully Anticline I ,2A i \\1/ ,,,a----~,.L~q \ / \'i/~ T .'IN " /F v" ,jii .0 \ I "50 I -100 • 100 .,5o " ~ bl
• 150
- 200 -200 I/~ 25O 25O m m Golden Age Mine Garden Gully Anticline
Fig. 9. Vertical approximately east west cross-sections of the Garden Gully Anticline, looking north• from the Bendigo Goldfield (see Fig. 1 for locations and Fig. 7 for (A). In both (A) and (B) the dark layers and areas represent significant quartz hinge reefs. (A) Confidence Extended Mine showing transition of folded layers from chevron shape into m-l-bid, then rounded hinge fold, then boxfold. Note the peculiar hinge reefs. (B) Golden Age Mine showing transition of folded layers from chevron shape upwards into flat topped lblds with subsidiary folds. (A) and (B) are from Transverse Sections Nos. 220.15 and 212A-1, respectively, Bendigo Office of the Geological Survey of Victoria, Victorian Department of Mines (undated). 138 T.J. Fowler, C.N. Winsor/ Tectonophysics 258 (1996) 125-150
4.3. Hinge "jamming" during boxfold hinge migra- The transformation to chevron shape, as described tion above, was usually aborted if hinge jamming became permanent. However in the case where one of the As noted above in point (2), hinge migration was boxfold hinges jammed, the entire median segment interrupted or slowed at some stages of shortening. could be rotated around the jammed hinge and be
Fig. 10. Examples of plasticine multilayer experiments yielding chevron folds. White layers are stiff plasticine, darker layers are soft plasticine. Embedding medium is a stiff plasticine-putty mixture. In all cases the layer interfaces are very lightly brushed with oil. (A) Note sinuosity of axial planes. Bottom edge is 4 cm long. (B) Note the transition in chevron fold (right of centre) upwards into boxfold and m-fold. Chevron on the right edge shows features of asymmetric chevrons formed by fusion of median segment and one limb, following jamming of one boxfold hinge. Also note bulbous hinge features. Bottom edge is 4.2 cm long. (C) Note large interlimb angle of chevrons on the right. The latter folds formed by sharpening of hinges of earlier rounded sinusoidal folds. Bottom edge is 5.5 cm long. T.J. Fowler, C.N. Winsor/ Tectonophysics 258 (1996) 125 150 139 incorporated onto one fold limb yielding an asym- showed median segment reduction (as described metric chevron. The second (mobile) boxfold hinge above), but not at a rate sufficient to mitigate buckle was represented as a local curvature maximum on shortening of the median segment leading to the this opposing limb (Fig. 10B). Similar features are formation of open rounded subsidiary folds (Fig. shown in experiments by Ghosh (1968). 10B). The subsidiary folds maintained large inter- limb angles (typically > 90 °) even at large values of 4.4. Unsuccessful transitions from boxfi)lds to e. Similar features are shown in multilayer experi- c'hel,rons ments by Ghosh (1968). The boxfold outer layers showed shortening behaviour analogous to mode 1 Whereas initial sinusoidal folds successfully kink amplification (Weiss, 1980) which involves ro- transformed to chevrons at larger values of overall tation of the kink axial plane. With mode 2 dominat- shortening, some initial kink/boxfolds did not. These ing in the boxfold core and mode 1 dominating in latter initial fold shapes gave rise instead to residual the outer layers of the same fold, curved kink axial boxfolds, or boxfolds with buckled median seg- planes will be the result. Weiss (1980) regarded ments, especially those showing m-folded (or w- modes 1 and 2 as contemporary competing mecha- folded) hinge zones, as described below. nisms. The delayed onset of mode 2 in outer layers While inner layers of a boxfold generally success- may be due to the initially lower limb dips in outer lully transformed to chevrons, the overlying layers layers. sometimes failed to do so. The outer layers initially Occasionally inner boxfolded layers failed to
Eiiii ii iiiiii iiiiiii iii iiiiiiiii i!! iii iii iiiiiiiiiiiiiiiiiiiii iiiiiiiiiii i iiiii i ¸¸ 13 H
~ A ~"
w
Fig. I I. (A)-(D) Development of chevron fold shapes from initial sinusoidal folds in mullilayer with layer interfaces well-brushed with oil. leading to moderately easy interlayer slip. (E)-(H) Development of a kink band which changes to a sinusoidal fold in its lower layers presumably due to easier slip, but transforms to a boxfold in its upper layers. Numerous interlayer voids have formed by the stage shown in (H), to accommodate the different fold shape change hislories, which nevertheless are ultimately assembled within one anticlinal fold. 140 T.J. Fowler, C.N. Winsor / Tectonophysics 258 (199¢5) 125-150
Fig. 12. (A)-(D) Progressive changes from boxfold shape to chevron. Note the formation of hinge voids associated with arching of the median segment and voids at boxfold hinges. Successful chevron transition has occurred in the inner layers. (E)-(L) Hinge migration in boxfold during layer shortening. Note the asymmetric boxfolded layer in the 8th white layer from the top. Also note the m-folded innermost layers and the median segment-wide void (truncated at the right edge of the photo), and the history of development of the synclines (involving axial plane rotation) adjacent to the boxfold anticline in the centre. Layer interfaces were very lightly brushed with oil. leading to limited ease of interlayer slip.
transform to chevrons, forming, e.g., m-folds, in- (see Discussion). Apart from the immediately overly- stead (Fig. 12). In these cases the reason appeared to ing layers, in most cases chevron shapes were at- be either hinge jamming, or crumpling of the core tained in the layers above the m-folded core.
Fig. 13. Measurements of limb length, limb dip, median segment length and dip tor experimental boxfolded layers transforming to chevrons, plotted against bulk shortening e% (as defined in Fig. 18). Note the stepwise increases in limb dip and length. (A) and (B) 6th and 8th grey layers respectively, from the top of the multilayer in Fig. 12E L (data points are labelled here E-L, each letter corresponding with photographs in Fig. 12). Measurements were made on photo-enlargements. R = right limb, L = left limb. T J. Fowler, C.N. Winsor/Tectonophysics 258 (1996) 125-150
24 24 l LIMB LENGTH t LIMB LENGTH~ R 22 22
20 20
18 18
16 I J 16 22
14 i" 20 14 20 G
12 I J 18
.5 ~0 16 10
8 14 ~
g 12 6 12
lO 4 10
8 2
10 20 30 40 50 10 20 30 40 50 -e% -e%
LIMB DIP LIMB DIP
70 ~ 70'
60 ~ 70 60' 70 ° H l 60 ° =3 50 G ~ K 50' J
40 ~ 50° ~ 40' 50 ° ,.-
40 ° E: 30 ~ t40 ° E: ¢u 20 < 30 ° 30'20 ° I 30 °
10 c 420 ° 10 ° 20 °
10 20 30 40 50 10 20 30 40 50 -e% -e% LENGTH 24 E 24 ~"'~L MEDIANSEGMENT MEDIAN SEGMENT 22 22 LENGTH LENGTH & DiP [ LENGTH & DIP 20 20
18 18
II 16 16
14 14 t J • 10 ° 12
10 DIP I K +5 ° 10 .10 ° [ F ~ j o o. 8 H 0 ~ 8 .5 °
6 -5 ° 6
o o 4 -10 4 -5
2 2 _1o °
10 20 30 40 50 10 20 30 40 -e% -e % A B 142 T.J. Fowler, C.N. Winsor / Tectonophysics 258 (1996) 125-150
5. Discussion Faill (1973) and Stewart and Alvarez (1991) con- cluded that the natural chevrons they studied were 5.1. Comparison ~f experimental cherron .folds with formed by various processes of kink hinge fusion, the Bendigo-Castlemaine folds but not by the kink interference mechanism de- scribed by Patterson and Weiss (1966). Multilayer deformation models and experiments In contrast, in the multilayer folding model E of producing chevron folds provide clues to important Ramsay and Huber (1987, p. 418)(with high viscos- details of the mechanism responsible for the devel- ity contrast, moderate ratio of incompetent to compe- opment of these folds, including their nucleation, tent layer thickness), the chevron fold shape was propagation and amplification rates and processes evident from moderate shortening values onwards (Cobbold, 1976; Dubey and Cobbold, 1977; Dubey, (similar to the model of Ramsay, 1974). Ramsay and 1980), wavelength controls (Biot, 1964; Johnson and Huber's (1987) model F (with high viscosity con- Honea, 1975; Williams, 1980), interlayer slip activ- trast, low ratio of incompetent to competent layer ity (de Sitter, 1958; Ramsay, 1967, 1974; Chapple thickness), produced asymmetric chevrons from con- and Spang, 1974; Behzadi and Dubey, 1980), and jugate folds via the mechanism of kink band interfer- shape changes during amplification (Bayly, 1974; ence described by Patterson and Weiss (1966). De- Johnson and Ellen, 1974; Dubey and Cobbold, 1977; spite our model parameters being relevant to model Williams, 1980; Stewart and Alvarez, 1991). Experi- E (and possibly model F), none of our experimental mental folds which transformed from rounded con- chevrons showed these evolutionary paths. Ghosh centric or conjugate/boxfold shapes to chevron (1968) and Honea and Johnson (1976) have empha- shapes have been reported by Ghosh (1968), Bayly sised ease of slip on layers as a control on evolution (1970), Cobbold et al. (1971), Johnson and Ellen of fold shapes during amplification. The last three (1974), Johnson and Honea (1975), Honea and John- authors found that early formed folds were sinu- son (1976), Johnson and Page (1976), Dubey and soidal if interlayer slip was moderately easy, while Cobbold (1977) and Stewart and Alvarez (1991). boxfolds formed first if interlayer slip was more difficult or delayed. Honea and Johnson (1976) noted that both sinusoidal and boxfold shapes may form in the same multilayer perhaps at intermediate condi- tions of interlayer shear strength. Ghosh's (1968) multilayer experiments produced chevrons from boxfolds by hinge fusion, but also generated m-folds from boxfolds when hinge migra- tion was aborted. We suspect that the non-chevron folds in the Bendigo-Castlemaine goldfields have evolved from parent boxfolds. Since these are transi- tional into regular chevrons we argue that at least some of the chevron folds in this field have evolved from boxfolds. The remainder of our discussion con- centrates on the particulars of boxfold to chevron e/--- x transformations. In drawing conclusions we ac- knowledge other possible (but probably subordinate) Fig. 14. Representation of the various alternative shape change mechanisms for the development of multi-hinge folds histories for a single boxfolded layer during bulk shortening. (A) in the Bendigo-Castlemaine region. These alterna- hinge migration and fusion to produce a chevron. (B) buckling of tives include: the median segment forming an m-fold. (C) buckling of the (a) anomalous span folding (Watkinson, 1976) median segment forming a flat-topped fold with gentle subsidiary which forms as a result of out-of-phase encounters folds. (D) asymmetric chevron formed by the fusion of the median segment with one fold limb. (E) rigid median segment with between laterally propagating fold systems, or folds shortening accommodated by limb steepening. longitudinally propagating towards each other but T.J. Fowler, C.N. Winsor / Tectonophysics 258 (1996) 125 150 143 maintaining a lateral offset at closest approach of the form of earlier hinge-related features (fractures less than a half-wavelength (Dubey and Cobbold, and cleavages) which lie oblique to and are lblded 1977). Such a history may yield features shown in around the migrated hinge. The hinge migration in Fig. 7E, but cannot explain the axial plane bifurca- the model of Ramberg (1964) for the origin of tion and rejoining featured in Fig. 7B,C,F. Anoma- asymmetric parasitic folds, and kinkband hinge mi- lous span tblds may show m-folded hinges, however gration during growth of kink bands (Weiss, 1980) it is unlikely that such folds would be commonly leave little or no evidence for mobility of hinges traceable along their axial (or median) planes into [though Stubley (1989) may provide an exception to regular chevron folds, as is usually observed in the this]. Stewart and Alvarez (1991) pictured kink fold Bendigo goldfield. Additionally the arclengths of the mobile-hinge activity (leading to chevron develop- multi-hinge folds seen in mine sections are not ment), as a low strain very rapid mechanism. They anomalous in arclength; ingeniously estimated the hinge displacement rate to (h) parasitic folding of thinner competent beds at be approximately 100 times faster than pressure- the hinge. However, there is no correlation between solution controlled slip on a fault. layers showing multi-hinge folding and unusually In the Bendigo-Castlemaine region there is no thin competent beds. Subsidiary hinge folds usually direct physical evidence for earlier mobility of fold canno! be traced into parasitic folds on the limbs. hinges. Axial plane cleavages show no signs of Also the enveloping surfaces of the hinge folds are disturbance in hinge zones or on limbs, though the broad and horizontal. absence of such disturbances is not an argument The various fold shape change histories shown by against hinge migration. The early stripy cleavages individual boxfolded layers during shortening in our in sandstones of this region described by Yang and experiments are presented in Fig. 14. From this Gray (1994), which are overprinted by a later axial figure it is clear that shortening may be accommo- plane grain alignment cleavage, may equally be pre- dated via median segment length reduction (via hinge folding or represent the effects of homogeneous migration (Fig. 14A). buckling and/or homogeneous shortening of boxlbld median segments. This latter shortening (Fig. 14B.C), limb rotation (Fig. 14E), or possibility is consistent with the presence of uni- median segment rotation (Fig. 14D). Any boxfolded layer may respond to shortening by more than one of these mechanisms during fold evolution, though ' [],! buckling of the median segment is irreversible. Whereas the models of Ramsay (1967, 1974) for
60 ° 60 ° chevron amplification allow only limb dip steepen- r- ing as the response to fold shortening, boxfold-to- chevron transformations allow varying combinations _ 50 ° 50 ° of median segment reduction and limb dip steepen- ing during fold tightening. The relative contribution 40 ° 40 ° of the two responses may vary smoothly or in a 30 ° 30 ° stepwise manner (Fig. 13), and is controlled by the 20 ° 20 ° 10 ° j '10 ° rates of hinge migration. i i I0 i L 10 2O 3 40 5O % -e Fig. 15. Plot of limb dip A against bulk shortening e~.'~ ~br 6. Hinge migration in boxfolds boxfolds experiencing hinge migration during shortening. For all examples the fold arc length W = 875 m (therefore W/2 = 437.5 The lbrmer activity of hinge migration is not m) and T = 15 m. These are considered to be representative values easily recognised in natural folds. The hinge migra- for the Bendigo-Castlemaine folds. The broken curve labelled C tion described by Odonn~ and Vialon (1987) in is for a chevron told of same arc length, for comparison. Curve 1: L 0=262.5 m, K=297.5: Curve 2: L o=262.5 m, K-437.5: superposed folding and by Gray (1981) in transected Curve 3: Lo=112.5 m, K=617.5; Curve 4: L0=112.5 m, folds, is a late delbrmation effect leaving evidence in K 812.5. 144 T.J. Fowler, C.N. Winsor/Tectonophysics 258 (1996) 125-150 formly well-developed stripy cleavages in the broad limb steepening in a single boxfolded layer are dis- very gently folded hinge zones of some of the folds cussed below, with the assistance of a simple geo- of this region (e.g. Fig. 4), which are suspected to metrical model derived in the Appendix. This is represent former boxfolds. followed by consideration of the shape evolution and Intuitively, large reflex angles and gently rounded hinge dilations of stacked boxfolded layers. fold hinges would least impede hinge migration. Layer thickness irregularities and layer surface asper- 6.1. Interrelations between hinge migration rates ities may (perhaps temporarily) slow or jam hinge and limb dips in shortening o]: boxfolds migration. Our experiments show cessation of both hinge migration and interlayer slip (i.e., fold lock-up) As noted above, boxfolded layers accommodate at limb dips of about 50-60 ° . Reduced layer slip bulk shortening by median segment length reduction activity increases interlayer friction which may im- and limb dip steepening. The two responses may pede curvature changes in layers. vary in importance at different stages of shortening. The effects of varying hinge migration rates on The rate of reduction of median segment lengths
22: 2o i I 24 18 [7] MEDIAN SEGMENT 22 16 20 14 18 12 16 10 14 8 12 6 10 4 RIGHT LIMB LENGTH 8 2 6
10 20 30 40 50 4 -e%
10 20 30 40 50
22 -e%
20
18
16 80 °
14 700
12 60 °
10 50 °
400
30 °
LEFT 20 ° LIMB LENGTH 10 °
10 20 30 40 50 10 20 30 40 -e% -e% Fig. 16. Limb dip, limb length and median segment length calculated using the geometric model (shown in Fig. 18), lbr the data presented in Fig. 13A. Note the consistent under-estimation of limb dip for the left limb (L), and the corresponding systematic over-estimation for right limb dip (R). The value of K chosen was 20.7. T.J. Fowler, C.N. Winsor/Tectonophysics 258 (1996) 125-150 145 appears to be approximately linear with respect to rate). This section deals with some of the conse- bulk shortening e, at least for the experimental mul- quences of hinge migration and limb steepening for a tilayers (Fig. 13). This relation for limb length is tightening boxfolded multilayer, especially dilation represented simply as L = L o - eK (where L = limb in the hinge zones I a topic important for the length for any stage of bulk shortening of the box- understanding of the development of auriferous fold and L 0 and K are constants, see Appendix Eq. quartz hinge reefs in the folds of the Bendigo-Cast- (1)). In the Appendix, Eq. (2) expressing e in terms lemaine region. of A. L and 14//2, is derived from Fig. 18. Eq. (1) Assuming first that rates of hinge migration are and Eq. (2) are used to derive an expression relating uniform, steepening of limbs will require uniform e and A to K (Eq. (3)), which is used to graph -e% dilation in the hinge normal to the median segment against limb dip A (Fig. 15) for boxfolds with arc lengths of 875 m (i.e., typical for the Bendigo-Cast- A B lemaine folds), with various initial limb lengths, and for different values of K. The "layer" thickness T is set at 15 m which Jessell et al. (1994) suggest is the average spacing of continuous bedding slip hori- zons in the Bendigo-Castlemaine region. Limb dips are higher at any e for any boxfold compared to a chevron of the same arc length (Fig. 15), since limb lengths will always be shorter in the boxfolds. I I Eq. (3) in the Appendix was used to calculate the limb dip changes against e% for layer 6 in Fig. 13A. The relevant measured data for this layer are repro- duced for comparison with the calculated limb dips and lengths, and are shown on Fig. 16. Given values for W/2, L~ and T for this folded layer of 20, 8.5 \ and 1.4 mm respectively, a value of K = 20.7 pro- E , vides the best fit for the data. At shortening values above e = 10%, right limb lengths are under-esti- mated while left limb lengths approximate well. Below 10% shortening the match between measured and calculated data is poor. Right limb dips are systematically over-estimated by about 5 °, with left Fig. 17. Simplified sketch of several histories of evolution ti)r a limb dips under-estimated to approximately the same stack of boxfolded layers abstracted from our multilayer experi- degree. These systematic departures from the calcu- ments (see text for details). (A) Limb dip steepening leads to different angles (dots and circles) being subtended with the axial lated values are related to the slight asymmetry that plane by the limbs and median segment, and consequently dila- this boxfold preserved during evolution towards a tions form across the median segment. (B) Alternate or subsequent chevron shape. history to (A), where the anticline median segment has buckled during further shortening. Adjacent synclines form by rotation of 6.2. Hinge dilation and axial plane rotation re- initially fiat-lying layers around jammed hinges so as to minimise synclinal hinge dilations. (C) Alternate response to (A). where sponses to limb dip changes in stocks qj¢ box.fi~lded hinge dilations in outer layers are diminished by curving of the l(.IVel's axial planes, and where upward lifting of the hinge is substituted partly by crumpling in the fold core. (D) Alternative mechanism The above discussion of hinge migration rates and of formation of chevrons from boxfolded layers where the syncli- related limb dip changes is relevant to individual nal axial planes have jammed, by rotation of the synclinal axial planes to steeper dips. (E) Jamming of one boxlold hinge leading layers in a boxfolded multilayer (though for all of to rotation of the median segment and its incorporation wholly the layers in a single boxfold ideally the limb lengths onto one limb of the fold. The resulting fold is asymmetric and and limb dips are common and grow at the same has a curved axial plane. 146 T.J. Fowler, C.N. Winsor / Tectonophysics 258 (1996) 125 150
(as for example in Fig. 17A, right edge of pho- ers (Fig. 12). As multilayer shortening proceeded, tographs in Fig. 12G-J), unless the boxfold axial the boxfold anticline amplified by hinge migration planes are able to rotate to a lower dip (discussed and limb lengthening as described above, until a later). Hinge dilations are necessary in order to stage was reached where limb steepening required maintain continuity of layers across the axial plane, dilation in the anticlinal hinges. The adjacent un- when the axial plane is not a bisector of the boxfold folded layers generally did not develop interlayer reflex angle. A similar effect is seen in kink band voids at this stage, because the equivalent dilation growth (Ramsay and Huber, 1987, p. 431). The would require downward displacement of layers. The amount of dilation (expressed as the width of parting alternative response for these adjacent layers was for of layers of thickness T) is derived in the Appendix hinge migration to cease and for the earlier unfolded (see Fig. 19 and Eq. (4)), and is shown to increase layers to rotate around the immobilized hinges, mak- with increasing T and increasing difference between ing the angles between layers and the axial planes axial plane dip and limb dip. equal either side of the axial planes. This eliminated The extensive flat uniformly thick bedding-paral- the need for dilation and led to the development of lel laminated veins found wrapped over fold hinge synclines (Fig. 12H-L, 17B), and may explain why zones (e.g. Fig. 8A) in the Bendigo-Castlemaine the synclinal (trough) reefs are relatively unimpor- folds may be natural equivalent structures to these tant in the Bendigo-Castlemaine goldfields. experimental hinge voids. Equivalent bedding-paral- One of the results of jamming adjacent synclinal lel veins to these may be later folded during progres- hinges is that the arclengths of the outer layers of the sive shortening (Fig. 9A, 17B). In the multilayer enclosed boxfold anticline are too short to proceed to experiments it was noted that median segment buck- chevron shape via hinge migration alone. Our experi- ling was particularly likely if hinge migration had ments suggest that the jammed synclinal axial planes jammed. Jamming allowed compressive stresses to bodily rotate towards the flattening plane at roughly be transmitted to the median segment, especially the stage that the innermost layers of the adjacent when fold limbs were already steep and resisted anticline achieve a chevron shape (Fig. 17D). After further rotation to accommodate shortening. the layers in the core have become chevrons further The continuity of bedding-parallel veins over tightening of the core may only be accommodated by macroscopic fold hinges, which are formed as de- limb rotation, as in the model of Ramsay (1967) for scribed above, is therefore not a reliable criterion for chevron folds. Eq. (5) in the Appendix, and Fig. 20, the pre-folding origin of such veins. Moreover, the give the required steepened dip of the synclinal axial opening vector for the hinge voids developed during planes to allow all of the outer layers of the enclosed limb steepening is normal to layering. Bedding-nor- anticline to become chevrons at constant fold limb mal dilation has also been posited as evidence for dip. pre-folding (fluid-overpressuring = hydraulic jack- If the two hinges of a boxfold migrate at different ing) of veins (Fitches et al., 1990; Cosgrove, 1993). rates then the median segment is liable to be rotated An alternative to uplifting layers at the median during fold tightening (Fig. 10B, 17E). One conse- segment to accommodate dilations includes contrac- quence is the incorporation of the median segment tion or crumpling of the boxfold core (Fig. 17C). onto one limb of the fold yielding asymmetrical This yields multihinge folds in the core while the chevrons. median segment of outer boxfold layers remains unfolded. Rotation of the axial plane to lower dip for the outer boxfold layers may mitigate dilation in the outer layers by allowing approximately equal angles Acknowledgements to be subtended by the axial plane with the median segment and fold limbs (Fig. 17C). The authors acknowledge the kind assistance pro- In our multilayer deformation experiments, a box- vided by Prof. Subir Ghosh regarding advice on fold anticline formed at low values of multilayer preparation of thin plasticine multilayers. We thank model shortening and was flanked by unfolded lay- Clive Willman for making available unpublished re- T.J. Fowler, C.N. Winsor/Tectonophysics 258 (1996) 125-150 147 cent mapping of the Castlemaine area. The helpful I l comments and advice by P. Hudleston, T. Engelder, l J.-P. Gratier and R. Groshong, Jr. are thankfully I acknowledged.
Sin(A+a) ! ~ I'~ - 90°'B Appendix A -, I T Symbols used (see Figs. 18-20): A = limb dip (radians) B = dip of the boxfold axial planes e = bulk shortening of the fold as shown in Fig. 18. K = a constant relating limb length increases (or A median segment reduction) to overall fold short- Fig. 19. Part of the limbs and median segments of two successive boxfolded layers. The limbs (dipping A) and the median seg- ening e ments, subtend different angles with the axial plane. This leads to L = boxfold or chevron fold limb length dilations (width S) across the median segment in order to main- S = width of layer parallel voids representing dila- tain layer continuity across the axial plane. tion across the median segment as a result of boxfold limb dip changes T = layer thickness W = total fold arc length. Remains constant during A. 1. Calculation of the relations between limb dip A deformation and rate of hinge migration K for a boxfolded laver transforming to a cheeron
Considering the model shown in Fig. 18, where the limb length L is allowed to linearly increase proportionally to overall fold shortening e: L = L o - eK ( 1) T i (where e has negative sign), e may be calculated from Fig. 18: (1 + e)W/2= W/2- L + ( L- AT)cos( A) ~ + nA + Tsin(A) (2) A Substituting Eq. (1) in Eq. (2) gives: (k - AT + TTanA)CosA (1 +e)W/2= W/Z- (L o-eK) + (L o - eK- AT)cos(A) (1 +e)W/2 + Tsin(A) (l+e)W/2 = W/2 - L + (L - AT)CosA + TSinA Which simplifies to: e = { (L- AT)CosA + TSinA - L }/(W/2) e = {Lo[cOs (A) - 1] - arcos(A) + Tsin( A)} Fig. 18. Simple geometrical model representing one of the sym- metrical halves of a boxfolded layer. The model is used to ~((W/Z) + K[cos(a) - l]} (3) calculate the expected limb dip for the folded layer at any value of bulk shortening for the fold (e%), given hinge migration rate A is plotted against e for various values of K and constant K, L 0 in Fig. 15. 148 T.J, Fowler, C.N. Winsor/Tectonophysics 258 (1996) 125-150
A.2. Calculation of width of L,oids in the boxfold A.3. Calculation of the necessau dip (~" the synclinal median segment as a result of limb steepening axial planes which allows boxfolded layers to progress to chet'ron shape despite jamming of hinge Given limb dip A, and axial plane dip B, layer- migration in adjacent synclines. parallel voids (width S) develop between layers of thickness T in the boxfold median segment, in order As a result of jamming of synclinal axial planes to maintain layer continuity across axial planes. From (initially dipping B), outer layers of an enclosed Fig. 19: boxfold have insufficient arc length to proceed to S+ T= Tsin( B)/sin( a + B) form chevron fold shapes unless the synclinal axial planes rotate towards the flattening plane. Fig. 20 from which shows a situation where the innermost layer has S = Tsin( B)/sin( a + B) - T (4) adopted a chevron shape. The dip of the axial plane
\ , I \ (n-1)TCosB \ i Sin(A+B)
1 )TTanA
\ \
"- { R + (n-1)TCos. B Sin(A+B) 1)TTanA - AT }
= (n-1)T{ TanA - CosB/Sin(A+B) }
Tan(A+B'-90 °) = (n-1)T { TanA - CosB/Sin(A+B) } (n-1)T
= TanA - CosB/Sin(A+B)
Fig. 20. Figure (on the left) of a set of n successive boxfotded layers. Layers are not shown individually, but are represented by a total thickness nT. The innermost layer has a chevron shape. With further shortening the synclinal axial plane rotates from dip B to dip B' (righthand figure), allowing the outer boxfolded layers to attain chevron shape. See Appendix and text for details. T.Z Fowler, C.N. Winsor/Tectonophysics 258 (1996) 125-150 149
B' required for the remaining outer layers to become Lachlan Fold Belt in central Victoria, Australia. Ore Geol. chevrons (here simplified by keeping limb dip A Rev.. 6: 391-423. Cox, S.F.. Etheridge, M.A., Cas, R.A.F. and Clifford, B.A., constant), is calculated from the relation shown in 1991b. Deformational style of the Castlemaine area, Fig. 20: Bendigo-Ballarat Zone: Implications for evolution of crustal tan( a + B' - 90 °) = tan(A) - cos( B)/sin( A + B) structure in central Victoria. Aust. J. Earth Sci.. 38: 151-170. de Sitter, L.U., 1958. Boudins and parasitic folds in relation to from which cleavage and folding. Geol. Mijnbouw. 8: 277-286. B'= 180~+tan I{sin(A+B) Dubey, A.K.. 1980. Late stages in the development of folds as deduced from model experiments. Tectonophysics. 65:311- /[cosB-tanasin(a+B)]}-A (5) 322. Dubey, A.K. and Cobbold, P.R.. 1977. Non-cylindrical flexural- slip folds in nature and experiment. Tectonophysics, 38:223 239. References Dunn, E.J., 1896. Reports on the Bendigo Goldfield Nos. I and 2. Spec. Rep. Mines Dep. Victoria. Baragwanath, W., 1903. The Castlemaine Goldfield. Mem. Geol. Faill, R.T., 1973. Kink-band lblding, Valley and Ridge Province. Surv. Vict., 2. Dep. Mines, Victoria. Pennsylvania. Bull. Geol. Soc. Am., 84:1289-1314. Bayly, M.B., 1970. Viscosity and anisotropy estimates from mea- Fergusson. C.L., Gray. D.R. and Cas. R.A.F.. 1986. Overthrust surements on chevron folds. Tectonophysics, 9: 459-474. terranes in the Lachlan Fold Belt. southeastern Ausmdia. Bayly, M.B., 1974. An energy calculation concerning the round- Geolog]r. 14:519 522. hess of fblds. Tectonophysics, 24: 291-316. Fitches, W.R.. Cave. R., Craig. J. and Maltman, A.J.. 199(I. The Behzadi, H. and Dubey, A.K.. 1980. Variations of interlayer slip flexural-slip mechanism: Discussion. J. Struct. Geol,, 12: in space and time during flexural folding. J. Struct. Geol., 2: 1081-1087. 453 457. Fowler. T.J. and Winsor. C.N., 1992. The possible role of Iqexural Blot. M.A., 1964. Theory of internal buckling of a confined slip lolding mechanism in the development of natural chevron multilayer. Bull. Geol. Soc. Am., 75: 563-568. folds from the Bendigo-Castlemaine area. Victoria. Geol. Soc. Cas, R.A.F. and VandenBerg, A.H.M., 1988. Ordovician. In: J.G. Aust. Abs., 32: 227. Douglas and J.A. Ferguson (Editors), Geology of Victoria. Ghosh, S.K., 1968. Experiments of buckling of multilayers which Geol. Soc. Australia Inc.. pp. 63-102. permit interlayer gliding. Tectonophysics. 6:2(17 249. Ceplecha, I.C. and Wall, V.J., 1976. Chewton Goldfield and Gray, D.R,. 1981. Cleavage-fold relationships and their implica- Wattle Gully Mine: A model for gold-quartz mineralization in tions for transected folds: an example from southwest Vir- slate belts. Bull. Aust. Soc. Explor. Geophys., 7: 40. ginia, Li.S.A.J. Struct. Geol.. 3: 265-277. Chace. F.M.. 1949. Origin of the Bendigo saddle reefs with Gray, D.R.. 1988. Structure and Tectonics. In: J.G. Douglas and comments on the formation of ribbon quartz. Econ. Geol., 44: J.A. Ferguson (Editors), Geology of Victoria. Geol. Soc. 561-597. Australia Inc.. pp. 1-36. Chapple, W.M. and Spang, J.H.. 1974. Significance of layer-paral- Gray, D.R., Gregory, R.T. and Durney, D.W.. 1991a. Rock lel slip during folding of layered sedimentary rocks. Bull. buffered fluid-rock interaction in defirrmed quartz-rich tur- Geoh Soc. Am., 85: 1523-1534. bidite sequences, eastern Australia. J. Gcophys. Res.. 96: Cobbold. P., 1976. Fold shapes as functions of progressive strain. 19681 - 19704. Philos. Trans. R. Soc. London, A283: 129-138. Gray. D.R., Wilson, C.J.L. and Barton, T.J., 1991b. lntracrustal Cobbold, PR., Cosgrove, J.W. and Summers, J.M., 1971. Devel- detachments and implications tk)r crustal ew)lution within the opment of internal structures in deformed anisotropic rocks. Lachlan Fold Belt, southeastern Australia. Geology, 19: 574- Tectonophysics, 12: 23-53. 577. Cosgrove. J.W., 1993. The interplay between fluids, folds and Gray, D.R. and Wilhnan. C.E.. 1991a. Thrust-related strain gradi- thrusts during the defiwmation of a sedimentary succession. J. ents and thrusting mechanisms in a chevron-Iolded sequence, Struct. Geol., 15: 491-500. southeastern Australia. J. Struct. Geol.. 13:69 ] -710. Cox. S.F., 1987. Antitaxial crack-seal vein microstructures and Gray. D.R. and Willman, C.E., 1991b. Deformation in the Ballarat their relationship to displacement paths. J. Struct. Geol., 9: Slate Belt. central Victoria and implications |or the crustal 779 787. structure across southeast Australia. Ausl. J. Earth Sci., 38: Cox. S.F.. Etheridge, M.A. and Wall, V.J., 1986. The role of 171-201. fluids in syn-tectonic mass transport and the localization of Herman. H.. 1923. Structure of the Bendigo Goldfield. Geol. metamorphic vein-type ore deposits. Ore Geol. Rev.. 2: 65-86. Surv. Victoria Bull.. 47. Victorian Dep. Mines. Cox. S.F., Wall, V.J., Etheridge, M.A. and Potter, T.F., 1991a. Hills. E.S., 1972. Elements of Structural Geology. 2rid Ed. Chap- Deforrnational and metamorphic processes in the formation of man and Hall. London, 502 pp. mesothcrmal vein-hosted gold deposits -- examples from the Honea, E. and Johnson, A.M., 1976. A theory of concentric, kink 150 T.J. Fowler, C.N. Winsor/Tectonophysics 258 (1996) 125-150
and sinusoidal folding and of monoclinal flexuring of com- Stone, J.B., 1937. The structural environment of the Bendigo pressible, elastic multilayers. IV: Development of sinusoidal Goldfield. Econ. Geol., 32: 867-895. and kink folds in multilayers confined by rigid boundaries. Stubley, M.P., 1989. Fault and kink-band relationships at Mystery Tectonophysics, 30: 197-239. Bay, Australia. Tectonophysics, 158: 75-92. Jessell, M.W., Willman, C.E. and Gray, D.R., 1994. Bedding- Talent, J.A. and Thomas, D.E., 1973. Stratigraphy and structure of parallel veins and their relationship to folding. J. Struct. Geol., the Palaeozoic of west-central Victoria. In: J. McAndrews and 16: 753-767. M.A.H. Marsden (Editors), Regional Guide to Victorian Geol- Johnson, A.M. and Ellen, S.D., 1974. A theory of concentric, kink ogy, 2nd Ed. Univ. Melbourne, pp. 65-82. and sinusoidal folding and of monoclinal flexuring of com- Tanner, P.W.G., 1989. The flexural-slip mechanism. J. Struct. pressible, elastic multilayers. I: Introduction. Tectonophysics, Geol., 11: 635-655. 21: 301-339. Thomas, D.E., 1953. The Bendigo Goldfield. In: A.B. Edwards Johnson, A.M. and Honea, E., 1975. A theory of concentric, kink (Editor), Geology of Australian Ore Deposits. Fifth Empire and sinusoidal folding and of monoclinal flexuring of com- Mining and Metallurgical Congress, Aus.I.M.M., pp. 1011- pressible, elastic multilayers. III: Transition from sinusoidal to 1027. concentric-like to chevron folds. Tectonophysics, 27: 1-38. VandenBerg, A.H.M., 1978. The Tasman Fold Belt System in Johnson, A.M. and Page, B.M., 1976. A theory of concentric, kink Victoria. Tectonophysics, 48: 267-297. and sinusoidal folding and of monoclinal flexuring of com- Watkinson, A.J., 1976. Fold propagation and interference in a pressible, elastic multilayers. VII: Development of folds within single multilayer unit. Tectonophysics, 34: T37-T42. Huasna Syncline, San Luis Obispo County, California. Weiss, L.E., 1980. Nucleation and growth of kink bands. Tectono- Tectonophysics, 33: 97-143. physics, 65: 1-38. Keppie, J.D., Boyle, R.W. and Haynes, S.J. (Editors), 1986. Whitelaw, H.S., 1914. Hustler's Line of Reef. Bendigo. Geol. Turbidite-hosted Gold Deposits. Geol. Assoc. Can. Spec. Pap., Surv. Victoria Bull., 33. 32. Wilkinson, H.E., 1988. Bendigo reef nomenclature. In: Bicenten- Odonn& F. and Vialon, P., 1987. Hinge migration as a mechanism nial Gold 88, Excursion No. 2 Guide: Central Victoria. Geol. of superposed folding. J. Struct. Geol., 7: 835-844. Surv. Victoria, pp. 22-27. Patterson, M.S. and Weiss, L.E., 1966. Experimental deformation Williams, J.R., 1980. Similar and chevron folds in multilayers and folding in phyllite. Bull. Geol. Soc. Am., 77: 343-374. using finite-element and geometric models. Tectonophysics, Ramberg, H.. 1964. Selective buckling of composite layers with 65: 323-338. contrasted rheological properties, a theory for simultaneous Willman, C.E., 1988. Geology of the Spring Gully l:10,000 map formation of several orders of folds. Tectonophysics, 1: 307- area, Bendigo Goldfield. Geol. Surv. Rep., 85. Geol. Surv. 341. Victoria. Ramsay, J.G., 1967. Folding and Fracturing of Rocks. McGraw- Willman, C.E., 1995. Castlemaine Goldfield. Castlemainer-Chew- Hilt, New York, 568 pp. ton, Fryers Creek 1:10,000 maps geological report. Geological Ramsay, J.G., 1974. Development of chevron folds. Bull. Geol. Survey of Victoria, Report 106. Department of Energy Miner- Soc. Am., 85: 1741-1754. als, Victoria. Ramsay, J.G. and Huber, M.I., 1987. The Techniques of Modern Willman, C.E. and Wilkinson, H.E., 1992. Bendigo Goldfield. Structural Geology, Vol. 2: Fold and Fractures. Academic Geol. Surv. Rep., 93. Dep. Manufacturing and Industry Devel- Press, London, 394 pp. opment, Victoria. Ramsay, W.R.H. and Willman, C.E., 1988. Economic Geology: Yang, X. and Gray, D.R., 1994. Strain, cleavage and microstruc- Gold. In: J.G. Douglas and J.A. Ferguson (Editors), Geology ture variations in sandstone: implications for stiff layer be- of Victoria. Geol. Soc. Australia Inc., pp. 454-482. haviour in chevron folding. J. Struct. Geol., 16: 1353-1365. Stewart, K.G. and Alvarez, W., 1991. Mobile-hinge kinking in layered rocks and models. J. Struct. Geol., 13: 243-259.