A new mechanism for “dome and keel” geom - Department of ...

Precambrian Research 212-213 (2012) 139–154 

Contents lists available at SciVerse ScienceDirect 

Precambrian Research 

journal homepage: www.elsevier.com/locate/precamres 

Regional shortening followed by channel flow induced collapse: 

A new mechanism for “dome and keel” geometries in Neoarchaean 

granite-greenstone terrains 

Lyal B. Harris a,∗ , Laurent Godin b , Chris Yakymchuk b,1 

a Institut national de la recherche scientifique, Centre - Eau Terre Environnement, 490 de la Couronne, Québec, (Québec) G1K 9A9, Canada 

b Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, (Ontario) K7L 3N6, Canada 

article info 

Article history: 

Received 17 June 2011 

Received in revised form 18 April 2012 

Accepted 27 April 2012 

Available online 9 May 2012 

Keywords: 

Channel flow 

Folding 

Centrifuge modelling 

“Dome and keel” geometry 

Granite-greenstone terrains 

Archaean tectonics 

1. Introduction 

1.1. Research aims 

abstract 

Centrifuge analogue modelling is used to test a new hypothesis 

for the development of dome and keel structures in Archaean 

∗ Corresponding author. Tel.: +1 418 654 2568; fax: +1 418 654 2600. 

E-mail addresses: lyal harris@ete.inrs.ca (L.B. Harris), 

godin@geol.queensu.ca (L. Godin), cyak@umd.edu (C. Yakymchuk). 

1 Present address: Department of Geology, University of Maryland, College Park, 

MD 20742, USA. 

0301-9268/$ – see front matter © 2012 Elsevier B.V. All rights reserved. 

http://dx.doi.org/10.1016/j.precamres.2012.04.022 

The lateral flow and extrusive exhumation of ductile migmatitic gneisses due to a horizontal gradient in 

lithostatic pressure, a process termed “channel flow” or “lateral protrusion”, has previously been proposed 

as an important tectonic process in large hot orogens. Centrifuge simulation of (i) layer-parallel shortening 

followed by (ii) collapse of a cover sequence during ductile flow of underlying layers in an analogous manner 

to channel flow suggests a new mechanism for the development of structures within Neoarchaean 

granite-greenstone terrains. The centrifuge model incorporates an upper package of silicone-modelling 

clay microlaminates that simulate a greenstone sequence that overlies slightly less dense ductile silicone 

putties, whose rheological properties simulate migmatitic felsic gneiss. A low viscosity, low-density layer 

along half of the infrastructure–superstructure interface simulates the presence of granitoid melt. During 

initial layer-parallel shortening upright folds form in the upper “greenstone” package and the interface 

with underlying ductile layers is folded. In basal silicone layers, recumbent to overturned non-cylindrical 

and upright folds form with increasing distance from the moving end wall. Removal of material parallel 

to fold hinges at the ‘foreland’ end of the model (i.e. furthest from the ram used to shorten models) in 

several stages simulates erosion and the difference in gravitational loading thus created induces ductile 

flow and lateral extrusion of basal silicone layers. Models aim to reproduce features comparable to 

those developed during extrusive channel flow and focused exhumation of basement migmatitic gneiss 

in nature. Early-formed recumbent to inclined folds are then accentuated during simulated channel flow, 

while new recumbent isoclinal folds in basal layers develop. Broad antiforms and tight synforms similar 

to the “dome and keel” geometry that typifies many Archaean granite-greenstone belts are produced as 

a late feature in the model. Channel flow and collapse of a fold-thickened crust is therefore proposed as a 

potential alternative mechanism for the formation of structures in some Neoarchean granite-greenstone 

terrains. By analogy with other physical and numerical models, channel flow in the Neoarchaean may 

be enhanced by impingement of an upper mantle wedge into the base of the crust. Our results imply 

that both lithospheric shortening and non-diapiric gravitational instabilities may be responsible for the 

formation of some Archaean “dome and keel” structures and may account for the juxtaposition of some 

granite-greenstone and high-grade gneiss terrains in the Archaean. Model results also show similarities 

to structures produced during lateral flow and withdrawal of salt in fold belts. 

© 2012 Elsevier B.V. All rights reserved. 

granite-greenstone belts. The progressive development of structures 

is studied in a model scaled to represent deformation of a 

greenstone sequence of layered volcanic and sedimentary rocks 

upon basement granitoid gneisses. Models simulate two separate 

deformation stages: (1) initial folding and crustal thickening 

during layer-parallel shortening, followed by (2) a separate period 

of “collapse” and gravity-driven re-equilibration during lateral 

ductile flow and of a ductile substrate analogous to extrusive 

channel flow (see Section 1.3 for definition). The model presented 

in this paper also illustrates mechanisms for the formation of 

overturned to recumbent folds in granite-greenstone terrains, 

their relative timing of formation with respect to folds in the

140 L.B. Harris et al. / Precambrian Research 212-213 (2012) 139–154 

overlying greenstone sequence, and their spatial distribution. 

The likelihood for and factors promoting channel flow in the 

Neoarchaean and possible regional implications for juxtaposition 

of granite-greenstone and high-grade gneiss terrains are discussed. 

1.2. Previous models for dome and keel structures and recumbent 

folds in granite-greenstone belts 

The origin of tight synforms, “keels”, or “cusps” of 

(meta-)sedimentary and (meta-)volcanic rocks of greenstone 

belts between open, rounded antiforms, “arches”, or domes 

of tonalite–trondhjemite–granodiorite (TTG) gneiss that typify 

Archaean granitoid-greenstone belts (Condie, 1984; Windley, 

1995) is contentious. The following diverse mechanisms have been 

proposed: 

(i) Folding during one or more phases of regional shortening 

(e.g. Snowden and Bickle, 1976; Myers and Watkins, 1985; 

Blewett, 2002; Blewett et al., 2004, 2010). Weinberg et al. (2003) 

illustrate that granite-greenstone belt fold geometry may be 

influenced by early granitoid intrusions that acted as rigid bodies 

during shortening. 

(ii) Domes formed due to Raleigh–Taylor (Wilcock, 1991), “gravitational” 

instabilities created by density inversion. Domal 

structures similar to those developed in centrifuge models 

of Ramberg (1967a,b, 1981a,b) and Dixon and Summers 

(1985) may occur where a denser greenstone sequence (ca. 

2.7–3.0 g/cm 3 depending on the proportion of sedimentary 

rocks and fresh or altered ultramafic and mafic rocks; de 

Bremond d’Ars et al., 1999) overlies less dense (ca. 2.7 g/cm 3 ; de 

Bremond d’Ars et al., 1999) gneiss ± granitoid. Domes formed 

due to gravitational instability created by such density differences 

are proposed for many granite-greenstone terrains 

(e.g. Gorman et al., 1978; Condie, 1984; Hickman, 1984; Robin 

and Bailey, 2009) where they are commonly referred to as 

“Raleigh–Taylor diapirs” (e.g. Bouhallier et al., 1995), or simply 

“diapirs” 2 (where formation due to density inversion is 

implied). Shackleton (1995) contends that diapiric structures 

are essential elements of greenstone belt evolution. Granitoids 

preferentially intruding the core of domes may accentuate 

gravitational instabilities and hence their diapiric emplacement 

forming polydiapirs (Weinberg and Schmeling, 1992). 

Diapirism may occur after thrust stacking and nappe tectonics 

(e.g. Dirks and Jelsma, 1998). Early extensional faults may be 

inverted during shortening and diapirism (Hippertt and Davis, 

2000). The interplay between regional shortening, folding, and 

diapirism due to gravitational instabilities is documented by 

Park (1982), Bouhallier et al. (1995), Dalstra et al. (1998), Lin 

(2005), Parmenter et al. (2006), and Erickson (2010) and Lana 

et al. (2010b) describe development of the dome and keel structure 

of the Barberton granitoid-greenstone belt during a 30 m.y. 

period of crustal extension following crustal shortening. Nappelike 

structures, instead of simple, symmetrical domes, may also 

form due to gravitational instabilities (Ramberg, 1967b), especially 

where the initial interface between basement gneisses 

and overlying greenstones is inclined (c.f. models of Talbot, 

1974). 

(iii) “Vertical tectonics” incorporating both “positive” and “negative” 

(i.e. descending) diapirs (Bédard et al., 2003; Bédard, 

2006), subsiding troughs or “sagduction” (Gorman et al., 1978; 

2 Note that the term “diapir” is, however, non-genetic and may be used for 

piercement structures without gravitational instablity (c.f. Weinberg and Schmeling, 

1992), although its usage in Archaean granitoid-greenstone terrains is generally 

taken to imply a gravity-driven origin for domes. 

Goodwin and Smith, 1980; Chardon et al., 1996, 1998; François 

et al., 2012), or partial convective overturn (Van Kranendonk 

et al., 2004; Bodorkos and Sandiford, 2006). Vertical tectonic 

models of Bédard (2006) differ from simple diapir models in (ii) 

in that they include the complex interplay between deformation 

and petrological phase (and hence density) changes, crustal 

melting, magmatic processes, and mantle delamination. 

(iv) Core complex formation during regional extension (e.g. 

Williams and Whitaker, 1993; Kloppenburg et al., 2001; Zegers 

et al., 2001; Kisters et al., 2003; Lana et al., 2010a) that may 

follow an early period of folding and thrusting (e.g. Lobato 

et al., 2001). Late granitoids that intrude Archaean granitegreenstone 

terrains are interpreted by Kusky (1993) to result 

from decompression melting during collapse and core complex 

formation. 

(v) Intrusion. In addition to TTG gneiss domes described above, 

late, intrusive granites may also form ovoid, domal features 

(Williams and Whitaker, 1993; Hofmann et al., 2003). 

The genesis of Palaeoproterozoic dome-and-keel structures is also 

conjectural, with thrust-related, orogenic collapse/core complex, 

diapir, and modified models combining all of them proposed 

(Marshak et al., 1997; Tinkham and Marshak, 2004). Whilst the 

above discussion has concentrated on rocks of low to medium 

grade, differential loading and “gravitational redistribution” is 

proposed by Gerya et al. (2000) to explain the formation of domes 

of granulite facies gneiss in high-grade Precambrian terrains. 

The origin of recumbent folds in Archaean migmatitic gneiss 

domes is likewise contentious. Recumbent folds have been interpreted 

as forming during vertical gravity-driven tectonics (Gorman 

et al., 1978) and are developed in Ramberg’s (1967a,b, 1981b) and 

Talbot’s (1974) centrifuge models. Centrifuge models by Harris 

et al. (2002) and Harris and Koyi (2003) show that both recumbent 

folds and the upright folds that deform them may develop during 

regional, layer parallel extension and may explain fold geometries 

in Archaean terrains interpreted from deep crustal reflection 

seismic profiles (Blewett and Czarnota, 2007; Goscombe et al., 

2009; Bédard et al., 2012). Alternatively, recumbent folds may have 

formed during one or more periods of regional thrusting and fold 

nappe emplacement prior to dome formation, as proposed for the 

Pilbara Craton of Western Australia (Bickle et al., 1980; White et al., 

1998; Van Kranendonk et al., 2004) or for Greenland (Windley and 

Garde, 2009). 

1.3. Channel flow, and similar tectonic processes 

Channel flow in an orogen is referred to here as the lateral 

flow of a weak, viscous crustal layer between relatively rigid yet 

deformable bounding crustal slabs due to a horizontal gradient in 

lithostatic pressure created by differences in crustal thicknesses 

beneath the hinterland compared to the foreland, and/or by erosion 

and focused denudation (as summarized by Godin et al., 2006 

and Grujic, 2006). In this process the mid-crust can be extruded 

toward the surface within a channel bounded by an upper normalsense 

boundary and a lower thrust-sense boundary. Channel flow 

and the similar process of “laminar flow” (Dewei, 2008) have been 

applied to explain first-order deformation features in large or wide 

hot orogens such as the Himalaya (Beaumont et al., 2001, 2004, 

2006; Grujic et al., 2002; Jamieson et al., 2004; Burbank, 2005; Jones 

et al., 2006; Harris, 2007), the eastern Variscan belt (Schulmann 

et al., 2005, 2008; Dörr and Zulauf, 2010), the Ediacaran Petterman 

orogeny in Australia (Raimondo et al., 2009), the Southern British 

Colombia Cordillera (Brown and Gibson, 2006), the Appalachian 

Inner Piedmont (Hatcher and Merschat, 2006), and the Proterozoic 

Central Mozambique Belt in Tanzania/Southern Kenya (Fritz et al., 

2009). Vanderhaeghe (2009, Fig. 7b) illustrates contemporaneous

channel flow, granite diapirism, and magmatic intrusion within a 

Phanerozoic orogen. Additional examples and references are given 

in reviews by Godin et al. (2006) and Grujic (2006). A similar 

process to channel flow, involving flow of a ductile layer with constrained 

lateral boundaries beneath a brittle-ductile to brittle cover 

sequence, is termed protrusion tectonics (Leonov, 1994, 2008). 

Channel flow (Cagnard et al., 2006; Parmenter et al., 2006), gravity 

driven continental overflow tectonics (Bailey, 1999), and general 

deep crustal flow (Culshaw et al., 2006a) have also been proposed 

for the Archaean. Similarly, Chardon et al. (2011) propose that the 

structure of Archaean terrains is controlled by pervasive, threedimensional 

flow of the lower crust incorporating orogen-normal 

shortening, lateral constrictional stretching, and transtension. As 

thermal gradients were higher in the Archaean and hence the 

crust more ductile (Richter, 1985; Rey and Coltice, 2008; Flament 

et al., 2008; further enhanced through heating by mantle plumes; 

Choukroune et al., 1995), channel flow is likely to have then been 

an important tectonic process (Rey and Houseman, 2006). Bailey 

(1999) contends that in the Archaean a ductile layer would have 

been universally present beneath an upper, brittle crust, and that 

the ductile layer may flow and extrude laterally onto adjacent 

ocean basins due to lateral differences in crustal thickness. Lateral 

protrusion has been applied to explain mapped Archaean and 

Palaeoproterozoic structures in the Baltic Shield by Kolodyazhny 

(2007) and Leonov (2008). 

1.4. Limitations of previous 1 g physical modelling of channel flow 

in simulating structures formed in granite-greenstone terrains 

Previous physical models of channel flow and analogous ductile 

flow processes in orogens (e.g. Miller, 1982; Chattopadhyay 

and Mandal, 2002; Mukherjee and Koyi, 2010; Duretz et al., 2011; 

Mukherjee et al., 2012) deformed under normal gravity (1g) do 

not take into account the interaction between active folding of a 

cover sequence of mechanically anisotropic layers during flow of 

the ductile substrate (e.g. Yakymchuk et al., 2012). Miller’s (1982) 

and Mukherjee and Koyi’s (2010) models simulate only structures 

developed in the zone of extrusive ductile flow where, apart from at 

the extrusive front, the boundaries are artificially fixed and planar. 

The models of Duretz et al. (2011) use a basal indentor to extrude 

the lower ductile layers instead of differences in gravitational loading 

and, as their upper crustal layers are simulated by sand, do 

not simulate active folding in a cover sequence. Whilst producing 

some of the features on granitoid greenstone terrains, physical 

1g models of orogen-parallel flow by Cagnard et al. (2006) during 

bulk shortening do not produce the observed differences in folds 

within granite-gneiss domes in comparison to an overlying greenstone 

sequence. A high-acceleration centrifuge is therefore used to 

simulate the force of gravity to achieve dynamic scaling of physical 

models incorporating modelling clays and silicone “bouncing putties” 

(Hubbert, 1937; Ramberg, 1967a,b, 1970, 1981a,b; Dixon and 

Summers, 1985). The model presented herein aims to (1) simulate 

active folding of a cover sequence and ductile flow of basal layers 

induced by differential loading and (2) test a new hypothesis for 

the formation of dome and keel structures in Archaean terrains. 

2. Centrifuge modelling of channel flow 

2.1. Modelling procedure 

An upper crustal layered greenstone sequence of sedimentary 

and volcanic rocks (the “superstructure”, following the terminology 

of Wegmann (1935) and Culshaw et al., 2006b) is simulated 

using microlaminates of modelling clays, mixes of modelling clays 

and silicone, and silicone putty with an upper layer of low-density 

L.B. Harris et al. / Precambrian Research 212-213 (2012) 139–154 141 

modelling clay (Fig. 1). Silicone “bouncing putties” of different 

effective viscosities and densities, whose bulk density is slightly 

less than the superstructure (1.14 g/cm 3 ), are used to simulate 

basal felsic ± migmatitic gneisses comprising the lower ductile 

sequence or infrastructure. Polydimethylsiloxane (PDMS), a clear, 

low density (1.05 g/cm 3 ) and low viscosity polymer (Weijermars, 

1986; Boutelier et al., 2008) simulating granitoid melt is present 

along the superstructure-infrastructure contact over half the width 

of the model. The rheological properties and scaling of modelling 

materials used in the model presented herein, which are part 

of a larger program of centrifuge channel flow experiments, are 

addressed by Poulin (2006), Godin et al. (2011), Harris et al. 

(2012), and Yakymchuk et al. (2012) so are not repeated herein. 

Tomodensitometry (“CT scanning”) techniques and applications 

to modelling using silicone and modelling clay materials are 

described by Poulin (2006). The model presented (Fig. 1) is similar 

to models presented in Godin et al. (2011), Harris et al. (2008, 2009, 

2010), Harris et al. (2012), and Yakymchuk et al. (2012), however 

it differs in that the cover sequence simulating greenstones is 

made slightly denser than underlying layers simulating basement 

gneisses by incorporating a larger number of competent and denser 

modelling clay layers (simulating mafic volcanics in a greenstone 

sequence). 

The model (shown schematically in Fig. 1a) was first progressively 

shortened via a collapsing wedge whilst undergoing an 

acceleration of ca. 950g. Material was then cut from the end of 

the model furthest from the shortening ram down into the infrastructure 

(Fig. 1b) in several stages to simulate focused erosion. The 

model is then allowed to isostatically equilibrate when returned 

to the centrifuge at ca. 950g but without imposing bulk shortening 

or extension. CT scans after each increment of deformation 

enabled the progressive development of structures to be viewed 

along the same cross-section (Figs. 2 and 3), as orthogonal sections 

(Fig. 4), and reconstructed in 3D (Figs. 5 and 6; “fly through” animations 

after initial shortening and of the final model are provided 

as supplementary data files 1 and 2). The final model was chilled, 

sliced, and photographed (Fig. 7). 

2.2. Folding during layer-parallel shortening 

Line drawings based on CT scans for the same slice through 

the model at different increments of deformation (Fig. 3a–c) and 

3D reconstructions (Fig. 5) show that tight to open, chevron to 

rounded, upright folds develop in the upper sequence simulating 

greenstones during layer-parallel shortening. Parasitic folds are 

present in several antiforms and synforms (Fig. 3a–c). Folds are 

cylindrical to slightly conical and fold hinges are slightly curved. 

The fold style in basal silicone layers varies with distance from the 

ram used to shorten the model. Adjacent to the ram, initially open 

to close overturned folds become isoclinal and recumbent and are 

then refolded by open upright folds with progressive shortening 

(Fig. 3a–c). Silicone layers in the center of the model are initially 

only slightly deformed, but small folds overturned in the direction 

of shortening develop at higher strains. Toward the end of the 

model furthest from the ram folds in basal silicone layers in the core 

of an antiform in the upper layered sequence are tight and upright. 

When present, the PDMS infills the cores of antiforms and creates 

a planar interface with the underlying silicone layers (Fig. 2i–k) 

where in the PDMS-absent half of the model, the basal silicone layers 

infill antiform cores (Fig. 2b–d). Folds of larger wavelength are 

developed over the part of the model where PDMS décollement is 

present (Figs. 2 and 5). Air bubbles caught in the models (black spots 

in Figs. 2 and 5) are found in some antiformal cores and may have 

assisted nucleation of some of these structures, or simply migrated 

toward fold hinges.


Fig. 1. Centrifuge model. (a) Schematic diagram showing the model before shortening. Layers are assembled upon a curved base that matches the outer curvature ofthe 

centrifuge rotor. A wedge of modelling clay collapses during centrifugation, pushing a nylon plate to shorten the model parallel to initial layering, as illustrated in (b). (b) 

Schematic diagram of model after initial shortening. Material is incrementally cut from the end of the model furthest from the nylon plate to simulate erosion and the 

model is placed in the centrifuge to re-equilibrate. (c) Model composition. DMC = Demco ® modelling clay, DC 3179 = Dow Corning ® dilatant compound 3179 silicone putty, 

CMM = Crayola ® model magic modelling clay, a low density modelling material, DMC-60 = mix of 60% DMC and 40% CMM, Plasticine = Harbutts ® oil-based modelling clay, 

PDMS = polydimethylsiloxane, a low density and viscosity unfilled silicone, CATP = Crazy Aaron Enterprises ® “Thinking Putties” (silicone putties similar to Rhodorsil Gomme 

silicone putty, of lower viscosity in comparison to DC 3179). Physical and X-ray computed tomography (CT) scanning properties of materials are discussed by Poulin (2006), 

Godin et al. (2011), and Yakymchuk et al. (2012). Strontium europium-aluminate powder = “Glow in the dark” powder from Glow Inc. ® is added to enhance some layers on 

CT scans (Poulin, 2006). 

2.3. Collapse of the superstructure during channel flow 

When material is removed to simulate erosion (as shown 

schematically in Fig. 1b) lateral flow, extrusion, and isoclinal folding 

of basal silicone layers occur beneath the area where material is 

removed in a similar manner as proposed for extrusive channel flow 

(e.g. Godin et al., 2006). The crests of antiforms in upper microlaminate 

layers simulating greenstones collapse (Figs. 2, 3d, and 4a–d), 

forming broader flat- to open-crested, doubly plunging structures. 

Minor parasitic folds that were developed during initial layerparallel 

shortening are flattened and unfolded. The tight synforms 

remain relatively unchanged. The upright antiform in the basal 

silicone layers closest to the ram is similarly flattened, while recumbent 

isoclinal folds within it are accentuated. Silicone layers in the 

central part of the models are folded by foreland-verging to overturned 

folds (Figs. 3d and 4). 3D views (Fig. 6) show that hinges of 

tight synforms are curved. Orthogonal slices (Fig. 4) and details of 

3D reconstructions (Fig. 6e–g) show that local large curvature of 

(sometimes doubly plunging) fold hinges produces elongate domal 

structures cored by basal silicone layers, which are deformed by 

isoclinal recumbent folds that form sheath-like structures. 

Oblique photographs of the final model are shown in Fig. 7. Thin 

layers of modelling clay and modelling clay-silicone mixes indicate 

that parasitic folds are best preserved in synforms in comparison 

to antiforms and parasitic folds are not present in lowermost 

plasticine layers over the open antiforms. PDMS (that simulates 

granitoid in nature) has migrated both laterally and longitudinally 

to infill crests of antiforms (Fig. 7b and c). In part of the model 

(Fig. 7c) the microlaminate superstructural sequence is folded by 

overturned to recumbent folds on the flank of a dome such that it 

is overhung by both silicone and PDMS (= granitoid and gneiss in 

nature). It therefore can be envisaged that in nature, if erosion were 

to expose a granite-greenstone terrain at a level such as shown in 

Fig. 7b and c by a dashed green line, granitoid or migmatite domes 

flanked by, or with screens of complexly folded gneiss, between 

which a tightly folded greenstone sequence would outcrop. 

3. Discussion 

3.1. Folding and unfolding 

In our centrifuge model fold hinges in the lowermost superstructure 

and infrastructure that were only slightly curved after 

initial layer-parallel shortening are reoriented to become more 

curved during channel flow. In contrast, uppermost layers in 

the superstructure are little affected during the channel flow 

stage of the model resulting in marked variations of fold orientations 

with depth (e.g. Fig. 5). The broad, open antiforms 

with few parasitic folds cored by PDMS and underlying silicones 

and tight synforms with parasitic folds in final stages of models 

(Figs. 2–4, 6, and 7) resemble cross-sections through many 

Archaean granite-greenstone terrains. Broad antiforms observed 

on orthogonal sections (Fig. 4) indicate the presence of doubly 

plunging to domal structures. Our model implies that crests of 

domes extended during the collapse stage, and that early parasitic 

folds may have been unfolded similar to the process of unfolding 

during “opening out” of early folds during superposed deformation 

described by Sengupta et al. (2005). Unfolding (as reviewed by 

Harris et al., 2012) is also required or important in hinge migration 

during folding (Mercier et al., 2007), kink propagation (Price


Fig. 2. CT scans showing slices at two positions in the model (a) and (h) during layer parallel shortening of 19, 31, and 39% respectively (left side of figure) and during 

collapse during lateral ductile flow of ductile basal layers toward the end of the model where material is removed to simulate focused erosion (right side of figure; indicated 

by horizontal line with double arrows). Note: (i) Differences in fold geometry between upper layered sequence or superstructure and the basal silicone layers of the 

infrastructure, (ii) changes in the geometry of structures in basal silicone layers from refolded recumbent folds near the advancing end wall (right side of slices in (b)–(d) and 

(i)–(k)), and (iii) opening out and destruction of minor folds in antiforms and preservation of tight synforms during flow of basal ductile layers in (e)–(g) and (l)–(n). Initial 

model height = 15 mm; final height = 22 mm. 

and Cosgrove, 1990), rotation of a folded layer into the extensional 

field (Ramsay et al., 1987), ductile deformation in shear 

zones (Carreras et al., 2005), and during thrust duplex development 

(Boyer and Elliott, 1982). It is therefore likely, as discussed in detail 

by Harris et al. (2012), that local unfolding may also occur in nature 

in the context of layer-parallel extension during channel-flow 

induced collapse. This provides a new mechanism for the development 

of broad antiforms and tight synforms in granite-greenstone 

terrains. 

The development of recumbent isoclinal folds close to the 

advancing ram, overturned folds in the center of models, and 

upright folds furthest from the ram in basal silicone layers during 

upright folding of microlaminates was also produced in a 1g 

physical model of Bucher (1956; Figs. 2–4) where layers of different 

stitching waxes inter-layered with grease were shortened. In 

Bucher’s (1956) experiment, layers closest to the advancing wall 

were warmer, and hence more ductile, than layers further away 

from the advancing wall. As no lateral changes in viscosity were


Fig. 3. Line drawing of changes in fold geometry for a central slice through the model based on CT scans during layer-parallel shortening (a–c) of 19, 31, and 39% respectively 

and isostatic adjustments and ductile flow of basal silicone layers toward the end of the model where material was removed to simulate focused erosion in (d). The final 

height of the model in (d) is 22 mm whereas its initial height was 15 mm. 

Fig. 4. Orthogonal slices through the model after two stages of erosion-induced flow following layer-parallel shortening, illustrating closures in both sections across the 

central antiform due to curvature of tight synforms. Left: sections (a) and (c) illustrate successive stages of slices parallel to the flow direction; right: sections (b) and (d) 

slices orthogonal to the flow direction in basal layers (locations indicated on a and c). Note that a non-cylindrical, sheath-like geometry of folds in basal silicone layers is 

indicated from the eye-shapes and the opposed vergence of structures in sections perpendicular to the flow direction. Model height = 22 mm.


Fig. 5. 3D reconstructions from CT scans showing progressive stages in shortening of the model with and without an upper layer (blue). Model width=8cm.(a)Undeformed 

model. (b–c) 19% shortening, (d–e) 31% shortening, (f–h) Two views after 39% shortening; the face viewed in (h) and (i) contains no PDMS, whereas the face in the other 

images is the side with PDMS. Note that folds show straight to slightly curved axes at the final stage of layer-parallel shortening. Folds are more open and have a larger 

dominant wavelength above the PDMS layer. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.) 

present in our model, proximity to the ram is therefore more important 

in explaining the changes in fold style in basal ductile layers 

rather than slight lateral differences in viscosity. Folds produced in 

our model of channel flow also resemble those portrayed by Leonov 

(2008; Fig. 8c), which he interprets as having formed during extrusive 

ductile flow of underlying ductile crust (lateral protrusion). 

3.2. Comparison with diapir and vertical tectonic models 

In contrast to classical domes resulting from gravitational/ 

Rayleigh–Taylor instabilities (Wilcock, 1991) in granite-greenstone 

terrains, as in the centrifuge models of Ramberg (1967a,b, 1981a,b; 

see discussion in Section 1.2 (ii)), the basal silicone layers in our 

model have flowed to infill the cores of antiforms developed due to 

active folding and have subsequently undergone ductile flow parallel 

to the base of the model. No density driven upwelling into 

the upper sequence of microlaminates has taken place and there 

is no necking at depth of ductile basal layers as in diapir models. 

In our model, PDMS flowed toward antiformal hinges due to its 

lesser density as granite melt would in both our proposed channel 

flow and diapir models in nature. Sinking of synclines and their 

refolding is similar to that proposed in some vertical tectonic and 

incipient sagduction models (e.g. Bédard et al., 2003 and Chardon 

et al., 1996, respectively). In our model, vertical movements modify 

and accentuate the geometry of folds formed during layer-parallel 

shortening. Vertical movements do not, however, initiate the formation 

of folds as for diapir models. 

3.3. Implications for Archaean tectonics 

Structural styles and tectonic mechanisms may have been 

different in the Archaean in comparison to younger terrains 

(Marshak, 1999; Bédard et al., 2012) due to a higher geotherm, 

enhanced radiogenic heating, and lighter sub-crustal lithospheric 

mantle that favored gravitationally-driven tectonics and ductile 

flow (Rey and Houseman, 2006; Rey and Coltice, 2008). The 

necessity for an initial period of layer-parallel shortening of the 

model is supported by evidence for regional horizontal shortening 

in many Archaean terrains (see reviews by Cawood et al., 2006 

and Bédard et al., 2012). The observation in our model of collapse 

of the thickened superstructure during lateral flow of basal ductile 

layers is consistent with results of numerical simulations that 

suggest that thickened Archaean crust is likely to spread and flow 

under gravity and that thrusting typical of Phanerozoic orogens is 

inhibited (Rey and Houseman, 2006). These authors suggest that 

Archaean lithosphere may have deformed in a similar manner 

to that of modern and thermally mature “hot” orogens such as 

the Himalaya where crustal gravitational forces play a significant


Fig. 6. 3D reconstructions from CT scans showing modification of structures during two stages (a–b) and (c–i) of ductile flow of lower silicone layers with and without upper 

layer (blue). Model width=8cm.(e–i) illustrate enlargements of areas in the model illustrating curvature of fold axes in lowermost marker layers in the superstructure and 

isoclinal recumbent folds in the infrastructure. In contrast, uppermost layers (blue) show little change during flow of basal silicone layers. (For interpretation of the references 

to colour in this figure legend, the reader is referred to the web version of the article.) 

role and channel flow is postulated to be an important process 

(see Section 1.2). Indeed, other centrifuge models incorporating 

simulated channel flow scaled to model deformation in the 

Nepal Himalaya produce structures, such as hinterland-verging 

folds, comparable to those observed in the field (Godin et al., 

2011). 

Analysis of regional gravity data by Peshler et al. (2004) shows 

that granitoid-gneiss domes from a range of post-2.8 Ga terrains do 

not have the common mushroom-shaped form and deep root zone 

predicted by centrifuge models of diapirs (e.g. Ramberg, 1967a,b, 

1981a,b; Talbot, 1974; Dixon, 1975), whereas the geometry of 

pre-2.8 Ga batholiths and greenstones are consistent with diapiric 

models. Greenstone keels (that may extend deeper than adjacent 

batholiths) in post-2.8 Ga terrains are interpreted by Peshler et al. 

(2004) to result from folding during horizontal shortening and not 

diapirism. Their profiles across pre-2.8 Ga terrains show geometries 

similar to those for an equivalent erosion level in our model (Fig. 7b 

and c). An efficient, aggressive weathering system is proposed for 

the Archaean (Lowe and Tice, 2004; Hessler and Lowe, 2006) due 

in part to the absence of plants, the likelihood for strong chemical 

erosion due to acid rain, and the presence of an anomalously buoyant 

sub-continental mantle lithosphere, which would enhance 

exhumation (Groves et al., 2006). For example, high erosion rates 

are indicated from regional studies of the Witswatersrand Basin 

(Groves et al., 2006; Schoene et al., 2008). However, until ca. 

2.8–2.5 Ga, continents were unable to sustain topography >2500 m 

(Rey and Coltice, 2008) and it has also been proposed that, prior 

to the Neoarchaean, much of the Earth may have been submerged 

(Flament et al., 2008), limiting or precluding extensive erosion. 

Channel flow models require a more rigid upper crustal greenstone 

sequence, which again is reached only in the Neoarchaean (Rey 

and Coltice, 2008), in part due to erosion and removal of heatproducing 

element-rich upper crust through extensive erosion that 

resulted in an increase in lithospheric strength with time (Schoene 

et al., 2008). These factors suggest that channel flow may be more 

applicable for Neoarchaean granitoid-greenstone terrains and that 

one or more of the alternative mechanisms described in Section 1, 

the most likely being diapirism and/or partial convective overturn, 

may be more significant for older terrains. In the early Earth, even 

the felsic substrate may not have been strong enough to enable the 

formation of dome and keel structures (Hamilton, 2007a,b) and 

vertical tectonic models, such as Bédard et al. (2003), may be more 

applicable. 

Classical domes, sub-circular in map view, were not fully developed 

in our model. In Archaean terrains, however, our modelling 

suggests that increased curvature of synformal fold hinges and 

the formation of domal structures during channel flow may be 

enhanced by:


Fig. 7. Photographs of three slices through the model at the end of deformation. Parasitic folds are best preserved in synformal areas in upper layered sequence (superstructure). 

If the model were sliced horizontally along the pale green dashed line representing a possible erosion level, then antiforms and domes cored by clear PDMS with screens 

of yellow underlying silicone would be similar to granite and migmatitic gneisses in nature. Note that boudinage of denser, competent (blue) plasticine layers resulted from 

imperfections in model construction. Model height = 22 mm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of 

the article.) 

(i) The presence of or lateral changes in the thickness of a décollement 

horizon between greenstones and gneissic basement (c.f. 

models presented herein and by Yakymchuk et al., 2012). 

(ii) Lateral differences in dip of erosion fronts, or variable amounts 

of erosion that bring about ductile flow oblique to initial fold 

axes (as in similar models by Yakymchuk et al., 2012). 

(iii) Erosion fronts at a high angle to the trend of the orogen that may 

induce (or enhance, cf. Chardon et al., 2009, 2011) orogen parallel 

flow, potentially in addition to orogen normal or oblique 

flow, if erosion fronts of other orientation(s) were also present. 

(iv) Irregularities in the geometry of a basal indentor, such as a 

wedge of sub-crustal lithospheric mantle that (as in models of 

Duretz et al., 2011) may enhance channel flow. 

(v) Discontinuities in the flow pattern during multiple pulses of 

channel flow (c.f. Hollister and Grujic, 2006). 

Our modelling results suggest that a greenstone sequence may portray 

fold styles that differ greatly from underlying gneisses (e.g. 

Figs. 2–4). These differences, although formed in a single event, 

could easily be misinterpreted to suggest different tectonic histories. 

Recumbent folds in basement gneisses or in the core of 

gneiss domes may develop at the same time as upright folds in the 

greenstone sequence and may not necessarily be associated with 

regional thrusting. For example, the folds produced in the “channel” 

in our experiments resemble those depicted in Schulmann et al. 

(2008, especially their Fig. 10) during interpreted channel flow in 

the Moldanubian domain of the Bohemian Massif. Based on the 

localization of structures in our model, and that described above by 

Bucher (1956), early recumbent folds may not be expected across 

an entire granite-greenstone province but may better develop near 

the source of shortening, although during channel flow recumbent 

folds are produced in the entire channel. During channel flow, 

regional variations in the “ponding” of granitoid magma beneath 

a greenstone sequence (simulated by PDMS in our model) or in 

the orientation and extent of the erosion front with respect to initial 

fold axes in the superstructure may result in fold interference 

patterns previously interpreted as requiring superposed regional 

shortening events. Further experiments with variable orientations 

of the erosion front are planned to test this hypothesis. It is likely 

that structures previously thought to result from other mechanisms 

(e.g. superposed folds during two or more periods of bulk shortening, 

diapirism and/or partial convective overturn, extensional 

core-complexes, etc.; see Section 1.2) may be better explained by 

a combination of early folding during regional shortening followed 

by collapse during channel flow. 

Our model demonstrates that (often late) extensional shear 

zones documented in some Archaean granite-greenstone terrains 

(e.g. in Canada, Australia and Brazil: Kusky, 1993; Lobato et al.,


Fig. 8. (a) Schematic cross-section illustrating structures in Meso-Neoarchaean crustal blocks of West Greenland redrawn after Fig. 15e of Windley and Garde (2009). Simple 

“arches and cusps” in granite-greenstones in the upper crust and crustally derived granites are separated from complexly refolded folds in lower crustal, granulite facies 

gneisses beneath a décollement horizon. (No scale was included in the original figure.) (b) Postulated reconstructed initial configuration based on centrifuge model results. 

In this reconstruction, both upper and lower levels were initially deformed by folds of the same wavelength subsequently modified by ductile flow of lower crustal rocks 

along the décollement horizon interpreted by Windley and Garde (2009). (c) Schematic cross-section of the Karelian Massif modified after Leonov (2008) illustrating open 

antiforms and pinched synforms developed during interpreted ductile flow (termed “lateral protrusion” by Leonov, 2008) of underlying gneiss. 

2001) may not require regional extension, but that (i) extension 

may be confined to the superstructure and ductile infrastructure as 

a result of collapse during channel flow, and (ii) the rigid basement 

beneath the ductile infrastructure may not necessarily be extended. 

The model may be particularly applicable as an alternative to 

diapirism in cases where there has been initial thrusting leading 

to crustal thickening and anomalously hot crust (e.g. Zimbabwe 

Craton; Dirks and Jelsma, 1998) that would continue to deform in a 

ductile manner for a sustained period similar to large hot orogens 

in the Phanerozoic. 

Windley and Garde’s (2009, Fig. 15) schematic section 

across West Greenland reproduced in Fig. 8a shows granulite 

facies orthogneiss and gabbro beneath a detachment horizon 

folded by upright structures of regular wavelength that 

refold isoclinal recumbent folds. Above the detachment horizon, 

tonalite–trondhjemite–granodiorite (TTG) orthogneisses and 

volcanic rocks display open arches (rounded antiforms) and cusps 

(tight synforms) typical of granite-greenstone terrains whereas 

beneath the inferred décollement in Windley and Garde’s (2009) 

schematic representation of structures, antiforms and synforms 

are of similar, regular wavelength. Our centrifuge modelling suggests 

that structures beneath a décollement horizon may not be 

significantly modified during channel flow. Our modelling suggests 

that prior to displacement on the décollement horizon in Greenland 

across which fold geometries change (Fig. 8a), sequences above 

and beneath the décollement horizon may have been deformed by 

folds of similar wavelength during the period of crustal shortening 

interpreted by Windley and Garde (2009). A suggested, prechannel 

flow reconstruction is shown schematically in Fig. 8b. 

Our centrifuge model suggests that folds above the décollement 

in Greenland may have opened out during channel flow 

induced collapse during which ductile flow along the décollement 

occurred. It is likely that granitoids were preferentially emplaced 

into antiformal closures similar to migration of PDMS in our 

model. 

3.4. Mechanisms for regional shortening and potential 

implications for enhanced channel flow in the Neoarchaean 

Despite considerable research in Archaean terrains since early 

discussions on the applicability of plate tectonic models to 

the Archaean in Kröner (1981) and the recognition of extensive 

imbrication and tectonic juxtaposition of terrains in some 

granitoid-greenstone and granulite-facies gneiss belts (Windley,


Fig. 9. (a) Lithospheric cross-section redrawn from thermo-mechanical numerical model of Gray and Pysklywec (2010; Fig. 2B) showing structures developed during imposed 

shortening. Imbricate structures in the mantle lithosphere produced in models of Gray and Pysklywec (2010) would appear identical to those interpreted as fossil subduction 

zones on deep crustal seismic profiles across Archaean terrains. However, no subduction nor terrane accretion occurred in the numerical models. Upper crust (green on online 

version) = dolerite rheology, lower crust (pink) = felsic granulite rheology, lithospheric mantle = dry olivine rheology and lower mantle = wet olivine rheology (see Gray and 

Pysklywec, 2010 for details). (b) and (c) Postulated evolution of an enlarged region indicated in (a) based on our centrifuge model of erosion-induced channel flow and the 

indentation-driven channel flow model of Duretz et al. (2011). In a similar manner to physical models of Duretz et al. (2011), indentation of sub-crustal lithospheric mantle 

may assist channel flow leading to exhumation of high-grade felsic gneisses and development of arch and cuspate folds in greenstones in the upper crust. Juxtaposition of 

low-grade granite-greenstone and high-grade gneiss terrains in (c) would likely produce a similar expression on deep crustal reflection seismic profiles as those interpreted 

as juxtaposition of terranes above a fossil subduction zone. These figures suggest that different tectonic interpretations of seismic profiles may be made either supporting 

or contradicting Archaean plate tectonic models for terrane assembly. (For interpretation of the references to colour in this figure legend, the reader is referred to the web 

version of the article.) 

1998) there is still much debate as to the driving mechanisms for 

Archaean tectonics and the relative importance of horizontal versus 

vertical motions (e.g. Lin, 2005; Condie et al., 2006; Bédard et al., 

2012). Ramberg (1967b, chapter 17) proposes that mantle traction 

on the base of the lithosphere may result in areas of crustal shortening 

and extension. Similarly, cratonic mobilism in response to 

mantle flow acting on cratonic keels is proposed by Bédard et al. 

(2012) to explain the development of contractional and extensional 

structures in Archaean terrains. Bédard et al. (2012) suggest 

that there is little evidence for Archaean subduction zones and 

hence that “alpine-style” collisional tectonics and orogenesis did 

not occur in the Archaean. 

Although shallowly-dipping reflectors in the upper mantle on 

deep crustal reflection seismic profiles of Archaean terrains have 

been taken by some authors as evidence for “fossilised” subduction 

zones and interpreted as implying the existence of plate tectonics 

in the Archaean (e.g. Ludden et al., 1993; Calvert et al., 1995; 

Cawood et al., 2006), alternative possibilities exist to explain these 

reflectors. For example, similar mantle reflectors in the North Sea 

are interpreted as extensional shear zones by Reston (1990). Alternatively, 

Robin and Bailey (2009) propose that low-angle reflectors 

could correspond to density boundaries. A geometry resembling 

fossil subduction zones is indeed produced in centrifuge models 

of structures in orogenic belts by Ramberg (1967b, figs. 98–100) 

containing initial density boundaries. Ramberg’s initial models 

comprised (i) upper horizontal layers of different density and 

viscosity, (ii) a middle layer package bounded laterally by and 

overlying (iii) material of uniform and greater density. Deformation 

during centrifugation resulting solely from “rearrangement of 

such unstable mass distributions to a more stable state” (Ramberg, 

1967a,b), i.e. without bulk horizontal shortening, produced dipping 

shear zones in the uniform basal layer(s). Complex fold nappe 

and/or domal, “diapiric” 3 (c.f. Dixon, 1975 and Ramberg, 1980, 

1981a,b) structures were developed in overlying layers depending 

on initial model configuration. 

Shear zones in the upper mantle of equivalent geometry to 

those interpreted on seismic profiles as remnants of subduction 

zones are also produced in numerical models of bulk shortening 

of Neoarchaean continental lithosphere without subduction (Gray 

and Pysklywec, 2010). Gray and Pysklywec (2010) illustrate folding 

and thickening of layers where the upper crust (defined in their 

models by a dolerite rheology) overlies a ductile, felsic granulite 

lower crust, simultaneously with imbrication of a lithospheric mantle 

with a rheology of dry olivine (Fig. 9a). Erosion was not included 

in these numerical models. Similarly, given the geometry produced 

in Gray and Pysklywec’s (2010) model shown in Fig. 9a, indentation 

of the upper mantle into the lower crust may occur in nature during 

further bulk horizontal shortening (Fig. 9b). If this were to be 

coupled with focused erosion in a natural situation comparable to 

the numerical model of Gray and Pysklywec (2010), then channel 

flow may occur (Fig. 9c). In a similar manner to the indentor used 

to induce channel flow in models by Duretz et al. (2011), impingement 

of the wedge of sub-crustal lithospheric mantle portrayed 

in the numerical model into the base of the crust may enhance 

channel flow and exhumation of mid-crustal material (Fig. 9c). Our 

centrifuge model suggests that the fold geometry of greenstones 

may be modified during this enhanced channel flow to produce 

open antiforms and tight synforms (Fig. 9c). 

3 Note that the term “diapir” was not used by Ramberg (1967a,b) to describe the 

structures in his centrifuge models, although they have subsequently been called 

diapirs.


Fig. 10. Cross sections through the Penobsquis salt structure of the Moncton Basin, New Brunswick, Canada, based on depth-converted seismic reflection profiles and borehole 

logs; simplified after Wilson et al. (2006). (a) and (b) Folds developed above an evaporite layer that has acted as a ductile décollement (similar to the PDMS layer in half of 

our model). (c) Collapsed and vertically flattened anticline, interpreted by Wilson et al. (2006) as being formed due to modification of an initially taller salt structure (as in a 

and b) during gravity spreading of the underlying salt. 

A channel flow model, such as portrayed in Fig. 9c, may also provide 

an alternative interpretation for the juxtaposition of deformed 

Archaean granite-greenstone terrains and high-grade migmatite 

and granulite facies gneiss terrains of the same or similar metamorphic 

age attributed to collisional tectonics (Shackleton, 1995) 

or plume tectonics (Sharkov and Bogatikov, 2010). Instead of 

marking a fossil subduction zone beneath the contact between 

granite-greenstone and high-grade gneiss terrains, shallowlydipping 

reflectors on seismic profiles may highlight upper mantle 

shear zones resembling those produced in numerical models by 

Gray and Pysklywec (2010). 

No unique process may explain the dome and keel geometry of 

granite–greenstone terrains. Bodorkos and Sandiford (2006) suggest 

that different mechanisms may apply depending on the age of 

the crust (reflecting temporal changes in heat-producing elements 

and crustal rheology with time) and the thicknesses of greenstone 

sequences. Bodorkos and Sandiford (2006) use this to explain 

differences in geometries between elongate domes in the Neoarchaean 

Eastern Goldfields Province and more circular domes (in 

map view) in the Mesoarchaean East Pilbara in Western Australia. 

Age differences in deformation style and mechanisms are not, however, 

conclusive. Choukroune et al. (1997) note that whilst the 

Superior Province in Canada is dominated by elongate belts the 

younger Dharwar Craton in India is characterized by diapiric dome 

and basin features, which they attribute to a major thermal event 

such as the impingement of mantle plume leading to reheating of 

the lower and middle crust enhancing diapirism, a process common 

in the late Archaean (Windley, 1998). 

Lateral protrusion (a mechanism similar to channel flow; see 

Section 1.3) is proposed by Leonov (2008) to explain folding 

of Palaeoproterozoic rocks producing tight synforms and broad 

antiforms in the Karelian Massif of Baltica (Fig. 8c). This suggests 

that our modelling may also be applicable to some Palaeoproterozoic 

and possibly younger terrains. 

3.5. Implications for mineral exploration 

The sequence of development of structures proposed from 

our centrifuge modelling has implications for regional structural 

interpretation, localization of dilatational sites through “stress 

mapping” (e.g. Groves et al., 2000; Mair et al., 2001), and fluid 

flow modelling (e.g. Drummond et al., 2004) applied to mineral 

exploration in Archaean terrains. Gold mineralization in greenstone 

belts commonly post-dates the peak of regional metamorphism 

(Groves et al., 2000). For example, in the Barberton greenstone 

belt, a classic dome and keel province (Lana et al., 2010b), mineralization 

occurred during a long period of extensional doming 

and buoyant rise of the basement during orogen-parallel extension 

and infolding of the greenstone sequence between gneiss domes 

at least 200 m.y. after peak metamorphism and crustal thickening 

(Dziggel et al., 2010; Lana et al., 2010a,b). Dziggel et al. (2010) 

suggest hydrothermal fluid flow related to mineralization toward

sites of reduced mean stress occurred during late-stage doming 

and extension. Stress mapping to predict sites of reduced stress 

requires knowledge of the far-field stresses during mineralization 

(Groves et al., 2000). It is therefore important in the formulation 

of gold exploration strategies to either determine from field criteria 

whether formation of regional domes occurred due to diapirism 

during regional subhorizontal shortening (e.g. Dalstra et al., 1998), 

during regional extension, or in a stress field that may locally be 

highly oblique to that responsible for initial folds, due to local differences 

in crustal loading without regional shortening or extension, 

as in our model. Field criteria for the recognition that unfolding has 

occurred are described by Harris et al. (2012). 

3.6. Comparison with salt tectonics 

Model results also show similarities to structures developed in a 

sedimentary sequence in fold and thrust belts above a salt layer (as 

reviewed by Hudec and Jackson, 2007). Upright folds with rounded, 

salt cored anticlines similar to that produced in the contractional 

stage of our models are developed during shortening of a sedimentary 

sequence (that contains internal décollement horizons) upon 

a salt layer are observed in nature and produced in numerical and 

sandbox models (e.g. Colletta et al., 1995; Costa and Vendeville, 

2002; Rowan et al., 2004; Yamato et al., 2011). Without interlayered 

ductile horizons within the cover sequence, or where salt layers are 

thin (Rowan et al., 2004), deformation is dominated by thrusting 

and the development of box or kink folds (Cotton and Koyi, 2000; 

Costa and Vendeville, 2002; Sans, 2003; Yamato et al., 2011). 

The presence or absence of a basal PDMS layer in the model presented 

above and models in Yakymchuk et al. (2012) is a dominant 

factor in developing curved fold axes in overlying microlaminate 

layers. Curvature of fold axes (and associated thrusts) is also controlled 

by the geometry of a ductile décollement representing salt 

(Cotton and Koyi, 2000; Luján et al., 2003). In the Santos Basin, 

Brazil, Fiduk and Rowan (2012) document complex dome-andbasin 

fold geometries developed in the sedimentary cover sequence 

during (possibly differential or convergent) flow of underlying layered 

evaporites in which recumbent and sheath folds were formed. 

These structures formed during flow of the evaporates and resemble 

those in the centrifuge model we present above. The thick, basal 

evaporite sequence in the Santos Basin would correspond to our 

whole basal silicone package (and not just the thin PDMS layer 

present in part of our model). 

Similarities in the processes of channel flow and salt ‘escape’ 

or ‘extrusion’ in fold and thrust belts are suggested by McQuarrie 

(2004). Lateral flow of salt during salt withdrawal or salt nappe 

extrusion in the distal part of a sedimentary basin results in a broad 

zone of extension of a proximal sedimentary sequence (e.g. Rowan 

et al., 2004). In sedimentary basins and their simulation in sandbox 

(e.g. Adam and Krézsek, 2012) and numerical (e.g. Ings et al., 

2004) models, the layers above the salt generally deform in a brittle 

manner and normal faults develop. Folds are developed or modified 

due to rollover on faults and not through active folding, as in 

our models. Salt (or its model analogue) upwelling occurs in normal 

fault-controlled synforms, a process we do not see in the centrifuge 

model described herein nor in models of unfolding during collapse 

of the superstructure during channel flow in Harris et al. (2012).An 

example that does, however, resemble our model is the Penobsquis 

salt structure in the Moncton Basin of eastern Canada described 

by Wilson et al. (2006) as the cover sequence above an evaporite 

layer deformed in a ductile manner. The Penobsquis salt structure 

comprises elongate, asymmetric domes (Wilson et al., 2006). Cross 

sections based on depth-converted seismic reflection profiles and 

borehole data in Fig. 10 are interpreted by Wilson et al. (2006) as 

illustrating (i) formation of anticlines during crustal shortening, followed 

by (ii) collapse and vertical flattening of an initially taller 


salt structure during gravity spreading of the underlying salt, to 

produce the structure illustrated in Fig. 10c. Flattening and opening 

out of antiforms and formation of local tight, overturned folds 

in Fig. 10c is mechanically comparable to the process we envisage 

for the formation of some Neoarchaean dome and keel structures 

based on our centrifuge model. 

4. Conclusions 

Centrifuge model-generated broad, open elongate antiforms 

and domes with few parasitic folds and pinched synforms with 

abundant parasitic folds simulate structures within a typical 

Archaean greenstone sequence of sedimentary and volcanic rocks 

overlying migmatitic basement gneiss. A combination of regional 

shortening followed by collapse during channel flow parallel to 

the initial axis of shortening produces structures that resemble 

the dome and keel geometry typical in granite-greenstone terrains. 

Refolded recumbent folds developed during shortening and 

accentuated during channel flow in the in the basal ductile layers 

also resemble folds in Archaean granitic gneisses. It is suggested 

that similar processes may have taken place in the Archaean, especially 

in the Neoarchaean, when erosion, increased rigidity of the 

upper crust, horizontal tectonics, and channel flow are possible 

and may produce early folding and crustal thickening. We propose 

that the typical form of some granite–greenstone belts may 

be a late feature that is characteristic of the Archaean as rocks 

remain ductile after initial folding due to greater heat flow facilitating 

extrusive channel flow and collapse of a denser overlying and 

tectonically thickened greenstone sequence. Results of our study 

and the ensuing hypotheses do not constitute unique solutions 

nor preclude the likelihood for vertical tectonics and diapirism in 

some terrains, especially prior to the Neoarchaean. Whilst there 

are inherent limitations, such as the ability to change rheology 

with time and the limited size of models, centrifuge modelling 

combined with CT scanning is shown to be an excellent means 

of simulating tectonic processes for testing alternate hypotheses, 

providing “4D” visualization of the formation of structures. 

Model results have applications in mineral exploration as they suggest 

changes in tectonic regime during formation of structures in 

greenstone belts and possibly for juxtaposition of Archaean terrains 

of different metamorphic grade. Our model may also help 

explain the formation of structures in some fold and thrust belts 

where sedimentary layers above an evaporite horizon have undergone 

dominantly ductile deformation. Centrifuge modelling also 

suggests factors to include in future numerical models. Further 

modelling is required to develop the hypotheses presented herein, 

and to test the effects of different erosion geometries and to study 

differences in the geometry of structures developed during orogenparallel 

(e.g. Cagnard et al., 2006) and lateral constrictional (e.g. 

Chardon et al., 2011) ductile flow or protrusion tectonics (Leonov, 

1994, 2008), density differences between sequences, granitoid 

intrusion, and changes in rheology with time. 

Acknowledgements 

Acknowledgment is made to the Donors of the American Chemical 

Society Petroleum Research Fund for centrifuge modelling and 

CT scanning support to L. Harris and to NSERC for Discovery grants 

to L. Harris and L. Godin. Modelling was undertaken by C. Yakymchuk 

whilst recipient of an NSERC Summer Research Scholarship. 

CT scanning was performed by L.-F. d’Aigle in the Quebec Multidisciplinary 

Scanning Laboratory. J. Poulin and E. Konstantinovskaya 

assisted in characterization of model materials and development of 

CT scanning techniques. 3D reconstructions were undertaken using 

Osirix Open Source software. The laboratory for physical, numerical


and geophysical simulations was funded by CFI and MELS-Q grants 

to L. Harris with contributions from INRS-ETE, BEG (U. Texas at 

Austin), Sun Microsystems, Seismic Microtechnology, and Norsar. 

S. Marshak (who suggested comparisons with salt tectonics) and 

S. Mukherjee are thanked for their comments and reviews. 

Appendix A. Supplementary data 

Supplementary data associated with this article can be found, 

in the online version, at http://dx.doi.org/10.1016/j.precamres. 

2012.04.022. 

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