Mechanical stratigraphy as a factor controlling the development of a sandbox transfer zone: a...

UNCORRECTED PROOF

Mechanical stratigraphy as a factor controlling the development of a

sandbox transfer zone: a three-dimensional analysis

Antonio Ravagliaa,*, Claudio Turrinib, Silvio Senoa

a3D Modelling Laboratory, Dipartimento di Scienze della Terra, Universita di Pavia, Via Ferrata, 1, 27100 Pavia, ItalybTotal, 2 Place de La Coupole, 92078 Paris La Defense, France

Received 14 April 2003; received in revised form 29 January 2004; accepted 12 April 2004

Abstract

In thrust belts, fold–fault terminations are common features of the structural architecture and can pose complicated problems to unravel, in

particular when two or more terminations are in close proximity. Such terminations usually reflect pre-existing attributes. Amongst the many

factors, lateral variations in the mechanical stratigraphy can control along-strike geometry and kinematics of fault-related folds.

A displacement transfer zone was produced in a compressional sandbox model by means of two adjacent, mechanically different

stratigraphic domains. The experiment allowed two discrete chains to develop in the different domains, so that a complex structural setting

occurred in the connecting area. Periclinal folds, oblique thrust fronts and oblique ramps developed in the resulting transfer zone. The

interaction between periclines in the transfer zone produced lateral culminations in the folded structures. The analysis of displacement across

the structural domains revealed that a significant loss of slip along the faults occurred in the relay zone. In this area, imbricate faulting was

partially replaced by layer-parallel shortening. A linear relationship appears to exist between the bed length of the thrust sheet and the related

fault slip.

q 2004 Published by Elsevier Ltd.

Keywords: Displacement gradient; Lateral/oblique ramps; Fault-related folds; Plunging folds; Sandbox model; Strain partitioning; Transfer zones; Mechanical

stratigraphy; Critical-taper theory; Bed-length versus slip relationship; Lateral heterogeneity

1. Introduction

Displacement transfer zones are complex structural

domains characterized by rapid lateral and vertical changes

in the structural elements. In compressional settings,

oblique and/or curved thrust fronts, en-echelon plunging

anticlines, anastomosing fault patterns, tear faults and

lateral ramps are all evidence of transfer zones. Each of

these geological structures transfers and accommodates

displacement along the strike (Dahlstrom, 1970; Blay et al.,

1977; Wilkerson et al., 2002). Where two or more

overlapping fault segments occur, the slip across a fault

surface decreases and, finally, dies out towards its tip,

replaced by the increasing slip on the contiguous fault.

Displacement profiles between overlapping structures

(Rowan, 1997; Burbank et al., 1999; Nicol et al., 2002)

often display loss of slip that may suggest various

mechanisms of shortening accommodations such as folding,

faulting and layer parallel shortening. In fact, structures may

be the result of the strain accommodated by the arrangement

of these three main mechanisms, which mutually act both

vertically and laterally. At the thrust tips, whether frontal or

lateral, there are localized zones of layer parallel shortening

and layer thickening as well (Coward and Potts, 1983;

Geiser, 1988; Butler, 1992). A correct evaluation of the

shortening accommodation may improve the cross-section

construction and validation process. Besides, the close

proximity of more faults and folds poses some questions

regarding the geometrical compatibility of the structures in

three dimensions. Interference occurs at the lateral tips of

faults and folds where the development of low level and

generally younger thrusts may cause refolding and strain to

occur on the upper and older sheet (Coward and Potts,

1983).

In nature, transfer zones have been widely recognised

and greatly studied in different tectonic settings. In fold-

and-thrust belts, the role played by the lateral variation of

0191-8141/$ - see front matter q 2004 Published by Elsevier Ltd.

doi:10.1016/j.jsg.2004.04.009

Journal of Structural Geology xx (0000) xxx–xxx

www.elsevier.com/locate/jsg

* Corresponding author. Tel.: þ39-0382-505857; fax: þ39-0382-

505890.

E-mail address: [email protected] (A. Ravaglia).

SG 1559—7/7/2004—17:02—MNICHOLLS—110000— MODEL 5 – pp. 1–14

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http://www.elsevier.com/locate/jsg

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the mechanical stratigraphy is frequently considered one of

the most important factors explaining the presence of lateral

and oblique ramps, tear faults and pericline terminations

(Fischer and Woodward, 1992; Letouzey et al., 1995;

Corrado et al., 1998; Philippe et al., 1998; Thomas and

Bayona, 2002, among others).

Several physical models have been built to simulate and

analyse transfer zones. The induced lateral variations were

obtained by imposing:

† variable thickness of the initial stratigraphy (Marshak

and Wilkerson, 1992; Marshak et al., 1992; Corrado et al.,

1998; Marques and Cobbold, 2002; Soto et al., 2002,

2003);

† vertical passive offset of the basement (Calassou et al.,

1993; Corrado et al., 1998);

† horizontal passive offset of the backstop (Calassou et al.,

1993);

† variable basal friction (Colletta et al., 1991; Calassou

et al., 1993; Cotton and Koyi, 2000; Schreurs et al., 2001;

Turrini et al., 2001; Lickorish et al., 2002; Bahroudi and

Koyi, 2003; Luian et al., 2003);

† presence of stationary ‘foreland’ obstacles (Corrado

et al., 1998; Turrini et al., 2001; Lickorish et al., 2002;

Gomes et al., 2003);

† syntectonic sedimentation or erosion (Barrier et al.,

2002; Marques and Cobbold, 2002), and

† non-homogeneous (i.e. interbedded layer composition)

mechanical stratigraphy (Corrado et al., 1998; Turrini

et al., 2001).

In this work we analyse the influence of mechanical

stratigraphy in the construction of a transfer zone and the

interactions occurring between faults and folds, while

evaluating the complexity of the resulting structural style.

We present a detailed analysis of a compressional sandbox

model, being part of a set of experiments focusing on

transfer zone simulation with lateral variation of the

mechanical stratigraphy. We reconstruct the three-dimen-

sional geometry of folds and faults in the resulting transfer

zone, giving a relationship between the lateral variation of a

thrust sheet and the slip along its fault. Finally, we assess the

partitioning of strain both along strike and vertically.

2. The experiment

For a complete historical review of modelling techniques

and a complete literature list, see Koyi (1997), Cobbold and

Castro (1999), Ranalli (2001) and Schellart (2002). We

reproduced a displacement transfer zone by simultaneously

deforming two adjacent, mechanically different, strati-

graphic domains within a sandbox apparatus (Fig. 1). The

first domain was a multilayer (non-homogeneous domain)

composed of sand and glass microbeads beds; the second

was a sand-only domain (homogeneous domain). The

thickness was constant across the model. The boundary

(B) between the two sectors was parallel to the shortening

direction and was located along the central part of the

model.

The initial model was 42 cm long, 30 cm wide and

1.8 cm high. We used two types of granular materials with

different physical parameters: sand, and glass microbeads.

The sand has an angle of internal friction (f) of 338 and a

grain size of 100–300 mm. In the near side of the model, i.e.

from 0 to 15 cm along-strike, two layers of 3 mm each of

glass microbeads replaced the sand, at 6 and 12 mm from

the base of the model. Glass microbeads are suitable for

simulating natural rocks because they enable low basal

friction detachment (Sassi et al., 1993) and inter-strata slips

(Turrini et al., 2001) to occur. Glass microbeads have

f ¼ 248, due to their high sphericity and rounding

(Schellart, 2000), and a grain size of 300–400 mm. For

this reason, in the non-homogeneous domain, f had an

average value of 308. The two domains were shortened over

the same basal detachment, this having a friction angle (f)

of 328.

The foreland side of the box was not closed. As soon as

sand started falling down from the foreland edge of the box,

but only in the far left-hand side (homogeneous domain), the

total shortening was 18.5 cm (44%) and the experiment was

Fig. 1. Initial model configuration. (a) Block diagram of the experimental

conditions (not to scale). (b) Sketch of the mechanical stratigraphy of the

two domains, with the inter-strata detachment indicated. B ¼ boundary

line; H ¼ homogeneous domain; NH ¼ non-homogeneous domain; MB ¼

glass microbeads; TL2 and TL4 ¼ top layers used for measurements;

L5 ¼ layer 5.


A. Ravaglia et al. / Journal of Structural Geology xx (0000) xxx–xxx2

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considered finished. During the experiment, neither erosion

nor sedimentation were simulated. The analysis of 1-cm-

spaced sections across the final model, combined with a

progressive snap-shot of the evolution in plan, aided the

three-dimensional reconstruction of the obtained defor-

mation geometries through time and space. The opportunity

to see clear cut-offs allowed the dip length of the thrust sheet

and slip along the faults to be measured accurately.

Measurements were performed on two stratigraphic levels.

The model transfer zone was analysed by 11 cross-

sections, taken every centimetre from 10 to 20 cm from the

near side of the sandbox apparatus. The interpreted

structures were then measured and contoured to reconstruct

the three-dimensional deformation distribution across the

modelled transfer zone. Structure contour maps for faults

and layering were constructed based on digitized interpret-

ations of cross-sections.

3. Results

Two distinct thrust belts develop in each domain (Figs. 2

and 3). In the non-homogeneous domain, far from the

boundary line, first-order thrust sheets develop, along with

second-order thrust faults, these being detached over the

shallow glass microbeads level (Fig. 4d–g). Conversely, in

the homogeneous domain, only first-order thrusts occur. In

both the domains, only small backthrusts appear, which are

slightly more developed in the homogeneous domain.

The model kinematics follows a generic piggyback

sequence, from the hinterland to the foreland, without

significant out-of-sequence events. The main feature of the

experiment was the alternate development of the four

external thrusts (labelled 5–8; Fig. 2) in the two compart-

ments of the model. After the continuous growth of thrusts 3

and 4 over the entire width of the sandbox, thrusts 5–8

developed as related to discontinuous periclinal faulted

folds plunging towards the centre of the model. Thrusts 5

and 7 formed in the homogeneous domain, then rapidly

propagated laterally with fronts curved towards the middle

of the model, and crossed the boundary for a distance of

4.2 cm. Thrust faults 6 and 8 developed in the non-

homogeneous domain, then propagated along-strike with

oblique fronts across the boundary. Such thrusts extended

into the opposite domain only for 2 cm.

3.1. The transfer zone

A 6.2-cm-wide transfer zone parallel to the shortening

direction formed in the centre of the model, representing the

connection between the homogeneous and non-homo-

geneous domains. This zone was characterized by the

contemporaneous occurrence of all faulted folds. The close-

up photograph of the transfer zone in plan view (Fig. 3)

reveals a braided pattern of thrust fronts. Thrusts 3 and 4

were almost parallel to the backstop away from the

boundary line and are slightly oblique in the transfer zone.

For instance, thrust 4 in the transfer zone is oblique and has

an acute angle of 728 with respect to the shortening

direction. Thrust faults within the transfer zone tend to form

with strike directions oblique (rather than perpendicular) to

the shortening direction, and do not tend to rotate during the

temporal evolution of the thrust system. The oblique front

connects two thrusts, different both in bed length and fault

slip. Structures from 5 to 8 are nearly confined to the domain

where they grew, and terminate towards the centre of the

model with an oblique front. All of the pericline-related

thrust surfaces branch from the base of the model and

laterally join the adjacent and earlier surface (thrusts 5–8;

Figs. 3 and 4).

Fault surfaces in the transfer zone are oblique ramps

connecting frontal ramps on both sides (Figs. 5 and 6a).

Structural contours of faults 6–8 exhibit the geometry of the

oblique ramps that are normally associated with plunging

anticlines. The oblique ramp along fault 6 dips at 258 and the

angle between the strike of the ramp and the transport

direction is 558. Along fault 7, the oblique ramp is very

narrow and similar to a lateral ramp. The oblique ramp dips

at 288, and the angle between the strike of the ramp and the

transport direction is 268. Fault 8 shows a wide oblique ramp

dipping at 258; the angle between the strike of the ramp and

the transport direction is 508. At depth, thrust fault 8

branches off the footwall of the adjacent thrust fault 7 (Fig.

4b). Similarly, thrust surface 6 branches off the footwall of

fault 5. The position of the oblique ramps shows that the

transfer zone, or the interference area, is more developed in

the non-homogeneous domain than in the homogeneous

one.

3.2. Thrust sheet geometry

Except for structures in the hinterland, which are

continuous throughout the model, the four more external

folds are periclines plunging towards the centre of the model

(Figs. 3, 6b and 7). The bed lengths of such thrust sheets

strongly decrease along-strike, and become zero approach-

ing their tip in the opposite domain (Fig. 8a). In the non-

homogeneous domain, thrust sheets have the shape of

recumbent thrust related folds (see sheets 6 and 8, Fig. 4d–

g), whereas in the homogeneous domain they are upright

thrust related folds (see sheets 5 and 7; Fig. 4a–c).

Only in the non-homogeneous domain do two orders of

structures develop; first-order faulted folds branch from the

basal detachment, and second-order smaller thrust sheets

detach over the shallow glass microbeads bed. The second-

order structures are poorly developed because of the mutual

competition between the two weak layers. Indeed a higher

basal friction decollement would have likely produced a

greater second-order structures occurrence (Turrini et al.,

2001, see their fig. 18). However, second-order thrusts

terminate as they approach the homogeneous domain (Figs.

2 and 3).


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The four external periclines have been individually

contoured at the top of layer 4 (labelled TL4 in Figs. 1 and

4). Interference patterns reveal that interplay occurred

between different age structures, especially for old thrust

sheets; for instance, towards the depression of fold 5 (Figs.

6b and 7a) a subculmination exists, probably due to the

uplift of fold 6. Fold 6 is influenced by its lateral and

underlying fold 7, and also exhibits a subculmination (Fig.

7b). Folds 7 and 8 do not display any lateral subculmina-

tions. Fold 8 reveals a very strong deflection of the main

hinge at the end of the experiment (558 with respect to the

shortening direction; Fig. 7d) and is parallel to the thrust

front. Finally, thrust sheets developed in the non-homo-

geneous mechanical stratigraphy seem to have a greater bed

length (Fig. 8a).

4. Discussion

4.1. Transfer zone arrangement

The model transfer zone is a broad structural domain that

develops parallel to the shortening direction. It is charac-

terized by the occurrence of fold terminations, axial plunge

and oblique fold hinges Faults and folds in the transfer zone

undergo more than a single phase of deformation. As a

result, faults reveal secondary deformation due to uplift

(Figs. 5a and 9a), and periclines display lateral subculmina-

tions towards their depressions (Fig. 7a and b). The related

pericline fault surfaces appear to be physically linked (Fig.

9b). Older folds do not display a clear horizontal deflection

of the hinge line, but they seem to maintain their original

pattern throughout the entire experiment (compare with Fig.

2).

4.2. Thrust fault displacement

We measured thrust front displacement in plan view

along profiles 10, 15 and 20 (Fig. 10) during different steps

of the deformation, using the reference grids in the hanging

wall and footwall of each thrust. The displacement pattern is

similar in the domains far from the boundary line (compare

Fig. 10a and c), yet the displacement seems greater in those

thrusts developed in the non-homogeneous domain. Such

divergence is clearer along the central profile (Fig. 10b);

undoubtedly, structures 6 and 8 accommodate larger

displacement relative to structures 5 and 7. The mechanical

stratigraphy, different in each domain, seems to control the

displacement (activity) of thrusts.

We also measured and plotted fault slip in the vertical

Fig. 2. Map view of the deformation kinematics. The dotted line (B) represents the boundary between the two domains. The white square grid is 5 cm. (a) Initial

state; (b) after 5 cm of shortening (11.9%); (c) after 8 cm of shortening (19%); (d) after 9 cm of shortening (21.4%); (e) after 13 cm of shortening (31%); (f)

after 14.5 cm of shortening (34.5%). B ¼ boundary line; H ¼ homogeneous domain; NH ¼ non-homogeneous domain.

Fig. 3. Detailed map view showing the braided pattern of thrust fronts in the displacement transfer zone at the end of the experiment; traces of seven cross-

sections are shown. Dotted lines represent second-order thrust faults. S ¼ cross-section position from the near side, in centimetres. Glass microbeads layers are

indicated with arrows. B ¼ boundary line; H ¼ homogeneous domain; NH ¼ non-homogeneous domain.



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UNCORRECTED PROOF

plane of shortening for each section in the final model. Fault

slip diminishes toward pericline depressions (Fig. 8b): the

cumulative amount of slip for all of the faults exhibits a

significant loss in the transfer zone: nearly 41% with respect

to the ‘normal’ areas. As for thrust sheet length, slip also

seems to be greater in the non-homogeneous than in the

homogeneous domain.

From the wedge theory perspective (Davis et al., 1983),

the critical taper simplified for a dry cohesionless wedge

relies on the friction of the basal decollement (according to a

direct relationship) and on the friction of the analogue

materials (according to a reverse relationship). In our model,

basal friction is the same in both domains (f ¼ 328), the

friction angle in the homogeneous domain is f ¼ 338 (only

sand) whereas in the non-homogeneous domain the use of

glass microbeads lowers the average angle of internal

friction of the material (f ¼ 308). Applying the approxi-

mated equation for a dry cohesionless wedge (Liu et al.,

1992) and using values of the two domains, we obtain a

higher theoretical critical taper in the non-homogeneous

domain (a þ b ø 128) with respect to the nearby homo-

geneous domain (a þ b ø 10.68), made up of sand solely

(f ¼ 338). As always, in the analogue experiment, the early

steps of deformation involve building up the topographic

slope to the critical angle; as a consequence, in both

domains the early shortening was absorbed by closely

spaced thrust faults. Thus, the critical value was exceeded

first in the homogeneous domain because it requires a lower

critical taper, and as a result, the deformation shifted

forward to produce a longer thrust sheet (thrust 5; Fig. 2c);

in contrast, at the same time in the non-homogeneous

domain the topographic slope was not high enough to

deform the foreland. Once the critical angle was surpassed

in the non-homogeneous domain as well, the front of

deformation progressed towards the foreland (Fig. 2d).

Fault kinematic analysis involves measurement of the

Fig. 5. Contoured fault surfaces. Elevation from the base of the model is in

centimetres, the contour interval is 0.2 cm. (a) 5, (b) 6, (c) 7 and (d) 8.

FR ¼ frontal ramp; OB ¼ oblique ramp. The dotted line represents the

boundary (B) between the non-homogeneous (NH) and the homogeneous

(H) domains. Plots are not overlapped.

Fig. 4. Seven cross-sections cut in the transfer zone. Dotted lines represent

second-order thrust faults. S ¼ cross-section position from the near side, in

centimetres. Glass microbeads layers are lightest grey and are indicated

with arrows. TL2 and TL4 ¼ top layers used for measurements; B ¼

boundary line; H ¼ homogeneous domain; NH ¼ non-homogeneous

domain.



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UNCORRECTED PROOF

slip of each cut-off using the footwall ramp as a pin (Fig.

11). Slip measures are then contoured and projected onto a

vertical plane. The result is a map of the distribution of slip

on the fault surface. Such analysis allows improved three-

dimensional interpretation of faults and their interrelation-

ship: for instance, it is useful in the investigation of

branching areas (Needham et al., 1996). All of the maps

show that slip varies both along strike and along dip. The

greatest slip normally occurs nearly in the middle of the

stratigraphy and where the thrust front is more advanced

(compare Figs. 3 and 11). Faults display a small amount of

slip approximately along cross-section 15, which corre-

sponds to the oblique ramps. Fault surfaces in the

homogeneous domain (faults 5 and 7; Fig. 11b and d)

seem to display a greater lateral gradient of slip than fault

surfaces in the non-homogeneous domain (faults 4 and 6;

Fig. 11a and c). On every fault surface, the along-strike

gradient of slip depicts linkage between two faults.

4.3. Thrust sheet bed length versus slip

As previously shown, thrust sheet bed length and slip

along the faults decrease simultaneously towards the

pericline depressions (Figs. 8a and b and 11). The plot of

thrust sheet bed length against slip, measured on each cross-

section (Fig. 8c), displays a good linear relationship and,

mainly, a similar coefficient of the regression line for

periclines 5–7 (here we consider these three thrusts as a

single series, not having appreciable differences). Despite

both the slightly greater bed length and map view

displacement measured in the non-homogeneous with

respect to the homogeneous domain, thrusts are not enough

to suggest any general rule in order to distinguish between

the two domains. This plot can help to predict the three-

dimensional geometry of a ‘steady’ thrust sheet, provided

that either the footwall or hanging wall are known or, in

exchange, the slip is determined. Also, it may afford the

geologist a test of the activity of a thrust when compared

with others of the same belt. The plot shows that slip

S ø 0.65L or, alternatively, thrust sheet bed length

L ø 1.54S, at least for this experimental configuration.

Fig. 6. Three-dimensional visualization of faults (a) and folds (b) in the

transfer zone. The dotted line represents the boundary (B) between the non-

homogeneous (NH) and the homogeneous (H) domains. SC ¼ are lateral

sub-culminations; T ¼ lateral terminations.

Fig. 7. Contoured fold surfaces, top layer TL4. Elevation from the base of

the model is in centimetres, the contour interval is 0.05 cm. (a) 5, (b) 6, (c) 7

and (d) 8. The dotted line represents the boundary (B) between the non-

homogeneous (NH) and the homogeneous (H) domains. Plots are not

overlapped. SC ¼ sub-culminations.



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Only thrust sheet 8 greatly departs from this trend, as its slip

is low with respect to its bed length; the slope of the

correlation line is shallow and reveals its young age; indeed,

this structure was the youngest fault and still active at the

end of the experiment.

4.4. Strain partitioning

The loss of slip along the faults (Fig. 8b) reveals that a

complicated strain partitioning may occur in the model

transfer zone domain. As a consequence, it is necessary to

analyse exactly how the strain is accommodated across the

transfer zone. The kinematics of layer-parallel shortening

has been described in two-dimensional sandbox models

(Mulugeta and Koyi, 1992; Koyi, 1995). Assuming that the

model deformation can be partitioned into three main

mechanisms, i.e. layer-parallel shortening, faulting and

folding, we measured bed shortening (Fig. 12a) using the

following methodology (see also Mulugeta and Koyi,

1987):

Input:

Fault heave (Fh);

Fig. 8. Plots of (a) the lateral variation of thrust sheet bed length (L)

measured along each cross-section; (b) the lateral variation of the slip (S)

measured along each cross-section; (c) the thrust sheet bed lengths (L)

against slip (S) for each pericline showing a good linear relationship (filled

line). Only thrust sheet 8 diverges from this trend due to its younger age

(dotted line). Here the regression line has been constructed using thrusts 5–

7 as a single series. The layer considered is TL2 of Figs. 1b and 4.

Fig. 9. Simplified contour diagrams. (a) Upwardly curved fault surface 5

due to a second phase of deformation. (b) Hard-linked relationship between

pericline fault surfaces 6–8. The greyscale represents the elevations from

the base of the model (in centimetres). The dotted line represents the

boundary (B) between the non-homogeneous (NH) and the homogeneous

(H) domains.



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UNCORRECTED PROOF

Thrust sheet bed length (Lb);

Initial length of the model (Li);

Final length of the model (Lf).

Output:

Faulting ¼ (Fh);

Folding ¼ Lb 2 (Lf þ Fh);

Layer-parallel shortening ¼ Li 2 Lb.

Because we measured the heave as the horizontal

distance between cut-offs, and we know that older and

internal thrust faults are rotated, we certainly measured a

heave value that is less than the actual. Consequently, the

calculated amount of folding could be slightly greater than

the actual. Further measurements have been done taking

into account the vertical rotation of thrust faults. Results

show an increase in the amount of faulting and a decrease in

folding, yet the pattern of the curves remained the same.

Despite the imperfections in the method, what we wished to

emphasize was not an absolute amount of partitioned strain,

but the relative difference between the two domains.

In the zone where the thrust fronts are oblique, some out-

of-plane motion took place, as displayed by the distortion of

the white grid at the topographic surface (Figs. 2 and 3).

Nevertheless, the amount of slip loss, nearly 41% with

respect to the adjacent ‘normal’ area, is too high to be

explained by out-of-plane motion only. The strain partition-

ing analysis shows that (Fig. 12b):

† far from the boundary line, faulting accounts for 50–

60% of shortening and is greater in the homogeneous

domain than in the non-homogeneous;

† layer-parallel shortening is 30–40% and is slightly

greater in the non-homogeneous domain;

† folding is relatively low, but is also greater in the

Fig. 10. Displacement diagrams showing the kinematic evolution measured

in map view. (a) Thrusts 4, 6 and 8 along profile 10 in the non-homogeneous

domain (NH). (b) Thrusts 4–8 along the boundary line (B). (c) Thrusts 4, 5

and 7 along profile 20 in the homogeneous domain (H).

Fig. 11. Maps of the slip on the faults. (a) 4, (b) 5, (c) 6, (d) 7 and (e) 8.

Contour interval is 0.4 cm. Black dotted lines represent the lower and upper

limits of the footwall cut-offs. Black hatched lines (BL) represent branch

lines between faults. Vertical grey dotted lines represent the boundary (B)

between the non-homogeneous (NH) and the homogeneous (H) domains.



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UNCORRECTED PROOF

non-homogeneous domain because of the strength

contrast (Erickson, 1996).

A comparison between deep and shallow structural levels

allows the following observations:

† faulting is greater at depth in the non-homogeneous

domain and, conversely, is greater near the surface in the

homogeneous domain;

† layer-parallel shortening is everywhere greater at deeper

structural levels;

† folding in the non-homogeneous domain is greater at

shallow structural levels, but no differences seem to

exist in the homogeneous domain.

Finally, the curves seem to indicate that the transfer zone

is more extensive in the non-homogeneous domain. The

most important feature is that, laterally along strike and

entering the transfer zone, imbricate thrusts tip out and are

replaced by layer-parallel shortening. Layer-parallel short-

ening is the dominant mechanism of deformation along the

boundary between the two domains. The distribution pattern

of the three mechanisms along the sections away from the

transfer zone, i.e. in the ‘normal’ area, exhibits some

differences with respect to the outcomes of Mulugeta and

Koyi (1987), as they found that layer-parallel shortening

accounted for 41%, and imbricate faulting for 44%, of the

final shortening.

In the bed length balancing method employed, short-

ening due to faulting is the only value directly measured on

the sections: folding and layer-parallel shortening are

calculated from other parameters. For this reason, we tried

to evaluate the model deformation better by measuring a

single layer thickness variation. We chose the shallow layer

5 (labelled L5 in Fig. 1b), which is made of glass

microbeads in the non-homogeneous domain. Contours of

the percentage thickness variation have been overlaid on

fold structure maps, in which both thickening and thinning

of beds is displayed (Fig. 13). The analysis of the plots

reveals that thickening:

† in general, increases towards periclinal depressions (over

oblique ramps);

Fig. 13. Contouring of the percentage of layer L5 thickness changes for

each pericline. (a) 5, (b) 6, (c) 7 and (d) 8. The greyscale refers to increase in

thickness, and the symbols scale refers to decrease in thickness (contour

interval 10%). The contour interval of folds (in grey) is 0.1 cm. Vertical

grey dotted lines represent the boundary (B) between the non-homogeneous

(NH) and the homogeneous (H) domains.

Fig. 12. (a) Bed length balancing method used in this paper. Sh ¼

shortening; Lb ¼ thrust sheet bed length; Fh ¼ fault heave; Lf ¼ final

length of the model; Li ¼ initial length of the model; L ¼ layer used as

reference. (b) Diagram showing how the strain is partitioned between

faulting (squares), folding (triangles), and layer-parallel shortening (circles)

for shallow (TL4, black lines) and deep (TL2, grey lines) structural levels.

Layer-parallel shortening greatly increases in the transfer zone and replaces

the amount of shortening due to faulting.



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UNCORRECTED PROOF

† in detail, is greater in the homogeneous than in the non-

homogeneous domain (Fig. 13a and c);

† locally, slightly affects thrust sheet footwalls.

Conversely, thinning:

† in general is more distributed in the non-homogeneous

domain because the glass microbeads layer is probably

affected by layer-parallel shear mechanisms (Fig. 13b

and d);

† occurs in all the fold forelimbs of both domains

because of the steep topographic slope.

Now, a question arises from these outcomes: is the layer-

parallel shortening in the sand partitioned in volume loss

and compaction (that in nature means porosity reduction,

cleavage and stylolite formation) or is layer-parallel short-

ening transferred in increased thickness of a layer, i.e. the

volume remains constant? Experimental studies on the

compaction of dry clean sands (Rutter and Wanten, 2000)

show almost no volume loss or, at least, less than 5%

(Nowak et al., 1998). Such experiments are usually

performed at higher strain than those produced in a sandbox

experiment and on confined samples, so the results can be

considered really extreme. Lohrmann et al. (2003) found

very little either positive or negative variations of thickness

(volume change) of various sand samples in their shear tests

at very low normal stress (comparable with sandbox

experiments). The difference is a function of the preparation

technique of the sample. Our sandbox models are more

likely suitable to dilation instead of compaction, according

the preparation technique we used.

Regarding direct measurements in sand models, Wilk-

erson et al. (1992) recorded thickness changes in sand layers

due to pure shear. They claimed the area increase had

“resulted from dilation accompanying frictional sliding

between sand grains and probably bear no direct relation to

dilation magnitude in real rocks”. On the contrary,

measurements of layer parallel compaction performed by

Koyi and Vendeville (2003) in sand wedges revealed an

area loss ranging from 2 to 5.8% (as the basal dip of the box

changes) ascribed to the reduction of porosity between the

sand grains, which corresponds to a layer parallel shortening

ranging from 9.5 to 15%. They conclude the deformation is

partitioned by both compaction and thickening.

Our measurements concerning area changes (performed

on all sections in the transfer zone) resulted in a widespread

increment of 4.6% on average, showing a moderate

maximum in the transfer zone. The dilation can be ascribed

to the disorder of the normal arrangement of sand grains due

to the deformation and also to the presence of faults. In sand

models faults are shear bands in which the normal

arrangements of grains changes, resulting in a decrease of

the bulk density (Colletta et al., 1991). This suggests that,

where faults are more numerous (in the transfer zone),

dilation is also greater. If any sand compaction locally

occurred, it would not be possible to detect with this

method. Such outcomes allow us to state that the calculated

layer-parallel shortening is partitioned in layers that thicken

variably from place to place; layer-parallel shortening

accommodates greater displacement in the transfer zone

(Fig. 13) and, along with dilation between sand grains,

globally resulted in a volume increase. So, once again, it is

worthwhile carefully considering the contribution of layer-

parallel shortening and layer thickening when dealing with

section balancing, and in particular in transfer zones where

both parameters reach greatest values.

We did not evaluate out-of-plane movement in the

accommodation zone, but we think it accounts for only a

very small amount of deformation. Plotting of the values of

ramp dip and strike in the diagram of Apotria et al. (1992)

(Fig. 8) resulted in an out-of-transport calculated deflection

plane of about 38. A numerical approach by Strayer and

Suppe (2002) demonstrates that, in their experimental

conditions, out-of-plane displacement of material can

occur but is very little, in particular “is an order of

magnitude or two less than in-plane displacement”.

In general, in the transfer zone, the cumulative slip of two

overlapping faults is less than in the ‘normal’ area because

part of the deformation is transferred from faulting (slip) to

layer-parallel shortening. Because we did not observe any

thrust front rotation during the kinematic evolution, we can

state that no significant along-strike extension occurred to

accommodate strain in the fold (Husson and Mugnier,

2003). We did not detect any tear faults, which in nature are

reported to possibly account for the accommodation of

displacement gradients along the strike of the structures

(Mueller and Talling, 1997).

4.5. Comparison with previous work

Laterally variable mechanical stratigraphy produced by

means of interbedded ‘weak’ analogue materials has been

only tested by Corrado et al. (1998). They used a Newtonian

silicon gum layer instead of a brittle one. They also

simultaneously applied a lateral thickness variation and a

vertical step in the basement. Therefore, more than a single

parameter influenced their modelled transfer zones. Never-

theless, in map view it is possible to note that the

interbedded Newtonian decollement resulted in a braided

architecture of thrust fronts and a diachronous kinematics

between the two compartments.

The general braided architecture of our transfer zone

looks similar to fold and fault geometries reported by other

authors and reproduced by means of different boundary

conditions (sand thickness variations: Marshak and Wilk-

erson, 1992; Marshak et al., 1992; Calassou et al., 1993).

There also appears to be a close similarity with a model by

Cotton and Koyi (2000), in which two different basal

detachments (frictional and ductile) were placed next to

each other. There the transfer zone also corresponds to a

structural domain separating two different styles of fold and



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UNCORRECTED PROOF

thrust belts. In fact, the use of Newtonian analogue materials

allows the development of counter regional thrusts, which

never occur in brittle stratigraphies. Differently from other

experimental set ups (e.g. Cotton and Koyi, 2000; Bahroudi

and Koyi, 2003), our lateral heterogeneity did not allow tear

faults to occur.

Surface contouring allowed a better geometrical descrip-

tion, showing that the dip of oblique ramps and frontal

ramps are similar, and comparable with the outcome

described by Calassou et al. (1993). In our experiment, we

emphasized the results of the interaction between different

fault surfaces and periclines in the transfer zone, resulting in

the development of lateral sub-culminations of folds and

convex upward fault surfaces never described before in

experimental literature.

The alternate thrust propagation on both sides of the

transfer zone closely resemble the kinematics resulting from

lateral variation of backstop geometry and described by

Calassou et al. (1993). More generally, the diachronous

kinematic development has been observed in all the

experiments characterized by any kind of lateral hetero-

geneities (both initial and sin-deformational) and braided

architectures of thrusts.

The along-strike strain partitioning analysis has been

performed before only by Liu and Dixon (1991) and Dixon

and Liu (1992) in centrifuge models. They analysed the

partitioning of strain using a different technique than the one

used above. Their results show that layer-parallel shortening

is the main deformation mechanism at deep structural levels

(63% of the shortening), whereas folding represents the

principal mechanism at shallow levels (55% of the short-

ening). Displacement transfer occurs along strike, as the

occurrence of pericline and en-echelon structures reveal, but

the low measurement resolution probably did not allow the

authors to relate layer-parallel shortening lateral variations

to the transfer zones.

5. Conclusions

The model along-strike mechanical stratigraphy zonation

strongly affected the experimental geometries and kin-

ematics. The final transfer zone separates two mechanical

domains being deformed, so that the following conclusions

result:

1. The kinematic sequence indicates a discontinuous

development of the deformation front, which propagates

intermittently and differently across the homogeneous

domain and the non-homogeneous domain.

2. The lateral mechanical anisotropy resulted in the

formation of oblique ramps. At the surface, oblique

ramps are connected to oblique thrust fronts. An oblique

thrust front can be (a) the linking structural feature

between two thrust fronts with different wavelengths or

(b) the lateral termination of a thrust sheet.

3. Analysis of the fault displacement suggests a compli-

cated displacement transfer across the model structures;

approaching the transfer zone, where the pericline

depressions occur, slip along the faults is substituted by

more layer-parallel shortening, which becomes the

greatest deformation mechanism in the transfer zone,

whereas folding remains nearly stable.

4. A linear relationship between thrust sheet bed length and

slip appears to exist in the model structures. This

relationship can help to predict the three-dimensional

attributes of an emplaced thrust sheet. In addition, it may

give some insights on the activity of a thrust sheet with

respect to others of the same belt.

5. Detailed measurements of layer thicknesses reveal an

increment towards the transfer zone. A decrease in

layer thickness (i.e. tectonic thinning) particularly

occurs in the forelimbs of the model folds.

The performed analogue model clearly reveals the

different features that might occur across a compressional

type, fold-fault related transfer zone. The resulting defor-

mation fabric provides a three-dimensional overview of the

possible architecture complexity arising within such a

structure domain. The analysis of the continuous set of

data across the experimental structures helps the quantitat-

ive evaluation of a transfer zone analogue to be performed.

The real time evolution of the model transfer zone, observed

on map view, suggests how deformation can progress within

a thrust belt system as it attempts to link different structures,

geometrically independent along-strike.

Application of the derived knowledge to natural transfer

zone situations might be used to reduce the uncertainty in

the reconstruction of the structure under evaluation.

Eventually, the ‘model’ criteria presented or discussed by

this study could represent alternative solutions to the

interpretation of those geometries, which are partially

exposed at surface or badly imaged at depth. As such, the

model results could be a valid support in the prediction and

prognosis of difficult targets, which might be suspected to

occur in a transfer-zone contraction related structure setting.

Acknowledgements

This work was supported by a PhD grant to A. Ravaglia

and is a portion of his dissertation. Constructive reviews by

Brent A. Couzens-Schultz and Nicholas B. Woodward

greatly improved this manuscript. David A. Ferrill is

especially thanked for his careful final review.

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Mechanical stratigraphy as a factor controlling the development of a sandbox transfer zone: a...

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