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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: aravaglia@manhattan.unipv.it (A. Ravaglia).
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UNCORRECTED PROOF
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.
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UNCORRECTED PROOF
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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UNCORRECTED PROOF
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UNCORRECTED PROOF
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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UNCORRECTED PROOF
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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