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Thrust wedges with décollement levels and syntectonic erosion: A view from analog models

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Analog sandbox models have been set up to study the impact of syntectonic erosion on thrust wedges with one and two décollement levels. Different accretion mechanisms are activated depending on interactions between surface processes and wedge mechanics: frontal accretion, backthrusting, underthrusting and underplating due to décollement induced duplex formation at depth. These mechanisms may function simultaneously, being located at different parts across the wedge. For all the experiments, a high friction is imposed at the base of models and the volume of eroded material remains equal to the volume of newly accreted material, maintaining a constant surface slope during the shortening. Erosion limits the forward propagation of thrust wedges and favors the underthrusting of basal layers allowing duplex formation. Erosion promotes development of major backthrusts in the thrust wedges without or with one décollement, but no backthrusts was formed in the wedges with two décollements. Slow erosion allows lower extent of basal underthrusting in comparison with regular-rate erosion. Variations in the erosion taper lead to changes in duplex geometry and exhumation rate in thrust wedges with one or two décollements. The 6° erosion taper promotes formation of antiformal stack at the rear part of thrust wedge, high rate of basal underthrusting and high extent of erosional removal. The cover layers are nearly completely eroded above the antiformal stack and form the synformal klippe in frontal part of thrust wedges. The 8° erosion taper allows development of individual ramp-anticlines and active forward thrusting of cover layers above the décollement and low rate of basal underplating below it, with consequent low extent of erosional removal. The results of our experiments support the observations on structural evolution and erosion in the Alberta Foothills of the Canadian Rockies.
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Thrust wedges with décollement levels and syntectonic erosion: A view from
analog models
E. Konstantinovskaya
a,
, J. Malavieille
b
a
Institut national de la recherche scientique, Centre Eau, Terre et Environnement (INRS-ETE), 490, de la Couronne, Quebec City (QC), Canada G1K 9A9
b
Géosciences Montpellier, CNRS UMR 5243, Université Montpellier 2, 34095 Montpellier, Cedex 5, France
abstractarticle info
Article history:
Received 19 August 2010
Received in revised form 7 January 2011
Accepted 14 January 2011
Available online 15 February 2011
Keywords:
Thrust wedge
Antiformal stack
Analog modeling
Erosion
Exhumation
Canadian Rockies
Analog sandbox models have been set up to study the impact of syntectonic erosion on thrust wedges with
one and two décollement levels. Different accretion mechanisms are activated depending on interactions
between surface processes and wedge mechanics: frontal accretion, backthrusting, underthrusting and
underplating due to décollement induced duplex formation at depth. These mechanisms may function
simultaneously, being located at different parts across the wedge. For all the experiments, a high friction is
imposed at the base of models and the volume of eroded material remains equal to the volume of newly
accreted material, maintaining a constant surface slope during the shortening. Erosion limits the forward
propagation of thrust wedges and favors the underthrusting of basal layers allowing duplex formation.
Erosion promotes development of major backthrusts in the thrust wedges without or with one décollement,
but no backthrusts was formed in the wedges with two décollements. Slow erosion allows lower extent of
basal underthrusting in comparison with regular-rate erosion. Variations in the erosion taper lead to changes
in duplex geometry and exhumation rate in thrust wedges with one or two décollements. The 6° erosion taper
promotes formation of antiformal stack at the rear part of thrust wedge, high rate of basal underthrusting and
high extent of erosional removal. The cover layers are nearly completely eroded above the antiformal stack
and form the synformal klippe in frontal part of thrust wedges. The 8° erosion taper allows development of
individual ramp-anticlines and active forward thrusting of cover layers above the décollement and low rate of
basal underplating below it, with consequent low extent of erosional removal. The results of our experiments
support the observations on structural evolution and erosion in the Alberta Foothills of the Canadian Rockies.
Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.
1. Introduction
Décollements are very common in foreland thrust belts and their
role has long been studied. They are responsible for duplex formation,
which evolution and geometry vary in styles and greatly inuence the
dynamics of thrust wedges. For examples, antiformal stacks with
forward thrusting in the cover and signicant underthrusting of basal
units are characteristics of the southern Pyrenees in Spain (Vergés and
Martinez, 1988), spaced ramp anticlines with folding in the cover and
small extent of basal underthrusting are typical for the San Andean
thrust belt in northern Argentina (Belotti et al., 1995), and duplex styles
may vary laterally along the same orogenic front as shown in the
Canadian Rocky Mountains (Fermor and Price, 1987; Lebel et al., 1996;
McMechan, 2001; Price, 1986, 2001; Price and Fermor, 1985; Soule and
Spratt, 1996; Stockmal, 2001). The synchronous play of frontal accretion
and underthrusting of duplex units at depth has been shown to be an
important mechanism for the material transfer in thrust wedges (e.g.
Glodny et al., 2005; Gutscher et al., 1996, 1998; Kukowski et al., 2002;
Platt, 1988, 1990). Combined with erosion, it allows exhumation of deep
rocks in compressional orogens (Avouac, 2003; Bonnet et al., 2007,
2008; Konstantinovskaia and Malavieille, 2005; Malavieille, 2010;
Osborn et al., 2006; Simoès et al., 2007). Thus, surface processes play a
major role in the growth of aerial thrust wedges inuencing their
dynamics and long term evolution. Numerous analog modeling studies
have been devoted to the understanding of these complex interactions
at different scales. Some consider thrust development at regional scale
(Barrier et al., 2002; Casas et al., 2001; Cobbold et al., 1993; Del Castello
et al., 2004;Larroque et al., 1995; Leturmy et al., 2000; Malavieille et al.,
1993; Marques and Cobbold, 2002; Merle and Abidi, 1995; Mugnier
et al., 1997; Persson and Sokoutis, 2002; Persson et al., 2004; Storti and
McClay, 1995; Storti and Poblet, 1997) or at the scale of the orogen
(Bonnet et al., 2007, 2008; Davy and Cobbold, 1991; Hoth et al., 2006;
Konstantinovskaia and Malavieille, 2005; Koons, 1989; Malavieille,
2010; Malavieille and Konstantinovskaya, 2010; Persson and Sokoutis,
2002), suggesting that synkinematic erosion and sedimentation
inuence fault propagation and geometry of thrust wedges favoring
a punctuated thrust activity, alternating frontal thrusting, out-of-
sequence thrusting and backthrusting.
Tectonophysics 502 (2011) 336350
Corresponding author. Tel.: +1 418 654 2559; fax: +1 418 654 2600.
E-mail address: Elena.Konstantinovskaya@ete.inrs.ca (E. Konstantinovskaya).
0040-1951/$ see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.tecto.2011.01.020
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journal homepage: www.elsevier.com/locate/tecto
Author's personal copy
Analog modeling approaches have been used for a long time in
complement with geological studies to analyze the effects of frictional
and ductile décollements on duplex styles. Presence of frictional (glass
microbeads) décollements in the accreted series of purely brittle
thrust wedge models allows underplating of thrust units developing
an antiformal stack, whose growth and location is favored by erosion
(Bonnet et al., 2007, 2008; Konstantinovskaia and Malavieille, 2005;
Malavieille, 2010). Presence of ductile décollements affects the
growth and evolution of sandsilicone experimental wedges. It was
shown that internal deformation style and forward propagation of
structures in brittleductile wedge models are strongly dependent
upon the brittleductile coupling (Bonini, 2003, 2007; Costa and
Vendeville, 2002; Mugnier et al., 1997; Smit et al., 2003). Strong
décollements in sandsilicone wedges with two décollements
(Couzens-Schultz et al., 2003) favor local underthrusting of the
cover, development of individual ramp-anticlines, internal deforma-
tion of thrust sheets and low layer parallel shortening, whereas weak
décollements enable forward thrusting of the cover, antiformal stacks,
coeval growth of structures, low internal strain and lower layer
parallel shortening that occur later. Otherwise, very weak silicone
décollements may produce a localization of deformation at long lived
detachment folds above a oor thrust tip (Bonini, 2003). It was
noticed that backthrusts (hinterland verging) and forethrusts (fore-
land verging) may develop in model thrust wedges depending on the
relative strengths of the basal décollement and overlying cover and on
basal friction (Bonini, 2007; Chapple, 1978; Davis and Engelder, 1985;
Mandl and Shippam, 1981).
The impact of sedimentation on thrust wedge geometry and fault
kinematics was studied by Bonnetet al. (2007), Konstantinovskaya et al.
(2009), Mugnier et al. (1997), Smit et al. (2010), Storti and McClay
(1995),andStorti et al. (2000). Diffuse sedimentation results in large
forethrust spacing and a more diffuse deformation style (Selzer et al.,
2007; Simpson, 2006; Storti and McClay, 1995). High-rate syntectonic
sedimentation towards the front of the wedge also lowers the taper
angle and focuses deformation towards the rearof the wedge (Simpson,
2006; Storti and McClay, 1995). The addition of syntectonic sediments
during the evolution of sandmica thrust wedges result in increase of
the retrovergent thrusting stage that occurred along a long-lived ramp
whose lowertip was located at the subduction slot (Storti et al.,2000). In
contrast, syntectonic sedimentation in foreland areas of thrust wedges
with décollements favors the activation of a weak décollement layer at
the base of a cover sequence, leading to the development of a piggyback
basin (Konstantinovskaya et al., 2009; Mugnier et al., 1997).
Erosion restrains the forward propagation of thrust wedge, induces
development of major back thrusts, enhances material transport across
the wedge, and focuses the internal deformation and exhumation close
to the backstop (Bonnet et al.,2007; Hoth et al., 2006; Konstantinovskaia
and Malavieille, 2005; Selzer et al., 2007; Simpson, 2006; Willett et al.,
1993). The application of both syntectonic erosion and sedimentation
(Konstantinovskaya et al., 2009; Mugnier et al., 1997)resultsin
development of passive-roof duplex(or triangle zones) in thrust
wedges with décollements. In thrust belts formed by a succession of
ramp anticlines, exhumation is induced by erosion that mainly
postdates the tectonics. In a passive-roof duplex, the exhumation is
mainly controlled by the erosion that continuously balances thrust
tectonics. A series of factors (relative strength of a décollement horizon,
strain rate andbasal friction) leadingto the developmentof backthrusts,
passive roof duplexes and triangle zones has been widely investigated
by physical modeling approaches (Bonini, 2007; Couzens-Schultz et al.,
2003; Gutscher et al., 1998; Koyi et al., 2000; Mulugeta and Koyi, 1987).
Our modeling study focuses on the impact of frictional décollements
on duplex formation, subsequent underthrusting (high basal coupling)
and underplating (different decoupling levels acting at different depths
within the wedge) and material transfer in eroded sand thrust wedges.
Results are then applied to better understand complex structures
observed in natural example of the Canadian Rocky Mountains.
2. Experimental method
We analyze the effects of syntectonic erosion on fault propagation,
duplex geometry, material transfer and exhumation in thrust wedges
using 2D sandbox experiments. This study focusing on the role of
décollements is complementary of an earlier work of Konstantinovskaia
and Malavieille (2005), in which the style of fault propagation and the
diversityof exhumation patterns were studied in eroded thrust wedges
as a function of type of basal friction (high and low), thickness of
accreted series and the presence of subduction window. In our previous
work it was shown that the uplift of material occurs along a cluster of
subvertical thrusts in the middle part of the eroded thrust wedge with
low basal friction. The material is exhumed along a series of inclined
(20°50°) thrusts in the rear of the high-friction wedge. The vertical
componentof exhumation is generally higher for the wedges with high
basal friction than for low-friction wedges, and it is amplied by the
presence of décollements. The addition of décollement layer in a sand
pack requires high basal friction in order to create a difference in rate of
lateral material transfer across the growing thrust wedge. Thus, only
high basalfriction wedges arediscussed here. Theeffect of slower rate of
erosion on fault kinematics and exhumation in a thrust wedge is newly
tested in the present study (MW3) in order to compare it to the
previously obtained model wedge (MW2) with regular erosion
(Table 1). The former experiment with a single décollement in model
wedge is reproduced in the present study (MW5) to be compared to
new experiments of model wedges with two décollements (MW67).
The basic device (Fig. 1) is made by a at basal plate bound by two
lateral glass walls (see detail in Bonnet et al., 2007, 2008;
Konstantinovskaia and Malavieille, 2005; Malavieille, 1984). To
reduce the amount of sidewall friction, a lubrication of glass walls was
done before sand deposition. The deformation box is 200 cm
(length)× 10 cm (width) providing 150 cm of total shortening. A
stepper motor moves a basal plastic sheet that is pulled beneath a
vertical rigid backstop at one side of the box. The rough surface of the
plastic sheet allows simulation of a high basal friction at the base of the
layered incoming sand. The stable backstop represents the upper plate
against which the thrust wedge develops. The analog granular materials
have frictional properties satisfying the Coulomb theory and they
correctly mimic non-linear deformation behavior of crustal rocks in the
brittle eld (Dahlen, 1984; Dahlen et al., 1984; Lohrmann et al., 2003).
The Aeoliansand used in the experiments is roundedwith a grain size of
less than 300 mm and a density of 1690 kg/m
3
. The internal coefcient
of friction is 0.57 and the cohesion Co= 20 Pa. A basal friction
corresponding to these parameters is around 24° for the high basal
friction models. Successive colored sand layers are put on the plastic
sheet simulating sedimentary sequences. The initial thickness of sand
layers is 3.6 cm. A proto-wedge is built before shortening to rapidly
obtain a thrust wedge at critical taper. It is 10.4 cm high and its slope is
15°. The passive marker particles composed of colored sand were
distributed at each 5 cm along the basal layer of model wedges. Tracing
of displacement paths of the marker particles from serial experimental
photos promotes reconstruction of material transfer through the thrust
wedge during continuous shortening.
The weak décollement levels are created by introducing thin
(12 mm) layers of glass microbeads at different levels of the sandcake.
They are a Coulomb material and their density and size are almost the
same as those of dry sand, however due to their close to perfect
roundness their coefcient of internal friction is about 23% smaller (0.44),
with cohesion almost negligible. The successive colored sand layers are
accreted in front of the proto-wedge developing a Coulomb thrust wedge
during convergence. Scaling factor is 10
5
: 1 cm in experiment is roughly
equivalent to 1 km in nature. Scaling, and characterization of models and
analog materials used in sand-box modeling are discussed in Dahlen
(1984),Dahlen et al. (1984),Davy and Cobbold (1991),Gutscher et al.
(1996, 1998),Kukowski et al. (2002),Lallemand et al. (1994),and
Lohrmann et al. (2003) with a synthesis given in Graveleau (2008).
337E. Konstantinovskaya, J. Malavieille / Tectonophysics 502 (2011) 336350
Author's personal copy
Erosion is performed by hand with a thin metal plate (the sand
being removed using a vacuum cleaner) to maintain the slope of the
wedge at a constant angle reecting the mean taper angle imposed by
wedge mechanics (Bonnet et al., 2007, 2008; Konstantinovskaia and
Malavieille, 2005). The erosion surface was projected on the glass wall
after 20 cm of shortening in the way that the prole runs along the
thrust wedge and cuts X axis at its toe. Erosion of the units was applied
in a constant manner, independently of their compositional nature, as
a function only of topography. Erosion is made gradually each 2 cm of
shortening. Between erosion steps, the wedge is allowed to try to
obtain its own critical taper. Thus higher topographies and topo-
graphic anomalies were eroded leading to erosion that is distributed
and linearly dependant on elevation. Generally this means that
erosion is increasing towards higher topographies. This is supported
by other analog models (e.g., Cruz et al., 2008; Hoth et al., 2006), and
also by observations from natural situations where erosion can be
positively correlated with elevation (e.g., Summereld and Hulton,
1994). The results of models are useful to discuss the effects of several
rst order mechanical parameters on the deformation and structural
evolution of orogenic wedges submitted to erosion.
There is no sedimentation applied in the experiments of the
present study. This setting likely corresponds to the orogen wedges
where eroded material is transported further away from the frontal
thrusts toward the foreland basin. In natural example of Canadian
Rockies compared to our experiments, the material eroded from the
Mesozoic thrust wedge of the Alberta Foothills is transported to the
east and deposited in the Alberta Planes.
The rst model(MW1) was set with no décollements, and no erosion
was applied through the experiment to establish a critical accreting
taper angle (8°) in the models with high basal friction (Table 1). Two
subsequent experiments were run to test the effects of erosion on the
thrust wedges without décollement (MW23, Table 1). The imposed
slope of the erosion surface was 8° determined from the rst
experiment. The erosion rate was two times slower in the experiment
MW3 in comparison to MW2 that was simulated by erosion and sand
removal each 4 cm of shortening (MW3) instead of 2 cm (MW2). The
experiment with slow erosion (MW3) could be compared with natural
cases in which a higher resistance of composing rocks and/or
unfavorable climatic conditions occurred. Three last experiments were
run to study effects of one (MW4) or two (MW56) décollements in a
thrust wedge under erosion. The erosion taper angle was 6° for the
thrust wedges with one (MW4) and two décollements (MW5) to reect
low basal friction at the base of the cover layers above the décollements
(Konstantinovskaia and Malavieille, 2005). The imposed slope of the
erosion surface was 8° in the experiment MW6 with two décollements.
The variation of boarding conditions in the six experiments (Table 1)
was set to study their effect on thrust wedge kinematics, fault
propagation, underthrusting and exhumation rates. The experiments
were repeatedly run to ensure that similar deformation is reproduced
under thesame setup and thatit was the changes in the initialsetup that
affected variations in model wedge evolution. The total amount of
shortening in the presented experimental runs varies about 105± 20 cm
(Table 1). The moment to stop the experiments was chosen once internal
wedge kinematics reached equilibrium and total length of sand layers
was accreted. The average value oftotal shortening 105 cm characterizes
the thrust wedge without erosion (MW0). The most important total
shortening (125 cm) is observed in eroded model wedge without
décollements (MW2) that is slightly higher than the one (120 cm) in the
wedge with slower erosion (MW3). The eroded wedges with one and
two décollements (MW46) demonstrate the lowest values of total
shortening that is 115 and 86 cm, respectively (Table 1).
3. Results
3.1. Experiments without and with erosion
3.1.1. No erosion
The model wedge MW1 (Fig. 2) is growing by continuous frontal
accretion and by slight basal underthrusting under the rear part of the
wedge. Forward propagation of thrust faults leads to constant lateral
growth of the model wedge. Frontal thrusts are relatively at (2025°)
and become stepper (3540°) with time being transferred and
Table 1
Initial setup characteristics, rates of erosion, duplex types, extent of basal underthrusting and erosional removal in the experimental thrust wedges.
Thrust
wedge
Erosion taper
angle, °
Erosion rate, cm
of shortening
Number of
décollements
Presence
of major
backthrust
Presence of duplex
and its type
Exhumation of
basal material
Extent of erosional
removal
a
,E
eros
,%
Ratio of basal
underthrusting
a
,
R
und
,%
Total
shortening,
cm
MW1 No No No Yes No No 0 6 105
MW2 8 2 No Yes No Yes 35 38 125
MW3 8 4 No Yes No Yes 28 33 119
MW4 6 2 1 Yes Antiformal stack Yes 26 25 115
MW5 6 2 2 No Antiformal stack No 23 30 86
MW6 8 2 3 No Upper ramp-anticlines
and lower normal duplex
No 18 16 86
E
eros
=S
erod
/(S
init
+S
inp
)×100%; S
erod
=S
init
+S
inp
S
nal
;R
und
=S
und
/S
nal
×100%, where S
init
, initial wedge area; S
inp
, total area added as inputat the front (or accreted at the
base) of the wedge; S
nal
, thrust wedge area at the end of shortening; S
erod
, area of eroded material, R
und
, ratio of basal underthrusting, S
und
, area of basal underthrusting; S
init
,S
nal
and S
und
are measured from digitized photos of model wedges, S
inp
is calculated from experiment parameters.
a
Values are taken at the end of shortening.
Fig. 1. Scheme of the experimental device. The initial thickness of sand layers is 3.6 cm. The height of the proto-wedge is 10.4 cm and its slope is 15°. The plastic sheet of rough surface
simulates high basal friction in the models. The angle of erosion slope αconstitutes 8° (critical taper) and 6° in different experiments. The cross sections presented in this study
(Figs. 27) are lateral photographs taken through the sidewall glasses.
338 E. Konstantinovskaya, J. Malavieille / Tectonophysics 502 (2011) 336350
Author's personal copy
deformed toward the rear part of the wedge. A major backthrust (31
38°) displaces the units ofthrust wedge over thetoe of the proto-wedge.
The accreted basal material is never exhumed remaining at the low
structural levels of the wedge (particles 1 and 2, Fig. 2).
The measured surface slope of the non-eroded thrust wedge MW1
is 8° that is used as a critical taper angle of erosion for other model
wedges with high basal friction.
3.1.2. Regular erosion (material removal each 2 cm of shortening)
The geometry of model wedge MW2 (length, thickness and slope
prole) remains constant (Fig. 3) once a critical erosion taper (8°) is
imposed. New material is accreted along frontal thrusts (1327°) and
transferred to the inner basal part of the wedge by underthrusting of
accreted units. Forward thrusts steepen (3038°) with depth and
toward the rear part of the wedge. Smaller backthrusts (3038°) are
associated with frontal forward thrusts. Through shortening, upper
sand layers are eroded and basal layers are piled up at the rear part of
the wedge where the domain of maximum exhumation is located
(Fig. 3). The early accreted basal material is uplifted to the high
structural levels of the wedge but does not reach the surface (particles
1 and 2, Fig. 3), while the later accreted basal material is exhumed
(particle 3, Fig. 3). Exhumation occurs along a series of steeply (50
64°) inclined forward thrust faults at the rear of the wedge (Fig. 3e).
The shallow seated backthrusts (4472°) cut the frontal part of the
proto-wedge at the back side of the exhumation area to accommodate
the backward displacement of the growing thrust wedge (Fig. 3be).
3.1.3. Slow erosion (material removal each 4 cm of shortening)
The thrust wedge MW3 (Fig. 4) is restrained to the constant
geometry with the beginning of erosion (8°), similar to the model
wedge MW2. Frontal thrusts (928°) become steeper (5562°) in the
central part of the wedge being inclined by rotation or by a series of
paired backthrusts (2534°). The backthrusts that develop at the rear
part of the wedge have a lower angle dip (4044°) in comparison to
the similar backthrusts of the model wedge MW2. The accreted
material is exhumed mostly in the central part of the wedge (particle
3, Fig. 4) along steep (5562°) forward thrusts.
The exhumation area has essentially the same width at the surface
in the experiments MW2 and MW3 but its location is different: it is
restricted to the rear of the wedge MW2 (Fig. 3e) and found in the
Fig. 2. Stages of deformation of the thrust wedge MW1, high basal friction, no erosion, no décollements: photos of the model (ad) and interpretation of the wedge structure at the
end of shortening (e). The thrust wedge grows laterally and thickens through the shortening (bd). No exhumation is observed. The wedge slope 8° (e) represents the critical
accreting taper angle for the thrust wedges with high basal friction.
339E. Konstantinovskaya, J. Malavieille / Tectonophysics 502 (2011) 336350
Author's personal copy
middle of the wedge MW3 (Fig. 4d). The different location of
exhumation is likely related to development of larger area backthrusts
in MW3 that demonstrate shallower dip (4044°) than in the
backthrusts in MW2 (4472°). The more extensive backthrust
development at the rear of the wedge MW3 is likely favored by
lesser rate of erosion that provided more time to the wedge to reach
critical taper between each step of erosion.
3.2. Experiments with erosion and presence of one and two décollements
The thrust wedges MW4 with one décollement (Fig. 5) and MW5
with two décollements (Fig. 6) were submitted to erosion along
prole of 6° to reect low friction along décollements at the base of
cover layers. The erosion in MW46 is applied each 2 cm of shortening
as in MW2.
3.2.1. Erosion 6° (one décollement)
The thrust wedge MW4 (Fig. 5) with one low friction décollement
(thin layer of glass microbeads) grows both by basal underthrusting
below the décollement and by frontal accretion of the cover layers
above it. At the beginning of shortening, the lower sand layers of the
thrust wedge accrete to form a duplex structure under the décolle-
ment in the rear part of the wedge (Fig. 5b). The cover sand layers
above the décollement are accreted along the forward propagating
thrusts (FT1FTn) to form an upper thrust wedge. The forward thrusts
(1620°) are associated with paired backthrusts in the upper wedge,
typical of thrust wedges with low basal friction (Konstantinovskaia
and Malavieille, 2005).
Continuous erosion (6°) leads to progressive growing of duplex
under the décollement to form an antiformal stack in the thrust
wedge MW4 (Fig. 5cd). The inclined forward thrusts (2050°) in the
duplex (Fig. 5c) with progressive shortening (Fig. 5d) become steeper
(6090°) and even get a negative plunge (15°) being arched
upward above the piled up basal tectonic slices. Both the early and the
later accreted basal material were brought to the surface in the
domain of major exhumation at the rear part of the thrust wedge
(particles 13, Fig. 5d). The upper thrust wedge of cover layers creeps
along the décollement (Fig. 5c) being nally compressed in a
Fig. 3. Stages of deformation of the thrust wedge MW2, the imposed erosion slope 8°, no décollements: photos (ad) and interpretation of the wedge structure at the end of
shortening (e). The thrust wedge retains the same geometry through shortening (bd). Exhumation of basal layers is observed at the rear part of the wedge at the end of shortening.
340 E. Konstantinovskaya, J. Malavieille / Tectonophysics 502 (2011) 336350
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synformal klippe in front of the growing antiformal stack (Fig. 5d).
New frontal thrust Fnthat develops in front of the synformal klippe is
merged to the décollement at the base of the cover sand layers
(Fig. 5d). The major backthrust (40°) grows at the rear of the wedge at
the nal stages of shortening, contributing to further basal material
exhumation (Fig. 5e).
3.2.2. Erosion 6° (two décollements)
The thrust wedge MW5 with two décollements (Fig. 6) is run with
the imposed erosion taper (6°), similar to the model wedge MW4. The
presence of two décollements in the wedge MW5 changes the fault
propagation inducing the development of independent system of
thrusts and pop-up structures (PU1 and PU2) above each décollement
(Fig. 6b, red square) with underplating of material at different
structural levels. The antiformal stack (Fig. 6c, d) is formed under
the lower décollement at the rear part of the thrust wedge, but it
remains wider and lower than the domal structure in the model MW4
(Fig. 5cd; Video 1). The upward transfer of basal material within the
lower duplex occurs along more at (3540°) thrusts comparing to
the one-décollement wedge MW4. Neither early, nor later accreted
basal material reaches the surface being uplifted to the medium depth
level of the wedge (particles 1 and 2, Fig. 6d). The area of maximum
exhumation is located at the rear of the thrust wedge (Fig. 6d). The
upper forward thrusts are very shallow dipping (3°) or even obtain a
negative plunge (12°) at the upper structural level of the wedge
MW5 (Fig. 6cd). The cover layers of the upper duplex (between two
décollements) are nearly completely eroded above the growing lower
duplex at the end of experiment (Fig. 6d). The upper thrust wedge
composed of the cover sand layers above lower décollement is
characterized by a series of paired forward and back thrusts (32°). The
upper wedge is compressed in the synformal klippe (Fig. 6d) at the
nal stages of shortening. No major backthrust occurs during the
thrust wedge formation.
3.2.3. Erosion 8° (two décollements)
The thrust wedge MW6 with 2 décollements (Fig. 7)was
submitted to erosion along the 8° slope taper that is a critical taper
angle for high basal friction wedges. The higher-angle erosion taper
applied to this thrust wedge leads to changes in fault propagation,
material transfer and exhumation if compared to other models with
décollements (MW4 and MW5).
The small-scale duplex is formed by underthrusting of basal
material under the lower décollement at the rear of the wedge, in the
domain of maximum exhumation (Fig. 7cd, Video 2). It composes
only about 1/3 of the wedge thickness at the end of experiment
(Fig. 7d). Paired forward and back thrusts develop in the cover layers
being frequently independent below and above the upper décolle-
ment (Fig. 7b, red square). With continued shortening, the cover
layers above the lower décollement are accreted along a series of
low-angle forward thrusts, which form individual ramp-anticlines
Fig. 4. Stages of deformation of the thrust wedge MW3, the imposed erosion slope 8°, no décollements: photos (left) and their interpretation (right). The erosion is two times slower
than in the experiment MW2 (Fig. 3). Exhumation of basal layers is observed at the middle part of the wedge at the end of shortening.
341E. Konstantinovskaya, J. Malavieille / Tectonophysics 502 (2011) 336350
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(Fig. 7cd). The frontal thrusts (32°) become steeper (43°) when
migrating toward the center part of the wedge with continuous
shortening. At the rear part of the model, the thrust faults in the cover
layers become at or get negative plunge (5°) being curved around
the growing lower duplex (Fig. 7d). The basal material is transferred
through the lower duplex but it is locked under the décollement and
never exhumed (particle 1, Fig. 7d). The higher duplex between two
décollements is not eroded in this model, only the cover layers above
the upper décollement being eroded (Fig. 7d). No major backthrust
occurs during the thrust wedge formation.
3.3. Marker particle displacement paths
The marker particles at the base of accreted sand layers were dis-
placed through thrust wedges with continuous shortening. The displace-
ment paths of marker particles were traced for each steps of shortening.
Fig. 5. Stages of deformation of the thrust wedge MW4, the imposed erosion slope 6°, one décollement (white glass microbeads) is introduced at the base of the model: photos (left)
and their interpretation (right). Exhumation of basal layers is observed in the dome-like antiformal stack at the rear of the wedge at the end of shortening (d). Thrust faults steeply
plunge down the section at the frontal part of the growing antiformal stack (cd). The cover layers above décollement are completely detached from the basal layers and compressed
in synformal klippe (ce).
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The thrust wedge MW1 without erosion is characterized by very
slight basal underthrusting. The marker particles (Fig. 2e) are
transferred along the base of the wedge to its rear part and then they
are slightly uplifted mostly by movement along the major backstop.
The thrust wedge MW2 with regular erosion at critical taper (8°)
demonstrates active material uplift and exhumation with shortening.
The basal material accreted during the rst stages of shortening is
uplifted along the steep forward faults in front of the proto-wedge,
but it does not reach the surface (particles 1 and 2, Fig. 3e). The basal
material accreted after the onset of erosion is exhumed along a series
of inclined forward thrusts at the rear of the wedge, in the domain of
maximum exhumation (particle 3, Fig. 3e).
The thrust wedge MW3 with slow erosion along critical taper prole
(8°) has the similar exhumation paths of the marker particles (Fig. 4d) if
compared to the wedge MW2 (Fig. 3e). But the domain of maximum
exhumation is stabilized in the central part of the thrust wedge.
The thrust wedge MW4 with one décollement demonstrates the
most pronounced exhumation of basal material in comparison to all
other experiments. Exhumation is started with the onset of erosion
along 6° prole. All marker particles accreted at the base of the thrust
wedge are exhumed at the rear part of the wedge, in the area of active
growing of the antiformal stack (Fig. 5cd).
The thrust wedges MW5 and MW6 with two décollements are
characterized by transfer of marker particles throughout the basal
duplexes but they stay locked under the lower décollements and
never reach the surface (Figs. 6d and 7d). The higher vertical uplift of
marker particles is observed for the model wedge MW5 (erosion 6°),
within which the antiformal stack was formed at the rear of the wedge
(Fig. 6d). The basal material is uplifted very close to the surface in this
model with upper duplex being nearly completely eroded. The model
wedge MW6 (erosion 8°) is characterized by formation of small-scale
basal duplex (Fig. 7d) and vertical transfer of basal material is limited
by about 1/3 of the wedge thickness.
3.4. Extent of erosional removal and ratio of basal underthrusting
As follows from the previous sections, erosion and variations of
critical taper angle affect the fault propagation and duplex geometry
Fig. 6. Stages of deformation of the thrust wedge MW5, the imposed erosion slope 6°, two décollements (white glass microbeads) are introduced in the model: photos (left) and their
interpretation (right). The independent system of thrusts develops above each décollement (red square). The basal layers below the lower décollement form antiformal stack
growing through shortening (bd) but they are not exhumed (d). The cover layers above the lower décollement are nearly completely eroded above the stack (d). Thrust faults
steeply plunge down the section at the frontal part of the growing antiformal stack (d). (For interpretation of the references to color in this gure legend, the reader is referred to the
web version of this article.)
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in the model thrust wedges. To quantify the effect of these parameters
on basal underplating and exhumation in the thrust wedges, we
calculated extent of erosional removal (E
eros
) and ratio of basal
underthrusting (R
und
)(Table 1).
To estimate the extent of erosional removal (E
eros
), we identied
the following areas (Fig. 8a): (1) S
init
, initial wedge area; (2) S
inp
, total
area added as inputat the front (or accreted at the base) of the
wedge; (3) S
nal
, thrust wedge area at the end of shortening; and
(4) S
erod
, area of eroded material. S
init
and S
nal
are measured from
digitized photos of the model wedges. S
init
is equal the sum of the area
of proto-wedge and of the undeformed sand layers. The length of
undeformed sand layers corresponding to S
init
is taken at the tip of the
nal thrust wedge. S
inp
is calculated from experimental parameters
being equal the product of thickness (3.6 cm) multiplied by length of
the accreted sand layers. The length of accreted layers is calculated by
analyzing the serial experimental photos of model evolution taking
into account the displacement of passive marker particles of colored
sand distributed at each 5 cm along the basal layer.
The extent of erosional removal (E
eros
) is estimated as follows:
Eeros =Serod.Sinit +Sinp

× 100%;
where the area of eroded material (S
erod
) is calculated as follows:
Serod =Sinit +SinpSfinal :
The ratio of basal underthrusting (R
und
)reects the part of
accreted basal material with respect to the nal area of eroded wedge
(Fig. 8b). The ratio is calculated as follows:
Rund =Sund =Sfinal × 100%;
Fig. 7. Stages of deformation of the thrust wedge MW6, the imposed erosion slope 8°, two décollements (white glass microbeads) are introduced in the model: photos (left) and their
interpretation (right). Thrusts are frequently independent below and above the upper décollement (red square). The basal layers below the lower décollement form small-scale
normal duplex slightly growing through shortening (bd). Basal material is never exhumed (d). The cover layers form individual ramp-anticlines (cd). Thrust faults are slightly
inclined down the section at the frontal part of the growing basal duplex (d). Only cover layers above the upper décollement are eroded above the duplex (d). (For interpretation of
the references to color in this gure legend, the reader is referred to the web version of this article.)
344 E. Konstantinovskaya, J. Malavieille / Tectonophysics 502 (2011) 336350
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where the area of basal underthrusting (S
und
)ismeasuredfrom
digitized photos of model wedges. It is dened as the area composed
of basal layer material within eroded wedge. The basal layer is detected
due to its specic color or it is situated below the lower décollement
(Fig. 8b).
The extent of erosional removal (Fig. 9a, Table 1) increases with
shortening for all eroded thrust wedges. This parameter is higher in
the thrust wedges without décollements (MW23) than in the thrust
wedges (MW46) with décollements. The highest quantity (35%) of
removed material occurred in the thrust wedge MW2 that was eroded
regularly (each 2 cm of shortening), higher than E
eros
(28%) in the
wedge with slow erosion (each 4 cm of shortening). Between the
thrust wedges with décollements, the higher erosional removal was
occurred in the wedges MW45 (with one and two décollements)
eroded at the 6° erosion taper (Fig. 9a). The thrust wedge MW6 (with
2 décollements) eroded along the 8° slope has the smallest extent of
erosional removal between all eroded thrust wedges.
The ratio of basal underthrusting (Fig. 9b) is the highest in the
eroded thrust wedges without décollements (MW23) with lower
value of R
und
in the wedge submitted to slow erosion (MW3). The
ratio of basal underthrusting is lower in the thrust wedges with one or
two décollements (MW45) than in the wedges without décolle-
ments (MW23). The thrust wedge with one décollement (MW4) is
characterized by stabilization of R
und
at the end of shortening (Fig. 9b)
because the basal material reached the surface and began to be eroded
at these stages (Fig. 5d). Between all eroded model wedges, the
smallest basal underthrusting occurred in the thrust wedge with two
décollements (MW6) and eroded along the 8° slope (Fig. 9b).
The thrust wedge without erosion (MW1) has no erosional removal
of materialand it is characterized by thelowest vertical transfer through
the model wedge (Fig. 2e) and the smallest ratio of basal material
accretion (Fig. 9) in comparison with all other experiments.
Variations in ratio of basal underthrusting are in positive linear
correlation with the extent of erosion removal in the examined thrust
wedges (Fig. 9c). When more material is removed by erosion from the
surface, then more material is accreted at the base of model wedges.
4. Discussion
4.1. Structural evolution of model thrust wedges
Two main tectonic processes account for the growth of thrust
wedges without erosion: frontal accretion, which is more pronounced
for the low basal friction wedges, and underthrusting that char-
acterizes the high basal friction wedges (e.g. Lallemand et al., 1994;
Malavieille, 2010). The last case is well illustrated by the experiment
without erosion, where the thrust wedge MW1 grows constantly by
underthrusting of tectonic slices (Fig. 2). Repeatedly during the
shortening, structural thickening of the wedge leads to increase of
shear stress at its base, and deformation propagates forward with
initiation of a new frontal thrust. The major backthrust at the rear of
the wedge and internal diffuse deformation helps to reach the critical
taper.
Thrust wedges submitted to constant erosion never reach the
critical equilibrium in their internal part because wedge thickness
remains constant and the deformation cannot propagate toward the
foreland (Mugnier et al., 1997). It is conrmed by our experiments,
where the eroded thrust wedges MW26 do not grow laterally
preserving their constant geometry (slope, thickness and length)
through progressive shortening (Figs. 37).
It was observed that slower erosion (MW3) leads to lesser extent
of erosional removal and lesser extent of basal underthrusting (Fig. 9)
in comparison with the experiment under regular erosion (MW2).
Correspondingly, the increase of erosional removal promotes further
increase of basal underthrusting. This conclusion supports the
idea of a strong feedback connection between surface processes and
internal wedge dynamics (Avouac, 2003; Bonnet et al., 2007, 2008;
Konstantinovskaia and Malavieille, 2005; Malavieille, 2010; Osborn
et al., 2006; Simoès et al., 2007).
A combination of low angle (6°) erosion and high basal friction in
the thrust wedges with décollements (MW45) favors the develop-
ment of highly exhumed antiformal stacks (Figs. 5 and 6). The wedges
are in constant increase of structural thickening by basal underplating
Fig. 8. (a) Proportions of initial (S
init
), eroded (S
erod
), and nal (S
nal
) areas in model wedges used to calculate extent of erosional removal (E
eros
). S
erod
=S
init
+S
inp
S
nal.
(b) Ratio
of basal underplating (R
und
) calculated as a ratio of the area of basal layers (S
und
) with respect to nal area (S
nal
) in model wedges. S
init
,S
nal
and S
und
are measured from digitized
photos of the model wedges. S
inp
is calculated from experimental parameters. Critical taper angle of our model wedges is equal to the imposed erosional slope (α) because basal
detachment is horizontal. (c) Critical taper angle (α+β) composed of the dip angle of basal detachment βand the taper angle αdetermines the geometry of a thrust wedge,
modied after Davis et al. (1983).
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in order to reach the critical taper (8°) required by high basal friction.
The tectonic slices within the stacks are piled up and arched above
each other (Figs. 5e and 6d). Antiformal duplexes are interpreted to be
formed when the amount of displacement along a thrust fault is
essentially equal to the length of the next lower thrust slice (McClay,
1992). Such a relationship seems to characterize the development of
antiformal stacks in the model wedges MW45.
Erosion along the critical slope (8°) applied to the thrust wedge
with décollements (MW6) promotes forward propagation of thrusts
in the cover layers located between and above two décollements and
favors the formation of individual ramp-anticlines (Fig. 7). Being in
equilibrium between the basal friction and the erosion slope, the
wedge does not need to grow vertically. Thus, less structural
thickening occurs under the lower décollement, where the normal
duplex is growing mostly laterally (Fig. 7cd). Tectonic slices in the
duplex dip gently backward, in the direction opposite to the overall
thrust transport. Length of tectonic slices in normal duplexes is known
to be greater than the slip along thrust faults in contrast to forward-
dipping duplexes where the slice length is lesser than the displace-
ment along thrust faults (McClay, 1992). It may be inferred that the
high basal friction in thrust wedges eroded along the equilibrium
critical slope (8°) favors small thrust slip and development of normal
duplexes while low basal friction under the same circumstances could
produce forward-dipping duplexes.
Thus, the experiments with model wedges constructed with
décollements (MW46) provide the evidence that the erosion slope
may inuence kinematics of fault propagation and duplex geometry.
Different accretion mechanisms (Fig. 5) are then activated depending
on interactions between surface processes and wedge mechanics:
frontal accretion, duplexing and underplating, and backward thrust-
ing. These mechanisms may function simultaneously, being located at
different parts across the wedge (Fig. 5e).
4.2. Natural example: Southern Foothills of the Canadian Rocky Mountains
The multiple detachments and duplexes of different structural
styles are well known in the southern Foothills of the Canadian
Rockies (Langenberg et al., 2006; Lebel et al., 1996; McMechan, 2001,
2002; Price, 1986, 2001; Price and Fermor, 1985; Price and Monger,
2000; Soule and Spratt, 1996; Stockmal, 2001).
Fig. 9. Diagrams showing the variations of the extent of erosional removal E
eros
(a) and ratio of basal underthrusting R
und
(b) in thrust wedges with continuous shortening given for
each step of deformation as a percentage of total shortening reached in each experiment (Figs. 27,Table 1). Correlation between parameters E
eros
and R
und
(c). E
eros
and R
und
are
higher in the eroded model wedges MW23 without décollements if compared to the eroded wedges MW46 with décoll ements (ab). Only very slight basal underthrusting occurs
in the model wedge MW1 without erosion (b). The model wedges MW45 eroded along 6° prole (antiformal stacks) are characterized by higher basal underplating than the model
wedge MW6 eroded along 8° prole (small scale normal basal duplex). Higher erosion removal promotes higher basal underplating (c).
346 E. Konstantinovskaya, J. Malavieille / Tectonophysics 502 (2011) 336350
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In the southern Alberta Foothills, the supracrustal wedge consists
of a very thin Lower Paleozoic to Middle Jurassic platform succession
that is overlain by a thicker late Jurassic to Paleocene foreland basin
succession (Price and Monger, 2000). The structure of the Foothills
(Fig. 10a) is dominated by closely spaced, steeply dipping, imbricate,
listric thrust slices, which give rise to the topography dominated by
ridges that are held up by sandstones and valleys that are eroded in
shales (Price and Monger, 2000). At the surface, the thrust faults are
closely spaced and generally dip steeply (N45°) to the southwest
(Fig. 10ab); but at depth they atten and merge with each other,
with a regional detachment near the top of the Paleozoic rocks, and
eventually, with the basal detachment of the foreland thrust and fold
belt, below which the Paleoproterozoic crystalline basement and
overlying supracrustal rocks are undeformed. The basal detachment
in the Alberta Foothills dips gently (β= 3°) to the southwest that
together with the taper angle (α= 2°) determines the geometry of
the thrust wedge in the frontal part of the Canadian Rockies (Fig. 10a).
The critical taper angle of this foothills thrust wedge (α+β=5°) is
close to that one of the eroded model wedges MW 45 with
décollements (Figs. 5 and 6), in which imposed erosional taper
angle is (α=6°) and basal detachment is horizontal (Fig. 8c). The
critical taper of the whole Canadian Rockies thrust belt is estimated to
be 5.2° in case of syntectonic and 8.9° in case of post-tectonic erosion
(Osborn et al., 2006).
The Moose Mountain and Limestone Mountain Culminations
(Fig. 10bd) in the western Foothills belt represent the examples of
the structures with antiformal stacks (Begin and Spratt, 2002;
Newson and Sanderson, 1996; Price and Monger, 2000; Repol et al.,
2010; Soule and Newson, 2000). The stacks are formed by Paleozoic
(Cambrian to Carboniferous) limestone thrust sheets that are anked
by Jurassic and Cretaceous clastic sequences. Hydrocarbon reservoirs
(Fig. 10bc) are associated with Paleozoic carbonates in the
antiformal stack of the Moose Mountain culmination (Price and
Monger, 2000; Soule and Newson, 2000). The oil and gas pools are
Fig. 10. (a) Structural cross-section across the southern Alberta Foothills, south of the Bow Valley, after Ollerenshaw (1978); Price and Monger (2000); 1, Tertiary; 2, Upper
Cretaceous; 3, UpperLower Cretaceous and Jurassic; 4, Mississippian; 5, Upper Devonian; and 6, Cambrian strata. (b) Simplied structural sections: Jumping Pound, and Jumping
Pound West gas elds, after Bruce, et al. (1995); Price and Monger (2000); 1, Tertiary; 2, Upper Cretaceous; 3, Lower Cretaceous and Jurassic; 4, Mississippian and Upper Devonian;
and 5, Cambrian strata. Wells are shown. (c) Simplied structural section through the Moose Mountain Culmination, after Newson and Sanderson (1996), Soule and Newson (2000).
The Moose Mountain thrust cuts down section over the highest point and the leading edge of the basal duplex; 1, Jurassic to Cretaceous; 2, Mississippian; 3, Devonian; and
4, Cambrian strata. Wells are shown. (d) Structural cross-section across Limestone Mountain Culmination, after Begin and Spratt (2002). The antiformal stack is a foreland-dipping
duplex. 12, Cretaceous Edmonton (1) and Belly River (2); 3, Cretaceous Blairmore and Blackstone and Jurassic Fernie and Kootenay; 4, Mississippian; 5, Devonian; and 6, Cambrian
strata. LD Low Detachment.
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sealed by the evaporitic Mississippian Formation emplaced by the
Moose Mountain thrust (Soule and Newson, 2000).
The antiformal stack of the Moose Mountain culmination is limited
at the base by a regional main detachment level (Fig. 10c), lodged at
the base of the Cambrian (Newson and Sanderson, 1996). The Moose
Mountain Thrust Fault separates the antiformal stack from the upper
imbricate system (Newson and Sanderson, 1996; Repol et al., 2010;
Soule and Newson, 2000). The Moose Mountain thrust surface (Soule
and Newson, 2000) ramped up over the western ank and top of the
Moose Mountain culmination and cuts down into the footwall duplex
on its eastern ank (Fig. 10c).
It was suggested that the Moose Mountain thrust developed
simultaneously (alternatively) with the underlying duplex, with the
majority of displacement on the thrust occurring before the footwall
duplex emplacement (Soule and Newson, 2000). Price (2001) also
noted that displacement in the Front Ranges north of Banff, Alberta
occurred simultaneously on several major faults that are distributed
across the thrust wedge. These observations are in good correspon-
dence with our experimental models, in which displacement occurs
simultaneously along two major décollements and in the underlying
duplexes (Figs. 57).
The structure of the Limestone Mountain Culmination (Fig. 10d) is
dominated by two lithotectonic packages separated by a major
detachment (Begin and Spratt, 2002). The lower structural package is
represented by a SW-plunging antiformal stack of four thrust sheets of
the Paleozoic carbonate platform rocks. The upper structural package
comprises Mesozoic siliciclastic foreland basin rocks, deformed into
an NE-verging thrust-and-fold belt (Begin and Spratt, 2002). The
antiformal stack is detached from the upper package by a decoupling
surface of the Roof Thrust. The imbricate fan of the upper package is
passively folded and tilted toward the foreland above the antiformal
stack. Our model wedges with one and two décollements also
demonstrate the passive folding and tilting of the cover layers
above the roof décollements toward the frontal (foreland) side of the
antiformal stacks (Figs. 57), similar to natural structures of Moose
Mountain and Limestone Mountain Culminations (Figs. 10cd).
The effects of syn- and post-tectonic erosion on exhumation in the
Canadian Rocky Mountains were studied by Osborn et al. (2006) by
comparison of physiography between end-of-Laramide time and the
present day. It was concluded that erosion triggered exhumation in the
rear of the thrust wedge with a subsequent forward shift of active thrust
faults. For example, once Paleozoic rocks had been exposed in the
western front ranges (Bourgeau thrust sheet), a new, more easterly
interface between mountains and foothills was created. This interface
likely jumped still farthereastward to the McConnell thrust sheet as the
Mesozoic rocks were progressively eroded (Osborn et al., 2006).
In our experiments, the domains of maximum exhumation are
located at the rear part of the accretionary wedges while the frontal
thrusts continue to propagate toward the foreland (Figs. 57), similar
to the Canadian Rockies (Osborn et al., 2006)(Fig. 10ab). The upper
imbricate wedge located above the roof décollement is folded into the
synformal klippe between the exhumed basal layers and the newly
accreted frontal tectonic slices (Fig. 5). The thrust slices in the upper
thrust wedge above the décollement propagate much farther toward
the foreland if compared to the thrust slices of the basal antiformal
duplex (Fig. 5cd). The similar suggestion was made by Repol et al.
(2010) for the Moose Mountain culmination: the thrust system
underlying the Moose Mountain Thrust (Fig. 10c) is characterized by
thrusts of smaller size and it likely suffered the signicantly less
tectonic transport when compared to the thrust system above the
Moose Mountain Thrust, for which persistent structural emplacement
along strike is observed.
The model proto-wedge in presented experiments (Fig. 6d) may
represent in nature the Paleozoic strata above the McConnel Thrust
(Fig. 10a) that consist of the older thrust wedge formed previously to
the Mesozoic imbricate thrust sheets of the Alberta Foothills (Osborn
et al., 2006). The antiformal stack composed of Paleozoic arched thrust
slices is localized in front of the McConnel Thrust, similar to the model
wedges MW45, in which antiformal stacks of basal layers are
exhumed in front of the proto-wedge toe (Figs. 5c and 6d). It seems
that proto-wedge localizes the subsequent deformation ahead of its
toe, without being affected signicantly by thrust faults during model
deformation. Such a relationship is likely observed in the frontal part
of the Canadian Rockies, where the older thrust wedge of Paleozoic
strata above the McConnel Thrust is not affected by the deformation
propagating more externally into the Mesozoic strata of the Alberta
Foothills as the accretionary wedge grows. The exhumation of
Paleozoic antiformal stack occurs at the rear part of the newly formed
thrust wedge, in front of the older wedge toe.
The Canadian Rockies are characterized by the typical presence of a
frontal passive-roof duplex (or triangle zone) structures (Fig. 10ab)
(Gordy et al., 1977; Hiebert and Spratt, 1996; Lebel et al., 1996; Price,
1981; Stockmal et al., 2001). The formation of triangle zones in sand
thrust wedges was previously obtained mostly in presence of ductile
décollements, being sensitive to basal shear stress, variation in strain
rate, strength and dip of the décollement, strength of the wedge, and
the presence of syntectonic erosion (Bonini, 2007; Cotton and Koyi,
2000; Couzens-Schultz et al., 2003; Gutscher et al., 2001; MacKay,
1995; Mugnier et al., 1997). In the present study, no triangle zone was
obtained in eroded model wedges contained or not frictional glass
microbeads décollements. However, triangle zones were observed in
eroded sand thrust wedges with frictional glass microbeads décolle-
ments (Konstantinovskaya et al., 2009). That experiment set differs
from the present study by the simultaneous application of syntectonic
erosion and sedimentation and by dip of basal detachment (β=2°)
and décollements, although the critical taper 67° (basal detachment
dip β=+ erosional slope α=34°) was the same as in the model
wedges MW45(α= 6°). Syntectonic sedimentation in foreland
areas of thrust wedges favors the activation of a weak décollement
layer at the base of a cover sequence (Konstantinovskaya et al., 2009;
Mugnier et al., 1997), while syntectonic erosion delays forward
propagation of the deformation front and results in the development
of out-of-sequence thrusting, duplexing and exhumation of the rear
part of wedges, (Bonnet et al., 2007; Davis et al., 1983; Horton, 1999;
Hoth et al., 2006; Konstantinovskaia and Malavieille, 2005; Willett
et al., 1993). The combination of the both processes in thrust wedges
with slightly inclined décollements is favorable for the triangle zone
development.
Based on the example of Canadian Rockies, Osborn et al. (2006)
calculated that in case of the post-tectonic erosion, the critical taper of
the thrust belt might have been 8.9°, and 68 km of material could
have been eroded with the erosion rate 0.13 mm/yr. In case of
syntectonic (syn-orogenic) erosion, the critical taper of the thrust belt
might have been 5.2°, and 24 km of material could have been eroded
with the erosion rate 0.065 mm/yr. Thus, the erosion slope likely
affects the extent of material removal in the orogenic belt.
Our experiments support the conclusions of Osborn et al. (2006).
Firstly, variation in erosion slopes applied to identical models with two
décollements (MW56) resulted in changes of fault kinematics (Figs. 6
and 7, see text above) and consequently in changes of the extent of
basal material underplating and erosional removal (Fig. 9). The 6° angle
of erosion prole promoted formation of antiformal stack (Fig. 6d) with
high extent of basal underplating (MW5), and the 8° erosion slope
provided formation of individual ramp-anticlines and small scale basal
duplex (Fig. 7cd) with low extent of the basal underplating (MW6).
Secondly, higher rate of erosion (Fig. 9) leads in the models wedges
(MW23) to higher extent of basal material exhumation.
5. Conclusions
Interactions between surface processes and wedge mechanics
have important consequences on the structure and evolution of
348 E. Konstantinovskaya, J. Malavieille / Tectonophysics 502 (2011) 336350
Author's personal copy
foreland thrust belts. Different accretion mechanisms are thus com-
bined to account for wedge growth: frontal accretion, backthrusting,
underthrusting and underplating due to décollement induced duplex
formation at depth. These mechanisms may function simultaneously,
being located at different parts across the wedge.
Our experiments clearly reect these complex feedback mecha-
nisms. A thrust wedge without erosion constantly grows by basal
underthrusting and forward propagation of frontal thrusts. The major
backthrust at the rear of the wedge helps to rapidly reach the critical
taper. Addition of erosion limits thrust wedge forward propagation
and thickening. The eroded thrust wedges in our experiments do not
grow laterally preserving the constant geometry (slope, thickness and
length) through shortening. The higher rate of erosion leads to higher
extent of exhumation of basal material. Slow erosion leads to lesser
extent of basal underthrusting. Variations in erosion slopes led in
changes of fault kinematics and of the extent of basal material
exhumation. The imposition of erosion taper with lower (6°) angle
than the critical value (8°) in the models with décollements promoted
formation of antiformal stack with high extent of exhumation. The 8°
erosion prole (equal to critical slope) provided formation of
individual ramp-anticlines in the upper wedge above the décollement
and small scale normal duplex below it with low amount of basal
underplating. The results of our experiments conform well to a natural
example of Canadian Rockies.
Acknowledgments
We particularly acknowledge Stephane Dominguez for help and
constructive comments during experimental work and Christian
Romano for technical support. Elena Konstantinovskaia beneted of
grants from the French MENRT to fund the Associate Professor
position in Montpellier (20002003). Our sincere thanks to F. Storti,
M. Bonini and an anonymous reviewer for useful and constructive
comments that aid us to improve the manuscript.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
doi:10.1016/j.tecto.2011.01.020.
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... This theory predicts that in supercritical orogenic wedges, deformation tends to propagate from the hinterland to the foreland, accumulating more complex deformation in the hinterland compared to the foreland (Dahlen, 1990), while in subcritical wedges are characterized by a lack of propagation and internal shortening via duplexing or basement-involved thrusting. Higher topography at the back of orogenic wedges can enhance erosion, and locally induce out-of-sequence thrusting, thus leading to the exposure of structures formed at relatively deep conditions within the wedge during relatively short periods of time (Konstantinovskaya and Malavieille, 2011). ...
... Although it intuitively seems that low angle stacking of thrust sheets should be less efficient than high angle reverse faults to uplift rocks and induce significant exhumation, Konstantinovskaya and Malavieille (2011) have shown in physical analog models that regular erosion rates, or localized erosion in an orogenic wedge favor underplating over propagation of deformation to the toe, thus inducing the formation of duplexes and antiformal stacks. As a result of such localized underthrusting within the wedge, a really restricted topographic uplift forms at the surface, increasing erosion and the exhumation rate of the rocks, thus causing thrusting and a fast exhumation. ...
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... Based on analog experiments and natural examples (e.g., Decelles and Mitra, 1995;Bonnet et al., 2007;Cruz et al., 2008;Wu and McClay, 2011;Graveleau et al., 2012 and references in), increased surface processes will transform the thrust belt and foreland basin system from a critical state to an unstable state (i.e., subcritical or supercritical state). This transformation will lead to the uplift and erosion of wedges, change in the geometry of wedges, multi-stage reactivation of faults, thermal convection, and unstable geomorphic construction processes (Willett et al., 1993;Persson et al., 2004;Hoth et al., 2006;Cruz et al., 2008;Konstantinovskaya and Malavieille, 2011). Cruz et al. (2008) suggested wedge erosion increases the density of fore-shears and controls the wedge geometry and its topographic divide migration. ...
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Topographic elevation of orogenic belts in the results of several processes and contributions occurring at different temporal and spatial scales, from short-to long-term and from the surface up to the mantle. Disentangle surface evolution on key areas may then encode for deeper ongoing processes. In this concern, the Anti Atlas of Morocco is a prominent mountain belt with moderate relief and with a long deformation history not recorded during the Cenozoic. The modern topography of the Anti-Atlas is resulted from late Miocene uplift driven by mantle processes. In this study, we address the question if mantle processes are still affecting the topography of the Anti-Atlas by providing new clues on the late Miocene-Quaternary uplift history of the north-western margin of the mountain belt. We analyze and compare the river long profiles and related morphometric features of channels draining the Souss Basin of the Anti-Atlas in western Morocco with analog and numeric similar scenarios. We find that the steepness of the rivers, the χ, ksn, slope and local relief show limited values. Moreover, the channels are characterized by Quaternary fluvial conglomerates lying at high elevation. The projection of the non-re-equilibrated river profile runs below the lower part of the channel. We explore the mechanism that could produce this feature by both numerical and analog models. The analogue and numerical long river profiles from modeling show similar morphometric features compared to the natural case when the uplift rate is diminished or set to zero. We speculate that in the Anti Atlas, uplift may have waned over Quaternary time leading to a spatial change in the critical point of sedimentation, triggering valley infilling at high elevation, and in turn a slow burying of the Anti-Atlas.
... Models must be scaled by their geometry, kinematics, dynamics, and rheology to realistically reproduce a given geological process (e.g., Hubbert, 1937;Ramberg, 1981;Weijermars & Schmeling, 1986). Considering a length scaling factor, l* = l model /l nature , that ranges between 10 −5 and 10 −6 (e.g., Cruz et al., 2008;Davy & Cobbold, 1991;Konstantinovskaya & Malavieille, 2011;Persson et al., 2004;Schreurs et al., 2006;Storti et al., 2000), 1 cm in the model correspond to 1-10 km in nature. The gravitation acceleration model-to-nature ratio is g* = g model / g nature = 1. ...
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In convergent systems, the interplay between tectonics, erosion, and sedimentation controls the orogenic evolution. The nature of the interactions between these factors is still elusive due to the complex feedbacks that operate across different temporal and spatial scales. Here, we investigate these feedbacks with analog models of landscape evolution designed to account for both tectonic forcing and surface processes, using a water‐saturated granular material that allows to simulate contemporary brittle deformation and surface processes. The deformation is imposed by the movement of a rigid backstop, and surface processes are triggered by simulated rainfall and runoff. We vary the convergence velocity, rainfall rate, and basal angle of the box, testing how different boundary conditions affect the balance between tectonics and surface processes. We measure the competition between input fluxes (tectonics) and output fluxes (erosion) of material, showing how sedimentation strongly affects the balance between these fluxes. The results suggest that the experimental equilibrium between tectonics and erosion can be achieved, in the analog models, only for low convergence rates (about 10 mm hr⁻¹) and/or for high basal angle (>2°, limited sedimentation). If the foreland is overfilled with sediments and/or if convergence velocity is higher, channels decrease their erosional efficiency, moving the dynamic equilibrium between tectonics and erosion toward the former.
... When the system reaches a steady-state equilibrium, the slab-top peeling process is predicted to occur with a strikingly constant periodicity on the order of several millions of years (Menant et al., 2020;Figs. 8B, 11C), giving rise to a very thick and stable accretionary edifice (e.g., Konstantinovskaya and Malavieille, 2011). The growth and preservation of such an extremely thick wedge (>35 km thick) formed by protracted basal accretion events as recorded in south-central Chile during the late Paleozoic (Western Series; e.g., Willner, 2005) may appear at first sight as a quite unusual phenomenon in Earth's Phanero zoic subduction history. ...
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Several decades of field, geophysical, analogue, and numerical modelling investigations have enabled documentation of the wide range of tectonic transport processes in accretionary wedges, which constitute among the most dynamic plate boundary environments on Earth. Active convergent margins can exhibit basal accretion (via underplating) leading to the formation of variably-thick duplex structures or tectonic erosion, the latter known to lead to the consumption of the previously accreted material and eventually the fore-arc continental crust. We herein review natural examples of actively underplating systems (with a focus on circum-Pacific settings) as well as field examples highlighting internal wedge dynamics recorded by fossil accretionary systems. Duplex formation in deep paleo-accretionary systems is known to leave in the rock record (i) diagnostic macro and microscopic deformation patterns as well as (ii) large-scale geochronological characteristics such as the downstepping of deformation and metamorphic ages. Zircon detrital ages also proved to be a powerful approach to decipher tectonic transport in ancient active margins. Yet, fundamental questions remain in order to understand the interplay of forces at the origin of mass-transfer and crustal recycling in deep accretionary systems. We address these questions by presenting a suite of 2D thermo-mechanical experiments that enable unravelling the mass flow pathways and the long-term distribution of stresses along and above the subduction interface as well as investigating the importance of parameters such as fluids and slab roughness. These results suggest the dynamical instability of fluid-bearing accretionary systems causing either episodic or periodic character of subduction erosion and accretion processes as well as their topographic expression. The instability can be partly deciphered through metamorphic and strain records, thus explaining the relative scarcity of paleo-accretionary systems worldwide despite the tremendous amounts of material buried by the subduction process over tens or hundreds of Ma timescales. We finally stress that the understanding of the physical processes at the origin of underplating processes as well as the forearc topographic response paves the way for refining our vision of long-term plate interface coupling as well as the rheological behavior of the seismogenic zone in active subduction settings.
... Being aware of the impact of erosion in natural and analog fold-and-thrust belts (e.g. Konstantinovskaya and Malavieille, 2011), we again invoke the Fars province to justify the use of just syncontractional sedimentation. In the Fars province, continuous sedimentation filled the Fars foreland from Campanian times where erosion localized in the crest of growing salt bodies and anticlines. ...
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The presented work studies salt-detached fold-and-thrust belts involving minibasins by means of physical analogue modeling. The experimental set up consist of a series of minibasins and diapirs built by downbuilding into a regular polygonal framework. The minibasins-diapir framework were then submitted to contraction, and for some examples accompanied by different rates of syncontractional sedimentation. We aimed at evaluating the influence of an initial salt basin geometry (i.e. equal thickness vs. along-strike tapered) on the development of the salt-sediment system, and how this influenced the geometries and kinematics of fold-and-thrust belts. We also tested how these are influenced by different syncontractional sedimentation rates. Results show that major differences on the early salt structures occur during downbuilding as a result of original salt budget (i.e. from pillows to diapirs), with a positive correlation between amount of original salt and salt structure development. Initial contractional deformation is localized on the weaker salt bodies, favoring salt extrusion. Shortening is then transferred forwards once vertical salt feeders are welded (i.e. secondary welds), while basal (primary) welds are sheared, rolled or delaminated. Changes on structural styles occur abruptly along-strike as controlled by degree of development of the precontractional salt structures. Relatively low syncontractional sedimentation rate delays forward propagation of deformation and promotes minibasins tilting. With larger sedimentation rates, a thicker cover inhibits minibasins deformation and secondary welding and, promotes a more coherent mechanical beam detached on the basal weld. Our modeling is compared to natural fold-and-thrust belts such as the Zagros and the European Alps.
... In summary, the results of the centrifuge and normal gravity experiments show that the presence of a low-friction basal décollement significantly influenced the evolution and pattern of our experimental thrust wedges by promoting forelandward thrust propagation and lower topography tapers (Fig. 10). Similar results have been suggested in previous analogue models investigating the role of frictional décollement (e.g., Teixell and Koyi, 2003;Massoli et al., 2006;Konstantinovskaya and Malavieille, 2011;Graveleau et al., 2012 and references therein). ...
Article
Centrifuge analogue modelling has provided significant insights into the evolution and architecture of fold and thrust systems. However, all previous works focused on the deformation of viscous and/or plastic layers, and did not analyze the development of structures in the brittle crust. In this work, we present the results of analogue centrifuge models reproducing compression of purely brittle upper crustal layers. We run enhanced gravity models investigating the role exerted by frictional décollements on the evolution and architecture of thrusting and compared the results with new normal-gravity models with similar set-up. In line with the results of previous experimental works, our models show that the presence of a low-friction basal décollement significantly influences the evolution and pattern of thrust systems. By reducing the basal friction of the experimental wedge or the sector of the wedge where the low-friction décollement is located, this layer promotes experimental wedges with low tapers and low heights in the inner portion. Moreover, when a low-friction basal décollement occurs, the tectonic wedge is characterized by an increase in wavelength of thrust sheets toward the foreland, compared to the purely brittle models. Results show a good comparison between centrifuge and normal gravity models, indicating that the centrifuge technique can be successfully used to model shortening in the brittle crust and therefore to analyze the evolution and architecture of thrust systems.
... Our data also identified the Caynarachi frontal thrust (7) with very high SL/K values and very high Neogene-Quaternary slip. These data show that the Huallaga basin in part experiences classical in-sequence propagation of frontal thrusts (e. g., Konstantinovskaya and Malavieille, 2011), but with the added peculiarity of the Biabo detachment fold-fault system. Our SL/K data also confirms the existence of the active, west-verging Jepelacio backthrust (2), which was also mapped by the Peruvian Geological Survey INGEMMET (see Fig. 2a), but which extends even further north and south in the basin than previously known. ...
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Satellite image-based lineament mapping in combination with the normalised Stream Length Gradient Index (SL/K-index) is one of the most widely used methods to identify active faults in areas where other verification methods, such as faulted strata, geophysical subsurface data, or seismic activity in the upper crust, are unavailable, and to classify their degree of activity. The accuracy of this approach, however, has not yet been assessed rigorously. We assess its accuracy by comparing the results against 27 thrust fault segments that were identified from six seismic reflection profiles with a total length of 340 km. Over 106,000 SL/K points were calculated from 815 river profiles on basis of the SRTM v.3.0 DEM. Standard deviations (SD) of < -2SD (class 1) to > +2SD (class 5) were calculated from the lognormalised SL/K data and five SL/K classes were made and assigned to the nearby lineaments. A comparison with the fault segments from the seismic profiles showed a good coincidence between the presence of knickpoints, SL/K classes 4 and 5 and to a lesser extent class 3. Lineaments with SL/K classes 1 and 2 never corresponded to actual faults. We identified seven master thrusts of which three are very active to highly active, likely older, structures with important amounts of accumulated slip. The other thrusts are probably younger faults with relatively little accumulated slip, yet they are also active to highly active faults. Minor strike-slip faults were identified as very active to highly active structures. Published slip rates shows that at least some of the SL/K class 4 faults experience slip rates of ~1 to ~4 mm a⁻¹ and the SL/K class 5 strike-slip faults may experience slip rates as high as one metre over the course of several months. Most active faults occur where the basal evaporite layer is abnormally thick.
... In physical modeling, the rheology and mechanics of the natural system are simulated by various materials (such as clay, quartz, silicone, and water) and technical equipment. Traditional physical modeling experiments mainly use dry sand (Babault et al., 2005;Barrier et al., 2002;Bigi et al., 2010;Bonnet et al., 2007;Corrado et al., 1998;Cotton & Koyi, 2000;Dahlen et al., 1984;Davis et al., 1983;Deng et al., 2017;Persson et al., 2004;Pichot & Nalpas, 2009;Simoes et al., 2007;Sun et al., 2003Sun et al., , 2016Wu et al., 2014) and simulate syn-sedimentation by adding material to the sandbox via sifting (Barrier et al., 2013;Bonnet et al., 2007;Pichot & Nalpas, 2009) and erosion by removing material via a vacuum cleaner (Konstantinovskaya & Malavieille, 2011;Malavieille, 2010). In order to reproduce channels, alluvial fans, and other associated landform features, a kind of material was tested Graveleau et al., 2011;Reitano et al., 2020) to deeply investigate the mechanisms between structural deformation and surface processes under a mesoscale setting. ...
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Structural deformation affects surface processes and the consequent distribution of tectonic stress and structural deformation patterns via crustal isostasy. The associated processes and mechanisms are widely concerned topics in tectonic geomorphology. In this study, we discuss the interaction between tectonic deformation and surface processes in fold‐and‐thrust belts by sandbox experimental modeling. The tectonic deformation patterns under different conditions are studied, including the occurrence of detachment and the different crustal shortening rates. Furthermore, we discussed the influence of surface erosion and sedimentation on the spatial distribution, morphology, amplitude, and process of tectonic deformation. The results show that the occurrence of plastic detachment links the interaction of surface erosion and tectonics such as deformation patterns. Thin‐skinned structures develop when plastic detachment occurs, thus amplifying surface processes. In turn, the erosion and deposition affect the position and morphology of new structures as well as the lateral propagation and connection of isolated structures. Different shortening rates lead to different propagation distances and structure arrangements. These different tectonic conditions result in diverse drainage evolution and the specific processes of surface erosion and deposition. In general, structural development is controlled by both endogenic forces and surface erosion and deposition processes. The physical properties of detachments and crustal shortening rates play an important role in linking deep tectonic activity and surface processes. These results are comparable to natural examples, such as the fold‐and‐thrust belts along the Tianshan Mountains, thus providing new clues for understanding the mechanism of the interaction between endogenic tectonic activity and surface processes.
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Rapid midcrustal cooling (>10 °C/m.y.) is typical of Phanerozoic orogens, but it is less commonly reported from Precambrian orogenic belts. Abundant new 40Ar/39Ar (predominantly plateau) dates reveal a period of late, rapid cooling following slow postpeak metamorphic cooling during the evolution of the Paleoproterozoic Cape Smith belt, a greenschist- to amphibolite-facies foreland thrust belt in the ca. 1.83 1.76 Ga Trans-Hudson orogen. We conducted 40Ar/39Ar step-heating analyses on biotite, hornblende, and/or muscovite from 38 samples sourced from the thrust belt and its footwall basement, the Archean Superior craton. The 40Ar/39Ar dates from the Cape Smith belt and re equilibrated Superior craton ranged ca. 1948–1708 Ma in biotite, ca. 1801–1697 Ma in muscovite, and ca. 1764–1694 Ma in hornblende. Of these, ~70% were ca. 1740–1700 Ma plateau dates, which we interpret as cooling ages following Cape Smith belt metamorphism; gas-release spectra of older outlying dates exhibit characteristics of excess Ar. Following the metamorphic thermal peak, the belt cooled at slow rates of up to ~1 °C/m.y. until ca. 1740 Ma. Concordant biotite, muscovite, and hornblende cooling dates of ca. 1740–1700 Ma require fast, late cooling of the belt (≥4 °C/m.y.) through upper midcrustal levels (~500–300 °C), and they allow for very rapid cooling rates (≤200 °C/m.y.). Accelerated cooling rates may have been triggered by uplift in response to detachment of lower crust or subcontinental lithosphere, facilitated by the postcollisional relaxation of isotherms and structural uplift in basementinvolved folds. In Superior craton basement, ca. 2704–2667 Ma 40Ar/39Ar hornblende plateau dates reflect undisturbed cooling ages following Neoarchean metamorphism, whereas younger and wide-ranging 40Ar/39Ar biotite dates (ca. 2532–1743 Ma) with variable gas-release spectra suggest spatially heterogeneous degrees of Ar resetting in biotite during Cape Smith belt tectonism. Partially reset 40Ar/39Ar biotite dates in the Superior craton up to ~100 km south of the belt suggest that the pre erosional thrust wedge extended at least that far south, and that it imposed a widespread low-temperature (<300 °C) and/or short-lived thermal overprint on the footwall basement. Integration of multimineral 40Ar/39Ar data with structural and metamorphic constraints for the Cape Smith belt indicates that modern-style postcollisional exhumation and rapid cooling were viable processes during the middle Paleoproterozoic.
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Corte compensado del Pirineo oriental: Geometria de las cuencas de antepais y edades de emplazamiento de 10s mantos de corrimiento (1) C/ Consell de Cent, 599, atic, la. 08026 Barcelona (2) C/ Enamorats, 132, 2011, la. 08026 Barcelona RESUMEN El manto del Pedraforca se ha dividido en dos unidades: el man-to superior del Pedraforca predominantemente constituido por ma-teriales del Cretftcico inferior y el manto inferior del Pedraforca formado principalmente por una serie de Cretacico superior dis-cordante por encima del Jurásico. La parte aflorante del manto superior del Pedraforca se ha interpretado como una rampa de blo-que superior. Su edad de emplazamiento es Maastrichtiense terminal. La dirección de transporte N-S de 10s mantos, en especial del manto inferior del Pedraiorca puede deducirse de la disposición de sus rampas oblicuas con direcciones NNE-SSW y NW-SE. Estruc-turas frontales de dirección E-W pueden observarse en todo el man-to. En el manto del Cadi, las estructuras tienen dirección E-W normal a la direccibn de transporte. El corte geológico se ha construido paralelamente a la dirección de transporte, a través de una zona en la cua1 no se presentan ram-pas oblicuas. El acortamiento minimo del Pirineo oriental es de 50 km para el conjunto de mantos. Esto representa el 68%. La velocidad media de desplazamiento del conjunto de mantos de corrimiento para esta zona del Pirineo fue de 1 mm/a., siendo 3.3. mm/a. la velo-cidad media establecida para el manto inferior del Pedraforca. La velocidad de migración de 10s depocentros aumenta hacia el ante-país, siepdo 4.9 mm/a. la velocidad deducida para las secuencias deposicionales superiores (Milany-Solsona) de edad Bartoniense-0-ligoceso inferior. Palabras clave: Pirineo. Corte compensado. Antepais. Alpino. Manto de corrimiento. ABSTRACT The Pedraforca thrust sheet has been divided i; two main structural units: the upper Pedraforca thrust sheet consisting of a lower Cretaceous and the lower Pedraforca thrust sheet where the upper Cretaceous directly overlying the Jurassic. The upper Pedraforca thrust sheet is interpreted as a hangingwall ramp. It was emplaced in the late Maastrichtian. The La Garrotxa thrust sheet formed by lower Eocene is interpreted as the lowest thrust sheet outcroping in the Eastern Pyrenees. These thrust sheets have been incorporated into a N-S balanced cross-section between the Llobregat and the Segre rivers. The minimum shortening calculated is 50 km. The average rate of transport for the thrust sheets in this part of the Pyrenees was 1 mm/y and 3.3 mm/y for the lower Pedraforca thrust sheet. The rate of depocenters migration increases towards the fareland. The migration rate for the last depositional sequences was 4.9 mm/y (Milany-Solsona).
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A set of experiments has been conducted to determine the role of erosion and friction during the growth of a frontal ramp. Experiments are scaled to simulate shallow geological as thin-skinned thrust and fold belts. The experimental device is made of a flat over which horizontal layers of silicone and sand are poured. Ahead of this flat, the horizontal layering abuts against a rigid ramp. At the back, the layering is horizontally compressed by a moving wall pushed by a screw-jack. Results show that motion along the ramp is rapidly inhibited and stopped in the lack of erosion of relief formed over the ramp. The geometry of structures formed over the ramp is highly dependent on both friction along the ramp and erosion. -from English summary
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Near Pincher Creek, Alberta, the leading edge of the Canadian Cordillera approximates a triangle zone geometry with both wedge and duplex characteristics. Shortening across the wedge area is estimated at approximately 35%, most of the displacement having occurred across low-angle thrust ramps and flats that offset Jurassic to Upper Cretaceous-age strata. A detachment extends eastward from the wedge tip for a minimum of 14 kilometres under the foreland, across which several kilometres of shortening may have been accommodated by backthrust and duplex mechanisms within the hanging wall of the upper detachment. Folded Tertiary-age strata indicate that triangle zone development continued through Paleocene time. Other wedge geometries have been identified west of the leading edge, suggesting that triangle zone development occurred throughout the Laramide Orogeny. Locally, no commercial hydrocarbon production occurs from the triangle zone; however, both source and clastic reservoir rocks are carried in the wedge, with potential trapping geometries.
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The triangle zone structure examined in this study is a deformed wedge inserted along a delaminated horizon in the basal Upper Brazeau Formation (Upper Cretaceous clastic unit). The surface expression of the triangle zone is an antiform, with an east-dipping thrust cutting through the core of the antiform. Surface antiforms delineate three en échelon structures in the Grease Creek area, with a strike length of approximately 20 km. A comparison of three balanced cross-sections suggests the en échelon surface geometry reflects the changing position of the lower bounding detachment of the intercutaneous wedge. From south to north the structures consistently step eastward. A "relict" triangle zone is interpreted beneath the west flank of the Grease Creek Syncline based on interpreted seismic data. The surface culmination of the Wildcat Hills triangle zone, located at the deformation front south of the study area, can be traced to the position of this relict triangle zone, demonstrating that there are two separate en échelon intercutaneous wedges. An important implication of the en échelon geometry is that it produces closure of the structural traps along strike because individual structures terminate along strike. A composite two-dimensional model of the triangle zone is presented from the published models and the Grease Creek study. Important features of this model include a ramp and flat geometry of the upper detachment and upper and lower detachments of the intercutaneous wedge that do not merge. Instead, the toe of the wedge is a zone between the upper and lower detachments rather than a distinct tip line. The internal geometry of the triangle zone structure is hinterland-dipping in the study area.