Download Decoupling along plate boundaries: Key variable controlling the

Decoupling along plate boundaries: Key variable controlling the
mode of deformation and the geometry of collisional mountain belts
Ernst Willingshofer, Dimitrios Sokoutis
Faculty of Earth and Life Sciences, VU University Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, Netherlands
gence rates (Faccenda et al., 2008). In addition,
the increasing resistance of buoyant continental material to subduction may strengthen the
stress coupling between the foreland and the
orogenic wedge (Ziegler et al., 2002). In this
study we discuss the orogenic wedge–foreland
plate interaction as a consequence of increasing mechanical coupling between the orogen
and the foreland. Our findings have implications for collisional mountain belts in general,
and their relevance is discussed for the case of
the European Alps in particular.
cm
MODELING SETUP
The experimental continental lithosphere
consists of three layers representing the brittle
crust, ductile crust, and ductile mantle. (See
Fig. 1 for the geometrical setup, the analogue
materials used, and the length and time scales.)
Material properties and scaling parameters were
described in Willingshofer et al. (2005). The
2.5
13
nt
In
de
ic
ge
n
ro
O
10 cm
Yield strength (Pa)
15 cm
Brittle crust
Brittle crust
Visc. crust Dry quartz sand
Viscous crust
& mantle Silicone mix I
Strong viscous
mix I Silicone mix II
upper mantle 30o
Low-viscosity lower mantle lithosphere
+ Asthenosphere (liquid)
Zone of decoupling
0
Depth (cm)
1.0
1.5
15 cm
Indenter
Orogenic wedge
er
dg
e
pl
la
re
Fo
0
we
at
e
18
Lateral confinement
nd
cm
22
Depth (cm)
INTRODUCTION
Decoupling within plates or at plate boundaries has implications for transmission of
stresses within the lithosphere and the resulting
strain, which is a reflection of the different deformation processes at work. Vertical decoupling
within the lithosphere is mainly a function of its
composition and thermal state. Investigations of
the consequences of decoupling at or close to
the Moho (e.g., Ellis, 1996; Royden, 1996) have
emphasized that crust-mantle decoupling affects
the shape, height, and length scales of collisional mountain belts. Horizontal decoupling,
however, relates to strain accommodated along
weak plate contacts, such as subduction channels (e.g., Stöckhert, 2002) or weak intraplate
faults (Handy et al., 2005). The weakness at
plate interfaces seems to be critical for the exhumation of high- and ultrahigh-pressure rocks
(e.g., Gerya et al., 2002), the development and
stability of one-sided terrestrial subduction
(Faccenda et al., 2008; Gerya et al., 2008), or
the amount of supported upper-plate topography (Lamb, 2006). The results of deep seismic
profiling, imaging the crustal-scale geometry of
mountain belts like the Himalayas (Zhao et al.,
1993), also demands strong decoupling to enable
underthrusting of the Indian continent.
The strength of the interface between plates
or between plates and orogenic wedges as
considered in this study can change through
time because lubrication between the plates
may become inefficient as the incoming plate
carries less sediment and the availability of
water decreases, or as shear heating and hence
thermally driven softening of the plate interface
decrease as a function of slowing of conver-
strength of the model lithosphere varies laterally such that a wedge-shaped weak zone is
bordered on both sides by strong lithospheres,
referred to as foreland plate and indenter,
respectively. In analogy to natural orogenic systems, the weak zone represents imbricated sediments and upper crustal slices, which have been
scraped off the incoming plate. Lateral changes
of lithospheric strength were incorporated in
the model by varying the thickness of the brittle
and ductile crustal layers and the rheology of
the ductile upper mantle. This study discusses
cases where the weak wedge is strongly coupled
to the adjacent plates (experiment A) and where
the same weak wedge is decoupled from the
strong plate along its inclined boundary (experiment B). It is argued that strong decoupling
along the wedge–foreland plate interface is conditioned by shear heating (e.g., Faccenda et al.,
2008) or by fluid activity in response to the
dehydration of cover sediments of the incoming
plate, similar to what is known from subduction
systems (e.g., Peacock, 1990). Strong decoupling
is achieved by lubricating the inclined boundary
with a weak 2-mm-thick silicone layer, differing from previously published lithosphere-scale
analogue models where crust-mantle coupling
has been implemented by varying the shortening
velocity (e.g., Davy and Cobbold, 1991). Solid
walls confined three sides of the model while the
fourth, a layer of silicone putty labeled “lateral
confinement” (Fig. 1), allowed limited amounts
of lateral escape. During modeling, the surfaces
of all experiments were scanned with a threedimensional video laser every 30 min, and the
data were converted to digital elevation models
(DEMs). All experiments were performed under
normal gravity conditions.
300 600 900
1.0
1.5
2.5
Yield strength (Pa)
0
Depth (cm)
ABSTRACT
The consequences of decoupling between weak orogenic wedges and strong adjacent foreland plates are investigated by means of lithospheric-scale analogue modeling. Decoupling
is implemented in the three-layer models by lubrication of the inclined boundary between a
strong foreland and a weak orogenic wedge. Plate boundaries are orthogonal to the convergence direction. Experimental results show that strong decoupling between the foreland and
the orogenic wedge leads to underthrusting of the former underneath the orogenic wedge
and deformation of the orogenic wedge by folding, shearing, and minor backthrusting. Shortening is mainly taken up along the main overthrust, the decoupled boundary, and within
the orogenic wedge, leaving the indenter devoid of deformation. In contrast, strong coupling
between the foreland and the orogenic wedge favors buckling, involving both the weak zone
and the strong plates. The results of these end-member models have implications for collision
zones, for example, the Eastern Alps in Europe, such that the switch from localized deformation within the orogenic wedge during the Oligocene–middle Miocene to orogen-scale uplift
and deformation during the late Miocene–Pliocene involving the foreland and indenter plates,
respectively, is interpreted as reflecting a change from a decoupled to a coupled system.
300 600 900
1.0
1.5
2.5
Length scale: 1cmEXP ~ 20 kmNATURE
Shortening velocity: 0.7 cm/hr
Figure 1. Modeling setup and initial strength variation among modeling domains as shown
by strength envelopes.
© 2009 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected].
GEOLOGY,
January
2009
Geology,
January
2009;
v. 37; no. 1; p. 39–42; doi: 10.1130/G25321A.1; 2 figures; Data Repository item 2009010.
39
MODELING RESULTS
Experiment A—Strong Coupling
Deformation commenced at the vertical
boundary between the weak wedge and the
indenter as backthrusting (thrust 1 in Fig. 2A).
Subsequently, deformation stepped into the
weak wedge, leading to the activation of
forethrust 2, which together with backthrust
1 defines a pop-up structure. At ~10% bulk
shortening, deformation within the brittle
crust jumped to the other side of the orogenic
wedge, where backthrusts and forethrusts (4, 5,
and 6) developed above the initially inclined
boundary (Fig. 2B). From 12% bulk shortening
onward most of the shortening within the brittle crust was taken up by forethrust 1′, which
coincides at the surface with former backthrust
1 (cf. Figs. 2B and 2C). Thrusting thereafter
remained confined to near the transition zones
between the weak and adjacent strong plates,
leading to narrowing of the zone between
thrusts 1 and 3 (Fig. 2D).
The cross section reveals the geometry of a
folded lithosphere (Fig. 2E), showing that the
process of folding controls the locus of thrust
formation (at the inflection points of folds, as
also shown by Davy and Cobbold, 1991; e.g.,
thrusts 1′ and 8), leading to the rotation of
thrusts into a steep dip (thrusts 1–3) and the
development of pop-down–type structures similar to those described by Sokoutis et al. (2005).
Although the rheological contrast between the
weak zone and the adjacent plates is on the order
of one magnitude, coupling between the plates
is strong, as documented by the lack of shear
across the inclined and vertical boundaries and
the fact that deformation affected all parts of the
model throughout the model run (Fig. 2F). The
latter can be deduced from the DEMs in Figures
2A–2D and the topographic profiles (Fig. 2F),
which show that the weak zone developed from
a broad uplifted region with thrust-controlled
relief (topographic profile after 5% bulk shortening) to form a single antiform. Broad antiform highs are separated by narrow longitudinal
basins and the asymmetry in the model is conditioned by the advancing wall.
Experiment B—Strong Decoupling
In experiment B, deformation started at the
decoupled boundary (thrust 1 in Fig. 2G). Thereafter, shortening was accommodated within the
weak zone by folding and by forethrusting and
backthrusting (thrust 2 in Fig. 2G and thrusts
6 and 10 in Figs. 2H–2J). Close to the transition to the lateral confinement, shortening of the
weak zone was also accommodated by minor
strike-slip faulting. In contrast to experiment
A, deformation remained localized on thrust 1
throughout the simulation, and contraction of
the weak wedge caused overturning of the fold
in the hanging wall to thrust 1.
40
Sections reveal an asymmetric lithospheric
structure, which is dominated by underthrusting
of the foreland plate (Fig. 2K). As shortening
is mainly taken up by motion along thrust 1, and
to a lesser extent within the brittle layer of the
weak wedge, the indenter remained nearly undeformed. The foreland-facing side of the weak
zone forms a recumbent fold cored by the ductile
crust. The foreland and indenter plates are separated by a broad ductile shear zone. The Moho
topography of the foreland plate documents
bending, whereas that of the indenter remained
flat. At the end of the experiment (24% bulk
shortening) a mature foreland-type basin had
developed on the underthrusting plate (DEMs
in Figs. 2H–2J, 2L). The evolving topography
is asymmetric, with steeper slopes facing the
foreland plate where migration of thrust 1 and
the progressive deepening of the basin can be
deduced from the topographic profiles.
INTERPRETATION OF
MODELING RESULTS
Our results show that decoupling between
orogenic wedges and adjacent strong plates is
a variable that can steer the structural and topographic evolution of collision zones as it controls the mechanisms by which the crust and
lithosphere deform (see GSA Data Repository
Fig. DR11). Our end-member models indicate
that strong mechanical coupling between plates
and orogenic wedges favors deformation of the
lithosphere by folding, involving the strong
indenter and foreland plates as well as the
weak orogenic wedge, differing in the relative
amount of vertical motion as a function of plate
and wedge strengths and the distance to the
stress source (moving wall). Consequently, stress
transfer from the indenter to the foreland plate is
hampered by the intervening weak wedge, leading to lower amplitude folding of the latter with
respect to the indenter. Hence, lateral strength
variations within the continental lithosphere
do not necessarily lead to decoupling between
plates or parts of plates, but merely influence
the wavelength and amplitude of deformation.
In contrast, strong decoupling between the
orogenic wedge and the foreland plate leads to
crustal- and lithospheric-scale thrusting. As most
of the shortening is taken up by shear along the
decoupled boundary, minor backthrusting along
the indenter–orogenic wedge boundary does
not lead to the development of a mature retroforeland basin. Similarly, internal deformation
of the indenter and foreland plate is negligible,
1
GSA Data Repository item 2009010, Figure DR1
(cartoon illustrating the consequences of decoupling
and coupling between the foreland and an orogenic
wedge), is available online at www.geosociety.
org/pubs/ft2009.htm, or on request from editing@
geosociety.org or Documents Secretary, GSA, P.O.
Box 9140, Boulder, CO 80301, USA.
except for bending of the latter due to loading by
the evolving orogen. Our results are consistent
with numerical modeling studies of Faccenda
et al. (2008) and Burov and Yamato (2008), who
emphasized that decoupling between colliding
plates produces one-sided mountain belts (our
experiment B), whereas strong coupling favors
the formation of two-sided orogens, with largescale folding being the dominant deformation
mechanism (our experiment A).
IMPLICATIONS FOR THE EVOLUTION
OF COLLISIONAL MOUNTAIN BELTS
AND THEIR FORELANDS
The models discussed in this study represent end-member scenarios with constant
mechanical boundary conditions throughout
the simulations. In nature mechanical coupling
between orogenic wedges and adjacent plates
is likely to be subject to changes through time
as the amount of water lubricating the contact
zone decreases, the buoyancy of the incoming
continental lithosphere, resisting subduction,
increases, or the effect of shear heating at plate
contacts diminishes as shortening velocities
decrease (Faccenda et al., 2008). In such cases,
an increase of coupling among the orogen and
the foreland plates is expected to occur, leading to the change of the dominant deformation mechanism through time. Folding of an
orogenic wedge with a complex rheological
stratification is difficult to reconcile in natural
examples on the basis of structural observations
because folding is typically disharmonic and is
accompanied by thrusting in the brittle layers of
the lithosphere (Davy and Cobbold, 1991). The
pattern of vertical motions in the mountain belt
and the surrounding plates, however, may help
to constrain increasing coupling between the
wedge and the foreland plates (Ziegler et al.,
2002). Taking the Eastern Alps (German, Austrian, Italian) as an example, we argue that the
following events in the evolution of the Alps
reflect a change from a decoupled to a coupled
system. Following consumption of a small
ocean (Alpine Tethys), late Eocene underthrusting of the distal European continental margin,
now exposed in the core of the mountain belt,
the Tauern Window (Frisch et al., 1998;
Lammerer and Weger, 1998), occurred under
weakly coupled conditions with calcareous
clays and silts from the ocean and the sedimentary cover as decoupling material. Loading of the European plate is reflected by strong
subsidence in the foreland basin during the late
Eocene–early Oligocene (Genser et al., 2007)
as the response to the northward-propagating
mountain belt. Experiment B depicts such a
situation where the foreland plate is thrust under
the orogenic wedge, leading to crustal thickening and uplift of the internal part of the orogen
and subsidence of the foreland plate.
GEOLOGY, January 2009
Experiment B - strong decoupling
Experiment A - strong coupling
1
1.6% bulk shortening (bs) ~ 14 km
Digital elevation model
Indenter
Weak wedge
Foreland plate
2
1.6% bs ~ 14 km
Lateral confinement
1 2
2
1
2
1
22
A
4 21
5 3
4
10% bs ~ 84 km
10% bs ~ 84 km
6
1
21
4
G
8 3 7
5
8
2
5
2 6
1
62
8 3
7
5
3
5
8
2
4
6
5
6
1 2
H
B
6 4 421'
5
15% bs ~ 126 km
83 7 8
8
20% bs ~ 174 km
3
64
17
42
62
8
1'
5
1
3
6
8
7
8
5
8
1 5
3
I
6 4 31'
56 2
22% bs ~ 182 km
17
64 2
3
1'
5
837 8
10
9
1 5 6
24% bs ~ 210 km
6
3
8
A
A
6 4
Brittle crust
Strong viscous upper man
4 cm
tle
3 2 1'
2
1
D
8 6 1
8
10
5
9
5 6
J
6 10
1
1
A
Initially inclined boundary
Viscous crust and upper mantle
Topographic evolution
7
9
8
8
17
6
C
Model Moho
4 cm
K
E
Relief along cross section A after:
0%, 5%, 10%, 15%, 20%, 22% bulk shortening
Vertical exaggeration: 2x
Topographic evolution
Relief along cross section A after:
0%, 5%, 10%, 15%, 20%, 24% bulk shortening
Vertical exaggeration: 2x
Steep slopes facing the foreland
Gentle slopes facing the indentor
4 cm
4 cm
Uplift
F
Subsidence
Thrust fault
Strike-slip fault
Normal fault
Migration of thrust front (1)
Anticline/syncline
Outline of weak wedge
L
Black colors = non-active
Figure 2. A–F: Structural and topographic evolution of experiment A. G–L: Structural and topographic evolution of experiment B. Original
top-view photographs, digital elevation models (DEMs), structural interpretation and cross sections along line A, and snapshots of evolving
topography along same section are displayed. Numbers indicate sequence of deformation. Grid spacing is 4 cm. Green rectangles in A and
G delineate areas covered by DEMs.
GEOLOGY, January 2009
41
The coupling between the orogen and the
foreland plate thereafter increased gradually, as
documented by the following.
1. Oligocene–Miocene strong internal deformation of the wedge led to folding and stacking
of tectonic units, now exposed along the central
axes of the Eastern Alps (Lammerer and Weger,
1998). During this period of time the strongest
vertical motions occurred in the core of the orogen, as documented by pressure-temperaturetime paths from the Tauern Window (e.g., Cliff
et al., 1985; Fügenschuh et al., 1997). Convergence was partitioned into a component of
north-south shortening and concomitant eastwest extension (Ratschbacher et al., 1991).
Eastward material transfer was conditioned by
the Periadratic fault system, leading to crustalscale decoupling between the orogen proper and
the indenter (Handy et al., 2005).
2. The switch from dominant north-directed
thrusting of the orogenic wedge onto the European foreland to south-directed thrusting and
internal deformation of the indenter occurred
during the late Miocene (ca. 12–10 Ma). Such
behavior is consistent with coupled examples as
shown in experiment A and the experiments by
Sokoutis et al. (2005).
3. Isotope data and, in particular, apatite
fission track data from the Eastern Alps
compiled by Luth and Willingshofer (2008)
document a switch from localized uplift and
cooling in the core zone of the orogenic wedge
in pre–middle Miocene time to regional,
orogen-scale uplift from the late Miocene
onward. This important change is also reflected
in the increase of sediment discharge from the
Alps (Kuhlemann et al., 2001).
4. Middle to late Miocene cooling probably
led to strengthening of the orogenic wedge,
which facilitated the stress transmission from
the indenter to the northern foreland, triggering
its uplift, which started ~6 m.y. ago as documented by subsidence analyses from the foreland basin (Genser et al., 2007). Folding of the
foreland plate (experiment A) in cases of
mechanical coupling between the foreland and
the orogenic wedge is a potential mechanism
to explain the late-stage uplift of the foreland
basin without the need to call upon weakly constrained processes like delamination of the lithospheric root (Genser et al., 2007). Based on our
results and in accord with the regional studies of
Ziegler et al. (2002), we argue that collisional
mountain belts in general are expected to evolve
from a decoupled to a coupled system, leading
to a change of the dominant deformation mechanism as portrayed in the vertical motions of the
orogenic wedge and the foreland.
42
ACKNOWLEDGMENTS
We thank T. Gerya and M. Bonini for their constructive reviews, and R. Stephenson for improving
the English. Funding by the Netherlands Research
Centre for Integrated Solid Earth Sciences (ISES) is
acknowledged.
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Manuscript received 20 July 2008
Revised manuscript received 16 September 2008
Manuscript accepted 19 September 2008
Printed in USA
GEOLOGY, January 2009