Int J Earth Sci (Geol Rundsch)
DOI 10.1007/s00531-012-0747-6
ORIGINAL PAPER
Granite magma migration and emplacement along thrusts
Eric C. Ferré • Olivier Galland •
Domenico Montanari • Thomas J. Kalakay
Received: 10 August 2011 / Accepted: 4 January 2012
! Springer-Verlag 2012
Abstract This paper investigates the influence exerted by
brittle tectonic structures in the emplacement of granite
plutons in contractional settings. We address both cases
where contractional tectonics and magma intrusion are (1)
coeval, to study how active contractional tectonics controls
the transport of magma, and (2) diachronous, to study the
role of pre-existing structures on the transport of magma. In
light of new experimental models, we show that magma
can rise along thrusts ramps and flats. This phenomenon
occurs for both low-viscosity magma (basalts to andesite)
and high-viscosity magma (dry granite). The experimental
results also allow the evaluation of the role played by
magma viscosity in determining pluton geometries. In
addition, a review of literature demonstrates a spatial and
causal relationship between granites and thrusts and highlights the geometric control of magma pathways in the
pluton final shape. The abundance of subhorizontal and
E. C. Ferré (&)
Department of Geology, Southern Illinois University,
Carbondale, IL 62901-4324, USA
e-mail: eferre@geo.siu.edu
O. Galland
PGP, Universitet i Oslo, Sem Selands vei 24,
0316 Oslo, Norway
O. Galland
Géosciences-Rennes (UMR6118 du CNRS),
Université de Rennes, Rennes Cedex, France
D. Montanari
Institute of Geosciences and Earth Resources,
Via Moruzzi 1, Pisa, Italy
T. J. Kalakay
Rocky Mountain College, 1511 Poly Drive Billings,
Billings, MT 59102, USA
tabular granitic intrusions indicates that the location of
inflating granitic sills along thrust flats can be common. We
argue that active and pre-existing flats-and-ramps thrusts
provide a preferential continuous planar anisotropy susceptible to become a granitic magma migration pathway.
Keywords Granite ! Pluton ! Thrust ! Flats-and-ramps !
Analogue modeling
Introduction
Magma emplacement along thrusts in convergent tectonic
settings is an important issue for example to understand
earthquakes along subduction zones (e.g., Anma 1997). A
number of granite emplacement models have been proposed for different tectonic settings (reviews in Hutton
1988; Clarke 1992; Paterson and Fowler 1993; Pitcher
1993; Ingram and Hutton 1994; Vigneresse 1995; Bouchez
1997; Castro et al. 1999; Petford et al. 2000). The interplay
between emplacement mechanisms such as diking, ballooning, and stoping during granite emplacement makes
each pluton a unique case. Most studies, however, point to
the feedback relationship between felsic plutonism and
regional deformation (e.g., Guineberteau et al. 1987;
Paterson et al. 1990; Tommasi et al. 1994; Neves et al.
1996; Aranguren et al. 1997; Archanjo et al. 1999; Wilson
and Grocott 1999; Vigneresse et al. 1999; Benn et al. 2001;
Ferré et al. 2002). Therefore, to better understand granite
ascent and emplacement processes it is important to
simultaneously consider both the plutons and the crustal
discontinuities that surround them.
The spatial relationships between faults (or shear zones)
and granite plutons has been intensely discussed (Paterson
and Schmidt 1999; Schmidt and Paterson 2000; Richards
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Int J Earth Sci (Geol Rundsch)
2001; Weinberg et al. 2004, 2005, 2006; Paterson 2006).
The main question is whether or not magma migration is
controlled by faults (or shear zones) and if such a spatial
relationship causes plutons to be statistically located close
to faults (or shear zones). This debate has focused mainly
on steep strike-slip faults (and shear zones).
A good way of addressing the complex processes of
interactions between plutons and faults is through analogue
modeling. For example, Román-Berdiel et al. (1997),
Román-Berdiel (1999), and Corti et al. (2003, 2005)
investigated how plutons intruded in extensional to strikeslip regimes, showing how magma intrusion was controlled
along faults and how faults were affected by the intrusion
of magma. Benn et al. (1998, 2000) simulated the intrusion
of plutons in a transpressional tectonic regime. In this latter
work, the rates of deformation and intrusion were coupled,
such that it was not possible to control independently
magma intrusion and deformation.
In compression, the ascent of magma is expected to be
less favorable than in extension due to the adverse orientation of principal stresses and the horizontal attitude of
tension sites that may act as magma pathways (Fig. 1;
Petford et al. 1994; Watanabe et al. 1999; Sibson 2003).
This leads some authors to surmise that magma cannot rise
through crust, while it is being subjected to compression
(e.g., Hamilton 1994; Watanabe et al. 1999). However, in
examples where the dominant deformation regime is
shortening, such as the Andes and Japan, the occurrence of
voluminous volcanic activity shows that magma does rise
through contracting crust even in places where the overlying crust is very thick (up to 70–80 km in the Andes;
Yuan et al. 2000). Nevertheless, the key question remains:
how can magma rise in such compressional settings?
Recent observations show that active volcanism can be
spatially associated with thrust faults, for example, El
Reventador (Ecuador; Tibaldi 2005, 2008) and Tromen
(Argentina; e.g., Galland et al. 2007b) volcanoes. In deeper
systems, a number of field-based structural studies show
that granitic plutons emplaced during orogenic shortening
commonly exhibit tabular shapes, which appear to be
closely related to flats and ramps of active thrusts
(e.g., Hutton 1988; Karlstrom et al. 1993; Searle 1999;
Blenkinsop and Treloar 2001; Kalakay et al. 2001; Spanner
and Kruhl 2002; Aranguren et al. 2003; Musumeci et al.
2005; Naibert et al. 2010). Thus, it is plausible that during
periods of crustal compression, thrust faults form preferential magma migration pathways. In addition, pre-existing
faults and fractures representing weaknesses in rocks (e.g.,
Sibson 2003), it is likely that pre-existing and inactive
thrust faults also represent preferential magma pathways.
To test whether active and pre-existing compressional
tectonic structures form preferential magma pathways, we
investigate the spatial and genetic relationships between
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Fig. 1 Stress regime, shear fractures, and tension fractures in the
upper crust. a In extension, shear fractures are normal and steeply
dipping, while tension fractures are near-vertical (dikes). b In
compression, shear fractures are reverse and gently dipping, while
tension fractures are near-horizontal (sills). Magma ascent is expected
to be less favorable in compression than in extension due to the
adverse orientation of principal stresses and the horizontal attitude of
tension sites that may act as magma pathways
plutons and thrusts (or low-angle reverse shear zones). In
this paper, we present an integrated approach combining
recent results of analogue modeling and a brief review of
case studies of plutons that were emplaced in compressional settings. The experimental results come from two
complementary techniques that simulate the injection of
either low-viscosity or high-viscosity magma in deforming
crust (Galland et al. 2006, 2007a; Mazzarini et al. 2010;
Montanari et al. 2010a, b; Musumeci et al. 2005). The
experimental results are subsequently compared to the
structures of representative field examples of plutons that
were emplaced along thrust faults.
The consistency between the experimental results and
the field-based structural data allows us to propose a general model of granite emplacement in actively shortening
orogens (although it would also be applicable to previously deformed regions). Our model is constrained by
the geometry and kinematics deduced from analogue
Int J Earth Sci (Geol Rundsch)
experiments, the temporal constrains provided by case
studies and the low aspect ratio of many granitic plutons
inferred from gravimetric studies (e.g., Vigneresse et al.
1999). This model shows that flats-and-ramps of thrusts
can be fundamental migration pathways for magmas of
various viscosities.
Experimental models of intrusions along thrusts
In order to investigate the emplacement of granitic magma
in convergent tectonic settings, we resorted to experimental
modeling. Former attempts have been carried out to study
the emplacement of very viscous magma in transpressional
tectonic regimes. For example, Benn et al. (1998, 2000)
performed experiments in which a stiff silicone intruded
into a deforming crust. However, in their experimental
apparatus, the rates of intrusion and deformation were
coupled. Therefore, it was impossible to study the mechanisms of magma emplacement by considering independently the rates of deformation and injection; obviously,
diachronous deformation and intrusion could not be
addressed either to study the effect of pre-existing faults on
magma transport.
In this paper, we present results from two complementary experimental approaches of intruding magma into a
shortening crust, where intrusion and deformation rates
were independent. Indeed, with the two different deformational apparatuses used and described in the current
paper, it was possible to run experiments with oil injection
only, deformation only, and coeval and diachronous
deformation and injection. This a major difference with the
experiments of Benn et al. (1998, 2000). In geological
systems with both active magma intrusion and tectonic
deformation, it is relevant to estimate the relative contribution of intrusion with respect to that of deformation.
Thus, we define the dimensionless ratio R = (mpV)/(L0Q),
where mp, V, L0, and Q are the piston velocity, the initial
volume and length of the model, and the volumetric flux of
oil injection, respectively. When R is large, tectonic
deformation dominates the process, whereas, when R is
small, intrusion dominates the process. In the first approach
(Model 1), the models simulate the emplacement of lowviscosity magma (100–105 Pa s), whereas in the second
approach (Model 2), the models simulate the emplacement
of a more viscous magma (about 1017 Pa s). In the following sections, the scaling for each experimental technique and the experimental results were briefly developed.
Model 1: Low-viscosity magma
Model 1 (Figs. 2, 3; Tables 1, 2) consisted of cohesive
fine-grained silica flour and low-viscosity molten vegetable
oil simulating brittle host rocks and low-viscosity magma
(100–105 Pa s), respectively. After the experiments, the
vegetable oil solidified at room temperature in the models.
The scaling and the experimental method have been
described in detail by Galland et al. (2006) and used by
Galland et al. (2007a). The Model 1 was performed at the
Modeling Laboratory of Geosciences Rennes, University
of Rennes 1, France.
The silica powder is sufficiently fine-grained (20 lm) to
prevent oil percolating through it. After compaction, its
density is 1,300 kg m-3. In a Hubbert-type shear box (e.g.,
Schellart 2000), Galland et al. (2006) demonstrated that the
silica flour fails according to a Coulomb criterion: the true
cohesion is *300 Pa and the angle of internal friction is
38". The vegetable oil is solid at room temperature and a
Newtonian fluid when molten. At a temperature of 50"C,
we measured a density of *900 kg m-3 and a viscosity of
2 9 10-2 Pa s (in a rotary viscometer).
Model 1 simulated processes in the shallow upper crust.
For scaling upward from model to nature (e.g., Ramberg
1981), the length ratio l* between experiment and nature
was taken from 10-5 to 10-4 (10 mm : 0.1–1.0 km) and
the density ratio q* ranged between 0.5 and 0.7 (Table 1).
The cohesion of the model rock is properly scaled down if
C* = q*g*l*, where g is the acceleration due to gravity.
This dictated that cohesion of model crust was in the range
50–6,500 Pa. Silica powder, with its cohesion of *300 Pa,
is therefore appropriate for the models.
The viscosity of the magma is properly scaled if the
viscosity ratio g* = C*h*/vm*, where h and vm are the
thickness of the intrusion and the velocity of magma,
respectively. Values from Table 1 provide values for g*
ranging between 5 9 10-11 and 6.5 9 10-4. The viscosity
range for basalt to rhyolite magma being 100–105 Pa s, this
imposes that the scaled viscosity of the model magma
should be in the range 5 9 10-9–65 Pa s. In nature, the
viscosity of rhyolitic magma actually increases with
solidification to much higher values but then the magma is
no longer able to flow; therefore, the range considered here
is realistic. The viscosity of the vegetable oil being
2 9 10-2 Pa s, the vegetable oil is an appropriate material.
Notice that the range for model magma viscosity is very
large. This has little effect as long as the viscous stresses
are small with respect to cohesion (Galland et al. 2006,
2007a), which is the case with such values.
The experimental setup consisted of a rectangular box
(0.6 9 0.4 9 0.2 m) with a 75-mm-thick layer of compacted silica flour (Fig. 2a). For experiments on magmatic
intrusion, a steadily moving piston caused horizontal
shortening and vertical thickening of the silica flour
(0.075 m thick), while a pump steadily injected molten oil
through an orifice (0.005 m in diameter) at the base of the
box (Fig. 2a). A first electric motor controlled the preset
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Int J Earth Sci (Geol Rundsch)
Fig. 2 a Sketch of the
experimental setup of Model 1
(low-viscosity magma, see text
for explanations). b Photograph
of LV1 model. Both straight and
arcuate thrusts can be observed.
The white straight line shows
the location of the cross-section
in (c). c Photograph and
corresponding line drawing of
cross-section of LV1 model
(location shown in b). Internal
horizontal markers (continuous
lines) are offset by faults
(dashed lines). The solidified
intrusion (red) lies at the base of
the model. Numbers indicate the
sequence of thrust development
from 1 oldest to 5 youngest
speed of the piston and a second motor controlled that of
the pump.
In Model 1, we ran experiments with varying values for
R. In experiment LV2, there was only intrusion but no
deformation, so that R = 0 (Table 2). The resulting intrusion was an axi-symmetrical sill with a flat bottom,
emplacing at the base of the box, connected to steeply
inclined sheets (Fig. 3a). The thickness of the intrusion was
typically 1–3 mm. The overburden was slightly uplifted due
to the overpressure of the oil. This geometry is a typical
saucer-shaped sill that forms in non-deforming sedimentary
basins (Galland et al. 2007a, 2009; Polteau et al. 2008).
In experiment LV3, there was only deformation but no
injection, so that R = ? (Table 2). In this experiment,
123
shortening resulted in thrusts rooted of the base of the
piston (Fig. 3b). In map view, thrusts had straight traces,
which means that lateral friction on the model was negligible. The first thrust formed close to the piston. As
deformation proceeded, the following thrusts formed successively away from the piston. Each thrust was separated
by a constant gap of 0.05–0.06 m, resulting in a thrust
wedge of constant surface slope (Fig. 3b). Such a geometry
is typical of thrust wedges in a laterally homogeneous crust
(e.g. Dahlen 1990; Smit et al. 2003).
In experiments with coeval deformation and injection
(LV1, Fig. 2b, c), the value of R = 7.5, that is, it was
intermediate between the two former end-members
(Table 2). Those experiments consisted of two successive
Int J Earth Sci (Geol Rundsch)
Fig. 3 a Sketch of experiment LV2 with diachronous deformation
and injection. Deformation was first, and injection was second. The
basal intrusion (red) branched into a complex zone were porous oil
flow occurred (hachured red). This porous injection overprints early
formed backthrusts. b Sketch of experiment LV3 with a first stage of
coeval shortening and injection, and subsequent injection only
Table 1 Experimental parameters and scaling ratios for Model 1 (low-viscosity magma)
q (kg m-3)
C(Pa)
vm (m s-1)
Nature
2,000–2,700
7
10 –10
Model 1
Ratios
1,300
0.48–0.65
300
3 9 10-6–3 9 10-5
8
h (m)
g (Pa s)
10 –1
1–100
100–105
10-3–10-2
10-3–1
10-3–10-2
10-2–10-5
2 9 10-2
2 9 10-7–2 9 10-4
-2
See text for definitions of physical parameters
stages: a short first stage of shortening without injection to
load the models, followed by a second stage with both
shortening and injection. During the first stage of the
experiment, the first thrusts to form were straight forethrusts against the piston. Subsequently, as deformation
proceeded the oil was injected. Then, the next thrust to
form was strongly arcuate in the middle of the models; the
domain between the straight and the arcuate thrusts formed
a poorly deformed plateau (Fig. 2b, c). Such a structure is
significantly different from that of experiment LV3
(Fig. 3b). The oil formed a flat-lying asymmetric sill
intrusion; it was much thicker (0.01 m) than in experiment
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Int J Earth Sci (Geol Rundsch)
Table 2 Values of experimental parameters for Model 1
-3
vp (10
-1
ms )
-6
Q (10
3
-1
m s )
R
Shortening
(%)
33
LV1
0.27
0.78
7.5
LV2
0.00
0.27
0
0
LV3
0.17
0.00
?
33
LV4
0.22, then 0.00
0, then 0.35
? then 0
33
LV5
0.18, then 0.00
0.78, then 0.78
5.3 then 0
33
See text for definitions
LV2 (Fig. 3a). Its leading edge away from the piston
coincided with the root of the arcuate thrust fault (Fig. 2c);
in addition, the oil migrated along the arcuate thrust. The
sizes and shapes of the plateaus were identical to those of
the sills (Galland et al. 2007a). The latter showed that the
final geometry of the experiments depended on R: models
with high values of R led to small plateaus and sills, and
vice versa. In models with small R, oil erupted along the
lateral ramps of the arcuate thrusts (Fig. 2b; Galland et al.
2007a).
An important question is whether the transport of
magma is affected by pre-existing faults. This can happen,
for example, for magma rising after the main orogeny (see
examples in ‘‘Examples of plutons emplaced along thrusts’’
section). In order to test the effect of pre-existing faults on
magma transport and emplacement, we performed an
experiment (LV4, Fig. 3c) where (1) the model was
deformed first (R = ?) and (2) oil was injected after
deformation stopped (R = 0; Table 2). After deformation,
the model was similar to that with deformation only, that is,
a classical thrust belt with straight thrust faults (Fig. 3c).
This means that oil intruded into an already developed, but
inactive, thrust wedge. After the second stage with injection only, the intrusive body exhibited a strongly asymmetric shape. It consisted of a basal sill of a few
millimeters thick, which branched laterally and upwards to
a backthrust zone (hachured on Fig. 3c), toward the piston.
This zone consisted of a mixture of oil with silica flour, but
was different from the intrusion observed, for example, in
experiment LV1 where the intrusion was almost pure oil.
We thus interpret this zone as a network of oil paths that
followed an intensely damaged host due to a wide backthrusting zone. Notice that the dip angle of back-thrusts is
steeper (*45") than that of fore-thrusts (*30"). In consequence, the intrusion in LV4 is steeper than in experiments LV1 and LV5. The difference in dip angle is caused
by the asymmetric nature of the model and by a slight
rotational component of the experiments.
The rates at which magmas are transported are known to
vary through time. Good examples are periodic volcanic
eruptions occurring in active tectonic areas (e.g., El
123
Reventador Volcano, Ecuador; e.g. Tibaldi 2005): during
the volcanic episodes, the magmatic rates become temporarily much faster than the tectonic rates, leading to R ? 0,
although the compressional stresses are still active. To test
this effect, we performed an experiment (LV5, Fig. 3d),
during which oil injection flow rate was varied during two
stages (Fig. 3b). (1) First, shortening and injection were
coeval and constant, resulting in a similar structure to that
of experiment LV1, with an arcuate thrust bounding a
poorly deformed plateau. During this stage, R = 5.3
(Table 2). At the end of this first stage, we had no access to
the internal structure of the model. (2) Second, shortening
was stopped while injection went on; this corresponded to a
fast injection of magma with respect to tectonic timescales, that is, volcanic eruption, while stresses were
assumed to be still active (R ? 0). This second stage
ended only a few seconds after it started, with the eruption
of the oil along the trace of the arcuate thrust. In crosssection, the intrusion exhibited a more complex shape than
in experiment LV1. It consisted of a cm-thick basal sill at
the base of the poorly deformed plateau, connected to a
mm-thick inclined dike (Fig. 3d). The leading edge of the
thick basal sill coincided with the root of the arcuate thrust.
By comparison with experiment LV1, we infer that this
thick basal sill formed during the first stage of the experiment. In contrast, we infer that the thin inclined dike
formed during the second stage of the experiment. Notice
the remarkable superposition of this thin inclined dike with
the arcuate thrust. The eruption point is not visible on this
cross-section as it occurred a few centimeters away from
the central cross-section.
Model 2: High-viscosity magma
The second study (Fig. 4; Tables 3, 4) consisted of sand/
silicone models, where a basement of quartz-sand layers,
and a low-viscosity mixture of silicone and oleic acid
represented brittle host rocks and high-viscosity granitic
magma, respectively. Models were constructed with initial
dimensions of 0.6 9 0.45 m and a thickness of 0.05 m.
Geometrical, rheological, kinematical, and dynamical
similarity ensured scaling to the natural process under
investigation (Hubbert 1937; Ramberg 1981), with a length
ratio of l* * 1 10-5 (10 mm in the model represents *1 km in nature) and a velocity ratio of v* * 6.5
10-3 (the *20 mm/h of model shortening corresponds to a
natural shortening rate of *27 mm/year, see Table 2);
density of analogue materials imposes a density scale ratio
of about 0.5 (Table 3). The analogue magma is a lowviscosity Newtonian fluid with a viscosity of about 7.102
Pa s, resulting in a scaled viscosity of about 1017 Pa s
(Table 3), suited to simulate a high-viscosity crystal-rich
magma being emplaced at shallow crustal levels. The
Int J Earth Sci (Geol Rundsch)
Fig. 4 Deformation and emplacement patterns of Model 2 experiments (high-viscosity magma). a Schematic model 3D view showing
the position of the injection point in relation to the thrust wedge.
b Final top-view photograph of the reference model (GITD 03)
performed with coeval shortening and magma injection. c Cross-
section and line drawing (d) of model GITD03. e Cross-section of
model GITD19 with injection subsequent to the shortening phase.
f Cross-section of model GITD12 with deformation post-dating
magma injection. Here, late-formed thrusts exploit magma/countryrock contact
Table 3 Experimental parameters and scaling ratios for Model 2 (higher viscosity magma)
q (kg m-3)
Nature
7
2,700
Model 2
10
1,300
Ratios
V/(m s-1)
C(Pa)
65
-6
0.48
6.5 9 10
h (m)
g (Pa s)
8.6 9 10
-10
1,000
1017
5.6 9 10
-6
0.01
6.5 9 10
-3
700
-5
1 9 10
7 9 10-15
See text for definitions of physical parameters
Table 4 Values of experimental parameters for Model 2
-6
vp (10
-1
ms )
Q (10
-6
3
-1
m s )
R
GIDT 00
5.6
0.004
?
GIDT 03
5.6
0.004
46
GIDT 06
0.0
0.004
0
GIDT 19
5.6 then 0
0 then 0.004
? then 0
GIDT 12
0 then 5.6
0.004 then 0
0 then ?
See text for definitions
scaling and the experimental procedure have been described in detail by Del Ventisette et al. (2006) and used by
Montanari et al. (2010a).
As in Model 1, a piston moving at a constant speed
deformed the models, and the experiments consisted of (1)
a first phase with shortening only and (2) a second phase
with coeval shortening and constant flow injection of the
analogue magma. The experiments were performed at the
Tectonic Modeling Laboratory of the CNR-IGG (Institute
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Int J Earth Sci (Geol Rundsch)
of Geosciences and Earth Resources) and of the Earth
Science Department of Florence University (Italy). Magma
injection during thrusting was performed by means of a
special apparatus consisting of a piston and a magma distribution system made up of pipes and a fixed injection
point (10 mm in diameter) on the basal plate of the
deformation apparatus (Fig. 4a). A reference experiment
with no injection (exp. GITD 00, R = ?) was performed,
resulting in a classical thrust wedge, in which thrusts had
straight traces. Moreover, in static conditions (i.e., without
deformation, exp. GITD 06, R = 0) the experimental
magma intrusion resulted in an almost circular shape in
map view (Montanari et al. 2010a).
In the case of syn-deformation magma intrusions (exp.
GITD 03), the sand/silicone models (simulating highviscosity magma) were characterized by a strong spatial
correlation between thrusts and related folds and synkinematic pluton geometries (both in map view and in
cross-section, Fig. 4b–d and Montanari et al. 2010a).
Magma preferentially migrated away from the injection
point and ascended along thrust surfaces, showing that the
intrusion was strongly controlled by deformation structures. The model magma accumulated in the low-pressure
area developing within the thrust-related anticline (Fig. 4c,
d). The cross-sectional shape of the model plutons was
asymmetric and controlled by the interplay between compressive structures, with the intrusion long axis controlled
by magma migration along a major fore-thrust and the
short axis oriented according to a minor back-thrust
(Fig. 4c, d). The exact shape of the model pluton would
have required tomographic analysis to be determined, and
this option was not available in the experimental setup.
However, it is important to keep in mind that fluids migrate
along the r2 axis. In map view, the intrusions were elongated parallel to the strike of the thrusts and anticlines
more markedly when deformation rates dominate over
injection velocity (Montanari et al. 2010a); the models
were characterized by an increase of the intrusion aspect
ratio (length/width) increasing the relative importance of
shortening over magma injection (Montanari et al. 2010a).
In the experiment GITD 03, with coeval deformation and
injection, the value of R was 12 (Table 2).
To test the role of simultaneity between magma injection
and deformation to determine the final geometry of the
intrusions, we also performed two additional new models. In
the first one, magma injection started only after the end of
deformation (exp. GITD 19, Fig. 4e), while in the second,
deformation occurred only after magma injection was stopped (exp. GITD 12, Fig. 4f). Values of the dimensionless
ratio R for these two model are given in Table 2. The crosssectional shape of the pluton in the model GIDT 19 was quite
symmetric and bound by the compressive structures,
exhibiting a delta-like geometry. This geometrical
123
arrangement clearly suggests that the presence of the preexisting compressive structures determined the localization
of the analogue magma intrusion, the latter being bound by
thrust and back-thrust faults. Pre-existing compressive
structures acted as barriers for lateral magma migration,
allowing the uprising of magma toward shallower levels. In
cross-sectional view, the Model GIDT 12 (Fig. 4f) with
deformation post-dating magma intrusion, clearly exhibits
characteristic features related to magma injection into a
static, undeformed model (please compare with Figure 2 in
Montanari et al. 2010a). The circular dome formed above the
intrusive body (in cross-section in Fig. 4f) and related to the
sand being uplifted by the intruding analogue magma is still
visible at the surface. The pre-kinematic shape of magma
intrusion is clearly deformed by shortening of the models
performed after the end of the magmatic phase. The final
cross-sectional shape of the analogue pluton was asymmetric, evidencing the fore-thrust localization related to the
presence of magma acting as a weakness zone. The pluton
was also clearly deformed in accordance with the late
shortening of the model. In this experiment, the magma
experienced greater degrees of transport and uprising in
comparison with the other previously described models, with
the top of the intrusion being near the surface. This feature
developed mainly during the injection phase and only to a
lesser extent during the successive shortening of the model.
In both models, even with significantly different
geometries and arrangements, a geometrical correlation
between shear zones and magma intrusions still remains. It
is important to note that in both experiments, the elongation of the intrusions parallel to the thrust fault did not
develop, demonstrating that only simultaneous deformation
and intrusion of magma leads to elongation of the intrusive
body parallel to the thrust plane. This feature characteristic
may provide an additional way to discriminate in the field
between syn-kinematic magma intrusions and post-kinematic intrusions.
Summary of modeling results
The Model 1 and 2 simulated the emplacement of magmas
of very different viscosities (mafic to andesite magma
versus granites). The resulting intrusions thus exhibited
distinct overall shapes: low-viscosity magma intrusions
were very thin and tabular, whereas high-viscosity magma
intrusions were more rounded. Such differences are in good
agreement with theory (Rubin 1993).
Although each experimental method simulated the
emplacement of magma of very different viscosities, they
provided similar results, sharing the same modalities and
geometries of magma emplacement during thrusting. (1)
Fluid injection resulted in gently dipping to flat-lying
intrusions. (2) When the magma reached a pre-existing or a
Int J Earth Sci (Geol Rundsch)
still active thrust, it migrated upward and followed the
thrust, at times accumulating within the thrust-related
anticline. (3) When a thrust formed after the magma has
started intruding, it nucleated within the intrusion or even
at the boundary between the pluton and the country rocks.
Such consistency suggests that whatever the viscosity
of magma intruding into a shortening crust, the transport of
magma may be strongly influenced by the presence of
thrust faults, whether active or not. In addition, the comparison between these two experimental series highlighted
that high-viscosity magmas are able to migrate along
thrusts and emplace within thrust-ramp anticlines exhibiting elongated geometries parallel to the strike of structures,
whereas decreasing the viscosity of magma facilitates its
upward migration. In this latter case, magma may come to
the surface by uprising along thrusts as evidenced in nature
by the development of several volcanoes at active-convergent margins (e.g., Gonzalez et al. 2009).
Examples of plutons emplaced along thrusts
In the following, we briefly review representative examples
of syn-kinematic and post-kinematic granite plutons, of
different age and composition, that were emplaced along
thrusts. Many other examples of granite plutons that could
also be interpreted as emplaced during thrusting were not
included in this review due to space limitations but share
the same characteristics (e.g., Hutton 1988; Karlstrom et al.
1993; Searle 1999; Blenkinsop and Treloar 2001; Spanner
and Kruhl 2002; Aranguren et al. 2003; Musumeci et al.
2005).
The early proterozoic Chilimanzi granites, Zimbabwe
Craton (Fig. 5a)
Biotite-hornblende tonalite-granodiorite-granite plutons
were emplaced at 2,620 Ma (D2) after an early continental
collision event (D1) dated at 2,670 Ma (Dirks and Jelsma
1998). Granites s.l. intruded, under greenschist facies
conditions, a crust with D1-inherited nappe-thrust geometry. Figure 3b of Dirks and Jelsma (1998) diagrammatically shows that the emplacement of granite sheets
parallels D1-flats with ascent paths probably provided by
D1-ramps. This is an example of post-kinematic granite
emplacement along thrusts and ramps where magma was
guided by pre-existing structures.
The palaeozoic Wyangala granites, Lachlan Fold Belt,
Australia (Fig. 5b)
The Wyangala biotite tonalite-granite plutons were emplaced around 415 Ma (Cycle II) shortly after an early
continental collision event (Cycle I) of Middle Silurian age
(Tobisch and Paterson 1990; Paterson et al. 1990). For the
Yarra pluton specifically, the structural data of Tobisch and
Paterson (1990) is consistent with magma ascent in a steep,
ductile, reverse shear zone, followed by thrusting during
granite cooling. This is an example of syn-kinematic
granite emplacement along a reverse shear zone.
The neoproterozoic Rahama granite, Nigeria-Brazil
transaharan belt (Fig. 5c)
In northern Nigeria, the Rahama biotite-hornblende monzonite-monzogranite pluton was emplaced at 580 Ma (D3)
after an early collision event (D1) dated ca 640 Ma (Ferré
et al. 1997, 2002). Magma emplacement occurred under
upper amphibolite facies conditions in a crust that had a
D1-inherited nappe-thrust structure. Granitic magmas were
drained along reverse inclined shear zones interpreted as
thrust ramps. Initial intrusion-related fabrics are overprinted by late, but still magmatic, strike-slip, and reverse
shear deformation. In Brazil, which is the southern extension of the same orogen (e.g., Caby 1989), similar tectonic
and plutonic events prevailed with granites emplaced both
in frontal and lateral thrust ramps (Corsini et al. 1991).
The late cretaceous: miocene granites of the Hidaka
Belt, Japan (Fig. 5d, e)
Peraluminous S-type tonalites intruded the Hidaka belt,
Hokkaido, along two structural discontinuities: along a
basal décollement and along a ramp to roof thrust of a
duplex (Shimura 1992; Toyoshima et al. 1994). A model of
granitic magma ascent along ramps and granite emplacement along flats is illustrated in Fig. 14b of Shimura (1992)
and Figure 13 of Toyoshima et al. (1994). This example is
geochronologically less well-constrained but also demonstrates the emplacement of granitic magma is guided by
thrust faults.
The miocene granites, High Himalaya, Tibet (Fig. 5f)
S-type biotite ± tourmaline leucogranite sills were emplaced around 24–22 Ma after a major collision (Harrison
et al. 1997). Anatexis is explained by shear heating and wet
melting along a thrust flat with a shear stress of about
30 MPa. The sills, situated above the South Tibetan
Detachment (STD) and below the Main Central Thrust
(Searle 1999), predate the STD. The cross-section in
Figure 3 of Murphy and Harrison (1999) displays subhorizontal granite sheets (sills) connected to oblique deformed
granite dikes, which may be interpreted as feeders. This is a
clear example of syn-kinematic magma emplacement along
thrust flats and ramps.
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123
Int J Earth Sci (Geol Rundsch)
b Fig. 5 Geological examples of granite plutons of various ages
emplaced in thrust systems. a The Early Proterozoic Chilimanzi
granites, Zimbabwe Craton (Dirks and Jelsma, 1998); b the Palaeozoic Wyangala granites, Lachlan Fold Belt, Australia (Tobisch and
Paterson 1990; Paterson et al. 1990); c the Neoproterozoic Rahama
granite, Nigeria-Brazil Transaharan Belt (Ferré et al. 1997, 2002);
d, e the Late Cretaceous to Miocene granites of the Hidaka Belt,
Hokkaido, Japan (Shimura 1992; Toyoshima et al. 1994); f the
Miocene granites, High Himalaya, Tibet (Harrison et al. 1997); and
g the Late Cretaceous granites of southwest Montana, USA (Kalakay
et al. 2001)
The late cretaceous granites of southwest Montana,
USA (Fig. 5g)
Biotite-hornblende granite plutons were emplaced between
85 and 55 Ma during thrusting in the Sevier orogen
(Kalakay et al. 2001). Thrust ramps are interpreted as part
of the magma plumbing system. Buckling at the hanging
wall of the thrust flat provides space for the intrusion.
Deformation at the hanging wall of an inflating laccolith is
necessary to accommodate magma but may not be driven
only by magma pressure (Scaillet et al. 1995; Acocella
2000). The development of a ramp anticline might also
provide space for magma input.
In summary, the fact that many granite plutons are
emplaced along thrusts is a direct consequence of magmatism occuring during or shortly after shortening (e.g.,
Harrison et al. 1997; Grocott and Taylor 2002; Oberli et al.
2004). More specifically, early continental plate collision is
characterized by horizontal shortening, expressed in the
middle and upper continental crust by thrusts and folds
(DeCelles and DeCelles 2001). In contrast, late convergence during post-lithospheric thickening can be accommodated by crustal extension or even orogenic collapse
(Malavieille 1993; Vanderhaeghe and Teyssier 2001).
Many orogenic belts experience episodic plutonism from
early to late stages of their development (e.g., Grocott and
Wilson 1997; Harrison et al. 1997; Ferré and Leake 2001;
Paquette et al. 2003).
A model of magmatic intrusion in flats-and-ramps
The idea that plutonism would be less likely in contractional settings comes from the preconception that compression is expected to prevent the ascent of magma
through the continental crust (e.g., Hamilton 1994;
Watanabe et al. 1999). However, the literature review in
the second part of this paper shows that granites may in fact
intrude into a shortening continental crust. Further, many
granitic plutons exhibit tabular shapes, either horizontal or
inclined. The experiments discussed in this paper simulated
the emplacement of magmas of various viscosities, and all
the resulting intrusions exhibited horizontal to inclined
tabular shapes. The similarity between geological observation and experimental results strongly suggest that the
experiments properly simulate the emplacement of granites
during crustal shortening.
In the experiments as well as in geological examples,
many intrusions exhibit flat-lying geometries. This is due
(1) to the compressional state of stress where r1 is horizontal and r3 vertical, controlling the opening of horizontal
tension or shear fractures (Hubbert and Willis 1957; Sibson
2003) and (2) to the strong mechanical interface between
the basal plate of the model and the model crust (Figs. 2, 3,
4; Galland 2005; Musumeci et al. 2005; Galland et al.
2006, 2007a). In nature, we also expect the compressional
stresses to control the horizontal spreading of magma (e.g.,
Menand et al. 2010). In addition, the rheological boundaries, such as the brittle-ductile transition, represent strong
rheological barriers for ascending magma (Hogan et al.
1998; Gerya and Burg 2007; Galland et al. 2009; Mazzarini
et al. 2010). Numerical simulations indeed show that the
brittle-ductile transition in a compressional tectonic setting
is strong enough to prevent the ascent of magma, which
spreads laterally to form flat-lying intrusions (Watanabe
et al. 1999). Therefore, both compressional stresses and
rheological boundaries naturally enhance the formation of
flat-lying intrusion in contractional tectonic settings. This is
corroborated by geological and geophysical observations of
flat-lying magmatic bodies at the brittle-ductile transition
in orogens such as the central Andes (e.g., Yuan et al.
2000) or the Spanish Hercynian Belt (e.g., Tornos and
Casquet 2005).
In the experiments, parts of the intrusions exhibit gently
dipping segments that closely follow the thrusts faults
(Figs. 2, 3, 4). The consistency between the location of
intrusions and thrusts occurred when thrusts were active
(exp. LV1, Fig. 2c; exp. GITD 03 Fig. 4c, d, and GIDT 12
Fig. 4f) as well as when thrusts were inactive (exp. LV4,
Fig. 3c and LV5, Fig. 3d; exp. GITD 19, Fig. 4e). In such
settings, faults, such as thrusts, represent zones of weakness compared to their intact country rock (e.g., Sibson
2003), which may provide preferential pathways for
magma. In nature, such consistency between thrusts and
intrusions, and even between thrusts and active volcanoes
in the Andes, has been described (Kalakay et al. 2001;
Lageson et al. 2001; Musumeci et al. 2005; Tibaldi 2005,
2008 Galland et al. 2007b). We therefore infer that thrust
faults in nature may potentially control magma transport
and ascent along dipping sheets.
The intrusion shapes in our models are not consistent
with the classic inverted teardrop shape of plutons that
would result from diapiric intrusion processes (e.g., Cruden
1988). They are not consistent either with subvertical
magma intrusion geometries as often represented for
batholiths. In contrast, our experiments suggest that the
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Int J Earth Sci (Geol Rundsch)
transport and emplacement of magma along flats and ramps
exhibit large-scale lateral transport from the source of the
magma to its final emplacement level. This result has major
implications for inferring the locations of magmatic sources in orogens, as well as in subduction zones.
In the experiments, the final shapes of model intrusions
and thrusts varied with varying deformation and injection
rates (Musumeci et al. 2005; Galland et al. 2007a;
Montanari et al. 2010a). In fact, we observe systematic
differences between experiments of injection with and
without active deformation. In Model 1, injection without
active deformation resulted in thin intrusions (1–3 mm
thick; exp. LV2, LV4 and second stage of LV5; Fig. 3). In
contrast, injection during active deformation resulted
in cm-thick intrusions at the base of the uplifted plateau. In
Model 2, the silicone migrated vertically when deformation
was not active (exp. GITD 19; Fig. 4e), whereas it accumulated at the core of the thrust-related anticline when
deformation was active (exp. GITD 03; Fig. 4c, d). We
thus infer that the tectonic uplift of the thrust hanging wall
favors magma accumulation and intrusion thickening at the
base of the hanging wall (Musumeci et al. 2005; Galland
et al. 2007a; Montanari et al. 2010a). Indeed, Galland et al.
(2007a) showed that when thrusts form above intrusions,
the tectonic uplift of the thrust’s hanging wall triggers a
drop in the magma pressure. Such an emplacement
mechanism is thus different from the laccolithic emplacement model, where overpressurized magma makes it own
room by lifting up its overburden (e.g., Corry 1988;
Galland et al. 2009). This is consistent with field observations (Kalakay et al. 2001; Roig et al. 1998; Musumeci
et al. 2005), where intrusions have been emplaced at the
core of growing anticlines. However, we cannot rule out
that magma overpressure contributes to the uplift of the
thrust hanging wall.
In a contractional orogenic setting, the evidence needed
to distinguish between pre-, syn-, or post-kinematic intrusions may not be preserved due to overprinting by magma
moving along the ascent pathway. In all three cases, preexisting planar anisotropies (faults, bedding planes, or
foliations) provide a path along which magma can be
injected from a feeder dike and upon which magma internal
pressure may assist in pushing the country rocks, hence
creating additional room. Magma emplacement can be
achieved through a single pulse or through multiple pulses
(e.g., Michel et al. 2008; Bons et al. 2001, 2010). The
multiple pulse case represented in Fig. 6a, b illustrates
successive steps in the process of pluton growth. The orientation of the feeder dike is not necessarily determined by
the regional stress field (large ellipses) but can be locally
controlled by pre-existing faults. The experiments
Pre-kinematic, syn-kinematic, and post-kinematic
emplacement
One of the fundamental uncertainties related to natural
plutons is whether granitic magmas ascend along preexisting thrusts or, alternatively, whether thrusts form in
response to granite pluton emplacement. Understanding the
interplay between felsic magmatism and deformation is
fundamental to explain the evolution of thrust-and-fold
belts. Even with modern radiometric dating methods, the
time relationships between felsic plutonism and orogenic
deformation are not always clear; it is therefore highly
important to obtain constraints from field observations.
Here, we present a brief summary of arguments that could
be used as field criteria for determining the pre-, syn-, or
post-kinematic nature of plutons.
123
Fig. 6 Three kinematic models illustrating the geometric relationships between granitic plutons and thrusts and the prevailing state of
stress in the crust. a Folding followed by pre-thrusting emplacement
of a pluton. b Syn-kinematic emplacement in thrust discontinuities
(flats). c Post-kinematic emplacement in inherited thrust
discontinuities
Int J Earth Sci (Geol Rundsch)
discussed here did not consider episodic magma emplacement but do not preclude it either.
In the case of magma emplacement before thrusting, the
intrusion locally modifies the mechanical strength of the
crust into which it is intruded. The model magma, while
being still liquid, lubricates the developing thrust plane and
leads to greater strain, a phenomenon already associated
with tectonic surges by Hollister and Crawford (1986).
Because of the rheological heterogeneity due to the intrusion, the deformation pattern is expected to be modified
around the intrusion, such as the arcuate thrusts observed in
models 1 and 2 (exp. LV1 and LV5; Figs. 2, 3d; Galland
et al. 2007a), as well as in nature (e.g., Kalakay and John
1997; Lageson et al. 2001; Tromen Volcano, Argentina;
Galland et al. 2007b). Finally, experiment GITD 12
(Fig. 4f) shows that pre-kinematic injection resulted in a
non-elongated intrusion that has been squeezed during
shortening. The fact that the intrusion is not elongated
parallel to the thrust is an argument that the magma did not
intrude along the fault.
Three kinematic models illustrating the geometric and
temporal relationships between granitic plutons and thrusts
are presented (Fig. 6). In the case of magma emplacement
contemporaneous with thrusting (syn-kinematic in
Fig. 6b), viscous-plastic shear along the hanging wall can
be frozen in marginal plutonic rocks and host rocks in some
cases. Evidence for syn-emplacement shear may also
continuously have been locally eroded but preserved in
mylonitic wall rock-granite xenoliths. The metamorphic
aureole surrounding this type of pluton is expected to
present a syn-kinematic fabric, that is, with elongated
porphyroblasts. Overall, the models 1 and 2 show that synkinematic intrusions exhibit elongated and asymmetric
shapes along the main thrusts. In addition, in the case of
syn-kinematic intrusion models, magma is likely to accumulate under thrust-related anticlines in the hanging wall.
Thus, both elongated asymmetric shape and magma accumulation in the thrust’s hanging wall are good criteria for
identifying syn-kinematic plutons. Finally, because the
least principal stress r3 is subvertical, we expect magma to
preferentially migrate along flats.
In contrast, in the case of magma emplacement after
thrusting (post-kinematic in Fig. 6c), the experiments show
that the presence of pre-existing planar anisotropy (faults)
should exert a strong control on the final shape and orientation of the granite pluton. This is corroborated by our
experiments (Figs. 2, 3, 4). However, the experiments
show systematic differences between syn- and post-kinematic intrusions. In Model 1, for example, post-kinematic
intrusions are much thinner than syn-kinematic intrusions
(Figs. 2, 3). In contrast with syn-kinematic intrusions (in
both models 1 and 2), there is no tectonic uplift accommodating tectonic shortening for post-kinematic intrusions,
so that we do not expect magma accumulation in thrustrelated anticlines, as for syn-kinematic intrusions. Therefore, we expect post-kinematic plutons to be thinner than
syn-kinematic plutons.
In models 1 and 2, we observe that post-kinematic intrusions are steeper than syn-kinematic intrusions (compare
Figs. 2, 3d with c, 4c with e). The major difference between
syn- and post-kinematic intrusions is the ambient stress: the
minimum principal stress r3 for syn-kinematic intrusions is
vertical, whereas this is likely to not be the case for postkinematic intrusions. It is well-known that fluids, such as
magma, are preferentially transported in conduits that are as
perpendicular to r3 (e.g., Sibson 2003). For syn-kinematic
plutons, we thus expect magma to propagate horizontally or
along gently dipping planes, such as fore-thrusts. In contrast,
for post-kinematic plutons, we expect magma to propagate
along steeply dipping planes or even strike-slip faults.
Therefore, subhorizontal plutons and plutons elongated
parallel to main thrust faults in the field are likely synkinematic intrusions, whereas steep or vertical plutons are
likely post-kinematic intrusions. Furthermore, the metamorphic aureole is most likely static, that is, porphyroblasts
do not exhibit a linear shape-preferred orientation.
Conclusions
Discussions regarding pluton emplacement processes
illustrate the complexity of the issue and the main goal of
this contribution is not to provide a universal solution to
this long-standing problem. Our review of numerous plutons in contractional orogens shows that they tend to be
spatially associated with thrusts. In a similar manner to the
debate on spatial relationship between plutons and vertical
shear zones (Paterson and Schmidt 1999; Schmidt and
Paterson 2000; Weinberg et al. 2004), one could argue that
many plutons in contractional orogens do not preferentially
occur near a thrust. Yet, the three proposed models for prekinematic, syn-kinematic, and post-kinematic granite
emplacement should be regarded while considering the
important feedback relationships established between
deformation in shear zones and magma emplacement
(Neves et al. 1996). We conclude that thrusts provide
convenient ascent paths to granitic magmas and also offer
an alternative to the two ascent models of diking and diapirism. In the dike ascent model, magma is driven upwards
along a propagating crack by an upward-decreasing pressure gradient. In the diapiric model, magma is driven
upwards by the difference in density between magma and
host-rock. Both models have limitations related to the
increase in viscosity due to crystallization. The model in
this contribution provides an inclined pathway for magma
to ascend along a major crustal structural discontinuity.
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Int J Earth Sci (Geol Rundsch)
An upward-decreasing pressure gradient is still needed to
drive magma up along thrusts, but the pressure of magma
just needs to be equal to lithostatic pressure to keep the
pathway open. Also, local dilational jogs along the thrust
plane may play a role in providing additional room for
magma. Further, the emplacement of granitic plutons along
flats accounts for the rather tabular aspect ratio of most
felsic intrusions (Vigneresse 1995). The proposed flat-andramp granite emplacement model (Fig. 6b) also emphasizes the role played by pre-existing mechanical anisotropies in guiding magma emplacement, as aspect often
overlooked in recent studies.
Acknowledgments This contribution is dedicated to the memory of
Wally Pitcher (1919–2004) who, by his remarkable intuition and
balanced judgment, influenced our views on granite emplacement so
durably. Part of the experiments were performed at GéosciencesRennes in collaboration with Peter R. Cobbold, Erwan Hallot, and
Jean de Bremond d’Ars who are kindly thanked for their insights. We
are indebted to the Editor in Chief Dr. Wolf-Christian Dullo, the
Subject Editor Dr. Marlina A. Elburg, and the two reviewers Dr. Ryo
Anma and Dr. Paul Dirk Bons whose comments contributed to clarify
our views.
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