Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/185
Comunicações Geológicas (2014) 101, Especial III, 1429-1432
IX CNG/2º CoGePLiP, Porto 2014
ISSN: 0873-948X; e-ISSN: 1647-581X
Analog modelling of strike-slip fault (lateral) propagation from
an elastic to a viscous medium: insights from trial experiments
Modelação análoga da propagação lateral de uma falha de
desligamento de um meio elástico para um meio viscoso: novos
dados a partir de experiências preliminares
F. M. Rosas1,2*, P. Terrinha1,3, J. Duarte4, L. Baptista3, C. Kullberg1,2, J. Almeida1,
F. Barata1, B. Carvalho1, V. Grigorova1, R. Tomás1, P. Almeida2
Artigo Curto
Short Article
.
© 2014 LNEG – Laboratório Nacional de Geologia e Energia IP
Abstract: A series of analog modeling experiments were carried out
to investigate lateral strike-slip fault propagation vs. attenuation
across a discrete rheology boundary, and to gain insight on the
mechanics governing the generation of associated characteristic
structural patterns. Experiments were conceived to simulate the
lateral propagation of a strike-slip fault through a Mohr-Coulomb
medium (dry quartz sand), from a zone overlying a rigid basement
(where the fault is pre-cut) to another one where the sand lies on top
of a viscous (Newtonian) silicone layer. Comparison of the obtained
results with the natural example of the Gloria Fault – Tore-Madeira
intersection (Eurasia-Nubia plate boundary, NE Atlantic) revealed a
striking first-order resemblance, allowing speculation about a
compliant tectono-magmatic evolution, and stressing the relevance
for future enhanced modeling.
Keywords: Analog modelling, Strike-slip fault propagation,
Rheological boundary, Gloria Fault, Tore-Madeira rise.
Resumo: Experiências de modelação análoga foram levadas a cabo
com o objetivo de investigar a propagação lateral de falhas de
desligamento através de uma fronteira reológica abrupta (transição de
um meio rígido para um meio viscoso), e de melhor compreender a
geração das estruturas geológicas associadas. As experiências foram
concebidas para simular a propagação lateral de uma falha de
desligamento através de um meio de comportamento mecânico do
tipo Coulomb-Mohr (areia quártzica desidratada) sobrejacente a uma
fronteira reológica abrupta simulada pela transição lateral entre duas
placas rígidas (de acrílico) e uma camada de silicone transparente (de
comportamento viscoso Newtoniano). A comparação com o exemplo
natural representado pela intersecção da Falha da Glória com a crista
Tore-Madeira (fronteira de placas Eurásia-Núbia no Atlântico NE)
relevou uma semelhança estrutural (geométrica e cinemática)
assinalável, permitindo especular sobre uma evolução tectonomagmática controlada por pressupostos mecânicos-reológicos
semelhantes aos assumidos experimentalmente, e sublinhando a
necessidade de continuar a aperfeiçoar futura modelação.
Palavras-chave: Modelação análoga, Propagação de falhas de
desligamento, Fronteira reológica, Falha da Glória, Crista ToreMadeira.
1
Departamento de Geologia, Faculdade de Ciências, Universidade de Lisboa
(FCUL), 1749-016 Lisbon, Portugal.
2
Instituto Dom Luiz-IDL, Universidade de Lisboa, 1749-016 Lisbon, Portugal.
3
Instituto Português do Mar e da Atmosfera, 1749-077 Lisbon, Portugal.
4
School of Geosciences, Monash University, Melbourne, Victoria 3800,
Australia.
*
Corresponding author / Autor correspondente: [email protected]
1. Introduction
Physical (analog) modeling of strike-slip faulting and
wrenching deformation has been the focus of a great number
of contributions in recent years, contemplating different
tectonic scenarios, and targeting different specific subjects at
different scales (e.g. Mandl et al., 1977; Tapponnier et al.,
1982; Richard et al., 1995; Mandl, 1988; Ratschbacher et
al., 1991; Schreurs, 1994). Recent examples include: a) the
use of spatiotemporal evolution of simple shear-band
configuration (and complying strain distribution) as a
qualitative/quantitative rheological proxy (e.g. Schrank et
al., 2008; Treagus & Sokoutis, 1992); b) investigating the
influence of varying (crustal/lithospheric–scale) rheological
stratigraphies (e.g. Dooley & Schreus, 2012 and references
herein);
and
c)
studying
of
high
oblique
transpressional/transtensive tectonic settings, including
oblique basin inversion reactivation (e.g. McClay & Bonora,
2001; Schreurs & Colletta, 2003).
Conversely, in this work we focus on the problem of
lateral strike-slip fault propagation as a function of lateral
(instead of vertical) varying rheological configuration. We
present preliminary results of new trial experiments
simulating the lateral propagation of a strike-slip fault
through a Mohr-Coulomb medium (dry quartz sand), from a
zone overlying a rigid basement (where the fault is pre-cut)
to another one where the sand-cake lies on top of a viscous
(Newtonian) silicone layer. Our purpose is to investigate
fault propagation vs. attenuation of a strike-slip fault across
such a discrete rheology boundary, gaining insight on the
fundamental mechanics at stake, and predicting the expected
structural pattern under such conditions. Furthermore, we
compare our results with the Present day morphotectonic
configuration of the Gloria Fault in NE Atlantic (e.g. Argus
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F. M. Rosas et al. / Comunicações Geológicas (2014) 101, Especial III, 1429-1432
et al., 1989), where this is known to intersect the so called
Tore-Madeira rise (Geldmacher et al., 2006), within a
scenario that we speculate could have resulted from an
evolution similar to the one simulated in our experiments.
2. Experimental apparatus and methodology
All the experiments were carried out in the deformation rig
depicted in the inset of figure 1A (orientated relatively to the
X, Y, Z coordinated axis as illustrated). In this Perspex rig
two vertical (fixed) side walls confine two horizontal
longitudinally juxtaposed basal plates (plates A and B in
Fig. 1), which can move relatively to each other along the X
direction in compliance with strike-slip kinematics.
In the initial state of all the experiments a 1cm thick
layer of transparent silicone putty (PDMS) is placed inside
the rig in front of the basal (rigid) Perspex plates with the
same thickness (inset of Fig. 1A). In the half area in front of
plate A the contact surfaces between the silicone layer and
the underlying Perspex base, as well as between the same
viscous layer and the adjacent vertical wall, are abundantly
lubricated with neutral liquid soap to prescribe free slip
boundary conditions. On the contrary, the same contact area
in front of plate B is non-lubricated, hence guarantying total
adherence of the silicone to the Perspex base and adjacent
fixed vertical wall (inset of Fig. 1A). Additionally a 2cm
thick sand-layered cake is also mounted inside the rig
covering (in frictional contact) both the underlying Perspex
plates and the silicone layer (Fig. 1A). This is done using a
moving elongated funnel to ensure the perfectly flat
deployment of the sand. In some experiments, a very thin
(~2mm thick) silicone layer was also placed between the
basal Perspex plates (A and B) and the overlying sand cake,
to maintain the same frictional boundary condition along
this interface. Equally spaced lines are imprinted on the
model top surface orientated parallel to the Y direction to
monitor strain in the XY plane.
During the experiment a computer driven stepping motor
pushes plate A to the right at a constant velocity of 3cm/hr,
moving it along the X direction relatively to plate B in
accordance with dextral strike-slip kinematics. The
movement of plate A is conveyed both upwards to the
frictionally overlying sand, and laterally to the half of the
sand-silicone cake lying in front of this plate, thus
heterogeneously deforming the whole model. Successive top
view photographs are taken at regular time intervals (Fig.
1B-D).
3. Experimental results
Since relatively early experimental stages a brittle
deformation band is formed in the sand (along the X
direction) above the contact between the rigid plates A and
B (Fig. 1B). This band is formed by typical wrenching
structures essentially comprising early formed Riedel
faults and P-shears, which are latter cut by Y-faults
defining an overall anastomosed pattern exhibiting a series
of aligned, sigmoidal shaped, fault-bounded domains with
a clear topographic expression (Fig. 1B and C). Such
topographic expression in (brittle) deformation bands was
previously interpreted as resulting from dilation in the sand
accommodated by a reverse-fault component of movement
along the Riedel faults, although in the total absence of
any external compressive stress (Le Guerroué & Cobbold,
2006).
As deformation increases during a certain amount of
time (until an absolute offset value of ~2.6 cm), the lateral
propagation of this brittle deformation band from the sand
overlying the rigid Perspex plates into the part of the sandcake lying above the silicone putty practically does not
occur (Fig. 1B). Instead, progressive thrusting and
associated topographic bulging occurs in the sand-silicone
cake in front of moving plate A (Fig. 1B and C).
As the absolute offset between plates A and B
continues to increase (for values greater than ~2.6 cm), the
brittle deformation band (now mostly represented by a
single Y-strike slip fault) is finally propagated into the
sand-silicone cake, where it first bends as it “passes by”
the main thrust (bulge), and progressively splays into two
different fault segments (Fig. 1C and D).
Finally (for offset values > ~6.8 cm), the strike-slip
fault is propagated across the entire length of the sandsilicone cake (leaving the topographic obstacle behind),
redeveloping from one of the previously formed faults
segments, and restoring a straighter narrower/discrete
geometry (Fig. 1D).
4. Discussion and comparison with natural examples
Obtained experimental results show that while the rightlateral offset between plates A and B is accommodated in
the overlying sand by the generation of a brittle (dextral
wrenching) deformation band (up until absolute offset
values of ~2.6 cm), in the sand above the silicone layer the
same implied rightwards movement of plate A is chiefly
accommodated by the build-up of a progressively bulging
thrust structure. This means that the viscous nature of the
silicone layer hampers stress propagation, strongly
concentrating the strain close to the frontal edge of plate A
where considerable shortening is accommodated by
complying fault assisted vertical extrusion (i.e. thrust
edification). Hence we conclude that lateral propagation of
the wrenching (brittle) deformation band in the overlying
sand-cake is strongly obstructed by the prescribed (abrupt)
rheological boundary, separating the underlying rigid
Perspex basal plates from the adjacent silicone layer.
From a threshold amount of absolute offset onwards
(around 6.8 cm in our experiments) the brittle deformation
band (Y strike-slip fault) starts to propagate into the sandsilicone cake, undergoing some degree of clockwise
rotation/bending. This could result from a refraction effect
associated to the crossing of an abrupt rheological boundary,
also testified by some degree of observed fault splaying. In
this case, however, this bending is mostly the result of
centrifugal deflection imposed by the topography (and
mass) anomaly represented by the developing thrust bulge.
Accordingly, when such an obstacle is overcome the
continuing propagation of the strike-slip fault restores its
original linear discrete geometry.
Analog modeling of lateral fault propagation
The obtained experimental structural configuration
seemingly complies with the overall tectonic set recognized
for the segment of the Gloria Fault (Fig. 1E) where this
major right-lateral strike-slip intersects the Tore-Madeira
rise, which is thought to represent the bathymetric
expression of a drifting plume-like feature (e.g. Geldmacher
et al., 2006). As in the experiments, the Gloria Fault trace is
observed to bend and splay with similar geometry while
crossing over the Tore-Madeira rise (where a number of
tectonically aligned volcanic plugs and cones are also
observed), and likewise regaining a seemingly discrete
nature after having crossed such a morpho-rheological
anomaly (Fig. 1D and E).
This NE Atlantic domain records the spatial (lateral)
1431
transition of the Eurasia-Africa plate boundary, from a zone
where it exhibits a distinctively discrete nature represented
by the active Gloria Fault strike-slip (with high magnitude
instrumental earthquakes, e.g. M=7.1 and M=8.4 of
08/05/1939 and 25/11/1941, respectively implying surface
ruptures of the order of 250-300 km, Buforn et al., 1988), to
another further to the east, corresponding to the Gulf of
Cadiz domain (in the SW Iberia offshore), where it assumes
a much more diffuse nature (Sartori et al., 1994). However,
the present preliminary experimental results only provide us
with a first order insight on the possible mechanics at stake
within such a tectonic scenario, while more definite
conclusions regarding this specific natural example must
rely on future fully scaled experiments.
Fig. 1. Obtained experimental results:
A – Top (XY) view of the
experimental initial state (inset:
perspective view of the deformation
rig without the top sand layer); B to D
- Top (XY) views of successive states
of deformation for incremental
amounts of absolute strike-slip offset
values (see detail explanation in the
text); E- Comparison of the final
experimental result with the natural
tectonic setting of the Gloria Fault Tore-Madeira
rise
intersection
(Eurasia-Nubia plate boundary, NE
Atlantic).
Fig. 1. Resultados experimentais
obtidos: A - Vista de de topo (plano
XY) referente ao estado inicial (caixa:
representação esquemática da prensa
de deformação usada); B a D Vista de
topo (planos XY) dos sucessivos
estádios
de
deformação
correspondentes
a
diferentes
incrementos
de
rejeito
em
desligamento direito (vide explicação
detalhada no texto); E - Comparação
do estádio final da deformação
experimental com o exemplo natural
referente à configuração tectónica da
intersecção da Falha da Glória com a
montanha submarina Tore-Madeira
(fronteira de placas Eurásia-Núbia,
Atlântico NE)
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F. M. Rosas et al. / Comunicações Geológicas (2014) 101, Especial III, 1429-1432
5. Conclusions
In view of the obtained experimental results the following
main conclusions can be drawn:
a) Lateral propagation of a strike-slip fault throughout a
Coulomb-Mohr material can be strongly hampered by
an underlying rheological boundary, marking an abrupt
lateral transition from a rigid medium to a viscous
(Newtonian) one;
b) Such a rheological boundary works as an obstacle to
the mentioned lateral strike-slip propagation in two
fundamental ways: i) inhibiting stress propagation
along the viscous medium, and hence strongly
concentrating strain near the rigid/viscous mechanical
boundary; ii) creating as a consequence a prominent
morphological anisotropy, which obstructs strike-slip
fault propagation changing the local stress-field and
promoting its centrifugal deflection;
c) Once such an obstacle is surpassed, the strike-slip fault
system regains a narrow discrete geometry, displaying
a trail configuration in which the fault trace seemingly
contours (goes around) the mass/topographic anomaly.
d) These experimentally obtained structural features are
strikingly comparable with the tectonic setting ascribed
to NE Atlantic domain, where a segment of the active
GFZ strike-slip fault intersects the Tore - Madeira rise.
Nevertheless, more definite conclusions based on such
a comparison require improvement of analog modeling
experimental work.
Acknowledgments
This work was sponsored by the Fundação para a Ciência e a
Tecnologia
(FCT)
through
projects
PestOE/CTE/LA0019/2011-12. The authors wish to express their
gratitude to two anonymous reviewers for their thorough
revision of the original manuscript.
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