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Patrice Rey
Room: 407
[email protected]
Tel: 12067
Lecture 5:
Structural geology of convergent plate boundaries
Aims: To characterize
the geometry of mountain
belts
Plate boundaries at work
Contrasting models of shortening
A/ Reverse listric faults merging on a single
décollement.
C/ Flats and ramps along a thrust surface.
E/ Folding above a weak zone (e.g. salt) or a
décollement.
B/ Blind reverse fault => fault propagation fold.
D/ Blind version of C/
Thrust surface in 3D and lateral termination of thrust faults.
A transfrom fault, transfers the
deformation from one region to another.
Transform fault
Thrusting
Different types of fault ramps (hanging wall
removed).
Thrusting
Contrasting type of mountain belts
Mountain belts result form the
shortening of weak continental
regions. Mountains may
therefore develop form the
shortening of...
... continental margins...
…basins...
…and transform boundaries
A simple active margin
In simple case the subducting plate is moving perpendicularly to the margin. Sediments are scraped off
the oceanic plate to form an accretionary prism due to the action of folds, reverse faults and thrust. At
first, folds and faults have an opposite vergence (direction of movement) to that of the subducting
plate. As the prism growth wider and higher it resists deformation and new folds and faults develop
verging in the same direction than that of the subducted slab. The prism develop a fan symmetry.
Antithetic fault
Synthetic fault
From active margin to continent-continent collision.
Eventually the subducting plate will drag a continental lithosphere into the subduction zone triggering
the collision of two continental margins, and the formation of a crustal accretionary prisme. This wedge
develops a fan shape with folds, reverse faults and thrusts verging in opposite direction away from the
axis of the mountain. It is the balance of forces,
A acting on the mountain belt, that dictated the
outward migration of deformation. As the crust
growths thicker it resists deformation. In the
meantime the gravitational force increases.
Eventually the forces opposing convergence and
thickening will balance the tectonic force that
drives them. When this happens, deformation
migrates towards adjacent areas.
B
C
Idealized sections through a collisional orogen
Foredeep
Metamorphic zone
Foreland fold
& thrust belt
Symmetric belt
Ophiolitic sutures
Asymmetric belt
Volcanic arc and
accreted terrane
Hinterland fold & thrust
belt
Collisional Mountain belts
The himalayas result from the closure of
the Thetys ocean and the collision between
India (detached from Gondwana 80Ma
ago) and the Eurasian plate. The collision
started 50Ma ago between India and Tibet
(a microplate), and is still ongoing.
During the collision, India was
underthrusted beneath Tibet. Devonian
sedimentary rocks from the Indian
platform were uplifted and now form the
higher peak on Earth: The Mount JolmoLungma (8851m).
The Main Central Thrust brings the highly metamorphosed
Indian platform (in blue) onto a less metamorphic domain
of schistose low-grade rocks. The MCT and the Main
Boundary Thrust have accommodated a few hundred
kilometers of convergence between India and Tibet since
the beginning of the collision.
Collisional Mountain belts
Collisional mountain belts involve the staking of several crustal segments. In the upper part of the
crust, sedimentary layers can be transported over a few hundred kilometers on top of the basement.
This is made easier when a weak layer such as salt exists at the base of the sedimentary pile. The fold
and thrust belt affecting a paleozoic sedimentary sequence (orange/yellow) in the foreland of the
Canadian cordillera is a good example. In the Foot Hills the Paleozoic rocks are thrusted on top of
mesozoic sediments (blue and green).
Palaeozoic and mesozoic sedimentary nappes
are translated on top of the Proterozic crystalline
(gneisses and granites, in pink) basement.
Collisional Mountain belts
The European Alps is one of the most
studied mountain belt in the world. This
mountain resulted from the collision
between the South-Alpine and the
European continents following the closure
of the Alpine ocean. The collision started
40Ma ago. Large nappes of the South
Alpine continent were thrusted on top of
sediments of the European
platform. During the
collision these nappes
were folded and eroded.
The Mont Cervin (or
Matterhorn) is a klippe of
South Alpine continent
standing above a flat
ophiolite bearing suture
zone.
Mountain belt on a transform fault.
The Pyrénées is a 400 km long mountain belt along the boundary between Spain and France. In the
middle Cretaceous the opening of the North Atlantic ocean induced the sinistral translation of the iberic
peninsula along the North Pyrenean Transform Fault. This lithospheric scale strike-slip fault created a
weak zone in the continental lithosphere, and controlled the deposition of pull-apart basins and the
intrusion of mantle rocks (Lherzolite). In the Eocene (40 Ma ago) convergence occurred between France
and the Iberic peninsula closing the basins and developing a symmetric mountain belt.
Iberic peninsula
France
Mountain belt on a subduction zone.
The Andes is the best example of a mountain belt
associated with a subduction zone. This belt is
composed of folded and faulted mesozoic
sediment (in blue) of an intracontinental
basin deposited on top of a basement.
This basement is made of deformed
Palaeozoic rocks on top of a proterozoic
crust (pink). This belt started to...
South Peru
Mesozoic
A
…develop 80 Ma ago.
It presents a simple fan symmetry.
The belt is intruded by long (up to 1000km) granitic batholiths,
and covered by volcanic rocks.
A
B
B
North Bolivia
Numerical modelling applied to continent-continent collision
In the last two decades progress in computer technology allowed the development of cost-effective
numerical modelling techniques. Numerical modelling consists in designing a simplified geological
system (geometry, density, viscosity, temperature...) and applying to it dynamical (forces) or kinematic
(velocities) conditions at its boundaries. The evolution of the model is controlled by the equations
governing the heat balance and the stress balance. These equations are solved on a number of point
covering the entire model. What follows show an example of such modelling applied to mountain belt.
Model geometry and boundary conditions
Depth dependent viscosity
The distribution of viscosity and its evolution through time strongly influence the way the lithosphere
deforms. Both models have been shortened by 750 km. The only difference is the viscosity profile.
Depth dependent viscosity
What happens in 3D?
Analogue modelling:
Scaled analogue models are also quite popular amongst tectonicists. This technique enables the
investigation of 3D models, and allows to include both ductile and brittle layers. However it offers less
flexibility in the set up of boundaries conditions, and thermal aspect remains difficult (but not impossible)
to integrate. Critical in this approach is to choose appropriate analogue materials that closely mimic the
behavior of the geological system under investigation. In this experiment Tapponnier et al., investigated
the development of large scale strike slip faults observed in Asia. They used plasticine to model the
response of the Euroasiatic continental lithosphere to the impingement of India.
Tapponnier et al., 1982
In this experiment, a 4 layers model is used to investigate the Euroasiatic lithosphere deformation: a
brittle layer (sand for the upper crust) overlying two ductile layers (low viscosity silicone for the lower
crust, a more viscous silicone for the upper mantle). Honey is the analogue for the asthenosphere. This
model shows the development of lithospheric scale folding as well as the development of large-scale
strike slip faults. Both structures are observed...
From Ph. Davy
Lithospheric folding: In front of the Nanga Parbat syntaxis.
In recent years a number of scientific studies have investigated the possibility of lithospheric scale
folding. Field observations and numerical tests seem to give strong support to this hypothesis. These
folds correspond to large scale undulations that tend to localize the initiation of thrusts.
Some structural characteristics of collisional mountain belts
Collisional mountain belts
result from the closure of
ocean basins and the collision
of continental margins.
Folds, reverse faults, thrusts,
and steeply deeping foliation,
are the consequences of a stress
regime in compression. The
combined action of folding and
faulting is responsible for the
contraction and the thickening
of the continental crust.
Deformation migrates from the hinterland to the forelands.
In front of the collision zone, strike-slip faults may develop to accommodate part
of the convergence through the escape of lithospheric blocks in front of the
collision zone.