1. Folds, Faults and Mountains
Patrice Rey
Room: 407
Tel: 12067

2. the geometry of mountain belts
Lecture 5:
Structural geology of convergent plate boundaries
Aims: To characterize
the geometry of mountain belts

3. Plate boundaries at work

4. Contrasting models of shortening
A/ Reverse listric faults merging on a single décollement.
B/ Blind reverse fault => fault propagation fold.
D/ Blind version of C/
C/ Flats and ramps along a thrust surface.
E/ Folding above a weak zone (e.g. salt) or a
décollement.

5. 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).

6. Contrasting type of mountain belts
... continental margins...
Mountain belts result form the shortening of weak continental regions. Mountains may therefore develop form the shortening of...
…basins...
…and transform boundaries

7. 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

8. 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, 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. A B C

9. Idealized sections through a collisional orogen
Foreland fold & thrust belt
Hinterland fold & thrust belt
Metamorphic zone
Volcanic arc and accreted terrane
Ophiolitic sutures
Foredeep
Symmetric belt
Asymmetric belt

10. 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 Jolmo-Lungma (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.

11. 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.

12. 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.

13. 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

14. Mountain belt on a subduction zone.
Mesozoic A
South Peru
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...
…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

15. Numerical modelling applied to continent-continent collision
Model geometry and boundary conditions
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.

16. Depth dependent viscosity

17. 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?

18. 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

19. 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

20. 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.

21. 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.