1. Continental collision mountain belts: the Arabia-Eurasia system
Paolo Ballato,

2. Today's class contents
1) Continental collision: a brief outlook (definition, causes, implications…..)
2) How is tectonics deformation accommodated within the Arabia-Eurasia collision zone?
3) A case study from the Alborz mountains, an intracontinental mountain belt linked to Arabia-Eurasia collision (the record from foreland basin deposits)
a) When did the deformation related to continental collision start in the Alborz mountains?
b) How did deformation evolve?
c) What can we learn from foreland basin deposits (i.e. climate vs tectonic) ?

3. PART 1
1) Continental collision: a brief outlook (definition, causes, implications…..)

4. Pre-collisional setting
Lower
plate
Upper plate
Prior to a continental collision, the landmasses are separated by oceanic crust, formed during an earlier episode of sea-floor spreading
As the continental blocks converge, the intervening sea floor (lower plate) is subducted beneath the upper plate
The descending oceanic slab generates a volcanic arc
Upper plate deformation is limited (tectonics stress is not transferred far away from trench) and shortening is mainly accommodated along plates interface (accretionary wedge)
India-Asia convergence rate decreased from 160 to 50 mm/yr in the last 70Ma
www2.bc.edu/~kafka/ge180.f03/PT_4.ppt

5. Collisional setting
As the continental lithosphere of the lower plate approaches the upper plate subduction terminates, suturing occurs, and continents are amalgamated
Tectonics stress is progressively transferred to the upper plate, where deformation is accommodated across a broad region (thousands of km from the suture zone). Possibly reactivation of structures forming old orogenic belts
A fold and thrust belt develops in the lower plate
Mountain range/ranges are formed and several km of crust can be exhumed
www2.bc.edu/~kafka/ge180.f03/PT_4.ppt

6. Why does continental collision occur?
Continental lithosphere ( g/cm3 )
crust (density ca. 2.7 g/cm3)
mantle (density ca. 3.3 g/cm3)
Oceanic lithosphere ( g/cm3)
crust (density ca. 2.9 g/cm3)
mantle (density ca. 3.3 g/cm3)
Cloos, 1993
Oceanic lithosphere is denser than continental lithosphere, so it tends to sink (subduction) into the asthenosphere when convergence takes place
When the continental crust reach the subduction zone the buoyancy forces oppose resistance to the slab pull forces; subduction ends and the pulling slab will break off sinking into the asthenosphere

7. How is it tectonics deformation absorbed in the upper plate?
1) Crustal thickening (exhumation)
2) Extrusion tectonics (lateral transport of crustal blocks)
Tapponnier et al., 1982, 1986

8. 3) Large scale folding (lithospheric buckling)
Burg et al, 1999
Difficult to demonstrate….is it an efficient mountain building process?
4) “Intra-collision zone subduction” (subduction of denser microplates located in the collision zone)
Matte et al, 1997

9. PART 1…..Summarizing
1) Continental collision occurs when plate convergence cannot absorbed anymore via subduction process
2) Continental collision takes place because buoyancy forces do not allow large amount of continental subduction
3) During continental collision tectonics deformation is not anymore localized along the plate margin (accretionary wedge), but affects a large area in the upper plate and propagate cratonward in the lower plate (fold and thrust belt)
4) Deformation in the upper plate is absorbed via :
Crustal thickening
Lateral extrusion of rigid blocks
Possibly via lithospheric buckling
“Intra-collision zone subduction”
5) Intracontinental deformation is generally localized along crustal weakness (i.e. old orogenic belts)

10. PART 2 2) How is tectonics deformation accommodated within the
Arabia-Eurasia collision zone?

11. The Arabia-Eurasia collision zone
Caucasus
Black Sea
Eurasia
Aspheron
Anatolia
Casp
T-I plateau
Kopeh Dagh
Alborz
Aegean
Central Iran
Hellenic
Cyprus
Zagros
Lut
Helmand
Nubia
Makran
Red Sea
Arabia
Owen FZ
Gulf of Aden
India
Eastern African Riften
Somalia

12. Arabia-Eurasia system: from oceanic subduction to continental collision
Opening of the Gulf of Aden
McQuarrie et al., 2006

13. Active tectonics of the Arabia-Eurasia collision zone: seismicity
Reilinger et al., 2006

14. Active tectonics of the Arabia-Eurasia collision zone: quantifying present-day deformation with GPS data
Subduction of a denser microplate (Southern Caspian Basin)
Westward extrusion of Anatolia
(escape tectonics)
Crustal thickening
Reilinger et al., 2006

15. Intra-collision zone subduction
Active deformation in North Iran
Alborz
Intra-collision zone subduction
South Caspian Basin crust is thinner and denser than adjacent regions
Brunet et al., 2003
Guest et al., 2007

16. The Arabia-Eurasia collision zone: kinematics model GPS based
Black numbers:
3 strike
(3) dip slip
White numbers:
plate velocities
Reilinger et al., 2006

17. Active deformation takes place along crustal heterogeneity (i. e
Active deformation takes place along crustal heterogeneity (i.e. old suture zone and orogenic belts)
Horton et al., 2008

18. PART 2……..Summarizing
1) Deformation is accommodated along seismic belts (mountain chains and large intracontinental strike-slip faults) bounding aseismic blocks
2) Deformation in the upper plate is absorbed via :
Crustal thickening (Zagros, Alborz, Caucasus, etc.)
Lateral extrusion of rigid blocks (Anatolia and smaller crust blocks)
Possibly via lithospheric buckling (Alborz-South Caspian basin system?)
“Intra-collision zone subduction” (South Caspian basin)
3) Intracontinental deformation is localized along crustal weakness like inherited structures (paleosutures and old orogenic belts)

19. PART 3
3) A case study from the Alborz mountains, an intracontinental mountain belt linked to Arabia-Eurasia collision (the record from foreland basin deposits)
a) When did the deformation related to continental collision start in the Alborz mountains?
b) How did deformation evolve?
c) What can we learn from foreland basin deposits (i.e. climate vs tectonic) ?

20. Plate deflection (flexural subsidence)
Foreland basin anatomy and sedimentary facies distribution
Tectonic load (crustal shortening and thickening; exhumation of crustal section)
Grain-size decrease
Plate deflection (flexural subsidence)
DeCelles and Giles, 1996
Coarse-grained facies are generally confined in proximity of the fold and thrust belt front.
However in some cases they can prograde into the foreland for tens of km……Why?

21. Progradation of gravel facies during a major thrusting phase
Lateral and vertical sedimentary facies evolution in a foreland basin system: syn-thrusting progradation of coarse-grained facies
Stable thrust front
Progradation of gravel facies during a major thrusting phase
Distance from the thrust front (km)
Burbank et al., 1988

22. Lateral and vertical sedimentary facies evolution in a foreland basin system: post-thrusting progradation of coarse-grained facies
Time (Ma)
Flemings and Jordan 1990

23. Lateral and vertical sedimentary facies evolution in a foreland basin system: climatic forcing
Zhang et al., 2001

24. Simplified tectonostratigraphy of the Alborz Mountains
36 Ma (end of magmatism)
The Alborz range is characterized by a complex crustal fabric, with inherited structures related to both compression and extension since Paleozoic time
Guest et al., 2006

25. ca. 4 mm/yr of left-lateral shearing
Central Alborz Mountains
ca. 4 mm/yr of left-lateral shearing
ca. 6 mm/yr of shortening
Modified after Geological maps of Tehran, Semnan, Saveh, Sari, Qazvin and Amol 1: , Geological Society of Iran, and Guest et al., 2006

26. Eyvanekey stratigraphic section
5 km
ASTER satellite image, bands 731-RGB

27. Unit 1 Unit 1C: braided river dep. system
Unit 1B: distal river dep. system N 4m S 4m
Unit 1A: playa lake dep. system N S
70m

28. Unit 2 Unit 2B: braided river dep. system
Unit 2A: playa lake dep. system N S

29. Unit 3 Unit 3C: alluvial fan dep. system 3C 3B
Unit 3B: braided river dep. system N S 5m
Unit 3A: playa lake dep. system N S

30. Stratal geometric relationship
ASTER satellite image, bands 321-RGB

31. Magnetostratigraphy Main prerequisites: Fine-grained lithologies
Normal Polarity
Fine-grained lithologies
Continuous sedimentation
Reverse Polarity
Independent age constrains
Reference MPTS

32. Magnetostratigraphy
In 75% of samples a Characteristic Remanent Magnetization (ChRM) was isolated
Ballato et al., 2008

33. Magnetostratigraphic Correlation
Ballato et al., 2008

34. Sediment accumulation rates
Coarse-grained sed.
Fine-grained sed.
Coarse-grained sed.
Fine-grained sed.
Coarse-grained sed.
Fine-grained sed.
Ballato et al., 2008

35. 6.2 Ma
PART 3…..Concluding
Sed.acc.rate = 0.65 mm/yr
7.5 Ma
a) When did deformation related to the Arabia-Eurasia continental collision start in the Alborz mountains?
Sed.acc.rate = 0.58 mm/yr
At ca Ma the basin records a sharp increase in sedimentation rate (0.04 to 0.58 mm/yr).
This increase reflect onset of flexural subsidence related to crustal shortening and thickening
17.5 Ma
Sed.acc.rate = 0.04 mm/yr
36 Ma

36. Tectonic vs climate: retrogradation of coarse-grained facies
Increase in slip rate +100% = increase in subsidence and sed. flux
Decrease in precipitation -50% = decrease in sed. flux
Post-perturbation
Post-perturbation
Pre-perturbation
Pre-perturbation
Pre-perturbation
Post-perturbation
Pre-perturbation
Pre-perturbation
Facies retrogradation
Facies retrogradation
Sediment flux
Sediment flux
Time (Myr)
Time (Myr)
Time (Myr)
Distance from fault (Km)
Time (Myr)
Distance from fault (Km)
In both cases retrogradation of sedimentary facies is recorded in the basin.
However, when precipitation decrease the sedimentation rate does not change since there is no perturbation in subsidence
Densmore et al., 2007

37. Tectonic vs climate: progradation of coarse-grained facies
Decrease in slip rate -50% = decrease in subsidence and sed. flux
Increase in precipitation +50% = increase in sed. flux
Post-perturbation
Post-perturbation
Pre-perturbation
Pre-perturbation
Pre-perturbation
Post-perturbation
Pre-perturbation
Post-perturbation
Facies progradation
Facies progradation
Time (Myr)
Sediment flux
Sediment flux
Time (Myr)
Time (Myr)
Distance from fault (Km)
Time (Myr)
Distance from fault (Km)
In both cases progradation of sedimentary facies is recorded in the basin. However, when precipitation increase the sedimentation rate does not change since there is no perturbation in subsidence
Densmore et al., 2007

38. Unit 1

39. Stratal geometric relationship
ASTER satellite image, bands 321-RGB

40. Unit 2
ca. 5 km
ca. 25 km

41. Unit 3
ca. 5 km
ca. 25 km

42. PART 3…..Concluding a) How did deformation evolve?
The locus of deformation moved forth and back, without a predictable pattern on a time scale ranging from 2 to 0.6 Ma
b) What can we learn from foreland basin deposits (i.e. climate vs tectonic) ?
In a medial-distal part of a foreland basin high sediment accumulation rates coincide with fine-grained sediments and reflect an increase in subsidence due to tectonic loading
Low sediment accumulation rates coincide with coarse-grained sediments and reflect decrease in subsidence related to intraforeland uplift
Progradation of coarse grained sediments during a moderate to high subsidence rate seems be related to an increase in sediment flux possibly triggered by enhanced precipitation

43. Thank you
With the contribution of Angela Landgraf, Manfred Strecker, Cornelius Uba, Norbert Nowaczyzk, Anke Friedrich, and many others…