Transcript Slide 1

Continental collision mountain belts: the Arabia-Eurasia system
Paolo Ballato, 11-02-2009
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) ?
PART 1
1) Continental collision: a brief outlook (definition, causes, implications…..)
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
India-Asia
convergence
rate
decreased
from 160 to
50 mm/yr in
the last
70Ma
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)
www2.bc.edu/~kafka/ge180.f03/PT_4.ppt
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
Why does continental collision occur?
Continental lithosphere (3.1-3.2 g/cm3 )
•crust (density ca. 2.7 g/cm3)
•mantle (density ca. 3.3 g/cm3)
Oceanic lithosphere (3.3-3.2 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
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
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
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)
PART 2
2) How is tectonics deformation accommodated within the
Arabia-Eurasia collision zone?
The Arabia-Eurasia collision zone
Black Sea
Anatolia
Hellenic
Eurasia
Casp
Alborz
Central
Iran
Cyprus
Helmand
Nubia
Makran
Arabia
India
Somalia
Arabia-Eurasia system: from oceanic subduction to continental collision
Opening of the
Gulf of Aden
McQuarrie et al., 2006
Active tectonics of the Arabia-Eurasia collision zone: seismicity
Reilinger et al.,
2006
Active tectonics of the Arabia-Eurasia collision zone: quantifying presentday deformation with GPS data
Westward
extrusion of
Anatolia
(escape
tectonics)
Subduction
of a denser
microplate
(Southern
Caspian
Basin)
Crustal
thickening
Reilinger et al.,
2006
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
The Arabia-Eurasia collision zone: kinematics model GPS based
Black numbers:
3 strike
(3) dip slip
White numbers:
plate velocities
Reilinger et al.,
2006
Active deformation takes place along crustal heterogeneity (i.e.
old suture zone and orogenic belts)
Horton et al., 2008
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)
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) ?
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?
Lateral and vertical sedimentary facies evolution in a foreland
basin system: syn-thrusting progradation of coarse-grained facies
Stable thrust front
Distance from the thrust front (km)
Burbank et al., 1988
Time (Ma)
Lateral and vertical sedimentary facies evolution in a foreland
basin system: post-thrusting progradation of coarse-grained facies
Flemings and Jordan 1990
Lateral and vertical sedimentary facies evolution in a foreland
basin system: climatic forcing
Zhang et al., 2001
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
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: 250 000, Geological Society of Iran, and
Guest et al., 2006
Eyvanekey stratigraphic section
5 km
ASTER satellite image, bands 731-RGB
Unit 1
Unit 1C: braided river dep. system
N
S
5m
Unit 1B: distal river dep. system
N
S
4m
4m
Unit 1A: playa lake dep. system
N
70m
S
Unit 2
Unit 2B: braided river dep. system
S
N
Unit 2A: playa lake dep. system
N
S
Unit 3
Unit 3C: alluvial fan dep. system
N
3B
S
3C
Unit 3B: braided river dep. system
N
S
5m
Unit 3A: playa lake dep. system
N
S
Stratal geometric relationship
ASTER satellite image, bands 321-RGB
Magnetostratigraphy
Normal Polarity
Main prerequisites:
Fine-grained lithologies
Continuous sedimentation
Reverse Polarity
Reference MPTS
Independent age constrains
Magnetostratigraphy
In 75% of samples a Characteristic
Remanent Magnetization (ChRM) was
isolated
Ballato et al., 2008
Magnetostratigraphic Correlation
Ballato et al., 2008
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
6.2 Ma
Sed.acc.rate = 0.65 mm/yr
PART 3…..Concluding
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. 17.5 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
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
Post-perturbation
Pre-perturbation
Time (Myr)
Distance from fault (Km)
Time (Myr)
Time (Myr)
Sediment flux
Facies
retrogradation
Pre-perturbation
Sediment flux
Pre-perturbation
Time (Myr)
Facies
retrogradation
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
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
Post-perturbation
Time (Myr)
Pre-perturbation
Distance from fault (Km)
Time (Myr)
Facies
progradation
Post-perturbation
Sediment flux
Sediment flux
Time (Myr)
Pre-perturbation
Time (Myr)
Facies
progradation
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
Unit 1
Stratal geometric relationship
ASTER satellite image, bands 321-RGB
ca. 5 km
Unit 2
ca. 25 km
ca. 5 km
Unit 3
ca. 25 km
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
Thank you
With the contribution of Angela Landgraf, Manfred Strecker, Cornelius Uba, Norbert
Nowaczyzk, Anke Friedrich, and many others…