Download - 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) ?
www2.bc.edu/~kafka/ge180.f03/PT_4.ppt
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)
Pre-collisional setting
India-Asia convergence
rate decreased from 160 to 50 mm/yr in
the last 70Ma
Lower plate Upper plate
www2.bc.edu/~kafka/ge180.f03/PT_4.ppt
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
Collisional setting
Why does continental collision occur?
Oceanic lithosphere (3.3-3.2 g/cm3)•crust (density ca. 2.9 g/cm3)
•mantle (density ca. 3.3 g/cm3)
Continental lithosphere (3.1-3.2 g/cm3 )•crust (density ca. 2.7 g/cm3)
•mantle (density ca. 3.3 g/cm3)
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
Cloos, 1993
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)
The Arabia-Eurasia collision zone
Nubia
Somalia
Arabia
Eurasia
India
Zagros
Alborz
Kopeh Dagh
Caucasus
Gulf of Aden
Red S
ea
Makran
Black Sea
Casp
Central IranHellenic Cyprus
AnatoliaT-I plateau
Ow
en F
Z
HelmandLut
Aegean
Eastern African Riften
Aspheron
McQuarrie et al., 2006
Arabia-Eurasia system: from oceanic subduction to continental collision
Opening of the Gulf of Aden
Active tectonics of the Arabia-Eurasia collision zone: quantifying present-day deformation with GPS data
Reilinger et al., 2006
Westward extrusion of
Anatolia(escape
tectonics)
Crustal thickening
Subduction of a denser microplate (Southern Caspian Basin)
Guest et al., 2007
Active deformation in North IranAlborz
Brunet et al., 2003
Intra-collision zone subduction
South Caspian Basin crust is thinner and denser than
adjacent regions
Reilinger et al., 2006
The Arabia-Eurasia collision zone: kinematics model GPS based
Black numbers:3 strike(3) dip slip
White numbers:plate velocities
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
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) A case study from the Alborz mountains, an intracontinental mountain belt linked to Arabia-Eurasia collision (the record from foreland basin deposits)
Foreland basin anatomy and sedimentary facies distribution
Tectonic load (crustal shortening and
thickening; exhumation of crustal
section)Plate deflection (flexural
subsidence)
Grain-size decrease
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?
Burbank et al., 1988
Lateral and vertical sedimentary facies evolution in a foreland basin system: syn-thrusting progradation of
coarse-grained faciesStable thrust front
Distance from the thrust front (km)
Progradation of gravel facies during a
major thrusting phase
Flemings and Jordan 1990
Tim
e (M
a)Lateral and vertical sedimentary facies evolution in a foreland
basin system: post-thrusting progradation of coarse-grained facies
Lateral and vertical sedimentary facies evolution in a foreland basin system: climatic forcing
Zhang et al., 2001
Simplified tectonostratigraphy of the Alborz Mountains
Guest et al., 2006
The Alborz range is characterized by a
complex crustal fabric, with inherited
structures related to both compression and
extension since Paleozoic time
36 Ma (end of magmatism)
Modified after Geological maps of Tehran, Semnan, Saveh, Sari, Qazvin and Amol 1: 250 000, Geological Society of Iran, and Guest et al., 2006
Central Alborz Mountains
ca. 6 mm/yr of shorteningca. 4 mm/yr of left-lateral shearing
5m
N S
Unit 1C: braided river dep. system
Unit 1
SN
70m
Unit 1A: playa lake dep. system
Unit 1B: distal river dep. systemN
4m
SN
4m
Unit 3C: alluvial fan dep. systemN S
Unit 3
3B 3C
Unit 3A: playa lake dep. systemN S
Unit 3B: braided river dep. systemN S
5m
Magnetostratigraphy
Normal Polarity
Reverse Polarity
Reference MPTS
Main prerequisites:
Fine-grained lithologies
Continuous sedimentation
Independent age constrains
Magnetostratigraphy
In 75% of samples a Characteristic Remanent Magnetization (ChRM) was
isolated
Ballato et al., 2008
Ballato et al., 2008
Sediment accumulation rates
Fine-grained sed.
Fine-grained sed.
Fine-grained sed.
Coarse-grained sed.
Coarse-grained sed.
Coarse-grained sed.
PART 3…..Concluding
a) When did deformation related to the Arabia-Eurasia continental collision start in the Alborz mountains?
36 Ma
17.5 Ma
7.5 Ma
6.2 Ma
Sed.acc.rate = 0.04 mm/yr
Sed.acc.rate = 0.58 mm/yr
Sed.acc.rate = 0.65 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
Densmore et al., 2007
Tectonic vs climate: retrogradation of coarse-grained faciesS
ed
imen
t flu
x
Time (Myr) Time (Myr)
Tim
e (
Myr)
Distance from fault (Km)Distance from fault (Km)
Tim
e (
Myr)
Sed
imen
t flu
x
Increase in slip rate +100% = increase in subsidence and sed. flux Decrease in precipitation -50% = decrease in sed. flux
Pre-perturbation
Post-perturbation
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
Post-perturbation
Pre-perturbation
Facies retrogradation
Pre-perturbation Post-perturbationPre-perturbation Pre-perturbation
Facies retrogradation
Densmore et al., 2007
Tectonic vs climate: progradation of coarse-grained faciesS
ed
imen
t flu
x
Sed
imen
t flu
x
Time (Myr) Time (Myr)
Tim
e (
Myr)
Tim
e (
Myr)
Distance from fault (Km) 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
Pre-perturbationPre-perturbation
Post-perturbationPost-perturbation
Pre-perturbation Post-perturbation Pre-perturbation Post-perturbation
Facies progradation
Facies progradation
Decrease in slip rate -50% = decrease in subsidence and sed. flux Increase in precipitation +50% = increase in sed. flux
a) How did deformation evolve?
b) What can we learn from foreland basin deposits (i.e. climate vs tectonic) ?
PART 3…..Concluding
The locus of deformation moved forth and back, without a predictable pattern on a time scale ranging from 2 to 0.6 Ma
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