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) ?

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

PART 1

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)

PART 2

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

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

Reilinger et al., 2006

Active tectonics of the Arabia-Eurasia collision zone: seismicity

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

ASTER satellite image, bands 731-RGB

Eyvanekey stratigraphic section

5 km

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 2

S N

Unit 2B: braided river dep. system

Unit 2A: playa lake dep. systemN S

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

Stratal geometric relationship

ASTER satellite image, bands 321-RGB

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

Magnetostratigraphic Correlation

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

Unit 1

Stratal geometric relationship

ASTER satellite image, bands 321-RGB

Unit 2

ca.

5 k

m

ca. 25 km

Unit 3

ca.

5 k

m

ca. 25 km

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

Thank you

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