Formation of retro-wedges during collision

(1)

INTR ODUC TION

Formation of retro-wedges during collision

INTRODUCTION

We challenge the generally accepted view that continent-continent collision results in doubly verging orogenic wedges with well-developed retro-wedges on the overriding plate. In fact we argue that retro-wedge formation is restricted to specify rheological conditions within the lower and upper plates as well as the plate contact; thus being the exception rather than the rule during collision.

In this contribution we use simple lithospheric-scale analogue experiments to infer favourable rheological conditions for the development of retro-wedges. In intermediate temperature

lithospheres represented by three layer models (brittle crust, ductile crust and upper mantle), the contact between the colliding and neutrally buoyant continents is weak and represents the

inheritance of a former subduction boundary. The degree of plate coupling however is not constant and is together with the rheological structures of the lower and upper plates, in particular the

presence of decoupling horizons, key variable in this study.

Ernst Willingshofer & Dimitrios Sokoutis

Faculty of Geosciences, University of Utrecht, The Netherlands Corresponding author: e.willingshofer@uu.nl

Numerical Model of Collision Zones

CONCL USIONS

EXPERIMENT S SERIES - A EXPERIMENT S SERIES - B

Faculty of Geosciences

0 50 150 250

1

2

3 Upper crust Lower crust Strength (Pa)

D epth (cm) Mantle lithosphere

Weak lower crust

EXPERIMENTS WITH DECOUPLING ZONES

antiform/synform

Reference level

thrust fault normal fault Uplift Subsidence

Experiment B3

40 cm

0 0,8 1,3 cm 2,8

brittle crust ductile crust

weak ductile crust/

decoupling material

ductile lithospheric mantle

Weak plate interface

Model Layer Density Coefficient of Viscosity Power

(kg/m

³

) friction (Pa s) n

Brittle crust 1300 0.7

Ductile crust 1400 4.8 x 10

⁴

1.9 Table2: Mechanical properties of analogue materials - series B

Weak ductile crust 1400 1.8 x 10

⁴

1.3 Weak plate interface 1400 1.8 x 10

⁴

1.3 Asthenosphere 1450 7.2 x 10

⁵

Ductile upper mantle 1550 1.2 x 10

⁵

1.3 Velocity: 1.9 x 10

^-6

(ms

^-1

); Length Scale: 1cmModel = 30kmNature

4 6 3 2

1'

8

4

80 km 4 cm

22% bs ~ 182 km

1 5 6 10

24% bs ~ 210 km 9

80 km 4 cm

Initially Inclined Boundary

Brittle Crust

Viscous Crust and Upper Mantle

Model Moho

Ductile Shear Zone Initially Decoupled Boundary

Viscous Upper Mantle

0 1.01.5

2.5

Depth (cm)

15 cm 10 cm 15 cm 2 cm

22 cm

15 cm 3 cm

low viscosity lower mantle lithosphere + asthenosphere (liquid)

strong viscous upper mantle viscous crust

brittle crust

Lateral Confinement

Fo re lan d P late

Ind en ter

viscous crust mantleand mix I

0 1.0 1.5

2.5 strength (Pa)

B

0 1.5 1.0

2.5 strength (Pa)

depth (cm)

A

brittle crust

A B

dry quartz sand silicone mix I silicone mix II

Experiment A1

600 300 900 300 600 900

30

^o

Experiment A2

Pre -e xis tin g W ea k Z on e

Experiment A3 Experiment A4

0 1.0 1.5

2.5 strength (Pa)

depth (cm)

C

600 300 900

C

20% bs ~ 160 km

1 2

landlocked sedimantary basin

80 km 4 cm

Low Viscosity Layer

Initially Decoupled Boundaries

0 60

120 180 240km

1 1 2 3

4 5

2 Stage 1 Block Uplift

S Stage 2

Pro-wedge Retro-wedge

S Stage 3

Minimum Taper Maximum Taper

Minimum Taper

S

EXPERIMENTS WITH A WEAK ZONE FAVOURABLE CONDITIONS FOR THE

FORMATION OF RETRO-WEDGES

Decoupling Layers

Decoupling between the orogenic wedge and the foreland (A1, A2)

80 km 4 cm

26% bs ~ 218km

2 1 3

Initially Inclined Boundary

45

^o

45

^o

Decoupling at two inclined boundaries and at the brittle-ductile transition (A3, A4)

Experiment B1

Experiment B4

Experiment B2

20% bs ~ 220 km

Decoupling in the mantle along the plate contact

Propagation of deformation

upper crust weak plate interface / lower crust

mantle lithosphere

strong coupling at the plate interface

(experiments A1, B1)

weak layers within the crust of the upper plate

(experiment B4)

lateral increase of lower crust rheology

(experiment B2)

weak orogenic wedge (experiments of series A) 4

1 2 3

Conceptual Model of Collision Zones Pro-wedge Retro-wedge

fold-thrust belt metamorphic core fold-thrust belt

Model Layer Density Coefficient of Viscosity Power (kg/m

³

) friction (Pa s) n

Brittle crust 1510 0.85

Viscous crust 1520 1.8 x 10

⁵

1.8 Visc. upper mantle 1540 7.2 x 10

⁵

2.0 Table1: Mechanical properties of analogue materials - series A

antiform/synform Reference level

thrust fault normal fault

Uplift Subsidence

Legend

2 1 3

4

A

0 60

120 180 240km

4 cm

6 2 1 6

0 60

120 180 240km

2 1

3 4 5 6

A

λ 0

60

120 180 240km

Decoupling at the plate interface (B1, B2)

Crust-mantle dcoupling on the upper and lower plates (B3, B4)

Pro-wedge Retro-wedge

1 2 3

4 Willingshofer & Sokoutis (2009)

Sokoutis & Willingshofer (2011)

Luth et al. (2010)

Luth et al. (2013) Willingshofer et al. (2013)

4 c REFERENCES

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