Lithosphere extension as a function of mantle strength Frédéric Gueydan, Christina Morency & Jean Pierre Brun

Transcription

1 Lithosphere extension as a function of mantle strength Frédéric Gueydan, Christina Morency & Jean Pierre Brun Gueydan et al., Tectonophysics, 2008

2 Role of rheological layering on lithosphere deformation EXTENSION Distributed extension COMPRESSION Single Rifts Jackson (2002) Buck (1991) High lithosphere strength Low lithosphere strength Narrow rift Wide rift Mechanical role of each rheological layers? No high strength mantle Crustal strength >> Mantle strenght Crustal and mantle contribution to lithosphere strength?

3 1- Mantle and crustal contribution to lithosphere strength Wet quartz, dry olivine Crustal thick=30 km

4 1- Mantle and crustal contribution to lithosphere strength Wet quartz, dry olivine Crustal thick=30 km

5 1- Mantle and crustal contribution to lithosphere strength Wet quartz, dry olivine Crustal thick=30 km

6 2D modelling- Model Set-up Finite-element code: SARPP- F. Gueydan & Y. Leroy

7 2D modelling- Model Set-up Finite-element code: SARPP- F. Gueydan & Y. Leroy Small perturbation (100m)- at Brittle-Ductile Transitions In order to initiate strain localization

8 2D Results- Role of mantle strength Model 1 Model 2

9 2D Results- Role of mantle strength Three patterns of lithosphere stretching Gueydan et al, Tectonophysics, 2008

10 Transition between the 3 patterns of lithosphere extension Coupled crust-mantle Décollement Wide Rift

11 Transition between the 3 patterns of lithosphere extension

12 Map of lithosphere extension modes Transitions Coupled/décollement TM~500 C At larger T M for larger crustal thickness Décollement/wide rift TM~700 C Absence of high strength mantle

13 Map of lithosphere extension modes Transitions Coupled/décollement TM~500 C At larger T M for larger crustal thickness Décollement/wide rift TM~700 C Absence of high strength mantle

14 3- Natural examples SINGLE RIFT Rhine, Baikal TWO BRANCHES RIFTS EAR Limagne- Bresse REGULARLY SPACED RIFTS Basin&Range Aegean Tibet

15 3- Natural examples SINGLE RIFT Rhine graben Brun et al., 1992 Baikal Deverchere & al.,2001

16 3- Natural examples TWO BRANCHES RIFTS Limagne Bresse Michon & Merle,2003!" #!"

17 3- Natural examples REGULARLY SPACED RIFTS Evvia-Corinthe Latorre et al., 1992 Post-orogenic extension Extension of pre-thickened crust Basin & Range Tibet $ % "&!"& " $& '

18 Conclusions Existence of a high strength mantle Strength of the deep crust Lithosphere necking Amount of crust/mantle coupling Rift spacing is a direct function of mantle strength

19 Enjeux de la modélisation numérique Allemand et al 1989 Modélisation analogique en extension

20 Enjeux de la modélisation numérique Allemand et al 1989 Modélisation analogique en extension Beslier & Brun, 1991

21 Enjeux de la modélisation numérique Allemand et al 1989 r lage Modélisation analogique en extension grande déformation grand déplacements horizontaux Beslier & Brun, 1991 parallèle aux limites lithologiques rhéologies complexes thermo-mécanique couplé,

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23 Rift spacing

24 Rift topography

25 Map of lithosphere extension modes 1 cm/a 1 mm/a Role of the applied velocity

26 Map of lithosphere extension modes Role of the rheological parameter Dry Olivine Wet Olivine

27 Role of the high strength uppermost mantle? Narrow rift Rhine, Brun et al., 1992 Wide rift Moho offset Existence of a high strength mantle Flat Moho No high strength mantle Objective Role of the rheological layering (thermal layering) on the patterns of lithosphere extension- Continuum analysis Focus on the role of the high strength mantle

28 2D Results- Role of the applied velocity Coupled crust-mantle/crustal décollement Crustal decollement Wide rift Decrease of the velocity Decreases the deep crust strength Increases the amount of Mantle/Crust decoupling Crustal decollement mode triggered for lower T M More localized deformation (cf. Wide Rift)

29 2D Results- Role of the crustal thickness Crustal décollement Crustal decollement Coupled crust-mantle Crustal decollement Increase of crustal thickness Increase of lithosphere strength (uppermost mantle) Increase the amount of Mantle/Crust Coupling