Toward a plate tectonics model of Alpine orogeny

1. 8th Workshop Alpine Geological Studies 10./12.10.07 Davos Toward a plate tectonics model of Alpine orogeny Edi Kissling, ETH Zürich In collaboration with: S. Schmid, M. Handy, J.-P. Burg, A. Thompson & T. Diehl, D. Lombardi, M. Spada (Poster Friday) (Poster + oral pres. today)

2. to further our understanding of past and current orogenic driving forces to aim at an Alpine-plate tectonics model Summary of presentation Present structure of lithosphere-asthenosphere system => Role of buoyancy forces across Moho and LAB Lippitsch et al. 2003 Objectives of study Application of principles to different lithosphere plate situations => derivation of generic plate models Application of generic plate models to episodes of Alpine evolution conclusions

3. Two lithospheric slabs: European and Adriatic with an eastward propagating tear beneath the Ivrea body Lippitsch et al. 2003 ECORS-CROP Profile Lippitsch et al. 2003 European lithospheric slab probably all continental mantle lithosphere

4. Perspective View of 3D Alpine Lithospere Slabs European plate Adriatic plate Apennine slab East Alpine slab

5. Moho reflectors Waldhauser et al. 1998 => significant crustal root! Moho topography and 3D crustal model

6. Alpine Lithosphere Isostasy (1) 2km topographic mass approximately compensated by 14 km crustal root (observed 28km thick root) 14 14 t Since erosion and uplift eventually remove the crustal root, two questions remain in the case of the Alps: After all the previous uplift and erosion, how comes the Alps are still strongly overcompensated? How did the crustal root get so deeply buried? => Lithosphere „root“

7. Alpine lithosphere isostasy (2) Airy for topo load + elastic for slab load observed Moho topography Elastic plate (Vening Meinesz) isostasy Airy crustal isostasy Despite low topography and very large crustal root, Alps are in (lithosphere!) isostatic equilibrium. Yolanda Deubelbeiss, Diploma thesis ETH 2005

8. Summary of other observables Delacou et al. 2004 MNT filtré (50 km) courtesy Spakman et al. 2007 Present stress field in the crust Current convergence rates of only 1- 4 mm/a Very large crustal root, moderate uplift rate (2mm/a), moderately high mountains, significant lithosphere slab, very small convergent rate

9. In collision, continental lithosphere delaminates near Moho And: Local earthquake tomography suggests no significant amount of crustal material subducts. (see Poster by T. Diehl et al.) Compare location of suture at Moho level, geometry of lithosphere slab, and results of analogue modelling Analogue models by Shemenda et al. 2000 Lippitsch et al. 2003

10. Unlike Andian type orogens, the Western and Central Alps ride on the subducting plate Adria is following rolling back European slab Lippitsch et al. 2003 Eurasia India suture Flysch and Molasse on same side of orogen

11. S-vergent subduction of E- mantle lithosphere beneath Central Alps Compare with post-collisional crustal shortening of 140 km (Schmid & Kissling 2000) => negative bouancy of continental mantle lithosphere Lippitsch et al. 2003

12. Evolution of Alpine Collision (western section) Figures by D. Fulda Lippitsch et al. 2003

13. Central Alpine isostatic uplift,stress regime, and mantle lithosphere delamination erosional unloading delaminational unloading

14. Conclusions “Alpine lithosphere slab structure” Lippitsch et al. 2003 Crustal structure links near-surface features and processes with mantle driving forces European slab retreated during subduction-collision Negative buoyancy of mantle lithosphere (relative to asthenosphere) Oceanic lithosphere has been detached upon collision Continental lithosphere delaminates near Moho Eurpean mantle lithosphere slab is mechanically weak Thick crustal root balances topography and lithosphere slab, NW-ward propagation of delamination near Moho causes NW migration of isostatic uplift

15. 2.9 2.85 3.30 3.25 Principle of approach: Lithosphere isostasy in isostatic equilibrium floating plate experiences divergent buoyancy forces across Moho

16. Lithosphere isostasy and mechanics (2) Mature oceanic lithosphere Young continental lithosphere For oceanic lithosphere: single mechanically strong layer! For continental lithosphere: weak point at Moho levels!

17. Evolution and subduction of Penninic ocean: the story of the Sesia nappes (1) Pre - 160 Ma 160 - 140 Ma Modified from reconstruction by Capitanio & Goes 2006 Around 100 Ma 1. Spreading in Ligurian-Piemont oceans 2. “Freezing” of Ligurian-Piemont oceanic lithosphere => no intra-oceanic plate boundary existing Re-interpretation of findings by Babist et al. 2007

18. Evolution and subduction of Penninic ocean: the story of the Sesia nappes (2) Around 95 Ma 160 - 140 Ma 100 Ma Around 85 Ma Re-interpretation of findings by Babist et al. 2007

19. Evolution and subduction of Penninic ocean: the story of the Sesia nappes (3) 80 Ma Re-interpretation of findings by Babist et al. 2007 Subduction of Penninic ocean initiated at its southern border, near Adriatic continental margin

20. Evolution and subduction of Penninic ocean: the story of the Sesia nappes (4) Re-interpretation of findings by Babist et al. 2007 Penninic slab roll back and N to NW movement of Adria

21. 3D crustal structure: lower crustal wedges and Penninic nappes Penninic nappes system TRANSALP lithosphere slabs Lower crust indenters Schmid et al. 2004 Pfiffner et al. crustal traverses document: non-cylindric structure, wedge tectonics dominantat crustal levels

22. Strongly extended formerly continental lithosphere (Penninic nappes, Brianconnais domain etc. ): strongly thinned lower continental crust, pieces of upper continental crust overlying newly formed (oceanic) mantle lithosphere => locally strong divergent buoyancy forces across Moho, easily detached in subduction and exhumed as nappes. Lithosphere isostasy and mechanics (3) Opening of Penninic ocean

23. Slab retreat and nappes formation- exhumation 80 My today likely configuration 80 My ago 40 My 40 My 19 My 32 My present configuration

24. Conclusions(1) - western and central Alps are situated on lower plate - Eastern Alps are similar to Himalaya on upper plate (similar to Apennines, different from Andes, Himalaya) -non-cylindrical structure at all crustal levels -dual subduction-collision zone with opposing slabs E. Kissling Alpine Workshop Davos 10.10.2007 Deep Alpine Structure

25. Conclusions(2) Alps are in isostatic equilibrium. With continued delamination and with current erosion rate, Alps will be of same height for another 8 My! main forces currently shaping the Alps are lithosphere delamination and surface erosion, i.e. buoyancy forces!

26. Conclusions(3) -subducting European (Penninic) slab roll back and retreat -slab roll back and buoyancy facilitate nappes exhumation E. Kissling Alpine Workshop Davos 10.10.2007 Lithosphere Isostasy and Mechanics -continental lithosphere: weak point at Moho levels! (Sesia and Penninic nappes)