1. Efficient exhumation of (ultra) high-pressure rocks by slab extraction Zhao, Z., P.D. Bons, Eberhard Karls University Tübingen, Germany E. Gomez-Rivas, University of Aberdeen, Scotland, UK A. Soesoo, Tallinn University of Technology, Tallinn, Estonia E. Burov, Université P. & M. Curie (Paris VI), Paris, France

2. Outline Introduction to slab extraction Definition Divergent double subduction zone setting Numerical simulations with FLAMAR Cases in geological records Paleotethys in Qiangtang Suture, Tibet D'Entrecasteaux eclogites, Papua New Guinea Conclusion

3. "Normally" suture between plates has reverse movement When plates diverge – rollback, or – slab extraction Slab extraction

4. Opposing slab pulls: F 2 > F 1 F resistance shear force at base and overlying oceanic lithosphere When F 2 > F 1 + F resistance Short slab may be pulled up by long slab Related to L 1 -L 2, temperature-density relationships, x,  1,2, rheology of slab and mantle, etc. Slab extraction is likely to happen when x becomes small x L1L1  L2L2  F1F1 F2F2 Divergent double subduction zone (DDSZ)

5. It solves simultaneously Newtonian dynamic equations of motion, in a Lagrangian formulation, coupled with visco-elasto-plastic constitutive equations, heat transport equations and state equation (Burov et al., 2001; 2003; Burov and Yamato, 2008)  C p (  T/  t + u  T) –  (k  ) - H r - H a - frac  II  II /  t = 0  = f(P,T) The parameters A, n, Q are experimentally determined material properties Numerical simulations: FLAMAR

6. 40km 110km Length differences170km (variable) 10km 80km Geometry: divergent double subduction zone Material properties from Angiboust et al. (2012) Parameters varied: V x, ∆L and rheology of slab ∆L VxVx Starting model and settings

7. Simulations V x =0

8. Simulations The-Depth of passive markers Exhumation rate of passive markers

9. Simulations V x =-2cm/y

10. Simulations The-Depth of passive markers Exhumation rate of passive markers

11. Slab gets extracted when – x is short – ∆L large – subducted lithosphere strong – Lubricate layer is necessary (serpentinite) High-pressure rocks exhumed rapidly – brought in contact with sediments/LP rocks – in locally strong extensional setting in case of plate divergence – rapid exhumation of HP rocks on both sides – opening of pull-apart basin Results

12. HP rocks in direct contact with non-metamorphic sedimentary mélange Consistent top-to-south shear Sedimentation and volcanism – no extensive erosion to exhume HP rocks Case 1: Qiangtang suture, Central Tibet (Zhao et al., 2015).

13. Young eclogites 4.6-5.6 Ma Exhumation rate ~2 cm/yr NW movement of Solomon sea plate New pull-apart basin (Baldwin et al., 2004; DesOrmeau et al., 2014; Korchinski et al., 2014). Case 2: D'Entrecasteaux eclogites

14. Extraction is an efficient mechanism for rapid exhumation of (U)HP rocks – Brings (U)HP rocks in contact with LP rocks – No need for erosion Close to eduction 1 and extension model 2 – Extracted slab can be negatively buoyant (ocean) Requires divergence of subducted slab and overriding plate – Single suture (Western Gneiss, Norway) – DDSZ (Qiangtang, d'Entrecasteaux Islands) Conclusion Refs.: (1) Duretz et al., 2012; (2) Brueckner et al., 2013

15. Thank you Zhao, Z., Bons, P., Wang, G., Soesoo, A., and Liu, Y.: Tectonic evolution and high- pressure rock exhumation in the Qiangtang Terrane, Central Tibet, Solid Earth, 2015.