Nan Zhang, Wei Leng, Shijie Zhong, Department of Physics, University of Colorado at Boulder

1. An 1-2-1 model for mantle structure evolution and its implications for mantle seismic and compositional structures and supercontinent process Nan Zhang, Wei Leng, Shijie Zhong, Department of Physics, University of Colorado at Boulder Zheng-Xiang Li, Department of Applied Geology, Curtin University of Technology, Australia Acknowledge help from Allen K. McNamara School of Earth and Space Exploration, Arizona State University Funded by NSF-EAR CIDER workshop, 2009

2. Degree-2 Structure in the Lower Mantle: African and Pacific Superplumes/Chemical Piles Origin: Controlled by plate motion [Hager & O’Connell, 1981; Lithgow-Bertelloni & Richards, 1998; Bunge et al., 1998]. Degree-2 structure: Dziewonski et al. [1984], van der Hilst et al. [1997], Masters et al. [1996, 2000], Romanowicz and Gung [2002], and Grand [2002]. Vs at 2300 km depth from S20RTS [Ritsema et al., 1999] [McNamara & Zhong, 2005] Using the past 119 Ma plate motion history [Lithgow-Bertelloni & Richards, 1998].

3. 1/30 1 100 km 670 km CMB Dynamic origin of long-wavelength mantle convection from radially stratified mantle viscosity Originally showed by Jaupart & Parsons [1985], Robinson & Parsons [1987] in 2-D models and Zhang & Yuen [1995] in 3-D spherical models. Bunge et al. [1996]. uniform Largely at degree 6 Depth otherwise constant viscosity X30 However, the exact mechanism is still an open question [see Zhong & Zuber, 2001; Lenardic et al., 2006]. What is the mantle structure for the past?

4. Supercontinent Pangea (330 -- 175 Ma) and Supercontinent Rodinia (900 -- 750 Ma) 750 Ma [Li et al.2008; Hoffman, 1991, Dalziel, 1991, and Torsvik 2003]. [Smith et al., 1982, and Scotese, 1997]

5. Time (Ga) Frequency of magmatism events/100 Ma Supercontinent events dominate tectonics and magmatism Original eruption sites of large igneous provinces and hotspots Torsvik et al. [2006] Major mountain belts: Ural and Appalachians Always degree-2 [Burke et al., 2008]. However, notice that the oldest event in this figure is the Siberia Trap (ST) at 252 Ma. Bleeker & Ernst [2007]

6. Previous dynamic models for supercontinent cycles 2-D dynamic model What if 3-D short-wavelength convection? Gurnis [1988]

7. 1/30 1 100 km 670 km CMB Depth Movie: Evolving to degree-1 convective structure Viscosity: h(T, depth). hlith>~200hum & hlm~30hum hr X30 Independent of Ra, heating mode, & initial conditions. Cause supercontinent formation over the downwelling?

8. An 1-2-1 model for the evolution of mantle structure modulated by continents [Zhong et al., 2007] Degree-1 convection when continents are sufficiently scattered. One major upwelling system. forming a supercontinent Degree-2 convection after a supercontinent is formed. Two antipodal major upwelling systems, including one under the supercontinent. breaking up the supercontinent Mantle structure: 121 cycle. At the surface: supercontinent cycle.

9. Time (Ga) Frequency of magmatism events/100 Ma Implications of the 1-2-1 model [Zhong et al., 2007] Vs at 2300 km depth from S20RTS • Continental magmatism: reduced level during the • supercontinent assembly, but enhanced after. • The African and Pacific superplumes are antipodal • to each other (i.e., degree-2). • The African anomalies are younger than Pangea • (330 Ma), but the Pacific anomalies are older.

10. Testing the 1-2-1 model predictions or hypotheses How? Using present-day seismic structure, and geological observations of continental motion for the past 500 Ma. ? [Scotese, 1997] After 119 Ma, Lithgow-Bertelloni & Richards [1998]

11. Results: Thermo-chemical structures at different times (i.e., when Pangea was formed) depth 2700 km depth L G Pangea

12. Power spectra Power Time (Ma) @2700 km depth

13. Comparison with present-day seismic structure S20RTS @2750 km depth @2700 km depth

14. Test 1: Always Degree-2? (Burke et al., 2008) Using present-day modeled thermochemical structure (degree-2) as initial condition.

15. Test 2: Downwellings in the Pacific hemisphere? After 220 Ma Initial condition includes a downwelling In the Pacific hemisphere. After 320 Ma After using the past 120 Ma plate motion. After 420 Ma

16. Implications: Plume-related volcanism and Siberian Flood Basalts Residual temperature at 350 km depth at 250 Ma • Oceanic plateaus formed on the Pacific (Panthalassic) and subsequently joined to the Asian and American continents [Maruyama et al., 1997; Safonova et al., 2009]. 2) Siberian flood basalts induced by two adjacent subduction zones?

17. Implications: Plume-related volcanism and its relation to the chemical piles Chemical pile at 2600 km depth (present-day) Residual temperature at 1000 km depth Plumes derived from chemical piles are indeed at the pile boundaries.

18. Implications: recycled crust vs primordial materials Crustal tracers (zero buoyancy) Primordial (dense) 1000 km depth 2600 km depth young old

19. Degree-1 or hemispherically asymmetric structures for the Earth and other planetary bodies? Pangea Surface topography on Mars Icy satellite Enceladus Crustal dichotomy Tharsis

20. Summary • 121 cyclic model for the evolution of mantle structure modulated by supercontinent cycle. • Tested the model with plate motion history and present-day seismic structures. • Implications for a) seismic structures (the African and Pacific superplumes and chemical piles – the Pacific pile is older!), b) plume-related volcanism (locations of plumes, Siberia flood basalt). c) primordial vs recycled crust as the source for the piles.

22. Power spectra Power @2700 km depth Time (Ma) Power Time (Ma)

24. Movie 2: A supercontinent turns initially degree-1 to degree-2 structures