An 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

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

Dynamic origin of long-wavelength mantle convection from radially stratified mantle viscosity Bunge et al. [1996]. Originally showed by Jaupart & Parsons [1985], Robinson & Parsons [1987] in 2-D models and Zhang & Yuen [1995] in 3-D spherical models. However, the exact mechanism is still an open question [see Zhong & Zuber, 2001; Lenardic et al., 2006]. Largely at degree 6 uniform X30 CMB 670 km 100 km 1/301 Depth otherwise constant viscosity What is the mantle structure for the past?

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

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

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

Movie: Evolving to degree-1 convective structure Independent of Ra, heating mode, & initial conditions.  lith >~200  um &  lm ~30  um Viscosity:  (T, depth). Depth CMB 670 km 100 km 1/301 rr X30 Cause supercontinent formation over the downwelling?

An 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. Degree-2 convection after a supercontinent is formed. Two antipodal major upwelling systems, including one under the supercontinent. forming a supercontinent breaking up the supercontinent Mantle structure: 1  2  1 cycle. At the surface: supercontinent cycle.

Implications of the model [Zhong et al., 2007] Vs at 2300 km depth from S20RTS 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. Time (Ga) Frequency of magmatism events/100 Ma Continental magmatism: reduced level during the supercontinent assembly, but enhanced after.

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

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

Power spectra Time (Ma) km depth

Comparison with present-day seismic km depth km depth

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

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

Implications: Plume-related volcanism and Siberian Flood Basalts Residual temperature at 350 km depth at 250 Ma 2) Siberian flood basalts induced by two adjacent subduction zones? 1)Oceanic plateaus formed on the Pacific (Panthalassic) and subsequently joined to the Asian and American continents [Maruyama et al., 1997; Safonova et al., 2009].

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

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

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

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.

Power spectra Time (Ma) Power Time (Ma) km depth

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