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. McNamaraSchool 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 km1/30 1

Depth otherwise constant viscosity

What is the mantle structure for the past?

Supercontinent Pangea (330 -- 175 Ma)

[Smith et al., 1982, and Scotese, 1997][Li et al.2008; Hoffman, 1991, Dalziel, 1991, and Torsvik 2003].

750 Ma

and Supercontinent Rodinia (900 -- 750 Ma)

Supercontinent events dominate tectonics and magmatism

Tim

e (G

a)

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>~200um & lm~30um

Viscosity: (T, depth).

DepthCMB

670 km

100 km1/30 1 r

X30

Cause supercontinent formation over the downwelling?

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.

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: 121 cycle. At the surface: supercontinent cycle.

Implications of the 1-2-1 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.

Tim

e (G

a)

Frequency of magmatism events/100 Ma

• Continental magmatism: reduced level during the supercontinent assembly, but enhanced after.

Testing the 1-2-1 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)

Pow

er

@2700 km depth

Comparison with present-day seismic structure

@2700 km depth

S20RTS @2750 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 bytwo 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 MarsIcy satellite Enceladus

Crustal dichotomy

Tharsis

Summary

• 121 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)

Pow

er

Time (Ma)

Pow

er@2700 km depth

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