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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, - PowerPoint PPT Presentation
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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