Transcript Zhong_chemical_pile_degree1

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
Degree-2 Structure in the Lower Mantle:
African and Pacific Superplumes/Chemical Piles
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].
Origin:
Controlled by plate motion [Hager &
O’Connell, 1981; Lithgow-Bertelloni &
Richards, 1998; Bunge et al., 1998].
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].
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].
1/30
1
100 km
uniform
X30
670 km
Largely at
degree 6
CMB
Depth
otherwise
constant viscosity
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?
Supercontinent Pangea (330 -- 175 Ma)
and Supercontinent Rodinia (900 -- 750 Ma)
750 Ma
[Smith et al., 1982, and Scotese, 1997]
[Li et al.2008; Hoffman, 1991, Dalziel,
1991, and Torsvik 2003].
Supercontinent events dominate tectonics and magmatism
Frequency of magmatism events/100 Ma
Time (Ga)
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]
Previous dynamic models for supercontinent cycles
2-D dynamic model
What if 3-D short-wavelength
convection?
Gurnis [1988]
Movie: Evolving to degree-1 convective structure
Viscosity:
h(T, depth).
hlith>~200hum
& hlm~30hum
1/30
1 hr
100 km
670 km
X30
Independent of Ra, heating mode, & initial conditions.
CMB
Cause supercontinent formation over the downwelling?
Depth
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.
Implications of the 1-2-1
model [Zhong et al., 2007]
Frequency of magmatism events/100 Ma
Time (Ga)
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.
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]
Results: Thermo-chemical structures at different times
(i.e., when Pangea was formed)
depth
2700 km depth
L
G
Pangea
Power
Power spectra
@2700 km depth
Time (Ma)
Comparison with present-day seismic structure
S20RTS @2750 km depth
@2700 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?
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
Implications: Plume-related volcanism and Siberian Flood Basalts
Residual temperature at 350 km depth at 250 Ma
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].
2) Siberian flood basalts induced by
two adjacent subduction zones?
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.
Implications: recycled crust vs primordial materials
Crustal tracers (zero buoyancy)
Primordial (dense)
1000 km depth
2600 km depth
young
old
Degree-1 or hemispherically asymmetric structures for the
Earth and other planetary bodies?
Pangea
Surface topography on Mars
Crustal dichotomy
Tharsis
Icy satellite Enceladus
Summary
• 121 cyclic model for the evolution of mantle structure
modulated by supercontinent cycle.
• Tested the model with plate motion history and presentday 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
Power spectra
@2700 km depth
Power
Time (Ma)
Time (Ma)
Movie 2: A supercontinent turns initially degree-1 to
degree-2 structures