Download Structure and Dynamics of Oceanic Lithosphere and Upper

April 10, 2018 | Author: Anonymous | Category: , Science, Earth Science, Plate Tectonics
Share Embed


Short Description

Download Download Structure and Dynamics of Oceanic Lithosphere and Upper...

Description

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

View more...

Comments

Copyright © 2017 HUGEPDF Inc.