The role of lower crustal flowin the formation of
subaqueous Archaeancontinental flood basalts
Nicolas Flament a, b, Patrice Rey a, Nicolas Coltice b, Gilles Dromart b & Nicolas Olivier b
a The University of Sydney – b Université de Lyon
AESC 2010, Perth, July 7th
DYNAMIC EARTH: Restless Earth and Earth structure
Subaqueous Archaean flood basalts 2Flament et al.
Archaean pillow basalts in 5 to 15 thick Archaean Large Igneous Provinces
In the Pilbara, WA, at ~ 3.5 Ga
In Isua, Greenland, at ~ 3.8 Ga
Photo P. Rey
Photo M. Boyet
Subaqueous Archaean flood basalts 3
Predominance of subaqueous continental flood basalts in the Archaean
Subaqueous flood volcanism on continental platforms is:
• common in the Precambrian
• rare to absent in the Phanerozoic
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Kump & Barley (2007)
Arndt (1999)
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What was different in the Archaean?The radioactive heat production was 2 to 3.5 times greater
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€
A t( ) = H i exp−t
τ i
⎛
⎝ ⎜
⎞
⎠ ⎟
i
∑
Heat production from Th, K and U
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Abundant mafic and ultramafic rocks indicate high eruption temperatures
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What was different in the Archaean?The mantle was ~ 200 ± 100°C hotter
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What was different in the Archaean?The volume of the continents was between 20 and 80% of present
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Higher sea levels in the ArchaeanPresent-day configuration
Flament, Coltice& Rey (2008)
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Higher sea levels in the ArchaeanPossible late-Archaean configuration
Flament, Coltice& Rey (2008)
Continents flooded by ~ 1km of water does not fully explain subaqueous CFBs 5 to 15 km thick
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Continental geotherms: the Archaean paradox
Nisbet (1984)
Hot continental crust?Or not?
Cold lithosphere?Archaean diamonds?
England & Bickle (1984)
Richardson et al. (1984)
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Model setup - geometry
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• Ellipsis, particle-in-cell, finite element (Moresi et al., 2003)
• Instantaneous emplacement of CFB at t0 = 0
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Model setup – geotherm & rheology
€
η =η0 exp −γ T( )
• Visco-plastic
• T-dependent viscosity:
• 1D, steady-state
• H(0) for present-daycratons
Taylor & Mclennan (1995)€
kd2T
dz2 + ρ ccH t( ) = 0
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Relaxation of the topographic anomalySubsidence by gravity-driven lower crustal flow
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t = 1 Myr
t = 2 Myr
• CFB 6 km thick• TMoho ≈ 800ºC
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Characteristic relaxation time τ
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w t( ) = w0 exp−t
τ
⎛
⎝ ⎜
⎞
⎠ ⎟
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Scaling laws for τ as a function of TMoho
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τ ∝ exp −TMoho( )
Estimate τ from the subsidence history of a CFB to estimate TMoho
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Case study: the Fortescue Group
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GSWA
Thorne & Trendall (2001)
• 6.5 km thick
• 2775-2630 Ma
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Structure of the Meentheena Centrocline
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• Emplaced on top of a dome and basin structure
• Absence of syn-depositional deformation
• Structure consistent with accommodation by gravitational subsidence
Williams & Bagas (2007)
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Directions of palaeostress σH and gravitational subsidence
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An exceptional stratigraphic column
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w t( ) = w0 exp−t
τ
⎛
⎝ ⎜
⎞
⎠ ⎟
w0 ≥ 600 m
w ti( ) ≈ w t f( ) ≈10 m
t ≤ 11 Myr
⇒ τ ≤ 3 Myr
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Cooling of the crust in the East Pilbara
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• At 2.7 Ga: TMoho ≥ 700ºC• Present-day: TMoho ≈ 480ºC (surface heat flow)• Cooling by ~ 220ºC
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Estimation of the transient component
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Coltice et al. (2007, 2009)
• High temperature under supercontinents• Zimvaalbara: Pilbara, Yilgarn, Gawler, Zimbabwe, Kaapvaal, Congo, São Francisco & Dharwar cratons(Aspler and Chiarenzelli, 1998)
• If 10% of the Earth’s surface:
anomaly ≤ +75ºC
The secular cooling is ≥ 150ºC over 2.7 Ga in the East Pilbara
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Conclusions
• The subsidence history of CFBs can be used to constrain the Archaean geotherm
• The Maddina Formation basalts constrain the secular cooling of the East Pilbara crust as ≥ 150ºC
• The flow of hot, ductile lower continental crust was a key process that maintained Archaean CFBs below sea level
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