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

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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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