Why Some Things May Have Looked Different in the Archean

Andrew Hynes, McGill University

4000 3500 3000 25 00 20 00 1500 1000 500 00

1

2

3

4

5

6

7

A ge (M a)

E ar th R ad iogen ic H eat Pr oduct ion

chon dritic K /U

crustal K/U

E xp onen t m

0 1 2 3 4 5 6 7 8 9 1 00

1

2

3

4

5

6

7

8

H eat-Flow as Function o f Potential Tem perature

q Tq TR R

=[ ]m

T T

crust

plate mantle

sub-plate mantle

crust

plate mantle

sub-plate mantle

Uniform Stretching (McKenzie, 1978)

T

plate

sub-plate

204060

80100

120140

160180

200

85 .9

1 5 3 .4

0200 400 600 800 1000 1200 1400 1600 1800

0204060

80100

120140

160180

200

C ont inen tal G eotherm sTempe rature (°C )

Exponent m

du e to s tr e tchin g

+3 0 0 °+1 0 0 °

- 0 .8 4 2 m o d er n ( s tr e tch o n ly)

Initial Elevation C hange ( =2); Double Heat Flowβ

0 1 2 3 4 5 6 7 8 9 10-2.5

-2-1.5

-1-0.5

00.5

11.5

22.5

crust

plate mantle

sub-plate mantle

crust

plate mantle

sub-plate mantle

Uniform Stretching (McKenzie, 1978)

less dense than sub-plate

more dense than sub-plate

1000 1200 1400 1600 1800 2000

0

20

40

60

80

100

120

Temperature (°C)

mantle solidus

mantle liquidus

1300°

0

5

10

15

20

25

30

0 1 2 3 4 5 6 7 8 9 10Expone nt m

β=2

+ 30 0°

+1 00 °

Melt Production with Stre tching

Exponent m

- 0 .8 4 2 m o d er n ( s tr e tch o n ly)

Initial Elevation C hange ( =2); Double Heat Flowβ

0 1 2 3 4 5 6 7 8 9 10-2.5

-2-1.5

-1-0.5

00.5

11.5

22.5

due to m el t

+300°

+100°

Exponent m

du e to s tr e tchin g

+3 0 0 °+1 0 0 °

- 0 .8 4 2 m o d er n ( s tr e tch o n ly)

Initial Elevation C hange ( =2); Double Heat Flowβ

0 1 2 3 4 5 6 7 8 9 10-2.5

-2-1.5

-1

-0.5

00.5

1

1.52

2.5

due to m el t

+300°

+100°

Exponent m

du e to s tr e tchin g

+3 0 0 °+1 0 0 °

- 0 .8 4 2 m o d er n ( s tr e tch o n ly)

Initial Elevation C hange ( =2); Double Heat Flowβ

0 1 2 3 4 5 6 7 8 9 10-2.5

-2

-1.5

-1-0.5

00.5

11.5

2

2.5

due to m el t

+300°

+100°

com bined

+100°

+300°

T T

crust

plate mantle

sub-plate mantle

crust

plate mantle

sub-plate mantle

Uniform Stretching (McKenzie, 1978)

2 .4 6

β=2

+ 10 0 °

+3 0 0 °

m od ern

0

0.5

1

1.5

2

2.5

3

3.5

4

01 2 3 4 5 6 7 8 9 10

Exponent mTher mal Subsidence

Te m p era tur e

M ode rn

A rc he an

Exponent m0 1 2 3 4 5 6 7 8 9 10

0

10

20

30

40

50

60

70

80

q =2.0R

300°

100°

Uniform stretching=2

;β

Net R e duction in Accom m odation Space

Exponent m0 1 2 3 4 5 6 7 8 9 10

0

10

20

30

40

50

60

70

80

q =2.0R

300°

100°

Uniform stretching=2

;β

Net R e duction in Accom m odation Space

D ouβle stretchingin m antle

204060

80100

120140

160180

200

85 .9

1 5 3 .4

0200 400 600 800 1000 1200 1400 1600 1800

0

204060

80100

120140

160180

200

C ont inen tal G eotherm sTempe rature (°C )

0 50 100 150 200 250 300 350 400 450 500-50

0

50

100

150

200

250

300

350

Age (M a)

Ridge Push a s Func tion of Age

0 50 10 0 150 200 250 300 350 400 450 500- 10

0

10

20

30

40

50

60

70

80

Age (M a)

Sla b-Pull for 500 km Slab

Archean

0 50 10 0 150 200 250 300 350 400 450 500- 10

0

10

20

30

40

50

60

70

80

Age (M a)

Sla b-Pull for 500 km Slab

Archean

Archea n; 2x cr us ta l thickness

0 50 10 0 150 200 250 300 350 400 450 500- 10

0

10

20

30

40

50

60

70

80

Age (M a)

Sla b-Pull for 500 km Slab

ArcheanArchean; 2x subduction rate

Archea n; 2x cr us ta l thickness

u

(viscosity ) m

hLl

hAasthenosphere

F

u = F h 1Ll 2 2 + 3 hμ LhA

after Turcotte & Schubert

lithosphere

Log e (viscosity) (Pa s )

40 45 50 55 60 65 700

50

100

150

200

450

250

500

300

550

350

600

400

650

(vi scosity of o livi neusi ng T and P dependencef r om K irby (1983))

A rch e na 10 0 M a

76 k m0.5E20 Pa s

m ean0.3E20 Pa s Low -V iscosity Channels

150 km T hi ck

112 k m3. 3E20 Pa s

m ean1.9E20 Pa s

Log e (viscosity) (Pa s )40 45 50 55 60 65 70

0

50

100

150

200

450

250

500

300

550

350

600

400

650

(vi scosity of o livi neusi ng T and P dependencef r om K irby (1983))

A rch e na 10 0 M a

L ow -V iscosity Channels D ef ined byThreshold V iscosity

110 km3. 0E20 Pa s

m ean2.1E20 Pa s

m ean1. 2E20 Pa s

62 k m3.0E20 Pa s

0 50 100 150 200 250 300 350 400 450 5000123

45

6

789

10

C om parison of P late Speed in M odern and Archean, with Half D riving Force

Plate Age ( M a)

a st he nosp her e 150 km t hic k

as th enos phe re w he re v isc osit y < 3.0 E2 0 P a s

Conclusions• Stretching at Archean passive margins would have resulted in markedly

thinner passive-margin sedimentary sequences.

• Passive margins would have been characterized by voluminous mantle-derived melts.

• The voluminous melts would have approximately restored crustal thicknesses to those preceding stretching.

• Development of thick lithospheric roots would have resulted in passive margins similar to modern ones, due to the resulting cooler geotherms.

• Driving forces for plate motion would have been half those today but resistive forces would have been reduced by much more.

• Subduction rates would have been more than twice those today, perhaps leading to universally erosional subduction zones.

age

heat loss from boundary-layer cooling

heat loss fromconvective transport

asthenosphere

oceanic plate

~2.8 Ga Volcanic-dominated rift margin, western Superior Province

Thick (250+ km) lithospheric keel beneath Kaapvaal (James et al. 2001) was present prior to 3.0-2.9 Ga passive margin formation

Thick Kaapvaal lithosphere at 3.3-2.9 Ga<2.88>2.76 Ga Witwatersrandconglomerates (Klerksdorp;Kositcin et al. 2001) containdetrital diamonds (Hallbauer et al. 1980)

Diamonds form at 150-250 kmdepth. Their age constrainstiming of formation of thicklithospheric keels

Kaapvaal diamond inclusionsyield ages of 3.3-3.2 Ga(Sm-Nd) and 2.9 Ga (Re-Os)(Richardson et al., 1984; Pearsonet al. 1998)