Why Some Things May Have Looked Different in the Archean
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Why Some Things May Have Looked Different in the Archean Andrew Hynes, McGill University
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Why Some Things May Have Looked Different in the Archean. Andrew Hynes, McGill University. Conclusions. Stretching at Archean passive margins would have resulted in markedly thinner passive-margin sedimentary sequences. - PowerPoint PPT Presentation
Transcript of Why Some Things May Have Looked Different in the Archean
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)
