Post on 08-Feb-2016
description
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)