Transcript Selective survival of crust - The University of Texas at Dallas

Selective Survival of Crust
Paul Morgan
Department of Geology
Northern Arizona University
Flagstaff, Arizona, USA
Penrose Conference
Lander, WY; 14-18 June 2006
Conclusions


As we go back in time, intrinsic crust
radiogenic heat production becomes
an important factor in the selective
survival of crust.
Different amounts of the heat
producing elements preserved in
“average” Archean and later
continental crust may be able to
explain the Hadean-Archean and the
Archean-Proterozoic transitions.
Plate Tectonic Basics

1968: Beatles, Bellusov, Rigid Body Rotations
Well-located Earthquakes with Magnitude >5.1
From: Sandwell, Anderson & Wessel, Global Tectonic Maps, in press.
ftp.topex.ucsd.edu
When did Plate Tectonics Start?
1928/1929!
Arthur Holmes, Proc. Phil. Soc. Edinburgh, 1928/29
Thermal Constraints on Selective
Survival of Crust (Lithosphere)
• Use



Observations of “average” intrinsic
radiogenic heat production in samples of
surviving crust
Known decay constants for the unstable
isotopes that contribute to this radiogenic
heat production (232Th, 235, 238U, 40K)
Laboratory data for estimates of strength
parameters of rocks of different
compositions to give lithospheric strength
profiles
Calculate

Geotherms
• For lithospheres with different crustal
thicknesses and heat production as a function
of time back to 4.5 Ga


Crustal strength profiles for these
geotherms
Whether these lithospheric sections have
enough integrated strength to maintain
thickness, or spontaneously thin making
them susceptible to subduction.
Intrinsic Crustal Heat Production


“Low” heat production (Archean) crust (microW/kg)
Total U 147 x 10-6; Th 150 x 10-6; Total K 52.5 x
10-6
“High” heat production (Proterozoic and younger)
crust (microW/kg) Total U 274 x 10-6; Th 281 x
10-6; Total K 98.0 x 10-6
[Source: Taylor and McLennan, 1985]

These values are consistent with the average
difference in surface heat flow between Archean and
younger provinces
Data at t=0 from Taylor & McLennan, 1985
Geotherms vs Time for Archean Crust
Temperature, °C
0
500
1000
1500
0
10
0 Ga
1 Ga
2 Ga
20
3 Ga
4 Ga
30
40
Depth, km
Assume only
intrinsic
lithospheric
radiogenic heat
production
changes with
time: Backcalculate
geotherms
through time
50
60
70
80
90
100
Use back-calculated geotherms to calculate
lithospheric strength curves through time
Low Heat Production (Archean) 40 km Crust
Compressional Yield Strength MPa
0
0
Depth km
25
50
75
100
500
1000
1500
2000
2500
0 Ma
1 Ma
2 Ma
3 Ma
4 Ma
Calculating Differential Pressure vs Thinner Crust
Conclusions 1


Any evolved crust is likely to have
been so hot and weak prior to about
4 Ga that it would have been
incapable of withstanding spreading
stresses relative to background 20
km crust.
If 20 km crust was being subducted,
most evolved 20 km crust would be
subducted or extensively reworked at
4 Ga
Conclusions 2


After ~ 4 Ga, low heat production
crust would have been able to have
start generating crust thicker than
~30 km with respect to spreading
stresses relative to 20 km crust, but
normal heat production crust would
still have been too hot and weak
This change possible represents the
Hadean to Archean transition
Conclusion 3


By 2 to 3 Ga, normal heat production
crust had sufficiently cooled to
thicken sufficiently to form crust
thicker than ~30 km stable relative
to 20 km thick crust, and intrinsic
radiogenic crustal heat production
ceased to be an important factor in
selective crustal survival
This change possibly represents the
Archean to Proterozoic transition
finis