Selective Survival of Crust Paul Morgan Department of Geology Northern Arizona University Flagstaff, Arizona, USA Penrose Conference Lander, WY; 14-18.

Selective Survival of CrustSelective Survival of Crust

Paul MorganPaul MorganDepartment of GeologyDepartment of Geology

Northern Arizona UniversityNorthern Arizona UniversityFlagstaff, Arizona, USAFlagstaff, Arizona, USA

Penrose ConferencePenrose Conference

Lander, WY; 14-18 June 2006Lander, WY; 14-18 June 2006

ConclusionsConclusions As we go back in time, intrinsic As we go back in time, intrinsic crust radiogenic heat production crust radiogenic heat production becomes an important factor in becomes an important factor in the selective survival of crust.the selective survival of crust.

Different amounts of the heat Different amounts of the heat producing elements preserved in producing elements preserved in “average” Archean and later “average” Archean and later continental crust may be able to continental crust may be able to explain the Hadean-Archean and explain the Hadean-Archean and the Archean-Proterozoic the Archean-Proterozoic transitions. transitions.

Plate Tectonic BasicsPlate Tectonic Basics 1968: Beatles, Bellusov, Rigid Body Rotations1968: Beatles, Bellusov, Rigid Body Rotations

From: Sandwell, Anderson & Wessel, Global Tectonic Maps, in press.ftp.topex.ucsd.edu

Well-located Earthquakes with Magnitude >5.1

When did Plate Tectonics Start?When did Plate Tectonics Start?

1928/1929!1928/1929!

Arthur Holmes, Proc. Phil. Soc. Edinburgh, 1928/29

Thermal Constraints on Selective Thermal Constraints on Selective Survival of Crust (Lithosphere)Survival of Crust (Lithosphere)• UseUse

Observations of “average” intrinsic Observations of “average” intrinsic radiogenic heat production in samples radiogenic heat production in samples of surviving crustof surviving crust

Known decay constants for the unstable Known decay constants for the unstable isotopes that contribute to this isotopes that contribute to this radiogenic heat production (radiogenic heat production (232232Th, Th, 235, 235,

238238U, U, 4040K)K) Laboratory data for estimates of Laboratory data for estimates of strength parameters of rocks of strength parameters of rocks of different compositions to give different compositions to give lithospheric strength profileslithospheric strength profiles

CalculateCalculate

GeothermsGeotherms• For lithospheres with different crustal For lithospheres with different crustal thicknesses and heat production as a thicknesses and heat production as a function of time back to 4.5 Gafunction of time back to 4.5 Ga

Crustal strength profiles for these Crustal strength profiles for these geothermsgeotherms

Whether these lithospheric sections Whether these lithospheric sections have enough integrated strength to have enough integrated strength to maintain thickness, or spontaneously maintain thickness, or spontaneously thin making them susceptible to thin making them susceptible to subduction.subduction.

Intrinsic Crustal Heat ProductionIntrinsic Crustal Heat Production

““Low” heat production (Archean) crust Low” heat production (Archean) crust (microW/kg) Total U 147 x 10(microW/kg) Total U 147 x 10-6-6; Th 150 x 10; Th 150 x 10-6-6; ; Total K 52.5 x Total K 52.5 x 1010-6-6

““High” heat production (Proterozoic and younger) High” heat production (Proterozoic and younger) crust (microW/kg) Total U 274 x 10crust (microW/kg) Total U 274 x 10-6-6; Th 281 x ; Th 281 x 1010-6-6; Total K 98.0 x 10; Total K 98.0 x 10-6-6

[Source: Taylor and McLennan, 1985][Source: Taylor and McLennan, 1985]

These values are consistent with the average These values are consistent with the average difference in surface heat flow between Archean difference in surface heat flow between Archean and younger provincesand younger provinces

Data at t=0 from Taylor & McLennan, 1985

Assume only Assume only intrinsic intrinsic

lithospheric lithospheric radiogenic heat radiogenic heat

production production changes with changes with time: Back-time: Back-

calculate calculate geotherms geotherms

through timethrough time

Geotherms vs Time for Archean Crust

0

10

20

30

40

50

60

70

80

90

100

0 500 1000 1500

Temperature, °C

Depth, km

0 Ga

1 Ga

2 Ga

3 Ga

4 Ga

Use back-calculated geotherms to calculate Use back-calculated geotherms to calculate lithospheric strength curves through timelithospheric strength curves through time

Low Heat Production (Archean) 40 km Crust

0

25

50

75

100

0 500 1000 1500 2000 2500

Compressional Yield Strength MPa

Depth km

0 Ma

1 Ma

2 Ma

3 Ma

4 Ma

Calculating Differential Pressure vs Thinner CrustCalculating Differential Pressure vs Thinner Crust

Conclusions 1Conclusions 1

Any evolved crust is likely to Any evolved crust is likely to have been so hot and weak prior have been so hot and weak prior to about 4 Ga that it would have to about 4 Ga that it would have been incapable of withstanding been incapable of withstanding spreading stresses relative to spreading stresses relative to background 20 km crust.background 20 km crust.

If 20 km crust was being If 20 km crust was being subducted, most evolved 20 km subducted, most evolved 20 km crust would be subducted or crust would be subducted or extensively reworked at 4 Gaextensively reworked at 4 Ga

Conclusions 2Conclusions 2 After ~ 4 Ga, low heat production After ~ 4 Ga, low heat production crust would have been able to have crust would have been able to have start generating crust thicker start generating crust thicker than ~30 km with respect to than ~30 km with respect to spreading stresses relative to 20 spreading stresses relative to 20 km crust, but normal heat km crust, but normal heat production crust would still have production crust would still have been too hot and weakbeen too hot and weak

This change possible represents This change possible represents the Hadean to Archean transitionthe Hadean to Archean transition

Conclusion 3Conclusion 3

By 2 to 3 Ga, normal heat By 2 to 3 Ga, normal heat production crust had sufficiently production crust had sufficiently cooled to thicken sufficiently to cooled to thicken sufficiently to form crust thicker than ~30 km form crust thicker than ~30 km stable relative to 20 km thick stable relative to 20 km thick crust, and intrinsic radiogenic crust, and intrinsic radiogenic crustal heat production ceased to crustal heat production ceased to be an important factor in selective be an important factor in selective crustal survivalcrustal survival

This change possibly represents the This change possibly represents the Archean to Proterozoic transitionArchean to Proterozoic transition

finis