Selective Survival of Crust Paul Morgan Department of Geology Northern Arizona University Flagstaff, Arizona, USA Penrose Conference Lander, WY; June 2006

Conclusions As we go back in time, intrinsic crust radiogenic heat production becomes an important factor in the selective survival of crust. 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. 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 1968: 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? 1928/1929! Arthur Holmes, Proc. Phil. Soc. Edinburgh, 1928/29

Thermal Constraints on Selective Survival of Crust (Lithosphere) UseUse Observations of “average” intrinsic radiogenic heat production in samples of surviving crust 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 ( 232 Th, 235, 238 U, 40 K) Known decay constants for the unstable isotopes that contribute to this radiogenic heat production ( 232 Th, 235, 238 U, 40 K) Laboratory data for estimates of strength parameters of rocks of different compositions to give lithospheric strength profiles Laboratory data for estimates of strength parameters of rocks of different compositions to give lithospheric strength profiles

Calculate Geotherms Geotherms For lithospheres with different crustal thicknesses and heat production as a function of time back to 4.5 GaFor lithospheres with different crustal thicknesses and heat production as a function of time back to 4.5 Ga Crustal strength profiles for these geotherms 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. 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 ; Th 150 x ; Total K 52.5 x “Low” heat production (Archean) crust (microW/kg) Total U 147 x ; Th 150 x ; Total K 52.5 x “High” heat production (Proterozoic and younger) crust (microW/kg) Total U 274 x ; Th 281 x “High” heat production (Proterozoic and younger) crust (microW/kg) Total U 274 x ; Th 281 x ; Total K 98.0 x [Source: Taylor and McLennan, 1985] [Source: Taylor and McLennan, 1985] These values are consistent with the average difference in surface heat flow between Archean and younger provinces 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

Assume only intrinsic lithospheric radiogenic heat production changes with time: Back- calculate geotherms through time

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

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. 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 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 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 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 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 This change possibly represents the Archean to Proterozoic transition

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