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Selective Survival of Crust

Selective Survival of Crust. Paul Morgan Department of Geology Northern Arizona University Flagstaff, Arizona, USA Penrose Conference Lander, WY; 14-18 June 2006. Conclusions.

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Selective Survival of Crust

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  1. Selective Survival of Crust Paul Morgan Department of Geology Northern Arizona University Flagstaff, Arizona, USA Penrose Conference Lander, WY; 14-18 June 2006

  2. 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.

  3. 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

  4. When did Plate Tectonics Start? 1928/1929! Arthur Holmes, Proc. Phil. Soc. Edinburgh, 1928/29

  5. 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

  6. 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.

  7. 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

  8. Data at t=0 from Taylor & McLennan, 1985

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

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

  11. Calculating Differential Pressure vs Thinner Crust

  12. 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

  13. 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

  14. 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

  15. finis

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