1. Did Modern Subduction Zones Appear in the Late Archean? Kent C. Condie Department of Earth and Environmental Science New Mexico Tech Socorro, NM 87801 USA

5. POSSIBLE PLATE TECTONIC RETURNS 1. Asymmetric Subduction 2. Symmetric Subduction 3. Plate Sinking

7. OPHIOLITES Pillow basalts Sheeted dykes Mafic intrusions Harzburgite-dunite Serpentinite Plagiogranite Podiform chromite Pillow basalts Sheeted dykes Mafic intrusions Harzburgite-dunite Serpentinite Plagiogranite Podiform chromite Pillow basalts Mafic-ultramafic intrusions Picrites Komatiites Minor felsic volcs Pillow basalts Mafic-ultramafic intrusions Picrites Komatiites Minor felsic volcs OCEANIC PLATEAUS OCEANIC PLATEAUS Pillow basalts Mafic intrusions Andesites Breccia-tuff Felsic volcs Graywackes Boninites Pillow basalts Mafic intrusions Andesites Breccia-tuff Felsic volcs Graywackes Boninites ISLAND ARCS ISLAND ARCS

9. LATE ARCHEAN GREENSTONES Mafic Plain (Plume) Types Arc Types Modern Arcs

10. LATE ARCHEAN GREENSTONES Mafic Plain (Plume) Types Arc Types

11. ARC CALDERA VOLCANSIM & LATE PLUME ACTIVITY ABITIBI GREENSTONE BELT 2700 Ma Courtesy of Wulf Mueller

12. Arc Plume n = 1 n = 9 n = 28 n = 25 n = 35 n = 18 > 3 Ga1.0 - 2.5 Ga2.5 - 3 Ga EVOLVING TECTONIC REGIMES

13. OCEANIC PLATEAUS

14. LATE ARCHEAN GREENSTONES Plume-Type Arc-Type

15. Arc Plume n = 1 n = 9 n = 16 n = 11 n = 19 n = 6 > 3 Ga1.0 - 2.5 Ga2.5 - 3 Ga Zr/Nb-Nb/Th in Greenstone Basalts (n = 2)

16. SIGNIFICANCE OF CONVERGENT PLATE ASSEMBLAGES 1)A Few occurrences does NOT necessarily mean modern subduction was global 2)A high frequency of occurrence reflects widespread distribution (BUT this assumes a representative sample)

17. 00.51.01.52.02.53.0 AGE (Ga) TECTONIC SETTINGS IN TIME 3.5 Ga 3.0 Ga 2.0 Ga2.5 Ga ? Oceanic Plateaus Island Arcs Ophiolites

18. FLAT TO STEEP SUBDUCTION Abbott et al. 1994 Geology 22: 937-940 FLAT TO STEEP SUBDUCTION Abbott et al. 1994 Geology 22: 937-940

19. Symmetric Subduction Steep Subduction Flat Subduction > 4.5 Ga ? 3.0 - 2.5 Ga 4.5 - 3.0 Ga

20. Temperature (ºC) Depth (km) Pressure (Gpa)

21. SUMMARY Plate tectonics, in some form, probably existed on planet Earth from the magma ocean stage onwards. Modern type subduction became important beginning in the Late Archean. Ancient tectonic regimes should be constrained only by a convergence of multidisciplinary evidence. It is easier to criticize someone else’s tectonic model than to create your own.