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

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Presentation transcript:

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

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

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

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

LATE ARCHEAN GREENSTONES Mafic Plain (Plume) Types Arc Types

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

Arc Plume n = 1 n = 9 n = 28 n = 25 n = 35 n = 18 > 3 Ga Ga Ga EVOLVING TECTONIC REGIMES

OCEANIC PLATEAUS

LATE ARCHEAN GREENSTONES Plume-Type Arc-Type

Arc Plume n = 1 n = 9 n = 16 n = 11 n = 19 n = 6 > 3 Ga Ga Ga Zr/Nb-Nb/Th in Greenstone Basalts (n = 2)

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)

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

FLAT TO STEEP SUBDUCTION Abbott et al Geology 22: FLAT TO STEEP SUBDUCTION Abbott et al Geology 22:

Symmetric Subduction Steep Subduction Flat Subduction > 4.5 Ga ? Ga Ga

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

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.