23 - Plate Tectonics and Geological Time

Onset of Plate Tectonics

  • Definition and Introduction: The onset of plate tectonics marks a significant shift in geological processes and has occurred over various geological timescales.

Geological Time Scale

  • 0 Ga (Present Day): Start of the Phanerozoic eon, which encompasses the most recent geological time period, characterized by abundant fossil evidence.

    • 0.54 Ga: Onset of the Neoproterozoic era.

    • 0.30 Ga: Assembly of the supercontinent Pangaea.

    • 0.54 Ga: Assembly of the Gondwana supercontinent.

    • 0.62 Ga: Oldest unequivocal ultrahigh-pressure (UHP) eclogite samples are dated from this period.

    • 0.90 Ga: Earliest knees in the geological record, associated with blueschist formations.

Supercontinent Assemblies

  • 1.23 Ga: Rodinia supercontinent assembly occurs during the Mesoproterozoic.

  • 1.80 Ga: Columbia (or Nuna) supercontinent assembly takes place.

  • 2.70 Ga: Formation of Kenorland supercontinent.

  • 2.10 Ga: Oldest unequivocal subduction-related high-pressure (HP) eclogite discovered.

  • 2.80 Ga: Debated evidence of subduction-related HP eclogite.

Archean Eon

  1. Age Segmentation:

    • Neoarchean: ~2.8 Ga

    • Mesoarchean: ~3.2 Ga

    • Paleoarchean: ~3.6 Ga

    • Eoarchean: ~4.0 Ga

  2. Research Findings:

    • 3.2 Ga: Brenner et al. (2020) findings.

    • 3.5 Ga: Greber et al. (2017) findings.

    • 3.8 Ga: Komiya et al. (1998) findings.

    • 4.0 Ga: Maruyama et al. (2018) results.

    • 4.2 Ga: Hopkins et al. (2008) findings.

  3. Hadean Eon:

    • 4.56 Ga: Beginning of the Hadean, signifying the Earth's formation.

    • 4.02 Ga: Discovery of the oldest unequivocal crust (Acasta gneiss).

    • 4.28 Ga: Oldest debated crust found in the Nuvvuagittuq belt.

    • 4.40 Ga: Oldest terrestrial crystals (Jack Hills zircon) recorded.

Geodynamic Regimes

  • Two Fundamental End-Member Geodynamic Regimes:

    • Mobile Lid Regime:

    • Characterized by high surface mobility.

    • Surface velocity ranges from 0.8 to 1.8 times that of the internal velocity.

    • Plate tectonics exemplifies this regime.

    • Stagnant Lid Regime:

    • Displays limited horizontal movements at the surface with restricted active yielding.

    • Vertical mass transport occurs, allowing limited mixing between the surface and the interior.

Mantle Potential Temperature Data

  • Temperature Trends in Geological Eras

    • Phanerozoic: 1800 °C to 1200 °C.

    • Proterozoic: Gradation discussed in Korenaga et al. (2008a, b); Condie et al. (2016); Davies (2009); and other research.

    • Archean: Varied mantle temperatures across geological layers.

MgO Content and Crustal Composition

  • MgO Content Observations:

    • Low-MgO pillow lava: ~7–14 wt%.

    • Archean basaltic and komatiitic lavas: Shares notable characteristics with sheeted dykes found in Yellowknife.

    • Gabbro and troctolite compositions noted with varying depths and associated MgO content.

  • Crust Composition Variation:

    • Comparisons of current juvenile continental crust distribution derived from various studies including Dhuime et al. (2012) and Spencer et al. (2017) showing growth trends adjusted for biases.

Metamorphic Gradients and T/P

  • Apparent Peak Metamorphic T/P Gradient Analysis:

    • Measured Gradients:

    • Granulite and UHT: 760 °C/GPa

    • Blueschist and eclogite: 440 °C/GPa

  • Graphical Representation and Depth Considerations:

    • Various types of eclogites across geological formations presented with their respective ages demonstrate distinct thermal and pressure gradients.

Volcanism and Lithospheric Dynamics

  • Volcanism Categories and Implications:

    • Different types of volcanism driving geological processes include submarine and island arc volcanism.

    • Important interactions between hydrated oceanic crust and the mantle's dynamics are described, including water (H₂O) roles in the subduction processes.

  • Tectonic Framework:

    • The formation conditions and settings (low-density vs. high-density) impacting lithospheric architecture and metamorphosed materials examined.

Zircon U-Pb Age Distribution

  • Zircon Analysis:

    • Global zircon U-Pb age distribution with implications for the understanding of modern-day tectonics.

  • Evolutionary Milestones:

    • The period dubbed the "Boring Billion" denotes the onset of mobile-lid tectonics approximately between 3.0 and 2.9 Ga, showcasing fundamental tectonic shifts in Earth's geological history.