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Tomas Naeraa - One of the best experts on this subject based on the ideXlab platform.

  • hafnium isotope evidence for a transition in the dynamics of continental growth 3 2 gyr ago
    Nature, 2012
    Co-Authors: Anthony I S Kemp, Tomas Naeraa, Anders Schersten, Minik T Rosing, J E Hoffmann, Thomas F Kokfelt, Martin J Whitehouse
    Abstract:

    Earth's lithosphere probably experienced an evolution towards the modern plate tectonic regime, owing to secular changes in mantle temperature. Radiogenic isotope variations are interpreted as evidence for the declining rates of continental Crustal growth over time, with some estimates suggesting that over 70% of the present continental Crustal reservoir was extracted by the end of the Archaean eon. Patterns of Crustal growth and reworking in Rocks younger than three billion years (Gyr) are thought to reflect the assembly and break-up of supercontinents by Wilson cycle processes and mark an important change in lithosphere dynamics. In southern West Greenland numerous studies have, however, argued for subduction settings and crust growth by arc accretion back to 3.8 Gyr ago, suggesting that modern-day tectonic regimes operated during the formation of the earliest Crustal Rock record. Here we report in situ uranium-lead, hafnium and oxygen isotope data from zircons of basement Rocks in southern West Greenland across the critical time period during which modern-like tectonic regimes could have initiated. Our data show pronounced differences in the hafnium isotope-time patterns across this interval, requiring changes in the characteristics of the magmatic protolith. The observations suggest that 3.9-3.5-Gyr-old Rocks differentiated from a >3.9-Gyr-old source reservoir with a chondritic to slightly depleted hafnium isotope composition. In contrast, Rocks formed after 3.2 Gyr ago register the first additions of juvenile depleted material (that is, new mantle-derived crust) since 3.9 Gyr ago, and are characterized by striking shifts in hafnium isotope ratios similar to those shown by Phanerozoic subduction-related orogens. These data suggest a transitional period 3.5-3.2 Gyr ago from an ancient (3.9-3.5 Gyr old) Crustal evolutionary regime unlike that of modern plate tectonics to a geodynamic setting after 3.2 Gyr ago that involved juvenile crust generation by plate tectonic processes.

  • hafnium isotope evidence for a transition in the dynamics of continental growth 3 2 gyr ago
    Nature, 2012
    Co-Authors: Anthony I S Kemp, Tomas Naeraa, Anders Schersten, Minik T Rosing, J E Hoffmann, Thomas F Kokfelt, Martin J Whitehouse
    Abstract:

    Hafnium isotope ratios obtained from zircons in southern West Greenland suggest that Earth’s ancient Crustal growth changed around 3.2 Gyr ago to a modern geodynamic regime involving juvenile crust generation by plate tectonic processes. The theory of plate tectonics is fundamental to modern geology, but there is considerable debate as to when the large-scale motions of Earth's crust that it describes actually started. This study tackles the question by measuring in situ uranium–lead, hafnium and oxygen isotope data from zircons of basement Rocks in southern West Greenland across a crucial time period in which the modern plate-tectonic regime might have initiated. The results are consistent with a transition from an ancient Crustal evolutionary regime, unlike that of modern plate tectonics, between 3.9 billion and 3.2 billion years ago. Thereafter a juvenile crust was generated, and plate tectonics as we know it became the norm. Earth’s lithosphere probably experienced an evolution towards the modern plate tectonic regime, owing to secular changes in mantle temperature1,2. Radiogenic isotope variations are interpreted as evidence for the declining rates of continental Crustal growth over time3,4,5, with some estimates suggesting that over 70% of the present continental Crustal reservoir was extracted by the end of the Archaean eon3,5. Patterns of Crustal growth and reworking in Rocks younger than three billion years (Gyr) are thought to reflect the assembly and break-up of supercontinents by Wilson cycle processes and mark an important change in lithosphere dynamics6. In southern West Greenland numerous studies have, however, argued for subduction settings and crust growth by arc accretion back to 3.8 Gyr ago7,8,9, suggesting that modern-day tectonic regimes operated during the formation of the earliest Crustal Rock record. Here we report in situ uranium–lead, hafnium and oxygen isotope data from zircons of basement Rocks in southern West Greenland across the critical time period during which modern-like tectonic regimes could have initiated. Our data show pronounced differences in the hafnium isotope–time patterns across this interval, requiring changes in the characteristics of the magmatic protolith. The observations suggest that 3.9–3.5-Gyr-old Rocks differentiated from a >3.9-Gyr-old source reservoir with a chondritic to slightly depleted hafnium isotope composition. In contrast, Rocks formed after 3.2 Gyr ago register the first additions of juvenile depleted material (that is, new mantle-derived crust) since 3.9 Gyr ago, and are characterized by striking shifts in hafnium isotope ratios similar to those shown by Phanerozoic subduction-related orogens10,11,12. These data suggest a transitional period 3.5–3.2 Gyr ago from an ancient (3.9–3.5 Gyr old) Crustal evolutionary regime unlike that of modern plate tectonics to a geodynamic setting after 3.2 Gyr ago that involved juvenile crust generation by plate tectonic processes.

Martin J Whitehouse - One of the best experts on this subject based on the ideXlab platform.

  • hafnium isotope evidence for a transition in the dynamics of continental growth 3 2 gyr ago
    Nature, 2012
    Co-Authors: Anthony I S Kemp, Tomas Naeraa, Anders Schersten, Minik T Rosing, J E Hoffmann, Thomas F Kokfelt, Martin J Whitehouse
    Abstract:

    Earth's lithosphere probably experienced an evolution towards the modern plate tectonic regime, owing to secular changes in mantle temperature. Radiogenic isotope variations are interpreted as evidence for the declining rates of continental Crustal growth over time, with some estimates suggesting that over 70% of the present continental Crustal reservoir was extracted by the end of the Archaean eon. Patterns of Crustal growth and reworking in Rocks younger than three billion years (Gyr) are thought to reflect the assembly and break-up of supercontinents by Wilson cycle processes and mark an important change in lithosphere dynamics. In southern West Greenland numerous studies have, however, argued for subduction settings and crust growth by arc accretion back to 3.8 Gyr ago, suggesting that modern-day tectonic regimes operated during the formation of the earliest Crustal Rock record. Here we report in situ uranium-lead, hafnium and oxygen isotope data from zircons of basement Rocks in southern West Greenland across the critical time period during which modern-like tectonic regimes could have initiated. Our data show pronounced differences in the hafnium isotope-time patterns across this interval, requiring changes in the characteristics of the magmatic protolith. The observations suggest that 3.9-3.5-Gyr-old Rocks differentiated from a >3.9-Gyr-old source reservoir with a chondritic to slightly depleted hafnium isotope composition. In contrast, Rocks formed after 3.2 Gyr ago register the first additions of juvenile depleted material (that is, new mantle-derived crust) since 3.9 Gyr ago, and are characterized by striking shifts in hafnium isotope ratios similar to those shown by Phanerozoic subduction-related orogens. These data suggest a transitional period 3.5-3.2 Gyr ago from an ancient (3.9-3.5 Gyr old) Crustal evolutionary regime unlike that of modern plate tectonics to a geodynamic setting after 3.2 Gyr ago that involved juvenile crust generation by plate tectonic processes.

  • hafnium isotope evidence for a transition in the dynamics of continental growth 3 2 gyr ago
    Nature, 2012
    Co-Authors: Anthony I S Kemp, Tomas Naeraa, Anders Schersten, Minik T Rosing, J E Hoffmann, Thomas F Kokfelt, Martin J Whitehouse
    Abstract:

    Hafnium isotope ratios obtained from zircons in southern West Greenland suggest that Earth’s ancient Crustal growth changed around 3.2 Gyr ago to a modern geodynamic regime involving juvenile crust generation by plate tectonic processes. The theory of plate tectonics is fundamental to modern geology, but there is considerable debate as to when the large-scale motions of Earth's crust that it describes actually started. This study tackles the question by measuring in situ uranium–lead, hafnium and oxygen isotope data from zircons of basement Rocks in southern West Greenland across a crucial time period in which the modern plate-tectonic regime might have initiated. The results are consistent with a transition from an ancient Crustal evolutionary regime, unlike that of modern plate tectonics, between 3.9 billion and 3.2 billion years ago. Thereafter a juvenile crust was generated, and plate tectonics as we know it became the norm. Earth’s lithosphere probably experienced an evolution towards the modern plate tectonic regime, owing to secular changes in mantle temperature1,2. Radiogenic isotope variations are interpreted as evidence for the declining rates of continental Crustal growth over time3,4,5, with some estimates suggesting that over 70% of the present continental Crustal reservoir was extracted by the end of the Archaean eon3,5. Patterns of Crustal growth and reworking in Rocks younger than three billion years (Gyr) are thought to reflect the assembly and break-up of supercontinents by Wilson cycle processes and mark an important change in lithosphere dynamics6. In southern West Greenland numerous studies have, however, argued for subduction settings and crust growth by arc accretion back to 3.8 Gyr ago7,8,9, suggesting that modern-day tectonic regimes operated during the formation of the earliest Crustal Rock record. Here we report in situ uranium–lead, hafnium and oxygen isotope data from zircons of basement Rocks in southern West Greenland across the critical time period during which modern-like tectonic regimes could have initiated. Our data show pronounced differences in the hafnium isotope–time patterns across this interval, requiring changes in the characteristics of the magmatic protolith. The observations suggest that 3.9–3.5-Gyr-old Rocks differentiated from a >3.9-Gyr-old source reservoir with a chondritic to slightly depleted hafnium isotope composition. In contrast, Rocks formed after 3.2 Gyr ago register the first additions of juvenile depleted material (that is, new mantle-derived crust) since 3.9 Gyr ago, and are characterized by striking shifts in hafnium isotope ratios similar to those shown by Phanerozoic subduction-related orogens10,11,12. These data suggest a transitional period 3.5–3.2 Gyr ago from an ancient (3.9–3.5 Gyr old) Crustal evolutionary regime unlike that of modern plate tectonics to a geodynamic setting after 3.2 Gyr ago that involved juvenile crust generation by plate tectonic processes.

Anthony I S Kemp - One of the best experts on this subject based on the ideXlab platform.

  • hafnium isotope evidence for a transition in the dynamics of continental growth 3 2 gyr ago
    Nature, 2012
    Co-Authors: Anthony I S Kemp, Tomas Naeraa, Anders Schersten, Minik T Rosing, J E Hoffmann, Thomas F Kokfelt, Martin J Whitehouse
    Abstract:

    Earth's lithosphere probably experienced an evolution towards the modern plate tectonic regime, owing to secular changes in mantle temperature. Radiogenic isotope variations are interpreted as evidence for the declining rates of continental Crustal growth over time, with some estimates suggesting that over 70% of the present continental Crustal reservoir was extracted by the end of the Archaean eon. Patterns of Crustal growth and reworking in Rocks younger than three billion years (Gyr) are thought to reflect the assembly and break-up of supercontinents by Wilson cycle processes and mark an important change in lithosphere dynamics. In southern West Greenland numerous studies have, however, argued for subduction settings and crust growth by arc accretion back to 3.8 Gyr ago, suggesting that modern-day tectonic regimes operated during the formation of the earliest Crustal Rock record. Here we report in situ uranium-lead, hafnium and oxygen isotope data from zircons of basement Rocks in southern West Greenland across the critical time period during which modern-like tectonic regimes could have initiated. Our data show pronounced differences in the hafnium isotope-time patterns across this interval, requiring changes in the characteristics of the magmatic protolith. The observations suggest that 3.9-3.5-Gyr-old Rocks differentiated from a >3.9-Gyr-old source reservoir with a chondritic to slightly depleted hafnium isotope composition. In contrast, Rocks formed after 3.2 Gyr ago register the first additions of juvenile depleted material (that is, new mantle-derived crust) since 3.9 Gyr ago, and are characterized by striking shifts in hafnium isotope ratios similar to those shown by Phanerozoic subduction-related orogens. These data suggest a transitional period 3.5-3.2 Gyr ago from an ancient (3.9-3.5 Gyr old) Crustal evolutionary regime unlike that of modern plate tectonics to a geodynamic setting after 3.2 Gyr ago that involved juvenile crust generation by plate tectonic processes.

  • hafnium isotope evidence for a transition in the dynamics of continental growth 3 2 gyr ago
    Nature, 2012
    Co-Authors: Anthony I S Kemp, Tomas Naeraa, Anders Schersten, Minik T Rosing, J E Hoffmann, Thomas F Kokfelt, Martin J Whitehouse
    Abstract:

    Hafnium isotope ratios obtained from zircons in southern West Greenland suggest that Earth’s ancient Crustal growth changed around 3.2 Gyr ago to a modern geodynamic regime involving juvenile crust generation by plate tectonic processes. The theory of plate tectonics is fundamental to modern geology, but there is considerable debate as to when the large-scale motions of Earth's crust that it describes actually started. This study tackles the question by measuring in situ uranium–lead, hafnium and oxygen isotope data from zircons of basement Rocks in southern West Greenland across a crucial time period in which the modern plate-tectonic regime might have initiated. The results are consistent with a transition from an ancient Crustal evolutionary regime, unlike that of modern plate tectonics, between 3.9 billion and 3.2 billion years ago. Thereafter a juvenile crust was generated, and plate tectonics as we know it became the norm. Earth’s lithosphere probably experienced an evolution towards the modern plate tectonic regime, owing to secular changes in mantle temperature1,2. Radiogenic isotope variations are interpreted as evidence for the declining rates of continental Crustal growth over time3,4,5, with some estimates suggesting that over 70% of the present continental Crustal reservoir was extracted by the end of the Archaean eon3,5. Patterns of Crustal growth and reworking in Rocks younger than three billion years (Gyr) are thought to reflect the assembly and break-up of supercontinents by Wilson cycle processes and mark an important change in lithosphere dynamics6. In southern West Greenland numerous studies have, however, argued for subduction settings and crust growth by arc accretion back to 3.8 Gyr ago7,8,9, suggesting that modern-day tectonic regimes operated during the formation of the earliest Crustal Rock record. Here we report in situ uranium–lead, hafnium and oxygen isotope data from zircons of basement Rocks in southern West Greenland across the critical time period during which modern-like tectonic regimes could have initiated. Our data show pronounced differences in the hafnium isotope–time patterns across this interval, requiring changes in the characteristics of the magmatic protolith. The observations suggest that 3.9–3.5-Gyr-old Rocks differentiated from a >3.9-Gyr-old source reservoir with a chondritic to slightly depleted hafnium isotope composition. In contrast, Rocks formed after 3.2 Gyr ago register the first additions of juvenile depleted material (that is, new mantle-derived crust) since 3.9 Gyr ago, and are characterized by striking shifts in hafnium isotope ratios similar to those shown by Phanerozoic subduction-related orogens10,11,12. These data suggest a transitional period 3.5–3.2 Gyr ago from an ancient (3.9–3.5 Gyr old) Crustal evolutionary regime unlike that of modern plate tectonics to a geodynamic setting after 3.2 Gyr ago that involved juvenile crust generation by plate tectonic processes.

J E Hoffmann - One of the best experts on this subject based on the ideXlab platform.

  • hafnium isotope evidence for a transition in the dynamics of continental growth 3 2 gyr ago
    Nature, 2012
    Co-Authors: Anthony I S Kemp, Tomas Naeraa, Anders Schersten, Minik T Rosing, J E Hoffmann, Thomas F Kokfelt, Martin J Whitehouse
    Abstract:

    Earth's lithosphere probably experienced an evolution towards the modern plate tectonic regime, owing to secular changes in mantle temperature. Radiogenic isotope variations are interpreted as evidence for the declining rates of continental Crustal growth over time, with some estimates suggesting that over 70% of the present continental Crustal reservoir was extracted by the end of the Archaean eon. Patterns of Crustal growth and reworking in Rocks younger than three billion years (Gyr) are thought to reflect the assembly and break-up of supercontinents by Wilson cycle processes and mark an important change in lithosphere dynamics. In southern West Greenland numerous studies have, however, argued for subduction settings and crust growth by arc accretion back to 3.8 Gyr ago, suggesting that modern-day tectonic regimes operated during the formation of the earliest Crustal Rock record. Here we report in situ uranium-lead, hafnium and oxygen isotope data from zircons of basement Rocks in southern West Greenland across the critical time period during which modern-like tectonic regimes could have initiated. Our data show pronounced differences in the hafnium isotope-time patterns across this interval, requiring changes in the characteristics of the magmatic protolith. The observations suggest that 3.9-3.5-Gyr-old Rocks differentiated from a >3.9-Gyr-old source reservoir with a chondritic to slightly depleted hafnium isotope composition. In contrast, Rocks formed after 3.2 Gyr ago register the first additions of juvenile depleted material (that is, new mantle-derived crust) since 3.9 Gyr ago, and are characterized by striking shifts in hafnium isotope ratios similar to those shown by Phanerozoic subduction-related orogens. These data suggest a transitional period 3.5-3.2 Gyr ago from an ancient (3.9-3.5 Gyr old) Crustal evolutionary regime unlike that of modern plate tectonics to a geodynamic setting after 3.2 Gyr ago that involved juvenile crust generation by plate tectonic processes.

  • hafnium isotope evidence for a transition in the dynamics of continental growth 3 2 gyr ago
    Nature, 2012
    Co-Authors: Anthony I S Kemp, Tomas Naeraa, Anders Schersten, Minik T Rosing, J E Hoffmann, Thomas F Kokfelt, Martin J Whitehouse
    Abstract:

    Hafnium isotope ratios obtained from zircons in southern West Greenland suggest that Earth’s ancient Crustal growth changed around 3.2 Gyr ago to a modern geodynamic regime involving juvenile crust generation by plate tectonic processes. The theory of plate tectonics is fundamental to modern geology, but there is considerable debate as to when the large-scale motions of Earth's crust that it describes actually started. This study tackles the question by measuring in situ uranium–lead, hafnium and oxygen isotope data from zircons of basement Rocks in southern West Greenland across a crucial time period in which the modern plate-tectonic regime might have initiated. The results are consistent with a transition from an ancient Crustal evolutionary regime, unlike that of modern plate tectonics, between 3.9 billion and 3.2 billion years ago. Thereafter a juvenile crust was generated, and plate tectonics as we know it became the norm. Earth’s lithosphere probably experienced an evolution towards the modern plate tectonic regime, owing to secular changes in mantle temperature1,2. Radiogenic isotope variations are interpreted as evidence for the declining rates of continental Crustal growth over time3,4,5, with some estimates suggesting that over 70% of the present continental Crustal reservoir was extracted by the end of the Archaean eon3,5. Patterns of Crustal growth and reworking in Rocks younger than three billion years (Gyr) are thought to reflect the assembly and break-up of supercontinents by Wilson cycle processes and mark an important change in lithosphere dynamics6. In southern West Greenland numerous studies have, however, argued for subduction settings and crust growth by arc accretion back to 3.8 Gyr ago7,8,9, suggesting that modern-day tectonic regimes operated during the formation of the earliest Crustal Rock record. Here we report in situ uranium–lead, hafnium and oxygen isotope data from zircons of basement Rocks in southern West Greenland across the critical time period during which modern-like tectonic regimes could have initiated. Our data show pronounced differences in the hafnium isotope–time patterns across this interval, requiring changes in the characteristics of the magmatic protolith. The observations suggest that 3.9–3.5-Gyr-old Rocks differentiated from a >3.9-Gyr-old source reservoir with a chondritic to slightly depleted hafnium isotope composition. In contrast, Rocks formed after 3.2 Gyr ago register the first additions of juvenile depleted material (that is, new mantle-derived crust) since 3.9 Gyr ago, and are characterized by striking shifts in hafnium isotope ratios similar to those shown by Phanerozoic subduction-related orogens10,11,12. These data suggest a transitional period 3.5–3.2 Gyr ago from an ancient (3.9–3.5 Gyr old) Crustal evolutionary regime unlike that of modern plate tectonics to a geodynamic setting after 3.2 Gyr ago that involved juvenile crust generation by plate tectonic processes.

Anders Schersten - One of the best experts on this subject based on the ideXlab platform.

  • hafnium isotope evidence for a transition in the dynamics of continental growth 3 2 gyr ago
    Nature, 2012
    Co-Authors: Anthony I S Kemp, Tomas Naeraa, Anders Schersten, Minik T Rosing, J E Hoffmann, Thomas F Kokfelt, Martin J Whitehouse
    Abstract:

    Earth's lithosphere probably experienced an evolution towards the modern plate tectonic regime, owing to secular changes in mantle temperature. Radiogenic isotope variations are interpreted as evidence for the declining rates of continental Crustal growth over time, with some estimates suggesting that over 70% of the present continental Crustal reservoir was extracted by the end of the Archaean eon. Patterns of Crustal growth and reworking in Rocks younger than three billion years (Gyr) are thought to reflect the assembly and break-up of supercontinents by Wilson cycle processes and mark an important change in lithosphere dynamics. In southern West Greenland numerous studies have, however, argued for subduction settings and crust growth by arc accretion back to 3.8 Gyr ago, suggesting that modern-day tectonic regimes operated during the formation of the earliest Crustal Rock record. Here we report in situ uranium-lead, hafnium and oxygen isotope data from zircons of basement Rocks in southern West Greenland across the critical time period during which modern-like tectonic regimes could have initiated. Our data show pronounced differences in the hafnium isotope-time patterns across this interval, requiring changes in the characteristics of the magmatic protolith. The observations suggest that 3.9-3.5-Gyr-old Rocks differentiated from a >3.9-Gyr-old source reservoir with a chondritic to slightly depleted hafnium isotope composition. In contrast, Rocks formed after 3.2 Gyr ago register the first additions of juvenile depleted material (that is, new mantle-derived crust) since 3.9 Gyr ago, and are characterized by striking shifts in hafnium isotope ratios similar to those shown by Phanerozoic subduction-related orogens. These data suggest a transitional period 3.5-3.2 Gyr ago from an ancient (3.9-3.5 Gyr old) Crustal evolutionary regime unlike that of modern plate tectonics to a geodynamic setting after 3.2 Gyr ago that involved juvenile crust generation by plate tectonic processes.

  • hafnium isotope evidence for a transition in the dynamics of continental growth 3 2 gyr ago
    Nature, 2012
    Co-Authors: Anthony I S Kemp, Tomas Naeraa, Anders Schersten, Minik T Rosing, J E Hoffmann, Thomas F Kokfelt, Martin J Whitehouse
    Abstract:

    Hafnium isotope ratios obtained from zircons in southern West Greenland suggest that Earth’s ancient Crustal growth changed around 3.2 Gyr ago to a modern geodynamic regime involving juvenile crust generation by plate tectonic processes. The theory of plate tectonics is fundamental to modern geology, but there is considerable debate as to when the large-scale motions of Earth's crust that it describes actually started. This study tackles the question by measuring in situ uranium–lead, hafnium and oxygen isotope data from zircons of basement Rocks in southern West Greenland across a crucial time period in which the modern plate-tectonic regime might have initiated. The results are consistent with a transition from an ancient Crustal evolutionary regime, unlike that of modern plate tectonics, between 3.9 billion and 3.2 billion years ago. Thereafter a juvenile crust was generated, and plate tectonics as we know it became the norm. Earth’s lithosphere probably experienced an evolution towards the modern plate tectonic regime, owing to secular changes in mantle temperature1,2. Radiogenic isotope variations are interpreted as evidence for the declining rates of continental Crustal growth over time3,4,5, with some estimates suggesting that over 70% of the present continental Crustal reservoir was extracted by the end of the Archaean eon3,5. Patterns of Crustal growth and reworking in Rocks younger than three billion years (Gyr) are thought to reflect the assembly and break-up of supercontinents by Wilson cycle processes and mark an important change in lithosphere dynamics6. In southern West Greenland numerous studies have, however, argued for subduction settings and crust growth by arc accretion back to 3.8 Gyr ago7,8,9, suggesting that modern-day tectonic regimes operated during the formation of the earliest Crustal Rock record. Here we report in situ uranium–lead, hafnium and oxygen isotope data from zircons of basement Rocks in southern West Greenland across the critical time period during which modern-like tectonic regimes could have initiated. Our data show pronounced differences in the hafnium isotope–time patterns across this interval, requiring changes in the characteristics of the magmatic protolith. The observations suggest that 3.9–3.5-Gyr-old Rocks differentiated from a >3.9-Gyr-old source reservoir with a chondritic to slightly depleted hafnium isotope composition. In contrast, Rocks formed after 3.2 Gyr ago register the first additions of juvenile depleted material (that is, new mantle-derived crust) since 3.9 Gyr ago, and are characterized by striking shifts in hafnium isotope ratios similar to those shown by Phanerozoic subduction-related orogens10,11,12. These data suggest a transitional period 3.5–3.2 Gyr ago from an ancient (3.9–3.5 Gyr old) Crustal evolutionary regime unlike that of modern plate tectonics to a geodynamic setting after 3.2 Gyr ago that involved juvenile crust generation by plate tectonic processes.