Evolution and Growth of Laurentia: Proterozoic Earth History

1. Chapter 9 Precambrian Earth History—The Proterozoic Eon

2. The Length of the Proterozoic • ~2 billion years • 42.5% of all geologic time • yet we review this long episode of Earth and life history very briefly

3. Archean-Proterozoic Boundary • Geologists have rather arbitrarily placed the Archean-Proterozoic boundary at 2.5 billion years ago • it marks the approximate time of changes in the style of crustal evolution

4. Age of Continental Crust

5. Archean vs. Proterozoic • Proterozoic is characterized by • differing crustal evolution • less metamorphism • plate tectonics similar to the present • less heat!

6. Evolution of Continents • Archean cratons assembled during collisions of island arcs and minicontinents • nuclei around which Proterozoic crust accreted • much larger landmasses formed • Proterozoic accretion at craton margins probably took place more rapidly than today • because Earth still possessed more internal heat • but the process continues even now

7. Focus on Laurentia • Large landmass that consisted of what is now: • North America • Greenland • parts of northwestern Scotland • some of the Baltic shield of Scandinavia • Laurentia originated and underwent important growth between 2.0 and 1.8 billion years ago • During this time, collisions among various plates formed several orogens • linear or arcuate deformation belts in which many of the rocks have been metamorphosed and intruded by magma

8. Proterozoic Evolution of Laurentia • Archean cratons were sutured • along deformation belts called orogens, • thereby forming a larger landmass • By 1.8 billion years ago, much of what is now Greenland, central Canada, and the north-central United States existed • Laurentia grew along its southern margin by accretion

9. Southern Margin Accretion • Laurentia grew along its southern margin • by accretion of the Central Plains, Yavapai, and Mazatzal orogens

10. Grenville Orogeny • A final episode of Proterozoic accretion occurred during the Grenville orogeny

11. Building North America • By this final stage, about 75% of present-day North America existed • The remaining 25% accreted along its margins, particularly its eastern and western margins, during the Phanerozoic Eon

12. Style of Plate Tectonics • The present style of plate tectonics • involving opening and then closing ocean basins • had almost certainly been established by the Early Proterozoic • In fact, the oldest known complete ophiolite • providing evidence for an ancient convergent plate boundary • is the Jormua complex in Finland • It is about 1.96 billion years old • similar to younger well-documented ophiolites

13. Jormua Complex, Finland • Metamorphosed basaltic pillow lava

14. Early Supercontinent • Possible configuration of the Late Proterozoic supercontinent Rodinia • before it began fragmenting about 750 million years ago

15. Ancient Glaciers • Very few times of widespread glacial activity have occurred during Earth history • Most recent one during the Pleistocene (1.6 Ma - 10 Ka): The Ice Age • we also have evidence for Pennsylvanian glaciers • two major episodes of Proterozoic glaciation • How do we recognize past glacial periods?

16. Proterozoic Glacial Evidence • Bagganjarga tillite in Norway • overlies striated bedrock surface

17. Ediacaran Fauna • The Ediacaran fauna of Australia Tribrachidium heraldicum, a possible primitive echinoderm Spriggina floundersi, a possible ancestor of trilobites

18. Ediacaran Fauna Pavancorina minchami • Restoration of the Ediacaran Environment

19. Ediacaran Fauna • Geologists had assumed that the fossils so common in Cambrian rocks must have had a long previous history • little evidence to support this conclusion • The discovery of Ediacaran fossils dramatically increased our knowledge about this chapter in the history of life

20. Represented Phyla • Three present-day phyla may be represented in the Ediacaran fauna: • jellyfish and sea pens (phylum Cnidaria) • segmented worms (phylum Annelida) • primitive members of the phylum Arthropoda (the phylum with insects, spiders crabs, and others) • One Ediacaran fossil, Spriggina, has been cited as a possible ancestor of trilobites • Another might be a primitive member of the phylum Echinodermata

21. Distinct Evolutionary Group • However, some scientists think these Ediacaran animals represent an early evolutionary group quite distinct from the ancestry of today’s invertebrate animals • Ediacara-type faunas are known from all continents except Antarctica • collectively referred to as the Ediacaran fauna • widespread between 545 and 670 million years ago • fossils are rare (lacked durable skeletons)