Wilson Cycle Notes

Wilson Cycle: Opening and Closing of Ocean Basins

Based on J. Tuzo Wilson's 1968 publication, the Wilson Cycle describes the cyclical process of ocean basin formation, expansion, contraction, and eventual closure over geological time. Each stage is characterized by distinct rock types and structures.

Stages of the Wilson Cycle:

  1. Embryonic Stage:

    • Characterized by uplift and crustal extension in continental areas.
    • Formation of rift valleys due to tensional forces.
    • Example: The East African Rift System is a prime example of this stage.

  2. Young Stage:

    • Rift valleys evolve into spreading centers.
    • Thin strips of oceanic crust begin to form between rifted continental segments.
    • Results in a narrow, parallel-sided sea.
    • Example: The Red Sea, which is opening between Northeast Africa and Arabia.

  3. Mature Stage:

    • The basin widens, developing into a major ocean.
    • Continental shelves flank the ocean.
    • Continuous production of new, hot oceanic crust along the mid-ocean ridge system.
    • Example: The Atlantic Ocean is in a mature stage.

  4. Declining Stage:

    • The expanding system becomes unstable.
    • Oldest oceanic lithosphere sinks back into the asthenosphere, forming an oceanic trench subduction system.
    • Associated island arcs or Andean-type volcanism may develop.
    • The onset of subduction at the ocean boundary marks this stage.
    • Example: The Pacific Ocean exhibits this declining stage.

  5. Terminal Stage:

    • Subduction rate exceeds the formation of new crust at the constructive boundary (mid-ocean ridge).
    • The ocean begins to contract.
    • Island arc complexes collide, forming young mountain ranges around the periphery of the contracting ocean.
    • Example: The Mediterranean Sea is in the terminal stage.

  6. End Stage:

    • All oceanic crust between continental masses has subducted.
    • Continents converge, resulting in a collision zone.
    • An active fold mountain belt, like the Himalayas, characterizes this stage.
    • Relict scars, known as sutures, remain as evidence of the former ocean basin.

  7. Suture/Inactive Stage:

    • The plate boundary becomes inactive after continental collision.
    • The suture zone, the site of the join between continental masses, represents a zone of weakness in the lithosphere.
    • This zone has the potential to become the site of a new rift, initiating a new Wilson Cycle.

  8. Erosion Stage:

    • The continent erodes, thinning the crust.
    • Eventually, the process may begin again.

Key Differences Between Stages:

  • Stage 1 vs. Stage 2: Stage 1 involves continental rifting and thinning without the creation of oceanic crust (e.g., East African Rift), while Stage 2 involves the formation of a narrow sea with oceanic crust (e.g., Red Sea).
  • Stage 3: The ocean widens due to seafloor spreading at the mid-ocean ridge (MOR) (e.g., Atlantic Ocean).
  • Stage 4: The ocean shrinks because of a subduction zone, but the MOR remains active (sea floor spreads). However, the rate of generation of oceanic lithosphere is less than the rate of subduction of the oceanic lithosphere. The distance between the MOR and the trench (subduction zone) diminishes (e.g., the Pacific Ocean).
  • Stage 5: Eventually the MOR also get subducted (e.g., the Mediterranean Sea).
  • Final Stage: All oceanic lithosphere gets subducted, and two continents collide to form mountain chains like the Himalayas (formed by the collision of India with Tibet after the Tethys Ocean was completely subducted).

Super-Continent Cycle

  • The geologic time scale shows some important events in the history of the continents. (Ma, million years ago.)