The Wilson cycle

What is the Wilson Cycle? State which geological process it begins with and which event it ends with.

The Red Sea and the Atlantic Ocean represent different stages of the Wilson Cycle. State which of the two is at an earlier stage and justify your answer.

What is a continental rift and at what type of plate boundary does it originate?

What is an oceanic ridge and at which stages of the Wilson cycle does it appear? What type of rock is generated at it?

The Atlantic is considered a "mature" ocean. Name two characteristics that justify this consideration, such as its width or the type of margins that border it.

What is a passive continental margin and how does it differ from an active margin? Relate each type to a stage of the Wilson cycle.

The East African Rift valley shows volcanism, grabens and elongated lakes. Explain which stage of the Wilson cycle it represents and what could eventually happen if extension continues for millions of years.

Which two types of plate boundary are involved, respectively, in the opening and in the closing of an ocean within the Wilson cycle? Justify your answer.

What is the maximum age of the rocks of the present-day ocean floor and why are there no ocean floors as old as the oldest cores of the continents?

The five panels in the diagram (a–e) show five stages of the Wilson Cycle out of order. Place them in chronological order, from the opening of a rift to the formation of a mountain range, and give a present-day example of each stage.

The Pacific Ocean is the largest on Earth and yet it is considered a declining ocean within the Wilson Cycle. Explain what is happening at its margins to justify this statement.

When subduction completely consumes the ocean floor separating two continents, what event brings the cycle to an end and what type of landform is produced? Give a present-day example.

Ophiolites are slices of oceanic crust and mantle incorporated into mountain ranges. Explain what they are, how they reached dry land (obduction) and what they demonstrate about the existence of vanished oceans.

An ocean opens at a rate of $3\ \text{cm/year}$ per flank for $60\ \text{Ma}$. Calculate the width reached at the end of that period, expressed in kilometres, and comment on which present-day ocean it would approach.

Mélanges, ophiolites and high-pressure metamorphic rocks are described at the sutures of ancient collisions. Explain what a suture zone is and why those three elements reveal a closed ocean.

The Mediterranean retains active subduction in Calabria and in the Hellenic arc. Place this sea within the Wilson cycle and explain what evidence allows it to be assigned to that stage.

A geological prospecting team maps within a continental cratonic mass a linear belt several hundred kilometers long where the following coexist:

1. An alignment of serpentinized peridotites, layered gabbros, and pillow lavas embedded within deformed pelagic sediments.

2. Blueschists and garnet-bearing eclogites marking high-pressure, low-temperature metamorphism ($P > 1,5\ \text{GPa}$, $T < 450\ ^\circ\text{C}$).

3. On both sides of this belt, shallow continental shelf sedimentary rocks containing Paleozoic marine fossils that belong to completely distinct biological provinces with no connection between them.

1. a) Demonstrate, reasoning from each of these three lines of evidence, why this alignment represents a fossil suture zone corresponding to the complete closure of a Wilson cycle (as in the Urals or the Appalachians) and not a simple intraplate fracture or incipient continental rift.
2. b) Reconstruct the relative chronology of the tectonic processes that took place in this region, explaining what additional data (for example, the position of remnants of a magmatic arc or an accretionary prism relative to the suture) would allow the initial polarity of the subduction to be established, and analysing the role of lithospheric buoyancy in the culmination of the orogen.

The Mediterranean Sea is commonly classified in the Wilson cycle as a basin in a stage of senescence or terminal decline. However, current geodynamic data show an apparent contradiction:

1. In the Eastern Mediterranean, relics of Mesozoic oceanic lithosphere persist, being consumed at the Hellenic trench.

2. In the Western Mediterranean (Tyrrhenian Sea and Balearic Basin), there are small basins with very young oceanic crust (less than $10\ \text{Ma}$) and active lithospheric thinning.

3. Convergence between Africa and Eurasia is very slow (scarcely $4-5\ \text{mm/yr}$ towards the NW).

1. a) Resolve the apparent paradox: how can the opening of back-arc oceanic basins coexist with an overall context of closure and continental convergence?
2. b) Predict the geodynamic evolution of the Mediterranean over the next $50\ \text{Ma}$: detail what will happen to the oceanic relics, to the marginal basins, and what type of orogen will result from the final collision between North Africa and Southern Europe.
3. c) What rock types and tectonic structures do you expect to find in the future orogen as evidence of the ancient ocean floor and the present-day Aeolian volcanic arc?

Along a cross-section of the Central and Western Alps, three major superimposed tectonic units are identified in the form of thrust nappes:

1. At the base, sheets of deformed European continental crust with intermediate-pressure metamorphism.

2. Thrusting over the European crust, a complex of greenschists, blueschists, and eclogites with serpentinized peridotites and metaradiolarites.

3. At the top, thrust sheets containing continental rocks derived from the Apulian microcontinent (African promontory).

1. a) Interpret the origin of the intermediate unit: why do eclogite-facies rocks (high pressure and low temperature) coexist with sequences of oceanic affinity, and which stage of the Wilson cycle do they record?
2. b) Justify why there is currently no active subduction volcanism in the Alpine range (unlike in the Andes), deducing the exact stage of the Wilson cycle in which the Alps are today.
3. c) Given that oceanic lithosphere is dense and tends to sink into the mantle, explain the physical mechanism by which fragments of the ancient Tethys seafloor managed to rise to an elevation of more than $3\,000\ \text{m}$ in the mountain range instead of being entirely subducted.