Exercises: 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?

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 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.

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.

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.

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.

Two boreholes in the North Atlantic oceanic crust along a profile perpendicular to the Mid-Atlantic Ridge are analyzed:

• Borehole A ($600\ \text{km}$ west of the axis): basaltic crust dated at $40\ \text{Ma}$, covered by $250\ \text{m}$ of pelagic sediments.

• Borehole B ($1\,800\ \text{km}$ west of the axis, adjacent to the North American passive margin): basaltic crust dated at $150\ \text{Ma}$, covered by $3\,200\ \text{m}$ of sediments.

A student asserts that, because the crust at B is almost four times older than at A, the basin spreading rate must have quadrupled in the last $40\ \text{Ma}$.

1. a) Critically evaluate the student's assertion by calculating and comparing the average half-spreading rates in the intervals $0-40\ \text{Ma}$ and $40-150\ \text{Ma}$. Has the opening dynamic remained approximately constant?
2. b) Justify why the sediment thickness at B is more than twelve times greater than at A (not proportional solely to age), relating it to the thermal subsidence of oceanic lithosphere as it matures in the Wilson cycle.
3. c) At margin B, the oceanic lithosphere is already denser than the underlying asthenosphere. Why does it not subduct spontaneously and immediately? Explain what mechanical threshold must be overcome to transition into the closing phase.

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?

The transverse geological cross-section of a typical Atlantic continental margin reveals the following tectonostratigraphic superposition:

• At the base, listric normal faults delimiting tilted blocks (half-grabens) filled with continental red beds and Upper Triassic–Lower Jurassic evaporites.

• Above, sealing the faults and unconformably overlying both the continental blocks and the adjacent basaltic oceanic crust, lies a thick wedge of undeformed Cretaceous and Cenozoic siliciclastic and carbonate marine sediments.

1. a) Deduce which stages of the Wilson cycle correspond to the basal sedimentary sequence ('syn-rift') and the upper sequence ('post-rift' or 'drift'), justifying what changes in the thermal regime and stress state explain this transition.
2. b) A geodynamic model suggests that in the next $30\ \text{Ma}$ this passive margin will transform into an active margin through spontaneous subduction. Discuss the mechanical feasibility of this process: what resisting forces prevent lithospheric rupture, and in which specific area of the margin is the plate most likely to decouple to nucleate subduction?
3. c) Predict the sequence of tectonic, magmatic, and metamorphic events that this margin would undergo if a subduction zone dipping toward the continent were established prior to final collision.

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.

Beneath a young collisional mountain range with an average elevation of $4\,000\ \text{m}$, seismic refraction profiles locate the Mohorovičić discontinuity at a depth of $65\ \text{km}$. In the adjacent undeformed craton, the Moho is at $35\ \text{km}$ and the relief is at sea level. Consider constant average densities of $\rho_c = 2\,700\ \text{kg/m}^3$ for the crust and $\rho_m = 3\,300\ \text{kg/m}^3$ for the lithospheric mantle.

1. a) Apply Airy's principle of isostasy to verify whether the mountain range is in isostatic equilibrium. Analytically calculate what crustal root ($r$) a relief of $4\,000\ \text{m}$ would require and compare the theoretical total crustal thickness with the seismically observed one.
2. b) Over the next $40\ \text{Ma}$, erosion removes a surface rock thickness of $3\,000\ \text{m}$. Considering only the Airy isostatic response to this unloading, without including the readjustment of the excess root detected in the first part, demonstrate why the mountain range will not lose $3\,000\ \text{m}$ of average elevation: calculate the value of the compensating isostatic rebound and the resulting final elevation.
3. c) Reason out the long-term petrological and structural consequences of this process on the orogenic root: what rocks and metamorphic facies will outcrop once the mountain range is turned into a peneplain?

An oceanographic research vessel records magnetic anomalies and bathymetric profiles along a $900\ \text{km}$ transect from an oceanic ridge to the continental slope of a basin lacking ocean trenches. The data reveal:

1. The average half-spreading rate at the ridge has been constant and equal to $1,5\ \text{cm/year}$.

2. At $900\ \text{km}$ from the axis, the oceanic lithosphere has a calculated age and a thermal thickness of $90\ \text{km}$, having cooled enough to acquire a density greater than that of the underlying asthenosphere.

3. The contact between the oceanic and continental crust shows no seismicity or shear deformation.

1. a) Calculate the age of the crust at the contact and deduce which stage of the Wilson cycle the basin is in, justifying why the absence of trenches does not contradict the fact that the oceanic lithosphere is already gravitationally unstable.
2. b) Assume that intraplate compressive stresses induce the rupture of the margin and the onset of subduction. Predict the successive changes that will be recorded in the bathymetry of the margin, the thermal regime, and the deformation style of the adjacent continental crust.
3. c) How will the net drift velocity of the oceanic plate change once the cold lithospheric slab penetrates deeply into the asthenosphere? Justify your answer by comparing ridge push and slab pull forces.

Compare two convergent sectors of the Pacific Ocean margin:

• Sector A (Mariana Trench): subducts Jurassic oceanic lithosphere ($\approx 160\ \text{Ma}$); trench depth close to $11\,000\ \text{m}$; Benioff plane dip $>70^\circ$; moderate plate-locking seismicity and an actively extending back-arc basin with newly formed oceanic lithosphere.

• Sector B (Central Chile Trench): subducts Miocene oceanic lithosphere ($\approx 20\ \text{Ma}$); trench depth of $8\,000\ \text{m}$; Benioff plane dip of $20^\circ-30^\circ$; interplate megathrust earthquakes and a high continental orogen with strong compression.

1. a) Justify how the age and thermal density of the subducting plate determine the differences in the dip of the Benioff plane, the depth of the trench, and the back-arc tectonic regime between both sectors.
2. b) Explain the exact thermodynamic mechanism that triggers partial melting in the upper mantle beneath the Mariana island arc, refuting the misconception that 'magma originates from the frictional melting of the subducting oceanic crust'.
3. c) Based on the balance between lithosphere creation at Pacific ridges and its destruction at the convergent margins bounding it, justify why the Pacific exemplifies the declining stage of the Wilson cycle and predict the global paleogeographic reorganization once its basin is consumed.