Plate Tectonics: Convergence, Subduction, and Lithospheric Dynamics
Convergence: Ocean Between Two Plates
When oceanic lithosphere cools and thickens as it moves from a ridge, it eventually breaks and subducts. Characteristics of subduction zones:
- Great bow
- Intense magmatism resulting in volcanic islands
- Tectonic erosion
- Merging of oceanic crust into the sublithospheric mantle
- Low pressure from the overriding plate on the subducting plate, leading to:
- Easy subduction of sediments
- Formation of a deep ocean trench
Convergence: Oceanic and Continental Lithosphere
Characterized by:
- Subducting plate sinking at a lesser angle
- Magmatism and volcanism
- Obduction
- High pressure from the subducting plate on the overriding plate, leading to:
- Difficult subduction of sediments
- High seismicity
- Thickening of the continental plate
Orogen: Mountain chain formed by compression and thickening of continental lithosphere.
Convergence: Two Continental Plates
Features:
- Interrupted subduction after continental collision
- Both continental lithospheres embed and ride around each other
- Formation of an orogenic suture between the plates
- Presence of ophiolites
- Major faults in the continental lithosphere
- Intense deformation and metamorphism in the suture zone
Sublithospheric Dynamics
Two effects occur on the subducting plate during oceanic subduction:
- Dehydration: The plate loses almost all water absorbed by subducted sediment.
- Partial Melting: Materials become liquid and form magma similar to granite, which rises to the surface.
D’’ and Thermal Plumes
Thermal plumes are columns of lower-density material rising through the lower mantle. Upon reaching the lithosphere’s base, they heat it, creating a hot spot with volcanic activity.
Basalt Plateaus
Hot spot volcanism ejects high-temperature basaltic lava. Areas covered by this lava are called basalt plateaus or provinces.
1. Intraplate Volcanoes Linked to Failure: Islands
- Volcanic activity is discontinuous, unlike with thermal plumes.
- Volcanic activity is unevenly distributed.
- Seismic surveys haven’t detected hot material under the islands, unlike in hot spots.
The accepted hypothesis is that Canarian volcanism results from faults traversing the Atlantic lithosphere, caused by the African and North African plate collision that formed the Atlas Mountains.
Geological Processes in Intraplate Continental Lithosphere
When a thermal plume is under the thicker, colder continental lithosphere, it cannot easily puncture it. The continental lithosphere becomes less dense, expands, and is pushed up by the plume, creating blisters. This uplift causes stretching and fracturing of the crust. Basaltic magmas inject into the fractures (rifts), forming oceanic crust. The rift becomes an incipient ocean, starting the extension process.
1. Wilson Cycle
In 1960, geologist Wilson proposed that oceans eventually close. His theory is based on:
- Rifting: Beginning of continental rupture
- Ocean opening and separation into two continents
- Subduction and approach of continental edges
- Continental collision
2. Rifting in the Iberian Peninsula
The Iberian Peninsula is currently uplifting, evident in coastal and inland areas. Raised river terraces and erosive plains indicate that water systems adjust to ascending plains. There is a clear tendency towards fracturing.