Accretionary Wedge (aiMOOC)
Accretionary Wedge (aiMOOC)
Introduction
An accretionary wedge, also called an accretionary prism, forms at a convergent plate boundary where an oceanic plate is forced beneath another plate. As the denser plate subducts into the mantle, the top layers of sediment and crust are scraped off and plastered onto the edge of the overriding plate. This pile of deformed rock and sediment builds a wedge‑shaped body that may rise above sea level to form coastal mountain ranges. Accretionary wedges record the history of plate convergence and help geologists understand tectonic processes and earthquake hazards.
Formation and structure
When two plates converge, the down‑going plate carries sediments, oceanic crust and sometimes fragments of volcanic island arcs. At the trench, the high pressure and friction cause some of this material to peel off and stack against the overriding plate. Successive layers of scraped‑off sediment form thrust sheets that wedge one on top of another. As the wedge grows, it tends toward a stable triangular cross‑section known as the critical taper. The base of the wedge is a fault surface called the décollement, along which the wedge glides as it thickens.
Materials and composition
Accretionary wedges are composed of a mélange of rock types. They typically include ocean‑floor basalts, pelagic clays and siliceous ooze, turbidite sands transported from continents, volcanic ash from island arcs and fragments of the eroded forearc ridge. High‑pressure metamorphic rocks may be thrust to the surface within the wedge. In some cases, elevated portions of seafloor known as terranes become accreted to the continental margin, widening the landmass over geologic time.
Associated landforms and basins
Immediately behind an accretionary wedge lies a forearc basin—a low area that collects sediments eroded from both the wedge and the volcanic arc inland. Farther back lies the volcanic arc itself, formed when fluids released from the subducting slab melt the overlying mantle. Together the accretionary wedge, forearc basin and volcanic arc create a characteristic double‑range topography found in regions like the Cascadia subduction zone. These features provide insight into the geometry of subduction and the potential for large earthquakes and tsunamis.
Significance and examples
Accretionary wedges are important archives of plate boundary processes. They contain fossils, sediments and metamorphic rocks that record the environment of deposition and the history of deformation. Studying them helps geologists assess seismic hazard because the wedge can lock against the subducting plate and then slip suddenly during an earthquake. Notable accretionary wedges include the Barbados prism in the Caribbean, the Nankai Trough off Japan, and the Cascadia accretionary complex along the Pacific Northwest of North America. Ancient accretionary wedges have contributed to building continents such as the western margin of North America.
Interactive Tasks
Quiz: Test Your Knowledge
Where do accretionary wedges form? (At convergent plate boundaries where one plate subducts beneath another) (!At divergent plate boundaries where plates move apart) (!Along transform faults) (!On mid‑ocean ridges)
What happens to sediments on the subducting plate as it enters the trench? (They are scraped off and added to the overriding plate) (!They melt immediately) (!They stay attached and descend into the mantle) (!They are eroded by surface waves)
What shape does a mature accretionary wedge tend to have? (A triangular wedge with a critical taper) (!A circular dome) (!A flat plain) (!A spherical body)
Which of the following is NOT commonly found in an accretionary wedge? (Granite plutons from continental interiors) (!Ocean‑floor basalts) (!Pelagic sediments) (!Turbidite sands)
What lies between the accretionary wedge and the volcanic arc? (A forearc basin) (!A mid‑ocean ridge) (!A rift valley) (!An ocean trench)
Why are accretionary wedges important for assessing earthquake hazards? (They can lock with the subducting plate and release energy during earthquakes) (!They shield continents from tsunamis) (!They generate surface waves that calm the ocean) (!They eliminate volcanic activity)
Which example represents an active accretionary wedge? (The Cascadia subduction zone off western North America) (!The East African Rift) (!The Mid‑Atlantic Ridge) (!The Great Rift Valley in Iceland)
What is a terrane in the context of accretionary wedges? (A fragment of crust or island arc accreted to a continent) (!A type of volcanic lava) (!A deep‑sea trench) (!An oceanic current)
What is the décollement? (The basal fault surface along which the wedge glides) (!A chain of volcanoes in the volcanic arc) (!A mountain range formed by erosion) (!A sediment layer in the forearc basin)
Which region has an ancient accretionary wedge that added significant width to North America? (The western margin of North America) (!The east coast of Africa) (!The Australian interior) (!The Mediterranean Sea)
Memory
| Convergent boundary | Where two tectonic plates collide |
| Sediment scraping | Material peeled from a subducting plate |
| Forearc basin | Low area between wedge and volcanic arc |
| Critical taper | Stable triangular shape of a mature wedge |
| Décollement | Basal fault surface of the wedge |
Drag and Drop
| Assign the correct terms | Accretionary Features |
|---|---|
| Convergent boundary | Where accretionary wedges form |
| Sediment scraping | Process of removing material from the downgoing plate |
| Forearc basin | Depression between the wedge and volcanic arc |
| Terrane | Fragment of crust accreted to a continent |
| Décollement | Fault surface along which the wedge slides |
Crossword Puzzle
| Accretionary | Wedge of scraped‑off sediments at a subduction zone |
| Convergence | Meeting of tectonic plates causing subduction |
| Mélange | Chaotic mix of rock types in an accretionary wedge |
| Forearc | Basin between the wedge and volcanic arc |
| Décollement | Basal fault plane of the wedge |
| Terrane | Crustal block added to a continent |
LearningApps
Cloze Text
Open Tasks
Easy
- Illustrate a subduction zone: Draw and label the trench, accretionary wedge, forearc basin and volcanic arc.
- Plate boundary research: Identify modern convergent boundaries around the world and note which have accretionary wedges.
- Sediment experiment: Use layers of wet sand and clay to model how thrust sheets can stack and form a wedge when compressed.
Standard
- Case study analysis: Investigate a specific accretionary wedge (e.g., Nankai Trough) and summarise its structure and associated hazards.
- Historical terranes: Research how terranes accreted to western North America contributed to mountain building.
- Forearc basin sediments: Find out what kinds of fossils and sediments are typically preserved in forearc basins and what they tell us about past environments.
Hard
- Seismic hazard evaluation: Explain how locking and slipping along the décollement can generate megathrust earthquakes and tsunamis.
- Tectonic reconstruction: Using geological evidence, reconstruct the sequence of accretion events that built part of a continent.
- Resource assessment: Discuss how accretionary wedges may host hydrocarbons or mineral deposits and evaluate the risks of exploration.
Learning control
- Process comprehension: Summarise how oceanic crust, sediments and island arcs are incorporated into an accretionary wedge.
- Topographic relationships: Explain the relationship between the accretionary wedge, forearc basin and volcanic arc in a subduction setting.
- Hazard analysis: Evaluate why some subduction zones produce large megathrust earthquakes while others do not.
- Composition interpretation: Analyse how the composition of wedge rocks can reveal the history of subduction and sediment supply.
- Tectonic growth: Discuss how repeated accretion of terranes can increase the width of a continent over geologic time.
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