Accreted terrains, also known as exotic terrains, are pieces of continental crust that have been transported significant distances from their original locations and then "accreted" (added) to another landmass. Imagine a giant jigsaw puzzle where some pieces were moved around before being fitted together!
Here's a breakdown of key features:
1. Origin and Transport:
* Origin: Accreted terrains often originate from a different tectonic plate than the one they are now part of. This could be due to:
* Oceanic plate subduction: As oceanic plates subduct (slide) beneath continental plates, they can carry pieces of continental crust with them.
* Micro-continental fragments: Small pieces of continents can break off and drift independently for long distances.
* Island arcs: Volcanic island chains can be generated along subduction zones and may later collide with and accrete to continents.
* Transport: The movement of these terrains is driven by plate tectonics and can involve vast distances, often thousands of kilometers.
2. Accretion:
* Collision: The transported terrain eventually collides with another continental plate. This collision is a powerful process that folds, faults, and deforms the crust.
* Welding: Over time, the accreted terrain becomes physically and geologically attached to the larger landmass, forming a new and complex tectonic setting.
3. Characteristics:
* Distinct geological signatures: Accreted terrains often have a unique geological history and structure compared to the surrounding landmass, including:
* Different rock types: They may contain rock types found in different tectonic environments.
* Different ages: The rocks may be significantly older or younger than the surrounding crust.
* Different fossils: The presence of unique fossils can indicate different climates and environments.
* Complex tectonic history: The process of accretion can create complex structures, like mountains, metamorphic belts, and fault systems.
4. Examples:
* Terranes in Western North America: The western US and Canada are composed of numerous accreted terrains, including the Wrangellia, Stikinia, and Alexander terranes.
* The Himalayas: The Himalayan mountain range is a prime example of a collision zone where an accreted terrain (the Indian subcontinent) collided with Asia.
5. Importance in understanding Earth's history:
* Reconstructing continents: Studying accreted terrains helps us understand how continents have formed and changed over billions of years.
* Understanding tectonic processes: Accretion plays a crucial role in shaping the Earth's surface and is related to mountain building, earthquakes, and volcanic activity.
* Resource exploration: Accreted terrains can contain important mineral deposits and oil and gas reserves.
Accreted or exotic terrains provide a fascinating window into the dynamic nature of Earth's tectonic processes. They are a testament to the constant movement and reshaping of our planet's surface.
