5.4 Accretionary wedges and orogenic belts
4 min read•august 16, 2024
Accretionary wedges form at convergent plate boundaries, where sediments and crustal materials scrape off the subducting plate and pile up on the overriding plate. These massive structures play a crucial role in shaping Earth's crust and building continents.
Orogenic belts develop from collisions between tectonic plates, leading to and intense deformation. These processes contribute to continental growth, crustal thickening, and the formation of complex geological structures that shape Earth's landscape.
Accretionary wedges at convergent margins
Formation and characteristics of accretionary wedges
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- Accretionary wedges form large, wedge-shaped accumulations of sedimentary and metamorphic rocks at convergent plate boundaries ( zones)
- Develop as sediments and crustal materials scrape off the subducting plate and accumulate on the overriding plate
- Occur primarily at ocean-continent convergent margins where oceanic crust subducts beneath continental crust
- Extend for hundreds of kilometers along subduction zones and reach thicknesses of several kilometers
- Internal structure characterized by imbricate thrust faults, , and intense deformation of sedimentary layers
- Growth influenced by factors such as sediment supply, subduction rate, and subducting slab angle
Processes driving accretionary wedge development
- Sediment accretion transfers material from subducting plate to overriding plate through scraping, offscraping, and underplating
- Offscraping peels sediments off the top of the subducting plate and adds them to the front of the wedge
- Underplating detaches sediments and crustal materials from the subducting plate and accretes them to the wedge base
- Continued plate convergence and sediment accumulation drive deformation
- Critical taper principle governs wedge shape and stability, balancing gravitational forces and tectonic stresses
- Deformation styles include thrust , folding, and mélange zone development (chaotic mixtures of rock types)
- Fluid expulsion and overpressure significantly influence deformation and structural evolution
Sediment accretion and deformation
Mechanisms of sediment accretion
- Offscraping occurs at the wedge toe, adding sediments to the front
- Underplating takes place deeper in the subduction zone, accreting materials to the wedge base
- Sediment supply from the subducting plate influences accretion rates
- Accretion processes vary along the subduction zone, creating lateral variations in wedge structure
- Sediment thickness on the subducting plate affects the efficiency of accretion
- Accretion rates can fluctuate over time due to changes in plate convergence rates or sediment input
Deformation processes and structures
- Thrust faulting creates imbricate structures within the wedge
- Folding occurs at various scales, from microscopic to kilometer-scale
- Mélange zones form through intense shearing and mixing of different rock types
- Pressure solution and fluid-assisted deformation contribute to internal wedge deformation
- Deformation intensity generally increases with depth and proximity to the subduction interface
- Strain partitioning leads to the development of distinct structural domains within the wedge
- Syn-sedimentary deformation affects newly accreted sediments at the wedge front
Orogenic belt development from collisions
Stages of orogenic belt formation
- Initial collision follows ocean basin closure and subduction cessation
- Crustal thickening occurs through folding, thrusting, and imbrication of crustal rocks
- Isostatic adjustment leads to uplift and mountain building
- Suture zone forms, marking the boundary between collided continents
- Metamorphism and partial melting of crustal rocks result from increased pressure and temperature
- Foreland basins develop on either side of the mountain range
- Post-orogenic processes include extensional collapse and erosion, shaping final morphology
Structural and metamorphic features of orogenic belts
- Thrust fault systems accommodate crustal shortening and thickening
- Fold-and-thrust belts develop in the external parts of the orogen
- High-grade metamorphic rocks (gneisses, schists) form in the orogen core
- Ophiolite sequences may be preserved along suture zones
- Syn-orogenic magmatism produces granitic intrusions and volcanic rocks
- Metamorphic core complexes can form during late-stage extension
- Shear zones and mylonite belts accommodate large-scale deformation
Accretionary wedges and continental growth
Contributions to continental growth
- Accretionary wedges add sedimentary and crustal materials to continental margins over time
- Terrane accretion facilitates addition of exotic crustal fragments to continental margins
- Orogenic belts represent major zones of continental crust thickening and reworking
- Vertical and lateral continental growth occurs through orogenic processes
- Amalgamation of continental fragments during orogeny contributes to supercontinent assembly
- Accretionary orogens (formed by multiple terrane and island arc accretion) play significant role in long-term continental crust growth
- Erosion and sediment redistribution from orogenic belts contribute to continental material redistribution
Geodynamic and geochemical implications
- Tectonic and magmatic processes in accretionary wedges and orogenic belts influence continental crust geochemical evolution
- Subduction-related magmatism contributes to the growth and differentiation of continental crust
- Crustal recycling through subduction and delamination affects the composition of the mantle and crust
- Preservation of accretionary wedges and orogenic belts in the geological record provides crucial information about past plate configurations
- Continental evolution patterns can be inferred from the study of ancient accretionary and collisional orogens
- Isotopic and geochemical signatures in orogenic rocks offer insights into crustal growth mechanisms
- Accretionary and collisional processes influence global elemental cycles (carbon, water) through metamorphism and magmatism
Key Terms to Review (18)
Accretionary wedge: An accretionary wedge is a geological feature formed at convergent plate boundaries where sediment and crustal material are scraped off a subducting oceanic plate and accumulate in a wedge shape. This process creates a dynamic environment that influences the formation of mountain ranges and contributes to the overall complexity of orogenic belts, highlighting the interplay between tectonic activity and sedimentation.
Alpine Orogeny: Alpine orogeny refers to the mountain-building event that resulted in the formation of the Alps and related mountain ranges, primarily during the late Mesozoic to early Cenozoic eras. This tectonic process is characterized by intense compression and collision of tectonic plates, leading to the uplift and deformation of the Earth's crust, as well as the creation of complex geological structures such as folds and faults.
Backarc: A backarc is a geological region located behind a volcanic arc, formed by tectonic processes associated with subduction zones. It typically features extensional tectonics, leading to the creation of basins and rift valleys, which are crucial for understanding the dynamics of plate movements and the formation of orogenic belts.
Convergent Boundary: A convergent boundary is a tectonic plate boundary where two plates move toward each other, often resulting in one plate being forced beneath the other in a process known as subduction. This interaction leads to significant geological features and phenomena, including earthquakes, volcanic activity, and mountain building, reflecting the dynamic nature of Earth's lithosphere.
Earthquakes: Earthquakes are sudden releases of energy in the Earth's crust, resulting from tectonic movements that create seismic waves. These movements can occur at different types of plate boundaries, affecting geological formations and human structures alike, and they are often linked to various geological processes such as subduction, rifting, and faulting.
Faulting: Faulting refers to the process of fracturing and displacement of rocks within the Earth's crust, often resulting from tectonic stress. This process is a key mechanism in the generation of earthquakes, forming along different types of faults, and plays a crucial role in shaping geological features such as mountains and valleys.
Folding: Folding is a geological process that involves the bending and warping of rock layers due to tectonic forces, often resulting in the formation of ridges and valleys. This process occurs primarily at convergent boundaries where tectonic plates collide, causing immense pressure that leads to the deformation of the Earth's crust. The outcome of folding can create complex structures like mountain ranges, showcasing the dynamic nature of Earth's geology.
Forearc: The forearc is the region located between a subduction zone and the volcanic arc that forms as a result of the subducting tectonic plate. This area plays a crucial role in the tectonic processes associated with plate boundaries, often featuring accretionary wedges formed from sediment and other materials scraped off the subducting plate. Understanding the forearc helps to illuminate the complex interactions between tectonic plates and their influence on geological features like mountain ranges and ocean trenches.
Geological Survey: A geological survey is a systematic examination and mapping of the geological features of a specific area, focusing on the composition, structure, and processes that shape the Earth. This process is crucial for understanding the geological history and potential resources of an area, which includes studying the formation of accretionary wedges and orogenic belts, as they play significant roles in plate tectonics and mountain building.
Harry Hess: Harry Hess was a prominent American geologist and a key figure in the development of the theory of plate tectonics, particularly known for his contributions to understanding seafloor spreading. His work helped establish the mechanisms of plate movement and the formation of ocean basins, connecting various geological features and processes within the Earth's lithosphere.
John Tuzo Wilson: John Tuzo Wilson was a Canadian geophysicist and geologist known for his pioneering contributions to the understanding of plate tectonics and the concept of transform faults. His work helped explain the movement of tectonic plates and their role in forming accretionary wedges, rift valleys, and orogenic belts. He is also recognized for formulating the theory of the Wilson cycle, which describes the lifecycle of supercontinents and the cyclical nature of continental formation and breakup.
Mountain Building: Mountain building, also known as orogeny, is the process by which mountains are formed through tectonic forces, particularly at convergent plate boundaries. This process involves the collision and convergence of tectonic plates, leading to crustal thickening, isostatic adjustments, and the creation of various geological features such as mountain ranges, folds, and faults.
Orogenic belt: An orogenic belt is a region of the Earth's crust that has been significantly deformed and uplifted due to the collision of tectonic plates, resulting in mountain ranges and complex geological structures. These belts often contain a variety of rock types, including metamorphic, igneous, and sedimentary, formed through processes like compression, folding, and faulting, making them essential for understanding tectonic activity and geological history.
Paleozoic orogeny: Paleozoic orogeny refers to a series of mountain-building events that occurred during the Paleozoic Era, primarily between 541 and 252 million years ago. These orogenic events were characterized by the collision and convergence of tectonic plates, leading to the formation of extensive mountain ranges and significant geological features across the continents. The Paleozoic orogeny was instrumental in shaping the modern layout of continents and is linked to processes such as the formation of accretionary wedges and the development of various orogenic belts.
Subduction: Subduction is the geological process where one tectonic plate moves under another and sinks into the mantle as the plates converge. This process is crucial in shaping Earth’s features, influencing everything from the formation of oceanic trenches to the creation of mountain ranges and volcanic activity.
Tectonic plate: A tectonic plate is a large, rigid slab of Earth's lithosphere that moves and interacts with other plates on the planet's surface. These plates can vary in size and shape, and their movements are driven by forces such as mantle convection, gravity, and the Earth's rotation. The interactions between tectonic plates are responsible for many geological features, including mountains, earthquakes, and volcanic activity.
Transform boundary: A transform boundary is a type of tectonic plate boundary where two plates slide past each other horizontally. This movement creates friction and can lead to significant seismic activity, often resulting in earthquakes, as the plates get stuck and release energy suddenly when they finally move.
Volcanism: Volcanism is the process by which magma from beneath the Earth's crust escapes to the surface, resulting in volcanic eruptions and the formation of various volcanic features. This process plays a crucial role in shaping the Earth's surface and influencing geological processes, contributing to the creation of new landforms, affecting climate, and impacting ecosystems.