9.3 Structural geology and tectonic settings
3 min read•july 22, 2024
Plate tectonics shape Earth's surface through powerful forces. These forces create diverse landscapes, from towering mountains to deep ocean trenches. Understanding tectonic settings helps explain how our planet's crust deforms and changes over time.
Structural geology reveals the hidden stories within rocks. By studying folds, faults, and other features, geologists can piece together Earth's past. This knowledge helps us interpret the complex history of our planet's ever-changing surface.
Tectonic Settings and Crustal Deformation
Tectonic settings for crustal deformation
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- Convergent plate boundaries involve the collision of two plates
- Subduction zones occur where an oceanic plate descends beneath another plate
- Oceanic-continental convergence leads to the formation of volcanic arcs and accretionary wedges (Andes Mountains)
- Oceanic-oceanic convergence results in the formation of island arcs and deep-sea trenches (Mariana Trench)
- Continental-continental collision occurs when two continental plates collide, leading to the formation of high mountain ranges (Himalayas)
- Subduction zones occur where an oceanic plate descends beneath another plate
- Divergent plate boundaries involve the separation of two plates
- Mid-ocean ridges are underwater mountain ranges where seafloor spreading occurs, forming new oceanic crust (Mid-Atlantic Ridge)
- Continental occurs when a continent is stretched and thinned, leading to the formation of rift valleys (East African Rift)
- Transform plate boundaries involve plates sliding past each other horizontally
- Strike-slip faults accommodate the lateral displacement of crust (San Andreas , North Anatolian Fault)
Structural geology in geologic features
- Mountain ranges are formed by compressional forces at convergent plate boundaries
- Folding and faulting of rock layers occur due to compression
- Uplift and erosion shape the mountain range over time
- Basins are formed by extensional forces at divergent plate boundaries or within plates
- Subsidence occurs as the crust stretches and thins
- Sediment accumulation fills the basin over time
- Examples include foreland basins adjacent to mountain ranges and rift basins in extensional settings
- Other geologic features influenced by structural geology include:
- Plateaus, which are uplifted regions of flat-lying sedimentary rocks (Colorado Plateau, Tibetan Plateau)
- Domes, which are circular or elliptical uplifts of rock layers (Black Hills, Adirondack Mountains)
Stress Fields and Structural Features
Regional stress and structural development
- Stress fields influence the development of structural features
- Compressional stress leads to the shortening of crust and the formation of thrust faults and folds
- Extensional stress leads to the stretching of crust and the formation of normal faults and grabens
- Shear stress leads to the lateral movement of crust and the formation of strike-slip faults
- Structural features develop in response to stress fields
- Folds are formed by compressional stress
- Anticlines are upward-arching folds
- Synclines are downward-arching folds
- Faults are formed by different types of stress
- Normal faults form under extensional stress, with the hanging wall moving down relative to the footwall
- Reverse faults form under compressional stress, with the hanging wall moving up relative to the footwall
- Strike-slip faults form under shear stress, with lateral movement of fault blocks
- Joints are fractures in rock with no displacement, often forming in response to regional stress fields
- Folds are formed by compressional stress
Interpreting geologic history through structure
- Field observations are crucial for interpreting geologic history
- Orientation of rock layers provides information about past deformation events
- Identification of folds, faults, and other structures helps reconstruct the tectonic history
- Cross-sections are used to visualize the subsurface geology
- Interpretation of deformation events and their timing can be inferred from cross-sections
- Stereographic projections are used to represent 3D orientation data on a 2D surface
- Analysis of and fault geometries can be performed using stereographic projections
- Tectonic reconstruction involves integrating structural data with other geologic information
- Interpretation of past plate movements and deformation events is possible through tectonic reconstruction
- Reconstruction of paleogeography and tectonic settings helps understand the geologic history of an area (reconstruction of the supercontinent Pangaea)
Key Terms to Review (17)
Alfred Wegener: Alfred Wegener was a German meteorologist and geophysicist best known for proposing the theory of continental drift in the early 20th century. His ideas laid the groundwork for modern plate tectonic theory, influencing our understanding of structural geology and tectonic settings, as well as the characteristics of plate boundaries.
Cenozoic Era: The Cenozoic Era is the most recent geological era, spanning from about 66 million years ago to the present. It is known as the 'Age of Mammals' due to the significant evolution and diversification of mammals and birds after the extinction of the dinosaurs. This era also saw major changes in Earth's climate, tectonic activity, and the development of modern ecosystems.
Convergent boundary: A convergent boundary is a tectonic plate boundary where two plates collide, leading to one plate being forced beneath the other in a process known as subduction. This interaction often results in significant geological features such as mountain ranges, deep ocean trenches, and volcanic activity. The type of convergent boundary formed depends on the nature of the colliding plates, whether they are continental or oceanic.
Divergent boundary: A divergent boundary is a tectonic plate boundary where two plates move away from each other, creating new crust as magma rises to the surface. This process is crucial in forming mid-ocean ridges and rift valleys, significantly influencing geological features and activity on Earth. As these plates separate, they can lead to the formation of new oceanic crust and contribute to seismic activity.
Elastic deformation: Elastic deformation refers to the temporary change in shape or size of a material in response to applied stress, which can return to its original form once the stress is removed. This property is crucial in understanding how rocks behave under stress in various geological settings, as it helps explain the mechanisms behind faulting and earthquakes, as well as the stability of geological structures.
Fault: A fault is a fracture or zone of fractures between two blocks of rock, allowing them to slide past one another. This movement is caused by the stress that builds up due to tectonic forces, leading to various types of faulting that can influence the landscape and trigger seismic activity. Faults are integral in understanding how tectonic plates interact, how earthquakes occur, and how rock deformation processes shape the Earth’s crust.
Fold: A fold is a geological feature that occurs when rock layers are bent or curved due to tectonic forces, such as compression or shear stress. Folds can reveal important information about the Earth's history and the forces that have shaped its crust. Understanding folds is crucial in structural geology because they indicate how rocks respond to stress and strain in different tectonic settings.
Geological cross-section: A geological cross-section is a representation that shows the arrangement of different rock layers and geological features below the earth's surface as if a vertical slice has been made through the ground. This visual tool helps to illustrate the relationships between various rock types, structural features, and geological formations, which are crucial for understanding both structural geology and the interpretation of geologic maps.
Ocean trench: An ocean trench is a deep, narrow depression in the ocean floor formed by the convergence of tectonic plates, where one plate is forced beneath another in a process known as subduction. These trenches are some of the deepest parts of the Earth's oceans and play a crucial role in plate tectonics, influencing geological activity such as earthquakes and volcanic eruptions.
Orogeny: Orogeny refers to the process of mountain formation that occurs through tectonic forces, typically involving the collision and convergence of tectonic plates. This geological phenomenon can lead to the uplift, folding, and faulting of the Earth's crust, resulting in the creation of mountain ranges and other significant landforms. Orogeny is a crucial aspect of structural geology and is heavily influenced by tectonic settings, such as subduction zones and continental collisions.
Pangaea breakup: The Pangaea breakup refers to the process that occurred around 200 million years ago when the supercontinent Pangaea began to break apart into smaller landmasses, leading to the formation of the continents we recognize today. This event is crucial for understanding the geological history of Earth, as it triggered significant changes in oceanic and atmospheric circulation patterns, influenced climate, and shaped the distribution of biodiversity across the planet.
Plate tectonics theory: Plate tectonics theory is a scientific concept that describes the large-scale movement of Earth's lithosphere, which is divided into tectonic plates that float on the semi-fluid asthenosphere beneath. This theory explains the interactions of these plates, including their movements, boundaries, and the geological features and phenomena that result from these processes, such as earthquakes, volcanic activity, and mountain building.
Rift valley: A rift valley is a lowland region formed by the divergence of tectonic plates, characterized by steep sides and a central block that has subsided between two faults. These valleys are often associated with continental rifting, where the Earth's crust is being pulled apart, leading to geological activity such as earthquakes and volcanic eruptions.
Rifting: Rifting is the geological process where a tectonic plate or lithosphere stretches and pulls apart, leading to the formation of rift valleys or basins. This process plays a crucial role in shaping the Earth's crust, often associated with divergent tectonic boundaries where tectonic plates move away from each other, resulting in volcanic activity and seismic events.
Sedimentary rock: Sedimentary rock is a type of rock that forms from the accumulation and compaction of mineral and organic particles, often in layers, over time. These rocks are crucial for understanding Earth's history, as they often contain fossils and provide insights into past environments. Their formation is closely tied to processes like erosion, sedimentation, and diagenesis, which play a significant role in shaping geological features.
Stratigraphy: Stratigraphy is the branch of geology that studies rock layers (strata) and layering (stratification). It plays a crucial role in understanding Earth's history, the age of rocks, and the processes that formed them, linking various aspects of geology such as fossil analysis, geological time, and tectonic settings.
Transform boundary: A transform boundary is a type of plate boundary where two tectonic plates slide past each other horizontally. This movement can create significant geological features such as fault lines and is associated with earthquake activity due to the stress that builds up as the plates interact. Understanding transform boundaries is crucial for recognizing how they relate to structural geology and tectonic settings, as well as their specific characteristics in plate tectonics.