Which Best Represents A Plausible Thickness Of The Lithosphere

The lithosphere, the rigid outer layer of Earth, plays a crucial role in plate tectonics, geological activity, and the planet's overall dynamic processes. Understanding its thickness is fundamental to comprehending these phenomena. Estimating the lithosphere's thickness involves considering various factors such as temperature, composition, and mechanical properties, leading to a range of plausible values depending on the method and region studied.

Defining the Lithosphere

The lithosphere is typically defined in two primary ways:

• Thermal Lithosphere: This definition is based on temperature. The lithosphere is considered the region above a specific isotherm, usually between 1280°C and 1350°C, which corresponds to the temperature at which rocks begin to lose their rigidity and partial melting starts to occur.
• Mechanical Lithosphere: This definition focuses on the mechanical properties of the Earth's outer layer. The mechanical lithosphere is the rigid part that deforms elastically over long timescales, distinguishing it from the underlying, more ductile asthenosphere.

The distinction between these definitions is important because the thermal and mechanical boundaries may not always coincide. The mechanical lithosphere is often thinner than the thermal lithosphere, especially in areas with high heat flow or compositional variations.

Factors Influencing Lithospheric Thickness

Several factors influence the thickness of the lithosphere:

• Temperature: Temperature is the most critical factor. As temperature increases with depth, the rocks become less viscous, and the lithosphere-asthenosphere boundary (LAB) is defined by a specific isotherm.
• Composition: The composition of the mantle affects its rheology. For example, the presence of water can significantly lower the melting point of mantle rocks, reducing the lithospheric thickness.
• Age: Older oceanic lithosphere is generally thicker than younger lithosphere because it has had more time to cool and solidify.
• Tectonic Setting: Different tectonic settings, such as oceanic ridges, subduction zones, and continental cratons, have varying thermal and mechanical properties that affect lithospheric thickness.

Methods for Estimating Lithospheric Thickness

Estimating the thickness of the lithosphere involves various geophysical and petrological methods:

• Seismic Methods:
  • Surface Wave Tomography: This method uses the dispersion of surface waves (Rayleigh and Love waves) to infer the shear wave velocity structure of the Earth's subsurface. The LAB is often identified as a low-velocity zone (LVZ) where seismic velocities decrease significantly.
  • Receiver Functions: This technique analyzes the converted seismic waves (e.g., P-to-S waves) from distant earthquakes to identify sharp velocity contrasts in the upper mantle, which can indicate the LAB.
• Heat Flow Measurements:
  • Surface heat flow measurements can provide constraints on the thermal structure of the lithosphere. By combining heat flow data with thermal conductivity measurements of crustal and mantle rocks, it is possible to estimate the depth to a specific isotherm, such as the 1300°C isotherm, which is often used to define the base of the thermal lithosphere.
• Flexural Studies:
  • The flexure of the lithosphere under the load of large features such as volcanoes or ice sheets can be used to estimate its effective elastic thickness (Te). Te is related to the mechanical thickness of the lithosphere and provides insights into its strength and rigidity.
• Magnetotelluric (MT) Sounding:
  • MT sounding measures the Earth's electrical conductivity as a function of depth. The LAB is often associated with an increase in electrical conductivity due to the presence of partial melt or interconnected fluids.
• Petrological Studies:
  • Analysis of xenoliths (mantle rocks brought to the surface by volcanic eruptions) can provide information about the temperature and composition of the upper mantle. Geotherms (temperature-depth profiles) can be constructed based on the mineralogical and geochemical characteristics of xenoliths, which can be used to estimate the lithospheric thickness.

Plausible Thickness of the Lithosphere in Different Tectonic Settings

The thickness of the lithosphere varies significantly depending on the tectonic setting:

• Oceanic Lithosphere:
  • Young Oceanic Lithosphere (near mid-ocean ridges): The lithosphere is very thin, typically less than 10 km. At the mid-ocean ridges, new oceanic crust is formed, and the mantle is hot and partially molten, resulting in a thin thermal and mechanical lithosphere.
  • Old Oceanic Lithosphere (far from mid-ocean ridges): As the oceanic lithosphere moves away from the ridge, it cools and thickens. The thickness can increase to about 100 km or more in the oldest parts of the ocean basins (e.g., the western Pacific). The cooling process follows a square root of age relationship, meaning the thickness increases proportionally to the square root of the lithosphere's age.
• Continental Lithosphere:
  • Cratons (stable continental interiors): Cratons are the oldest and most stable parts of the continents. They are characterized by thick lithosphere, often exceeding 200 km. The deep lithospheric roots of cratons are cold, depleted in dense elements, and highly viscous, providing long-term stability.
  • Orogenic Belts (mountain ranges): Orogenic belts are regions of active or past mountain building. The lithosphere in these areas can be highly variable in thickness due to complex tectonic processes such as crustal thickening, delamination, and subduction. The thickness may range from 50 km to 150 km.
  • Rift Zones (regions of continental extension): Rift zones are areas where the continental lithosphere is being stretched and thinned. The lithospheric thickness in rift zones is typically reduced due to elevated heat flow and extensional deformation. The thickness may range from 30 km to 70 km.

Detailed Thickness Estimates

To provide a more comprehensive understanding, let's examine plausible thickness ranges for the lithosphere in different settings based on various studies:

• Oceanic Lithosphere:
  • Young Oceanic Lithosphere (0-20 million years):
    • Thickness: 5-30 km
    • Characteristics: High heat flow, shallow LAB, low seismic velocities.
    • Example: Mid-Atlantic Ridge, East Pacific Rise.
  • Intermediate-Age Oceanic Lithosphere (20-80 million years):
    • Thickness: 30-70 km
    • Characteristics: Moderate heat flow, gradual thickening of the lithosphere.
    • Example: Central Pacific Ocean.
  • Old Oceanic Lithosphere (80+ million years):
    • Thickness: 70-120 km
    • Characteristics: Low heat flow, well-defined LAB, high seismic velocities in the lithosphere.
    • Example: Western Pacific Ocean, old parts of the Atlantic Ocean.
• Continental Lithosphere:
  • Cratons:
    • Thickness: 180-250 km or more
    • Characteristics: Low surface heat flow, thick lithospheric root, high seismic velocities, depleted mantle composition.
    • Example: North American Craton, Siberian Craton, Kaapvaal Craton (South Africa).
  • Orogenic Belts:
    • Thickness: 50-150 km (variable)
    • Characteristics: Moderate to high heat flow, complex crustal and mantle structure, variable seismic velocities.
    • Example: Himalayas, Andes, Alps.
  • Rift Zones:
    • Thickness: 30-70 km
    • Characteristics: High heat flow, thinned crust and lithosphere, elevated mantle temperatures, active volcanism and seismicity.
    • Example: East African Rift System, Baikal Rift Zone.

Case Studies and Examples

• North American Craton: Seismic studies using surface wave tomography and receiver functions have shown that the lithosphere beneath the central part of the North American Craton extends to a depth of approximately 200-250 km. Xenolith studies from kimberlites in this region support these findings, indicating a thick, cold, and depleted mantle lithosphere.
• East African Rift System: Geophysical investigations, including seismic tomography and magnetotelluric sounding, have revealed that the lithosphere beneath the East African Rift System is significantly thinned, with a thickness of around 50-70 km. This thinning is associated with elevated mantle temperatures and active rifting processes.
• Himalayan Orogenic Belt: The lithospheric structure beneath the Himalayas is complex due to the collision of the Indian and Eurasian plates. Seismic studies suggest that the lithosphere is thickened in some areas due to crustal shortening and underthrusting, while other regions may have a thinner lithosphere due to delamination or removal of the mantle lithosphere. Thickness estimates range from 80-150 km.
• Western Pacific Ocean: The oldest oceanic lithosphere in the western Pacific has been extensively studied using heat flow measurements and seismic methods. The lithosphere in this region is approximately 100-120 km thick, reflecting its long history of cooling and thickening since its formation at the mid-ocean ridge.

Challenges and Uncertainties

Despite the advancements in geophysical and petrological techniques, several challenges and uncertainties remain in accurately determining the lithospheric thickness:

• Resolution of Geophysical Methods: Seismic tomography and other geophysical methods have limited resolution, particularly at greater depths. This can make it difficult to precisely locate the LAB and resolve fine-scale variations in lithospheric thickness.
• Ambiguity in LAB Definition: The LAB is not always a sharp boundary and can be gradational in some regions. This makes it challenging to consistently define and identify the LAB using different methods.
• Assumptions in Thermal Models: Thermal models used to estimate lithospheric thickness rely on assumptions about the thermal conductivity and heat production of crustal and mantle rocks. These parameters can vary significantly depending on the composition and geological history of the region.
• Spatial Variability: The lithospheric thickness can vary significantly over short distances, particularly in tectonically active regions. This makes it difficult to extrapolate thickness estimates from point measurements to larger areas.
• Data Integration: Combining data from different geophysical and petrological methods is essential for obtaining a comprehensive understanding of lithospheric thickness. However, integrating these data can be challenging due to differences in resolution, sensitivity, and data coverage.

The Role of the Asthenosphere

Understanding the lithosphere also requires understanding the asthenosphere, the ductile layer beneath it. The asthenosphere is characterized by higher temperatures and a small degree of partial melt, which reduces its viscosity and allows it to flow over geological timescales. The interaction between the lithosphere and asthenosphere plays a crucial role in plate tectonics and mantle convection.

• Viscosity Contrast: The large viscosity contrast between the rigid lithosphere and the more ductile asthenosphere is what allows the lithosphere to move as plates on the Earth's surface.
• Mantle Convection: The asthenosphere is part of the Earth's mantle convection system, where heat from the Earth's interior is transported towards the surface. This convection drives plate tectonics and influences the thermal structure of the lithosphere.
• Deformation Mechanisms: The asthenosphere accommodates the deformation caused by plate motions, allowing the lithosphere to deform elastically and transmit stress over long distances.

Future Directions in Lithospheric Research

Future research on lithospheric thickness will likely focus on:

• Improved Geophysical Imaging: Developing higher-resolution seismic and electromagnetic imaging techniques to better resolve the LAB and variations in lithospheric structure.
• Advanced Modeling: Creating more sophisticated thermal and geodynamic models that incorporate realistic material properties and account for the complex interactions between the lithosphere, asthenosphere, and mantle.
• Data Integration: Integrating data from multiple sources, including seismology, heat flow, magnetotellurics, and petrology, to obtain a more comprehensive understanding of lithospheric thickness and its relationship to tectonic processes.
• Global Mapping: Generating global maps of lithospheric thickness based on consistent datasets and methodologies to provide a global perspective on the structure and evolution of the Earth's outer layer.
• Deep Earth Observatories: Establishing deep Earth observatories, such as deep boreholes and seafloor observatories, to directly measure temperature, pressure, and other physical properties of the lithosphere and asthenosphere.

Conclusion

The thickness of the lithosphere is a fundamental parameter for understanding Earth's dynamic processes. While there is no single "best" value for lithospheric thickness, the plausible range varies significantly depending on the tectonic setting, age, and composition of the region. Oceanic lithosphere generally ranges from a few kilometers at mid-ocean ridges to about 120 km in the oldest ocean basins. Continental lithosphere can vary from around 50 km in rift zones to over 200 km in cratonic regions. Continued research using advanced geophysical and petrological techniques is essential for refining our understanding of lithospheric thickness and its role in plate tectonics and mantle dynamics. Understanding the range of plausible thicknesses, the methods used to estimate them, and the factors that influence them is crucial for advancing our knowledge of Earth's structure and evolution.