Lithosphere Rheology

Rheology: The study of deformation and the flow of materials under the influence of applied stress.

Crust: Divided into oceanic and continental crust.
- Oceanic Crust Thickness: 6 km.
- Continental Crust Thickness: 40 km to 100-150 km.
Mohorovičić Discontinuity (Moho): The boundary between the crust and the upper mantle.
Upper Mantle: Extends from the Moho to the transition zone, approximately 410 km deep.
- Composed of both lithosphere and asthenosphere.
Transition Zone: Depth from 410 km to 660 km, characterized by changes in mineral phases.
Lower Mantle (Mesosphere): Extends to 2891 km (Gutenberg Discontinuity).
Outer Core: Fluid layer beneath the mantle, from 2891 km to 5150 km.
Inner Core: Solid inner part of the Earth, extends from 5150 km to 6371 km.

The lithosphere consists of multiple layers of different rheologies.
Lithospheric Strength Envelopes: Graphical representation as plots of strength versus depth, accounting for variations in pressure, temperature, and mineralogy.

Governed primarily by frictional sliding on pre-existing faults.
- The upper crust is typically fault-rich.
Strength increases linearly with depth according to Byerlee’s Law:
- The equation: au = au0 + rac{(P - P0)}{m}, where au is shear stress, au_0 is the initial shear strength, P is the pressure, and m is the friction coefficient.

As temperature increases with depth, the mechanisms of deformation shift from brittle to ductile:
- Brittle Deformation: Dominant at shallower depths due to higher stress.
- Ductile Deformation: Dominant at greater depths due to increased temperature.
- Deformation Mechanisms:
- Diffusion Creep: Movement of atoms over a distance due to concentration gradients.
- Dislocation Creep: Movement of dislocations in a crystal structure, allowing for deformation.
The strength of the middle and lower crust is influenced primarily by quartz, which is the weakest (granitic) mineral.
- Strength envelope derived from flow laws for wet quartz, exhibiting an exponentially decreasing strength with increasing depth and temperature.

The point at which deformation shifts from predominantly brittle to ductile mechanisms is termed the brittle-ductile transition.
- This occurs at approximately 10-15 km depth in the continental crust.
- The strength envelope peaks in the continental crust at this depth, correlating with the maximum depth of most earthquakes and their nucleation depth.
- A second peak in the lithospheric strength envelope is noted just below the Moho, in the olivine-rich mantle.

Strength envelopes extend throughout the lithosphere by analyzing mantle strength characteristics.
- An abrupt increase in strength occurs at the Moho, where the weaker quartz-bearing lower crust is replaced by stronger olivine-bearing mantle below.

With increased temperature and pressure at greater depths, olivine experiences ductile flow, leading to reduced strength.
The transition to the underlying weak mantle indicates the lithosphere-asthenosphere boundary characterized by a "jelly sandwich" type of rheological layering, with strong layers separating weak layers.

The oceanic crust thickness averages 7 km.
- The mantle is positioned at a shallow depth beneath the oceanic crust.
Olivine, prominent at these depths, exhibits significantly greater strength compared to quartz in equivalent continental crust settings.
- Temperatures remain low enough (20-60 km depth, affected by temperature gradients) that olivine deforms in a brittle manner.

Unlike the continental lithosphere, which has a “jelly sandwich” rheological layering with an internal weak layer under ductile flow, the oceanic lithosphere behaves as a single rigid plate due to its high strength.
Deformation zones in continental lithosphere tend to be wider and more diffuse compared to those in oceanic lithosphere.

The lithosphere-asthenosphere boundary signifies a rheological boundary that varies with temperature.
The asthenosphere is a much weaker layer beneath the lithosphere that behaves fluidly in response to stress.
The location of this boundary often coincides with the seismic Low Velocity Zone (LVZ), representing a layer of low viscosity allowing for relative movement of the lithosphere and asthenosphere.