Importance of Structural Geology

Structural geology allows us to unravel deformational histories and their relationships to the Earth system as a whole, helping us to understand the mechanisms of plate tectonics and its expression as fault slip in the upper crust and as plastic flow at depth. It is used in quantifying hazards related to earthquake probability and concepts such as slope stability, and involves investigation of applied mechanics of natural materials, helping us understand rock strength and hydraulic fracture (fracking). Structural geology also has economic uses, helping to find resources deformed or created by geologic structures.

Earth's Mechanical Layering

The outer silicate shell of the earth is divided into three mechanical layers, the lithosphere, asthenosphere, and the mesosphere.

The lithosphere is the solid, rigid outer shell that behaves in a plate-like manner. It includes all of the crust (with rare exceptions), and usually the uppermost mantle as well. The base of the crust is known as the Moho, short for Mohorovičić Discontinuity, where seismic wave velocity jumps upon entering denser mantle rocks. The thickness of the lithosphere varies dramatically with type and location. The thinnest oceanic lithosphere, found near spreading centers, can be as little as seven kilometers thick: a simple layer consisting entirely of oceanic crust. The thickest oceanic lithosphere can be up to a hundred kilometers thick, where the upper mantle has cooled underneath a comparatively thin, old layer of oceanic crust.

Continental lithosphere is thinnest where it overlies the very hot, viscous upper mantle at the thinnest fringes of active continental rifts. At these locations, the lithosphere is composed only of crust and can be less than ten kilometers thick. The thickest continental lithosphere consists of approximately 40 km of crust overlying 100 to 150 km of cold, but somewhat buoyant, upper mantle, and is found in continental cratons (interiors). The thickest continental crust, however, is approximately 70 km thick and is found beneath orogenic plateaus in the Andes and Tibet. This thick crust is underlain by very little mantle lithosphere: in general, the lithosphere is thin beneath orogens, even if the crust is thick.

The asthenosphere is the solid but viscous layer beneath the lithosphere that flows in response to density variations, driving plate tectonics. Very hot crust in rifts or thick orogens may also flow sufficiently to act as asthenosphere. (An orogen is region of continental crust undergoing deformation, usually in response to plate collision; rifting and strike-slip dominated orogens also exist.)

The mesosphere consists of the lower mantle, where mineral phases retard flow relative to the asthenosphere. Downwelling oceanic lithospheric slabs may stall at the mesosphere/asthenosphere boundary. The mesosphere still participates in mantle convection.

Structure vs. Tectonics

A geologic structure is a geometric element of rock that records displacement or deformation. Examples of geologic structures include tilted strata, faults, folds, lineations, and foliations.

Tectonics is the study of deformational history as recorded by geologic structures, and includes investigation of processes such as formation of rift basins, development of mountain ranges, and collision of continents.

In many ways, the difference is a matter of scale. Tectonic problems will often encompass numerous geologic structures and their relationships to one another, both spatially and over time; but some geologic structures are important tectonic problems in their own right, such as a plate boundary fault, or crustal-scale tilting. Structural geology alone, however, is not enough to understand the full picture of how and why continents deform: interactions with oceanic lithosphere fundamentally influence continental deformation. Until these influences -- in the form of seafloor spreading and subduction -- were discovered, many rejected Alfred Wegener's theory of continental drift due to lack of explanation of a physical mechanism to drive the proposed motions.

Plate Tectonics

Plate tectonics describes the motion of rigid plates of lithosphere around the Earth's spherical shell. This assumption that plates are rigid works for almost all oceanic lithosphere and for continental lithosphere outside of orogens, which tend to host more distributed deformation and are often poorly described by rigid plate interactions.

Interactions between lithospheric plates come in several varieties.

Structural Models

Stresses and Sign Conventions

Forces in the earth are usually termed as stresses. A stress is like a force, but there are several important differences. Stresses have units of force per area, like pressures do. Stresses are also defined to be at mechanical equilibrium, meaning all forces are balanced: thus, stress is always represented as a pair of opposing vectors. A single vector is called a traction. Finally, stresses are generally not perpendicular to a plane in the earth, so they are considered in terms of two components: normal stress and shear stress. Normal stresses are positive when compressional and negative when extensional; shear stresses are positive when sinistral (counterclockwise) and negative when dextral (clockwise). General stress is a summation of a normal stress and a shear stress.

Strain

Deformation in the earth is formally described as strain. Strain is expressed as a ratio, such as the change in length over the original length, rather than just a measurement of displacement. Like stress, there are multiple components: extension (normal) strain and shear strain.

Elongation strain, \(e= \frac{\Delta L}{L_{0}}\), is the change in length divided by the original length. Elongation strain is positive for elongation and negative for shortening.

Summary