Structural geology

Structural geology is a subdiscipline of geology that studies how rocks deform in response to the stresses that act within the Earth. Rocks and their forming minerals accommodate the stresses acting upon them and keep a record thereof by developing at times truly spectacular geological structures, such as fractures, faults or folds.
Fig 1: Small scale folds (crenulations) in a mica-rich fault rock (phyllonite) from the Kvenklubben fault, Repparfjord, Finnmark (Photograph: E. Torgersen).

Spennende deformasjoner

The results of deformation can be observed and studied at all scales, be it in a thin section prepared from a rock to study it under a microscope (Figure 1), at the outcrop in the field (Figure 2) or even on aerial or satellite images (Figure 3).

Figur 2: En foldet sekvens med kalkrike bergarter i Repparfjord, Finnmark. (Foto: HJ Kjøll)
Figur 3: Et LIDAR-bilde av den nordvestlige delen av øya Bømlo i Hordaland, Vest-Norge. Den høye oppløsningen gjør det enkelt å identifisere kontinuerlige sett av sprekker på øya.

The analysis of geological structures is aimed at understanding why, how and when the Earth deformed. By means of structural investigations it is possible to constrain the mechanical laws that control how deformation is taken up by geological materials and to reconstruct the evolution of mountain chains in space and through geological time. Structural studies are also crucial for the compilation of modern geological maps and cross sections, that is, the tools that we use to illustrate how rocks are distributed on the surface of the Earth and at depth below it. 3D models can be constructed only with the help of structural geology (Figure 4)

Figure 4: 3D-modell av en fold i regional skala. Illustrasjon: Thomas Scheiber

The thorough understanding of the deformation processes that have shaped the Earth and the ability to predict how structures develop in three dimensions also at depth have major societal implications. Examples range from determining where valuable mineral resources and oil might be buried, to helping mitigate earthquake hazard, and to helping the sound planning of infrastructures such as roads, bridges and underground excavations.

Structural geologists follow commonly three different approaches when studying the structural evolution of an outcrop, of a region or of a hand specimen:

1) Descriptive and geometric analysis: geological structures are identified, characterized and measured in order to produce their accurate description and to understand their orientation in space (Figure 5).

Figur 5: Strukturkart over den østlige delen av Repparfjord tektoniske vindu i Finnmark, og en projeksjon av lagdelt sandstein i Saltvann-formasjonen (gule sirkler på kartet til venstre).

2) Kinematic analysis: faults are studied in order to determine the movement of the blocks that are juxtaposed along the fault plane, that is the surface where slip was localized during deformation (Figure 6).

Figur 6: Småskala kvartsklaster, som på grunn av sin asymmetri, kan brukes som kinematisk indikator innenfor Kalak dekkekompleks eksponert på Markopp i Repparfjord, Finnmark. (Foto: G. Viola)

3) Dynamic analysis: geological structures are used as natural gauges to derive information on the forces and pressures that were active at the time of deformation.

At a first approximation, deformation in rocks can be assigned to either brittle or ductile processes. Brittle processes act mostly in the uppermost parts of the Earth and lead to frictional deformation and the development of discrete, localized fractures and faults (Figure 7).

Figur 7: Et sett av sammensatte skjærbrudd gjennom en pegmatitt (hvitt omriss) innenfor en granittisk vertsbergart. Forskyvningen av pegmatitt-åren gir geologene en forståelse av hva som har skjedd. Bildet er tatt på øya Kalla, i nærheten av Olkiluoto i sørvestlige Finland, hvor de finske myndigheter bygger et dyp depot for langtidslagring av høyverdig atomavfall. Den strukturelle analysen av hvordan den sprø deformasjonen påvirker området er av betydning for utformingen av depotet og for vurdering av fremtidig mulig seismisk aktivitet. (Foto: G. Viola)

Ductile processes are instead active at higher temperatures, thus deeper in the Earth and cause deformational patterns that are volumetrically more distributed, such as shear zones and the spectacular folds that we can so often observe around us.