Structural geology

Structural geology is a critical part of engineering geology which is interested in the physical and mechanical properties of rocks. It provides major concepts for trying to understand the rock and lithosphere deformation. The study of structural geology tries to connect between current geometries such as folds and faults with its deformational histories. Deformation histories help us also to remodel the nature of the forces which are related to the formation.

Introduction

Structural geology is the study of the three dimensional distribution of large bodies of rock, their surfaces, and the composition inside the rock units to learn about their tectonic movements history, past geological events and environments that could have deformed them. These can be dated to know when the structural features formed. If the nature of these rocks can be determined, petroleum geologists can discover if oil or natural gas are trapped within the rocks.

Importance of Structural Geology

The study of structural geology has a primary importance in economic geology, both petroleum geology and mining geology. The main target of structural geology is to use measurements to understand the stress field that resulted in the observed strain and geometries. We can also understand the structural evolution of a particular area due to plate tectonics (e.g. mountain building, rifting).

An essential importance of structural geology is to know areas that contain folds and faults because they can form traps in which the accumulation and concentration of fluids such as oil and natural gas occur. Environmental geologists and hydrologists need to understand structural geology because structures are sites of groundwater flow and penetration which may have an effect on leakage of toxic materials from waste dumps or leakage of salty water into aquifers.

Objective

Structural geologists use different methods to measure rock geometries, and reconstruct their deformational histories. Then, the stress field that resulted in that deformation is calculated. 

Basic Branches of the Science

The study of structural geology can be divided into three parts which are strongly integrated and interlinked:

• The concepts of stress, strain and rheology of the lithosphere
• Description of structures from the grain scale over the outcrop scale to the mountain and tectonic scale.
• Interpretation of geological maps and identifying the structures on them such as folds and faults.

Stress fields

By understanding the relationships between stress and strain in rocks, geologists can put the observed patterns of rock deformation into a stress field. The following features are used to determine stress fields from deformational structures:

• In perfectly brittle rocks, faulting occurs at 30° to the greatest compressional stress.
• The greatest compressive stress is normal to fold axial planes.

Mechanical properties of lithosphere have a great importance for interpretation of deformation at all locations and time scales, from local scale to large-scale geodynamics and from seismic time scale to billions of years. 

Depending on loading conditions and time scale, lithosphere displays elastic, brittle (plastic) or viscous (ductile) properties. For very small strains and/or short time scales (e.g. seismic), rocks deform elastically under stress, atomic bonds can be broken at quite small strains leading to inelastic deformation. Inelastic brittle deformation results from fractures into a single frictional shear band (fault) at sufficiently high strains. In nature there is no pure elastic, viscous or plastic deformation; all types of deformation take place altogether but in different proportions.

Scales of structures 

The processes studied by structural geologists extent is at least 14 orders of magnitude (10E-6 –10E7 m). Even restricting our analysis to macroscopic structures by taking the outcrop-scale as a lower limit, the scale divide highlighted by Ramsay and Lisle spans magnitudes from millimetres to tens of thousands of kilometres.

Geological maps interpretation

Geological map is a medium of communication that uses graphic symbols to represent spatial relationships between geographical and geological features. The interpretation of geological maps goes beyond identifying the individual items showed on the map. The interpretation of geological maps is basically an attempt to visualize and understand the complex shapes of rock units in the subsurface. An understanding of the variety of geological structures is important, because it helps to determine the nature of subsurface structures from geological maps. Structural geology, therefore, is a major cornerstone of the art of geological map interpretation. We can learn how to interpret geological maps and know the structures in them such as folds and faults.

Folds are formed when the flat and planar surfaces are bent or curved as a result of plastic deformation. They vary in size from microscopic crinkles to mountain sized folds. Fault is a planar fracture in rock in which the rock on one side of the fracture has moved with respect to the rock on the other side. Large faults within the earth’s crust are the result of differential or shear motion.

Conclusions

Structural geology is obviously one of the most important subjects for geoscientists working in petroleum industry as they can identify the locations that may have traps such as folds and faults which are good for the accumulation of oil & natural gas.