Lecture 11 Structural Geology

* Rock deformation
* Descriptions of an inclined plane: strike and dip
* Folds in Rock
* Rock fractures: Joints and Faults
* Geologic maps

* Rock deformation

Rocks deform under stress. Deformation refers to all changes in volume and/or shape of a rock body. In Lecture 7 of Mechanical Properties of Rocks, we described how rocks deformed under applied load. The Earth is a dynamic system. Tectonic forces within the Earth are so enormous that thousands of kilometers of rock layers are bent, folded, overturned, and sometimes highly fractured. Structural geology examines the present state of crustal deformation and determines the original geologic setting and the nature and direction of the earth forces that produced these rock structures.

For deformation in the elastic range of a rock, the rock returns to its original shape when the stress is removed.

Folding in a rock occurs in the plastic range of the rock.

Faulting occurs when the rocks break.

* Descriptions of an inclined plane: strike and dip

In describing geologic structures, strike and dip is used to describe the orientation of an inclined plane such as an inclined rock layer or fault.

Strike is the direction of the line produced by the intersection of the inclined plane with the horizontal plane. It is expressed as the angle of the line from the north.

Dip is the angle between the inclined plane and the horizontal plane.

Strike and dip directions are always mutually perpendicular.

* Folds in Rock

A fold is a bent structure that originally was planar, such as a sedimentary bed. Folds may be produced by either horizontal compression or vertical forces in the crust, just as pushing in on opposite sides of a paper or up from below.

Terms used to describe the parts of a fold:

limb: The two sides of a fold are called limbs.
axis: A line drawn along the points of maximum curvature of a layer of a fold.
axial plane: an imaginary plane surface that divides a fold as symmetrically as possible.
plunge: If the fold axis is not horizontal, the angle of the axis with the horizontal plane is called plunge.

Types of folds

anticline: upfolds or arches of rock layers
syncline: downfolds or troughs of rock layers.
monocline: only one direction of dip prevails in a fold system.

symmetrical fold:  the axial plane is vertical with the limbs dipping symmetrically from the axis

asymmetrical fold: the axial plane is tilted from the vertical with one limb dipping more steeply than the other.

overturned fold: one limb is tilted beyond the vertical

recumbent fold:  this is an overturned fold "lying on its side" so that the axial plane is nearly horizontal.

Outcrop view of folds

Geologists typically work from available surface outcrops of rock formation to reconstruct subsurface structures.

In the field or on a map view of an anticline, the oldest beds are in the center and the beds become progressively younger in each direction. The opposite is true for synclines, where the youngest bed is in the center and the beds get progressively older in each direction.

Symmetrical folds have equal bed widths on opposite sides of the axial plane, but asymmetrical folds will have different bed widths on the opposite sides.

For a plunging anticline, the nose (formed by the intersection of the fold system with a horizontal plane) points in the same direction as the plunge. For a plunging syncline, the nose points in the direction opposite to that of the plunge.

* Rock fractures: Joints and Faults


A joint is a crack along which no appreciable movement has occurred. Joints can be found in almost every outcrop of rocks. Most joints are produced when rocks are deformed by tectonic forces. Joints can form from nontectonic processes, such as columnar joints from the cooling of the igneous rocks. Another example is sheeting from the gradual expansion when erosion removes the overlying load.

Faults (will be illustrated with pictures)

A fault is a fracture with relative movement of the rocks on both sides of it, parallel to the fracture.

Fault terminology:
strike, dip of fault plane, hanging wall, foot wall.

Types of faults:

dip-slip fault: normal fault, reverse fault, thrust fault

strike-slip fault: left lateral, right lateral

oblique-slip fault: has both strike-slip and dip-slip component. Note: The textbook calls it "translation fault", which is rarely used.

* Geologic maps

Geologic maps are among the best sources of information for preliminary site location and design. Thus, engineers need to become familiar with the construction and use of these maps. Elements of a geologic map include rock types, relative ages of the rocks, geologic contacts between different rock units, geologic structures (e.g., faults), and maybe topographic contours.