Geology Lab: Geologic Faults and Cross-sections

In order to create a geologic cross-section:

1. Draw a line of section across the map area of interest, making it very near perpendicular to the strike of the rock units. Many times, these section lines will already by provided. Published geologic maps may have section lines drawn on the map surface, and some have selected cross-sections included as part of the map. 2. Study the map area and familiarize yourself with the map symbols and units present. Make note of the geologic structures near your line of section. 3. Place a sheet of paper along the cross-section line and mark points of intersection of geologic structures. 4. Sketch the subsurface by interpreting the information the information provided on the geologic map. This may involve some guesswork as features may flatten or steepen with depth. 5. Connect similar formations with one another, maintaining the same width throughout the subsurface expression of the formation. 6. In most cases, geologic cross-sections do not employ vertical exaggeration; the horizontal scale of the map and the vertical scale of the cross-section will be the same in order not to distort the geologic relationship of the structures.

Right-lateral and Left-lateral Strike-slip Fault

A strike-slip fault is described as having either right-lateral or left-lateral displacement, depending on the relative motion of the blocks on either side of the fault. If you are standing on one side of the fault and look across to the other side, if the opposite side has been displaced to your right, the fault is called a right-lateral strike-slip fault. If the side opposite the fault has moved to the left, the fault is a left-lateral strike-slip fault.

Faults

Faults are common geologic features in many areas. They are easily distinguished from geologic contact lines and are shown with heavy dark lines. Faults cut across structures and disrupt outcrop patterns. Dip-slip faults, and the sense of movement, can be determined on a geologic map if the dip of the fault plane is known. Generally, any structure or contact that has been offset by faulting will migrate in the direction of the dip of the beds, and are also in the upthrown block.

Introduction to Geologic Faults

Faults are fractures or breaks in rock along which movement has taken place. They occur when rocks experience brittle deformation and/or when stress is applied rapidly. These geologic structures are important when studying geohazards such as earthquakes, tsunamis, and landslides; as movement along fault planes can trigger large scale natural disasters and affect people and structures near the fault movement and many times, hundreds of miles away.

Symbols on the map

From the symbols used on the fault, you already know by the U and D which side of the fault was the up-thrown side. You are also given the dip of the fault plane, and you know that the hanging wall is always above the fault plane, so you can determine what type of fault this is - a reverse fault. The red symbol in the middle of the map indicates a syncline. If that symbol was not on the map, you would still have a series repeating beds with the youngest bed in the middle (based on the letters symbolizing the geologic time period), and strike and dip symbols showing beds dipping towards each other - a syncline. Just by reading the map symbols and before you have committed anything to paper, you should already have some clues about the structural relationship.

Geologic Cross-Sections

Geologic cross-sections provide information about rocks and their structure found below the Earth's surface. The construction of a cross-section and the interpretation are made based upon information provided on the geologic map. The map symbols, age of the rock units, the application of the geologic principles such as superposition and cross-cutting relationships help geologists "see" the relationships that exist below the surface. Cross-sections are constructed in a similar manner to topographic profiles, although elevation will not be considered in this setting.

Economic Minerals and Materials

Geologic faults can be a source of economic minerals and materials, as the forces that initiate plastic deformation such as uplift, geologic stresses, metamorphic processes, as well as the emplacement of intrusive igneous bodies, all help aggregate ore minerals in heavily faulted areas. Hydrothermal fluids move relatively easily along these fracture planes, depositing economically important geologic materials for future mining activity.

Normal vs. Reverse Fault

If the hanging wall moves down in reference to the footwall, the fault is called a normal fault. It is "normal" because it is acting in concert with gravity. If the hanging wall has moved up in reference to the footwall, the fault is called a reverse fault. The hanging wall is moving in "reverse" of gravitational forces. A normal fault is when the hanging wall has moved down the fault plane, in concert with gravitational forces. A reverse fault is when the hanging wall has moved up the fault plane, against gravitational forces.

Figure about hanging and footwall

Many larger faults function as mine shafts for economically important geologic materials.

Strike-slip faults

Strike-slip faults result from horizontal displacement of rock units along the strike or trend of a fault; the rocks "slip" along the strike of the fault plane. Many strike-slip faults have near vertical fault planes; the infamous San Andreas Fault system along the west coast of the United States has experienced many hundreds of miles of displacement over geologic time.

Structures on maps

Study the hypothetical geologic map found on the following page and make note of the geologic structures you can observe in map view. Think about how these relationships might be portrayed beneath the surface. Some of the structures found on the map are: Dipping beds, Fault, Syncline, and Intrusive igneous rock.

Thrust faults

Thrust faults are specific types of low-angle reverse faults; generally the fault plane is inclined at an angle less than 45 degrees. Reverse and thrust faults are the result of compressional deformation; large scale examples of both are found in mountain ranges that formed at convergent plate margins such as the Appalachian, Ouachita, and parts of the Rocky Mountains. Some thrust faults displace large packages of rocks many tens to hundreds of miles from their original location. Reverse faults that have a fault plane that dips less than 45 degrees are referred to as thrust fault.

Types of Faults

Vertical displacement along a fault plane is referred to as dip-slip movement, the resulting faults are called dip-slip faults. These faults are described based on the relative movement of the fault blocks as they "slip" along the dip of the fault plane. These blocks are given names based on their orientation to the fault plane; the hanging wall is always above the fault plane, the footwall is below. The names were given by miners, who often descended along these fault planes to extract economically important resources. The miners would hang their lantern and tools above their work space, on the hanging wall; the footwall would be the surface where the miner would stand.

The Law of Cross-Cutting Relationships

You can also determine the relative geologic history of the map area using another geologic principle, The Law of Cross-cutting Relationships, which states that any geologic feature which cuts across another is the younger of the two features. In the map area below, the fault cuts across the syncline, and the intrusive rock cuts across the fault. When describing a geologic history, the oldest event is always listed first. Using the Law of Cross-cutting Relationships, their relative ages, from oldest event to youngest would be: folding and creation of syncline structure; faulting; and igneous intrusion.