What Are Rocks Below And Above A Fault Called

What are Rocks Below and Above a Fault Called? Understanding Fault Blocks and Their Geological Significance

Faults are fundamental features of Earth's crust, representing fractures along which significant displacement of rock masses has occurred. Understanding the terminology associated with faults is crucial for interpreting geological structures and processes. A key aspect of this understanding involves knowing the names given to the rock formations located above and below the fault plane. This article delves into the terminology, exploring the geological context, formation processes, and significance of these rock formations.

Defining the Key Terms: Hanging Wall and Footwall

The rock masses on either side of a fault plane are known as fault blocks. The terms used to describe these blocks are hanging wall and footwall. These names derive from mining terminology, imagining a miner working a vertical vein.

The Hanging Wall: Above the Fault

The hanging wall is the block of rock that lies above the fault plane. Imagine a miner working a steeply inclined vein; the rock above him, which would "hang" over his head, is the hanging wall. This block can move up, down, or sideways relative to the footwall, depending on the type of fault.

• Normal Faults: In normal faults, characterized by extensional forces, the hanging wall moves downward relative to the footwall. This movement creates a significant dip in the hanging wall block.
• Reverse Faults: In reverse faults, associated with compressional forces, the hanging wall moves upward relative to the footwall. This often results in steep slopes and overthrust sheets.
• Strike-Slip Faults: In strike-slip faults, the movement is predominantly horizontal, parallel to the strike of the fault. The hanging wall's vertical displacement relative to the footwall is minimal.

The Footwall: Below the Fault

The footwall is the block of rock that lies below the fault plane. Again, envisioning the miner, this is the block of rock under his feet. Like the hanging wall, the footwall's movement is dependent on the type of fault.

• Normal Faults: The footwall remains relatively stable compared to the downward-moving hanging wall, forming a tilted block.
• Reverse Faults: The footwall is pushed downward, beneath the rising hanging wall, creating a significant uplift.
• Strike-Slip Faults: The footwall's relative movement is horizontal, similar to that of the hanging wall.

Types of Faults and Their Impact on Fault Blocks

The movement and orientation of the fault plane dictates the type of fault, influencing the characteristics of both the hanging wall and footwall.

1. Normal Faults: Extensional Tectonics

Normal faults result from extensional stress, where the Earth's crust is pulled apart. The hanging wall moves down relative to the footwall, often creating a graben (a down-dropped block) between two uplifted blocks called horsts. The degree of displacement can range from centimeters to kilometers.

Geological Significance: Normal faults are commonly found in rift valleys, mid-ocean ridges, and areas experiencing crustal stretching. They play a crucial role in shaping the landscape, creating basins and ranges, and influencing groundwater flow. The exposed rock formations on the fault scarps provide valuable information about the geological history of the region.

2. Reverse Faults: Compressional Tectonics

Reverse faults are formed under compressional stress, where the Earth's crust is squeezed together. The hanging wall moves up and over the footwall, resulting in uplift and shortening of the Earth's crust. Reverse faults with a dip of less than 45 degrees are called thrust faults.

Geological Significance: Reverse and thrust faults are common in convergent plate boundaries, where tectonic plates collide. They are associated with mountain building (orogeny) and can lead to the formation of nappes—large sheets of rock that have been transported significant distances. They can also cause significant seismic activity.

3. Strike-Slip Faults: Shear Stress

Strike-slip faults occur when the movement is primarily horizontal, along the strike of the fault plane. The displacement is parallel to the fault trace. The most famous example is the San Andreas Fault. While there is minimal vertical displacement between hanging wall and footwall, the horizontal movement can lead to significant lateral offset.

Geological Significance: Strike-slip faults are frequently found at transform plate boundaries, where plates slide past each other. They can cause devastating earthquakes, as seen in the case of the San Andreas Fault. The lateral displacement affects drainage patterns, creating offset streams and valleys.

Beyond the Basic Terminology: More Complex Fault Systems

The simple hanging wall/footwall model is a starting point. Real-world fault systems are often far more complex. They can involve multiple fault planes, branching faults, and variations in displacement along the fault. Furthermore, the terms hanging wall and footwall are only relative to a specific fault plane. In a complex fault zone, a block might be a hanging wall relative to one fault and a footwall relative to another.

Fault Zones and Their Internal Structure

Fault zones are not simply single, planar fractures, but rather zones of deformation that can be meters to kilometers wide. These zones contain a complex network of fractures, breccia (broken rock), and altered rock materials. The internal structure of fault zones influences the movement of fluids, the occurrence of earthquakes, and the overall mechanical behavior of the fault.

Identifying Fault Blocks in the Field

Identifying and characterizing fault blocks in the field requires careful observation and geological mapping. Key indicators include:

• Fault scarps: Step-like features created by the vertical displacement of the fault blocks.
• Offset stratigraphic units: Disruptions in the layering of sedimentary rocks.
• Fault gouge: A finely pulverized rock material found along the fault plane.
• Slickensides: Polished and striated surfaces on the fault plane, indicating the direction of movement.
• Breccia zones: Areas of broken and cemented rock fragments.

The Importance of Studying Fault Blocks

Understanding the characteristics of fault blocks is essential for a variety of applications:

• Earthquake hazard assessment: The geometry and movement history of faults are crucial for predicting the potential for future seismic events.
• Resource exploration: Faults can control the movement of groundwater and hydrocarbons, making them important targets for exploration.
• Geotechnical engineering: Fault zones pose significant engineering challenges, requiring careful consideration in infrastructure design.
• Paleogeographic reconstructions: Studying the offset of rock units allows geologists to reconstruct the past movement of tectonic plates.
• Understanding tectonic processes: The characteristics of fault blocks provide valuable insights into the forces driving plate tectonics.

Conclusion: A Foundation for Geological Understanding

The simple yet powerful terms "hanging wall" and "footwall" provide a fundamental framework for understanding the geometry and kinematics of faults. By analyzing the characteristics of these fault blocks—their relative movement, displacement, and geological context—geologists can gain crucial insights into Earth's dynamic processes, from mountain building to earthquake generation. This knowledge is not only academically important but has vital implications for hazard mitigation, resource management, and infrastructure development. Continued research and advanced techniques in geological mapping and geophysical analysis further refine our understanding of these critical components of Earth’s structure. The complexity of fault systems underlines the need for ongoing investigation and a multifaceted approach to their analysis.