---
title: "Fault gouge, breccia and mylonite"
subject: "Engineering Geology I"
chapter: 5
key: "semester_3/engineering_geology_i:fault-gouge-breccia-and-mylonite"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/fault-gouge-breccia-and-mylonite
site: BCT Engineering
---

# Fault gouge, breccia and mylonite

Engineering Geology I · Chapter 5: Structural Geology

- Asked in: 2059 Chaitra (old course), 4 marks
- Video: [Fault Rocks](https://www.youtube.com/watch?v=mSbm5zL2DwQ) — Zak Stark
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/fault-gouge-breccia-and-mylonite
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-5

## Question

Differentiate between fault gouge, fault breccia and mylonite.

## Answer

| Point | Fault gouge | Fault breccia | Mylonite |
|---|---|---|---|
| Nature | Soft, clay-like, finely ground rock powder | Angular broken fragments of rock cemented together | Hard, fine grained, banded rock |
| Grain size | Very fine (clay to silt) | Coarse, angular fragments (>2 mm) | Very fine, crushed and recrystallised |
| Formation | Intense grinding at shallow depth along the fault | Crushing at shallow depth in brittle rock | Ductile shearing and recrystallisation at greater depth |
| Fabric | Unconsolidated, soft | Chaotic, no foliation | Foliated, banded |
| Strength | Very weak, plastic when wet | Weak to moderate | Fairly strong but brittle |
| Engineering | Impermeable but unstable; causes sliding and squeezing | Permeable; leakage and instability | Planar weakness along banding |

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which is soft, clay-like finely ground rock?
   A. Fault breccia
   B. Fault gouge
   C. Mylonite
   D. Basalt
   Answer: B. Gouge is very fine and soft. (Key point: Gouge)

2. What is fault breccia?
   A. A banded foliated hard rock
   B. Soft clay-like powder
   C. Angular broken rock fragments cemented together
   D. Rounded pebbles
   Answer: C. Breccia has coarse angular fragments. (Key point: Breccia)

3. How does mylonite form?
   A. By shallow crushing of brittle rock
   B. By cooling of magma
   C. By deposition in rivers
   D. By ductile shearing and recrystallisation at greater depth
   Answer: D. Mylonite forms at depth. (Key point: Mylonite formation)

4. Which causes sliding and squeezing, being very weak and plastic when wet?
   A. Fault gouge
   B. Mylonite
   C. Breccia
   D. Granite
   Answer: A. Wet gouge is weak and plastic. (Key point: Engineering)

## Related in Engineering Geology I

- [Fault versus joint](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Afault-versus-joint) — Engineering Geology I, chapter 5, asked 2×
- [Fault versus thrust](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Afault-versus-thrust) — Engineering Geology I, chapter 5