Overthrust vs. Thrust

Attribute	Overthrust	Thrust
Definition	A type of reverse fault where older rocks are pushed over younger rocks	A type of compressional stress that shortens and thickens the crust
Direction of Movement	Horizontal	Vertical
Resulting Landform	Overlying rocks are thrust over underlying rocks	Folds and faults are formed
Commonly Associated with	Mountain building	Mountain building

Definition

Overthrust and thrust are both geological terms used to describe the movement of rock layers in the Earth's crust. Thrust refers to a type of fault where older rocks are pushed up and over younger rocks, creating a reverse fault. Overthrust, on the other hand, is a specific type of thrust fault where the older rocks are pushed a great distance over the younger rocks, often resulting in significant displacement.

Formation

Thrust faults are typically formed by compressional forces in the Earth's crust, where tectonic plates collide or move towards each other. This compression causes the rocks to deform and slide over each other, creating the characteristic reverse fault. Overthrust faults, on the other hand, are formed by a similar process of compression but result in much larger displacements due to the rocks being pushed a greater distance.

Characteristics

Thrust faults are characterized by the older rocks being pushed up and over the younger rocks at a relatively shallow angle. This results in a ramp-like structure where the older rocks are stacked on top of the younger rocks. Overthrust faults, on the other hand, have a much shallower angle of inclination and can involve significant horizontal displacement of the rocks, sometimes over tens or hundreds of kilometers.

Examples

One of the most famous examples of a thrust fault is the Lewis Overthrust in Montana and Alberta, Canada. This fault is a classic example of a thrust fault where older rocks have been pushed up and over younger rocks. The overthrust fault is estimated to have moved rocks over 50 miles, creating a significant displacement. Another well-known example of an overthrust fault is the Himalayan mountain range, where the Indian plate is being thrust under the Eurasian plate, resulting in the uplift of the Himalayas.

Impacts

Thrust faults and overthrust faults can have significant impacts on the geology and topography of an area. Thrust faults can create mountain ranges, such as the Rockies in North America, by uplifting older rocks over younger rocks. Overthrust faults, with their larger displacements, can create complex geological structures and can even lead to the formation of oil and gas traps as rocks are pushed and folded over each other.

Conclusion

In conclusion, while both thrust and overthrust faults are types of faults caused by compressional forces in the Earth's crust, they differ in terms of the extent of displacement and angle of inclination. Thrust faults typically involve older rocks being pushed up and over younger rocks at a shallow angle, while overthrust faults involve larger displacements and shallower angles of inclination. Both types of faults play a significant role in shaping the Earth's geology and topography.