2.6 Transform Boundaries

Charlene Estrada

Block diagram of transform boundary sliding two plates of continental lithosphere past one another
Figure 2.6.1. A transform boundary causes a fault between two plates of the lithosphere, which will slide past one another. This motion does not create or destroy crust and will cause earthquakes, but no volcanoes.

A transform boundary occurs when two tectonic plates move past one another. Shear stress operates at transform boundaries, which involves sliding motion. No lithosphere is destroyed or created, and mountain chains are not built at transform boundaries. Although none of these events occur at transform boundaries, they are far from boring; the continuous stress that builds within the lithosphere from the sliding motion causes faulting and earthquakes.

Most transform boundaries are associated with the spreading spreading centers at mid-ocean ridges. They occur near these divergent boundaries because the spreading rate changes along a ridge. Because the surrounding rock along the ridge is hard and brittle, it accommodates these shifts in spreading rates with sliding motions. Perhaps the most famous transform boundaries, however, are those on the continental lithosphere with effects that are directly felt by nearby cities and towns.

Transform boundaries can cause both large faults and a series of smaller associated faults. Transform faults refer to the lateral displacement of large rock units due to the shearing motion caused by a transform boundary. The motion at a transform fault is classified into two categories: right-lateral and left-lateral. However, these categories are not determined by the composition of the lithosphere. An examination of the two transform faults below demonstrates that they are very similar. Can you notice the difference?

Right lateral fault, where the block on the right appears to be moving toward you if you were looking down the fault line.
Figure 2.6.2. Block diagram and horizontal view of a transform fault. The different colors represent rock layers. The scale is at the bottom.
Left lateral fault, where the block on the left appears to be moving toward you if you are looking down the fault line.
Figure 2.6.3 Block diagram and horizontal view of a transform fault. The different colors represent rock layers. The scale is at the bottom. How are these diagram different from the ones on Figure 2.6.2?

A type of transform plate motion can be identified by examining the two tectonic plates from a bird’s-eye view. For example, below is the Piqiang fault from China:

Piqiang fault in China where one set of layers on the left appear to be displaced at least 2km above the others along the fault line.
Figure 2.6.4. Google Earth imagery showing the transform Piqiang fault in China.

To determine what type of transform fault it is, follow these steps:

  1. Identify the boundary between the two rock units.
    Same image of the Piqiang fault as above, but with a red line superimposed over the fault line.
    Figure 2.6.5. The red color traces the fault line between rock units.
  2. Find features that have been displaced apart from one another due to the sliding motion of the plates or rock units.
    Piqiang fault image with red fault line drawn and blue circles indicating layers of rocks that appear to be displaced from one another.
    Figure 2.6.6. Blue circles indicate distinctive rock units that have been displaced from one another by the fault.
  3. Based upon those displaced features, decide which rock unit appears to be moving toward you. Which side is it on?
    Piqiang fault with purple arrows indicating the direction of displacement for each rock unit along the fault.
    Figure 2.6.7. Purple arrows show the direction the rock units (blue circles) are moving relative to a person looking down the red fault line.
  4. If the plate moving toward you is on the right side, the motion is right-lateral. If it is on the left side, it is left-lateral. This method always works, no matter from which direction you are looking at the boundary!
Piqiang fault with purple arrows indicating direction of rock displacement on each side of the fault line. On the left side, the rocks are moving toward the viewer. On the right side, the rocks are moving away from the viewer.
Figure 2.6.8. The unit of rock layers that moves toward the viewer relative to the red fault line is on the left. The Piqiang Fault is a left-lateral fault.


Share This Book