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# 3.8 Conservative plate boundaries and transform faults

Conservative plate boundaries and transform faults occur when plates slide past each other in opposite directions, but without creating or destroying lithosphere. Transform faults connect the end of one plate boundary to the end of another plate boundary, so there are potentially three types of transform fault:

• those that link two segments of a constructive boundary

• those that link two destructive boundaries

• those that link a destructive boundary with a constructive boundary.

Transform faults linking two constructive boundaries are the most common, and account for the displacements between adjacent segments of mid-ocean ridges. Accordingly, this type of ocean transform fault forms an integral part of constructive plate boundaries, and their position is made obvious by the jagged shape of parts of the ocean-ridge system that are split into several segments by series of so-called fracture zones. Examples can be easily seen on the Cocos-Nazca Ridge (also known as the Galapagos Spreading Centre), and the Pacific Ocean spreading ridge (i.e. East Pacific Rise) between 10°N and 10°S, and 40°S and 55°S respectively, or manifest as shorter segments along the Atlantic Ocean spreading ridge between 0°and 40°S. Generally, oceanic transform faults occur at right angles to spreading ridges and, therefore, their orientation is indicative of the direction of plate motion.

Transform faults are seismically active - but only where two different plates are adjacent to one another. In Figure 19, the fault trace marks the boundary between plates A and B. Plate A is moving towards the east while plate B is moving towards the west.

Figure 19 Diagram showing relative movements across an oceanic transform fault W, X, Y and Z off-setting a constructive plate boundary. The large arrows indicate the sense of plate motion away from the ridges XX' and YY'.

## Question 16

Describe the sense of relative movement along the length of the fault between W and X, X and Y, and Y and Z.

Between W and X, the fault separates different parts of plate B and so there is no differential movement. Between X and Y it separates plate A from plate B, which are moving in opposite directions. Between Y and Z it separates different parts of plate A and there is, again, no differential movement.

## Question 17

Which part of the fault between W and Z will be seismically active?