Fig.4.2.1: 3D illustration of a transform fault on the East Pacific Ridge according to the bathymetric map of the East Pacific (ETOPO1 2008) from 9° bis 11° N (Marine Geoscience data systems, 2008).

There are different types of transform faults, not just those between two segments of a mid-oceaniv ridge (Fig. 4.2.1). Short repetition: A transform fault always runs exactly from transformation point to transformation point and not beyond. The continuation of the transform fault, which is often clearly visible in the morphology of the seafloor, is the fracture zone, where the plates move in the same direction. This is therefore no longer a transform fault.

Now of course a transform fault does not only exist between two ridge segments, as we find in large numbers in the mid-oceanic ridge systems, but also when there are different plate boundaries, for example a mid-oceanic ridge on one side and a subduction zone on the other side, or between two subduction zones.

The short animation in Figure 4.2.2 shows two examples of transform faults located between a mid-oceanic ridge and a subduction zone. In contrast to rigde-ridge transform faults, such transform faults are rarely stable in length. On the contrary, the length of the transform faults changes depending on the polarity of the subduction zone and the respective plate velocities.

Above, the plate shown in light blue color is subducted beneath the greenish one, below it is the other way around. Above, the greenish plate becomes larger due to the formation of new oceanic crust at the mid-oceanic ridge, but nothing is taken away from it in the subduction zone because the other side is subducted and therefore the transform fault increases in length. This is different below, because the greenish plate subducts beneath the blue one, which is why the transform fault becomes shorter here. With different plate boundaries at the ends of a transform fault, the length of the transform fault increases or shortens, depending on the polarity of the subduction zone.

Fig.4.2.3: Evolution of the Juan de Fuca plate in the eastern Pacific (based on the bathymetric map of the eastern Pacific and topography of North America (after ETOPO1, 2008; Meschede, 2023, unpubl.).

An example for the shortening of a transform fault is the Mendocino transform fault between the Juan de Fuca plate and the Pacific plate (Fig. 4.2.3). About 40 million years ago, the Juan de Fuca Plate was part of the Farallon Plate. The Farallon Plate was a larger plate which at that time also included what is now known as the Cocos Plate and the Nazca Plate further to the south. The Mendocino transform fault was a normal ridge-ridge transform fault at the time. However, the Farallon plate continued to subduct beneath North America and about 30-35 million years ago the Mendocino transform fault reached the subduction zone, so that the ridge-ridge transform fault became a ridge-subduction zone transform fault. Since then, the Mendocino transform fault has become shorter and the Juan de Fuca plate has become smaller.

Figure 4.2.4 shows transform faults between two subduction zones. There are three ways in which transform fault may develop. We are either dealing with opposing subduction zones, which can lead to shortening or an increase in length. This depends on the respective plate motion velocities. However, if both subduction zones move in the same direction and subduct at the same rate, the transform fault will maintain its length. In the middle of the animation the situation is shown with two subduction zones in the same direction; here the transform fault remains constant. For subduction zones in opposite directions, depending on the direction of subduction, the transform fault will either shorten, as in the example on the left, or increase in length, as can be seen on the right. So you can see here too that it depends on the velocity of subduction and, above all, its polarity, whether there is a shortening or an increase in length at the transform fault.

Fig.4.2.5: Transform fault (Alpine transform fault zone, New Zealand) between the west-dipping subduction zone in the Hikurangi trough (Pacific plate under the Indo-Australian plate) and the east-dipping subduction zone SW of New Zealand (Indo-Australian plate under the Pacific plate) ( Frisch & Meschede, 2004, unpubl.)

A nice example of a transform fault connecting two subduction zones can be found in New Zealand. In northern New Zealand is the Hikurangi Trough, where the Pacific Plate is subducted beneath the Indo-Australian Plate (Fig. 4.2.5). Southwest of New Zealand it is the other way around, where the Indo-Australian plate is subducted under the Pacific plate. And between the two subduction zones lies the Alpine transform fault zone (named after the New Zealand Alps). In the current constellation, the Alpine Fault will probably become longer. Such a situation, in which the direction of subduction is reversed over a relatively short distance, is called a subduction flip.

Fig.4.2.6: San Andreas transform fault between the Pacific and North American plates (photo: pixabay).

Most transform faults are found between segments of mid-oceanic ridges. However, sometimes there are situations where transform faults cut through continental crust, which can then also be studied on land.

Example: San Andreas fault in California, USA: This transform fault is morphologically very pronounced, as you can see in the photo (Fig. 4.2.6). It stretches from the subduction zone in the north down to Palm Springs in southern California.

Fig.4.2.7: Plate boundary between the Pacific and North American plates with the San Andreas transform fault (based on the bathymetric map of the Eastern Pacific and topography of North America, after ETOPO1, 2008; Meschede, 2023, unpubl.)

In the north there is a triple junction between two transform faults (Fig. 4.2.7; these are the Mendocino fault, which was just mentioned before, and the San Andreas fault) and a subduction zone, this is the Cascadia subduction zone along which volcanic eruptions occur repeatedly, such as e.g., Mt. St. Helens in 1980. Triple junctions will be discussed later in a separate chapter. To the south, the San Andreas fault merges into a series of spreading zones and smaller transform faults that extend throughout the Gulf of California. 40 million years ago there was still a subduction zone along the coast in this area, but it disappeared due to the collision with the mid-oceanic ridge between the Farallon Plate and the Pacific Plate. This collision fundamentally changed the direction of plate movement in this zone. The convergence movement in the subduction zone became a lateral movement, as is typical in transform faults.

Fig.4.2.8: Plate boundary between the European and Anatolian plates with the North Anatolian transform fault (from Frisch & Meschede, 2021).

Example: North Anatolian fault in Turkey between the European and Anatolian plates (Fig. 4.2.8). Severe earthquakes occur regularly in this fault zone, most recently in 1999 with a severe earthquake in the Izmit region. However, the severe earthquake in Turkey in spring 2023 does not belong to this fault zone, but to the East Anatolian Fault Zone, which also represents a transform fault and which meets the North Anatolian Fault in the east of Turkey.