Fig.4.1.1: Bathymetric map of the Eastern Pacific (ETOPO 2008) with the spreading zones (red), transform faults (yellow) and fracture zones (dashed white) on the East Pacific Ridge and Galápagos spreading center.

On the bathymetric map of the Pacific Ocean (Fig. 4.1.1), the morphological structures of the mid-oceanic ridges and deep-sea trenches above the subduction zones can be clearly seen. It can be seen that in particular the mid-oceanic ridges are repeatedly interrupted and offset. This is exactly where the transform faults are located. They connect the offset ridge segments and thus form a continuous plate boundary together with the ridge segments. Structural elements that are a direct continuation of the transform faults are the fracture zones. However, there is no lateral displacement on them, only vertical compensatory movements occur here. This is one of the most frequently observed errors in the illustration of transform faults: the fracture zones are referred to as transform faults – but they are not.

Fig.4.1.2: Bathymetric map of the South Atlantic (ETOPO 2008) with the spreading zones (red), transform faults (yellow) and fracture zones (dashed white) on the Mid-Atlantic Ridge.

In the southern Atlantic, the morphology of the ocean floor is much more pronounced than in the Pacific. This is due to the spreading velocity, which is significantly lower in the Atlantic than in the Pacific, so that stronger morphological differences can emerge. On the map (Fig. 4.1.2) you can see the often large offset of the ridge segments, which can be over 900 km. The fracture zones, which also occur in the extension of the transform faults, are also very clearly pronounced here.

Fig.4.1.3: Opening of the Atlantic between South America and Africa. Left: situation shortly before the opening around 110-115 million years ago, right: current situation; yellow lines: spreading zones, red lines: transform faults (Meschede, unpubl., 2023).

The length of the transform faults is fixed from the beginning and does not change after the opening of the ocean, at least when considering transform faults connecting two ridge segments. There are also other situations where the length of the transform fault changes, which will be discussed in a later chapter. If one moves the lines of the ridge axis and the connecting transform faults along the fracture zones to the edges of the two continents, once west to the coast of South America and once east to the coast of Africa, one can see that they are in fairly good agreement with the coastlines on both sides.

In Fig. 4.1.3, the position of the two continents South America and Africa in the Middle Cretaceous on the left side is compared with the current situation on the right side. You can clearly see that the course of the ridge axis, together with the transform faults, corresponds exactly to the fault line between South America and Africa, where a graben was formed 115 million years ago.

model for spreading between Africa and South America

Additional material: PDF file for download

The pdf file contains an easy to create paper/fabric model of the spreading between Africa and South America. The opening of the ocean between the two continents can be simulated in a very simplified form.

‘the video animation of Fig. 4.1.4 shows in a very simplified sketch the process of the breakup of a continent. A continent breaks apart on an irregular fracture zone. First, a graben is formed in which, soon after the opening, new oceanic crust forms, which then forms an ever-widening ocean. The animation shows that the length of the transform fault remains constant throughout the spreading activity. The transform fault extends from the central rift graben of one ridge segment to the rift graben of the next ridge segment and not beyond. The transform faults in this animation are indicated by the three horizontal, purple lines.

Fig.4.1.4: Animation of the breakup of a continent with the opening of an ocean and the development of transform faults and fracture zones (Meschede, unpubl., 2022).

Fig. 4.1.5: Age of the oceanic crust (Meschede, unpubl., 2022, modified after Frisch & Meschede, 2021)

The age map of the oceanic crust clearly shows the offset of the ridge segments. The magnetic stripe patterns are always created parallel to the ridge axes, so they are inevitably offset.

Fig. 4.1.6: Model of a transform fault between two ridge segments of a spreading zone (from Frisch & Meschede, 2021)

Figure 4.1.6 shows a model of a transform fault between two ridge segments of a spreading zone. The transform fault is precisely defined with a beginning and an end: it runs from transformation point to transformation point. Beyond the transformation point, the directions of movement of the two plates are in the same direction, they run parallel and there is no longer any lateral displacement there, unlike in the transform fault. These zones are called fracture zones. They are not inactive because tectonic movements occur here too. However, this tectonic movement is exclusively vertical and is caused by the different rates of sinking of the oceanic lithosphere of different ages. In Chapters 3.2 and 3.3, which deal with the formation and aging of oceanic lithosphere, the topic that the average weight of the oceanic lithosphere increases with age and that it therefore sinks more and more into the asthenosphere was already discussed. The subsidence is always particularly rapid at the beginning of the spreading, because as the asthenosphere cools, first a lot and then less and less lithospheric mantle is added from below to the oceanic lithosphere. The elevation of the oceanic crust is accordingly equalized with age as seen in Fig. 4.16. The fracture zone is shown by the arrows of different lengths on the section; at the back is the slightly older, slower sinking oceanic crust and at the front is the younger, faster sinking oceanic crust.