Transform Faults Represent One of the Three

8 Transform faults ransform faults represent one of the three Because of the drift of the newly formed oceanic types of plate boundaries. A peculiar aspect crust away from ridge segments, a relative move- T of their nature is that they are abruptly trans- ment along the faults is induced that corresponds formed into another kind of plate boundary at their to the spreading velocity on both sides of the termination (Wilson, 1965). Plates glide along the ridge. Th e sense of displacement is contrary to fault and move past each other without destruc- the apparent displacement of the ridge segments tion of or creation of new crust. Although crust is (Fig. 8.1b). In the example shown, the transform neither created or destroyed, the transform margin fault is a right-lateral strike-slip fault; if an observer is commonly marked by topographic features like straddles the fault, the right-hand side of the fault scarps, trenches or ridges. moves towards the observer, regardless of which Transform faults occur as several diff erent geo- way is faced. Transform faults end abruptly in a metries; they can connect two segments of growing point, the transformation point, where the strike- plate boundaries (R-R transform fault), one growing slip movement is transformed into a diverging or and one subducting plate boundary (R-T transform converging movement. Th is property gives this fault) or two subducting plate boundaries (T-T fault its name. In the example of the R-R transform transform fault); R stands for mid-ocean ridge, T for fault, the movement at both ends of the fault is deep sea trench ( subduction zone). R-R transform transformed into the diverging plate movement of faults represent the most common type and are the mid-ocean ridge. Beyond the transformation common along all mid-ocean ridges (Fig. 1.5). point, the crust on both sides of the imaginary The length of R-R transform faults remains prolongation of the fault moves in the same direc- constant; however, in contrast the length of R-T tion with the same velocity. Th erefore, no strike-slip and T-T transform faults, with one exception, either movement occurs in the prolongation of the fault grows or shrinks (Fig. 8.1). Transform faults that beyond the transformation point. Th e dashed lines connect subducting plate boundaries typically cut in Figure 8.1b mark this fi ssure where younger through areas of continental crust. Examples in- crust is welded against older crust beyond the the clude some of the most notorious transform faults, transformation point. the San Andreas Fault in California (Fig. 8.7), the Th e best confi rmation for the mechanism of North Anatolian Fault in Turkey (Fig. 8.10), the oceanic transform faults is the earthquake occur- Jordan Fault in the Middle East (Fig. 8.6), and the rence and the fault-plane solutions of the quakes Alpine Fault on the southern island of New Zealand (Ch. 2). Only the segments of the active transform (Fig. 8.11). fault between the ridge segments indicate continu- ous seismic activity; seismicity ends abruptly at the Oceanic transform faults transformation points, the termination points of Mid-ocean ridges are intercepted by R-R transform the faults. Th e fault-plane solutions always indicate faults that normally are perpendicular to the ridge. horizontal displacement parallel to the fault with a Th us the ridges are subdivided into segments that sense of displacement which is consistent with the mostly have lengths of several tens of kilometers. prediction of the theory. At the East Pacifi c Rise west of Mexico, large transform faults occur at distances of several hundred Fracture zones in the ocean fl oor kilometers and smaller ones occur at distances of In spite of its abrupt end, the prolongation of a 10 or 20 km. In each case the fault connects two transform fault is topographically expressed on segments that appear to be dislocated. However, the ocean fl oor. Th is expression is caused by the the ridge segments maintain their distance from diff erent ages of crust across the prolongation. Th is each other and are not dislocated by the fault; in age diff erence creates an isostatic imbalance so the fact, the fault forms a connecting link of constant crust lies at diff erent depths. Across this prolonga- length (Fig. 8.1b). Although every transform fault tion, there is no plate boundary. With respective is a strike-slip fault, not all large strike-slip faults increasing age and increasing distance to the ridge, represent a plate boundary (Fig. 8.1a). the ocean fl oor subsides (Fig. 4.11). Younger crust W. Frisch et al., Plate Tectonics, DOI 10.1007/978-3-540-76504-2_8, © Springer-Verlag Berlin Heidelberg 2011 Licensed to jason patton<[email protected]> 8124 Transform faults Fig. 8.1 Sketch maps showing possible confi gurations a) of transform faults (b-g) compared to the geometry of a common strike-slip fault (a). All examples shown have right-lateral displacements. The movement velocity (v) is the relative movement between the two plates. Each respective lower image indicates the modifi cation to fault geometry over time. R: mid-ocean ridge, T: deep sea trench ( subduction zone). common strike-slip fault b) R - R c)R - T d) R - T v/2 v/2 v/2v/2 vv/2 v/2 v is at the same time subsiding faster than older crust. v Because the prolongation of the active part of the v v transform fault separates crustal segments of dif- v/2 v/2 ferent ages, relative vertical movements occur along this fi ssure (Fig. 8.2). Th erefore, these fi ssures are planes of movement, mostly vertical in nature, and are called fracture zones. Beyond a transformation point at a subducting plate boundary, a fracture zone cannot continue because plate segments are not welded together at such a location but rather are split; crust on either side is of the same age (Fig. 8.1c–g). constant in length shrinking by v/2 growth by v/2 Th e fracture zones on the ocean fl oor are clearly e)T - T f)T - T g) T - T v v v visible in submarine digital relief maps. Th ey represent zones of weakness within the oceanic plates v v v and are thus easily reactivated. Prominent examples v v v can be observed in the Eastern Pacifi c where several signifi cant fracture zones occur. Th ey are spaced at distances of approximately 1000 km and can be traced from the margin of the North American Plate westward into the ocean (Fig. 8.3). Also in the Atlantic, especially in the region between Western Africa and Brazil, a number of large fracture zones occur as the prolongation of transform faults. At the fracture zones, the magnetic stripe patterns appear constant in length shrinking by v growth by v displaced in the same dimension as the mid-ocean mid-oceanic ridge fracture zone ridges (Fig. 2.12). subduction zone transform fault transformation point Oceanic transform faults and fracture zones commonly form escarpments, ridges or deep Fig. 8.2 Geometry of transform faults at a mid-ocean troughs with relief greater than 2000 m. Th e in- ridge. Although the strike-slip movement ends abruptly at tervening segments of the mid-ocean ridge appear the transformation point, a vertical movement occurs in its as dome-shaped bulges (Ch. 5). Th e Puerto-Rico prolongation (“ fracture zone”) because the adjacent crustal Trough at the northern edge of the Caribbean Plate segments of diﬀ erent ages subside with diﬀ erent rates. is an example of a large depression along a transform fault and its dimensions approach those of a transformation point rift axis transform fault deep-sea trench at a subduction zone (Fig. 8.4). It fracture zone fracture zone obtains a maximum water depth of 9219 m. Along with vertical compensation the Murray fracture zone in the northeastern Pa- motion cifi c (Fig. 8.3), a trough with a water depth of more than 6000 m is developed; in contrast, the abyssal plain is 1000 m higher. Also along this fracture zone are elongate ridges that protrude more than 1000 m above the ocean fl oor. The occurrence of ridges and troughs along oceanic transform faults is a result of tensional and compressional processes. Tension or compression occurs along the fault when the plate motion Licensed to jason patton<[email protected]> Continental transform faults 1258 direction slightly changes. Commonly, the narrow, 140°W 120°W 100°W 80°W elongate stretched plate segments between the 40°N transform faults and fracture zones may be locally North America rotated further complicatig the fault geometry. Ten- Pioneer f.z. sion and compression may alternate at the same Murray f.z. location or appear spatially laterally along a fault zone. Tension and compression generate distinct fracture zone products on the ocean fl oor; tensional forces generate deep cracks in which sedimentary deposits form 20°N Molokai f.z. Hawaii and occasionally produce intraplate volcanism Clarion f.z. whereas compressional forces generate reverse transform fault faults or folds, particularly within soft sedimentary Pacific Ocean deposits of the troughs, and ridges result. Th ese spreading movements along the fracture zones are capable of Clipperton f.z. axis producing sporadic, small earthquakes. 0° Galapagos f.z. Th e vertical and lateral movements along oce- Galápagos anic transform faults and fracture zones cause the exposure of various kinds of oceanic rocks; East Pacific Rise these rocks are commonly strongly deformed 10°S Marquesas f.z.

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