Ridge To Concave Dextral Transform Fault

The theory of sea floor spreading proposes that oceanic lithosphere is created at mid-ocean ridges and is

Pleistocene

Pliocene

Miocene

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Paleocene

Maastrichtian

Campanian

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13 16

Coniacian o e c ta ret

Santonian

Turonian

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Albian

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Berriasian

Hauterivian M6 M8 M10

Valanginian M12

M16 M17 M18 M19

Tithonian M20 M21

Kimmeridgian M23 M24 M25

Oxfordian M28

Figure 4.13 A geomagnetic polarity timescale for the past 160 Ma together with oceanic magnetic anomaly numbers (after McElhinny & McFadden, 2000, with permission from Academic Press. Copyright Elsevier 2000).

balanced by the complementary destruction of oceanic lithosphere at subduction zones. While this theory neatly explains the geometry of lithospheric behavior in two dimensions, a problem arises when the third dimension is considered, namely where do ridges and trenches terminate horizontally? This problem was addressed by Wilson (1965), who proposed that the ends of these features were linked by a new class of faults which he called transform faults. At these faults there is neither creation nor destruction of lithosphere,

Figure 4.14 Relative positions of Europe and Africa with respect to North America illustrating their separation during the Mesozoic and Cenozoic. Ages of reconstruction shown in millions of years (redrawn from Pitman & Talwani, 1972, with permission from the Geological Society of America).

Figure 4.14 Relative positions of Europe and Africa with respect to North America illustrating their separation during the Mesozoic and Cenozoic. Ages of reconstruction shown in millions of years (redrawn from Pitman & Talwani, 1972, with permission from the Geological Society of America).

but rather the motion is strike-slip, with adjacent lithosphere in tangential motion.

The existence of large lateral relative movements of the lithosphere was first suggested from marine magnetic anomalies in the northeastern Pacific (Fig. 4.1), which were found to be offset along fracture zones. Combined left lateral offsets along the Mendocino and Pioneer faults amount to 1450 km, while the right lateral offset across the Murray Fault is 600 km in the west and only 150 km in the east (Vacquier, 1965).

However, in interpreting these fracture zones as large scale strike-slip faults, a major problem arises in that there is no obvious way in which the faults terminate, as it is certain that they do not circumnavigate the Earth to join up with themselves. Wilson (1965) proposed that the faults terminate at the ends of ridges or trenches, which they commonly meet at right angles.

Wilson termed this new class of faults transform faults, because the lateral displacement across the fault is taken up by transforming it into either the formation of new lithosphere at a terminated ocean ridge segment or lithosphere subduction at a trench. Figure 4.15 shows the plan view of an ocean ridge crest that has been displaced by transcurrent and transform faulting. The transcurrent, or strike-slip, fault (Fig. 4.16b) causes a sinistral offset along a vertical plane which must stretch to infinity beyond the ridge crests. The transform fault (Fig. 4.15a), however, is only active between the offset ridge crests, and the relative movement of the lithosphere on either side of it is dextral. Transform faults differ from other types of fault in that they imply, indeed derive from, the fact that the area of the faulted medium, in this case lithosphere, is not conserved at ridges and trenches.

Transcurrent Transform Fault

(a) Transform fault (b) Transcurrent fault

Figure 4.15 Comparison of transform and transcurrent faults.

(a) Transform fault (b) Transcurrent fault

Figure 4.15 Comparison of transform and transcurrent faults.

Transcurrent Faults

Figure 4.16 (a) Six possible types of dextral transform fault: (i) ridge to ridge; (ii) ridge to concave arc; (iii) ridge to convex arc; (iv) concave arc to concave arc; (v) concave arc to convex arc; (vi) convex arc to convex arc. (b) Appearance of the dextral transform faults after a period of time (redrawn from Wilson, 1965, with permission from Nature 207, 334-47. Copyright © 1965 Macmillan Publishers Ltd).

Figure 4.16 (a) Six possible types of dextral transform fault: (i) ridge to ridge; (ii) ridge to concave arc; (iii) ridge to convex arc; (iv) concave arc to concave arc; (v) concave arc to convex arc; (vi) convex arc to convex arc. (b) Appearance of the dextral transform faults after a period of time (redrawn from Wilson, 1965, with permission from Nature 207, 334-47. Copyright © 1965 Macmillan Publishers Ltd).

Wilson (1965) defined six classes of transform fault that depend upon the types of nonconservative features they join (Fig. 4.16). These may be an ocean ridge, the overriding plate at a trench or the underthrusting plate at a trench. Figure 4.16a shows the six possible kinds of dextral transform fault; a further six based on sinistral movement are also possible. Figure 4.16b shows how the transform faults would develop with time. Cases (i) and (v) will remain unchanged, cases (ii) and (iv) will grow, and cases (iii) and (vi) will diminish in length with the passage of time.

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