GLORIA Investigations of Oceanic Fracture Zones: Comparative Study of the Transform Fault Zone

Journal of the Geological Society, London, Vol. 143, 1986, pp. 743-756, 17 figs. Printed in Northern Ireland

GLORIA investigations of oceanic fracture zones: comparative study of the transform fault zone

R. C. SEARLE Institute of Oceanographic Sciences, Wormley, Godalming, Surrey GU8 5UB, UK

Abstrad: The tectonic patterns of some twenty spreading centre offsets have been determined using the long range sidescan sonar GLORIA and are reviewed in this paper. Offsets of less than about 20 km fail to produce transform faults but are accommodated by short sections of oblique spreading (with overlapping spreading centres in the fast spreading case). At slow slip rates such offsets can be associated with topographic fracture zones that areindistinguishable from those produced at true transform faults. Such features may account for a significant proportion of spreading centre offsets on slowly spreading mid-ocean ridges. Offsets larger than about 30 km produce a true ‘transform fault’. Thiscomprises a 1 to 5 km-wide band of individual faults that are parallel or subparallel to the spreadingdirection. The bandcorresponds to the ‘Transform Fault Zone’ (TFZ) of continental wrenchfaulting. It includes the ‘Principal Transform Displacement Zone’ (PTDZ) and secondary structures such as Riedel shears.A narrow TFZ is normally associated with a prominent FTDZ, which appears to be either a narrow furrow or a single scarp; on the other hand wider zones often fail to show a clearPTDZ on GLORIA.Both types are common on the slowly spreading Mid-Atlantic Ridge, and may even occur at different points along the same large offset transform (such as Romanche), although the narrow type appears to occur preferentially where the local Ridge axis is oblique to the spreading direction. Narrow TFZsand prominent PTDZs appearto be more common on the fast spreading East Pacific Rise. It is suggested that broader TFZs and less prominent PTDZs may form in response to the complicating effects of small components of compression across the transform fault, perhaps as a result of departure of the transform fault trace from a true small circle. Such compression would be more easily supported in the thicker, stronger lithosphere of slowly spreading ridges, but could be relieved by tension due to the ridge-push force where the ridge is oblique to the spreading direction.

Since 1971 the Institute of Oceanographic Sciences has used only about 20 km on either side of the ship. Resolution is the long-range sidescan sonar GLORIA (Somers et al. 1978) about 50m down-range, and along-track resolution de- to study a number of ridge-ridge transform faults that have creases from about 200 m near the track to 2 km at 30 km a variety of offsets and spreading rates. Onthe slowly range. spreading Mid-Atlantic Ridge these include the small-offset In the GLORIA records or ‘sonographs’ shown in this Kurchatov FractureZone (Searle & Laughton 1977) and paper, highlyreflective areas are white and areas of poor FAMOUSarea fracture zones (Laughton & Rusby 1975; acoustic return are black. Various studies, in which features Whitmarsh & Laughton 1975,1976), the larger offset on the sonographs were compared with the results of other Charlie-Gibbs (Searle 1981), Kane(Roest et al. 1984), technique-broad- and narrow-beam echosounding, seismic Oceanographer,Hayes, Atlantis, and Fifteen-Twenty profiling, photography,and manned observation-have fracture zones, and the very large offset Romanche Fracture enabled us to interpret the features with a good degree of Zone (M. V. Thomas, unpublished data; Parson & Searle confidence (e.g. Whitmarsh & Laughton 1976; Searle & this volume). Tamsett (1984) has made a comparative study Laughton 1977; Mougenot et al. 1984; Searle 1984). Long, of all of the fracture zones in the slowly spreading Gulf of narrow, stronglyreflecting lineaments are usuallyfault Aden, and I have also studied the fast slipping Quebrada, scarps, whileslightly broaderones are volcanicridges. Discovery and Gofar fracturezones on the East Pacific Rise Areas of very fresh seafloor containing rough, unsedimented (Searle 1983). Inaddition, we have imaged considerable lava, and other outcrops of lithified rock, generally give rise lengths of the various mid-ocean ridges wherethere are to broad regions of strong backscattering, whileheavily small offsets of the spreading centre but no well-developed sedimented areas appear much darker. We have thus been fracture zones. This paper provides an opportunity to review able to infer the major tectonic elements in fracture zones the findings of those studies and to compare andcontrast the from the sonographs. tectonic style of transform offsets in a variety of contexts. Short offsets GLORIA We start by looking at the smallest spreading-centre offsets, TheGLORIA Mark 2 system has a two-sided beam less thanabout 20km across, on slow spreading ridges. transmitted from a surface-towed vehicle, and can insonify These occur where the overall trend of the mid-ocean ridge up to a @-km wide swath. However, the maximum range is is only slightly oblique to the spreading direction. This is so usually more limited, owing to refraction of sound in the over much of the northern Mid-Atlantic Ridge between water column, and in most of the examples to be shown it is majorfracture zones. Agood example is the 45 “N area

743

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r - 45'06 ' 1 I 1 29'00' 28'00' 27'00'

Fig. 1. (a) GLORIA Sonograph mosaic; (b) inferred tectonic lineaments, on the Mid-Atlantic Ridge crest near 45 "N. After Laughton& Searle (1979).

(Fig. 1; Laughton & Searle 1979). Themedian valley is ends of these intrusion zones because the direction of the defined by inward dipping normal fault scarps, which are minimum compressive stress would be rotated about 45" by offset en 6chelon. Individual scarps are 10 to 20 km long, the shear stresses developing there. and their ends are often curved in a sigmoidal fashion. Short faults oriented at 45" to the spreadingdirection occur at the offsets. The overall fault pattern producedin this way is one Intermediate offsets of zones of straight faults approximately perpendicular to As the offset approaches 20 km (roughly the width of the the spreading direction, separated by short zones of faults median valley on slow spreading ridges), the arrangementof that are at about45" to it. Such zones of oblique faulting do oblique faults becomes more coherent andproduces a 20- to not seem to remain in the same place along the ridge axis, 30-km long well-defined zone of oblique spreading. Figure 3 but appear and disappear more-or-less at random provide to shows a single GLORIA swath across the axis of the a patchwork texture in which faults normal and oblique to medium spreadingCows-Nazca Spreading Centre or the spreading direction alternate, with a typical spacing of 'Galapagos Rift' in the eastern equatorialPacific, where this 10-20 km (Fig. 1). Within such a pattern,the zones of effect is quite clear. Here the position of the spreading axis oblique faulting may occasionally extend up to 50 km, but can be inferred from a band of bright backscattering (largely not more, along the spreading direction. hidden beneath the ship's track to theeast of the offset) and Searle & Laughton (1977) suggested that thispattern the line of symmetry betweenthe inward facing fault scarps. arises when short individual spreading centres or intrusion Note that there is no sign of any tectonic lineament that zones are staggered within the median valley floor (Fig. 2). might represent a transform fault along the N-S spreading Theoblique normal-faults would arisebetween the offset direction.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/5/743/4893072/gsjgs.143.5.0743.pdf by guest on 28 September 2021 I I I rotation of the stress ellipse under the increasing influence of the shear stresses within the transform fault zone (Crane I 1976; Searle & Laughton 1977; Lonsdale 1978; Fox & Gallo I 1984). It should be noted that the sense of curvature is the opposite of that whichwould be produced by faultdrag l across the transform. We observe this characteristic curving of the axial fault scarps at most of the fracture zones we have examined, regardless of offset or slip rate (Searle 1979, 1983). It has also been observed in the Arakapos fault zone in the Troodos ophiolite,which was interpreted by Simonian & Gass (1978) as an ancient transform. The curvature of dykes in the sheeted dyke complex at Arakapos indicates a dextral spreading centre offset (sinistral strike-slip motion on the transform) according to this hypothesis, although this is the opposite of that inferred by Simonian & Gass. We have observed major oblique escarpments similar to those at KurchatovFracture Zone in the walls of most slowly slipping fracture zones, though they are usually more clearly seen the inactive limbs than in the active transform a b in valley. We have not seen such major features in fast slipping Fig. 2. Sketch to show how normal and oblique faulting can fracturezones, although small-scale oblique scarps are alternate along a slightly oblique ridge axis, the oblique faults being common between closely-spaced, fast-slipping transforms produced by shear between the offset ends of adjacent intrusion (Searle 1983). zones. (a) Postulated intrusion zones and resultant stresses; (b) fault Figure 7 shows large,oblique scarps in theeastern pattern that might result from stresses in (a). After Searle & inactive limb of Charlie Gibbs Fracture Zoneon the northern Laughton (1977). Mid-Atlantic Ridge. I believe theseoblique escarpments

Such zones of oblique spreading are quite common on slow spreading ridges, and areoften associated with fracture-zone-like topography. This was first observed in the Kurchatov Fracture Zone which was surveyed in some detail in1975 (Searle & Laughton 1977). This is a typical oblique-spreading offset, with a ridge-ridge ofket of 20 km (Fig. 4). Figure 5 shows the offset region viewed with the sonar looking toward the north; this should be a favourable arrangement for seeing E-W features; but clearly there is no major lineament, or transform fault, parallel to the 100" spreadingdirection. Instead the offset region ischarac- terized by a 45" oblique linear trough. Although thereis no transform fault, there is a well developed fracture valley, which has a relief of about 2000 m and can be traced for 150 km (about 15Ma) fromthe ridge axis (Searle & Laughton 1977). Several such oblique offsets have been seen in the Gulf of Aden, and in one of these (at 48" 32' E) an earthquake mechanism confirms that normal faulting is occurring with an ENE-WSW strike, about 45" oblique to the NNE-SSW spreadingdirection (Tamsett 1984). In fact, of about 10 ridge offsets of around 20 km or more in the Gulf of Aden, only four clearly have transforms while at least five are oblique offsets (Fig. 6).Note, however, that even the oblique-spreading offsets can be associated withtypical fracturezone topography, i.e. linear ridges and valleys aligned along the spreading direction. I \ I I Thereare two otherimportant features of oblique P' spreading offsets. One is the curvature, toward the direction 20 km of offset, of the normal faults that parallel the spreading axis; the other is the existence of prominent 45" oblique Fig. 3. (a) Two-sided GLORIA swath across the axis of the escarpments in thefracture zone walls. Bothfeatures are medium spreading Cows-Nazca Spreading centre in the eastern well exemplified at Kurchatov Fracture Zone (Fig. 4). equatorial Pacific; (b) tectonic interpretation. The spreading axis It is now generally agreed that the curvature of normal (stipple) can be inferred both fromthe line of symmetry of opposing faultsnear fracture zones is aresult of the progressive fault scarps and from the axis of the zone of maximum reflectance.

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bl i

-4OO30.N fig. 4. (a) Part of mosaic of east- and west- looking GLORIA sonographs; (b) bathymetric chart (contour interval 250 m), over the central part of Kurchatov Fracture Zone, Mid-Atlantic Ridge. After Searle 30.w 29.W (1979).

I 20 km

b I

Fig. 5. (a) North-looking GLORIA sonograph; (b) bathymetric chart, of the spreading centre offset at Kurchatov Frac- ture Zone. Contour interval 250 m. Box on chart locates sonograph, and arrow shows direction of insonification. Stipple indicates inferred plate boundaries. Note that there is no clear tectonic lineament along the E-W spreading direction.

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Fig. 6. Topography of the Gulf of Aden (contour interval 500 fathoms), after Laughton (1970), with plate boundary inferred from GLORIA data. Transform faults are indicated by a single line and marked 'TF', spreading axes are indicated by double lines, and oblique spreading offsets are marked 'OS'.

29" 2 o 29" 8" 2 27" 6" 2 5" I

52 5 2"

I l 29" 2 8" 2 7" 2 6" 2 5"

b 29" 2 8" 2 7" 2 6" 2 5" t Fig. 7. (a) Two-sided GLORIA sonograph swath, (b) topography (in kilometres), of part of the east- em, inactive, limb of Charlie- Gibbs Fracture Zone, North Atlantic ocean (after Searle 1981). Note the clearly displayed oblique scarps (heavy lines in (b) in the walls of the 25" fracture valley.

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have an originsimilar to that of the curved axial faults active slip plane, but which may migrate back and forth with (Searle 1979). Oblique tension gashes or small normal faults time in the transform fault zone. are produced at 45" to the spreading direction in the oblique The critical length of offset needed to produce a offset region (or,at larger offset transforms, atthe transform fault on a slow spreading ridge appears to be ridge-transform intersections). Then, as seafloor is lifted up about 25 to 35 km: the largest oblique spreading offsets-at the sides of the median valley, movement on these faults Kurchatov Fracture Zone (Searle & Laughton 1977), in the serves to decouple the crust rising into the crestal mountains FAMOUSArea (Searle 1979), and in the Gulf of Aden from that remaining in the transform valleyfloor. At the (Tamsett 1984)-are 20-25 km, while the smallestoffset sametime, the vertical differential movement createsthe known to produce a well-developed transform (in the Gulf large (20-km long, 2000-m high) scarps. of Aden) is 35 km (Tamsett 1984). The most detailed GLORIA study of a large-offset Large offsets transform was made at Charlie-Gibbs Fracture Zone (Searle 1981). Figure 9 shows a detail of the southern transform We now look at features with larger offsets. Figure 8 shows zonethere. There are several points to notehere. The part of Hayes Fracture Zone on the Mid-Atlantic Ridge, transform fault zone is represented by a narrow band of which has a total offset of 110 km. This istypical of the lineaments that are almost parallel to the 095" slip direction. larger offset transforms on slow spreading ridges. It has a This band is 1.5 to 2 km wide. Towards the western end of well-developed band of tectonic elements striking parallel to the transform, where it is viewed optimally at mid-range on the WNW-ESE slip direction. The band isonly a few the sonograph, the zone is seen to consist of a single, kilometres wide; most of the lineaments within it are very narrow, sharply defined lineament (marked'A'), whichis straight and narrow, and we infer that they are low fault sometimes flankedby much shorter (2- to 7-km long), scarps. Majortranscurrent faults in continents are disconnected, parallel lineaments. The mainlineament commonly associated with similar bands of faults; these are appears to alternate between reflection and shadow, predominantly strike-slip faults andrepresent the surface suggesting that it represents a low scarp alternately facing expression of the transcurrent faults. By analogy, we take north and south. I interpretthis scarp as the trace of a major the bands of faults in oceanic fracture zones tobe the strike-slip fault that represents the PTDZ. Near 31" 1O'W physical embodiment of the 'transform fault', and call such the reflecting lineament widens slightly (to about 800 m) and bands the 'transform fault zone'. This zone is the has anadjacent shadow tothe south, suggesting the time-averaged trace of the transform fault,but within it presence of a narrow, linear ridge-possibly a serpentinite there is oftenonetrace-the 'Principal Transform diapir intrudedup the transform fault. Farther east the Displacement Zone' (PTDZ)-that represents the currently PTDZ is less clear as it runs into the shadow cast by the

Fig. 8. (a) Montage of north-looking GLORIA sonographs overpart of Hayes Transform at 34 "N on the Mid-Atlantic Ridge; (b) outline topography (contour interval500 m) and inferred tectonic featuresof the transform fault zone. The zone is obscured by shadow at the eastern end of the sonographs. In this and subsequent figures the sonographs have been chosen to illustrate details of the transform faultzone, and do not necessarily clearly depict other features in the fracturezones.

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a 32 31 30

Fig. 9. GLORIA sonograph (b) with topographic map (contour interval500 m), (c) and tectonic interpretation (a) of the southern transform fault of Charlie-Gibbs Fracture Zone, North Atlantic (after Searle 1981). Insonification is towards the south. 52 Features A and B are discussed in the text. Tectonic features outside the transform fault zone were inferred from other I I 32 31 30 sonographs (not shown).

northern wallof the transform valley, though it probably In 1981 10s ran GLORIA along the transform portions runs along the foot of the major scarp marked 'B'. of all the main Atlantic fracture zones between 35 "N and Although the transform fault zone is only 1-2 km wide, 15"N,i.e. Oceanographer, Hayes, Atlantic, Kane, and it lies within a zone about 10 km wide that contains more Fifteen-Twenty. Wefound that the picture seenat widely dispersed lineaments parallel or subparallel to the Charlie-Gibbs FractureZone is representative of one slip direction. This is the 'transform tectonized zone'. The extreme form of the transform fault zone, in which the zone transform valley here is about 40 km wide, so the transform is narrow, and asingle throughgoing lineament (probably the tectonizedzone occupies only thedeepest part of the PTDZ) can be recognized. Another good example of this transform valley. On the other hand, the axial normal fault type of transform is seen in Hayes Fracture Zone (Fig. 8), scarps produced at the Mid-Atlantic Ridge spreading centre wherethe PTDZ is anarrow lineament over most of its reach well down intothetransform valley without length. At Oceanographer Fracture Zone (not illustrated), interruption. the transform fault zone is also narrow, but over part of its Within thetransform tectonized zone are many short length the PTDZ follows the foot of a major escarpment, lineaments that are some 20" oblique to the slip direction. as at Charlie-Gibbs. Macdonald et al. (1986),using the Their linearity suggests thatthey are faultscarps, not Scripps Institution of Oceanography's Deep-Towinstru- random reflections from the ends of ridge-parallel highs. I ment,and Bowen & White(this volume), using the interpret them, on the basis of their orientation, as faults Cambridge deep-tow seismic profiler, have found that the that were initiated as Riedel shears; however, they do not PTDZ atthe Vema Fracture Zone is characterized by a offset any features that they cross, so theamount of small furrow. We have notseen any evidence of such strike-slip motion on them is very limited; it is possible that features inslow slipping fracture zones, but they maybe they wereinitiated as small-offset shearsand have below the resolution limit of GLORIA. subsequently suffered significant dip-slip motion. Whitmarsh Atthe opposite extreme from Charlie-Gibbs Fracture & Calvert (1986; Calvert & Whitmarsh, this volume) have Zone wefind atransform fault zone that isonly poorly shown that the outerlimit of this band of oblique lineaments defined on GLORIA images. A good example is Kane corresponds to the edgeof the zone of abnormally thin, slow FractureZone (Fig.lO), wherethere are few clear fault and low-density crust that is associated with the transform scarps but a numberof poorly defined lineaments parallel to fault. the slip direction. It is impossible to infer the precise nature .

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N \ 20 km 46OW 45OVJ

Fig. 10. (a) Montage of north-looking GLORIA sonographs overKane Transform at 24 "N on the Mid-Atlantic Ridge;(b) topography (contour interval 500 m) and inferred tectonic lineaments of the transform fault zone.

of these lineaments from the GLOFUA data alone; some lineaments, though present, are not continous. The latter is may be broad faultzones or larger escarpments, othersfault the case at Fifteen-Twenty. Herethere are several sharp blocks or igneous ridges,and still othersfault-bounded fault-like lineaments with the 5-kmwide transform fault sedimentponds. However, their general linearity and zone that look like the PTDZ as seen at Charlie-Gibbs or parallelism to the slip direction strongly suggest that they Hayes,but they arebroken into 15- to 30-kmlong are,or are veryclosely associated with, elements of the unconnected segments. Inaddition, there is anumber of transform fault zone. low, elongate ridges up to 4 km wide and several tens of Such indistinct transform fault zones appear to be rather kilometres long. They can beseen on the bathymetric wider than the very distinct Charlie-Gibbs Fracture Zone profiles of Collette et al. (1984). On the GLORIA images type. The one at Kane Fracture Zone is of variable width, they appear as long, lenticular bright patches, which arise up toa maximum otabout 4 km,and theone at from reflections on the sides of the ridges thatface the Fifteen-Twenty Fracture Zone is even wider (Fig. 11). In sonar. The farsides of these reflecting zones often show such wide transformfault zones thePTDZ isharder to sharp cut-offs at the crests of the ridges. I speculate that recognize, eitherbecause there are only poorly defined these may be diapiric ultrabasic intrusions, but they have lineaments parallel to the slip direction, or because sharp not been sampled.

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-20 km I

-1 5030 'N

&W 45; W

Fig. 11. (a) Two-sided sonograph swath over Fifteen-Twenty Transform, Mid-Atlantic Ridge;(b) topography (contour interval 500 m) and inferred lineaments of the transform fault zone.

Two other points should be mentionedwith regard to the transform fault zones ofslowly slipping transforms. First, TFifteen-Twenty they do not always follow the deepest part of the transform valley, as has been shown at Charlie-Gibbs Fracture Zone (Searle 1981) and Romanche Fracture Zone (Belderson et al. 1984; M. V. Thomas, pers. comm.1984). Secondly, there may be arelation between the regional direction of the mid-ocean ridgeand the width and complexity of the transform fault zone. Determination of the width of the transform fault zone from GLORIA data is somewhat uncertain, but the north l Atlantic data do suggest that the zone may be broader and Hayes morecomplex where the Mid-Atlantic Ridge is almost = 11 perpendicular to the spreading direction, as for example at Kane Transform (Fig. 12). On the other hand, where the regional trend of the ridge is highly oblique to the local slip Angle between MAR and spreading direction direction, as at Charlie-Gibbs, Oceanographer and Hayes, the transform fault zoneis much narrower and the PTDZ is more clearly displayed. I suggest that the controlling factor Fig. U. Plot of the width of the transform fault zone againstthe may be the direction of the ridge push force. In general, angle between the slip directionand the regiod direction of the there may be small and locallyvarying amounts of ridge axis, fora number of transforms on the Mid-Atlantic Ridge. compression across the transform (transpression-Harland Vertical bars indicate the range of width at each transform.Note 1971), for example as the result of small departures of the that the width of the transform fault zone decreases with increasing PTDZ from a perfect small circle about the pole of rotation obliquity of the Ridge.

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owing to localized lithospheric inhomogeneities. These As atslowly slipping fracture zones, the axial fault scarps compressional forces would tend to produce a more complex curve in the direction of the spreading centre offset, due to transform fault zone. The ridge push force is directed down shearing in the vicinity of the transform. However, here the the ridge flank, andtherefore its direction in thenorth transforms are so closely spaced that between them the Atlantic varies because of the marked sinuosity of the spreading fabric remains oblique to the spreading direction, Mid-Atlantic Ridge (Roest et al. 1984). Where the ridge is because it is still partially within the influence of shearing in oblique to the spreading direction this force tends toprovide the transforms. Unlike slowlyslipping fracture zones, a component of tension across the transform, so tending to however, these do not show major oblique scarps in the counteract any localized compression and therefore freeing fracture valley walls. the transform to develop in a simpler way. A recent survey of the fast slipping Garret Transform found asimilar arrangement of multiple transform faults Fast slipping transforms (Gallo et al. 1983), andit islikely that thismultiple transform styleis quite common in fastslipping fracture We now move on to fast slipping transforms. The GLORIA zones (Fox & Gallo 1984). Theseauthors havesuggested data from these are limited to one survey which covered the that it occurs because the much thinner lithosphere at fast Quebrada-Discovery-Gofar fracture zone complex on the spreading ridges allows the easy formation and movement of East Pacific Rise between 3.5 "S and 5 "S (Searle et al. 1981). thePTDZ within the transform tectonized zone. I Each of these three fracture zones, which were previously suggested, in addition, that the production of several closely ill-defined butappeared tobe single topographic valleys, spaced transforms may occur in response to small changes of was found to contain multiple transform faults: four at spreading direction, so that these multi-transform fracture Quebrada, two at Discovery, and threeat Gofar (Searle zones are the physical embodiment of the theoretical 'leaky 1983). The transform offsets range from 24 to 93 km, or 0.32 transform fault' (Searle 1983). to 1.23Ma. Figure 14 shows bathymetric profilesacross Quebrada Figure 13 showsa GLORIA swath overparts of FractureZone with the positions of the transform fault Quebrada and Discovery fracture zones. The southernmost transform shown in the figure is one of the two associated with Discovery Fracture Zone. The four separate transform faults (or strictly speaking, transform fault zones) associated with Quebrada Fracture Zone are clearly seen. They occupya band 25 km wide, and have indvidual spacings of between 5 and 10km.

L -FOSSIL 10 km TRANSFORM 1 I 1

ZU km

'4"s l I Fig. 13. (a) Two-sided GLORlA sonograph swathover parts of the fast slipping Quebradaand Discovery fracturezones on the East Fig. 14. Bathymetric profilesacross Quebrada Fracture zone, with Pacific Rise at 4 "S; (b) interpretation. Light lines, selected tectonic the positions of the four active transform fault zones (Ql-Q4) lineaments; heavy line, inferred plate boundary; half arrows, slip marked by solid arrows. Open arrows show the fossil traces of the directions; full arrows, spreading direction. Strainellipses are also transform fault zones,and inset shows the location of the profiles. shown. After Searle (1983). After Searle (1983).

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20 km N \

Fig. 15. (a) North-looking sonograph;(b) topography (contour interval 100 m), of the southernmost transform in the fast-slipping Gofar Fracture Zone at4.5 "S on the East PacificRise. The inferred plate boundaryis shown conventionally.

zones marked. Note that thereis a single topographic valley, clearly displayed. Lonsdale (1978) camed out a deep-tow but as in the Atlantic, the transforms do not necessarily run survey of the northernmost Quebradatransform and showed along thebottom of thetransform valley, butfrequently that the PTDZ there is a furrow some50 m deep and 150 m occupy terracespart way upthe sides. The strike of the wide. transform valley therefore does not give a true estimate of the spreading direction. Finally, we look in slightly more detail at the transform Summary and discussion fault zone on these fast slipping transforms. Compared with Spreading axis offsets of a few kilometres are frequent on the slowly-slipping Atlantic transforms, the transform fault slowly spreading ridges, and are accomplished by en kchelon zone here is always narrow and relatively simple, being no offsets, with short segments of 45" oblique normal faults more than 1.5 km wide (about the same as the narrowest between the offset segments. Such oblique faults are very Atlantic ones) and usually containing a fairly clear PTDZ common on slow- and medium-spread crust, andseem to trace. Figure 15shows the longest and most complex of representnatural a fracture mode of the oceanic these, the southernmost of the three Gofar transforms. It is lithosphere. Short zones of oblique faulting may be aligned 90 km long and has a 1.3 Ma offset. Its transform fault zone along the spreading direction, but rarely persist for more is quite similar to the simplest of the slow-slipping ones, than 50 km in one place. They aretherefore not as such as Hayes transform, although the PTDZ may be even persistent as the 'zero-' or 'short-offset' fracture zones of more continuous at Gofar. I attribute this narrowness and Schouten & White (1980) and Schouten & Klitgord (1982). simplicity to the thinness and weakness of the fast spread When the offset reaches about 20 km,the oblique lithosphere, allowing ready deformationby simple strike-slip faulting forms a well-defined zone of oblique spreading in faulting. the offset region, whichmay persist for manymillions of The form of the PTDZ is somewhat variable. Overmuch years. It appears tobe so easy for the lithosphere to fracture of Gofar Fracture Zone it seems to be a simple low scarp, along the 45" direction, that when the offset reaches about although in the centreof Fig. 15 there is a hint of a doubled thesame length asthese oblique faults, a stable oblique lineament,perhaps suggesting afurrow. In thenorthern spreading zone is formed.A fully-fledged fracture zone Discovery transform (Fig. 16) the PTDZ appearsto run valley is often associated withsuch features, even though along the foot of a major escarpment (as at Charlie Gibbs there is no transformfault (although it is possible that and Oceanographer transforms), and at Quebrada Fracture transform faults form intermittently inthem). However, it is Zone (Fig. 17) the double nature of the PTDZ lineament is clear that the fracture zone topography can persist without

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n the existence of a currently active transform fault. Such features, then, are clearly one form of ‘zero-’ or ‘small-offset’ fracture zone (Schouten & White 1980). Such offsets, which I referto as ‘oblique spreading offsets’, appearto be quite common on slowly spreading ridges. I believe that we have observed one on a medium spreadingridge, but they have notbeen seen on fast spreading ones. The fast spreading equivalent is probably the ‘overlapping spreading centre’ that has been described by Macdonald & Fox (1983), Lonsdale (1983),and 20 km Macdonald et al. (1984), although it is not yetknown whether overlapping spreadingcentres leave trails of f disturbed topography which might be analogous to fracture h \ zone traces. Overlapping spreading centreshave similar offsets to the slow spreadingoblique offsets, andare characterized by oblique tectonic lineaments in the offset region. However, the two spreading centres are able to overshoot the offset region and overlap, at least temporarily, because the fast spread lithosphere is thin and weak enough to undergo the deformation requiredby this process. The much thicker and stronger lithosphere on eitherside of slow spreading offsets, however, inhibitsthe propagation of spreading centres across the offset region (Fujita & Sleep 1978), so overlapping of the spreading centres cannotoccur there, and Fig. 16. (a) Part of south-looking sonograph mosaic; (b) the plates continue to behaverigidly. topography (contour interval 100 m), of part of the fast slipping In spite of this spreading rate dependence of a ridge’s Discovery Fracture Zone on the East Pacific Rise at 4 “S. The ability to support overlapping spreading centres, which, as inferred plate boundary isshown conventionally. argued above,is a function of the strengthof the lithosphere

20 km

103’ \

Fig. 17. (a) Part of south-looking sonograph mosaic; (b) topography (contour interval 100 m), of part of Quebrada FractureZone, East Paci- fic Rise, 3.7 “S. Inferred plate boundary shown conventionally. Box shows position of Lonsdale’s (1978) \ Deep-tow survey.

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bounding the offset region, it is remarkable that the critical in the inactive fracture zonelimbs. No equivalent of these has length of offset that must be attained before a transform been seen in fast slipping fracture zones. I believethis is fault is produced is very similar on fast and slow spreading because the oblique scarps owe their development to major ridges, at about 30 km. It is suggested that this critical length vertical offsets induced as the slowly spread seafloor on the is determined by the properties of the lithosphere within the fracturezone flanksrises from the median valley floor; oflset region, and particularly in the narrow crustal accretion clearly this cannot occur on fast spreading ridges, which lack zones at each end of a transform where strike-slip faulting median valleys. Minor oblique faults do occur there in must be continually renewed. It is shown elsewhere (Searle response to shearing, but are not subjectedto the major 1984) that the patterns of faulting associated with spreading vertical motions experienced by similar faults in slowly axes in an axial zone a few kilometres wide are almost spread crust. independent of spreading rate,and I suggested that this The transform fault zoneand PTDZdo not always implies that lithospheric properties within the axial zone are follow the line of deepest water in the transform valley, and almost constant. The constant lengthof the critical offset for may depart from it quite considerably. Care must therefore transform fault formation may be further evidence of the be exercised in using the topographic expressions of fracture same effect. zones as indicators of relative plate motion. For offsets of more thanabout 25-30 km, a through- Fast slipping fracture zones may contain multiple, closely going band of strike-slip faulting, the Transform Fault Zone, spaced transform faults. Such multiple transforms may form is formed. This band is up to 5 km wide and contains the in response to small changes in spreading direction, the currently active plateboundary or Principal Transform whole zonethus functioning as a ‘leaky transform’. It is Displacement Zone.The transform fault zone varies in easier for this to occur in fast spread lithosphere because the width between 1 and 5 km. In slowly slipping fracture zones lithosphere bounding the transform is thinner and weaker thezone is narrowerwhere the regional trend of the there. However, some moderately spaced transform faults spreading centre is oblique to the spreading direction. This are known on slow spreading ridges: the two transforms at may be because the ridge push force in such situations Charlie-Gibbs Fracture Zone are separated by 45 km (Searle supplies an additional component of tension across the 1981), and St Paul’s FractureZone in the equatorial transform fault zone, freeing it from the complicating effects Atlantic contains at least two transforms 25 km apart (M. V. of transpression. In fast slipping fracture zones the Thomas unpublished data). transform fault zone is comparable with or slightly narrower than the narrowest slowslipping example. Again, this Igratefully acknowledge the support of the GLORIA operation suggests very similar lithospheric properties in both fast and team, shipboard scientists, and ship’s crews of the many research slowslipping transforms where the strike-slip faulting is voyages that have contributed data to this study. I am particularly initiated (i.e. in the ridge-transform intersections). How- indebted to the many colleagues whohave contributed over the ever, because the lithosphere bounding the transform is years to the ideas presented here bytheir valuablediscussions, stronger in the slow slipping case, transpression there can suggestions, andcriticisms. I thankLindsay Parson for acritical complicate thestructure much more easily than in fast review of the manuscript, and Colin Jacobs and Jonathan Bryan for slipping zones. Narrower transform fault zones tend to have assistance. more clearly developed PTDZs, again presumably because the simpler stress patterns allow easier development of a References single through-going fault, whereas complex structures and stress fields produce a variety of small features of limited BELDERSON,R. H., JONES, E. J. W., GORINI,M. A., & KENYON,N. H. 1984. A long-range side-scan sonar (GLORIA) Survey of the Romanche active extent and variable orientation and tectonic type. transform in the equatorial Atlantic. Marine Geology, 56, 65-78. Macdonald er al. (1986) have studied the transform BOWEN,A. N. & WHITE,R. S. 1986. Deep-tow seismicprofiles from the fault zone at VemaFracture Zone.,using the Scripps VemaTransform and Ridge-Transform intersection. Journal of the Institution of Oceanography’s Deep-Tow system. They Geological Society, London, 143,807-17. CALVERT,A. J., & WHITMARSH,R.B. 1986.The structure of the found thatthe transform fault zonenear the ridge- Charlie-Gibbs FractureZone. Journal of the Geological Society, London, transform intersection comprises a larger number of 143, 819-21. anastomosing faults in a fairly broad zone, but that the zone COLLETTE,B. J., SLOOTWEG,A. P,, VERHOEF,J., & ROEST,W. R. 1984. becomes narrower and simpler farther from theintersection. Geophysicalinvestigations of the floor of the Atlantic Ocean between 10 and38 O (Kroonvlag-project). 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J., LONSDALE,P. & MADSEN,J. 1983.The We suspect that this variation reflects a varying stress field morphotectonic expression of the world’s fastest-slipping plate boundary: the Garret Transform. EOS, Transactions of the American Geophysical along the transform as a result of local asperities, small Union, 64, 85 (abstract). departuresfrom a puregreat circle, and other similar HARLAND, W. B. 1971. Tectonic transpression in CaledonianSpitzbergen. effects. Geological Magazine, 108, 27-42. In slowlyslipping fracture zones, major oblique scarps LAUGHTON,A. S. & RUSBY,J. S. M. 1975. Long-rangesonar and photographic studies of the median valley in the FAMOUS area of the are found in the valley walls. They are probably formed in Mid-Atlantic Ridge near 37“N. Deep-sea Research, 22, 279-98. the walls of the transform valley near the ridge-transform -& SEARLE,R. C. 1979. Tectonic processes on slow spreading ridges. In: intersections, but they tend to be most clearly recognizable TALWANI,M., HARRISON, C. G. & HAYES,D. E. (eds) Deep Drilling

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Resultsin the AtlanficOcean: OceanCrust. MauriceEwing Series, - 1981. The active part of Charlie-Gibbs Fracture Zone: a study using American Geophysical Union, Washington D.C., 2, 15-32. sonar and other geophysicaltechniques. Journalof Geophysical LONSDALE,P. 1978. Near-bottom reconnaissance of a fast-slipping transform Research, 86, 243-62. fault zone at the Pacific-Nazca Plate boundary. Journal of Geology, 86, - 1983. Multiple, closely-spaced transform faults in fast-slipping fracture 451-72. zones. Geology, 11, 607-10. - 1983. Overlapping Rift Zones at the 5.5"s Offset of the East Pacific -1984. GLORIA survey of the East Pacific Rise near 3.5"s: tectonic and Rise. Journal of Geophysical Research, 88, 9393-406. volcanic characteristics of a fast spreading mid-oceanrise. MACDONALD,K. C., CASTILLO, D., MILLER,S. P., Fox, P. J., KASTENS, K., & Tectonophysics, 101, 319-44. BONATTI,E. 1986. Deep-tow studies of the Vema Fracture Zone: I. The -, FRANCIS,T. J. G., HILDE, T.W. C., SOMERS,M. L., REVIE, J.,JACOBS, tectonics of a major slow-slipping transform fault and its intersection with C. L., SAUNDERS,M. R. & BARROW,B. J. 1981. 'Gloria' side-scan sonar the Mid-Atlantic Ridge. Journal of Geophysical Research, 91, 3334-54. in the East Pacific. EOS, Transactions ofthe AmericanGeophysical - & Fox, P. J. 1983. Conjugate spreading centres and degenerate Union, 62, 121-2. transform faults on the East Pacific Rise. Nature, 302, 55-8. -& LAUGHTON,A. S. 1977. Sonar studies of the Mid-Atlantic Ridge and -, SEMPERE,J. C. & FOX, P. J. 1984. East Pacific Rise from Siqueiros to Kurchatov Fracture Zone. Journalof GeophysicalResearch, 82, Orozco Fracture Zones: along-strike continuity of axial neovolcanic zone 5313-28. and structure and evolution of overlapping spreading centers. Journal of SIMONIAN,K. D. & GASS,1. G. 1978. The Arakapos Fault Belt, Cyprus: a Geophysical Research, 89, 6049-69. fossil transform fault. Geological Society ofAmerica Bulletin, 89, MOUGENOT,D., KIDD,R. B., MAUFFRET, A., REGNAULD, H., ROTHWELL,R. 1220-30. G., & VANNEY,J. R. 1984.Geological interpretation of combined SOMERS, M.L., CARSON,R. M., REVIE,J. A., EDGE,R. H., BARROW,B. J. Seabeam, GLORIA, and seismic data from Porto and Vigo Seamounts, & ANDREWS,A. G. 1978. GLORIA 11-an improved long range Iberian continental margin. Marine Geophysical Researches, 6, 329-63. side-scan sonar. Proceedingsof the IEEEIIERE sub-conference on PARSON,L. M. & SEARLE,R. C. 1986. Strike-slip fault styles in slow-slipping offshoreinstrumentation and communications. BPSPublications Ltd, oceanic transform faults-evidence from GLORIA surveys of Atlantis London, J16J24. and Romanche Fracture Zones. Journalof the GeologicalSociety, TAMSETT,D. 1984. Spreading centres and fracture zones in the Guy of Aden. London, 143, 757-61. PhD Thesis, University of Newcastle-upon-Tyne. ROEST,W. R., SEARLE, R. C.& COLLE~E,B. J. 1984. Fanning of fracture WHITMARSH,R.B. & CALVERT,A. J. 1986. Crustal structure of Atlantic zones and a three-dimensional model of the Mid-Atlantic Ridge. Nature, fracture zones: I-the Charlie-Gibbs Fracture Zone. Geophysical Journal 308, 527-31. of the Royal Astronomical Society, 85, 107-38. SCHOUTEN,H. & KLITGORD, K.D. 1982. The memory of the accreting plate - & LAUGHTON,A. S. 1975. The fault pattern of a slow-spreading ridge boundary and the continuity of fracture zones. Earthand Planetary near a fracture zone. Nature, 258, 509-10. Science Letters, 59, 255-66. -& -1976. A long range sonar study of the Mid-Atlantic Ridge crest -& WHITE,R. S. 1980. Zero-offset fracture zones. Geology, 8, 175-9. near 37"N (FAMOUS area) and its tectonic implications. Deep-sea SEARLE,R. C. 1979. Side-scan sonar studies of North Atlantic fracture zones. Research, W, 1005-23. Journal of the Geological Society, London, 136, 283-92.

Received 5 July 1985; revised typescript accepted 2 December 1985.

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