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

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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 clear PTDZ 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, highly reflective 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/5/743/4893072/gsjgs.143.5.0743.pdf by guest on 28 September 2021 744 R. C. SEARLE a r - 45'06 ' 1 I 1 29'00' 28'00' 27'00' Fig. 1. (a) GLORIA Sonograph mosaic; (b) inferred tectonic linea- ments, 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 The oblique 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).
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