Journal of the Geological Society, London, Vol. 143, 1986, pp. 163-114, 9 figs. Printed in Northern Ireland
Fracture zones in the North Atlantic: morphology and a model
B. J. COLLETTE Vening Meinesz Laboratorium, Budapestlaan 4, P.O. Box 80.021, 3508TA Utrecht, The Netherlands
Abstrad: A morphology is presented for the typical cross-section of an inactive fracture zone in the Atlantic. Inactive fracture zones typically consist of an asymmetric valley to the young side of the fracture zone plane and a high wall or scarp to the old side. Large-offset fracture zones may be accompanied by a marginal valley on the other side of the high wall. This topography is superimposed on the depth-age step due to lithospheric cooling. A model is developed which accounts for this morphology. The model relates the inactive fracture zonemorphology to the topography found at present-day intersections of the spreading axis with transform faults. The asymmetry of the median valley near fracture zones plays an important role in explaining the typical fracture zone morphology. The existence of a median valley is related to the viscous delay of the upwelling mantle material at the spreading axis. Itsasymmetry near fracture zones can be accounted for by modelling the viscous drag exerted by the lithosphere on the asthenosphere. If the viscosity is low, as under Reykjanes Ridge and the East Pacific Rise, no median valley develops and a different morphology may be expected. The occurrence of marginal valleys is interpreted as the result of lithospheric warping when the graben walls in the transform domain, which are caused by tension due to horizontal thermal contraction. are excessively high.
Based on numerous observations in thecentral North what goes on at the Ridge axis, the detailed observations of Atlantic, a model has been developed for what is proposed the 70s in the FAMOUS area (e.g. Arcyana 1975; Heirtzler as the typical cross-section of an inactive fracture zone in & Van Andel 1977) on small-offset fracture zones and the slow-spreading oceans. The model consists of an asymmetric submersible dataonthe transform domains of the valley to the young side of the fracture zone plane and a large-offsetKane and Oceanographer FZ (Karson & Dick highwall or scarp tothe old side. This topography is 1983; Karson et al. 1984) are invaluable. Several findings of superimposed onthe theoretical depth-agestep due to the respective diving teams are incorporated in the model. lithospheric cooling. Inthe Atlantic this step frequently Vema FractureZone is characterized by a verylarge becomesmasked by the fracture zone topography and, in offset as are some of theother fracture zones in the general, by the roughness of the Atlantic ocean floor. The Equatorial Atlantic. Thesefracture zones showseveral model can be related tothe present-day morphology of specific features.Reference will be made to the present fracture zones at the Mid-Atlantic Ridge in the transform transform domain of Vema FZ as originallydescribed by domain. VanAndel et al. (1971) and to the inactivewestern limb The greater part of the observations was made in a zone which is covered by the Kroonvlag data. about 650 km wide between the English Channel and the Amodel will be presented whichaccounts for the north coast of South America during the Kroonvlag project generalized fracture zone morphology. The model combines (Collette et al. 1984). Thiszone comprises oceanic crust the effects of horizontal thermal contraction andcrustal ranging in age fromover 84 Ma (end of Cretaceous warping as lithospheric processes, and of viscousdelay of Magnetic Quiet Zone) to Present. In addition reference will the upwelling mantle material and drag exerted by the be madeto surveys of several fracture zones in the CMQZ on lithosphere theon asthenosphere as asthenospheric the African Plate (Twigt et al. 1983;Slootweg & Collette processes. 1985; see alsoVerhoef & Duin, this volume). The latter Recently, Sandwell (1984; cf. also Sandwell & Schubert investigations led to the development of a magnetic model 19826) proposed a thermo-elastic model for fracture zones for inactive fracture zones, which is not only valid for the based on a morphology which differs from what is found in CMQZwhere no reversals of theearths magnetic field the Atlantic. This brings us to a discussion of the fracture occur, but which appears to have general validity. The zones in the Pacific on which these authors base their model. gravity observations of these surveys provided an insight Roest et al. (1984) reported on the fanning pattern of the into the isostatic compensation of inactive fracture zones, transform fault directions in the North Atlantic. Since then, especially of the fracture zone ridges. theinterpretation of the anomalous structural directions With regard to the present transform domain, observa- near 1520'N has beenrevised (see Roest & Collette, this tions madeduring a Survey of the Fifteen-Twenty volume). It appearsthat about 8 Maago the North Fracture Zone(FZ) will be referred to (cf. Roest & American/South American plate boundary broke through Collette, this volume) as well as data on the Mid-Atlantic at this latitude. Nevertheless, indications remain that sig-
Ridge between 12 O and 20 ON (e.g. Collette et al. 1980). The moidality of very-large-offset fracture zones is a systematic Fifteen-Twenty FZ appears to be in a leaky mode. For the feature. This can only be explained by allowing for a certain remainder, the morphology of this fracture zone is not degree of three-dimensionality of the seafloor spreading essentially different from what has been found farther north process. in the central North Atlantic. For a good understanding of Finally,it is discussed under whichcircumstances the 763
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49U 48U 49U 46U 47U 45U 44U 43U41U 42U 40U 39U 38U 37U 36U34U 35U 33U 32U 31 U 25N 25N
241 24N
23N
22N
21N
20N
I9N FILTERED TOPOGRAPHY
18N
t 7N
16N I6N I 4 U Fig. 1. Filtered topography of a part of the central North Atlantic between 16" and 25"N. Open circles denote earthquake epicenters, solid bars indicate the median valley.
typical cross-section of inactive fracture zones is not Ridge axis: to the W the older side is situated N of the observed. It appearsthat changes of seafloor spreading fracture zone-axis, to the E it is situated S of it. direction play an important role in this. The typical asymmetric cross-sectionis absent in that part of the area where the azimuth of the fracture zones is The typical cross-section of inactive fracture zone south of west, see, for example, Kane FZ between 36" and an 39"W (Fig. 5). This occurs after an anti-clockwise change of Figure 1 is a projection of the seismic data of the Kroonvlag seafloor spreadingdirection. As illustrated inFig. 6, an tracks between 16" and 25W. A special presentation was anti-clockwise change of spreading direction leads for chosen, namely topography after reduction with the values dextral fracture zones to a shortening of the offsets and a of acontour file based on thetotal data set (cut-off complementary growth in length of the interjacent spreading wavelength 60 nm, roll-off 6dB/octave). Negative topo- segments.Evidently, the mechanism responsible forthe graphy has been shaded.Earthquake epicentres (open origin of the typical fracture zone morphology is not circles) delineate the Mid-Atlantic Ridge axis; the median effective under these circumstances, an observation that was valleyis indicated schematically by heavysolid lines. The curved E-W course of thefracture zones easilyis recognized. Kane FZ has the largest offset of the fracture 34w 36W 35w zones in the area. To the N one finds the Northern FZ, to the S, Snellius FZ, Luymes North and South FZ and Vidal t FZ. The asymmetric cross-section of thefracture zones becomes immediately evident in this presentation. Figure 2 23N shows a section of Kane FZ in more detail, Fig. 3 part of the 23N dextral Luymes South FZ and the sinistral Vidal FZ. Figure 4 showsLuymes South FZ where it transects the Mid-Atlantic Ridge near 46'40'W. Theterms dextral and sinistral refer tothe movement in thetransform domain (and not to the offset). Reconstruction of the area, based on bathymetric and seismic evidence and on magnetic dating by means of the seafloor spreading anomalies (cf. Collette et al. 1984), proves thatthe steep side of thefracture zone-valley is 22N 22N always oriented at the older sideof the fracture zone-axis. A 34w 36W 35w striking illustration is formed byFig. 4, which depicts the Fig. 2. Kane fracture Zone between 34" and 36"W. Negative change of asymmetry from W to E on passing the present topography has been shaded.
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44w 43w 44w 42W 41 W
18N I8N
17N 17N
Fig. 3. Luymes South and Vidal Fracture Zones between 41" and 44"30'W. The former fracture zone is sinistral, the latter dextral. 44w 43w 42W 41 W
first made for Kane FZ in the CMQZ (Twigt et al. 1983). FZ has an offset of 300 km which is more than of any of the The broad fracture zone-valley, which develops then and in fracture zones to thenorth. The South Wall and the which a transverse ridge may occur like in the case of the accompanying marginal troughstart developing about Kane FZ in the CMQZ, is rather atypical and difficult to half-way along the present transform domain. recognize as such on individual profiles. The fossil westward continuation of Vema FZ is shown All available structuraland bathymetric information in Fig. 7. Analysis of the magnetic anomalies shows that the indicates that the high wall to the old side of fracture zones magnetic contrast representing the fracture zone-axis lies to consists of uplifted seafloor spreadingcrust. The best the N of the high wall. This means that the relatively small illustrations of this are formed by the GLORIA recordings valley foundthere is thefracture zone-valley. The large in theFAMOUS area (Whitmarsh & Laughton 1976), depression to the S of the ridge is the equivalent of the on the major fracture zones in the central North Atlantic marginal trough in the transform domain.To the N, the wall (Roest et al. 1984) and on Charley-Gibbs FZ (Searle 1981). attains an average height of 1.5 km; the sediment covered SEABEAM recordings on the South Wall of Vema FZ also basement to the S, in the Demerara Abyssal Plain, is2.5 km exhibit seafloor spreadingtopography (H. D. Needham high. The difference represents the depth/age step, enlarged pers. comm.). The character of the topography of the high by the isostatic response to the sedimentary loading of the wall was, furthermore, confirmed by detailed surveys of the abyssal plain. Note thatthe South Wallshows large Mid-Atlantic Ridge between 12" and 18"N (e.g. Collette et variations in height along its axis. Near 47O3O'W it becomes al. 1980; see also Roest & Collette, this volume) and in buried by the turbidite sedimentation. submersible diving experimentson Kane and Oceanog- East of the Mid-Atlantic Ridge axis aNorth Wallis rapher FZ (Karson & Dick 1983; Karson et al. 1984). found which, however, does not attain the same heights as the South Wall. Walls which are comparable to the South Wall of Vema FZ occur along the old sides of fracture zones Vema Fracture Zone farther southin the Equatorial Atlantic (e.g.Romanche and Although also in the central North Atlantic thehigh wall on Chain FZ, GEBCO-sheet 5.08, 1982;cf. Bonatti 1978), in the old side of fracture zones may reach considerable the Indian Ocean (e.g. Owen FZ; Bonatti 1978) and in the heights (cf. Kane FZ, fig. 2), Vema FZ differs from the Pacific (see Discussion). more northerly fracture zones in so far as its South Wall In the model to be discussed, the morphological attains huge dimensions and is accompanied by a marginal differences with fractures zones with a smaller offset will be trough to the south (Van Andel et al. 1971, fig. 4). Vema accounted for.
48W 47w 46W 19N I9N
Fig. 4. Change of character of Luymes South Fracture Zone on passing the Mid-Atlantic Ridge (indicated by dashes). To the W the liesolder side towardsto the N, RU I8N , -,. the E it lies towards the S. 48W 47w 46W
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35w 34w 33w 34w 35w
29N 2 9.N
35w 34w 33w 34w 35w
Fig. 5. Loss of typical character in cross-section whena change of Fig. 6. Change of length of transform offsets and spreading spreading direction occurs (Kane FZ between 33" and 3520'W). segments when a change of seafloor direction occurs.
The transition from transform fault to inactive typical cross-section can be understood. The high wall at the fracture zone old side is a graben wall from the transform domain, the The intersection of spreading centre and fracture zone has asymmetric fracture zone-valley on the younger side of the been studied at several places in the central North Atlantic. fracture zone-axis develops, aswe will argue later, out of A typical feature is the asymmetry of the median valley near thedepressed lid of the newly formed lithosphere at the the intersection: at the inner corner of spreading axis and corner of spreading centre and the inactive section of a transformfault one finds, in general, an abnormally high fracture zone. inner wall while at the opposite side thetopography is lower The geology of the intersection has been described by than normal (Fig. 8). Beyond thefracture zone valley, Karson & Dick(1983) forOceanographer and Kane FZ. which has farther on in the transform domain the character of a graben, one finds the high graben wall, which comes from thejuxtaposed intersection of transformfault 44wand 45w spreading axis. The high fracture zone wall frequently shows I"'I""'i1 signs of rejuvenation of the topographic relief going from young to old along the transform domain. As examples of thesketched configuration are mentioned Oceanographer FZ (Fox et al. 1976), Kane FZ (e.g. Karson & Dick 1983), Fifteen-Twenty, Marathon and I OFFSET: 4000 MS I Mercurius FZ's (Collette et al. 1984) and VemaFZ 44w(Van 45w Andel et al. 1971). It is from this asymmetric settingthat the inactive Fig. 8. Asymmetry of median valley near a fracture zone (N of fracture zone emerges and from which at the same time its Marathon FZ).
50W 49w 48W 47w
12N 12N Fig. 7. The South Wall of Vema Fracture Zone to the west of the present transform domain. The sediments in Demerara Abyssal Plain to the south of the South Wall and in the fracture zone valley to the north have been shown as black areas. The de- pression to the south of the Wall
1 1 h1 is the continuation of the mar- 1 IN , I I. 50W 49w 48W valley. 47w ginal
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These authors observed that the high inner corner results suffer from this effect. Therefore, until now, gravity data has from the circumstance that the normal faults, which form not confirmed whether the crustal thinning towards fracture theinner walls of the median valley, have larger throws zones, as observed in seismic refraction and reflection close to the intersection. In addition they noticed that the experiments, is a general feature. The available admittance basalt flows become mainly directed towards the lower side, studies (Kogan & Kostoglodov 1981; Louden & Forsyth into the valley on the inactive part of the fracture zone. This 1982) point toa regional compensation of fracture zone means that in addition to an asymmetric topography, an topography, although Diament (1981) prefers a deep-seated asymmetryin the petrological constitution of theoceanic Airy type compensation. Regional compensation is equiv- crust develops. The latter finding has consequences for the alent to a wavelength-dependent covariance of the crustal understanding of the magnetic anomalies over inactive thickness with the topography and thus implies acertain fracture zones. degree of crustal thickening underthe highwall along fracture zones. Two-dimensional model computations Gravity over fracture zones indicate that the crust under the high walls along fracture zones is not thicker than normal (Cochran 1973; Robb & With regard to gravity and geoid anomalies over fracture Kane 1975;Twigt et al. 1983; Mulder & Collette 1984) or zones, one has to discern between the anomaly caused by even thinner (Whitmarsh & Calvert 1986). the depth/age step across fracture zones and the anomalies More detailed field work is needed to specify the relation relatedtothe specific fracturezone topography. The between fracturezone topography and crustal thickness depthlage step anomaly has been investigated in studies by from gravity data.The only relevantcurrent information Crough (1979), Sibuet & Veyrat-Peiney (1980), Detrick from gravity data suggests that the high walls along fracture (1981), Casenave et al. (1982) and Sandwell & Schubert zones cannotresult from a surplus buoyancy underthe (1982a) and appears to fit existing lithosphere models. The ridges which is seated in the crust, for this would require a interpretation of the shorter-wavelength anomalies related crustal thickening whichis least compatible withshallow to the local topography is less simple. The main reason for Airy-type compensation. this is thatthe shorter-wavelength anomalies are also affected by the spreading topography, both with regard to the observed gravity anomaliesand to the observed The model topography which goes into the modelling. Sufficiently dense two dimensional data sets, which are needed to overcome The main morphological features of a ‘typical’ fracture zone, this, do not yet exist. both in the transform domain and beyond, are summarized Bothstatistical methods-thoseusing theadmittance inFig. 9. We shall discusswhich processes or aspects are between topography and gravity-and model computations responsible for their formation.
b. f. 2
c-52 Marginal Volley I WALL volley - + wall WALL I.Z. 1.2. + volley WALL WALL WALL - Marginal Volley A B Q. 9. Schematic representation of the fundamental topography of large(A) and very large (B) fracture zones in the Atlantic. Sections a to farediscussed in text @v., median valley; f.z., fracture zone axis). Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/5/763/4893100/gsjgs.143.5.0763.pdf by guest on 03 October 2021 768 B. J. COLLETTE Fracture zone topography in the transform domain with (sections a and b, see Fig. 9) &(U3 WO = - This topography can best be described as a graben structure 40 with uplifted walls to both sides (Collette & Rutten 1972). where V, is a line load The shortening needed to produce a graben canbe found 114 adequately in thethermal contraction of the growing LY is the flexural parameter = lithosphere in the horizontal plane in the direction of the Eh3 spreading axis (Collette 1974; Turcotte 1974).With a D= temperature difference of1200" and a linear thermal 12(1 - OZ) expansion coefficient of 1.5 X 10-'/°C, theamount of contraction is about 2%.The involved tensional stresses with E - Young's modulus would be of the order of 20 kbar, which is much higher than u - Poisson's ratio brittle rock can stand. This will lead to stress relaxation by h - elastic thickness of the lithosphere rupture at discrete intervals (Turcotte 1974). Tensional pm,pw- specific density of mantle and seawater rupture of brittle rock alwaysoccurs perpendicular to the g - acceleration of gravity. maximum stress (Gramberg 1970).In view of the stress release which occurs along the spreading axis as an intrinsic W becomes zero for xo = (n/2)a. Taking the age of the aspect of the spreading process, the maximum stress lithosphere half-way along the offset at 12 Ma, and using the becomes directed along the spreading axis, and the related thickness/age relation of Bodine et al. (1981), one obtains tensional faulting perpendicular to it. for the thickness of the lithosphere 10.5 km and for X" The combination of primary faulting perpendicular to 60 km. the spreading direction with the development of thermal For Vema FZ, which has anoffset of 300km andan contraction faults perpendicular to the spreading axis age-contrast of about 24 Ma,the length of X, canbe explains the rhombic fabric of the ocean floor, with the estimated at 20 km (cf. Fig. 7). This is much less than the fracture zones functioning as transform faults. In the central predicted value. The equivalent lithosphericthickness h is North Atlantic the average spacing of the fracture zones is 2.5 km. This reflects an apparent age of about 1 Ma, with about 60 km. h = 3.la, inwhich a is the agein Ma (cf. Bodine et al. Can thermal contraction also explain the total deficiency 1981). However, the first signs of the South Wall only show of topographic bulk measured along the axis of the at a distance of about 100 km (or 8Ma) from the spreading Mid-Atlantic Ridge?A 10-km wide fracture zone would axis. The theoretical thickness of lithosphere of 8Ma old is consumeall thermal contraction overan axial length of 8.7 km. From thisit might be concluded that heating by 500 km.Therefore, secondary effectsmust also be intrusions along the fz-axis and/or frictionalong the considered. One such effect is that normal faulting parallel transform fault plane leads to lithosphericthinning. A to the thermal crack will occur, thus broadening the surface comparative study is needed to prove this suggestion. expression of the feature.Another is the insufficiencyof production of crustal material at theend of a spreading The median valley (section c) segment. Thinning of oceanic crust over fracture zones has been documented inmany experiments (for a review, see The medianvalley has the aspects of a graben, although herethe concept isfully inadequate to describe the White et al. 1984) and can be modelled as the end-effect of kinematics of the phenomena, However, the median valley the magma generation of a finite spreading centre. It will lead to a topographic depression parallel to the fracture exhibits a central depression and walls with normal faulting zone axis which is essentially symmetric. as in a graben. Several explanations havebeen brought forward to The opening of a crack atthe surface of a viscous medium with a solidifymg crust will lead to an upwelling of account for this configuration (for a review see Watts 1982). the substratum in isostatic response to the mass deficiency The problem is complicated since both the strength of the represented by the volume of the crack. For a small-offset lithosphere and the viscosityof the asthenosphere are fracture zone this volume is small and a simple uplift of the involved. In addition one must reckon with the presence of crust will result (section a). With increasing offset the crack magmachambers under the spreading axis.An integrated not, onlybecomes wider but also deeper. With the quantitive model has not yet been proposed. square-root depth/age relation of Parsons & Sclater (1977) Admittance studies which quantify the relation between this meansthat the cross-section of the crack increases gravity and topography in the frequency domain (McKenzie linearly with the age-gap of the offset. Also its length starts & Bowin1976; Cochran 1979; McNutt 1979; Bowin & counting, making the upwelling more two-dimensional. Milliganinpress) have been successful indeveloping These effects lead to a strong increase of the upwelling as a predictionfilters which yield the observedgravity from function of the age-gap. topographic data by adjusting the density of the topography In the reaction to the upward pressure of the substratum and the elastic thickness of the lithosphere. In this approach against the lithosphere the effect of the Aexural rigidity of the presence or absence of the median valley is a given fact. the lithosphere becomes perceptible in the form of crustal Different filters apply to different parts of the world ocean. warping, and a depression develops beyond the uplifted Tapponnier & Francheteau (1978)have proposed steady state necking and regional uparching of the lithosphere as lithospheric lid. The warping W is governed by the expression (Turcotte & Schubert 1982, p. 127) the processes accounting for the median valley. Both the gravity studies and the necking model reckon X with the lithosphere only, and not with the viscosity of the W = W, e-*'" cos - (1) LY asthenosphere which might, of course, be so low that no Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/5/763/4893100/gsjgs.143.5.0763.pdf by guest on 03 October 2021 FRA CT UR E ZONES IN THE NORTH ATLANTICNORTH THE FRACTURE INZONES 769 appreciable time ' effect results forthe response to the the introduction of a variable temperature-dependent unloading of the asthenosphere by the spreading process. viscositycoefficient andfor of non-linear effects like the Sleep & Rosendahl (1979) and Collette et al. (1980) treated differentialrising of partial melts in a moreviscous the problem as a purely viscous one, neglecting the strength asthenosphere (cf. Schouten et al. 1985) willbe neededin of the lithosphere. The latter authors modelled the median the flow model. valleyby finding a steady state solution forthe surface With regard tothe asymmetryin cross-section of the expression of a semi-continuouslyrenewed furrow in a medianvalley nearthe intersection point with a fracture viscous liquid, the furrow being the mass deficiency created zone, again two effects can be discerned, one related to the by the parting of the spreading plates. The decay viscous flow of the asthenosphere, the other mainly to the constant k of a 2-D sinusoidal distortion z,,cos ax of the reaction of the lithosphere. First consider the latter effect. surface of a viscous liquid is dependent on thewavenumber, The inner corner of the intersection of a spreading centre or and a transform fault isrelatively free from restraints to adjust in a vertical sense to its equilibrium level, due to the z = z, e-krcos MC (2) circumstance that along the transform fault plane no stress with build-up occurs. The oppositecorner is restrained by the process of welding of young lithosphere being created in the k=g 2va (2a) 7 median valley to the older lithosphere at the other side of the fracture zone. where g is the acceleration of gravity and v the coefficient of Although the welding process leads to asymmetry, it is viscosity. The steadystate solution consists of a narrow not thought that the effect is sufficient to explain why the depression superimposed on a broader upwelling,which inner corner is higher than normal, forming the highest together would form the median valley with its walls. The point in the surroundings, while at the same time the axis of result can be readily understood from expression (2a): small the medianvalley deepens towards the intersection. wavelengthsdecay slower than larger ones, which means Therefore, effects offlow of the asthenosphere must be that after a time t, a remnant of the former ones is still left involved as well. One cause of asymmetry of the flow when a new load is applied. pattern could be the viscous drag which is exerted by the A good fit with actual data for the Mid-Atlantic Ridge parting lithosphere at the surface of the asthenosphere. Due near 15"N was obtained with v = 1.5 X 10-19 stokes. For tothe drag the total outwardflow of material and, Reykjanes Ridge, where no medianvalley exists, a lower therefore, the total inflow from the asthenosphere, is several viscosity of 0.3 X W'' stokes would apply. For the fast times larger than the volume of the lithosphere, v,, which spreading East Pacific Rise Y would be still lower, i.e. has to be replaced per unit of time. In the 2-D case the total 0.4 X 10-l8 stokes. The variation in viscosity was related to flow can be expressed as v,(l + 2f,), where fd is the one-way the percentage of partial melt, the latter Ridges having a drag factor. For reasons of symmetry, thedrag factor larger heatcontent due to the presence of the Icelandic becomes zero along the transform fault plane. This means hotspot and to the faster spreading respectively. thatthe outward flow andthe inflow atthe intersection Collette et al. (1980) stressed that in reality the effect of reduce with vrfd to vl(l +fd). For the inner corner, where the lithospheric strength cannot be neglected. This seems to the drag is zero, this leads to an excess of upward directed be confirmedby the circumstance thatthe extinctridge flow. This results in a positive bulge (or a positive gravity crests in the Labrador and Coral Seas exhibit a frozen-in anomaly) and explains the high inner corner topography. At negative gravityanomaly (Watts 1982,fig. 4), which one the other side of the medianvalley the drag remains the would not expect in the case of a purely viscous behaviour. same with reduced inflow. This leads to a deficiency of mass However, since the modelsyield comparable results, (or a negative anomaly) and explains the depressed assessment of the relative importance of the effects of character of the topography at thisside of the median viscous lag and of lithospheric strength will be very difficult valley. in practice. In a steady state model, the foregoing result can also be understood by interpreting the juxtaposed high and low as a positive and a negative surface load; theseloads are necessary to compensate for the asymmetry in the drag at The median valley nearthe intersection with afracture the intersection point, caused by the change in the sense of zone (section d) movement at the surface. The problem of asymmetry of the When approaching the intersection with a fracture zone, the viscous drag,exerted by the lithosphere onthe astheno- median valley becomes deeper and increasingly asymmetric. sphere, near fracture zonelspreadingaxis intersections is the In the viscousmodel the deepening of the medianvalley subject of further study (B. J. Collette & J. Verhoef towards the intersection with a fracturezone follows unpublished). automatically asthe end-effectof the linear model. The driving forces from beyond the fracture zone fall away while the inflow remains the same, although it becomes tapered (see next section). Thisdemands locally a larger driving Topographyofinactivefracturezones(sectionseandf) forceand in the steadystate model a larger depression The typical cross-section of the inactive segments of fracture results. Further modelling is needed to check whether zones follows from the merging of the depressed axis of the theend effect also can account forthe occurrence of a median valley with the high graben wall deriving from the median valley near the intersection with a fracture zone at other spreading centre. Whenmoving outward, a line places on the East Pacific Rise, where otherwise no median originally situated along the median valley axis will recover valley occurs. It cannot be precluded that in a final model its isostatic equilibrium position by being transported up the Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/5/763/4893100/gsjgs.143.5.0763.pdf by guest on 03 October 2021 m B. J. COLLETTE inner wall of the median valley. However, near the fracture of the North American continent and the Udintsev FZ in the zone axis it is prevented from doing so since it becomes South Pacific. Sandwell proposes a morphology consisting of welded tothe opposite fracture zone wall. Forfracture a simple depthlagestep which becomes frozen in atthe zones with a large offset this results in section e, forfracture intersection of spreading centre and transform fault. Next zones with a very large offset, in section f. the step becomes rotated by the bending of the lithosphere The effectiveness of welding together both fracture zone when the depthiage step decreases with increasing age of sides isdifficult toevaluate. If it is perfect,the surplus the oceanic crust. An asymmetric cross-section would result buoyancy of the depressed younger side will have an effect whichis exactly theopposite of whatis observed in the on the wall at the older side, and the total topographic step Atlantic, showing a highwall on the younger side of the will remain intact. If not,part of thestep may get lost, fz-axis and a depressed valley on the older side. Sandwell although this doesnot seem tohappen under normal computes the evolution of the fz cross-section and compares circumstances. the outcome with the altimeter data, for which he derived thehorizontal derivative. He also shows results for Romanche FZ and neighbouring fracture zones in the EquatorialAtlantic, mentioning, however, that in the Discussion Atlantic different conditions prevail. Only the western limb and part of the transform domain of the Mendocino FZ has been preserved. However, Comparison with fracture zones in the Pacific comparison with bathymetric data (e.g. GEBCO-sheet 5.07 The model proposed for the Atlantic fracturezones is based 1982) reveals that the Mendocino Ridge lies to the south of on two fundamental elements: thetransform domain on theolder side of thefracture 1. the presence of a median valley to account for the zone-axis. The fz-axis lies about 70 km farther N than recorded asymmetry of inactive fracturezones, sinceit is indicated by Sandwell in his fig. 11. The continuity of the from the non-equilibrium position of the median valley axis altimeter anomaly indicates that farther W,as far as 135"W, that the depressed younger side of an inactive fracture zone the Ridge forms the older side. Thenegative anomaly to the is a relict, and S of the Ridge, which stands out on Haxby's map (1983), 2. a sufficiently large offset to account for the occurrence and therelated deep, represent a marginalvalley. This, of a high fracture zone wall to the older side of the fracture together with the existence of a median valley on the Gorda zone axis and of the accompanying marginal trough when Rise (cf. Heinrichs 1977) places the modem Mendocino FZ the high wall reaches extreme dimensions. in the class of Vema FZ and the other Equatorial Atlantic Therefore the model would not apply to the East Pacific fracturezones. West of135"W a different configuration Rise, which has no median valley. The morphology of the exists as confirmed by recent reports (H. W. Menard, pers. transformdomain of Quebrada, Discovery, GofarFZs comm.; cf. also Caress et al. 1985). Here the depth-age step (Lonsdale 1978; Searle 1983) and Wilkes FZ (Kureth & Rea model appears to apply. Actually, the sections used by 1981) tothe south of theGalapagos triple point, and Sandwell in his computations are situated over this part of Siqueiros FZs(Crane 1976) to thenorth of it indeed is the Mendocino FZ. different from what is found in the Atlantic (cf. Fox & Gallo For the Udintsev FZ the altimeter data show a complex 1984). No data exist on the inactive sections of these picture of the transform domain, withhigh fracture zone fracture zones. This paper will not speculate what the role of walls and accompanying marginalvalleys and a possibly thehorizontal along-axis component of thethermal intrusive transverse ridge in the transform valley, as follows contraction might beunder these circumstances. It only from comparison with unpublished bathymetric data points out that it cannot be precluded that, with a thinner (Eltanin cruise 33). There are no indications of a median lithosphere, a different relaxation mechanism takes over, as valley. The high fracture zone walls and the marginal valleys might be concluded fromthe absence of fracture zones continue to bothsides past the intersectionsof the transform between Galapagos triple junction near 2"N and the 2'50's with the spreading axis and appear to merge with the young Offset (Lonsdale 1977), i.e. over a distance of about 500 km. sides, which are high. This is also clear from the geoidal (Alsonote the absence of fracture zones on Reykjanes height map of thearea by Haxby (1983). A bathymetric Ridge, where Collete et al. 1980 inferreda higher heat section farther west shows a simple depth-age step. content of the mid-ocean ridge). It thusappears that the two large-offset fracture Tamayo FZ might be different again (Kastens et al. 1979; zones mentioned in the Pacific have many features in common MacDonald et al. 1979; Gallo et al. 1984). The spreading with the large-offset fracture zones in theEquatorial velocity is smaller than near Siqueiros FZ and the existence Atlantic, without being entirely identical.The modern of end-effect median valleys has beenreported near the Mendocino FZ is comparable with the Atlantic fracture intersectionpoints with the Rise.However, the tectonic zones,the older part is not. Udintsev FZ seems, in setting of Tamayo FZ, whichonly recently became freed principle, torepresent the depth-age step model, but the from the geometric constraints related to the early opening transform domain showsall the complexities of the phase of the Gulf of California, cautions against including Equatorial Atlantic fracture zones. It is not yet possible to this fracture zone in the scheme. comment upon the variations exhibited by the large Pacific Next to be discussed are several large-offset zones in fracture zones andnot documented in theAtlantic. It is the Pacific which have served in a study by Sandwell (1984; tempting to attribute all differences in morphology to see also Sandwell & Schubert 1982b) on the thermo-elastic variations in spreading velocity,in the viscosityof the behaviour of theoceanic lithosphere, using SEASAT asthenosphereand, perhaps also in the thickness of the altimeter data. These studies refer to Mendocino, Pioneer, lithosphere,thus continuing the approach of the present Murray, Molokai, Clarion and Clipperton FZs to the west paper. However, knowledge of the Pacific fracture zones is Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/5/763/4893100/gsjgs.143.5.0763.pdf by guest on 03 October 2021 FRA CT UR E ZONES IN THE NORTH ATLANTICNORTH THE FRACTURE INZONES 771 still far from complete. Also, the Atlantic Ocean may not expect thatmore examples willfollow from afurther yet have yielded all its secrets. It would not be surprising if analysis of altimeter data. the two oceans showed more fundamental resemblance than Three-dimensionality may therefore prove to be an generally assumed, differences being quantitativerather important aspect of the seafloor spreading process, although thanqualitative and related to physical processes, the the effects of itmay be difficult to distinguish from the parameters of which tell us more about the conditions in the occurrence of real spreading direction changes and from the different parts of the world ocean. leaking of differential movements of large segments of the A final question is what thermo-elastic adjustmentof the oceanfloor along what may be called plate boundaries of the depth differences over large-offset fracture zones will do to a second order. Careful analyses are needed to resolve this. A more complex fracture zone morphology. The effect of the problem which also deserves further attention ishow the adjustment will be superimposed on the typical fracture complementary divergence on the convex sides of mid-ocean zone topography as this topography was modelled in Fig. 9. ridges is compensated for, i.e. whether theneeded extension The scarps will be rotated the other way from Sandwell's can be realized in a diffuse way or whether outlets in the model and become somewhat steeper. The increasing form of plateboundaries of the second order play a thickness of the ageing lithosphere will lead toa larger fundamental role in this. flexural wavelength andrelateda broadening of the phenomenon,concurring withSandwell's observations regarding this point. In this context we recall what has been Magnetic anomalies over fracture zones said on lithosphericthinning in thetransform domain of Surveys of several fracture zones in the Cretaceous Vema FZ. Such thinning would smooth the step in elastic Magnetic QuietZone on the African Plate (Twigt et al. properties over the fracture zone axis. 1983; Slootweg & Collette 1985; see also Verhoef & Duin this volume) revealed an amazingly simple magnetic pattern over these fracture zones. The CMQZ records a period in which the polarization of theearth's magnetic fieldwas The three-dimensional character of seafloor spreading normal. When reduced to the pole, the anomalies become Theoccurrence of horizontalthermal contraction of the positive over the fracture zone valleys, i.e. to the younger oceanic lithosphere, appearing in fracture zones as such and side of the fracture zone axis. In about 30% of the sections more especially in the leaking of large-offset transforms in this anomaly is asymmetric and accompanied by a negative and, possibly, also beyond the transform domain (cf. Roest anomaly to the older side. Where the fracture zone has a & Collette this volume), characterizes seafloor spreading as double topographic expression, as in the case of Kane FZ a 3-D process. near 2lo40'N/29"00'W, the positive anomaly is also double. Roest et al. (1984; cf. also Collette et al. 1984) raised the The findings were interpreted as anindication of a higher question of whetherthree-dimensionality of the seafloor magnetization of thefracture zone valley, i.e.a stronger spreading process also might play a role in the fanning of normal magnetization under normal-field conditions, and a fracture zones. They arguedthat the divergence of the stronger reversed magnetization when the field is reversed. ridge-push over a sinusoidal mid-ocean ridge would cause a Following this principle, Collette et al. (1984) computed a small amount of convergent flow towards the concave side magnetic fracturezone model withchanging polarities, of the ridge and a complementary divergent flow towards which they applied successfully to the Kroonvlag dataset. the convex side. The convergence would be restricted to the A stronger magnetization can mean a higher degree of amount of thermal contraction in the horizontal plane. A magnetization of the same volume of rock or a larger bulk few degrees of fanning of the flowline pattern over a with the same magnetization. The outcome was related to curvilinear mid-ocean ridge might result in this way. The the asymmetric development of the median valley near the principle was applied to explain the fanning of transform intersection with a fracture zone. We recall that Karson & faultdirections theincentral North Atlantic from Dick(1983) reported an asymmetryin the deposition of Oceanographer to the Fifteen Twenty FZ. These directions basalts at the intersectionin present transform domains with appear to converge in such a way that the parting of the preferential flow of the basalts towards the inactive fz-valley American and African plates would be obstructed. and tectonicdenudation towards theother side, the high The revised interpretation of the structures near 15'20'N inner corner of the median valley with the transform fault. in terms of a northward jump of the North American/South We suggest thatthe latter process isreflected in those American plateboundary about 10Ma (Roest & Collette sections where the anomaly is preponderantly asymmetric. this volume) seems to solve part of the problem of how to An attenuation of the total magnetization couldoccur reconcile the diverging spreading directions. Nevertheless, a which would be symmetric with respect to the fz-axis and certain degree of convergence of spreading movement still which would reflect the recorded symmetric thinning of the seems present. In addition, the central North Atlantic is not crust. Such aphenomenon wouldinvolve larger wave- the only area suspected of 3-D flow. The transform fault lengths, which means that the effect is more attenuated by directions in Romanche FZ show a counter-clockwise the earth filter (Schouten & McKamy 1972) and, therefore, deviation of severaldegrees from thespreading direction less easy to recognize. according to Minster & Jordan's (1978) RM2 pole for the The correspondence of the magnetic model for South Atlantic (M. V. Thomas pers. comm.). In Quebrada intermediately-large-offsetfracture zones and the proposed FZ too, the transform faults make an angle of less than 90" morphological model reinforces the underlying concept of with thedirection of thespreading axis (Searle 1983). asymmetry. Few systematic investigations have been carried Furthermore, we mention the sigmoidal course of Udintsev out of magnetic anomalies over very-large-offset fracture FZ, which follows the same pattern. This is the first example zones. Cochran (1973) modelled a 20-kmwide non- for which the evidence is based on SEASATdata. We magnetized body in the western inactive limb of Romanche Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/5/763/4893100/gsjgs.143.5.0763.pdf by guest on 03 October 2021 772 B. J. COLLETTE FZ. Robb & Kane (1975) reported that the Vema FZ South direction and the existence of small offsets seem the major Wall is non-magnetic. If the transverse body in the eastern reasons why fracture zones do not always exhibit the typical limb of Kane FZ near2lo40'N/29"00'W is an intrusion asymmetric cross-section. deriving from the transform domain, then the finding that it A final comment concerns the occurrence of a marginal has a lower magnetization may helpfuture modelling valley along the highwall atthe opposite side from the efforts. transformfault. It was argued thatthe marginalvalleys result from crustal warping in a reaction to the emerging of Factors influencing typical fracture zone topography extremely high fracture zone walls. These occuralong Direction changes of seafloor spreading have a large impact very-large-offset fracture zones and were interpreted as on both the occurrence and disappearance of fracture zones being related to upwelling due to leaking. Leaking would be andon their appearance in cross-section. Menard & afundamental aspect of very-large-offset fracture zones. Atwater (1968, 1969) described how the necessity of ridge However, the transition from large to very large offsets is a offsets arises or vanishes with changing spreading direction gradual one. Therefore, it is interesting to note that some of (see also Fig. 6). The observations in thecentral North the larger-offset fracture zones, e.g. Kane FZ near 35" and Atlantic showed that the larger fracture zones to the north near 40"W, showsigns of an incipient marginalvalley at of Kane FZ obtained their offsets after the large changes of places where the high wall is somewhat larger than usual. spreading direction at the end of the Cretaceous Magnetic Quiet Zone. Until then the general course of the spreading axis in this part of the area did not deviate appreciably from Summary and conclusion the spreading meridian. Nevertheless, at the onset of the A model has been developed for fracture zone topography CMQZ the spreadingaxis was dissected by many small-offset which accounts for the asymmetric cross-section of inactive features,partly of the Kurchatov FZ type (cf. Searle & fracture zones as observed in the Atlantic Ocean. Contrary Laughton 1977) and perhapspartly also of the even less to whatmight be expected from a lithospheric cooling stable propagating rift offset type (e.g. Shih & Molnar 1975; model, one finds typically a high wall or scarp on the older Hey 1977). This finding concurs with Schouten & Kiltgord's side of thefracture zone axiswhile the younger side is observations (1977; see also Schouten & White 1980) on the depressed and forms an asymmetric valley. This cross- existence of zero-offset fracture zones from the analysis of section was related to the asymmetric development of the the M-series anomalies on the American Plate. topography to both sides of the medianvalley near Reconstruction of thespreading pattern from 124Ma intersections with fracture zones. until present (Collette et al. 1984; Slootweg & Collette 1985) Mechanisms have been discussed which may cause this showed that all thelarger fracture zones survived the latter asymmetry. It was related tothe fundamentally changes of thespreading pattern, with the exception of asymmetric boundary conditions which govern the involved Tydeman FZ, whichdissolved in the offsets of Fracture rheological processes. These concern both thestrength of Zones A, B, etc. in the FAMOUS area and the Northern the lithosphere and the viscosity of the asthenosphere. A FZ which not so long ago lost its character of an orthogonal quantitative model which deals with both effects, taking into fracture zone. Several of theother larger fracture zones account the fundamentally asymmetric rheology, has not yet underwent minor adjustments (e.g. Kane FZ which jumped been accomplished. 20 km to the south between anomaly 34 and 33). Between The asymmetry principle appears inapplicable or less these large fracture zones, one sees fracture zones appear effectivewhen a change of seafloor spreading direction and disappear again (or become small-offset features) as a occurs which necessitates the growth in length of the function of the spreading direction. spreading centres. Then broad atypical valleys result. Also The small-offset fracture zones of the Kurchatov type, when the sense of change of seafloor spreading direction is with oblique offsets of the order of 20 km, frequently exhibit opposite,irregular forms may originate. Small-offset the typical asymmetric fracture zone cross-section (also R. fracture zones of the Kurchatov Fracture Zone type may C. Searle pers comm.). However, they are difficult to trace also exhibit irregular cross-sections. in consecutive profiles since they are fundamentally Thegraben character of the transform domains of non-steady-state features and highly irregular. fracture zones indicates thatfracture zones are primarily A reason why larger-offset fracture zones are not always extension features.The extension is related to thermal easy to recognize from their cross-section is that the typical contraction of thelithosphere in the direction of the asymmetric expression deteriorates into a broadand atypical spreading axis. It was argued that the void which is created valleywhen aspreading direction change occurs whichis by contraction depends linearly on the age gap of the offset oppositeto the sense of movement along thetransform of afracture zone. When the offsetis sufficiently large, fault,e.g. a counter-clockwise spreadingdirection change mantle upwelling will occur. This explains the occurrence of with adextral fracture zone (see Fig. 6). This effectwas intrusive bodies in the transform domain of larger fracture mentioned whendiscussing Fig. 1.Under these cir- zones ('leaky transforms') and of extremely high fracture cumstances the spreading centres have to grow in their axial zone walls, and, as a secondary feature, theorigin of marginal direction. The observations show thatthen a different valleys as the effect of lithospheric warping. Consideration topography results. In the reverse situation the cross-section of the involvedflexural parameter suggests thatthe of the fracture zone valleymay become narrower and, as upwelling andthe intrusive processes may lead to observed for Kane FZ in the CMQZ, may disappear totally lithospheric thinning. althoughthe magnetic expression prevails and even is The model accounts for the majority of the observations stronger. on fracture zones in the Atlantic, including the very-large- There are no clear-cut explanations for these anomalous offset fracture zones in the Equatorial Atlantic. Comparison forms and it is concluded that changes of seafloor spreading with published and some unpublished dataon two large Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/5/763/4893100/gsjgs.143.5.0763.pdf by guest on 03 October 2021 FR A C T U R E ZONES IN THE NORTH ATLANTICNORTH THE FRACTURE INZONES 113 fracture zones in the Pacific indicated that these show the -, VERHOEF,J. & DEMULDER,A. F. J. 1980. Gravity and a model of the same type of features as theEquatorial Atlantic fracture Median Valley. Journal of Geophysics, 47, 91-98. zones. The modernMendocino FZ and thetransform CRANE,K. 1976. The intersection of the Siqueiros transform fault and the East Pacific Rise. Marine Geology, 21, 25-46. domain of Udintsev FZ donot conform tothe simple CROUCH,S. T. 1979. Geoid anomalies across fracture zones and the thickness depthjage step model. Farther from the spreading axis the of the lithosphere., Earth and Planefary Science Letters, 44, 224-30. bathymetry does obey this model. It is not yet understood DETRICK,JR, R. S: 1981. An anilysis of geoidanomalies across the what causes this change of character.The beliefwas Mendocino Fracture Zone: implications for thermal models of the lithosphere. Journal of Geophysical Research, 86, 11751-62. expressed that no fundamental differences between the DIAMENT,M. 1981. Etude de la rtponse isostatique en domain oceanique et Pacific and the Atlantic are involved and that we should aim applications (f trois zones defracture de I'Atlantiqu'e. Thbe doct. 36 cycle, for a total physical theory of the world ocean floor. Universitt de Paris-Sud, Paris. The model doesnot predict a definite fracturezone Fox, P. J. & GALLO,D. G. 1984. Tectonic model for Ridge-Transform- Ridge plate boundaries: implications for the structure of oceanic morphology inthe absence of median valley, as on the lithosphere. Tectonophysics, 104, 205-42. fastest spreadingsegment of the East Pacific Rise where one -, SCHREIBER,E., ROWLETT,H. & M~CAMY,K. 1976. The geology of the finds Quebrada, Discovery, Gofarand Wilkes Fracture Oceanographer Fracture Zone: a model for fracture zones. Journal of Zones. Although the offsets of these fracture zones compare Geophysical Research, 81, 4117-28. in length with thelarge Atlantic fracture zones, the age GALLO,D. G., KIDD,W. S. F., Fox, P. 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