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Fracture Zones in the North Atlantic: Morphology and a Model

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Fracture Zones in the North Atlantic: Morphology and a Model 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 zone morphology 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-offset Kane 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 been revised (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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/5/763/4893100/gsjgs.143.5.0763.pdf by guest on 03 October 2021 764 B. J. COLLETTE 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 valley is indicated schematically by heavy solid 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. 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 AC TU RE ZONES IN THE NORTH ATLANTICNORTH THE FRACTURE IN ZONES 765 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.
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