Sea MARC I side-looking sonar images and Sea Beam bathymetry along a 400-km stretch of the Juan deFuca Ridge crest provide evidence that excessive extrusive volcanism periodically builds a crestal ridge along the axis of seafloor spreading. An elongate summit depression (ESD), or rift valley, is commonly observed in the spine of this crestal ridge. The crestal ridge volcanic landform has a distinctive shape that is recognized in bathymetric contours both along the spreading axis and at least up to 30 km away from the axis. The landform has a plan-form shape that resembles a side view of an archer's bow with the long dimension of the bow form parallel to the strike of the ridge. In cross section, the bow form is flat on top and has steep flanks. These bow-form shapes can be explained by magma that rises into the crust at a discrete center and flows laterally into belts of ridge-parallel dikes, similar to Icelandic fissure eruptions. Both the variable dimensions of the ESD along axis of the Juan de Fuca Ridge and the relationship among volcanic flow morphologies within and beyond the ESD suggest the four different segments of the Juan de Fuca Ridge presented in detail here display different stages in a cycle of oceanic crust accretion. This cycle includes episodes in which there is (1) extrusive volcanic construction which widen the crestal ridge prior to the collapse of the summit depression, (2) collapse within the summit region of the crestal ridge to form an ESD during a phase of volcanic inactivity, and (3) renewed magmatism in the ESD as its floor widens by extension and brittle fracture of the upper crust. This episodic model implies that the width of the young seafloor affected by volcanic extrusion or dominated by tectonic stretching varies through time. 1977; Macdonald and Atwater, 1978] and a rotation of fault blocks [Davis and Lister, 1977; Harrison and Stieltjes, 1977; Laughton and Searle, 1979]. Several steady state models have been proposed to explain rift valleys on slow-spreading ridges as depressions caused by either (1) hydraulic head loss and viscous drag in a narrow conduit of upward flowing asthenospheric material [Sleep, 1969; Lachenbruch, 1973), (2) extension and necking of the lithosphere near the axis [Tapponnier and Francheteau, 1978], or (3) the disproportion in the widths of the axial extension zone and axial volcanic zone [Deffeyes, 1970; Palmasson, 1973; Anderson and Noltimier, 1973]. Investigators who have primarily used submersibles and remote cameras to examine in detail the axes of slow-, medium-, and fast-spreading centers have described ridgeparallel and symmetric zones characterized by either a predominance of relatively intact volcanic constructional features or mainly broken-up volcanic terrain cut by fissures and faults [e.g., Normark, 1976; Macdonald and Luyendyk, 1977; CYAMEX Scientific Team, 1981; Ballard et al., 1981, 1982; Choukroune et al., 1984; Lichtman et al., 1984]. Detailed examination of a short segment of the East Pacific Rise (-EPR) at 2 iøN (full-rate ~6 cm/yr...
The morphometric characteristics of the axial regions of oceanic spreading centers are determined by (1) the type of volcanic flows, (2) the relation beLween primary volcanic relief (on a scale of a few meters to tens of meters) and degree of sediment cover, and (3) the extent of surficial expression and timing of tectonic disruption of the young oceanic crust. Even within a single, continuous, linear spreading-ridge segment with relatively uniform axial valley dimensions over a distance of 50 or more kilometers, such as along the southern Juan de Fuca Ridge, the changes in morphometric characteristics along axis within the youngest crust indicate distinct variation in tectonic and volcanic activity over short distances within short time periods. An integrated analysis of Sea MARC I, Sea MARC II, and photographic data for the southernmost continuous segment of the Juan de Fuca Ridge shows that generalizations about tectonic and volcanic processes at spreading ridges must consider both the temporal scale of processes as well as the physical scales of observations if predictive models are to be successful. Comparison of the morphometric expression within the major hydrothermal vent area and the rest of the southernmost ridge segment suggests that the mapped distribution of hydrothermal vents may reflect the extent of survey effort rather than uniqueness of geologic setting. INTRODUCWION The Juan de Fuca Ridge (JFR) is an intermediate-rate (6 cm/yr full spreading rate) midocean ridge spreading center located off the coast of Oregon and Washington in the northeast Pacific Ocean (Figure 1). The JFR spreading system extends between the Blanco Fracture Zone to the south and the Canadian continental margin to the north and consists of several distinct segments [Delaney et al., 1981; Kappel and Ryan, 1986]. The southern part of the ridge system is divided into two segments, with a westward ridge jump at about 45øN [Embley et al., 1983; Morton et al., this issue]. The area that we focus on in this paper is the southernmost continuous segment of the JFR closest to the Blanco Transform.
RésuméLa profondeur des fonds marins est une donnée essentielle pour un grand nombre de domaines d'activité : scientifique, économique, politique. La bathymétrie -la science de la mesure des profondeurs de l'océan -a des applications aussi diverses que la gestion des ressources minérales et vivantes, l'aménagement des habitats sous-marins, l'implantation des câbles sousmarins et autres pipe-lines, l'extension des juridictions des pays riverains dans le cadre de l'article 76 de la convention des Nations Unies sur le droit de la mer, etc. Sur le plan scientifique, des questions d'ordre fondamental -telles que la formation des fonds océaniques ou le rôle du relief sous-marin sur l'évolution du climat -passent par une connaissance globale et homogène de la topographie du plancher océanique.Les cartes globales de bathymétrie actuellement disponibles sont inadaptées pour bon nombre de ces applications, car de vastes zones océaniques demeurent inexplorées. Les sondeurs acoustiques multi-faisceaux offrent la résolution adéquate, mais il faudrait plus de 200 années-navire pour couvrir l'ensemble des grands fonds, à un coût de l'ordre de plusieurs milliards d'euros. En revanche, l'altimétrie satellitaire permettrait d'obtenir un modèle global de bathymétrie satisfaisant pour de nombreuses applications, en moins de six ans, à un coût inférieur à la centaine de millions d'euros. En effet, dans l'océan, les masses d'eau ont tendance à s'accumuler au dessus des montagnes sous-marines à cause de l'attraction causée par les reliefs. La surface de l'océan au repos (en l'absence de toute perturbation océanique) correspond au géoïde, une surface équipotentielle en tous points perpendiculaire à la force de pesanteur locale.
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