Deep-sea hydrothermal vents and cold seeps are submarine springs where nutrient-rich fluids emanate from the sea floor. Vent and seep ecosystems occur in a variety of geological settings throughout the global ocean and support food webs based on chemoautotrophic primary production. Most vent and seep invertebrates arrive at suitable habitats as larvae dispersed by deep-ocean currents. The recent evolution of many vent and seep invertebrate species (<100 million years ago) suggests that Cenozoic tectonic history and oceanic circulation patterns have been important in defining contemporary biogeographic patterns.
Side-scan sonar imaging of the Central Lau Basin (SW Pacific) has revealed a Central Lau Spreading Centre (CLSC) propagating southwards at the expense of an Eastern Lau Spreading Centre (ELSC) with a small Intermediate Lau Spreading Centre (ILSC) forming a ‘relay’ between the two. Volcanic rocks sampled along these spreading centres, and from two adjacent seamounts, are glassy to fine-grained pillow lavas and sheet flows of basalts, ferrobasalts and andesites. The evolved rocks are mostly confined to the propagating tip of the CLSC and can be explained by a high rate of cooling relative to magma supply, as invoked for magma genesis at propagating ridges elsewhere. Compared with equivalent rocks from the eastern Pacific, the most evolved members of the CLSC suite require similarly high degrees (>90%) of fractional crystallization from their basaltic parents. Fractional crystallisation cannot, however, account for the compositional differences between CLSC and ELSC lavas. Whereas the composition of the CLSC lavas lies just within the compositional spectrum of typical N-MORB, the ELSC lavas are distinctly enriched in alkali and alkaline earth elements, reach oxide and apatite saturation at lower Fe, Ti and P concentrations and generally show greater vesicularity despite slightly greater depths of eruption, all indicative of a water-rich subduction component. They also have lower contents of Ni, Sc, Na and Fe and higher contents of Si at a given MgO concentration that indicate a more depleted and more hydrous mantle beneath the ELSC compared with the CLSC. These results provide further evidence that, beneath the Central Lau Basin, the source composition changes progressively from MORB-type to island arc tholeiite type as the subduction zone is approached, both eastwards from the CLSC to ELSC and southwards along the ELSC to the Valu Fa Ridge. They also indicate that the composition of the subduction component may vary systematically away from the arc, with Th, LREE, Ba, Rb and H (as H 2 O) all present close to the arc, only Ba, Rb and H 2 O present at intermediate distances and just H 2 O perceptible at the furthest distances.
[1] We have investigated the relations between volcanic, tectonic, and hydrothermal activity on Lucky Strike Seamount (37°17 0 N, Mid-Atlantic Ridge) using a nested survey strategy involving collection of data from different deep-sea mapping systems. The highly tectonized seamount summit consists of three volcanic cones surrounding a relatively flat depression with a young lava lake in its center. Hydrothermal activity is focused mainly within the summit depression with most of the vents located proximal to the lava lake. Isolated active and inactive chimneys and mounds are widespread throughout the summit depression and occur on both volcanic (pillow lava) and hydrothermal (sulfide rubble and hydrothermally cemented breccias) substrates. The large volume of sulfide rubble, together with the nature of the sulfide structures, indicates that hydrothermal activity has been episodic but ongoing for a long period of time (hundreds to thousands of years). On the basis of the distribution of hydrothermal deposits, we propose a model of alternation between tectonic and volcanic control on hydrothermalism at Lucky Strike Seamount. Midsegment melt focusing produces a spatially and temporally stable heat source that sustains focused high-temperature hydrothermal activity over long time periods. During periods of amagmatic extension, active faulting within the summit depression provides multiple, near-surface fluid flow pathways for discharge of high-temperature fluids and widespread deposition of massive sulfides. During eruptive events, rapid effusion of very hot lava creates a lava lake and hyaloclastite deposits. The new sheet flows form a cap on the hydrothermal system, and fluid upflow is reorganized. Discharge of high-temperature fluids is restricted to isolated sites with relatively high permeability, for example, the edges of the lava lake. Much of the upwelling hydrothermal fluid pools in the subsurface, conductively cools, and mixes with entrained seawater before discharging as widespread low-temperature diffuse flow. Hyaloclastites become cemented, further augmenting the sealing of the system. Present-day activity at Lucky Strike Seamount represents this locally volcanically controlled phase of activity, despite the segment as a whole being dominantly amagmatic.
[1] On-axis deep tow side scan sonar data are used together with off-axis bathymetric data to investigate the temporal variations of the accretion processes at the ultra-slow spreading Southwest Indian Ridge. Differences in the length and height of the axial volcanic ridges and various degrees of deformation of these volcanic constructions are observed in side scan sonar images of the ridge segments. We interpret these differences as stages in an evolutionary life cycle of axial volcanic ridge development, including periods of volcanic construction and periods of tectonic dismemberment. Using off-axis bathymetric data, we identify numerous abyssal hills with a homogeneous size for each segment. These abyssal hills all display an asymmetric shape, with a steep faulted scarp facing toward the axis and a gentle dipping volcanic slope facing away. We suggest that these hills are remnants of old split axial volcanic ridges that have been transported onto the flanks and that they result from successive periods of magmatic construction and tectonic dismemberment, i.e., a magmato-tectonic cycle. We observe that large abyssal hills are in ridge sections of thicker crust, whereas smaller abyssal hills are in ridge sections of thinner crust. This suggests that the magma supply controls the size of abyssal hills. The abyssal hills in ridge sections of thinner crust are regularly spaced, indicating that the magmato-tectonic cycle is a pseudoperiodic process that lasts $0.4 m.y., about 4 to 6 times shorter than in ridge sections of thicker crust. We suggest that the regularity of the abyssal hills pattern is related to the persistence of a nearly constant magma supply beneath long-lived segments. By contrast, when magma supply strongly decreases and becomes highly discontinuous, regular abyssal hills patterns are no longer observed.
The active southern Havre Trough (35020 '-37øS) backarc basin is interpreted to form from the evolution and interaction of migrating cross-arc magmatism, and the progressive development of longitudinal rift grabens. The proposed migration of the proto-Kermadec arc front from the Colville arc at-5 Ma to the active Kermadec arc margin is recorded by the construction of arc edifices trailed across the intervening, and contemporaneously rifling, backarc complex. Migration trails are identified for at least four arc volcano sources. The Rumble V arc migration trail is the most prominent, forming a continuous, high-standing, magmatic arc ridge. These postulated arc trails segment the backarc region and initially limit riff development to the intervening proto-arc crust blocks. Early rifling between the arc volcanoes forms fully developed riff grabens which, with progressive basin widening, propagate longitudinally across the trails of migrating constructional arc magmatism. A model of the balance between rates of constructional arc magma production M• and destructive back-arc (rifling) extension V• is proposed. When V• is high, M• is insufficient to keep abreast of destructional rifting, resulting in small, isolated arc massifs quickly dismembered by rifling. Conversely, when V• is low, M• is sufficiently greater than rifling to produce a continuous, high-standing cross-arc ridge which segments longitudinal riff development. Migrating arc magmatism will be best observed, and preserved, in rifling backarc basins when M• is Y 600 km 3 m.y. 4 1 O0 km 4 of plate boundary and V• is < 25 mm a -•. Introduction Following the recognition ofbackarc basins in the 1970s [e.g., Karig, 1970; Packham and Falvey, 1971 ], early structural and tectonic studies of these extensional settings generally interpreted seafloor spreading as the dominant mechanism of basin opening [e.g., Lawyer et al., 1976; Weissel, 1981 ]. However, more recent studies ofbackarc basins, principally based around seafloor swathmapping and Ocean Drilling Program (ODP) drilling, have demonstrated the importance of crustal rifting in backarc basin tectonism and magmatism [e.g., Taylor et al.These latter studies, from various western Pacific backarc basin sites, are beginning to establish a general, although by no means consistent, consensus of the characteristic tectonic and magmatic processes within the rifting "phase" ofbackarc opening. To date, the most consistent observations are (1) the similarity of backarc rift graben architecture (with alternating half-graben asymmetry) to continental rift systems, (2) the better initial develop-ment of the rift system between the flanking arc volcanoes of the volcanic front, (3) the eruption of mid-ocean ridge basalt (MORB)like (backarc basin basalt; BABB) and arc-related magmas throughout the rifting phase, (4) the development of"pseudo linear magnetic anomalies" from the emplacement of rift magmas into less magnetic arc crust, and (5) the evolution of the rifting "phase" through various stages, including a final stage of ...
[1] A continuous, domed detachment surface (FUJI Dome) has been imaged on the very slow-spreading southwest Indian Ridge using deep-towed side-scan sonar, and has been investigated by manned submersible and sea-surface geophysics. The Dome is morphologically similar to other oceanic detachments, core complexes or mega-mullions. In addition to bathymetric mullions observed in shipborne bathymetry, finer scale spreading-parallel striations were imaged with the side scan. On the detachment surface, metabasalt crops out near the termination, probably as part of a thin fault sliver. Gabbro and troctolite probably crop out near the summit of the dome. The rest of the detachment surface is covered with sediment and rubble which is basaltic except for a single sample of serpentinite. Most of the detachment surface dips toward the ridge axis at 10°-20°, but near the breakaway it is strongly rotated outward, and dips away from the axis at up to 40°. Normal, undeformed volcanic seafloor crops out adjacent to the detachment. Modeling of sea surface magnetic data suggest the detachment was active from 1.95 Ma for about 1 Ma during a period of reduced and asymmetric magmatic accretion. Modeling of sea surface and seafloor gravity requires laterally fairly uniform but high density material under the Dome, and precludes steeply dipping contacts between bodies with large density contrasts at shallow levels under the Dome.
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