Ph. Blondel Southampton Oceanography Centre, Empress Dock, Southampton SO14 3ZH, UK
L. M. Parson Southampton Oceanography Centre, Empress Dock, Southampton SO14 3ZH, UK
B. J. Murton Southampton Oceanography Centre, Empress Dock, Southampton SO14 3ZH, UK
C. R. German Southampton Oceanography Centre, Empress Dock, Southampton SO14 3ZH, UK
It has long been recognised that heat flow measurements at spreading centres could only be explained by convective cooling of the crust with circulating seawater (e.g. Elder, 1967). Understanding the importance of hydrothermal venting is therefore of prime importance for the study of plate tectonics and physical and chemical fluxes in the deep ocean. Submarine hydrothermal activity has been better documented since the beginning of systematic exploration of the deep oceans. So far, 139 deep sea hydrothermal sites have been identified, 65 of which are active. As emphasized in a recent review (Lowell et al., 1995), some of the most fundamental questions that need to be addressed concern the spatial characteristics of hydrothermal activity and its relationship with geology. Data recently acquired along the Mid-Atlantic Ridge (MAR) as a part of the BRIDGE Programme (Murton et al., 1994; German et al., 1996) are therefore most relevant, as they enable us to investigate the distribution of hydrothermal fields (9 new sites have been discovered), their spacing along the ridge and the relationship with local surface processes: tectonism, volcanism and sedimentation. The different datasets used for this study are located both in the region close to the Azores hotspot (36°N-38°N) and in a region free from any hotspot influence (27°N-30°N). Because of the size of the datasets involved in the Geographic Information System (several Gigabytes for the sonar imagery alone), they can only be processed efficiently with numerical techniques. These techniques also allow quantitative studies of the inter-relationship between the different processes.
Acoustic classification of TOBI sidescan imagery
Much information about seafloor geology can be conveyed by side-scan sonar imagery. TOBI is the side-scan sonar developed at SOC and provides high-resolution images of the seafloor with a resolution of 6 metres after processing. This is closely related to the scale of the local geologic structures. The acoustic energy backscattered from the seafloor to the sonar is directly linked to the intrinsic properties of the target, such as reflectivity, porosity, micro-scale roughness and geometry. Tonal properties of the images are expressed in terms of grey levels, and quantitatively described by mean values, contrasts, etc. (1st-order statistics). But the geological interpretation is mainly based on the local textures. These can be intuitively qualified as rough or smooth, local or regional, repetitive or random. Quantitative measurements, however, are best extracted from the image with stochastic methods such as Grey-Level Co-occurrence Matrices (GLCM). GLCMs address the average spatial relationships between pixels of small region (Haralick et al., 1973). They are described by statistical measures called indices. Their usefulness to the specific case of sonar images in mid-oceanic ridge environments has been assessed in conjunction with detailed ground-truthing using ROVs and submersibles (Blondel et al., 1993; Blondel, in press) and only two have been retained: entropy and local homogeneity. Entropy measures the lack of spatial organisation inside the region where the GLCM is computed. Entropy is high with rougher textures and low when the texture is more homogeneous. All geological features that might be encountered in sonar images are characterised by specific entropy signatures, varying with their degree of roughness. Conversely, local homogeneity quantifies the amount of similarities inside the computation window. It is more sensitive to organised and low contrast textures. In the 2-D measurement space formed by the entropy and local homogeneity signatures, three first-order geologic types are clearly distinguished. Sedimented areas correspond to lower entropies and higher local homogeneities. This fits with the nature of sediment cover, smooth and with quite homogeneous textures. Tectonised areas present a larger interval of local homogeneities, ranging from low to mid-range, and higher entropies. The differences in homogeneities are related to the spatial organisation of local textures, tectonised areas with preferential directions having higher homogeneities than heavily tectonised zones where no pattern is discernible. The higher entropies are due to the presence of faults and fissures. Volcanic processes are characterised by lower local homogeneities, and the higher entropies. This is related to the nature of volcanic features, rougher and with varying degrees of organisation. These classifications, cross-checked with ground-truthing (Blondel et al., 1993; Blondel, in press), have a high degree of reliability.
The MAR south of the Azores hotspot (36°N-38°N)
The Azores region is roughly constrained between latitudes 36°N and 40°N, at the site of a Triple Junction between the North American, Eurasian and African plates. The Azores Islands themselves are situated on a broad plateau elevated as a result of the thermal effect of a mantle "hot spot". The ridge is partitioned into short spreading segments, offset from one another by structural discontinuities. To address questions on the exact location and tectonic styles of the Azores spreading centre and Triple Junction, a multi-sensor survey was conducted in 1994 as part of the European Community MARFLUX project. The MAR was surveyed continuously along 200 km of its length, ensonifying 3,000 km2 of seafloor. TOBI side-scan sonar imagery was gathered concurrently with multibeam bathymetry and deep-tow measurements of physical and chemical anomalies in the water column. In addition to the hydrothermal sites already confirmed in this section of the MAR (Menez Gwen at 38°N and Lucky Strike at 37°17'N), our strategy led to the discovery of more than 8 new sites (German et al., 1996). Textural analysis was performed on the whole TOBI imagery, showing excellent agreement with manual interpretations. Areas affected by the different processes were computed every minute of decreasing latitudes. Entropy and local homogeneities were not computed along the edges of the swaths and points with ambiguous signatures (i.e. TOBI track, mixed lithologies) were not taken into account. Sediment cover is marked by very distinct maxima, mainly restricted to the Non-Transform Discontinuities (NTDs). Volcanic processes are principally confined within segments. Neo-volcanic activity declines with distance from the Azores, which is consistent with previous observations (eg Parson et al., 1994; Detrick et al., 1995; German et al., 1996). In contrast, tectonic activity increases with distance away from the hotspot but is not restricted to spreading segments. The northern segments are more affected by volcanism, and the southern segments by tectonics. This is attributed to the declining influence of the Azores hot-spot with distance (German et al., 1996; Blondel, in press). Cross-correlation of the three different processes with each other, with bathymetry, and with the distribution of known hydrothermal sites was further investigated. As expected, sedimentation is inversely correlated in most areas with the other geological processes that we are assessing. Bathymetric highs are correlated to volcanism, which is consistent with the construction of the ridge by magmatic processes. And tectonic and volcanic processes are inversely correlated in most of the segments. No consistent relationship can be observed between the three quantified geological processes and the distance to the closest hydrothermal site. Using our analyses, we speculate that it is significant that at a distance from the Azores hotspot, the hydrothermal sites are associated with an increase in tectonism rather than volcanism.
The MAR between Kane and Atlantis FZ (27°N-30°N)
Except for the Atlantis Transform, there are no transform offsets along this 300-km long portion of the plate boundary. Instead, the MAR is offset at intervals of 10-100 km by NTDs, usually located at depth maxima along the rift valley. The inner floor of the rift valley is shallowest near the middle of each segment and deepens toward the discontinuities. The only documented hydrothermal site is Broken Spur, discovered in 1993 (Murton et al., 1994; Murton et al., 1995). Textural analysis has been applied to the TOBI imagery acquired along the axial valley only. Preliminary analyses show variations in tectonism and volcanism from one segment to another, attributed to differences in the magmatic upwelling patterns. Similar variations can be observed inside the segments, consistent with observations from Sempéré et al. (1993) that upwelling is enhanced in the middle of segments. Tectonism increases with proximity to the NTDs. Relative variations of volcanism, sedimentation and tectonism in the NTDs suggest that the NTDs develop through evolutionary processes. The Broken Spur Vent Field (29°10'N) occupies the central part of a spreading segment. It lies at the crest of an axial volcanic ridge, within an axial graben and at the intersection of cross-cutting faults. From first analyses, hydrothermalism seems correlated with tectonism, but not as much with volcanism.
Quantitative seafloor classification through textural analyses shows that hydrothermal activity is spatially correlated with the local tectonics and not necessarily with neo-volcanism. This complements the current models, according to which the increase in activity associated with plate reorganisation may be related to tectonism which creates permeability by rigid plate deformation (Lowell et al., 1995), and the observations made in situ (e.g. Murton et al., 1994) and on sonar mosaics (e.g. Parson et al., 1994) that venting focuses in areas of important cross-cutting tectonics.
Blondel, Ph., Geol. Soc. Spec. Pub., (in press).
Blondel, Ph., Sempéré, J.C. & Robigou, V., IEEE-OES Oceans' 93, 209-213 (1993).
Detrick, R.S., Needham, H.D. & Renard, V., J. Geophys. Res. 100, B3, 3767-3787 (1995).
Elder, J.W., Geophys. Mono. Series, AGU 8, 211-239 (1967).
German, C.R., Parson, L.M. & HEAT Scientific Team, Earth Planet. Sci. Lett. 138, 93-104 (1996).
Haralick, R.M., Shanmugam, K. & Dinstein, R., IEEE Trans., SMC-3, 610-621 (1973).
Lowell, R.P., Rona, P.A. & von Herzen, R.P., J. Geophys. Res. 100, 327-352 (1995).
Murton, B.J, & CD-76 Scientific Party, Earth Planet. Sci. Lett. 125, 119-128 (1994).
Murton, B.J., Van Dover, C.L. & Southward, E.C., Geol. Soc. Spec. Pub. 87, 33-43 (1995).
Parson, L.M., Coller, D., German, C.R., Needham, H.D. & HEAT Party, Eos Trans., AGU 75, 658 (1994).
Sempéré, J.C., Lin, J., Brown, H., Schouten, H. & Purdy, G.M. Mar. Geophys. Res. 15, 153-200 (1993).
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