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doi:10.2204/iodp.proc.301.102.2005

Summary and conclusions

Several decades of mapping, seismic, and heat flow expeditions have made the eastern flank of the JFR one of the best-surveyed hydrothermally active regions in the world. This ridge flank provides numerous "type examples" of coupled fluid-heat processes within the oceanic crust and provides a useful reference to which other ridge flanks can be compared. Geophysical surveys have helped to identify numerous characteristic features and have facilitated planning for drilling operations to explore the nature, dynamics, and influence of fluid flow through upper oceanic crust. However, despite considerable effort, there remain important open questions regarding sediment deposition and modification processes; the scales, patterns, and rates of fluid circulation in basement; and magnitudes of lithospheric and advective heat loss in this region, all of which could be addressed with a modest amount of additional sediment sampling and geophysical data acquisition.

Swath mapping of this ridge flank illustrates how thick accumulations of turbidites and hemipelagic sediment blanket the seafloor across long distances, reducing the locations where basement is exposed directly to the ocean. The relative scarcity of basement outcrops on sites older than ~1.5 Ma has benefited several studies of ridge-flank circulation because it makes it possible to resolve first-order flow paths and flow rates between sites of fluid recharge and discharge. However, the specific roles of individual outcrops in extracting lithospheric heat in this area remain to be quantified, as do the crustal properties required to allow these outcrops to serve as efficient crustal ventilators. Several outcrops are good candidates for future work to assess the significance of these features to local and regional hydrogeologic conditions, including newly mapped features.

Seismic coverage across this region has been essential for determining sediment thickness and basement geometry, understanding relations between seafloor outcrops and underlying basement structure, and planning and interpreting thermal surveys. Seismic data have also allowed investigation of numerous sedimentary processes, particularly the influence of basement relief on sediment deposition and subsequent modification. High-resolution seismic data have helped to map out abrupt contrasts in sediment properties, many of which are associated with basement highs and fluid seepage at the seafloor, and provided information about relations with faulting (in sediments and basement), folding, and associated porosity anomalies.

Key issues to be addressed in future studies include the distribution of individual sedimentary units, many of which can be tracked across lateral distances of kilometers, and the influence of basement tectonics on reflector structure, sediment physical properties, and thermally driven fluid upflow. It will be particularly helpful to resolve first-order relations between the occurrence of acoustic washouts, basement relief, local and regional variability in sedimentary deposition, and deformation and fluid seepage. These features are known to occur in many ridge-flank settings, and their relations and significance remain to be delineated. In the First Ridge and Second Ridge areas, where ODP and IODP sites are located, further integration of seismic and borehole data will improve interpretations of stratigraphy and the spatial distribution and seismic signature of key reflectors.

Heat flow data collected in this region help to define several thermal and hydrologic regimes. The homogenization of upper basement temperatures in these areas, as a result of vigorous circulation in basement, can result in extreme variations in conductive heat flow even where the seafloor is relatively flat. For this reason, it is essential that heat flow and seismic data be colocated if thermal measurements are to be interpreted with confidence. Where sediment is absent or relatively patchy close to the active spreading center, heat flow is strongly suppressed. Open exchange of hydrothermal fluids in these areas serves to advect much of the lithospheric heat from the crust. In addition, local convection in basement may result in redistribution of heat flow at the seafloor; this process is most apparent in areas of buried basement relief.

Farther to the east, where sediment cover is nearly complete, seafloor heat flow patterns are dominated by a combination of lithospheric cooling and local advective redistribution. Heat flow in areas that are within a few kilometers of basement outcrops may also be influenced by hydrothermal discharge and recharge, resulting in seafloor heat flow being elevated or depressed, respectively. Unfortunately, it is not possible to determine with confidence the extent of a regional heat flow anomaly in this area because it is not clear what conductive reference should be used. An analysis of data that are thought to be far enough from areas of basement exposure and significant buried relief suggests that "background" heat flow in this area could be lower than predicted by standard lithospheric cooling models. An alternative explanation is that this apparent anomaly results from biases in sampling or data filtering. Resolution of this conundrum may result from a few carefully positioned heat flow profiles along existing seismic lines across relatively flat basement, tens of kilometers from the nearest outcrops. Additional modeling will also help us to understand what observed physical conditions mean for the hydrogeologic and thermal properties of oceanic crust, as well as the nature of hydrothermal processes within extensive basement aquifers.

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