Site surveys related to IODP Expedition 301: ImageFlux (SO149) and RetroFlux (TN116) expeditions and earlier studies

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Setting, planning, and goals. . . . . . . . . . . . . . . 1 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Results and discussion. . . . . . . . . . . . . . . . . . . . 7 Summary and conclusions. . . . . . . . . . . . . . . . 15 Acknowledgments. . . . . . . . . . . . . . . . . . . . . . 16 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 1Zühlsdorff, L., Hutnak, M., Fisher, A.T., Spiess, V., Davis, E.E., Nedimovic, M., Carbotte, S., Villinger, H., and Becker, K., 2005. Site surveys related to IODP Expedition 301: ImageFlux (SO149) and RetroFlux (TN116) expeditions and earlier studies. In Fisher, A.T., Urabe, T., Klaus, A., and the Expedition 301 Scientists, Proc. IODP, 301: College Station TX (Integrated Ocean Drilling Program Management International, Inc.). doi:10.2204/ iodp.proc.301.102.2005 2Department of Geosciences, University of Bremen, Bremen, Germany. 3Expedition 301 Scientists’ Addresses. 4Institute for Geophysics and Planetary Physics, University of California, Santa Cruz CA, USA. 5Pacific Geoscience Center, Geological Survey of Canada, Sydney BC, Canada. 6Lamont-Doherty Earth Observatory, Columbia University, Palisades NY, USA. Abstract Integrated Ocean Drilling Program Expedition 301 was preceded during 2000 and 2002 by three surveys that helped to delineate seafloor and basement relief, sediment thickness, and the nature of ridge-flank hydrothermal conditions and processes on the eastern flank of the Juan de Fuca Ridge. These surveys generated swath map, seismic, and thermal data used to select locations for primary and secondary drilling targets, building from several decades of earlier work. We show compilations and examples of data from several characteristic settings in and around the Expedition 301 work area and use these observations to evaluate sedimentation patterns and thermal conditions in basement. There remain important unanswered questions in this area concerning fluid circulation within the upper oceanic crust, the magnitude of lithospheric heat input, the quantitative significance of advective heat loss from the crust, and relations between basement relief, sedimentation, and sediment alteration. These questions may be resolved through collection of a modest amount of additional data focusing on a few critical locations. Setting, planning, and goals The Endeavour segment of the Juan de Fuca Ridge (JFR) generates lithosphere a few hundred kilometers west of the Olympic Peninsula, Washington State (USA) (Figs. F1, F2). The study area contains structural features common to most ridge flanks: extrusive igneous basement overlain by sediments, abyssal hill topography, high-angle faulting, and basement outcrops. The topographic relief of the young oceanic crust produces barriers to turbidites originating from the continental margin to the east, resulting in the accumulation of thick sediments that bury the eastern flank of the JFR (Davis et al., 1992; Underwood et al., 2005). Igneous basement is exposed to the west, where the crust is young, and the sedimented seafloor to the east is relatively flat, except over basement outcrops. Low-permeability sediment limits advective heat loss from most of the ridge flank, leading to strong thermal, chemical, and alteration gradients in igneous basement. Basement relief is dominated by linear ridges and troughs, oriented subparallel to the spreading center, and was produced mainly by faulting, variations in magmatic supply at the ridge, and off-axis volcanism (Davis and Currie, 1993; Karsten et al., 1998, 1986). Basement relief across this ridge flank tends to be smoother near doi:10.2204/iodp.proc.301.102.2005 Proc. IODP | Volume 301 L. Zühlsdorff et al. Expedition 301 site surveys In addition, Maurice Ewing expedition EW0207, led by S. Carbotte (Lamont-Doherty Earth Observatory of Columbia University), R. Detrick (Woods Hole Oceanographic Institution), and G. Kent (Scripps Institution of Oceanography), collected multichannel seismic (MCS) and swath-bathymetry data. This 2002 program included detailed surveys along the axis of the JFR and a suite of ridge-flank profiles (~1500 km total length), of which the northernmost crossed the Expedition 301 work area. We had originally hoped to complete the ImageFlux survey in advance of the RetroFlux survey, to allow time for processing and interpretation of swath-map and seismic data prior to collection of cores and heat flow data. The two surveys ended up being run at the same time, but we were able to coordinate activities at sea by email, radio, and a hard-copy data swap so as to avoid having both ships working too closely together and to take advantage of new discoveries as the surveys progressed. Methods Swath mapping Prior to Expedition 301 site surveys, vessel-based swath coverage of this area was limited mainly to isolated tracks across the ridge flank (by ships heading to/from the active spreading center to the west) and to small areas containing particular features such as basement outcrops. The Ridge Multibeam Synthesis Project (RMBS) compiled data from numerous surveys, but Generic Mapping Tools grid files included only the near-ridge area (ocean-ridge.ldeo.columbia.edu/general/html/home.html). Additional lines of swath data in the Expedition 301 field area were acquired during the 1996 Sonne survey (Rosenberger et al., 2000), but these data had never been merged with the RMBS synthesis. Bathymetric data collected by the Sonne in 1996 and 2000 were acquired at typical survey speeds of 5.5– 7.0 kt while shooting seismic profiles and 10–11 kt during transits between work areas. The hullmounted Hydrosweep DS2 system produces usable data up to two times water depth with a horizontal resolution on the order of 90 m (at 2700 m depth and depending on ship’s speed and other factors). To suppress refraction effects on the outer beams without knowing the local sound velocity profile, the Hydrosweep system uses a calibration mode to compare depth values of the central and outer beams in order to calculate a mean sound velocity. Using this configuration, residual depth errors are minimized to values <0.5% of the water depth, on the order of 5– 15 m for typical water depths in this area (Grant and Schreiber, 1990). the active spreading center (±100–200 m), and rougher (±300–700 m) to the east. Integrated Ocean Drilling Program (IODP) Expedition 301 is the first part of a two-expedition drilling program, with associated nondrilling experiments, intended to assess the nature of fluid pathways in the crust and the dynamic influences of fluid circulation on this hydrothermally active ridge flank. Expedition 301 and related experiments will help to identify the distribution of hydrologic properties in the crust; the extent to which crustal compartments are connected or isolated (laterally and with depth); linkages between ridge-flank circulation, alteration, and geomicrobial processes; and quantitative relations between seismic and hydrologic properties. These and other scientific questions, and the methods used to address them, are discussed elsewhere (Fisher et al., “Site U1301,” and “Expedition 301 summary” chapters). The main goals of this paper are to • Show locations where swath mapping, seismic, and heat flow data were collected during recent oceanographic expeditions in preparation for Expedition 301 and related experiments; • Present and describe examples of these data that are characteristic of local and regional hydrothermal environments; • Analyze selected sets of collocated seismic and thermal data to resolve hydrothermal conditions in shallow basement; and • Define important questions that remain to be resolved in this area. Rosenberger et al. (2000) and Davis et al. (1997a) summarized survey results through 1996, and several other papers have discussed subsets of available data, including drilling results from Ocean Drilling Program (ODP) Leg 168 (e.g., Davis et al., 1992, 1999; Underwood et al., 2005; Wheat et al., 2000; Wheat and Mottl, 1994). Readers interested in a detailed presentation of regional geology and hydrogeology are directed to these studies and to other references cited throughout this paper. There were two oceanographic expeditions in 2000 intended to provide site survey data for Expedition 301. Sonne expedition SO149 (ImageFlux) was led by V. Spiess, L. Zühlsdorff, and H. Villinger (University of Bremen, Germany) and collected swath-map, seismic, and heat flow data. Thomas G. Thompson expedition TN116 (RetroFlux) was led by A. Fisher (University of California at Santa Cruz), E.E. Davis (Pacific Geoscience Center), C.G. Wheat (University of Alaska Fairbanks), and M. Mottl (University of Hawaii) and collected mainly heat flow data and sediment cores, with a small amount of swath mapping.