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APPENDIX A: DATA PROCESSING METHODS
A1. Hull-mounted backscatter and bathymetry data
The KM0804 and KM0410 multibeam data were collected at slow ship speeds of ~1.5-2
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knots using closely spaced track lines designed for the IMI-120, so the much wider swaths of
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hull-mounted data overlap extensively, providing dense near-axis coverage [Martinez and
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Taylor, 2006; Martinez et al., 2008]. Swath backscatter data were first processed using MB-
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System software (mbbackangle, mbprocess) [Caress and Chaves, 2008] to minimize grazing
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angle and nadir artifacts. To take advantage of the high data density, individual backscatter and
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bathymetry pixels from the multibeam surveys were extracted using MB-System software. The
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ensemble bathymetry and backscatter datasets were each median filtered and gridded using GMT
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[Wessel and Smith, 1998] software. Although the inherent horizontal resolution of the 12 kHz
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multibeam data is ~100 m we take advantage of the dense overlapping coverage to decrease
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noise in the gridded backscatter and bathymetry data. For the backscatter data, this redundancy
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made it possible to filter out the majority of the remaining nadir and swath edge distortion,
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creating a smoother image that is much easier to interpret. Multibeam grids as fine as 10 m were
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produced from the dense data set to facilitate comparison at a similar grid spacing to the deep-
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towed data.
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A1.1. Deep-towed backscatter and bathymetry data
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The IMI-120 backscatter and bathymetry data were gridded at a cell size of ~2 m,
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providing highly detailed imagery of the axis and near-axis regions. Deep-towed data require
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navigational correction because the instrument’s position cannot be directly measured and is
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instead estimated using “layback” calculations based on the ship position from the GPS/inertial
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navigation system and the digitally-recorded payout of the tow cable. The layback method
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assumes that the tow vehicle follows the ship track, but the vehicle tends to cut inside the ship
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track during course changes, and there are variations due to deep-ocean currents and the
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hydrodynamics of the vehicle itself, causing positional errors of up to 100’s of m. Software
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packages developed by the Hawaiʻi Mapping Research Group (HMRG) at the University of
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Hawaiʻi at Mānoa [R. Davis, personal communication, 2009] were utilized to graphically correct
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the position by visually matching prominent features between shipboard multibeam data and
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deep-towed data. Due to differing data resolutions and the limitations of the program, it was
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possible to significantly improve the IMI-120 navigation, but inaccuracies and imperfect
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agreement on the order of ~100 m or less remain.
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The quality of the data approaching swath edges is variable due to smaller ensonification
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angle, which causes weaker returns, and local topography, which can cause acoustic shadowing.
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Automatic methods for blending overlapping edges or cutting them off at a fixed distance from
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the nadir cannot determine which of the overlapping swaths contains the “best” data. Therefore,
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following re-navigation, the backscatter data were hand edited to reveal the best imagery within
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overlapping swaths. This was done using a manual method employing image processing
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software (Adobe Photoshop CS3®) to selectively trim parts of overlapping swaths where the
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underlying swath showed more information.
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A2. Other data sets
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While backscatter and bathymetry were the primary data sets used for mapping and
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interpretation, other geophysical, geochemical, and oceanographic data from the region were
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utilized to gain more insight into the relationships between various processes.
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Miniature Autonomous Plume Recorders (MAPR’s) are small self-contained
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hydrothermal plume sensors that measure and internally record temperature, pressure, optical
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backscatter, and oxidation-reduction potential (∆Eh) in the water column [Baker and Milburn,
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1997; Baker et al., 2010]. They were attached to the sonar tow cable on the KM0410 and
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KM0804 deep sonar tows and used to map hydrothermal activity over most of the length of the
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ELSC and VFR [Baker et al., 2006, 2010].
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Seismic tomographic results from the L-SCAN (MGL0903) experiment [Dunn et al.,
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2009; Dunn and Martinez, 2011] and a previous MCS study along the ELSC [Harding et al.,
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2000; Jacobs et al., 2007] were utilized to correlate observations with changes in crustal
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structure and density and the extent and continuity of the axial magma chamber reflector.
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Publically available (www.earthchemportal.org) geochemical data from samples collected along
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the majority of the ELSC/VFR axis were examined mostly to help understand how subduction
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input varies along the axis and correlate this with changes in ridge morphology, structure, and
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volcanism [Jenner et al., 1987; Vallier et al., 1991; Loock, 1992; Pearce et al., 1994; Peate et
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al., 2001; Fretzdorff et al., 2006; Hergt and Woodhead, 2007; Escrig et al., 2009].
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APPENDIX B: GEOLOGIC MAPS
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Figure A1: Geologic map of ELSC1. Basemaps: a) 12 kHz bathymetry (.0001° cell size); b) 12
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kHz backscatter (.0001° cell size); c) IMI-120 backscatter (.00002° cell size). Geologic mapping
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was limited to portions of the segments covered by the KM0804 survey, where deep-towed IMI-
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120 backscatter data was available as a reference. Symbology: identified faults marked in red
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with ticks pointing down-dip, lava flows outlined in solid purple, off-axis volcanic cones
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outlined in dashed magenta, and mass-wasting features are mapped in orange. Bathymetric
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contours are marked in black at an interval of 0.01 km, and spreading axes are marked on all
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plots with thin solid red lines.
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Figure A2: Geologic map of ELSC2. Basemaps: a) 12 kHz bathymetry (.0001° cell size); b) 12
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kHz backscatter (.0001° cell size); c) IMI-120 backscatter (.00002° cell size). Geologic mapping
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was limited to portions of the segments covered by the KM0804 survey, where deep-towed IMI-
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120 backscatter data was available as a reference. See Fig. A1 for map symbols.
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Figure A3: Geologic map of ELSC3. Basemaps: a) 12 kHz bathymetry (.0001° cell size); b) 12
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kHz backscatter (.0001° cell size); c) IMI-120 backscatter (.00002° cell size). Geologic mapping
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was limited to portions of the segments covered by the KM0804 survey, where deep-towed IMI-
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120 backscatter data was available as a reference. See Fig. A1 for map symbols.
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Figure A4: Geologic map of ELSC4. Basemaps: a) 12 kHz bathymetry (.0001° cell size); b) 12
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kHz backscatter (.0001° cell size); c) IMI-120 backscatter (.00002° cell size). Geologic mapping
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was limited to portions of the segments covered by the KM0804 survey, where deep-towed IMI-
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120 backscatter data was available as a reference. See Fig. A1 for map symbols.
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Figure A5: Geologic map of VFR1. Basemaps: a) 12 kHz bathymetry (.0001° cell size); b) 12
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kHz backscatter (.0001° cell size); c) IMI-120 backscatter (.00002° cell size). Geologic mapping
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was limited to portions of the segments covered by the KM0804 survey, where deep-towed IMI-
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120 backscatter data was available as a reference. See Fig. A1 for map symbols.
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Figure A6: Geologic map of VFR2. Basemaps: a) 12 kHz bathymetry (.0001° cell size); b) 12
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kHz backscatter (.0001° cell size); c) IMI-120 backscatter (.00002° cell size). Geologic mapping
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was limited to portions of the segments covered by the KM0804 survey, where deep-towed IMI-
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120 backscatter data was available as a reference. See Fig. A1 for map symbols.