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Site survey data

Three cruises surveyed and sampled the Louisville Seamount Trail before and in preparation for Expedition 330. In 1984 Lonsdale (1988) made a transit along the entire trail while collecting 3.5 kHz and magnetic anomaly data as well as the first multibeam swath bathymetry along the chain. In that cruise 25 guyots and 12 other large volcanoes were mapped, and at least one single-channel seismic reflection profile was collected across their summits. Following that initial cruise, a limited set of dredge samples (blue circles in Fig. F1B) were used for total fusion 40Ar/39Ar age dating (squares and triangles in Fig. F4A) and geochemistry (triangles in Fig. F6A).

In November 2002, Cruise 167 of the F/S Sonne (SO167; Stoffers, 2003) surveyed the Louisville Seamount Trail between the Tonga Trench and the 169°W bend. Subaerial lava and volcaniclastics were dredged from 11 guyots at 39 different stations (gray circles in Fig. F1B). Inductively coupled plasma–mass spectroscopy results indicate that the dredged basalt is alkalic basalt, whereas preliminary 40Ar/39Ar age data indicate a sometimes complex age history for the oldest seamounts in the trail (circles in Fig. F4A) (O’Connor et al., submitted). These samples reveal little geochemical variation along the Louisville Seamount Trail, except for a single large guyot that overlies the Wishbone fracture zone in the Pacific plate (Beier et al., 2011).

During the 2006 AMAT02RR site survey cruise the SIMRAD EM-120 echo-sounding system was used to map 72 seamounts and guyots, many with full coverage and all with at least 80% multibeam coverage. Multichannel seismic reflection data were collected along the oldest one-third of the seamount trail (Fig. F1B) using two 45–105 in3 generator-injector air guns and an 800 m 48-channel streamer, resulting in 79 seismic lines with 69 crossing points on 22 seamounts. From these data we selected four primary and seven alternate drill sites on seven seamounts that (1) fall within the age constraints of the comparative Leg 197 experiment we proposed to carry out, (2) have a sufficient sedimentary cover of at least 10 m, based on 3.5 kHz subbottom profiling and side-scan reflection data, and (3) show consistent reflectors below these sediments representing basaltic basement. In addition, 29 sites were dredged on 21 seamounts and guyots (green circles in Fig. F1B). From these dredge hauls 42 groundmass and mineral separates from 17 seamounts were successfully age dated using the 40Ar/39Ar incremental heating technique (diamonds and plateau diagrams in Fig. F4) (Lindle et al., 2008; Koppers et al., 2011). Major and trace element concentrations were determined for 61 samples, and Sr-Nd-Pb isotope analyses were carried out for 49 samples (circles in Fig. F6A) (Vanderkluysen et al., 2007).

Magnetic surveys were also conducted of two seamounts and the small guyot at 168.6°W that was targeted for drilling (Site U1377 on Hadar Guyot). The magnetic anomaly pattern for Hadar Guyot unfortunately has very low amplitude (Fig. F7) and yields an unreasonable paleopole position, but the complexity of the anomaly pattern suggests that dual polarities might be present within this volcanic structure. In contrast, the 35.8°S seamount (located 1.1° north of prospectus Site LOUI-3B) has a well-defined (root mean square crossover error = 3 nT) and simple magnetic anomaly pattern with the normal polarity, presumably reflecting formation during Chron 26n at 57.5–57.9 Ma (Cande and Kent, 1995). Seminorm inversions (Parker et al., 1987) yield paleopole positions that are relatively stable over a range of misfits (Fig. F8) and generally compatible with the Pacific apparent polar wander path (Sager and Pringle, 1987). Finally, these inversions give a paleolatitude of ~49° ± 7°S, similar to the present-day 50.9°S latitude of the Louisville hotspot. Despite the ambiguity in the interpretation of seamount magnetic anomalies (Parker, 1991) and its relatively large 1σ uncertainty, this result may suggest that no (or little) discernible paleolatitude shift has occurred since this Louisville seamount formed around 58 Ma and over a time interval in which the contemporary Suiko Seamount in the Emperor Seamounts showed at least a 6° paleolatitude shift.