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doi:10.2204/iodp.sp.330.2010

Site survey data

Data acquisition

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, collecting the first multibeam swaths and 3.5 kHz and magnetic profiles. During this cruise, 25 guyots and 12 other large volcanoes were mapped, and at least one single-channel seismic reflection profile was collected across their summits. A limited set of dredge samples (light blue circles in Fig. F2C) were used for total fusion ⁴⁰Ar/³⁹Ar age dating (Fig. F4A) and geochemistry (Fig. F6).

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 lavas and volcaniclastics were dredged from 11 guyots at 39 different stations (gray circles in Fig. F2C). Preliminary inductively coupled plasma–mass spectrometry results indicate that the dredged basalts are all alkali basalts, whereas preliminary ⁴⁰Ar/³⁹Ar age data indicate a sometimes complex age history for the oldest seamounts in the trail (orange circles in Fig. F4A).

In 2006, the SIMRAD EM-120 system was used during the AMAT02RR site survey cruise to map 72 seamounts and guyots, many with full coverage and all with at least 80% multibeam coverage. Multichannel seismic (MCS) data were collected along the oldest third of the seamount trail (Fig. F2B) using two 45–105 in³ generator-injector air guns and an 800 m 48 channel streamer that generated 79 seismic lines with 69 crossing points on 22 seamounts. From these MCS data we selected four primary and seven alternate sites for IODP drilling on seven seamounts that (1) fall within the age constraints for the comparative Leg 197 experiment we propose to carry out, (2) have a sufficient sediment cover of at least 10 m based on 3.5 kHz profiling and sidescan reflection data, and (3) show consistent reflectors below these sediments representing basaltic basement. Additionally, 58 groundmass and mineral separates from 47 samples of dredge hauls from 29 sites on 21 seamounts and guyots (green circles in Fig. F2C) dredged during the AMAT02RR cruise were age dated using the ⁴⁰Ar/³⁹Ar incremental heating technique (red diamonds and plateau diagrams in Fig. F4) (Lindle et al., 2008). Major and trace element analyses have been carried out on 61 samples, and Sr-Nd-Pb isotope analyses have been carried out on 49 samples (Fig. F6) (Vanderkluysen et al., 2007).

Detailed magnetic surveys of two seamounts and the small guyot at 168.6°W that is targeted for drilling (proposed Site LOUI-4B) were conducted in preparation for Expedition 330. The resulting magnetic anomaly for 168.6°W is very low amplitude (Fig. F9) and yields an unreasonable paleopole position, but the complexity of the anomaly pattern suggests that dual polarities may be present. If cored, these changing polarities could provide a more robust paleolatitude estimate. In contrast, the 35.8°S seamount (located 1.1° north of proposed Site LOUI-3B) has a well-defined (root mean square crossover error = 3 nT) and simple anomaly with a normal polarity, presumably reflecting formation during Chron 26n from 57.5 to 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. F10) and generally compatible with the Pacific apparent polar wander path (Sager and Pringle, 1987). These inversions give a paleolatitude of ~49° ± 7°S, similar to the present-day 50.9°S latitude of the Louisville hotspot but with a relatively large 1σ uncertainty estimate. Despite ambiguity in the interpretation of seamount anomalies (Parker, 1991), this result seems to suggest that only little discernible paleolatitude shift has occurred since the seamount formed at ~58 Ma, at which time the contemporary Suiko Seamount in the Emperor Seamount Trail shows at least a 6° paleolatitude shift.

Seismic interpretation

Interpretation of MCS data collected during the AMAT02RR site survey cruise is complicated because the Louisville seamounts have not been drilled before—neither by DSDP/ODP nor using piston coring. In fact, samples and data collected during Expedition 330 will be essential in groundtruthing the seismic interpretation and improving the final seismic images of this group of intraplate seamounts. Nevertheless, the available MCS data provide us with the first-order information needed to meet the objectives and goals of Expedition 330.

Seismic imaging and 3.5 kHz data show that at all primary sites the overall thickness of the pelagic sediment cap is <20 m, underlain by a <55 m thick sequence of volcaniclastics and followed by what largely appears as "opaque" volcanic basement without any significant reflectors. The intermediary sequence of volcaniclastics themselves appear to be intercalated with some lava flows and/or carbonates because they show strong reflectors dispersing outward from the centers of the targeted seamounts. In fact, on many of the larger guyots (not targeted for drilling) in the Louisville Seamount Trail, these layered sequences (most probably volcaniclastics) have substantial thicknesses of as much as several hundred meters that dip and thicken toward the margins of the guyots. Dredge samples from depths corresponding to the outcrop of this unit have recovered volcaniclastic sediments, including rounded cobbles from supposedly shallow beach deposits (SO167 cruise report; Stoffers, 2003). Based on the sparse information presented above and the observations of the Emperor seamounts made during Leg 197, we therefore interpret this unit to be an outward-thickening sequence of volcaniclastics. The bottom of this sequence provides us with the depth at which we expect to begin coring 350 m into lava flow–dominated basaltic seamount basement. Note that similar layered sequences drilled from the Nintoku Seamount during Leg 197 proved to be lavas with some (minor) intercalated sediments/paleosols (Kerr et al., 2006).

The thickening of the volcaniclastic sequence also has been imaged by the seismic refraction experiment carried out during the German SO195 cruise (Grevemeyer and Flüh, 2008). During this experiment, a single 370 km long refraction line was conducted orthogonally to the overall northwest trend of the Louisville Seamount Trail, crossing the summit of the 27.6°S guyot, which is located one seamount down and about 1.1° south of proposed Site LOUI-1C. Based on the outcome of this refraction experiment (using 35 ocean-bottom seismometer stations spaced every ~10 km), Contreras-Reyes et al. (2010) were able to image the internal structure of this seamount, the oceanic crust underneath it, and the flexed shape of the MOHO (Fig. F7). Although their data do not provide sufficient resolution for the uppermost 500 m of this seamount to image individual volcanic sequences or seismic reflectors, the data give us a good idea of the overall velocity structure of the 27.6°S guyot, including (1) a sequence of "basaltic extrusives" (i.e., lava flows of 4.0–6.0 km/s seismic velocity) extending to shallower regions and to a significantly <0.5 km basement depth in the center of this seamount and (2) a thickening sequence of "volcaniclastic infill" (i.e., 2.0–3.0 km/s seismic velocities), starting with a very thin layer at the seamount summit that substantially thickens outward, particularly on the seamount flanks and in its flexural moat. This outcome lends confidence to our interpretation of the AMAT02RR seismic reflection profiles and our placement of drill sites away from the shelf edges of the guyots and the center of these volcanic structures.

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