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Seismic interpretation

Interpretation of the multichannel seismic data collected during the AMAT02RR site survey is complicated because the Louisville Seamounts have never been sampled by piston coring or drilling, and the seismic survey itself provided only limited velocity information. In fact, samples and data collected during Expedition 330 are essential in ground truthing the seismic interpretation and improving the final seismic images of this group of intraplate seamounts. Despite some ambiguity in interpretation, the available data provided us with the first-order information needed to successfully meet the objectives and goals of Expedition 330.

Seismic imaging and 3.5 kHz data show that the overall thickness of the pelagic sediment cap is <20 m at all primary sites, underlain by a <55 m thick sequence of volcaniclastics and followed by what largely appears to be “opaque” volcanic basement with no significant reflectors. The intermediary volcaniclastic sequences show strong reflectors dipping outward from the centers of the targeted seamounts and are interpreted to represent intercalated lava flows and sediments. Many of the larger guyots (not targeted for drilling) in the Louisville Seamount Trail show a substantial thickness (up to several hundred meters) in these sequences that similarly dip and thicken toward their margins. Dredge samples from depths corresponding to outcrops of this unit recovered various volcaniclastic sediments, including rounded cobbles from supposedly shallow beach deposits (SO167 cruise report). On the basis of Leg 197 observations of the Emperor Seamounts, the bases of these dipping volcaniclastic sequences were taken as the contacts with lava flow–dominated basaltic basement (Kerr et al., 2006).

The thickening of the volcaniclastic sequence also has been imaged by a 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 carried out orthogonal to the overall northwest trend of the Louisville Seamount Trail and crossing the summit of the 27.6°S guyot, which is located ~1.1° south of Site U1372. On the basis of the outcome of this refraction experiment (with 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. F5). Even though their data do not provide sufficient resolution for the uppermost 500 m of this seamount, the data give us a good idea of the velocity structure of the 27.6°S guyot, with (1) a sequence of “basaltic extrusives” (i.e., lava flows and 4.0–6.0 km/s seismic velocities) extending to shallower regions and reaching <0.5 km basement depth in the seamount center and (2) a sequence of “volcaniclastic infill” (i.e., 2.0–3.0 km/s seismic velocities) starting with a very thin layer on top of the seamount and substantially thickening outward, particularly on the seamount flanks and in its flexural moat. This outcome provided confidence in our interpretation of the AMAT02RR seismic reflection profiles and our placement of drill sites away from the shelf edges of the guyots and toward the center of the smaller volcanic structures.