Evidence for Off-Axis Volcanic Relicts of the Bouvet Hotspot and its Interaction With the Southwest Indian Ridge

We conducted a detailed geophysical mapping and rock dredging program of the Southwest Indian Ridge (SWIR) between 9 deg E and 16 deg E in December 2000-January 2001. The study area encompasses features that are interpreted to be associated with the off-axis trace of the Bouvet hotspot when it interacted with the SWIR during the last 20 Ma. The mantle Bouguer anomaly map, which reflects the gravity field arising from sub-seafloor density anomalies in the crust or mantle, reveals negative anomalies over three distinct areas, each of apparently different volcanic origin. The most negative mantle Bouguer anomaly (- 50 mGal) is centered over a cone-shaped topographic high on the Shaka Ridge at 10.5 deg E and 52.25 deg S (the Woods Hole Seamount). This result suggests that the Shaka Ridge, which lies on the northwestern flank of the Shaka Fracture Zone, is associated with anomalously thick crust possibly produced by hotspot volcanism that might be associated with the early Bouvet hotspot trace. This interpretation is also consistent with the petrography of the rock samples dredged from relict volcanic cones along the Shaka Ridge. The second mantle Bouguer gravity low (- 20 mGal) lies over a topographic high at 11.3 deg E and 52.8 deg S (the Joseph Mayes Seamount). The northern and southern flanks of this topographic high have positive crustal magnetization, suggesting that this is a region of recent volcanism on the SWIR ridge axis. The third significant mantle Bouguer low (- 20-30 mGal) is associated with a well-developed spreading segment centered at 14.7 deg E and 52.15 deg S (a Narrowgate-like segment). This ridge segment and its off-axis relicts have the shallowest topography and most coherent fine-scale abyssal hill morphology along the section of ridge between 9.5 deg E and 16 deg E, as well as high crustal magnetization, suggesting it is the most robust spreading segment in our survey region. In contrast, a 150-km long stretch of the SWIR ridge axis between 11.75 deg E and 13.75 deg E is associated with slightly positive mantle Bouguer anomalies (5-10 mGal) and poorly developed magnetization reversal patterns, indicating this is a region of anomalously thin crust. The new results from this cruise together with our earlier study of off-axis geophysical anomalies suggest the possibilities that the flux of the Bouvet hotspot varies in time or/and that the hotspot is a recent melting anomaly enhanced by complex mantle upwelling patterns at the Bouvet triple junction.