LEG 121

Broken Ridge and Ninetyeast Ridge

During Leg 121, four sites (Site 752 to Site 755) were drilled on Broken Ridge to test competing hypotheses about rifting tectonics and lithospheric extension. Operations documented that sedimentation occurred in increasing water depth before the rifting event, consistent with rift initiation by intraplate stress. The uppermost strata in the dipping and truncated sequence, pelagic carbonate ooze, are middle Eocene in age. The youngest sediments deposited on the truncation surface are upper Eocene oozes containing reworked assemblages of shallow-water foraminifers with coarse sands and gravels. The winnowed Oligocene to Pleistocene pelagic cap records the intensity of ocean currents flowing over the ridge. These results indicate > 1,000 m of uplift in response to a middle Eocene rifting event. The short duration of the rifting event (3-7.5 m.y.) and low present-day heat flow suggest a mechanical rather than a thermal mechanism for uplift. The overall response of the ridge to rifting is consistent with theoretical explanations for the uplift of rift flanks or shoulders as a flexural rebound effect following tectonic unloading of the lithosphere during extension. The large amounts of volcanic ash documented in the section are similar in composition to Kerguelen and Ninetyeast lavas, and represent distal fallout of hot spot volcanism a few hundred kilometers to the southwest. At the K/T boundary, Cretaceous chalks are succeeded by an ash-rich layer overlain by Paleocene chalks. This ash layer represents an interval of > 1 m.y. during which the rate of carbonate deposition was as little as 1/8 the prevailing rate immediately above and below, indicating abnormally low planktonic productivity following an ecological crisis at the end of the Cretaceous.

Drilling at three sites (Site 756 to Site 758) on the Ninetyeast Ridge investigated the evolution of the Kerguelen/Ninetyeast hot spot and problems related to rapid northward movement of the Indian Plate during the Late Cretaceous and Tertiary. The Ninetyeast Ridge lavas are transitional between normal mid-ocean ridge basalts and oceanic island basalts, specifically Kerguelen lavas. Apparently, the Kerguelen/Ninetyeast hot spot remained near the Indian-Antarctic spreading center during construction of the Ninetyeast Ridge, as is evidenced by the transitional composition of the basalts, the locally compensated gravity signature of the ridge, and the pattern of magnetic anomalies on either side of the ridge which requires asymmetrical spreading on at least one segment of the Indian- Antarctic ridge. The large volume of basaltic ash on Ninetyeast and Broken Ridge is an important feature of the Kerguelen/Ninetyeast hot spot relative to other hot spot volcanoes. Those ashes formed in shallow marine environments as individual volcanic centers emerged and then subsided again. Apparently, Ninetyeast Ridge was built by discrete, mostly subaerial, volcanic centers. Marginal to these centers, volcanic activity was limited to deep-water pillow and sheet flows.