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doi:10.2204/iodp.proc.304305.101.2006

Scientific objectives

Atlantis Massif has several key features that make it an ideal target for drilling an OCC:

• Its young age (<2 m.y. old) indicates that seafloor weathering and erosion have not degraded (macro-)structural relationships;
• The hanging wall is interpreted to be in contact with the footwall of the detachment; and
• Mantle seismic velocities have been interpreted to be present at several hundred meters depth below seafloor of the domal core (Fig. F3), potentially affording access to fresh in situ peridotite with conventional drilling.

The scientific objectives address fundamental questions related to (1) the nature and evolution (alteration) of the oceanic lithosphere accreted at slow-spreading ridges and (2) the formation of OCCs.

The hypotheses tested by drilling during Expeditions 304 and 305 include the following:

• A major detachment fault system controlled the evolution of Atlantis Massif;
• late flexure (rolling hinge model) is the dominant mechanism of footwall uplift;
• The nature of melting and/or magma supply contributes to episodes of long-lived lithospheric faulting;
• Expansion associated with serpentinization contributes significantly to uplift of the massif core;
• The Mohorovicic discontinuity (Moho) at Atlantis Massif is a hydration front; and
• Positive gravity anomalies at Atlantis Massif indicate relatively fresh peridotite.

If long-lived normal faulting and displacement are responsible for the evolution of the massif, uplift of the core may be the result of isostatic adjustment (Vening Meinesz, 1950) and thin-plate flexure (Spencer, 1985; Wernicke and Axen, 1988; Buck, 1988; Lavier et al., 1999; Buck et al., 2005). Differential rotation between the footwall and hanging wall blocks is predicted by thin-plate theory, so we can apply results from Expeditions 304 and 305 to investigate whether either the core and/or logging data show evidence of such history. Logging data provide continuous (oriented) images of fracture patterns in the borehole wall. These can be compared with fractures and veins measured in the cores from the same depth interval. Paleomagnetic inclination data are incorporated to determine any history of rotation of the footwall to depths >1 km. The pressure-temperature evolution of alteration reflects the tectonic and magmatic history as well, with cooling rates and water/rock ratios being controlled by intrusions, the amount of unroofing, and the degree of fracturing.

All detachment models predict that hanging wall rocks initially reside structurally above the footwall. If this is the case, petrologic and geochemical results may show a genetic relationship between footwall rocks and basalts of the hanging wall, dependent on the longevity and/or magnitude of fault slip.

Finally, the processes responsible for the development of OCCs appear to be episodic, with one factor being the level or style of magmatic activity at the local spreading center. Detailed study of the igneous sequence and structural relationships therein will be used to address the evolution of melting, intrusion, and cooling during the formation of Atlantis Massif. Comparison of our findings with those from ODP Legs 118/176, 153, and 209 will provide a means for assessing the similarities and differences in conditions that prevailed at the slow-spreading centers where the lithosphere drilled at these sites was initially formed.

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