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doi:10.2204/iodp.pr.305.2005
Atlantis Massif has several key features that make it an ideal target for OCC drilling: it is <2 m.y. old, so 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 reported to be present at several hundred meters depth below seafloor of the domal core, potentially affording access to fresh in situ peridotite with conventional drilling.
The scientific objectives, as outlined in the drilling proposal and the Expeditions 304 and 305 Scientific Prospectus (Blackman, John, Ildefonse, MacLeod, Ohara, Miller, and the Expedition 304/305 Project Team, 2004), address fundamental questions related to (1) the formation of OCCs and (2) the nature and evolution (alteration) of the oceanic lithosphere accreted at slow-spreading ridges.
The hypotheses to be tested by drilling during Expeditions 304 and 305 were:
1. A major detachment fault system controlled the evolution of Atlantis Massif.
2. Significant unroofing occurred during formation of this OCC.
3. Plate flexure (rolling hinge model) is the dominant mechanism of footwall uplift.
4. The nature of melting and/or magma supply contributes to episodes of long-lived lithospheric faulting.
5. Expansion associated with serpentinization contributes significantly to uplift of core.
6. The Mohorovicic discontinuity (Moho) at Atlantis Massif is a hydration front.
7. Positive gravity anomalies at Atlantis Massif indicate relatively fresh peridotite.
The hypothesis that the Moho at Atlantis Massif coincides with an alteration front could not be directly tested, as Hole U1309D did not penetrate into rock with seismic velocity of ~8 km/s.
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). Differential rotation between the footwall and hanging wall blocks is predicted by thin-plate theory, so we can apply results from IODP Expeditions 304 and 305 to investigate whether the core–logging data show evidence of such history. Logging data provide continuous (oriented) images of fracture patterns in the borehole wall. These are compared with fractures and veins measured in the cores from the same depth interval. Paleomagnetic data are incorporated to determine any history of rotation of the upper footwall. 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.
The processes responsible for the development of an OCC appear to be episodic, with one factor being the level and/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.