168 Prospectus

Site: HT-3A

Priority: 1
Position: 47°54.52´N; 128°45.00´W
Water Depth: 2637 m
Sediment Thickness: 140 m
Total Penetration: 180 mbsf
Seismic Coverage: Line 95-15, SP 430 (Fig. 4, Appendix 1A)

Objectives: The Hydrothermal Transition site comprises a suite of four holes spanning a transition from open to sediment-sealed hydrothermal circulation. Holes are sited from 200 m to 20 km away from the nearest basement outcrop. Heat flow across the transect increases from about 15% to more than 80% of expected lithospheric values; upper basement temperatures increase from a few degrees to more than 40°C; hydrothermal fluid composition changes from normal to altered seawater; upper crustal seismic velocities increase from approximately 3.5 to more than 5.5 km/s. This reentry hole will intersect basement at approximately 20°C and will be completed with a CORK installation.

Drilling Program: RCB only (two trips, both through reentry cone), with approximately 40 m of basement penetration. Casing will be set after initial RCB drilling reaches first competent basement.

Logging and Downhole: Approximately five temperature probe trips, packer/flowmeter testing, and CORK installation

Nature of Rock Anticipated: Turbidite sediments; weakly altered basalt

To Site HT-4A

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