Technical Note 20/4

LEG 176

Return to Hole 735B

Modified from Proposal 300-Rev Submitted By
H.J.B. Dick, S. Hart, J.H. Natland, P. Robinson, R. Stephen, & R.P. von Herzen

Staff Scientist: Jay Miller
Co-Chief Scientists: Henry Dick & James Natland


Leg 176 will deepen Hole 735B, originally drilled during ODP Leg 118 on the Southwest Indian Ridge off the southeast coast of Africa, to a nominal depth of 2 km. This endeavor takes advantage of the unique outcrop of the lower oceanic crust exposed along a 15-km-long wave cut terrace 18 km east of the Atlantis II Transform Fault. At 500 m below seafloor, Hole 735B represents the deepest penetration into plutonic basement in the world's oceans. Further drilling operations will improve our understanding of the nature of the ocean crust. Studies engendered by this project will directly address recent geophysical investigations, placing regional and potentially global constraints on ocean crustal structure and evolution.

The principal objective of Leg 176 is to recover a representative section through the lower ocean crust and potentially into the upper mantle by deepening Hole 735B to at least 2 km, possibly making this the deepest hole ODP has ever drilled. A complete logging suite will complement the downhole data acquired during Leg 118, as well as serve as a singular example of the utility of logging tools in deep basement drilling. Assuming the nearly complete recovery achieved on Leg 118 continues on this project, a representative suite of all lithologies in this section will be sampled. This lithologic suite will be used to develop an internally consistent model of the igneous, metamorphic, and tectonic processes that occurred during the formation and subsequent exhumation of this section through the ocean crust. If drilling conditions prevent deepening Hole 735B, an alternate strategy originally envisioned as the second of a two-leg drilling plan, will be adopted. This alternate strategy will test the lateral and temporal variability in the deep ocean crust by drilling a series of 500-m-deep holes, offset by 100,000 yr increments along a lithospheric flow line.

To Leg 176 Proposed Site Information

Table of Contents

Publications Home

ODP Home