Drilling Strategy | Table of Contents


During Leg 205, we will return to Leg 170 drill sites; the Leg 205 sites are 1039R, 1040R, and alternate Site 1043R. Science objectives for Leg 205 have two primary foci. The first is the igneous and alteration history of the basement at reference Site 1039R on the incoming plate. The second is on the three hydrological systems: in basement at Site 1039R; in the uppermost section of the subducting sediment section at Site 1040R; and along the décollement and upper conduit at Site 1040R. An alternate site, Site 1043R, has the same objectives as those at Site 1040R. These goals will be accomplished as described in detail below by limited coring of selected intervals, downhole temperature measurements, logging at Site 1039R, the installation of long-term observatories to monitor temperature and pressure, and sampling fluids and gases at key hydrological intervals.

Site 1039R Science Objectives
During Leg 205, coring and sampling will begin at Site 1039R within the carbonates above the sill encountered during Leg 170, will continue through the sill and the previously undrilled sediments beneath the sill, and ~100 m into basement. The scientific objectives to be addressed through coring, sample analysis, and logging at Site 1039R are as follows.

A long-term borehole observatory (i.e., a modified CORK) will also be emplaced at Site 1039R to sample fluids and to monitor temperature and pressure within the uppermost permeable basement. The science objectives for this are to

Site 1040R Science Objectives
Limited coring through the décollement and into the uppermost underthrust section at Site 1040 and installation of modified CORKs to monitor and sample within the area of maximum flow of deeply sourced fluids in the décollement and in the underthrust sediment section address the following scientific objectives.

A lower-priority target at Site 1040R is the fluid conduit (perhaps a thrust) encountered at 180-220 mbsf during Leg 170 coring. Pore fluid chemistry profiles across this horizon show advective flow of deeply sourced fluids, similar to those along the décollement. If necessary, the objectives above for the décollement could also be addressed at this horizon.

Alternate Site 1043R Science Objectives
Site 1043R has been approved as an alternate site. Installation of a modified CORK with a sampling interval in the décollement here would accomplish all the décollement science objectives at Site 1040. Penetrations at both Sites 1040 and 1043 would permit close comparison of hydrological and compositional characteristics along a section of the décollement 1 km apart.

Drilling Strategy | Table of Contents