Drilling Strategy | Table of Contents


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

Site 1039R-A Science Objectives
During Leg 205, coring and sampling will begin at Site 1039R-A 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-A are as follows.

1.Quantify the amount of carbonate in the subducting sediment and uppermost altered basaltic crust for comparison with CO2 fluxes out of the volcanoes to evaluate carbon recycling through the arc.
2.Determine the distribution of metalliferous carbonates above the sill and above basement and determine the concentrations of elements such as Cu, Cr, Ni, V, and Pb to construct element fluxes into the trench and to constrain their flux out of the basement.
3.Determine the extent of sill emplacement and their contribution to the bulk composition of the subducting igneous crust.
4.Determine the igneous and alteration mineralogy, petrology, and geochemistry in the uppermost 100 m of the oxidative alteration zone and characterize the original volcanic structure within the basement. Use the geochemical data to calculate subduction fluxes. Attention will be paid to low-temperature alteration features that may result from near trench fluid flow as well as that deriving from ridge-crest and near off-axis hydrothermal circulation.
5.Determine physical properties in the core and borehole that may affect estimations of basement composition and lithologic variation or that relate to fluid flow and deformation such as porosity, density, permeability, fracture distribution and orientation, and strength.

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

1.Use pressure, temperature, fluid, and gas compositions and fluid flow rates together with downhole measurements to characterize the fluid and heat fluxes responsible for the abnormally low heat flow in the vicinity of this site because of seawater incursion to basement.
2.Evaluate the thermal, hydrological, and chemical implications of this extensive fluid circulation for the thermal structure of the uppermost part of the subducting plate, the hydrological pathways available in the shallow subduction zone and overlying prism, and global element fluxes.

Sites 1040R-A, 1040R-B, and 1040R-C Science Objectives
Limited coring of the décollement 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.

1.Determine physical properties of the décollement horizon from further structural experiments on whole-round samples to constrain hydrological modeling and permit integration of fluid flow and deformation models.
2.Determine chemistry of pore fluid profiles from décollement whole rounds to compare with profiles measured during Leg 170 and to evaluate possible heterogeneity.
3.Determine pressure, temperature, and composition of fluids and gases along the décollement and evaluate any possible changes through time for hydrologic modeling. This same data set will constrain the flux of elements out of the downgoing sediment section along the décollement to evaluate the role of fluid egress on element fluxes to the ocean and its corollary, changing composition of the residual slab because of fluid loss.
4.Use selected elements, element ratios, and isotopic compositions in the fluids from the décollement in an attempt to constrain dehydration reactions at the updip and, perhaps, downdip limits of the seismogenic zone.
5.Determine pressure, temperature, and fluid composition in the zone of compaction dewatering beneath the décollement to constrain pathways of fluid return to the surface and to evaluate the effects of this flow system on element fluxes.
6.Collect whole-round samples from the décollement under appropriate conditions for postcruise microbiological investigations to determine the resident microbial ecology of the zone for comparison with eventual microbial experiments on fluids collected from the décollement.

A lower-priority target at these sites is the fluid conduit (perhaps a thrust) encountered at 180-220 mbsf during Leg 170 coring at Site 1040. 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.

Drilling Strategy | Table of Contents