There is a scarcity of accurate mass-balance estimates at convergent margins due to (1) the complexity of both the sedimentary and structural processes at these margins, (2) the poor structural imaging of the deeper parts of forearc regions, and (3) the need for reliable age estimates that generally require drilling. To establish the mechanisms of accretion and underplating, tectonic erosion, and deformation and dewatering, essential observations must include (1) the rate (positive and negative) of prism growth as a function of incoming sediment volume and type; (2) the partitioning of frontal offscraping, underplating, internal prism deformation, and subduction erosion; and (3) the effects of fluids. Excellent control is required of material mass-balance and residence time in the prism, in addition to detailed structural geometry of the complex interior regions of forearcs. Such control and imaging is not well established in any convergent margin, despite expending a great amount of effort to understand these margins. The convergent margin off Costa Rica (Fig. 1) satisfies all the requirements necessary to determine accurate mass-balance and flow estimates, except for knowledge of the age and residence time of the prism material. The trench is devoid of turbidites here and the convergence rate is known. Recently acquired 2D and 3D seismic reflection data across the margin provide excellent control of the internal structure of the forearc, and they define boundaries between the accretionary prism and the overlying slope cover, as well as between the prism and the subducting plate. These data show that the slope cover extends to within 3-5 km of the trench, so it protects the accreted mass from erosion and conserves its volume. Consequently, the growth rate of this prism can be calculated accurately when the emplacement age of the accreted material is determined by drilling through the basal slope cover and top of the prism.

Five sites were drilled during Leg 170. The major objective of Site 1039 was to serve as a reference site for drilling in the Costa Rica subduction complex. Site 1039 was chosen for its location in the trench axis and away from fault scarps (Figs. 1, 2, 3, 4). Thus, any turbidites entering the trench would be expected to be found at Site 1039. The site was located on seismic Line CR-20, acquired on the R/V Fred Moore in 1987. Two major units were identified from the seismic data. An upper unit of about 180 m was thought to consist of hemipelagic sediment, whereas the lower layer was thought to be pelagic carbonate ooze, based on earlier drilling off Central America. Seismic data indicated that the strata at Site 1039 decrease in thickness beneath Proposed Site CR-2, so it is important to understand the properties of the sediment section prior to its subduction. In addition, Site 1039 provides a geochemical reference for comparison with that of the active volcanic arc, including chemical tracers such as Be-10 and Ba. In addition to standard coring and core analyses, we used logging-while-drilling (LWD) logs to obtain high-resolution porosity and density measurements.

Site 1040 was located 1.6-km landward of the toe-of-slope off the Nicoya Peninsula, Costa Rica. The objective of this site was to pass through the sedimentary wedge near the toe, the décollement, and the underthrust sedimentary section beneath the décollement. Specific objectives included determining (1) the nature and age of the wedge material; (2) the physical properties of the wedge and underthrust section; (3) the chemistry of pore waters within the wedge, décollement, and underthrust section; and (4) the implications for the balance of mass and fluids between the trench sequence and the underthrust and wedge sequence. The specific location of Site 1040 was chosen because in the seismic records the wedge appeared to be entirely sedimentary, the underthrust section was traceable from the reference section and appeared to be thinned, and the basement was also accessible to the drill. In addition, the décollement reflector was clearly of reversed polarity, indicating an inversion of seismic impedance, possibly the result of elevated pore-fluid pressure. Furthermore, surface studies in this area measured very low heat flow, and we hoped to learn the cause of this anomaly as well to obtain a better definition of the problem with depth. LWD was a high priority for Site 1040, as it was for Site 1039. Comparison of LWD data from Site 1040 and Site 1039 could provide us with a record of density and porosity of unprecedented detail between a reference site and an underthrust sedimentary section.

Site 1041 is located on the midslope of the Costa Rica margin, about 12 km up from the toe-of slope off the Nicoya Peninsula. The margin here consists of a prism section underlying much of the slope, and a sedimentary apron of 500-600 m thickness that overlies the prism. A major uncertainty exists as to whether the high-amplitude reflection underlying the slope apron off Costa Rica represents ophiolitic rocks of the onshore Nicoya peninsula, or sediment accreted by underplating of the underthrust Cocos Plate stratigraphy. The prime objective of this site was to determine the nature and composition of the prism material underlying the slope apron. Additional objectives included (1) determining the fluid-flow regime of the midslope; (2) comparing the apron material with that of the lower wedge above the décollement drilled at Site 1040; and (3) penetrating through an out-of-sequence fault zone, marked by a well-developed reflection at about 1.4 s (two-way traveltime) beneath the seafloor. Using velocities obtained from Shipley et al. (1992), we estimated that the reflector depth was ~1200 mbsf. New seismic velocities from McIntosh and co-workers (unpubl. data), indicated this reflector more likely lies at a depth of 1500 to 1600 mbsf. We were unable to sample the prism beneath the apron at this site because of difficult hole conditions. For this reason, we obtained permission to drill Site 1042 where the prism reflector occurs at an estimated depth of ~300 mbsf, i.e., 200 m shallower than at Site 1041 (Fig. 5). Site 1042 had the same objectives as Site 1041.

Site 1043 was designed to test whether or not offscraping is temporally and spatially intermittent. At Site 1040, rotary core barrel (RCB) drilling disturbance essentially destroyed much of the primary structural fabric above the décollement. Site 1040 showed much less offscraping (~1%) than expected based on seismic reflection data (~10%). At Site 1043, we anticipated less drilling disturbance using extended core barrel (XCB) coring because the décollement is much shallower (approximately 150 mbsf) than at Site 1040 (371 mbsf). The rate of change of thickness of the underthrust hemipelagic sequence (Units U1 and U2) appeared to be highest at Site 1043. This rate of change may correspond to a high rate of dewatering of the underthrust section as well. The hemipelagic section shows evidence of anomalous dips at Site 1040, suggesting deformation of this section beneath the décollement. Site 1043 was also designed to test whether deformation of the underthrust hemipelagic sediments develops soon after subduction. LWD was a high priority and comparison of LWD data from Sites 1043 and 1039 has provided us with an unprecedented, detailed record of density and porosity between a reference site and an underthrust sedimentary section.

To 170 Results - Site 1039

To 170 Table of Contents

Publications Homepage

ODP Homepage