Volatiles and Fluids in Subduction Zones

Investigations in the 4th week were concentrated offshore Central Costa Rica in the area of the upper plate opposite the seamount province of the oceanic plate. The margin here is sculptured by the furrows of seamounts subducting with the lower plate. First target was the Christmas Ridge. OFOS observations revealed massive outcropping carbonates at the whole top area of this local high (1 mile in diameter). Neither at the top nor at the western flanks we found sufficient recent venting activity that would have justified further investigations.

The next target was the uplift landward of Jaco Scarp, caused by a subducting seamount. The top of the structure, the area of highest uplift, is characterised by numerous faults. We ran two OFOS missions across these areas characterised by high backscatter in TOBI images. OFOS 10 revealed a seafloor either paved by carbonates, or covered by fragmented and donut-shaped carbonates. In the whole area we observed scattered bivalve shells. OFOS 11 verified that most of the lineated high backscatter areas are caused by faults indicated by small depressions with depths on the order of 5 to 15 m. Only at the rim of the depressions talus boulders were found. The top of the uplifted area is structured by massive chemoherm-like carbonates. A dense field of vesicomyid clams were observed at a place, where the massive chemoherm complex were probably cut by fractures.

Generally, all OFOS deployments had to cope with relatively strong currents. Planning the tracks often had to follow operational rather than geological requirements. With our OFOS missions so far we investigated prominent geological structures such as mounds or seamount scars. All observations indicated the ocurrence of massive carbonates, which are associated with fluid venting, throughout these structures. In order to look for sites with recent or ongoing fluid venting activity, we will concentrate the forthcoming OFOS deployments of this cruise on tectonically influenced structures which are considerably younger.

Starting on April 3 in the morning, we continued to survey the uplifted and faulted area upslope Jaco Scarp with the side-scan sonar and the subbottom profiler of the DTS-1. Gaining more and more experience after several launch and recovery operations, the deployment was carried out smoothly in less than 15 minutes. The whole uplifted area with the pronounced fault pattern was imaged with 8 parallel profiles each around 10 miles in length. The northeastern flank of the uplifted area, showing a steady slope in the bathymetric data, turned out to be a series of rather steep, staircase like precipicies. As the DTS-1 vehicle is towed at a height of just 100 m above the seafloor, steaming uphill required total attention of the DTS-1 watchkeepers. The survey across the uphill area of Jaco Scarp was completed with a track operating the DTS-1 system in the high-frequency mode. The faults and cracks on top of the structure, and the headwall of Jaco Scarp were imaged in great detail.

Work continued in the morning of April 5 with a TV-grab in order to sample donut-shaped carbonates observed by OFOS 10 two days ago. The grab felt over again, yet it yielded eroded carbonate chimneys and calcitic cemented sediments forming partly rounded boulders. In addition, more than 90 specimen of the seep-typical bivalve Acharax (Solemyidae) were extracted from the sediment indicating a weak venting activity at this site.

The mosaic of the processed TOBI data recorded during SO-144 in 1999 show a small area with striking high backscatter between Jaco und Parrita Scarp. This structure was investigated with an OFOS mission on the afternoon of the same day. Beside very infrequent clamshells in soft sediment along the whole track we observed no changes in the sediment texture or lithology which could explain the high backscatter patterns.

In the evening of the same day, following the OFOS survey, TOBI was again launched for more than 4 days mapping of the lower part of the margin and the deformation front. Three parallel profiles, each about 80 miles in length, will extend the investigated area southward until Osa Peninsula.

In the night from sunday to monday we observed rain, for the first time of this cruise, reminding us after so many sunny days that weather can change. A tropical thunderstorm brought really heavy rain, the noise of the drops on the water surface were well recorded in the TOBI images.

With best regards of all aboard
Wilhelm Weinrebe