FS SONNE Fahrtbericht SO159 = Cruise Report SO159 SALIERI - South American Lithospheric Transects Across Volcanic Ridges ; Guayaquil - Guayaquil, August 21 - September 17, 2001

Abstract

The SALIERI experiment is a cooperation between GEOMAR and GEOAZUR to study the Carnegie Ridge and the active margin of Ecuador and Colombia. The cruise started on 22 August in Guayaquil on-board R/V Sonne and ended on 16 September in Guayaquil. During this cruise -4000 nautical miles (nm) o f multibeam bathymetry were acquired most of them together with magnetics. Two wide-angle seismic lines were shot on the Carnegie Ridge, three in the Gulf of Guayaquil and one on the Colombian margin for a total of 130 OBS deployments. Profiles P02, P05 and P06 were shot along pre-existing multichannel seismic reflection lines acquired during the SISTEUR cruise in 2000. Most o f the OBS data are of excellent quality with clear arrivals at offsets greater than 100 km on the oceanic crust. Two approximately N-S lines were shot across the Carnegie Ridge. The western profile (POl) on Carnegie Ridge exhibits a 16-km thick crust whereas the eastern one, immediately west o f the subduction zone, shows a crust as thick as 25 km. Modelling indicates that crustal thickening is mainly related to a thickening of oceanic layer 3. Velocity structures resemble those described beneath other oceanic plateaus formed near a spreading centre especially the Cocos and Malpelo ridges. In the Gulf of Guayaquil the dip seismic line P02 constrains deep structures of the overriding plate and the subducting oceanic plate. On the margin the thick Guayaquil sedimentary basin imaged from MCS during the Sisteur cruise is at least 6 km thick. The crust of the South American plate extends westwards almost to the trench with seismic velocities increasing from -4.0 km/s to -4.5 km/s. A clear shadow zone (0.5 to 1.0 s of offset along P03) possibly outlines sediments subducted beneath the margin. Profile P06 across the North Ecuadorian-Southwest Colombian margin exhibits clear arrivals from the upper plate and the down-going plate, which will allow to model the convergent margin using both wide-angle and previous SISTEUR MCS data. Multibeam bathymetry acquired during the cruise show that the Carnegie Ridge is a highly asymmetric dome feature with a steep, faulted northern flank and a gently sloping southern one intruded locally by large, flat-topped and elongated seamounts. Among the most conspicuous features on the Ridge are fields of circular closed depressions distributed over its flanks at depths of 1500 to 2600m, and two EW-trending alignments of reflective basement highs outlining a graben-like structure in its eastern part. These data also show that the inner trench slope off Guayaquil ( latitude 3°S) is unstable, affected by mass wasting and incised by a major canyon carrying sediment into the trench. Incipient accretion is evident at the toe of the margin. The Grijalva Fracture Zone is marked by an overall N50E scarp with a narrow linear N60E trending Ridge. Offshore Esmeraldas near 1° 40'N, the upper margin section is smooth and sub-horizontal. Its lower section has a rough morphology and is deeply incised by a major canyon dumping sediment into the Colombia trench to form a noticeable sedimentary fan. Between 1° 40'N and 0° 30 N, incipient accretion is locally evident along a generally non-accretionary margin. Magnetic anomalies across Carnegie Ridge are generally related to the topography, in particular large seamounts and local volcanic domes and basement faults. South of the Carnegie Ridge possible seafloor spreading anomalies are observed. The margin profiles in SW Colombia and the Gulf of Guayaquil are magnetically quiet presumably due to the homogeneous and weak magnetic composition o f the upper plate.