Seismic reflection profiles acquired in S19 cruise: the image of VTR and PTDZ tectonics

Four N-S seismic reflection profiles were acquired across the Vema transform to investigate the structural style of the VTR and PTDZ (Fig. 2). Our seismic lines were ex- tended to a roughly E-W oriented scarp, which runs 65 km south of the Vema trans- form. This stmcture named ("Lema" FZ KASTENS1998 is markedly asymmetric and probably marks a former transform plate boundary. We will describe the major features observed along one of these seismic sections (Vema-07M, for location see Fig. 2) that runs S-N within the S. American plate, from the Lema FZ to the Vema transform valley. A seismic image of Lema FZ is visible between shot 200 and shot 750 (Fig. 6a): the north-facing flank bounds a small sedimentary basin. The basin infilling appears to be undisturbed, and onlapping the fault scarp and the oceanic crust. The sea floor to the south of this structure (the hanging-wall) lies about 1000 m above the oceanic crust on the northern side (foot-wall). Moving northward (from shot 800 to shot 1300, Fig. 6b) a series of mounds are present. They may represent sedimentary deposits derived from slope instability along the southern wall of VTR. The complex geometry of the reflections, the chaotic seismic faeies and scoop-shaped reflectors suggest that slumpings and/or mega-slumpings may have occurred on the southern wall of VTR. The VTR rises between shot 1300 and 1750 (Fig. 6c) reaching here a minimum depth of about 1500 m below the sea level. The northern flank is terraced with a major slope break at shot 1590. The seismic profile shows, between shots 1410 and 1490, a southward dipping reflector; it could represent the base of basaltic layer 2a. The middle scarp terrace represents the top of the gabbro sequence AUZENDE1989. Rugged topography is present at the base of VTR slope, suggesting active phenomena of gravity instability along low angle detachment surfaces. These deposits pass laterally to the Vema valley sedimentary sequence, between shot 1850 and shot 2250 (Fig. 6d). The transform valley infilling shows fine layering, alternated by regularly spaced stronger reflectors, Tectonic deformation is concentrated in a 4 km wide sector at the transform valley center (Fig. 6d) where the PTDZ is located. Here evidence of compressive deformations can be observed throughout the sediment column down to the valley basement. The median ridge is visible buried below 0.5 sec of sediments.

Figure: .