THE VOLCANIC ARGENTINE CONTINENTAL MARGIN: CRUSTAL ARCHITECTURE AND SEDIMENT DISTRIBUTION

In 1998/1999 we collected an extensive MCS (>12000km) and refraction seismic data set at the Argentine continental margin. Using additional tie-lines and wells from the industry we were able to map five main seismic unconformities along the shelf, slope and continental foot. The final break-up of the South Atlantic at approximately 132 Ma was preceded by a phase of crustal extension in Permian times. From drilling results in the Colorado Basin we infer an Early Permian age for this extensional episode. The Early Creta-ceous continental break-up and initial sea-floor spreading were accompanied by large-scale, transient volcanism emplacing voluminous extrusive constructions on the conjugate outer conti-nental margins of the South Atlantic. Along the Argentine margin this volcanic activity is documented by a 60-120km wide complex structured wedge of seaward dipping reflectors (SDRS). Between 40°S and 44°S the internal structure of the SDRS wedge is subdivided into at least three separate sequences. Each of these sequences is bound by strong main unconformities and shows distinct variations in reflection characteristics (reflector curvature, reflector length/continuity). The outer seaward dipping wedges are build up by shorter reflectors indicating an emplacement under sub-marine conditions, whereas the emplacement of the inner wedges took place during subaerial conditions. From all these observations we infer that the formation of the dip-ping layers along the volcanic margin of the South Atlantic was episodic. The refraction/wide-angle reflection data show a high velocity (7.5km/s) body underlying all three SDRS wedges. These high velocity lower crustal sections were also found at the conjugated African margin indicating a rather symmetrical evolution of the South Atlantic volcanic margin. From the mapping of the different sediment unconformities we found indications for different depositional environments and varying subsidence history of the SDRS wegde along the margin. In several locations the oldest, most landward SDRS are covered by sediments of > 4 s TWT thickness.