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FIGURE 1 Overview map of Antarctica and surrounding ocean floor using a combined BEDMAP (bedrock topography with ice sheet removed) (Lythe et al., 2001) and satellite-derived bathymetry grid (McAdoo and Laxon, 1997). White areas on the continent are below sea level if the ice sheet is removed (without isostatic compensation). Yellow lines mark boundaries between the six crustal breakup sectors of respective conjugate continents in this paper. Red lines show the tracks of offshore multichannel seismic profiles of the SCAR Seismic Data Library System (SDLS) (compiled by M. Breitzke, AWI). Black lines mark the locations of offshore deep crustal seismic refraction profiles. WS = Weddell Sea sector; DML = Dronning Maud Land sector; EL = Ellsworth-Lambert Rift sector; WL = Wilkes Land sector; MBL = Marie Byrd Land sector; APE = Antarctic Peninsula-Ellsworth Land sector.

STRETCHING AND BREAKING: PASSIVE MARGIN TYPES

Weddell Sea (WS) Sector Conjugate to South America

The complex tectonic development of the Weddell Sea sector (Figure 2) has more recently been reconstructed by Hübscher et al. (1996a), Jokat et al. (1996, 1997, 2003, 2004), Leitchenkov et al. (1996), Ghidella and LaBrecque (1997), Golynsky and Aleshkova (2000), Ghidella et al. (2002), Rogenhagen and Jokat (2002), and König and Jokat (2006). Deciphering of the crustal types in the central Weddell Sea is still hampered by the lack of deep crustal seismic data. In the southern Weddell Sea seismic refraction data reveal a thinned continental crust of about 20 km thickness beneath the northern edge of the Filchner-Ronne ice shelf (Hübscher et al., 1996a). It can be assumed that this thinned crust extends northward to a boundary marked by the northern limit of a large positive gravity anomaly (Figure 2). König and Jokat (2006) associate an east-west rifting of this crust (stretching factor of 2.5) with the motion of the Antarctic Peninsula from East Antarctica as the earliest event in the Weddell Sea plate circuit at about 167 Ma prior to the early Weddell Sea opening in a north-south direction at about 147 Ma. It is not clear, however, whether some of the crustal extension is also associated with this early Weddell Sea opening. The crust between the northern end of the large positive gravity anomaly and the magnetic Orion Anomaly and Andenes Anomaly (Figure 2) is interpreted as a COT with the Orion Anomaly suggested to represent an extensive zone of volcanics that erupted during the final breakup between South America and Antarctica (König and Jokat, 2006). Deep crustal seismic refraction data across the Orion and Andenes anomalies and the assumed COT south of it are needed in order to constrain their crustal composition and type. Although the Orion Anomaly may provide a hint toward a volcanic-type margin, the few seismic data do not allow a complete characterization of the COT in the southern Weddell Sea. Identified magnetic spreading anomalies (oldest is M17) show evidence that oceanic crust exists north of the Orion and Andenes anomalies with the prominent T-Anomaly marking supposedly the changeover from slow to ultraslow spreading-type crust (König and Jokat, 2006).

The Weddell Sea margin along the east coast of the Antarctic Peninsula is still rather enigmatic due to missing data. König and Jokat (2006) show that it rifted from the western Patagonian margin as part of the earliest plate motion in the Weddell Sea region at about 167 Ma. They follow Ghidella



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