Seafloor spreading in the Weddell Sea from magnetic and gravity data

A re-compilation of magnetic data in the Weddell Sea is presented and compared with the gravity field recently derived from retracked satellite altimetry. The previously informally named 'Anomaly-T,' an cast-west trending linear positive magnetic and gravity anomaly lying at about 69degreesS, forms the southern boundary of the well-known Weddell Sea gravity herringbone. North of Anomaly-T, three major E-W linear magnetic lows are shown, and identified with anomalies cl2r, c21-29(r) and c33r. On the basis of these, and following work by recent investigators, isochrons c13, 08, c20, c21, c30, c33 and c34 are identified and extended into the western Weddell Sea. Similarly, a linear magnetic low lying along the spine of the herringbone is shown and provisionally dated at 93-96 Ma. Anomaly-T is tentatively dated to be M5n, in agreement with recent tectonic models. Although current tectonic models are generally in good agreement to the north of T, to the south interpretations differ. Some plate tectonic models have only proposed essentially north-south spreading in the region, whilst others have suggested that a period of predominantly east-west motion (relative to present Antarctic geographic coordinates) occurred during the mid-Mesozoic spreading between East and West Gondwana. We identify an area immediately to the south of T which appears to be the southerly extent of N-S spreading in the herringbone. Following recent work, the extreme southerly extent of the N-S directed spreading of the herringbone is provisionally dated M9r/M10. In the oldest Weddell Sea, immediately to the north and east of the Antarctic shelf, we see subtle features in both the magnetic and gravity data that are consistent with predominantly N-S spreading in the Weddell Sea during the earliest opening of East and West Gondwana. In between, however, in a small region extending approximately from about 50 km south of T to about 70degreesS and from approximately 40degrees to 53degreesW, the magnetic and gravity data appear to suggest well-correlated linear marine magnetic anomalies (possible isochrons) perpendicular to T, bounded and offset by less well-defined steps and linear lows in the gravity (possible fracture zones). These magnetic and gravity data southwest of T suggest that the crust here may record an E-W spreading episode between the two-plate system of East and West Gondwana prior to the initiation of the three-plate spreading system of South America, Africa and Antarctica. The E-W spreading record to the east of about 35degreesW would then appear to have been cut off at about M10 time during the establishment of N-S three-plate spreading along the South American-Antarctic Ridge and then subducted under the Scotia Ridge.

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