SOUTH GEORGIA REVISITED: HOW AND WHY DID IT SEPARATE FROM SOUTH AMERICA?

Professor Robert H. Dott Jr. posed a challenging question in the ninety-sixties: what is the island of South Georgia doing at the eastern end of the North Scotia Ridge? The remote island was reported to be composed mainly of Cretaceous turbidites. Granitic plutons, and a quartz-rich gneissose basement had also been recorded. This was clearly no Hawaii despite its isolated oceanic location.

In the early seventies we were able to study the turbidites. Together with our work in southernmost South America, this made it clear to us that the South Georgia microcontinent originated in what is now a ‘hole’ in the South American continental shelf immediately east of Cape Horn.

South Georgia, however, remains enigmatic in two respects. First, the mountains of the Allardyce Range along the spine of the island are anomalously high compared with their along-strike counterparts on Isla Navarino, Tierra del Fuego, ~3000m vs ~1000m. Second, it is not apparent how the South Georgia microcontinent was displaced eastward with respect to the South American continent. Let alone why. The displacement must have occurred after the mid-Cretaceous compressional event that inverted the Rocas Verdes marginal basin. The compression obducted oceanic parts of the former basin floor as the Tortuga and Larsen Harbour ophiolitic complexes of Isla Navarino and South Georgia respectively, and deformed the turbiditic basin infill, thereby initiating the tectonic uplift of the Andean Cordillera.

There are three possibilities for the displacement of South Georgia: transcurrent faulting along the Scotia-South America plate boundary, motion on transform faults associated with Cenozoic seafloor spreading that generated the Scotia plate, and a combination of the two. The anomalously high elevation could be explained by compression at a restraining bend of a transform fault. These enigmas are more than local issues when we consider why the displacement and uplift occurred. The Scotia arc may be a focus of asthenospheric ‘return flow’ from the closing Pacific basin to the opening Atlantic basin. Hence, the tectonic history of the island that captured Bob Dott’s attention forty years ago, despite the fact that few geologists even knew of its existence, may yet hold important clues to deep Earth processes for future generations to appreciate.