THE IMPORTANCE OF TRENCH PULL IN BREAKING UP PANGAEA AND OPENING THE ATLANTIC (Invited Presentation)

We consider the importance of trench pull for breaking up Pangaea and opening the Atlantic Ocean basins since the Jurassic. Present-day plate motions are to 90% explained by trench pull, attributable to the relative lengths and orientations of subduction zones at the plate boundaries. Hence a straightforward hypothesis is that trench pull also dominated plate motions in the geologic past and therefore broke up Pangaea, but this was not borne out by accepted palaeo-trench configurations. Eastward-dipping subduction zones are widely thought to have lined the continental margins of North and South America since the Atlantic started to rift (~200 Ma) and open (~170 Ma). Such trenches would have exerted no westward slab pull on the Americas during their westward drift.

For North America, we have challenged this prevailing view of margin-hugging trench locations because massive slab walls of subducted lithosphere in the lower mantle are located more than 1000 km west of the reconstructed Pangaean margin of North America. These slabs would have accumulated beneath an offshore island archipelago while the continent was part of Pangaea. Hence intra-oceanic subduction initially pulled North America westward, away from Pangaea, until the continent gradually overrode those trenches and their arc terranes, and subduction flipped to eastward beneath the continental margin. Indeed, the lower parts of these slabs bear no geometric imprint of the North American margin, as might be expected from margin-hugging subduction, whereas their upper reaches do.

We show that slab geometries indicate an analogous archipelago offshore Central America and the northern part of South America. Hence South America was also pulled westward by intra-oceanic subduction during the early opening of the South Atlantic. The modern, Cenozoic phase of Andean thrusting corresponds to the times since this northern Andean archipelago was overridden by South America.

We map out the extent of the Mezcalera Ocean, the westward-subducting basin that was pulled into the intra-oceanic trenches ahead of the Americas, based on the shapes of the slab walls that now hold its lithosphere and are imaged by seismic tomography.