The stress regime associated with continental break-up

Extensional syn-rift structures at passive margins indicate that continental break-up occurs in response to horizontal deviatoric tension in the continental lithosphere. An abrupt change in the state of stress occurs at the onset of the post-rift (break-up) unconformity when the large tension ceases. The stress regime at the time of continental break-up has been modelled by finite element analysis with a view to simulating the observed features. A local compression is associated with thinned crust and an opposing local tension is produced by hot, low density upwelled asthenosphere. The resulting stress associated with stretching and thinning of the lithosphere is relatively insignificant and is likely to be compressive unless the normal continental crust is very thin. One possible source of break-up tension is the large tensional loading stress associated with an underlying hot, low density upper mantle, such as now occurs in the present day uplifted continental rift systems. It is uncertain whether such local tension could readily cause break-up within the present compressional stress regime of normal continental regions. Another possible source of wide-spread continental tension would be the occurrence of subduction on opposite sides of a large continental mass such as Pangaea, giving rise to instability. At such times, splitting could more readily propagate outwards from a domed plume region leading to break-up, thus involving both active and passive factors. The radical change in stress regime at the onset of seafloor spreading is readily modelled as the change in stress associated with the development of a new weak plate boundary.