STRAIN EVOLUTION AND THE RELATIVE ROLE OF LITHOSPHERIC HETEROGENEITIES DURING CONTINENTAL RUPTURE

The evolution of strain patterns during continental rupture can be quite complex, and may be due to either changes in far-field stresses, and/or lithospheric heterogeneities. The West Antarctic Rift system (WARS) is an example of a rupture system that experienced a distinct switch in deformation style. During the first stage extension was broadly distributed throughout most of the Ross Sea region. Later, the style of extension changed and was focused primarily in the Terror Rift, near the boundary with the East Antarctic craton. Although absolute timing is difficult to determine, this switch in strain style may be coeval with volcanism and/or oblique faults in the area. These possible temporal and spatial associations may indicate that a change in the stress field is responsible for the change in strain.

To explore the relative role of far-field stresses and lithospheric heterogeneities on the evolution of the WARS we have developed a 2-d finite element model to discern the processes and conditions responsible for the observed evolution of rifting. Model results indicate that the transition from broadly distributed extension to focused rifting can evolve naturally without requiring external changes in either the regional stress regime or thermal state.

The initial stage of modeled diffuse extension throughout West Antarctica results from a prescribed uniformly weak West Antarctic lithosphere (thinner, hotter) versus a prescribed stronger East Antarctic lithosphere (thicker, colder). However, the strength of the West Antarctic lithosphere increases during thinning, and eventually is no longer the weakest region. At this time, strain moves to a weaker narrow region near the boundary with the East Antarctic Craton. Thinning focuses in this region, and runaway necking leads to rupture. Thus, no change in plate motion directions or rates nor impingement of a plume is required to explain the evolution of strain patterns.