THE ROLE OF HETEROGENEITIES IN CRUSTAL STRENGTH DURING CONTINENTAL RUPTURE: A NUMERICAL MODELING APPROACH

Subduction zones often transition into continental rifts with accreted terranes becoming the locus of extension. The lateral variations in crustal strength associated with these accreted terranes must play a role in the patterns of the subsequent extensional evolution. To explore the affect of heterogeneities in crustal strength on the evolution of continental rifting we use a 2-D geodynamic model to simulate the evolution of a lithospheric cross section of accreted terranes juxtaposed to a craton. The model domain consists of a thick, strong craton on the right side and a thinner, weaker lithosphere of accreted terranes on the left side. A stronger crustal block is emplaced within the accreted terranes to simulate the affect of strength heterogeneities of accreted terranes.

The experiment looked at the impact of varying three factors: i) the location of the strong block, ii) the rate of crustal heat production in the strong block, and iii) the spreading velocity. Model results fall into three different extensional styles; 1) rifting and eventual rupture of the lithosphere occurs at the left edge of the model 2) lithospheric rupture is located between the strong block and the craton 3) the locus of extension migrates from the edge of the model to the region between the strong block and the craton.

All model simulations begin with extension at the left edge of the model. In all cases with a high heat production within the strong block, regardless of location or spreading rate, extension eventually migrates to the region between the strong block and the craton. Simulations with moderate to low heat production in the strong block, and with the strong block located nearer the model edge, the extension immediately migrates to the region between the strong block and the craton. Simulations with moderate to low heat production in the strong block, and with the strong block located far from the model edge, with high spreading velocity, extension and lithospheric rupture occurs at the model edge.

The results of this study suggest that strength heterogeneities systematically control the evolution and final geometry of a continental rift and should be addressed when studying continental rupture. These results may provide insights to the evolution of complex continental rifts such as the Basin and Range Province and the West Antarctic Rift System.