Structural Style of Sedimentary Basin and Passive Margin Formation: Roles of Strain Softening and Surface Processes

Ritske S. Huismans¹ and Chris Beaumont^2
1 Departments of Oceanography, Dalhousie University, Halifax, NS
² Dalhousie University, Halifax, NS

We use thermo-mechanical models to study lithosphere extension and passive margin formation. We focus on two aspects of passive margin formation: 1) factors controlling the (a)symmetry of lithosphere extension, and 2) the role of surface processes in affecting the style of lithosphere extension.

We examine the role of material strain softening of the crust and upper mantle and its influence on asymmetric extension through the self-consistent development of localized shear zones. Our results show that the amount of asymmetry depends on the amount of strain softening and on the extension rate. We compare the models of extension with simplified material properties with simplified analytical predictions of the rate of work in a system with a strain softening plastic layer and a viscous layer below and derive criteria that control the style of deformation.

Summarizing the main model results: 1) Asymmetry is favored by strain softening of a coupled, cold frictional upper lithosphere, at low rifting velocities. 2) At higher rift velocities and in cases where the lower crust is weak the asymmetric mode is suppressed. Model results including the effects of erosion and/or sedimentation suggest a strong sensitivity of the rift mode to surface processes. 3) Strong erosion appears to enhance the asymmetry, whereas 4) complete sedimentation on the other hand suppresses the asymmetry.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005