Strain Localization within Fault – Related Folds , with Applications to Mars

Insight into the mechanics of brittle strain localization and fault propagation is gained by investigating the growth of deformation bands and fault–related deformation band damage zones. In porous granular rocks such as sandstone or tuff, the onset of brittle strain localization is characterized by pore space collapse along nucleating deformation bands, then by shear displacements along propagating deformation bands. Numerical models of local stress orientation and magnitude (via strain energy density criteria) are shown to systematically predict characteristic compressive and extensional stepover geometries between overlapping deformation bands. These models also successfully predict observed geometries, intensities and propagation tendencies of fault– related deformation band damage zones. The methods developed here provide a framework for investigating the growth of faults and fault–related folds in porous granular rock and provide insight into structural controls of reservoir–scale fluid flow. Numerical models of fault–related folds are also used to investigate two long– standing questions on the evolution of the Martian lithosphere; the nature of lithospheric rheology at the base of the Tharsis volcanic load during the Late Hesperian (c. 3.0–3.6 b.y.a.), and the state of lithospheric stress during this time. The distribution of vergence directions of Late Hesperian thrust faults reveal that the base of the Tharsis load was brittle at the time of faulting and that this deformation was the result of thick–skinned deformation involving the entire brittle thickness of the lithosphere. Minor 100–km–scale patches of mechanically well–stratified crust are found to have occurred at this time due to the presence of near–surface volatile reservoirs. These patches however, are not characteristic of the global lithospheric rheology. Wide–spread thrust faulting during the