STRIKE-SLIP AND OBLIQUE-SLIP FAULTING ASSOCIATED WITH OBLIQUE COLLISION: EXPERIMENTAL INSIGHTS ON STRAIN LOCALIZATION AND SLIP PARTITIONING

Existing mechanical models of strike-slip partitioning on oblique active margins generally assume a rigid crustal wedge moving along a pre-existing strike slip fault if a critical convergence obliquity is exceeded. Though there is general agreement that the critical obliquity positively depends on basal slope and friction on the interfaces, the solutions, based on force balance or work minimization, are very diverse.

Analogue tectonic modeling combined with digital image correlation gives high resolution constraints on the kinematics of (crustal) deformation. Such data are particularly useful for quantification of slip partitioning in oblique collision through detailed constraints on fault slip vectors as well as the amount of localized vs distributed deformation. In this presentation, I highlight the strong control of erosional unloading on strain localization and fault kinematics in oblique collision, as well as the role of localization itself. Even very simple analogue models consisting of a single strain-weakening brittle-frictional layer and kinematic boundary conditions that are constant in time and space produce distinct kinematic stages and a particularly complex fault slip evolution, that existing mechanical models fail to explain.

The model series without erosion show a two-stage evolution. An initial, transient “oblique wedge” stage with oblique slip on pro- and retro-shear is followed by a steady state stage of “strain partitioning”, in which slip is partitioned between strike-slip on a newly formed sub-vertical fault above the velocity discontinuity and oblique slip (20-30° obliquity, depending on the convergence angle) on the pro-shears. In this stage, the retro-shear accommodates near-orthogonal slip (5-10° obliquity).

In the model series with erosion, a strike-slip fault develops as well, straddling the retro-shear. However, a larger component of the boundary-parallel displacement remains to be accommodated on the pro-shears. Slip on the retro-shear remains highly oblique for as long as it is active. The pro-shears show a very conspicuous kinematic evolution: upon initiation, the slip is oblique (30-50° depending on convergence angle). It progressively rotates to even larger obliquity, attaining near-strike-slip in the case of the models with 70°-80° convergence obliquity.