The role of elasticity in normal faulting and the development of axial topography in the oceanic lithosphere

In this study we compare 2D numerical simulations of lithospheric extension with and without elasticity in order to investigate its role on the development of normal faults and axial topography at oceanic spreading centers. Specifically, we use a finite difference / marker-in-cell technique to model visco-elasto-plastic (VEP) and visco-plastic (VP) deformation of the lithosphere under extension. Simulated fault zones form spontaneously as the system evolves and the associated strain localization is achieved by reducing the cohesion in proportion to the accumulated plastic strain in regions undergoing yielding. We investigate the development of different fault modes (e.g. growth of multiple faults vs. a single large-offset fault) both in a VP and a VEP lithosphere for a range of lithospheric thicknesses, spreading rates, and rates of cohesion loss. In our simulations, fault-induced bending of a VP lithosphere occurs on a larger wavelength and with less overall vertical deflection than in a VEP lithosphere. Flexural rotation of long-lived, initially steep faults does not require elasticity, but appears to have a strain-rate-dependent wavelength in a VP lithosphere. We find that thinner lithosphere and rapid weakening promote the growth of large-offset faults in both a VEP and a VP lithosphere. The effect of neglecting elasticity appears greater in thicker lithosphere, where a VP rheology favors the growth of multiple steep faults instead of a few large-offset faults. We also note that a VP lithosphere requires more total extension to achieve the same faulting pattern as a VEP lithosphere. This may be due to distributed viscous deformation taking up a portion of the extension in the VP case. To further quantify our numerical results, we develop scaling relations describing the build-up of topographic and bending stresses in a faulted VP lithosphere and compare them to those previously derived for a VEP lithosphere. These relations are then implemented in a force balance model to predict the characteristic heave and spacing of normal faults as a function of (mainly) lithospheric thickness and the fraction of extension that may be accommodated by magma injection. This study provides a first step in evaluating the suitability of visco-plastic numerical codes for modeling rifting, as well as insight into the dominant modes of deformation in young oceanic lithosphere.