Stressed Thrust Ramps Localize Fault Injectites on the Naukluft Thrust, Namibia

Timothy J. Sherry, McGill University
Christie D. Rowe, McGill University

Thrust ramps influence static stress change fields and control the style of fault deformation and damage. However, it is not fully understood how static stress change fields influence the style of fault rock deformation and off-fault damage during coseismic slip. Previous works have calculated Coulomb stress changes associated with fault slip and related these to areas of wall rock deformation or triggered slip on neighboring faults. We build on this by focusing on fault rock injectites, which are sensitive to both on-fault and off-fault stress conditions. Fault rock injectites record strain and are filled with pseudotachylyte or clay gouge on the scale of centimeters to meters in height, respectively. Here we correlate large (50-70 m) fault rock injectites located around a thrust ramp with modeled static normal stress changes to determine the influence of fault geometry on the formation of these structures. We mapped the area of the thrust ramp with differential GPS and generated a topographic surface model of the fault. Fault surface cross sections, parallel the southeast hanging wall transport direction, were input into USGS Coulomb3 software to model the geometric influence on static stress changes. The thrust ramps influence static normal stress change during fault motions by focusing stress change gradients at ramp inflection points. Along the length of the ramp, the static stresses perpendicular to the fault are in tension as the fault dilates to move past the ramp. Injectites tend to correlate with areas of sharp static stress change gradients between compressive and tensional stresses. These stress gradients are likely controls on injectite nucleation sites. Geometrically controlled static stress changes reinforce and localize coseismic dynamic stresses where areas of compressive static normal stress squeeze fluidized material towards zones of tension and wall rock fracture. We can also use the volume of injectites to constrain aspects of coseismic pressurization on the fault surface. The volume of material filling the injectites is related to the volume of fluidizing CO2 dissociated through frictional heating from the source rock. A simplified model is applied to estimate earthquake slip patch area for mixtures of CO2 - H2O fluidized slurries. The compressibility factor of water does not greatly influence earthquake magnitude estimates, with Mw ~ 4.2. Thrust ramps are therefore important sites for potential injectite localization and earthquake nucleation.