Stress, strain, and fault behavior at a thrust ramp: Insights from the Naukluft thrust, Namibia

We report observations from a kilometer-scale thrust ramp on the Naukluft thrust, Namibia. The Naukluft thrust is a low angle thrust that was active at subgreenschist facies conditions and accommodated several tens of kilometers of displacement at the base of the Naukluft Nappe Complex in the Pan-African Damara Orogeny. The fault zone is generally planar and a few meters thick, comprising predominantly a dolomite-rich cataclasite. At the ramp, the fault-rock assemblage increases in thickness, and the hanging-wall, which elsewhere is relatively intact, contains a high density network of inclined quartz veins, subvertical dolomite and calcite veins, breccia zones, as well as injectites of cataclastic fault rock emanating from the fault surface. The geometry of the hanging-wall structures indicates local subhorizontal extension. Local tensile stress can be explained by bending in the hanging-wall as it deformed to slide above the ramp structure. High fluid pressures created dynamically during fast slip by decarbonation of carbonate fault rock, and by dewatering of the footwall under an impermeable fault during interseismic periods, led to additional reduction in local effective compressive stresses. In this location, the ramp is more optimally oriented for slip in the inferred regional stress field, and therefore likely to fail before the contiguous thrust flats that are subparallel to the maximum principal stress. As such, the ramp represents the likely location for nucleation of fault slip, which could both trigger dynamic failure of the adjacent thrust faults, and produce hanging-wall extensional structures.