Cyclic frictional-viscous slip oscillations along the base of an advancing nappe complex: Insights into brittle-ductile nappe emplacement mechanisms from the Naukluft Nappe Complex, central Namibia

The Naukluft Nappe Complex (NNC) forms a far-traveled fold and thrust belt klippe of the Panafrican Damara Belt in central Namibia. Estimates of the SE directed displacement range between 50 and 80 km. The entire nappe stack was thrust along an out-of-sequence, nearly planar, horizontal structure, the "Naukluft Thrust." The thrust zone consists of several distinct lithological components whose typical distribution, when all present, from bottom to top is (1) a massive, ochre-yellow weathering dolomite; (2) a polymict "gritty dolomite" (called in the past "Sole Dolomite"), (3) strongly foliated and isoclinally folded calcmylonites, and (4) an upper massive dolomite. A very discrete (<50 mm thick, often <10 mm thick) planar brittle fault (component 5) can occur at any level within this sequence. Our investigations show that the gritty dolomite forms by progressive cataclasis of the massive dolomite (component 1). Moreover, clasts of gritty dolomite are observed randomly oriented within a similar gritty dolomite matrix, suggesting multiple pulses of brecciation and self-brecciation. The gritty dolomite locally forms injection veins into the calcmylonites, and these veins are themselves boudinaged, indicating broadly coeval cataclastic and ductile deformation. The evolution of structures within the thrust zone is linked to the presence and flow of overpressured pore fluids. Field observations suggest that several pulses of fluid-induced brittle deformation overprinted, in a cyclic fashion, ductile structures formed during the emplacement of the nappe edifice. A "fault valve" behavior is suggested for the basal detachment of the NNC, with bulk shortening being accommodated by incremental slip during a history of combined viscous and frictional flow. Copyright 2006 by the American Geophysical Union.