Clay-smear continuity and normal fault zone geometry - First results from excavated sandbox models

摘要

The continuity of clay-rich fault gouge has a large effect on fluid transmissibility of faults in sand-clay sequences, but clay gouge continuity and composition in 3D are not well known. We report observations of 3D clay smear continuity in water-saturated sandbox experiments where the sheared clay layers were excavated after deformation. The experiments build on existing work on the evolution of clay gouge in similar 2D experiments where interpretations were made in profile view. We used well-known model materials ("Benchmark" sand and uncemented kaolinite-sand mixtures) that were further characterized using standardized geotechnical tests and triaxial compression experiments at effective pressures corresponding to the sandbox experiments. Results show a nonlinear failure envelope of the sand, in agreement with existing models. Unconfined compression experiments with the clay show cohesion around 50 Pa and brittle behavior. A sheared, ductile clay layer embedded in sand above a 70° dipping basement fault reveals a complex, natural-looking clay gouge architecture with relay ramps, breached relays and fault lenses. The clay gouge shows clear variations in composition and thickness and becomes locally discontinuous at throw-thickness ratios above 7, in contrast to our earlier 2D observations where discontinuous clay-gouge only formed in cemented clay layers. In addition to tectonic telescoping in the relays, the thin, continuous parts of the clay gouge were transformed from an initial pure clay by mechanical mixing of sand and clay. We also discuss the applicability of these results to the evolution of normal fault zones and deformation bands in sand-clay sequences at effective pressures below the onset of cataclasis and conclude that in fault zones a higher degree of internal segmentation reduces the probability of the formation of discontinuities.