Analogue experiments of normal fault formation in multi-layers of alternating strength

 As normal faults accumulate displacement, smearing of weaker fine-grained materials, such as clays, along their fault plane can reduce fault permeability and thus affect fluid flow in subsurface reservoirs, making clay smear development relevant for groundwater, geothermal and CO₂ storage applications. Here we use analogue experiments to investigate the potential of smearing of weaker layers along fault planes in a multi-layer sequence of granular materials.  

The natural prototype is the interbedded limestone and marl sedimentary units of the Malm formation in a quarry in southern Germany. The normal faults in the quarry have small offset (usually < 50 cm) and dip between 40° – 65° predominantly trending NE – SW. We observe discontinuous marl smearing along the fault planes, which are surrounded by deformation zones with a dense tensile fracture population. Average limestone and marl bed thicknesses on both footwall and hanging wall is 32 cm and 4.5 cm, and 33 cm and 2.5 cm respectively.

Our analogue experiments are scaled to represent layers at quarry scale. We tested several sand and gypsum plaster mixtures using empirical and ring shear methods to find cohesive strength contrasts suitable for simulating the limestone-marl sequences. The material tests show that with increasing plaster content and confining pressure, cohesion increases, while the angle of internal friction shows a non-linear behaviour for plaster/sand mixtures. We here use sand for marl layers and gypsum for limestone. We sieve the materials in a 50 x 30 cm box of which half the base plate can drop down along a prescribed angle. We analyse deformation from 2D-timelapse and 3D-CT image data, using PIV and image analysis.

Models with sand (marl) layers within gypsum (limestone) without overburden show numerous mode I fractures at the free surface with localized fault planes. Shear zones are steep with dip angles in the range of 66° - 84°. Models with overburden form shear zones with dips ranging from 65° - 83°, forming less mode I fractures, but instead mainly shear fractures that cut across each cohesive layer. Sand smearing is observed to vary in models without overburden, while it is a consistent component of the fault zones at depth in models with overburden. We find that the quantity of sand smear is a function of the thickness of the embedded sand layers. The sand pours into large openings formed between cohesive gypsum powders with simultaneous mixing of the materials during fault displacement. This process causes an accumulation of sheared granular materials along the fault zone and in turn expands the shear zone width.

The experiments with overburden show steep dipping fragmented fault zones, as well as the formation of tensile fractures that form in, and cut through cohesive beds, similar to what is observed in the quarry. Sand smearing processes of rolling and mixing in dilatant portions during displacement is however more brittle in nature than ductile smearing observed in the quarry.