Pressure solution seams in carbonatic fault rocks: mineralogy, micro/nanostructures and deformation mechanism

We have investigated mineralogy and micro/nanostructures of pressure solution seams in four different carbonatic faults with kilometric displacement, cropping out in the Northern Apennines, Italy. Disregarding the different protoliths and deformation conditions, the stylolite-filling material has almost constant mineralogical characteristics, being invariably formed by an ultrafine matrix that encloses relic insoluble grains, among which quartz, feldspars and detritic micas. The ultrafine matrix also hosts syn- and post-dynamic phases (e.g., foliation-parallel goethite flakes and apatite euhedral nanocrystals in random orientation). The ultrafine matrix is formed by smectitic clays in nanosized (001) lamellae, showing pervasive interlayer fissuring, layer bending and preferred orientation parallel to the slipping surface. Stylolite mineralogy and micro/nanostructures may affect deformation mechanisms and permeability properties of the fault rock. In particular, we propose that the extremely low friction coefficient of smectite would favour frictional sliding along the faults and that the fissured and oriented nanostructure of the smectite-dominated seams would enhance the sealing attitude of the structures in the fault-perpendicular direction.