Quantification of fabrics in clay gouge from the Carboneras fault, Spain and implications for fault behavior

Clays in fault rocks have the potential to control fault behavior. The formation of frictionally-weak clays in fault rocks can lead to fault zone weakening, and the development of fabrics through authigenesis and mechanical rotation in clay-rich fault rocks can influence fault-zone permeability structures. The left lateral Carboneras Fault of southwestern Spain is a ca. 1 km wide fault zone with anastomosing layers of clay-rich gouge up to ~ 50 m thick. The gouge, derived largely from mica schist, is composed of neoformed illite and chlorite. Anisotropy of magnetic susceptibility (201 measurements) and X-ray pole figures (144 measurements) show comparable oblate clay fabrics, with the axis of maximum anisotropy sub-parallel to strike. This dimensional fabric contrasts with previously published maximum permeability results that are sub-vertical. This observation, coupled with the generally poor clay fabric measured in the samples indicates that the orientation of clays is not likely to be the primary control of the permeability anisotropy of clay-rich fault gouge, becoming important only when gouge zones are 10s of meters thick, which is very unusual. Even in a fault zone like that of the Carboneras fault, with thick, very clay-rich gouge zones with a visible foliations and slip surfaces, the degree of clay preferred orientation is low above the millimeter scale, showing that any strong clay fabric associated with the foliation are extremely localized phenomena. This suggests that processes operating below the millimeter scale may significantly influence fault behavior, a result compatible with earlier characterizations of faults in a variety of geologic settings, highlighting the importance of understanding how the formation and fabric of fault-related authigenic clays influence fault behavior.