FABRICS AND AGE OF CLAY GOUGE IN THE MOAB FAULT, UTAH, USA

The Moab normal fault is located in east-central Utah, and has accommodated displacement along a gouge zone that is 1-2 m wide at the study site. The gouge is bound by an ~3m wide zone of cataclastically deformed and altered protolith. Analyses of X-ray powder diffraction patterns indicate significant change in clay mineral assemblages, with gouge enriched in illite and kaolinite relative to protolith, and altered protolith enriched in illite/smectite relative to undeformed protolith and gouge. The degree of phyllosilicate preferred orientation in these fault rocks, measured using X-ray texture goniometry, is low, decreasing >65% from that in undeformed protolith. Thus fluids were likely restricted to the gouge zone from adjacent protolith not by a clay fabric, but by lower-permeability illite/smectite-bearing rocks adjacent to the gouge. This permeability contrast provides a mechanism by which high-pressure fluids can be focused in the fault zone, and offers a mechanism for elevated fluid pressure and fault weakening. Transmission electron microscopy shows that 1Md polytype illite formed from alteration of preexisting detrital 2M1 muscovite and illite. The concentration of discrete illite is 80-90% in both protolith and gouge, but the amount of detrital illite relative to authigenic illite, represented by the proportion of 2M1, decreases from ~60% in protolith to a few percent in the gouge zone. This implies that gouge is composed of >50% neocrystallized illite. Preliminary illite age analysis using 40Ar/39Ar dating of size fractions of gouge samples yields a Tertiary age for the gouge fabric, which agrees with earlier K-Ar illite age analyses of ~50-60 Ma from a location to the northwest where different lithologies are juxtaposed. Whereas a Mesozoic age for Moab faulting associated with salt tectonics is well established from field relationships, this new age result shows that fabric development and reactivation along the Moab fault occurred as late as Tertiary.