Overprinting faulting mechanisms during the development of multiple fault sets in sandstone, Chimney Rock fault array, Utah, USA Nicholas C. DAVATZES, Atilla AYDIN and Peter EICHHUBL Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115 Abstract. The deformation mechanisms producing the Chimney Rock normal fault array (San Rafael Swell, Utah, USA) are identified from detailed analyses of the structural components of the faults and their architecture. Faults in this area occur in four sets with oppositely dipping fault pairs striking ENE and WNW. The ENE-striking faults initially developed by formation of deformation bands and associated slip surfaces (deformation mechanism 1). After deformation band formation ceased, three sets of regional joints developed. The oldest two sets of the regional joints, including the most prominent WNW-striking set, were sheared. Localized deformation due to shearing of the WNW-striking regional joints formed WNW-striking map-scale normal faults. The formation mechanism of these faults can be characterized by the shearing of joints that produces splay joints, breccia, and eventually a core of fault rock (deformation mechanism 2). During this second phase of faulting, the ENE-striking faults were reactivated by shear across the slip surfaces and shearing of ENE-striking joints, producing localized splay joints and breccia (similar to deformation mechanism 2) superimposed onto a dense zone of deformation bands from the first phase. We found that new structural components are added to a fault zone as a function of increasing offset for both deformation mechanisms. Conversely, we estimated the magnitude of slip partitioned by the two mechanisms using the fault architecture and the component structures. Our analyses demonstrate that faults in a single rock type and location, with similar length and offset, but forming at different times and under different loading conditions, can have fundamentally different fault architecture. The impact by each mechanism on petrophysical properties of the fault is different. Deformation mechanism 1 produces deformations bands that can act as fluid baffles, whereas deformation mechanism 2 results in networks of joints and breccia that can act as preferred fluid conduits. Consequently, a detailed analysis of fault architecture is essential for establishing an accurate tectonic history, deformation path, and hydraulic properties of a faulted terrain.