Clay neomineralization in fault zones: Extracting information on fault properties and timing.

Abstract

The ability to distinguish fault-related clays from non-fault-related clays in fault zones offers important insights into the influence of clays on fault behavior. The ability to quantitatively characterize and date clay fabrics in fault rocks, and comparison of those fabrics with laboratory data, permit inferences about the ways in which clay-controlled fabrics may or may not influence faulting. Many faults appear to be weaker than predicted from laboratory experiments, and clays, due to their low strength and role in permeability structure, offer a possible explanation for this behavior. This study documents fault-related clay neomineralization, which can be distinguished from pre-faulting and post-faulting growth by mineralogical characterization. In exhumed fault zones, post-faulting alteration may be significant, such as observed in the Punchbowl Fault. Phyllosilicates from the minimally deformed rocks of the SAFOD Pilot Hole provide complementary results to establish a base line for recognizing fault-related phyllosilicate growth and provide a bridge between studies of exhumed fault zones and fault zones at depth. The identification of fault-related neomineralization indicates that the formation of clay gouge in faults is not strictly a mechanical process; rather, fault-related crystallization plays a central role, as shown in the Moab Fault. Clay fabrics are weakly developed in fault zones, indicating that the permeability structure of fault zones is not significantly controlled by preferentially oriented clays. This is observed by comparisons of X-ray and magnetic anisotropy fabrics from the Carboneras Fault with laboratory permeability measurements. Variations in the types of fault-related, neoformed clays may modify the permeability structure of fault zones, allowing the core of the fault to be isolated from the wall rock, creating fluid pathways that are restricted to the fault zone. The ability to quantify detrital and fault-related illite in fault rocks also provides the means to directly date the timing of its growth in these rocks. Using Ar encapsulation dating, radiometric ages show that fault fabrics are ancient, such as in the Moab Fault, and that ages of neoformed illite in fault rocks of the Sevier foreland of the US Rockies record periods of widespread fault activity.