Transformations in Clay-Rich Fault Rocks: Constraining Fault Zone Processes and the Kinematic Evolution of Regions.

Abstract

The apparent mechanical weakness of faults in the brittle regime (<300 °C) conflicts with theoretical and laboratory predictions that active faults should be strong and not slip at the low angles observed in the field. Clay-rich fault rocks have been suggested as a reason for this weak behavior, and clay mineralogy of brittle faults in a variety of tectonic environments is the central theme of this thesis. A systematic study of the mineralogy of fault rocks in the western US distinguishes three sets of mineral transformations: 1.) Detrital chlorite  chlorite-smectite and smectite; 2.) Detrital chlorite  Mg-rich assemblage of sepiolite, talc lizardite or palygorskite. 3.) Detrialt muscovite  authigenic 1Md illite, or detrital feldspar  authigenic 1Md illite. Fabric intensity measurements of these fault rocks demonstrate that clay fabrics are uniformly weak in comparison with other phyllosilicate-rich rocks. Fabric studies of experimentally-produced gouges indicate that fabric intensity is sensitive to shear strain and normal stress, yet is largely independent of total clay mineral content. The observation that 1Md illite growth in gouges is common motivated the development of a robust fault gouge dating technique for these rocks. Study of the Sierra Mazatàn metamorphic core complex demonstrates the reliability of the clay dating method that quantifies the relative abundances of illite polytypes and dating by the Ar-Ar method. The ages of both authigenic and detrital components are shown to be fully consistent with geologic constraints. Application of 1Md illite dating in gouges from the Spanish Pyrenees demonstrates ‘pulses’ of deformation during the Paleogene and lateral variation in the timing and style of deformation. Gouge ages from the Ruby Mountains, Nevada, show that low- and high-angle normal faults were active coevally, requiring that this detachment system was active at dips <30°. Gouges from the Rwenzori Mountains of Uganda formed in the top 2 km of the crust, where significant mineral transformations did not occur. The results of these studies address the controversial issues of fault strength and orientation, produce a new method to date brittle gouge with potentially wide application, and constrain the spatial-temporal window of clay transformations in fault gouge.