Formation of Authigenic and Detrital Clay Gouges –Examples from the US Cordillera and Uganda's Rwenzori Mountains, East African Rift

The recognition that authigenic clay growth in fault zones is a fundamental process in shallow crustal rocks is relatively recent. To assess the extent of clay mineral transformations in gouges from a variety of crustal depths, we conducted a systematic XRD study of clay gouges from 30 exposures of 17 low-angle normal faults (LANFs) in the American southwest and 10 brittle faults in the Rwenzori Mountains horst block of the East African Rift.

In the LANF gouges, extensive clay mineral transformations occurred in one or more clay horizons in 29 of the 30 faults sampled. Only 1 LANF site consisted entirely of detrital gouge that was formed by cataclasis without neoformation of clay. Three categories of LANF gouge transformations are observed: 1.) ‘Retrograde diagenesis' transformations of an early, detrital chlorite-rich gouge to authigenic chlorite-smectite and/or Mg-smectite. 2.) Reaction of detrital chlorite-rich gouges with Mg-rich fluids at 50-150 °C and late in the deformation history to produce localized lenses of one of two assemblages: sepiolite + saponite + talc + lizardite or palygorskite +/- chlorite +/- quartz. 3.) Growth of authigenic 1Md illite, either by transformation of detrital 2M1 illite (or muscovite), or growth of authigenic illite by dissolution of K-feldspar. Illitization of detrital illite-smectite is also observed, but is uncommon.

The Rwenzori Mountains gouges, by contrast, show little evidence for authigenic clay growth. These exhumed gouges consist entirely of reworked material that is mineralogically indistinguishable from wall-rock, occasionally with some authigenic smectite, and formed in the upper few km of crust.

The combined observations of mineral transformations indicate that authigenic-dominated clay gouges require fluid fluxes and geologic temperatures of at least 60-70 °C, whereas lower temperatures produce only gouges that are cataclasis-dominated. These conditions place limits on the formation depths of authigenic clay-rich gouges and their mechanical role in faulting.