How clays affect fault strength and slip behavior: Lessons from SAFOD

The strength and slip behavior of upper crustal faults has been attributed to (i) values of normal stress, (ii) pore-fluid pressure, and (iii) frictional properties. Direct observations on natural fault rocks provide compelling evidence for the role of localized neomineralization, as demonstrated by our work on samples from the San Andreas Fault Observatory at Depth (SAFOD) drillhole at Parkfield, California. Mudrock samples from fault zones at ~3066 m and ~3296 m measured depth (MD) show variably spaced and interconnected networks of displacement surfaces that consist of host rock particles that are abundantly coated by polished films with occasional striations. Transmission electron microscopy and X-ray diffraction study of the surfaces reveal the occurrence of neocrystallized thin-film clay coatings containing illite-smectite (I-S) and chlorite-smectite (C-S) phases. X-ray texture goniometry shows that the clay crystallographic fabric of these faults rocks is characteristically low, in spite of an abundance of clay phases. 40Ar/39Ar dating of the illitic coatings demonstrate recent crystallization and reveals the initiation of an “older” fault strand (~8 Ma) at 3066 m MD, and a “younger” fault strand (~4 Ma) at 3296 m MD. Today, the younger strand is the site of active creep behavior, reflecting continued activation of clay-weakened zones. We propose that fault creep is controlled by the localization of thin (< 100nm thick) nanocoatings on fracture surfaces that are sufficiently smectite-rich and interconnected to allow slip with minimal breakage of stronger matrix clasts. Displacements are accommodated by frictional slip along coated particle surfaces, in combination with intracrystalline deformation of the mineral lattice, resulting from crystal dissolution, mass transfer and growth of expandable clays. The highly localized concentration of both I-S and C-S minerals does not require volumetrically large mass transfer. A scenario is proposed where initial cataclasis (seismic behavior) and associated fluid infiltration create nucleation sites for neomineralization on displacement surfaces, which eventually creates a sufficiently connected framework that dominates subsequent fault slip behavior (creep). The role of newly grown, ultrathin, hydrous clay coatings on displacement surfaces in the San Andreas Fault emphasizes the importance of clay neomineralization in fault zones and contrasts with scenarios of reworked talc/serpentine phases as an explanation for weak faults and creep behavior at these depths. The occurrence of C-S phases to temperatures above the stability of I-S indicates that smectitic clay phases can affect fault behavior to depths well below the SAFOD borehole penetration, possibly to the base of the brittle zone.