CLAYS AND FAULT CREEP IN THE SAN ANDREAS FAULT

Along today’s San Andreas Fault, several fault segments experience creep behavior, which is variably attributed to factors including (i) low values of normal stress, (ii) elevated pore-fluid pressure, and (iii) low frictional strength. The San Andreas Fault Observatory at Depth (SAFOD) drillhole in Parkfield, California provides new evidence for the role of frictional properties on fault strength, as demonstrated by our recent mineralogical work on mudrock samples from fault zones at ~3066 m, ~3194 m and ~3296 m measured depths. X-ray diffraction (XRD) results show illite, illite-smectite (I-S) and chlorite minerals dominating at 3186.7 m to 3196.3 m, and 3294.9 m to 3297 m measured depth, whereas abundant chlorite-smectite (C-S) and corrensite (50:50 C-S) minerals are mostly restricted to well-defined intervals in the center of the two fault strands between 3196.3 m to 3198.1 m, and 3297.5 to ~3305 m. Importantly, the fault zone rocks investigated are abundantly coated by polished thin-films with occasional striations. Transmission electron microscopy (TEM) and XRD studies of these surface coatings reveal the occurrence of neocrystallized thin-films containing both illite-smectite (I-S) and chlorite-smectite (C-S) phases. We propose that the majority of fault creep is controlled by the high density of these thin (~ 100 nm thick) nanocoatings on fracture surfaces, which are sufficiently interconnected to allow slip with minimal breakage of stronger matrix clasts. Displacements are accommodated by localized frictional slip along coated particle surfaces and hydrated smectite phases, in combination with intracrystalline deformation of the clay mineral lattice, associated with extensive crystal dissolution, mass transfer and growth of expandable layers. The localized concentration of smectite in C-S minerals likely extends to greater depths (< 10 km), with cataclasis and fluid infiltration creating nucleation sites for neomineralization on displacement surfaces in the San Andreas Fault. We suggest that the localization of smectitic clay minerals promotes today’s creep behavior in contrasts to previously proposed scenarios of reworked talc/serpentine phases as an explanation for weak faults and creep behavior.