Nanocoatings of clay and creep of the San Andreas fault at Parkfield, California

Abstract

Mudrock samples were investigated from two fault zones at ~3066 m and ~3296 m measured depth (MD) located outside and within the main damage zone of the San Andreas Fault Observatory at Depth (SAFOD) drillhole at Parkfi eld, California. All studied fault rocks show features typical of those reported across creep zones with variably spaced and interconnected networks of polished displacement surfaces coated by abundant polished fi lms and occasional striations. Electron microscopy and X-ray diffraction study of the surfaces reveal the occurrence of neocrystallized thin fi lm clay coatings containing illite-smectite (I-S) and chlorite-smectite (C-S) minerals. 40 Ar/ 39 Ar dating of the illitic mix-layered coatings demonstrated Miocene to Pliocene crystallization and revealed an older fault strand (8 ± 1.3 Ma) at 3066 m MD, and a probably younger fault strand (4 ± 4.9 Ma) at 3296 m MD. Today, the younger strand is the site of active creep behavior, refl ecting a possible (re)activation of these clayweakened zones. We propose that the majority of slow fault creep is controlled by the high density of thin (<100 nm thick) nanocoatings on fracture surfaces, which are suffi ciently smectite-rich and interconnected at low angles to accommodate slip with minimal breakage of stronger matrix clasts. Displacements occur by frictional slip along particle surfaces and hydrated smectitic phases, in combination with intracrystalline deformation of the clay lattice, associated with extensive mineral dissolution, mass transfer, and residual precipitation of expandable layers. The localized concentration of smectite in both I-S and C-S minerals contributes to fault weakening, with fracturing and fl uid infi ltration creating new nucleation sites for neomineralization on displacement surfaces during continued faulting. The role of newly grown, ultrathin, hydrous clay coatings contrasts with previously proposed scenarios of reworked talc and/or serpentine phases as an explanation for weak fault and creep behavior at these depths. GEOLOGICAL SETTING AND SAMPLESThe SAFOD drillhole is located along the creeping section of the San Andreas fault in central California (Fig. 1A). Northwest of the drillhole, the fault has a creep rate of 2.5-3.9 cm/yr (Titus et al., 2006); microearthquakes (Mw 0-2.0) are detected at shallow depths of 2-3 km (Nadeau et al., 2004). Drilling in summer 2005 successfully crossed the active trace of the San Andreas fault at ~3300 m MD with a measured temperature of ~112 °C

Frictional properties of the active San Andreas Fault at SAFOD: Implications for fault strength and slip behavior

“…Experiments on cuttings from the CDZ document a low coefficient of friction (μ < 0.25), low (near-zero) rates of frictional healing, and strong localization of these mechanical properties to the creeping gouge zone [Carpenter et al, 2011]. These behaviors are correlated with the presence of magnesium-rich clays, suggesting that clay mineralogy is an important control on the strength and healing behavior of the fault, at least at these depths [e.g., Schleicher et al, 2010;Bradbury et al, 2011;Holdsworth et al, 2011;Hadizadeh et al, 2012;Richard et al, 2014].…”

“…Wall rock to the NE of the CDZ is composed of highly sheared siltstones and mudstones, containing abundant quartz, feldspar, and calcite, with overall clay content decreasing with distance from the fault (Table S1). Examination of core containing the CDZ indicates the presence of both Mg-rich clay (saponite) coatings [Schleicher et al, 2010] and thick, Mg-rich clay zones throughout the active fault [Holdsworth et al, 2011].…”

Faulting processes in active faults – Evidences from TCDP and SAFOD drill core samples

“…Many previous studies suggest that the growth of Mg-rich phyllosilicates may play a key role in the mechanical behavior of the SAF and can be directly related to fault weakening (e.g. Rymer, 2007, 2012;Schleicher et al, 2010;Holdsworth et al, 2011;Bradbury et al, 2011;Lockner et al, 2011). Janssen et al (2011) suggest that the presence of abundant clay minerals is often related to the occurrence of nanoscale porosity that may indicates high fluid pressure.…”

A large mantle water source for the northern San Andreas fault system: a ghost of subduction past

“…Second, secondary hydrous minerals were observed in the drill core that penetrated the active segment of the fault, indicating that aqueous fluids were present in the zone at some time in the recent past. Some hydrous minerals, such as talc (Moore and Rymer 2007) and clay minerals (Schleicher et al 2006;Schleicher et al 2010) are weak shear-zone materials and may be responsible for fault creep behavior. Finally, our model does not preclude the possibility that fluid flow through the crust from the shrinking serpentinite mantle wedge could be episodic as affected by tectonic and seismic activities and that our estimates of long-term average water discharge rates do not reflect the possibility of transient fluid transport.…”