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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

International audienceA 1.6 km riser borehole was drilled at site C0009 of the NanTroSEIZE, in the center of the Kumano forearc basin, as a landward extension of previous drilling in the southwest Japan Nankai subduction zone. We determined principal horizontal stress orientations from analyses of borehole breakouts and drilling-induced tensile fractures by using wireline logging formation microresistivity images and caliper data. The maximum horizontal stress orientation at C0009 is approximately parallel to the convergence vector between the Philippine Sea plate and Japan, showing a slight difference with the stress orientation which is perpendicular to the plate boundary at previous NanTroSEIZE sites C0001, C0004 and C0006 but orthogonal to the stress orientation at site C0002, which is also in the Kumano forearc basin. These data show that horizontal stress orientations are not uniform in the forearc basin within the surveyed depth range and suggest that oblique plate motion is being partitioned into strike-slip and thrusting. In addition, the stress orientations at site C0009 rotate clockwise from basin sediments into the underlying accretionary prism

During the second phase of the Alpine Fault, Deep Fault Drilling Project (DFDP) in the Whataroa River, South Westland, New Zealand, bedrock was encountered in the DFDP-2B borehole from 238.5-893.2 m Measured Depth (MD). Continuous sampling and meso-to microscale characterization of whole rock cuttings established that, in sequence, the borehole sampled amphibolite facies, Torlesse Composite Terrane-derived schists, protomylonites, and mylonites, terminating 200-400 m above an Alpine Fault Principal Slip Zone (PSZ) with a maximum dip of 62°. The most diagnostic structural features of increasing PSZ proximity were the occurrence of shear bands and reduction in mean quartz grain sizes. A change in composition to greater mica:quartz+feldspar, most markedly below ~ 700 m MD, is inferred to result from either heterogeneous sampling or a change in lithology related to alteration. Major oxide variations suggest the fault-proximal Alpine Fault alteration zone, as previously defined in DFDP-1 core, was not sampled.