Direct dating of ductile shear zones and calculation of uplift/exhumation rates can be done using various radiometric dating techniques. But radiometric dating of shallow crustal faulting, which occurs in the crust's brittle regime, has remained difficult because the low temperatures typical of shallow crusted faults prevent the complete syntectonic mineral recrystallization that occurs in deeper faults. Both old (detrital) and newly grown (authigenic) fine-grained phyllosilicates are thus preserved in shallow fault zones and therefore their radiometric ages reflect a mixture of both mineral populations. Also, the loss of 39Ar during neutron irradiation in dating of clay minerals can produce erroneously old ages. Here we present a method of characterizing the clay populations in fault gouge, using X-ray modelling, combined with sample encapsulation, and show how it can be used to date near-surface fault activity reliably. We examine fault gouge from the Lewis thrust of the southern Canadian Rockies, which we determine to be approximately 52 Myr old. This result requires the western North America stress regime to have changed from contraction to extension in only a few million years during the Eocene. We also estimate the uplift/exhumation age and sedimentary source of these rocks to be approximately 172 Myr.
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
Based on lithological, structural and geophysical characteristics, the Proterozoic Grenville Orogen of southern Ontario and New York has been divided into domains that are separated from each other by ductile shear zones. In order to constrain the timing of metamorphism, U-Pb ages were determined on metamorphic and igneous sphenes from marbles, calc-silicate gneisses, amphibolites, granitoids, skarns and pegmatites. In addition, U-Pb ages were obtained for monazites from metapelites and for a rutile from an amphibolite. These mineral ages constrain the timing of mineral growth, the duration of metamorphism and the cooling history of the different domains that make up the southern part of the exposed Grenville Orogen. Based on the ages from metamorphic minerals, regional and contact metamorphism occurred in the following intervals:
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