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Isotopic studies of Canadian Rocky Mountain thrust faults preserve the timing and identity of orogenic fluids and their fault zone pathways. Using previously dated samples, we measure the O-and H-isotopic compositions of fault gouge. These nearly 100% neomineralized gouges and their associated damage zones act as primary orogenic fluid pathways. As such, they provide a specific and local look into the nature of the Late Jurassic to Early Eocene orogenic plumbing system in the Alberta Rockies. Considering clay polytype stability and regional temperature conditions, we obtain a range of geofluid isotopic compositions during Jurassic-Eocene thrust faulting: δ18Ofluid ranged from ~-3.3 to 9.2 ± 3.2?‰ δDfluid ranged from -119 to -46 ± 13‰ VSMOW. The range of O- and H-isotopic compositions reflects mixing of fluid sources, including the pervasive presence of surface-sourced fluids (up to ~90%). The interpreted prevalence of a surface fluid source in fault rocks is in agreement with regional isotopic trends previously observed in undated veins of fractured host rock. Our results confirm that thrust faults of the Alberta Rocky Mountains acted as major fluid-focusing conduits during orogenic activity. We further show that these faults incorporated both deeply sourced and surface-sourced fluids into zones of enhanced and dynamic permeability, heterogeneously distributing fluids along fault planes across the fold-thrust belt, promoting the growth of fault-zone weakening clay minerals. [ABSTRACT FROM AUTHOR]

Laser-ablation 40Ar/39Ar step-heating analyses of 20 pseudotachylyte veins from a single location along the exhumed central portion of the active Alpine fault of New Zealand yield total gas age values between 1 and 19 Ma. Evidence shows that they are genetically related and were formed during coeval episodes of seismogenic melting at shallow crustal depth, contrasting with a spread in formation ages. The total gas ages show an exponential decrease with increasing proportion of melt matrix and K content, reflecting incomplete degassing and mixtures of radiogenic Ar sources. Calculation of intercepts for all-melted matrix and all-clast end-member components indicate ca. 570 ka (Quaternary) friction-melting ages of ca. 332 Ma (Lower Carboniferous) source rock. Assuming an average exhumation rate of 6–9 mm/yr for uplift and erosion, these results imply that friction melts were generated during major slip episodes at ∼3.5–5 km crustal depth. We conclude that reliable dating of young pseudotachylyte can be accomplished by combining chronologic study with clast-matrix quantification of genetically related vein assemblages. [ABSTRACT FROM AUTHOR]

The sensitivity of smectite during brief and protracted heating intervals can provide crucial information about the temperature history of faults during seismogenic slip and creep. Pelagic-sourced smectite is the most abundant clay mineral that is incorporated into the slip zone that was drilled during the Japan Trench Fast Drilling Project (JFAST) Expedition 343 in the Japan Trench, located ~820 m below seafloor. This study investigates the potential for abundant smectite to preserve a record of coseismic frictional heating associated with the Tohoku earthquake in May 2011 by laboratory examination of mineral hydration and dehydration in JFAST drill core samples during brief (maximum of 5 min) and protracted (5 h) heating sequences. Using a real-time heating stage that is connected to an X-ray diffractometer, we observe that (1) both brief and protracted heating causes reduction of water interlayers in smectite, (2) smectite recovers faster to the original hydration state after quick heating than long heating, and (3) nonrecoverable collapse of all smectite occurs at >200 °C for brief and protracted heating rates. Because hydrated, smectite clays are widely present in the fault rocks, we conclude that frictional heating within the slip zone of the Tohoku fault zone cannot have reached significantly elevated temperatures. [ABSTRACT FROM AUTHOR]

Human activity causes vibrations that propagate into the ground as high-frequency seismic waves. Measures to mitigate the coronavirus disease 2019 (COVID-19) pandemic caused widespread changes in human activity, leading to a months-long reduction in seismic noise of up to 50%. The 2020 seismic noise quiet period is the longest and most prominent global anthropogenic seismic noise reduction on record. Although the reduction is strongest at surface seismometers in populated areas, this seismic quiescence extends for many kilometers radially and hundreds of meters in depth. This quiet period provides an opportunity to detect subtle signals from subsurface seismic sources that would have been concealed in noisier times and to benchmark sources of anthropogenic noise. A strong correlation between seismic noise and independent measurements of human mobility suggests that seismology provides an absolute, real-time estimate of human activities., (Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)

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.