The dating of shallow faults in the Earth's crust

Abstract

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.

Post-Caledonian brittle deformation in the Bergen area, West Norway: results from K–Ar illite fault gouge dating

“…The illite crystallinity is a measure of crystallite thickness, but is also influenced by structural distortion, e.g., dependent age spectra (e.g., Pluijm et al, 2001;Solum et al, 2005;Löbens et al, 2011;Bense et al, 2014;Torgersen et al, 2015) that have been explained by two main processes:…”

“…Therefore, the K-Ar ages are bulk ages for the dated fine fractions, meaning that any K-bearing mineral present in the fine fractions will contribute to the age. Two main sources of 'contami nati on' must be considered: (1) Finely crushed host-rock minerals: They can be inherited from both sedimentary and crystalline host rocks (e.g., Vrolijk & van der Pluijm, 1999;Pluijm et al, 2001;Zwingmann et al, 2010b;Torgersen et al, 2014). The finest fractions are generally less affected by, or even free from this type of contamination.…”

Geochronology: linking the isotopic record with petrology and textures — an introduction

“…Ages of both clay 'end-member' components can be determined due to differences in thermally activated Ar release between authigenic and detrital clay minerals, as revealed by laser-stepped heating (Onstott et al 1997). Dating combined with detailed X-ray diffi'action analysis of several grain size fractions of clays in fault gouges allows for the extrapolation of ages of clay mixtures to both pure authigenic and pure detrital 'end-member' ages (van der Pluijm et al 2001). While considerable complexities exist in the processes governing the authigenic growth of mica, such as Ostwald ripening (Eberl et al 1990;Kfibler & Jaboyedoff 2000) and the reaction kinetics of mineral homogenization (Livi et al 2002), these new techniques offer exciting prospects for the complicated dating of fault gouges and our understanding of brittle faults.…”

“…In order to improve our understanding of the evolution of (semi-)brittle fault rocks like fault gouges or pseudotachylytes, both in-situ and stepwise-heating (or even better their combined use) 4~ dating have recently been applied (Mfiller et al 2002;van der Pluijm et al 2001). Fault gouges, which predominantly consist of fine-grained4clay 3 minerals, pose enormous difficulties for Ar/-9Ar dating.…”

Influence of deformation and fluids on Ar retention in white mica: Dating the Dover Fault, Newfoundland Appalachians

“…Dating of fine grained minerals (< 2 µm) such as illite can provide constraints on timing of active retrograde fault movement in the shallow crust (van der Pluijm et al, 2001;Zwingmann and Mancktelow, 2004;Zwingmann et al, 2010, Pleuger et al, 2012. K-Ar dating is a reliable tool to constrain authigenic formation of fined-grain illite clay minerals in brittle faults to circumvent 39 Ar recoil by 40 Ar/ 39 Ar dating (Clauer et al, 2012, Torgersen et al, 2015.…”