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The dating of shallow faults in the Earth's crust

Nature volume 412, pages 172–175 (2001)Cite this article

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 difficult1,2,3,4 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 ∼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 ∼172 Myr.

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Figure 1: Representative Ar age spectra of clays in samples at the Lewis thrust for three grain size populations.
Figure 2: The Illite Age Analysis (IAA) plot correlates the percentage detrital component and age of a sample.

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Acknowledgements

D. R. Pevear has retired from ExxonMobil Upstream Research Company. We thank D. R. Peacor for assistance and several Cordilleran geologists for discussion, and the National Science Foundation and ExxonMobil Upstream Research Company for support of our fault gouge research.

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Authors and Affiliations

  1. Department of Geological Sciences, University of Michigan, Ann Arbor, 48109, Michigan, USA

    Ben A. van der Pluijm & Chris M. Hall

  2. ExxonMobil Upstream Research Co., Houston, 77252-2189, Texas, USA

    Peter J. Vrolijk, David R. Pevear & Michael C. Covey

Authors
  1. Ben A. van der Pluijm
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  2. Chris M. Hall
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  3. Peter J. Vrolijk
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  5. Michael C. Covey
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Correspondence to Ben A. van der Pluijm.

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van der Pluijm, B., Hall, C., Vrolijk, P. et al. The dating of shallow faults in the Earth's crust. Nature 412, 172–175 (2001). https://doi.org/10.1038/35084053

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