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Short-lived orogenic cycles and the eclogitization of cold crust by spasmodic hot fluids


Collision tectonics and the associated transformation of continental crust to high-pressure rocks (eclogites) are generally well-understood processes, but important contradictions remain between tectonothermal models and petrological–isotopic data obtained from such rocks. Here we use 40Ar–39Ar data coupled with a thermal model to constrain the time-integrated duration of an orogenic cycle (the burial and exhumation of a particular segment of the crust) to be less than 13 Myr. We also determine the total duration of associated metamorphic events to be 20 kyr, and of individual heat pulses experienced by the rocks to be as short as 10 years. Such short timescales are indicative of rapid tectonic processes associated with catastrophic deformation events (earthquakes). Such events triggered transient heat advection by hot fluid along deformation (shear) zones, which cut relatively cool and dry subducted crust. In contrast to current thermal models that assume thermal equilibrium and invoke high ambient temperatures in the thickened crust, our non-steady-state cold-crust model satisfactorily explains several otherwise contradictory geological observations.

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We especially thank H. Austrheim for all of his help in the field, hospitality, and discussions about the outcrop; M. Villeneuve for use of the ultraviolet-laser argon facility at the Geological Survey of Canada in Ottawa; and in particular S. Smith for technical assistance. In addition, M. Lund helped collect some samples and supplied Fig. 1, and S. Kelley and A. Perchuk provided comments on the manuscript. Comments by H. Austrheim, D. M. Carmichael, A. Clark, L. Godin, I. Parsons, C. Thompson, M. Villeneuve, S. M. Rigden and H. M. Klaschka on earlier versions of this paper are also acknowledged. This research was supported by the Natural Sciences and Engineering Research Council of Canada and the Australian Research Council.

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Correspondence to James K. W. Lee.

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Supplementary information

Supplementary Table S1

Representative electron microprobe analyses of amphiboles from sample Alv6 (XLS 20 kb)

Supplementary Table S2

40Ar-39Ar data for amphibole (XLS 31 kb)

Supplementary Table S3

Laser step heating 40Ar-39Ar analyses for phlogopite (XLS 41 kb)

Supplementary Table S4

Ultraviolet-laser 40Ar-39Ar analyses for phlogopite (XLS 25 kb)

Supplementary Table S5

40Ar-39Ar total fusion data for pyroxenes, garnet and olivine (XLS 24 kb)

Supplementary Table S6

Approximate 40Ar concentrations of various minerals separated from two peridotite lenses surrounded by eclogite (XLS 18 kb)

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Further reading

Figure 1: Geological map of Holsnøy island, northwest of Bergen, western Norway.
Figure 2: 40 Ar– 39 Ar release spectra for two step-heated amphibole aliquots of different grain size from the Alvfjellet lens (sample Alv6) on Holsnøy island.
Figure 3: 40 Ar– 39 Ar release spectra for step-heated phlogopite from the Alvfjellet (Alv6; Alv7) and Hundskjeften (Hunds14) lenses on Holsnøy island.
Figure 4: Cumulative probability diagram of integrated 40 Ar– 39 Ar ages for phlogopite from the Alvfjellet lens.
Figure 5: Apparent 40 Ar– 39 Ar age versus distance profiles across phlogopite from Alv6, Alv7 and Hunds14.
Figure 6: Relationship between time and temperature required for a 300-µm-diameter amphibole grain and a 1,500-µm-diameter phlogopite grain to incorporate 3.97 vol.% and 20 vol.% 40 Ar E , respectively.
Figure 7: Modelled pressure–temperature–time path for the Lindås nappe, Bergen arcs.


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