Variations in Alpine white mica 40Ar/39Ar dates from the cover units of the Siviez-Mischabel Nappe relate to regional variations in the thermal history of the nappe. We focus on three regions within the nappe: the central Siviez-Mischabel (CSM), the southern Siviez-Mischabel (SSM), and the eastern Siviez-Mischabel (ESM). Our approach weaves together observations of quartz and mica textures in thin section, the variation of 40Ar/39Ar date with grain size, considerations of the effective diffusion dimension (EDD) of argon in white mica, and a comparison of dates with diffusion model results. In the CSM, pressure solution of quartz and dislocation glide in mica accommodated Alpine deformation. Dates record mica growth during nappe emplacement from 40 to 36 Ma and do not vary with grain size. In the SSM and ESM, both mica and quartz show textures associated with dynamic recrystallization, and dates decrease with grain size. In the SSM, dates also agree with the timing of nappe emplacement, but in the ESM, dates significantly post-date the timing of nappe emplacement. A comparison of dates with diffusion model results supports inferences from rock fabrics that the SSM experienced higher peak temperatures than the CSM, even though dates from both units approximate the timing of mica growth. Dates obtained from the ESM, however, do not compare well with simple models, and the thermal evolution of this region of the nappe, in the neighborhood of the Simplon Fault Zone, is not well understood.
Bibliographical noteFunding Information:
Thanks to John Wheeler for use of DIFFARG, and to Bill Hames, Simon Kelley, and Dirk Slawinski for help modifying and running DIFFARG. Thoughtful, critical reviews by Peter Gromet, Kip Hodges, and Mike Williams significantly improved this manuscript. MJM thanks the American Association of University Women for support during the revision of this manuscript. Dating in this study was supported by the Swiss National Science Foundation (2000-056849). Field research was supported by the U.S. National Science Foundation (EAR 9206118).
- 40Ar/39Ar dates
- Alpine white mica
- Effective diffusion dimension
- Grain size
- Quartz deformation mechanisms
- Siviez-Mischabel Nappe