Structure and internal deformation of thrust sheets in the Sawtooth Range, Montana: insights from anisotropy of magnetic susceptibility

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McCarthy, Dave J.
Meere, Patrick A.
Petronis, Michael S.
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Geological Society of London
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Geological strain analysis of sedimentary rocks is commonly carried out using clast-based techniques. In the absence of valid strain markers, it can be difficult to identify the presence of an early tectonic fabric development and resulting layer parallel shortening (LPS). In order to identify early LPS, we carried out anisotropy of magnetic susceptibility (AMS) analyses on Mississippian limestones from the Sawtooth Range of Montana. The Sawtooth Range is an arcuate zone of north-trending, closely spaced, west-dipping, imbricate thrust sheets that place Mississippian Madison Group carbonates above Cretaceous shales and sandstones. This structural regime is part of the cordilleran mountain belt of North America, which resulted from accretion of allochthonous terrains to the western edge of the North American continent.Although the region has a general east-west increase in thrust displacement and related brittle deformation, a similar trend in penetrative deformation or the distribution of tectonic fabrics is not observed in the field or in the AMS results. The range of magnetic fabrics identified in each thrust sheet ranges from bedding controlled depositional fabrics to tectonic fabrics at a high angle to bedding.
Regional structure , Kinematic history , Strain variation , Rocky Mountains , Wyoming Salient , Disturbed belt , Finite strain , Fabrics , Foreland , AMS , Tectonic fabrics , Sawtooth Range , Sevier fold and thrust belt
McCarthy, D. J., Meere, P. A. and Petronis, M. S. (2020) 'Structure and internal deformation of thrust sheets in the Sawtooth Range, Montana: insights from anisotropy of magnetic susceptibility', in Bond, C. E. and Lebit, H. D. (eds.) Folding and Fracturing of Rocks: 50 Years of Research since the Seminal Text Book of J. G. Ramsay. Geological Society, London, Special Publications, 487, pp. 189-208. doi: 10.1144/SP487.6
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