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

Dave J Mccarthy,Michael S Petronis,Patrick A Meere

doi:10.1144/sp487.6

Abstract

AbstractGeological 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.

Highlights

The initial formation of a penetrative tectonic fabric or cleavage usually develops as a response to coaxial layer parallel shortening (LPS) in fold and thrust belts (Cooper et al, 1986; Mitra, 1994; Mitra et al, 1985; Yonkee and Weil, 2010)
In order to determine the extent of the development of this penetrative LPS fabric in the Sawtooth Range, anisotropy of magnetic susceptibility (AMS) data were collected on samples from five thrust sheets; all exposed along the Sun River in the Sawtooth Range
The variation in Pj values do not appear to correlate with changes in bulk susceptibility (Fig. 7a), which implies that Pj is controlled either by primary or tectonic fabrics, rather than the composition of the limestones

Summary

Introduction

The initial formation of a penetrative tectonic fabric or cleavage usually develops as a response to coaxial layer parallel shortening (LPS) in fold and thrust belts (Cooper et al., 1986; Mitra, 1994; Mitra et al, 1985; Yonkee and Weil, 2010). The central Sawtooth Range is an arcuate zone of predominantly north-south trending, closely spaced, west dipping, imbricate thrust sheets and associated folds comprised of Paleozoic and Mesozoic sedimentary rocks (Fig. 3; Holl and Anastasio, 1992) These eastward propagating thrusts typically placed dominantly Carboniferous Mississippian aged carbonate rocks of the Madison Group above Cretaceous shale and sandstones. Type 2: The first sign of an incipient tectonic fabric is typically weaker than the primary sedimentary fabric, the AMS ellipsoid may still be weakly oblate and conformable with bedding In this case, the K1 axes may start clustering in the direction of extension and defining a magnetic lineation parallel to the intersection of an incipient LPS fabric with bedding. This stage typically has flattened oblate AMS ellipsoids perpendicular to bedding

RESULTS

Discussion

Conclusion