Ductile Deformation in the Tobacco Root Mountains of Southwest Montana

Daniel Blednick
Department of Geology, Amherst College, Amherst, MA 01002-5000
Faculty Sponser: Tekla Harms, Amherst College

Quartz c-axis analyses have been applied to determine if penetrative simple shear occurred along the contact that juxtaposes the amphibolites, aluminous schists, and quartzites of the Spuhler Peak Metamorphic Suite (SPMS) against the quartzofeldspathic gneiss and amphibolite of the Indian Creek Metamorphic Suite (ICMS) and the Pony Middle Mountain Metamorphic Suite (PMMMS) in the Tobacco Root Mountains of southwest Montana. These Archean crystalline rocks have been subjected to intense deformation; the contact is currently folded in a km-scale, possibly non-cylindrical fold. Burger (1969) hypothesized that the contact may have originally been a regional scale fault. The fault model for the contact explains the juxtaposition of the contrasting lithologic assemblages of the SPMS and the ICMS and PMMMS. In contrast, Gillmeister (1971) hypothesized that the contact was originated as a depositional contact. This distinction is critical to the tectonic and metamorphic history of the Tobacco Root Mountains. Accordingly, it is necessary to test these interpretations of the SPMS/ICMS-PMMMS contact by determining if shear occurred along this contact; and, if so, identifying the direction and sense of that shear.

In this study, 40 oriented samples were taken from 30 outcrops of quartz rich rocks at the SPMS contact and up to 0.5 km from the contact on either side. The samples consisted primarily of fuchsitic quartzite from the SPMS and quartzofeldspathic gneiss from the ICMS and PMMMS. Microscopic analysis of quartz deformation fabrics requires a large number of grains to be analyzed in thin section, thus requiring the selection of fine grained samples. In order to sample a broad range of fine grained, quartz-rich rock, samples were taken from many points along the SPMS contact. Locations were chosen not only from the nose of the fold, as in the Kranenberg (1996) study, but also down both limbs of the fold. This enabled comparison between the deformed rocks of the nose and the deformed rocks along each limb. Each sample was cut to produce two mutually perpendicular thin sections: both perpendicular to foliation, one parallel to lineation.

In order to provide sufficient evidence for the strain history of these rocks, both textural analysis of samples in thin section as well as quartz C-axis analysis were completed. Initial petrographic analysis of these samples was followed by C-axis analysis in which the orientations of quartz C-axes were measured using a universal stage to determine if lattice preferred orientation (LPO) exists. The data were then plotted on a lower hemisphere projection and interpreted.
Quartz grains of varying size and shape occur within a single sample. Grain sizes range from <1mm to 5mm. The larger grains are elongate and have some undalose extinction and deformation lamellae. These elongate grains have complex, polygonal 120° grain boundaries with a high surface area to volume ratio, and are decorated along their boundaries with small, optically distinct, polygonal quartz grains. We interpret these elongate irregular shapes, which are present in rocks from all three units, to be inherited from originally strongly serrate ribbon grains (and subgrains). They record a history of dynamic recrystallization during simple shear followed by static thermal annealing. There is also a smaller percentage of samples that are dominated by more typical, equigranular metamorphic quartz grains.

C-axis measurements were plotted on a lower hemisphere projection and rotated with respect to regional lineations. Contour density plots support the hypothesis that penetrative simple shear occurred along the contact. Determining the orientation of the c-axis of quartz grains extends the work of Kranenberg (1996) who recovered asymmetric cross girdle patterns from C-axis analyses of rocks from the Sunrise Lake region of the ICMS. His patterns are consistent with a hypothesis of penetrative simple shear along the contact. Of the six samples plotted by Kranenberg, two produced interpretable results. This study has yielded plots with a similar variability, and some plots can be initially interpreted as having a cross girdle pattern. However, this study uses samples from a broader range of sample locations than those considered in the Kranenberg study. Other plots from this study did not produce recognizable cross girdle patterns, but rather small circle patterns opening the possibility of pure shear flattening along some parts of the contact.
Both the quartz microfabric and C-axis data, taken together, indicate that shear has indeed occurred along the contact. Analysis of these C-axis patterns and microfabrics supports the conclusions of Kranenberg (1996) and Burger (1969) that the contact represents a fault rather than a depositional contact.