Evolution of the Precambrian Rocks of Yellowstone National Park (YNP): Structural GeologyWyatt Fereday1, Sabina Kraushaar2, Erin Lynch3, David Mogk4, Darrell Henry5, Paul Mueller6, and David Foster6

1Colby College, 2Fort Lewis College, 3Bryn Mawr College, 4Montana State Univ., 5Louisiana State Univ., 6Univ. of Florida

Introduction Field Relations Structural Features Fold Events Mylonites Conclusions

10-WF-7-19-01. Mylonite fabric with chlorite alterations. XPL.

Distributed shear zones display extensive mylonitization, particularly in the igneous plutons, dikes and sills that intrude the JMS. Mylonites in the Blacktail Deer Creek and Garnet Hill areas contain sinistral shear-sense indicators and have an average orientation of 359/38. Mylonitization appears to post-date the folding events, and may be related to emplacement of associated injected igneous bodies. Quartz deformed mainly by sub-grain rotation, while feldspars showed both brittle and plastic deformation (temperatures ≈400-550 °C). Mica fish and quartz ribbons indicate simple-shear deformation.

The structural features have a generally consistent nature:

•Typically, the first foliation (S1) is NE-SW striking and is sub-parallel with bedding (S0)

• A second foliation (S2) is developed axial planar to F2 folds, which plunge gently to the SW and NE.

• Cross-cutting mylonites have an average strike and dip of 33.4/33.5 SE.

Figure 2. F2 kink folds in pelitic schist near Bear Creek.

10-WF-7-19-01. Sub-grain-rotation in quartz. XPL.

10-WF-7-13-01. Silimanite grain in foliation. PPL.

Pelitic schists experienced at least three folding events that include the following features:

•Rare isoclinal folds (F1) and at least two stages of crenulation folds (F2 and F3).

•F2 crenulation fold hinges and L2 mineral lineations plunge shallowly to the N-NE or S-SW.

•Meter- to regional -scale open fold system (F3) - identified based on poles to foliation with calculated fold axis with trend and plunge of 173/22.

•Rare L3 mineral lineations are SE-trending and nearly orthogonal to L2 and F2. This open fold system may be related to the finer-scale F2 crenulation and L2 mineral lineation.

Figure 6. Equal area stereonets show structural features of the Bear Creek area schists. F2 and F3

crenulation fold hinges are plotted on the left, n = 54. L2 and L3 mineral lineations are plotted on the right, n = 39.

Figure 4. Equal area stereonet shows poles to S0 bedding (solid symbols) and S1 schistosity (open symbols) of the Bear Creek area schists, n = 107. The star symbol represents the calculated regional fold axis.

Figure 9. Poles to the mylonite foliations: Brown diamonds = Garnet Hill area, n = 4. Purple squares = granite and gneisses North of the Lamar river, n = 9. Black square = mean vector. Circle = 95% confidence cone.

Figure 5. S1 Foliations across transect. Left: western half (Rattlesnake Butte to Blacktail Bridge). Avg. S & D = 026/55 SE, n = 171. Right: eastern half (Blacktail Bridge to Slough Creek). Avg. S & D = 006/45 SE, n = 306.

Metapelitic rocks are multiply deformed through three separate folding events. Granitic intrusive rocks are undeformed and thus post-date the major deformation events. However, late-stage ductile and brittle shear zones cut both the metasedimentary rocks and injections associated with the granite bodies.

The Precambrian rocks in northern Yellowstone National Park (YNP) exhibit a range of structures that relate to its deformational history. Earlier, Jablinski et al. (1992) found evidence for three folding events in the Precambrian metapelitic rocks in the Jardine area. However, little was known about the structural relations of the area in a broader sense. This study extends structural investigations from the Jardine to Garnet Hill areas (Fig. 1). The rocks include the Jardine Metasedimentary Sequence (JMS) with depositional ages of 2.8-2.9 Ga cut by 2.8 Ga granitic plutons.

The purpose of this study is to:

•characterize the structural features in the metasediments and in the plutons of YNP

•interpret their relation to the tectonic history of this region.

Figure 8. Sinistral shear-sense-indicator, felsic injection on south side of Garnet Hill.

Acknowledgements This project was supported through the NSF REU program, Division of Earth Science grants EAR 0852025, 0851752, and 0851934.

Special thanks to YNP staff, Christie Hendrix, Stacey Gunther, Carrie Guiles, Bridgette Guild and Hank Heasler for their support and interest.

Our studies have been aided by Jeff Marsh of Colby College and Kim Hannula of Fort Lewis College

References

• This area experienced three deformation events in late Archean time (prior to 2.8 Ga).

• Metapelites were folded in each of the deformation events.

• Plutons remain largely undeformed by the folding events and thus post-date the major deformation events. However, the plutons are cut by discreet mylonitic shear zones.

• Microstructures in mylonites imply temperatures of 400-550°C and sinistral, simple-shear deformation.

Jablinski, J.D. and T.B. Holst, 1992, Deformational History of Archean Metasedimentary Rocks of the Beartooth Mountains in the Vicinity of the Mineral Hill Mine, Jardine, Montana: The Journal of Geology, v. 100, p. 561-578.

Figure 3. Schist cut by mylonitized felsic dike at Garnet Hill.

Strike and Dip of Foliation

Trend and Plunge of Fold Axes

Figure 1. General structural trends of the Jardine Metasedimentary Sequence

Figure 7. Crenulation folds in schists.

Figure 10. Mylonitic microfabrics