Transcript Structural Geology

Evolution of the Precambrian Rocks of Yellowstone National Park (YNP): Structural Geology
Wyatt Fereday1, Sabina Kraushaar2, Erin Lynch3, David Mogk4, Darrell Henry5, Paul Mueller6, and David Foster6
1Colby
Introduction
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.
College, 2Fort Lewis College, 3Bryn Mawr College, 4Montana State Univ., 5Louisiana State Univ., 6Univ. of Florida
Field Relations
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.
Structural Features
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.
The structural features have a generally
consistent nature:
The purpose of this study is to:
•Typically, the first foliation (S1) is NE-SW
striking and is sub-parallel with bedding (S0)
•characterize the structural features in the
metasediments and in the plutons of YNP
Fold Events
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 SSW.
•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.
• A second foliation (S2) is developed axial
planar to F2 folds, which plunge gently to the SW
and NE.
•interpret their relation to the tectonic history of
this region.
• 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.
Strike and Dip of
Foliation
Trend and Plunge
of Fold Axes
Figure 1. General structural trends of the Jardine
Metasedimentary Sequence
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 3. Schist cut by mylonitized felsic dike at
Garnet Hill.
Mylonites
10-WF-7-19-01. Mylonite fabric
with chlorite alterations. XPL.
Conclusions
Figure 8.
Sinistral
shear-senseindicator,
felsic
injection on
south side of
Garnet Hill.
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 postdate 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.
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.
• 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.
References
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.
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.
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.
Figure 7. Crenulation folds in schists.
10-WF-7-19-01. Sub-grain- 10-WF-7-13-01. Silimanite
rotation in quartz. XPL.
grain in foliation. PPL.
Figure 10. Mylonitic microfabrics
Our studies have been aided by Jeff
Marsh of Colby College and Kim Hannula
of Fort Lewis College