Structural geology of the Quad-Wyoming-Line Creeks area, Beartooth Mountains, Montana

Edited by: Leonard H. LarsenMartin Prinz, and Vincent Manson



The Quad-Wyoming-Line Creeks area is in the northeastern part of the Beartooth Mountains of Montana. The rocks of the area consist mainly of banded migmatite, granitic gneisses, amphibolite, quartzite, and agmatite; small amounts of biotite schist and biotite gneiss, iron-silicate rocks, ultramafic rocks, mafic dikes, and felsic porphyries are also present. Quartzite outcrops continuously around the major folds, and agmatite is especially widespread in the axial zone of a major anti-form.

Two fold sets have been revealed by detailed mapping (scale 1:7200) and statistical analysis of the penetrative subfabric elements. The large later folds, F2, are the south-plunging Quad Creek synform and the south-southwest-plunging Wyoming Creek antiform; they are nearly upright and non-isoclinal. A metanorite intrusion is located in the axial zone of the Quad Creek synform. Small F2 fold style varies from similar in relatively ductile rocks to open, concentric folds in non-ductile rocks. Only one large F1, located immediately north of the metanorite pluton, has been mapped. F1 folds are characteristically similar in style. Refolding of small F1 folds by small F2 folds has been observed throughout the map area.

Statistical analysis of the compositional layering, So, and the axial planes of small F1 folds, S1, shows that the πS0- and πS1-axes constructed in the Wyoming Creek antiform are nearly coincident with each other and with south-southwest-plunging maxima of small F2 fold axes, B2. Both B2 and the axial planes of F2 folds, S2, are dispersed, showing that the later folds are noncylindrical and nonplane. The axes of small F1 folds, B1, are generally dispersed along well defined π-girdles. B1 and B2 maxima are generally coincident at some stations in the Wyoming Creek antiform-axial zone, suggesting that B1 and B2 are, at least locally, collinear. The long axes of hornblende crystals, L1, are also dispersed, but are not parallel to B1; the L1-subfabric appears to indicate that L1 developed during the F1 folding but plunged somewhat more steeply to the south-southwest to south than Bi and that the F2 folds are concentric rather than similar in the amphibolite. In the Quad Creek synform (F2), all of the above-mentioned fabric elements diverge around the north side of the metanorite body.

The Precambrian deformational history of the present map area, and probably the remainder of the eastern Beartooth Mountains, appears to be comprised of two phases of folding. During the first phase (F1) south-southwest plunging, isoclinal or nearly isoclinal folds formed by passive flow during metamorphism to at least the upper amphibolite facies; these F, folds were not upright and may have been nearly recumbent. L1 was formed during this phase. Metamorphic differentiation of a sedimentary sequence occurred at this time, but it is not clear whether any new material was added.

The second phase of deformation is characterized by upright, non-isoclinal folds which also formed about south-southwest to south plunging axes. Flexural flow was the dominant mechanism in the development of the Quad Creek synform and the Wyoming Creek antiform, but passive flow was important in layers of ductile rocks. Granitization was generally synchronous with the F2 phase; pegmatite dikes were emplaced later in a passive manner. Quartzite and amphibolite acted as resisters to granitization; their termination in granitic gneisses and migmatites is usually most adequately explained by refolding rather than granitization of the quartzite. It is possible that partial melting occurred in some areas such as the agmatized zones.

Time of emplacement of the metanorite body is still questionable, but it clearly predates the F2 folding; intrusion prior to F1 is probable because B1, B2, and L1 diverge in the vicinity of the body.

Study Area

Publication type Book chapter
Publication Subtype Book Chapter
Title Structural geology of the Quad-Wyoming-Line Creeks area, Beartooth Mountains, Montana
DOI 10.1130/MEM115-p1
Volume 115
Year Published 1969
Language English
Publisher Geological Society of America
Contributing office(s) Flagstaff Science Center
Description 21 p.
Larger Work Type Book
Larger Work Subtype Monograph
Larger Work Title Igneous and Metamorphic Geology
First page 1
Last page 18
Country United States
State Montana
Other Geospatial Beartooth Mountains
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