The Grasshopper basin of southwest Montana is a complex east-dipping graben containing five unconformity-bounded sequences of Tertiary sedimentary rocks. The Eocene-Oligocene basin lies within the northern Rocky Mountain Basin and Range province. Geologic mapping in five and a half 7.5 minute quadrangles indicates that at least three distinct phases of extension characterize the Cenozoic tectonic evolution of Grasshopper basin from approximately 46 Ma toMa.
The significant phases of extension in Grasshopper basin were phases 1 and 3. During the first phase of extension (46-27 Ma) the nonplanar Muddy-Grasshopper fault was initiated and 90% of the basin fill was deposited. At least 7 km of dip-slip displacement along this fault controlled the deposition of the Medicine Lodge beds (3.5 km thick) and development of a transverse fold train and a longitudinal anticline. The second phase of extension (late Eocene-early Oligocene) resulted in northwest-southeast trending extensional structures and was probably coincident with deformation along the Lemhi Pass fault (20 km to the southwest). The third phase of deformation (early Oligocene-middle Miocene) dismembered the once larger protobasin into smaller subbasins and tilted the northwest-dipping limb of the longitudinal anticline. The structures formed during this phase have north-south and northeast trends. Little sediment was deposited during phases 2 and 3. Overall >85% E-W extension accrued.
Extensional folds are common in Grasshopper basin and formed during all three phases of extension. One orthogonal fold set was recorded. Two-dimensional kinematic analysis of the longitudinal Bachelor Mountain anticline shows that this fold is a double-Ârollover that probably developed above a longitudinal ramp in the Muddy-Grasshopper fault. The transverse folds are the result of the changing strike of the downwardÂ-flattening Muddy-Grasshopper fault. A transverse syncline developed above a convex up part of the fault whereas a transverse anticline formed above a concave up part of the fault that reflects changes in the strike of the fault. Three-dimensional inclined shear probably created this geometry.
| Identifer | oai:union.ndltd.org:UTAHS/oai:digitalcommons.usu.edu:etd-7797 |
| Date | 01 May 2002 |
| Creators | Kickham, Julie C. |
| Publisher | DigitalCommons@USU |
| Source Sets | Utah State University |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | All Graduate Theses and Dissertations |
| Rights | Copyright for this work is held by the author. Transmission or reproduction of materials protected by copyright beyond that allowed by fair use requires the written permission of the copyright owners. Works not in the public domain cannot be commercially exploited without permission of the copyright owner. Responsibility for any use rests exclusively with the user. For more information contact digitalcommons@usu.edu. |
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