Structural and kinematic evolution of the Eocene-Oligocene Grasshopper extension basin, southwest Montana

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 indicates that at least three distinct phases of extension characterize the Cenozoic tectonic evolution of Grasshopper basin from approximately 46 Ma to <28 Ma.; The significant phases of extension in Grasshopper basin were phases 1 and 3. During the first phase of extension the nonplanar Muddy-Grasshopper fault was initiated and 90% of the basin fill was deposited. The second phase of extension resulted in northwest-southeast trending extensional structures. The third phase of deformation dismembered the once larger protobasin into smaller subbasins. 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.