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Temporal and Kinematic Evolution of Intrabasinal Extensional Folds in Grasshopper Basin, SW Montana

J. C. Kickham
Utah State University, Department of Geology, Logan, LIT

Only recently has the nature and importance of hanging-wall folding in rift basins been identified. With increasing frequency, folds that develop parallel (longitudinal) and perpendicular (transverse) to the main basin-bounding fault have been identified. These simple, open folds provide clues for understanding the four dimensional evolution of rift basins.

Grasshopper basin developed as an Eocene-Oligocene supradetachment basin and resides within the Rocky Mountain Basin and Range. Two sets of folds have been identified in Grasshopper basin, a transverse east-plunging syncline and a longitudinal anticline.

To identify the fault processes responsible for fold sets, a model (2D/3D Move from Midland Valley Exploration) will be constructed for each hypothesized fault geometry. Then the model will be exposed to each of the kinematic algorithms available (fault parallel flow, inclined shear, flexural slip, and variable heave) to determine which process(es) are responsible for folding. The culmination of structural modeling will be twofold: It will provide structural insight regarding the effect of fault geometry on fold development and identify the potential for subbasin development due to the interaction between intersecting folds.

Recent field studies have documented fanning dips that steepen toward the basin-bounding normal fault suggesting that complicated stratigraphic relationships, not explained by simple growth models, are abundant. By applying structural measurements (strike and dip data, fault slip direction, and fracture orientations) along with sedimentological observations to the models, a best-fit solution will be chosen to explain the surface and subsurface structural geology.

AAPG Search and Discovery Article #90902©2001 AAPG Foundation Grants-in-Aid