Analysis of the interchange between en echelon basin-margin Laramide folds adjacent to the Wind River Mountains, Wyoming

Abstract

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Uplift of the basement-cored Wind River Mountains during the Laramide orogeny (ca. 75- 45 Ma) produced a series of subsidiary en echelon NW-SE trending folds--Dallas Dome, Derby Dome, and Sheep Mountain anticline, along the southern margin of the Wind River basin. These folds are sub-parallel to the trend of the Wind River uplift and are consistent with NE-SW shortening that is generally inferred for the Laramide orogeny. However, the origin of the en echelon fold pattern, which is slightly oblique to the main uplifts, is unclear. It has been interpreted as: 1) a result of reactivated basement faulting, or 2) a result of late-stage N-S Laramide shortening. The origin and geometry of the en echelon interchanges between Dallas Dome and Derby Dome and between Derby Dome and Sheep Mountain Anticline is the focus of my study, which attempts to test the hypothesis that N-S shortening played a significant role in the formation of their complex interchange. To test this hypothesis, the study attempted to evaluate of the progressive development of fractures across the Derby Dome--Dallas Dome en echelon fold interface. An additional part of the study included a seismic experiment to evaluate the geometry of back-limb faulting on Derby Dome. Laramide fractures in the study area can be divided into four groups: 1) fractures that formed prior to the folding, 2) fold-induced fractures that formed during folding, 3) reactivated pre-existing fractures, and 4) newly formed fractures related to the formation of the interchange zone. The fold-related fractures, collected from the back-limb thrust hanging wall above the Dallas Dome-Derby Dome interchange, include a conjugate set bisected by a NE-SW striking extensional fracture set, which are consistent with regionally recognized NE-SW Laramide shortening. Two sets of extension fractures were produced during the folding, one parallel to the Dallas and Derby Dome axial planes and a second normal to the fold hinge lines. The regional fracture sets are rotated along with bedding that curves into the interchange zone. Rotation of the best developed conjugate fracture produced a late-stage set of NNE-SSW striking pinnate fractures. One subarea also contained complex curved fractures that appear to have formed in response to reactivation in the interchange zone, but the mechanics of formation of these fractures is unclear. The observations were insufficient to test the N-S shortening hypothesis. Both seismic refraction and reflection experiments were conducted to image the geometry of back limb fault duplexing, which was first interpreted by Brocka (2007). The duplexing of the thrust fault was successfully imaged, but with slight geometry and location differences from the previous interpretation. In this study, the fault duplexing is interpreted to propagate farther to the north and not turn to the east as it was initially mapped.