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Using a New 3-D Structural Model of the Wyoming Laramide Rockies Basement for Static Characterization and Kinematic Reconstruction

Abstract

To assist with basin screening and modeling, resource assessment, and as input for geomechanical analysis we have constructed a 3D model of the crystalline basement for the region of Wyoming where Laramide structures occurred. Using the model we are able to assess the static characteristics of this system and examine its temporal and spatial evolution. The model was built using published geologic maps and cross sections; 1200 2D seismic lines and 53000 well penetrations. Mapped structures include the principal arches and related back-thrusts and back-limb tightening anticlines. Geometric viability of the model is facilitated using 3D structural framework interpretation and kinematic validation methods. Static Characteristics. The structural relief of principal arches ranges to a maximum of 9.5km. The deepest depressions are at intersections of NW and E-striking principal arches. The controlling faults dip between 20–80° with a mean of 45° and don't appear to vary as a function of regional strike. The fault length to displacement ratios average 0.08 and fall within published thrust and strike-slip fault populations. High gradients in fault displacement occur where the tips of adjacent faults interact. Arches to the SW, (Wind River arch), are structurally mature with one master fault compared to the more embryonic Bighorn arch in the NE with its many flanking faults. Comparing cross sections between embryonic and mature cases it appears that arch folding predates significant fault slip. 75% of the structural relief for the Bighorn arch is from arch folding with minor fault slip. The Wind River arch has an equal degree of folding but 80% of its total offset is attributable to fault slip. Kinematic Analysis. The principal arches and their attached basins are divided for independent and fault-pinned restoration using flexural flattening and restored opposite to their structural vergence and according to fault fitting constraints. Assuming a fixed NA craton and NE-ward tectonic progression we infer a 4 stage history. The Green River basin moved NNE under-thrusting the Gros Ventre, Wind River, and Sierra Madre domains. These 4 domains then moved NE. Movement changed to ENE as the Beartooth and Owl Creek domains joined the allocthonous group. Final movement was eastward and included the Bighorn, Casper, and Laramie domains. Left-oblique movement occurred on E-W domain boundaries as an accommodation and a slight clockwise rotation of the SW domains is also inferred.