Abstract

The Wind River range is the largest Laramide uplift in Wyoming and is cored by Precambrian rocks thrust over sedimentary rocks of the Green River Basin to the southwest. The origin of these uplifts is in dispute; it has been ascribed to horizontal compression along thrusts or vertical movement along high-angle reverse faults. Therefore, the attitude of the bounding fault is critical to understanding the mechanics of deformation. A gravity study based on 1800 stations has been carried out to attack this question and to complement a COCORP deep crustal reflection study. Smoothed free-air gravity anomalies are highly positive over the Wind River range and near zero over the adjacent margins. Bouguer gravity anomalies range from -252 and -225 mgal in the Green River basin and Wind River basin, respectively, to -150 mgal in the basement core of the Wind River range. The bulk density of sedimentary rocks in the adjacent basins is the critical parameter for gravity interpretation of the deep structure and ranges from 2.25 to 2.65 g/cm 3 , based on density logs. Gravity modeling demonstrates that most of the +85 mgal gravity anomaly associated with the uplift is caused by the sediment-basement density contrast, but about 18 mgal has a deeper source. This can be accounted for by offset of more dense middle crust along a thrust fault dipping at about 40 degrees. Three-dimensional (3-D) modeling confirms this interpretation along the southern part of the range. No significant offset of the Mohorovicic discontinuity can be present. This suggests that the thrust either flattened out or the lower crust deformed by homogeneous ductile deformation. Gravity interpretation gives information on deep structure not found in the COCORP reflection data and further confirms the compressive nature of this Laramide uplift.

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