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The structural configuration and causal interpretation of foreland uplifts in the Rocky Mountain region have gained some clarity through recent petroleum exploration efforts. The most enlightening procedures have continued to be drilling, seismic recording, and surface mapping.
Drilling has confirmed the presence of an overturned limb of Paleozoic rocks beneath many foreland thrusts and a 20° to 30° angle of dip on most fault planes, two characteristics predicted by Berg in 1961 in his fold-thrust theory. Drilling has also revealed that some foreland thrusts do not have an overturned limb of Paleozoic rocks, and instead Precambrian rocks have been thrust directly over Eocene or Cretaceous rocks.
Seismic records have shown a relatively planar fault zone that does not appear to steepen at depth, and, in fact, frequently appears more horizontal, even with velocity corrections to depth. These records have also demonstrated thrust traces at angles ranging from 20° to 35°. Synthetic seismograms made from sonic logs recorded in wells that penetrated Precambrian rocks show zones of intense fracturing in both crystalline and metasedimentary rocks.
Surface mapping and biostratigraphic work on and adjacent to these
uplifts indicate that the forces that caused Laramide deformation may have changed through time. Compression was primarily east-west in latest Cretaceous and formed folds parallel to the Cordilleran thrust belt and probably formed wrench fault zones perpendicular to them. Later (Eocene) compression was more north-south and formed the east-west-trending uplifts and thrusts. South-southeast-trending wrench faults were activated in the foreland at this time, and eastward thrusting ceased in the Cordilleran thrust belt.
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