Paleomagnetic study of thrust sheet rotations in the Helena and Wyoming salients of the northern Rocky Mountains
Sarah Eldredge, Rob Van der Voo, 1988. "Paleomagnetic study of thrust sheet rotations in the Helena and Wyoming salients of the northern Rocky Mountains", Interaction of the Rocky Mountain Foreland and the Cordilleran Thrust Belt, Christopher J. Schmidt, William J. Perry, Jr.
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Paleomagnetic investigations throughout the overthrust belt in the Wyoming and Helena salients show that buttressing effects of the Rocky Mountain foreland have caused local rotations of thrust sheets along the margins of the salients. Two previous studies of Triassic, Upper Jurassic, and Lower Cretaceous rocks in the Wyoming-Idaho overthrust belt involved rocks from the Prospect, Bear, Darby, and Absaroka thrust sheets. These studies documented as much as 60° counterclockwise rotation in the northern edge, and as much as 30° clockwise rotation in the southeastern edge of the Wyoming salient. It has been suggested that these rotations were buttressing edge effects, since the rotations do not appear to have been transferred back into the interior of the Wyoming salient.
New work in the southern portion of the Helena salient in southwestern Montana has documented a similar situation. Here, samples from the Lower Cretaceous Kootenai Formation show a maximum of 54° clockwise rotation in the region of the McCarthy Mountain salient, 23° clockwise rotation within the southwest Montana transverse zone, and 35° clockwise and 30° counterclockwise rotations in the nose of the salient. These rotations vary greatly from site to site, suggesting that the thrust sheets were not deformed in a coherent fashion, but rather, they broke and rotated as individual pieces where the effects of buttressing against the Rocky Mountain foreland were the greatest.