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The AAPG/Datapages Combined Publications Database

CSPG Bulletin


Bulletin of Canadian Petroleum Geology
Vol. 15 (1967), No. 3. (September), Pages 219-229

High Angle Dips at Erosional Edge of Overthrust Faults

Gray W. Crosby


Passive crystalline basement, parallelism of fault and stratigraphic surfaces, and drill data in overthrust belts, indicate that major overthrust fault surfaces commonly dip less than 15° over broad areas. High dips of 30° to 80°, in many instances observed at the trace, are probably not representative of the fault as whole. Some proposed mechanisms of overthrusting, particularly gravitational gliding, may be suspect owing to frequency of observation of high dips. One reason for the occurrence of these steep dips is the tendency for fault segments subparallel with the ground surface to disappear quickly by erosion relative to steep segments in the toe and steps. Another is toreva block faulting at erosional edges of thrust plates. Overthrusting generally brings older, indurated beds over younger, weak strata. Frontal scarps steepen, toreva blocks form and rotate when displaced downward along the curved fault surfaces. Thus, the truncated overthrust fault is rotated and steepened.

As examples, the Absaroka overthrust fault is steepened from 18° to 90° in a large, half-toreva block 18 miles north of Kemmerer, Wyoming, and Cambrian rocks overlying the Hogsback overthrust are rotated at Hogsback Mountain near La Barge, Wyoming. Four additional toreva blocks are preserved in autochthonous Cretaceous Adaville Formation east of the Hogsback trace, indicating the overthrust surface has been repeatedly rotated and steepened as the erosional edge of the allochthon receded westward.

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