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Abstract

The northern Franklin Mountains and the Colville Hills, District of Mackenzie, are a series of ridges of divergent trends separated by broad, mostly drift-covered valleys. Some ridges are supported by thrust plates, and others by asymmetric anticlines. These structures, which represent shortening of the sedimentary cover, record tangential compression. Despite a variety of structural trends, there is no evidence for more than one phase of compression.

The structural province is characterized by enigmatic thrust reversals. Typically, one end of a range is underlain by a southwest-dipping fault and the other by a northeast-dipping fault. The abrupt transition takes place via a transverse fault which separates the opposing blocks, and which seems to require longitudinal shortening of the range in addition to the more obvious shortening perpendicular to it. Reversals along the trend of a particular range are inadequately explained, but the close geometric relation between reversals and transverse faults suggests an interrelated origin dependent on longitudinal shortening in conjunction with lateral shortening.

Most of the northern Franklin Mountains appear to be floored by a décollement zone in shale and evaporite beds of the Cambrian Saline River Formation. Structures above the zone probably are accentuated by tectonic thickening of the Saline River Formation. The décollement is assumed to extend beneath the Colville Hills about 175–200 mi (280–320 km) northeast of the Mackenzie Mountain front. In the McConnell Range on the south and the Mackenzie Mountains on the southwest, the décollement zone must be at a lower stratigraphic level, because beds older than the Saline River Formation are exposed in structures.

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