The Imperial anticline, a fault-bend fold above a bedding-parallel thrust ramp, Northwest Territories, CanadaGeological Survey of Canada contribution no. 1997169.

Abstract

The Imperial anticline, about 50 km west of Norman Wells, Northwest Territories, Canada is a fault-bend fold which overlies an unusual thrust ramp, totally dictated by stratigraphy and not at all like those in text books. The lower-flat/ramp/upper-flat geometry of the underlying thrust fault is decreed by the stratigraphic configuration, which is that of a southward dipping, gently flexed, Neoproterozoic Mackenzie Mountains Supergroup (MMS) succession truncated unconformably by a flat-lying Phanerozoic succession. The lower flat and ramp are controlled by a bedding-parallel glide zone in the flexed MMS, whereas the upper flat is controlled by another bedding-parallel glide zone in salt beds in the lower part of the flat-lying Phanerozoic. This study provides new insight into a type of stratigraphically dictated thrust ramp that may well be present in other thrust belts. Angular truncations at unconformities are to be expected at craton margins and stratigraphic configurations similar to the one documented here may be the cause for other large, low-angle thrust ramps. Understanding the Imperial anticline and its underlying thrust permits a logical linkage of an inferred deep detachment in Proterozoic strata beneath the Mackenzie Mountains with the much shallower detachment in Upper Cambrian salt beds beneath the northern Franklin Mountains. Represented on the seismic lines are at least 1.5 km of basal MMS strata that do not outcrop in Mackenzie Mountains. They probably correlate with the Pinguicula Group that is exposed in the Wernecke Mountains about 250 km to the west.