Abstract

Mount Bertha anticline is a major thrust-associated fold in the Rocky Mountains of western Canada, well displayed in a 900-m-thick middle Paleozoic carbonate sequence that forms the core of the fold. This carbonate sequence may be divided into three mechanical-stratigraphic units of roughly similar thicknesses: a lower, massive dolostone; a middle, layered dolostone; and an upper, micritic limestone. Substantial variations in structural geometry occur along strike. These variations correlate with changes in the thickness of the lower dolostone unit in the core of the fold that result from an oblique hanging-wall ramp.

A mode I fault-bend fold develops along most of the exposed strike length of Mount Bertha anticline. This configuration corresponds to regions where a full thickness of the lower dolostone unit occurs in the core of the fold. Where the lower dolostone unit is thin or absent in the hanging wall of the Mount Bertha thrust, the configuration of Mount Bertha anticline is dominated by second-order fault-propagation and detachment folds developed in the upper two mechanical-stratigraphic units. Along the length of the Mount Bertha structure, the limestone unit has been progressively attenuated and/or faulted in the lower forelimb region and overridden by the advancing thrust sheet.

The fold geometry of the central part of Mount Bertha anticline compares favorably with geometric models of fault-bend fold structures that allow forelimb thinning, except for parts of the fold affected by the tectonic removal of rock from the lower forelimb. Existing geometric models of fault-bend folds do not consider this process. A companion fold within the hanging wall of the Mount Bertha thrust, Cranswick anticline, is interpreted to be a mode I fault-bend fold that has been significantly modified by second-order fault-propagation folding during the waning phases of movement along the Mount Bertha thrust.

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