GEOMETRY AND EVOLUTION OF A FAULT-BEND FOLD - MOUNT-BERTHA ANTICLINE

Mount Bertha anticline is a major thrust-associated fold in the Rocky Mountains of western Canada, well displayed in a 900-m-thick middle Pa leozoic carbonate sequence that forms the core of the fold, This carbo nate 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 vari ations in structural geometry occur along strike, These variations cor relate with changes in the thickness of the lower dolostone unit in th e core of the fold that result from an oblique hanging-wall ramp. A mo de I fault-bend fold develops along most of the exposed strike length of Mount Bertha anticline, This configuration corresponds to regions w here a full thickness of the lower dolostone unit occurs in the core o f the fold, Where the lower dolostone unit is thin or absent in the ha nging wall of the Mount Bertha thrust, the configuration of Mount Bert ha anticline is dominated by second-order fault-propagation and detach ment 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 re gion and overridden by the advancing thrust sheet. The fold geometry o f the central part of Mount Bertha anticline compares favorably with g eometric models of fault-bend fold structures that allow forelimb thin ning, except for parts of the fold affected by the tectonic removal of rock from the lower fore limb, Existing geometric models of fault-ben d folds do not consider this process, A companion fold within the hang ing wall of the Mount Bertha thrust, Cranswick anticline, is interpret ed 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.