W.K. Wallace1, P.K. Atkinson1
(1) Geophysical Institute, University of Alaska, Fairbanks, AK
ABSTRACT: Evolution of Non-parallel Geometry of Detachment Folds in the Northeastern Brooks Range, Alaska
A wide range of interlimb angles represents different stages in the evolution of detachment folds in the Carboniferous Lisburne Limestone in the northeastern Brooks Range of Alaska. The Lisburne tends to form detachment folds because it is relatively thick (~500 m) and competent and overlies the much less competent Kayak Shale (~150 m). Detachment folds in the Lisburne are symmetrical angular folds, commonly with box-fold geometries in larger folds and chevron geometries in smaller folds. They range from open to isoclinal and have wavelengths in the hundreds of meters. Hinges of both synclines and anticlines are thickened relative to limbs, particularly in less competent intervals and at smaller interlimb angles. Thickening is accommodated by parasitic folds, minor faults, solution cleavage, internal strain, and fractures, with the local importance of each varying depending on shortening and other uncertain controls. The lower part of the upper Lisburne is the interval of maximum competency; thickening generally increases as competency decreases away from this interval toward the inner arc of a fold.
Bulk strain appears to have superseded flexural slip as the dominant mechanism of folding in folds with smaller interlimb angles. The amount of thickening varies for a given interlimb angle, which suggests that other factors besides shortening influence thickening, such as relief on the underlying detachment, relative thicknesses of the competent and incompetent intervals, and other variations in mechanical stratigraphy. The significant thickening in the hinges of these folds cannot be reconstructed with a simple parallel-fold geometry, particularly as interlimb angle decreases.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado