Contrasts in Structural Style Between the Northern and Northeastern Brooks Range: Folds vs. Thrust Faults in the Lisburne Limestone

By

W.K. Wallace, M.A. Jadamec, and P.K. Atkinson (University of Alaska, Fairbanks)

The structural style of the Carboniferous Lisburne Group changes dramatically across the boundary between the northeastern Brooks Range and the main axis of the northern Brooks Range. Lisburne limestone in the northeastern Brooks Range displays upright and symmetrical detachment folds that are rarely cut by thrust faults. Variations in fold geometry suggest that, as shortening increased, flexural-slip folding was gradually superseded by fold flattening (pure shear). Flattening was accommodated by parasitic folding and penetrative strain that resulted in changes in bed thickness across folds. South of the central Brooks Range front and its eastward continuation into the Brooks Range, imbricate thrust sheets in Lisburne typically display leading hangingwall anticlines and trailing footwall synclines. These folds probably formed as asymmetrical detachment folds that were later cut by thrust faults. Bed thickness appears to change less across these folds than in the northeastern Brooks Range, suggesting that flexural slip played a more dominant role in the evolution of the asymmetrical folds. Factors that may have influenced this abrupt change of structural style are mechanical stratigraphy, amount of sedimentary and/or structural overburden, and structure of the underlying rock units. Fold asymmetry clearly favored thrust breakthrough, but the causes of asymmetry remain unclear. The boundary between fold- and thrust-dominated structure in the Lisburne plunges westward beneath foreland basin deposits in the foothills. Thrust-dominated structures continue into the subsurface immediately north of the central Brooks Range front, but fold-dominated structures likely predominate north of the projected boundary and at least as far west as the Sagavanirktok River.

AAPG Search and Discovery Article #90008©2002 AAPG Pacific Section/SPE Western Region Joint Conference of Geoscientists and Petroleum Engineers, Anchorage, Alaska, May 18–23, 2002.