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shortening increased, flexural-slip folding was gradually superseded by fold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.