Evolution and Extension of Thrust-Truncated Detachment Folds in the Upper Marsh Fork Area of the Eastern Brooks Range, and Implications for the Eastward Extent of the Endicott Mountains Allochthon

By

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

In the eastern Brooks Range fold-and-thrust belt, the Carboniferous Lisburne Group and underlying Kayak Shale are separated into two structural domains by the east-trending Continental Divide thrust front. North of the thrust front, the northeastern Brooks Range is characterized by symmetric, upright, east-northeast-trending detachment folds formed by buckling of limstones of the Lisburne Group above a decollement in the Kayak Shale. South of the thrust front, folds in the Lisburne typically are asymmetric, trend eastnortheast, and are cut by thrust faults. Studies in the area of the upper Marsh Fork of the Canning River in the southern domain suggest that the folds evolved by thrust-breakthrough of asymmetric detachment folds rather than as fault-propagation folds, as is commonly assumed for strongly asymmetric thrust-related folds. A better understanding of the geometry and evolution of thrust-truncated detachment folds in the Marsh Fork area may be applicable to interpretation of subsurface structures in the Lisburne beneath the foothills north of the central Brooks Range. Steeply dipping transverse normal faults commonly cut thrust sheets and their hangingwall anticlines in the Marsh Fork area. These faults may be a consequence of late extension orthogonal to the north-northwest shortening responsible for thrust emplacement and fold truncation. The structural style of the Marsh Fork area is the same as that displayed by the Lisburne throughout the Endicott Mountains allochthon. This and the presence of deep-water Lisburne facies in the Marsh Fork area suggest that the Continental Divide thrust may correspond with the leading edge of the Endicott Mountains allochthon.

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.