Limitations on the Thermal Conditions During Formation of Fractures and Detachment Folds in the Northeastern Brooks Range

By

C.L. Hanks (University of Alaska, Fairbanks), M. Parrish (U.S. Geological Survey), and W. K. Wallace (University of Alaska, Fairbanks)

Fractured and detachment-folded Mississippian-Pennsylvanian Lisburne Group carbonates and overlying Permo-Triassic clastic rocks are potential exploration targets in the foothills of the Brooks Range of northern Alaska. Along-strike examples of these folds and associated fractures are well exposed in the nearby northeastern Brooks Range. While paleo-thermal indices in the host rock limit the conditions of folding to temperatures equal to or less than 250 deg. Celsius, field and petrographic relationships suggest that different fracture sets formed under different temperature-pressure conditions. Many early fractures that formed on fold limbs due to flexural slip were subsequently overprinted by penetrative strain. While these fractures may have developed at lower temperatures, subsequent penetrative strain probably occurred at the upper end of the temperature range, suggesting that folding culminated at relatively high temperatures. The apparent genetic relationship of early fractures to structures that formed at high temperatures, and the absence of oil-bearing fluid inclusions in cemented early fractures suggest that early fractures postdated any oil migration that might have occurred in the area.

Later pervasive extension fractures show little sign of subsequent deformation and probably formed at lower temperatures and pressures during the waning phases of folding and/or unroofing. These late extension fractures also likely postdated local oil migration

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.