Fracture paragenesis and microthermometry in Lisburne Group detachment folds: Implications for the thermal and structural evolution of the northeastern Brooks Range, Alaska

Abstract

The distribution, character, and relative age of fractures in detachment folded Mississippian–Pennsylvanian Lisburne Group carbonates and overlying Permian–Triassic clastic rocks in the northeastern Brooks Range of northern Alaska provide important clues to the thermal and deformational sequence experienced by these rocks. Although paleothermal indices in the host rock limit the conditions of folding to temperatures equal to or less than 280C, field and petrographic relationships suggest that different fracture sets formed at different times during the deformational history of the rocks, providing a record of deformation under changing temperature and pressure conditions. These rocks probably initially entered the oil-generation window (80–140C) during the Early Cretaceous formation of the Colville basin via thrust loading by the Brooks Range to the south. Regional fractures formed during this time as a result of high pore pressures and low in-situ differential stresses. Shortening in these rocks related to the advancing northeastern Brooks Range fold and thrust belt began during the Late Cretaceous to early Tertiary. Early phases of detachment folding were via flexural slip, with associated fracturing. With continued shortening and growth of detachment folds, structural thickening resulted in deeper burial of the bottom part of the deforming wedge. Early fold-related fractures were subsequently overprinted by penetrative strain during peak folding at temperatures of approximately 280C. Continued shortening resulted in uplift and erosional unroofing at approximately 60 Ma. Late fold-related fractures formed at about 150C. Subsequent uplift of the thickened wedge through 60C occurred after about 25 Ma. Late pervasive extension fractures related to unroofing and/or regional stresses formed at relatively shallow depths and low temperatures, overprinting all the earlier fractures and penetrative structures.