IN SITU STRESS VARIATIONS AT THE NORTHEASTERN BROOKS RANGE AND EASTERN NORTH SLOPE OF ALASKA: IMPLICATIONS FOR REGIONAL CHANGES IN TECTONIC SETTING

The northeastern Brooks Range of northern Alaska is an active, north-directed fold-and-thrust belt that is advancing on the Barrow Arch and the north-facing passive margin of the Arctic Basin. Density logs, leak-off tests and mud profiles from 57 wells from the northeastern North Slope constrain the magnitude of the present-day in situ stresses in the area. The resulting regional distribution of lateral and vertical variations in relative stress magnitude indicate that in situ stress varies significantly across this part of the North Slope, reflecting a complex interplay between basement structure, active tectonics and sediment loading.

Preliminary analyses suggest that two distinct stress regimes exist across this region of Alaska. Both strike-slip (S_Hmax > S_v > S_hmin) and normal stress regimes (S_v >S_Hmax >S_hmin) dominate areas adjacent to and above the Barrow Arch. These observed in situ stress regimes are consistent with observed fault patterns in the subsurface and with north-south extension along the Barrow Arch and the north Alaska margin.

In the south, the in situ stress regime immediately north of the northeastern Brooks Range thrust front reflects an active thrust fault regime (S_Hmax > S_hmin > S_v) at depths up to 6000 ft. This is consistent with active fold and thrust structures observed in surface exposures and in the subsurface in this area. However, at deeper depths (greater than 6000 ft), the relative in situ stress magnitudes suggest a transition from a thrust fault regime to a strike slip regime, possibly as a result of increasing vertical stress due to greater overburden. This observation is consistent with the few earthquake focal mechanisms in the area and suggests deep north-northeast oriented strike slip faults may underlie the western margin of the northeastern Brooks Range