Structure and thermochronology of the metamorphic core of the Brooks Range, Alaska

Detailed field studies were undertaken in two key areas of the Central Belt of the Brooks Range: (1) the north flank of Mt. Igikpak in the Survey Pass Quadrangle and (2) in the Shishakshinovik Pass area in the eastern Ambler River Quadrangle. In both areas structural, stratigraphic, petrologic, 40Ar/39Ar, apatite fission-track and U-Pb data were used to constrain the kinematic and thermal history of metamorphic rocks of those areas.; North of the Mt. Igikpak massif a crustal section ∼15 km thick is exposed. There are upper greenschist facies rocks in the deeper portions, and very low grade metamorphic rocks at higher structural levels. Two foliations are found: a higher grade relict S₁ fabric and a lower grade S ₂ fabric that controls the metamorphic layering. 40Ar/ 39Ar analyses from S₁ white mica in the low-grade rocks at the northern end of the transect indicate that peak M₁ metamorphism occurred before ∼112 Ma. We ascribe M₁ to shortening that occurred during collision of an island arc against the Arctic Alaska margin. S ₂ involved the retrogression of earlier assemblages. Kinematic indicators on S₂ are top-to-the-north. A rapid cooling event from 500 ± 50°C to 300 ± 50°C took place between ∼98 and ∼90 Ma. The driving mechanism for ductile deformation during S₂, and for rapid cooling documented by our thermochronologic data, was probably the gravitational collapse of the core of the orogen, over-thickened during the preceding collision.; At Shishakshinovik Pass there are Mississippian Lisburne Group strata surrounded by metamorphic rocks typical of the Central Belt of the Brooks Range. All the rocks at Shishakshinovik Pass are intensely deformed, so that one cannot distinguish between an autochthonous and an allochthonous sequence. Furthermore the Mississippian rocks, instead of being attached to the underlying basement, are in the hanging wall of a northwest dipping shear zone. Based on the variations in metamorphic grade and the 40Ar/ 39Ar thermochronology, we argue that this shear zone was an extensional structure active during the mid-Cretaceous orogenic collapse of the Brooks Range. A consequence of this structural interpretation is that the Endicott Mountains allochthon need not be restored south of the Shishakshinovik orthogneiss.