Subduction accretion, thermal overprinting, and exhumation of high-pressure/low-temperature metasedimentary rocks of the south-central brooks range

Abstract

ABSTRACT Lithological, petrological and structural observations and measurements of graphite thermometry, Si-in-phengite barometry, and 40Ar/39Ar phengite thermochronology were performed along two transects across the south-central Brooks Range of northern Alaska. The integrated data set reveals a history of accretion, protracted thermal overprinting of high-pressure/low-temperature (HP/LT) metamorphic assemblages, and subsequent exhumation. Tectonic domains characterized by upper greenschist facies metamorphic rocks display a dominant E-W axial planar cleavage/foliation fabric that overprints bedding and earlier metamorphic foliations. Raman maturation thermometry and Si-in-phengite barometry document a southward increase from greenschist facies conditions (ca. 360–400°C; 3–7 kbar) in the Doonerak window and Endicott Mountains allochthon to epidote-blueschist facies conditions (~515°C and 10–13 kbar) within the Schist Belt. Within the Central and Schist belts albite porphyroblasts overgrow the earlier deformation fabric and quartz microstructures exhibit evidence for diffusive recovery. This indicates thermal overprinting of deeper portions of the accretionary complex caused protracted (ca. 25 m.y.) greenschist facies overprinting of HP/LT assemblages within the Ar partial retention zone. The Schist Belt – Central Belt boundary is defined by a singular, penetrative foliation that displays a weak shear-band fabric. 40Ar/39Ar results along this boundary indicate that the fabric records deformation along the lower boundary of a nappe that began to move up the subduction channel as subduction ceased by ~135-130 Ma. Subsequent deformation along the Schist Belt – Phyllite Belt boundary reflects the exhumation of HP/LT rocks along a south-dipping normal-sense shear zone. Phengite 40Ar/39Ar age gradients coupled with the first arrival of coarse-grained Brooks Ranges detritus within the adjacent Yukon Koyukuk Basin indicate that tectonic denudation along this shear zone partly exhumed the Schist and Phyllite belts to mid-crustal depths by 110–105 Ma. Final exhumation of the HP/LT rocks was delayed until the Cenozoic which resulted in < 15 km of additional denudation.