EXHUMATION OF HP/LT METASEDIMENTARY ROCKS OF THE SCHIST BELT, SOUTHERN BROOKS RANGE AS REVEALED BY THERMOBAROMETRY AND 40AR/39AR THERMOCHRONOLOGY

The northward obduction of an island arc onto the Arctic Alaska continental margin in the latest Jurassic to earliest Cretaceous, led to shortening of the continental margin sedimentary prism, southward subduction of the underlying continental crust and high-pressure/low-temperature (HP/LT) metamorphism of the southern Brooks Range (BR). Since the recognition of a S-dipping, normal-sense, shear zone along the southern margin of the BR, tectonic models for the exhumation of the HP/LT rocks have invoked either syn-convergent extension or post-orogenic core-complex style exhumation. To determine the details of this history, structural, thermobarometric and thermochronologic data were collected from N-S transects through the orogen (Wiseman quadrangle). The dominant foliation in S-tectonites of the Central Belt (CB) and Schist Belt (SB) strike ~ E-W, and are axial plane to macro to microscopic tight to recumbent folds. Symmetrical to slightly N-vergent recumbent folds and porphyroclast microstructures observed within S-tectonites suggest a large component of pure shear flattening. Graphite thermometry and Si-in-phengite barometry display a southward increase in temperature and pressures from ~375 °C and 2 kbar in the northern CB to peak temperatures of ~525°C and 10-11 kbar in the central SB. South of peak P-T conditions, pressures and temperatures decrease to ~400 °C and 2 kbar toward the Phyllite Belt (PB). The decrease in PT, quartz grain-size reduction and S-vergent shear bands define a 5-6 km wide, greenschist facies normal-sense shear zone system that overprints the early SD fabric. ⁴⁰Ar/³⁹Ar of phengite from the metamorphic core exhibit an age pattern similar to the P-T data. ⁴⁰Ar/³⁹Ar ages increase southward from ~120-115 Ma to maximum ages of 136 – 132 Ma adjacent to the CB-SB boundary, before decreasing in age southward toward the shear zone. These results suggest an Early Cretaceous (~135 Ma) northward emplacement of the SB rocks over rocks of the CB that occured during southward subduction. Nappe emplacement led to the growth and/or thermal resetting of HP micas in the CB before cooling by 120-115 Ma. Overprinting of the early convergent fabrics along the southern margin of the BR suggests that a younger zone of late Early Cretaceous extension affected rocks of the southernmost Schist and Phyllite Belts.