THERMOCHRONOLOGIC EVIDENCE FOR CRETACEOUS STRIKE-SLIP DISPLACEMENT ON THE PORCUPINE SHEAR ZONE, ALASKA AND YUKON

The Porcupine shear zone in Alaska and Yukon played a critical role in the Paleozoic translation of the Arctic Alaska terrane along the Laurentian margin, but its Mesozoic history and relation to opening of the Canada Basin remain uncertain. Post-Mississippian strata exposed within the fault zone record polyphase brittle and ductile deformation related to both sinistral and dextral strike-slip displacement, but this deformation is absent in overlying Miocene basalt flows. Three samples were collected for ⁴⁰Ar/³⁹Ar dating from metasedimentary rocks within the shear zone and within 2 km of the Late Devonian Old Crow batholith to the north. Muscovite from two samples collected north of the main fault strand produced plateau or plateau-like segments of ca. 340-360 Ma, consistent with cooling after pluton emplacement, as well as younger steps of ca. 93 and ca. 156 Ma. In contrast, muscovite from metasandstone collected within the main fault produced complex argon spectra with single steps as old as ca. 1779 Ma and does not record heating during pluton emplacement despite close proximity to the batholith. Thus, the ⁴⁰Ar/³⁹Ar data reflects contrasting thermal histories across the main fault strand and partial resetting during or after the Cretaceous, most likely related to strike-slip displacement. The Cretaceous or younger argon loss and partial resetting is further constrained by new U/Pb calcite dates of ca. 112 Ma. Dated samples are from 2-5 cm wide blocky calcite veins associated with fault breccias and linked non-sigmoidal vein arrays cutting various Tonian-Devonian carbonate rocks. These field relationships and geochronological data provide the first unambiguous evidence for Early Cretaceous strike-slip displacement along the Porcupine shear zone, which is consistent with translational or hybrid models for the opening of the Canada Basin.