177 Chronology of Orogenesis: Unlocking the Timelines of Mountain Building

The focus of this session is the use of geochronological methods (including but not limited to U-Pb, Ar/Ar, FT, (U/Th)-He) to establish the timing of key events in mountain building, including different orogenic phases, exhumation, and uplift rates.

Tuesday, 21 October 2025
8:00 AM - 12:00 PM
HBGCC, 304C

Polyphase Cretaceous and Cenozoic deformation along the Porcupine Fault System of Yukon and Alaska

Paper number: 177-4
Program: Technical Sessions
Time: 8:29 AM – 8:49 AM

Authors

Abstract

The Porcupine Fault System (PFS) of northern Yukon and Alaska is a major craton-bounding fault zone that accommodated Paleozoic terrane translation of Arctic Alaska along the northern margin of Laurentia. The Mesozoic-Cenozoic history of the PFS remains poorly understood due to a lack of coupled structural and geo- thermochronological datasets required for new kinematic and paleogeographic models of the circum-Arctic, northern Pacific, and northern Cordillera. We couple regional low-temperature thermochronology, Raman spectroscopy of carbonaceous material (RSCM), and fault rock thermochronology to understand faulting and exhumation patterns. Faults within the PFS exhibit normal, thrust, and strike-slip kinematics and are commonly coated by hematite, epidote, and/or calcite which are amenable to (U-Th)/He and U-Pb thermochronology. Field and microstructural observations support synkinematic growth and deformation of these minerals, allowing us to target them to date deformation. RSCM analyses show similar peak temperatures of ~325°C across the various structural blocks in the PFS, supporting lateral juxtaposition of para-autochthonous and allochthonous rocks against autochthonous strata of the Laurentian margin. Zircon (U-Th)/He (He) dates across the PFS are ~115-75 Ma and apatite He dates are dominantly between ~80-50 Ma. Inverse thermal history modeling coupled with RSCM temperatures indicate two major periods of cooling in the Cretaceous and Paleocene. The Cretaceous cooling is coeval with preliminary calcite U-Pb dates from conjugate vein sets at ~120 Ma, recording deformation during opening of the Canada Basin. A thin tuff interbedded within nonmarine strata cut by PFS faults yielded a depositional age of ~61 Ma, overlapping with Paleocene cooling and highlighting the presence of previously unrecognized early Cenozoic synorogenic deposits in potential pull-apart basins. Hematite He yielded dates from ~8.5-0.7 Ma, and microstructural observations support that dates record hematite precipitation and deformation along brittle faults in the shallow crust. Our field observations and structural data integrated with regional and fault rock geo- and thermochronology support reactivation of the PFS during the Cretaceous and Paleogene with deformation as young as the Pleistocene, suggesting the PFS played a role in the opening of the Arctic Ocean and widespread Cenozoic strike-slip faulting in the northern Cordillera. The new results highlight the need for detailed and integrated multi-proxy studies to resolve the complex and long-lived displacement histories characteristic of major strike-slip fault systems.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-413088

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