285-1 Dating Detachment Faults Using 40Ar/39Ar Muscovite and Some Problems: Example From Albion-Raft River-Grouse Creek Metamorphic Core Complex (ARGMCC)

Session: Rock Deformation and the Dynamics of Mountain Building: A Session Honoring the Scientific Contributions of John P. Platt, Part II

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Abstract:

The absolute timing of deformation within MCCs of the western U.S. is key to understanding the geodynamic factors at play in the transition from shortening to extension in the Cordillera.  Dating of mylonite development and overlying detachment faults in MCCs using ⁴⁰Ar/³⁹Ar muscovite has proven challenging as the partial Ar retention zone of muscovite overlaps the T range of quartz deformation.  Prior work in the ARGMCC concluded that initial detachment faulting and denudation/cooling occurred during the Eocene (47-40 Ma) based on⁴⁰Ar/³⁹Ar ages of muscovite but regional and local evidence for significant extension and uplift is lacking during this period.  We investigated this contradiction with geologic mapping, structural studies and ⁴⁰Ar/³⁹Ar geochronology in a transect south from the 30 Ma Almo pluton (Albion Mts.) to Dove Creek Pass in the western Raft River Mts. where Eocene muscovite ages were previously reported. New ⁴⁰Ar/³⁹Ar muscovite, biotite and K-feldspar results help constrain the T-t history of the transect. The 30 Ma syn-extensional Almo pluton and its deformed and attenuated amphibolite facies country rocks yield muscovite, biotite and K-feldspar ages of 25, 22 and 20 Ma compatible with thermal relaxation about a large pluton intruded at 10-15 km depth based on metamorphic assemblages (Sill-Ky-And) and thermal modeling. Low-T thermochron and basin evolution indicate final normal fault related denudation and cooling took place in the Miocene. At Dove Creek Pass, the amphibolite facies foliation related to intrusion of the Almo pluton decreases to greenschist facies and overprints Mesozoic(?) amphibolite facies fabrics. Highly attenuated units yield ⁴⁰Ar/³⁹Ar total gas mica ages of ca. 46, 45, 43 and 42 Ma in structurally descending order that are best interpreted as partial Ar retention in Mesozoic(?) micas during transient heating related to the Almo pluton. K-feldspars yield ages like those in the Almo pluton. Low-T thermochron indicates Miocene denudation by normal faulting. Because neither region requires significant pre-40 Ma denudation of overlying crust, this integrated study suggests caution in interpreting ⁴⁰Ar/³⁹Ar muscovite ages as direct timing constraints for deformation and/or fault-related exhumation without additional supporting data.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8382

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