29-4 Rb-Sr Geochronologic Constraints on Deformation Timing in the Norumbega Fault System, South-Central Maine

Session: Rates, Dates, and Plates: Petrochronological Approaches to Unraveling Tectonometamorphic Histories (Posters)

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Abstract:

The Norumbega fault system (NFS) in Maine is a dextral transcurrent fault that was initiated during Middle-to-Late Devonian time due to a shift in regional kinematics. Within south-central Maine, the Sandhill Corner shear zone (SCSZ) was exhumed from depths of ~10–15 km and represents the longest continuous strand of the Norumbega system. Previous structural and geochronological studies of this shear zone and surrounding rocks identified multiple deformational events culminating in localized mylonite zones with abundant evidence of seismic activity (e.g., friction melts). The timing of these events is generally well-known, with the exception of the latest event that involved coseismic deformation. Current understanding of these events relies on ⁴⁰Ar/³⁹Ar thermochronology published two decades ago suggesting an age range for the latest event spanning ~380–290 Ma - a range that opens questions about continuous vs punctuated deformation. Recent studies have shown that Rb-Sr analyses of targeted mica textures in metamorphic rocks using LA-ICP-MS/MS can provide detailed thermochronological histories, with particular application to muscovite and biotite. Here we report in situ Rb-Sr dates from micas to redefine the deformational sequence for this region. Muscovite and biotite dates were performed across transects in and around the SCSZ, allowing for tighter time constraints on the structural and metamorphic events affecting the NFS. Preliminary Rb-Sr muscovite geochronology of one transect yields dates of 330–354 Ma, suggesting this technique can constrain the Paleozoic evolution of the shear zone. Our project explores the potential of this rapidly developing technique for improving our understanding of relations between deformation and metamorphism in shear zones.

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