33-3 Petrochronology of Composite Footwall Mylonites in the Whipple Mountains Metamorphic Core Complex

Session: Going with the Shear - New Insights into Lithospheric Extensional and Strike-Slip Systems (Posters)

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Abstract:

Metamorphic Core Complexes (MCCs), expressed by low-angle normal faults (LANFs), ductile footwall mylonites, and brittle distended hanging-walls, are characteristic of large-magnitude extension in a multitude of geodynamic frameworks. Yet, despite their significance, our understanding of the influence of inherited structures and anisotropy on ductile footwall deformation and brittle LANFs is often poorly resolved. The Whipple Mountains MCC (WMCC), part of a corrugated series of MCCs located in the Colorado River Extension Corridor (CREC), has been integral in our foundational knowledge of the temporal, geometric, and kinematic development of MCCs. The CREC underwent multiple stages of shortening and extension between Mesozoic shortening and most recent Miocene extension and MCC formation. End-member interpretations of footwall mylonitization in the WMCC vary from entirely syn-Miocene extension to the exhumation of earlier mylonitic fabrics concurrent to regional Mesozoic/Cenozoic tectonics (e.g. Laramide). Given evidence of the latter present in neighboring CREC MCCs, this study is a high-resolution focus on multiple outcrops, documenting the heterogeneous lithologies and strain distribution, and looking at the individual mylonitic facies through the lens of petrochronology to differentiate temporal and rheological evolution of the composite rheological fabrics. We expect to find that high-temperature (>450°C) mylonites represent the capture of a preexisting mylonitic zone unrelated to Miocene extension, while low-temperature (<450°C) mylonites correlate with ductile shearing associated with Cenozoic MCC formation and regional large-magnitude extension.  This preliminary data provides tantalizing new insights, corroborating that Mesoproterozoic metamorphic footwall rocks were heavily infiltrated by late Cretaceous plutons and dikes (~72 Ma). Importantly, new apatite U-Pb ages fall into two microstructurally distinct Eocene and Oligo-Miocene age groups, clustering at 23-28 Ma and 33-44 Ma respectively. These data indicate that high-temperature mylonites appear to be Laramide in age while lower-temperature mylonites and rocks at deeper structural levels are associated with Miocene extension and MCC exhumation.  While there is ample regional evidence for Eocene and Oligo-Miocene ductile shearling, this systematic study integrating microstructural fabric analysis and zircon and apatite U-Pb petrochronology demonstrates the composite nature of WMCC footwall mylonites and helps resolve important questions regarding the mechanical and thermal evolution of large-scale crustal extension and elucidate the impact of fabric inheritance on mylonitization in MCC exhumation.

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

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