32-13 EBSD and δ18O Analyses Record Fluid-Fault Interactions at the Brittle-Ductile Transition During the ca. 1.4 Ga Baraboo Orogeny

Session: Latest Research Advances in Structural Geology and Tectonics (Posters)

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The South Range Fault is one of many recently mapped structures in the Baraboo Hills (Wisconsin) that record deformation during the ca. 1.4 Ga Baraboo Orogeny. It is a shallowly NW-dipping thrust fault with a ~10 m thick damage zone of highly fractured Baraboo quartzite. The fault core is an anastomosing network of quartzite cataclasite up to 15 cm thick. Mesoscopic quartz + white mica + hematite veins developed within the fault zone, with some hematite-rich veins reactivated as subsidiary faults. Cathodoluminescence imaging reveals a network of multiple cross-cutting quartz microveins, some no more than several microns wide in the fault zone. Although the fault zone is dominated by brittle microstructures, evidence of ductile deformation, including irregular extinction, subgrains, and recrystallized grains, is locally preserved. In general, brittle structures overprint ductile features, although mutually cross-cutting relationships are locally preserved. Electron backscatter diffraction (EBSD) analyses of quartz and hematite and δ¹⁸O isotope analysis of quartz were conducted to explore relationships between brittle and ductile deformation and syndeformational fluid(s). Quartz does not record a crystallographic preferred orientation (CPO) in either the damage zone or core, due to brittle overprinting. Sheared hematite has a weak to strong CPO. Crystallographic vorticity axes (CVAs) in both quartz and hematite are consistent with top-to-the SE shear, matching the kinematics of brittle structures. Slip systems inferred from misorientation axis analysis are {r}<a> and {m}<a> for quartz and [c]<a> and {m}<a> for hematite, consistent with deformation up to the peak metamorphic temperature recorded in the Baraboo Hills: ~385°C. δ¹⁸O patterns differ between the damage zone and fault core. In the damage zone, host quartz grains record δ¹⁸O values of 11.8 – 12.4 ‰ and microvein values of 11.2 - 16.8 ‰. In contrast, host grains record 10.4 – 11.8 ‰ and microveins 10.6 – 11.6 ‰ in the fault core. The greater range of δ¹⁸O values in the damage zone, as well as the values themselves, record interaction with both lower and higher δ¹⁸O fluids. The fault core shows greater homogenization of δ¹⁸O values, consistent with more extensive exchange with a lower δ¹⁸O fluid (~4.5-7‰ at 350-380°C). Collectively, these data provide evidence of fluid-assisted localization of deformation in the fault core during the transition from ductile to brittle deformation.

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

doi: 10.1130/abs/2025AM-9667

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