FINDING STRAIN LOCALIZATION AND DATING THE ONSET OF DEFORMATION IN THE WESTERN IDAHO SHEAR ZONE USING TITANITE U-PB GEOCHRONOLOGY AND CRYSTALLOGRAPHIC VORTICITY AXIS ANALYSIS

Two unresolved challenges for studying shear zones are determining when deformation began and the spatial-temporal patterns of strain localization. Because of the temperature and pressure conditions at which it deforms and recrystallizes, titanite (sphene, CaTiSiO₅) can provide both timing and kinematic information about deformation in mid-crustal shear zones. We present high resolution U-Pb titanite petrochronology in conjunction with crystallographic vorticity axes from deformed orthogneiss as the foundation for a new method of direct tectonic fabric dating.

The Little Goose Creek complex, McCall, Idaho is a series of orthogneisses deformed by the western Idaho Shear zone (WISZ). Rocks in this complex are highly deformed and have gradational and/or obscured contacts. We sampled two units of the Little Goose Creek complex to address the question of whether WISZ deformation was broadly distributed across the shear zone and whether strain localization played a significant role in the deformation of this shear zone. Unit Gpg is a potassium feldspar megacrystic porphyritic orthogneiss and unit Gtg is a coarse-grained tonalite orthogneiss. Both units have the same macroscopic fabric orientation.

We dated geochemically characterized and microsampled intragrain titanite domains by isotope dilution thermal ionization mass spectrometry. Unit Gpg was intruded at ~105 Ma and deformed from ~100 Ma until ~93 Ma. Unit Gtg has a younger magmatic age (~91 Ma) and deformed for a shorter and younger interval of time (~88 Ma to ~85 Ma). These results suggest that the WISZ was long-lived and that deformation occurred in temporally and spatially discrete zones, as evidenced by younger units that deformed without resetting the U-Pb systematics of nearby, previously-deformed titanite.

Preliminary results from electron backscatter diffraction mapping give crystallographic vorticity axes (CVA) in titanite. We compare the vorticity of the titanite with the bulk vorticity of the rock to determine whether kinematics recorded by the titanite in the Little Goose Creek complex reflect the progressive fabric development of the shear zone and whether the two units record the same vorticity despite deforming at different times. This work shows that high-resolution U-Pb titanite ages can be directly related to deformation fabrics.