HEMATITE MICROTEXTURES AND (U–TH)/HE THERMOCHRONOMETRY OF THE HURRICANE FAULT, SOUTHWESTERN UTAH: EVIDENCE FOR A PALEOEARTHQUAKE?

Hematite-coated fault surfaces offer the potential to characterize and understand the mechanisms and timing of past deformation, including seismicity, in exhumed fault zones. We apply integrated micro- to nanoscale microscopy and geochemistry with hematite (U–Th)/He (He) thermochronometry dates to document hematite textural evolution and timing of fault slip on the seismically-active Hurricane fault in southwestern Utah. Here, hematite is preserved on a “polished” bedrock fault scarp that cuts the Triassic Moenkopi Formation. Hematite occurs in mm- to cm-thick veins oblique to the fault surface. Locally, hematite is present in elongate, striated, mm- to cm-scale lenses on the slip surface, and we target this material for thermochronometry. Scanning electron microscopy (SEM) shows hematite within ~100–200 μm of the fault surface comprises rounded hematite particles ~100 nm to 2 μm in diameter that lack grain boundaries. Away from the surface and beneath these nanoparticles are randomly-oriented, ~70–150 nm-thick hematite plates. Plate and rounded, “fused” particle morphologies likely reflect initial hematite crystallization from fluids and deformation, respectively. SEM imaging and energy dispersive X-ray spectroscopy (EDS) also reveal a featureless, ~3 μm-thick, Al-rich silica film enveloping the hematite nanoparticles at the fault surface, suggesting it is amorphous, opaline silica. This silica layer is exclusively found in contact with deformed hematite, implying association with fault slip. A preliminary mean hematite He thermochronometric date is 375 ± 54 ka (±1σ std. dev.; n = 11). This date is appreciably younger than previously-reported, regional apatite He thermochronometry data. This suggests hematite He data may record hematite formation or thermal resetting from friction-generated heat during fault slip. Ongoing hematite He analyses targeting and comparing the distinct textural domains will discriminate between these possibilities, and scanning/transmission electron microscopy will evaluate the crystallinity of the surface silica and hematite nanoparticles. Collectively, these data will allow us to decipher the timing and mechanisms of past deformation of the Hurricane fault and understand analogous relationships in other hematite-bearing fault zones.