IN SITU 40AR/39AR UV LASER ANALYSIS OF SUB-MILLIMETER MICROSTRUCTURAL DOMAINS IN MIOCENE PSEUDOTACHYLYTE

The South Mountains metamorphic core complex, AZ, USA, exposes the footwall damage zone of a major low-angle normal fault. Within this zone, Miocene-age granodiorite mylonite was overprinted by brittle structures, including pseudotachylyte-lined slip surfaces, which quenched following frictional melting at seismic slip rates. These pseudotachylyte veins exhibit heterogeneity across a range of scales. At the outcrop scale, veins vary in thickness (<1 mm to ~2.5 cm), development of foliation and lineation (fabric), and number and size of survivor clasts. Many veins are also internally heterogeneous, and can be divided into discrete structural domains on the basis of biotite crystallite fabric development and orientation, and the size, number, and shapes of survivor clasts. The boundaries between domains are subparallel to vein boundaries. These structural domains are likely homogenized during traditional step-heating ⁴⁰Ar/³⁹Ar analyses of 10 mg subsamples. To overcome this obstacle, we determined the ages of domains within cm-diameter polished sections of pseudotachylyte veins via in situ ⁴⁰Ar/³⁹Ar analysis with a 193 nm wavelength UV excimer laser and a multi-collector mass spectrometer operating with ion counting multipliers. Using a spot size of 150 μm, we explored host rock K-feldspar age variations and documented the effects of host rock contamination of pseudotachylyte. To determine the ages of pseudotachylyte-producing earthquakes, we targeted areas within each domain that were free of survivor clasts and rich inneocrystalline K-feldspar microlites visible in back-scattered electron images. Our results show that some microstructurally distinct domains within the same vein record different ⁴⁰Ar/³⁹Ar ages. Between 10 and 27 individual in situ dates, obtained from each of four domains (in two veins), reveal three distinct ages between 17.2 and 19.2 Ma. Our findings indicate that individual pseudotachylyte veins can record multiple earthquakes. Moreover, they demonstrate the potential of in situ geochronologic analyses, integrated with detailed microstructural observations, to address complex structural problems in relatively young systems, where high spatial resolution sampling is imperative.