LOW-ANGLE NORMAL FAULT SEISMOGENESIS SUPPORTED BY PSEUDOTACHYLYTE 40AR/39AR THERMOCHRONOLOGY AND STRUCTURE

Despite being unfavorably oriented for slip according to Anderson-Byerlee mechanics, low-angle normal faults (dips <30°) are common structures within extensional terranes worldwide. Some workers address this paradox by suggesting these faults fail aseismically and are unusually weak. Our study adds to this discussion through characterizing zones of densely spaced pseudotachylyte within the South Mountains metamorphic core complex, Phoenix, Arizona.

These spectacular exposures cut mylonitized South Mountains granodiorite (22 ± 4 Ma, Reynolds, 1985) at two sites situated below the primary detachment. Within pseudotachylyte zones, numerous cross-cutting veins locally include cataclasite. Stacked zones spaced approximately 2 m apart and comprised of veins up to 1.5 cm thick characterize the first site. At a second site, situated further beneath the paleodetachment, pseudotachylytes up to 3 cm thick are localized in a 1-3 m band below a shallowly dipping fault juxtaposing mylonitized granite and granodiorite. Collectively, these observations suggest significant and repeated seismogenesis. The average dips of pseudotachylyte generation surfaces at these sites are 14° and 19°, respectively. These dips are much lower than the lock-up angle of 30° predicted by Andersonian mechanics and are between orientations of host mylonite S- and C-surfaces. Veins are not localized along the dominant C-surfaces.

⁴⁰Ar/³⁹Ar analysis of vein material from the first site yields age plateaus of 16.27 ± 0.22 and 17.44 ± 0.33 Ma. Potassium is largely carried by unusually pure, neocrystalline K-feldspar. Ages are younger than and within error of apatite fission track ages for the host rock (c. 17.5 Ma, Fitzgerald et al., 1994). This indicates that they record the timing of quenching and closure to diffusion of argon at temperatures <110°C. Assuming an elevated geothermal gradient of 25-30°C/km, these data show that pseudotachylyte formed in the uppermost seismogenic zone, at <4 km depth. Pairing this thermochronologic constraint with pseudotachylyte orientations supports seismogenic slip on very low-angle fault surfaces.