EPPUR SI MUOVE: MIOCENE PSEUDOTACHYLYTE VEINS PRESERVE A RECORD OF EARTHQUAKES >M5.5 ON LOW-ANGLE NORMAL FAULTS (Invited Presentation)

Low-angle normal faults (LANFs) are poorly oriented for failure, yet the geologic record generally supports slip at dips <30°. The much shorter seismic record, however, suggests they do not produce earthquakes ≥M5.5. Wernicke (1995) hypothesized LANFs produce large earthquakes at intervals that exceed the seismic record. Others proposed LANFs were reoriented by isostatic rebound during exhumation, slipping at steeper dips before rotating into current shallow orientations. Livaccari, Geissman, & Reynolds (1993) used paleomagnetic data to test the latter hypothesis, showing Miocene rocks in the footwall of the South Mtn. detachment, a LANF in AZ, record no evidence of rotation. Opponents argue their data do not definitively prove LANF slip; our detailed analyses of pseudotachylyte fault veins (ave. dip = 14°) from the footwall damage zone of the South Mtn. detachment show they do.

A U-Pb CA-ID-TIMS weighted mean zircon age shows mylonite host rock formed from granodiorite intruded at 20.574 ± 0.027 Ma. An UltraViolet Laser Ablation MicroProbe (UVLAMP) ⁴⁰Ar/³⁹Ar weighted mean age of K-feldspar from mylonite, 20.16 ± 0.32 Ma, records rapid cooling. UVLAMP and step-heating analyses of cross-cutting fault veins (ave. 5% glass with <1 wt% K₂O and 32% K-feldspar crystallites with 15-17 wt% K₂O) yield 5 distinct ages between 18.88 ± 0.18 Ma and 16.24 ± 0.23 Ma. All veins analyzed by alternating field and thermal demagnetization experiments acquired paleomagnetic recordings at current shallow dips. Some veins have normal polarity, but others collected just meters away have reversed polarity and pass a reversal test. Seismic activity therefore spanned several geomagnetic polarity chrons, with an earthquake recurrence interval of 529 ± 117 ka.

We estimate slip magnitude from vein thickness measured at 10 cm intervals, fault dip, and variables such as wall rock temperature at the time of failure inferred from thermochronologic data. Calculated slip values fall within a range exhibited by ground-rupturing historic earthquakes (Wells & Coppersmith, 1983). We use these authors’ correlations to estimate earthquake magnitudes for 3 veins shown to have formed at current dips: M=6.37-7.19, 6.58-7.30, and 6.58-7.30. Collectively, our data show significant LANF seismicity occurred at intervals that far exceed the seismic record.