Pseudotachylyte records on low-angle normal faults - Josh Feinberg - 2021 MagIC Workshop (1/20/21)
Channel: Magnetics Information Consortium (MagIC)
Date: 2021-01-21

"Miocene pseudotachylyte preserves a record of earthquakes on low-angle normal faults in the South Mountains metamorphic core complex" by Josh Feinberg, Institution of Rock Magnetism, University of Minnesota.

Talk presented at the virtual 2021 MagIC Workshop (https://earthref.org/events/MAGIC/2021/) - Rock and Paleomagnetism through Time and Space. Jan 19th-21st, 2021.

Abstract:
Low-angle normal faults (LANFs) are poorly oriented for failure according to Andersonian fault mechanics. The geologic record generally supports slip at dips less than 30°, but the much shorter seismic record suggests such faults do not produce earthquakes greater than 5.5. One explanation for this discrepancy is that LANFs produce large earthquakes at unusually long intervals (Wernicke, 1995). Another is that faults were reoriented by isostatic rebound during exhumation, allowing them to slip at steeper dips before rotating into their current shallow orientations. In 1993, Livaccari, Geissman, & Reynolds published a paleomagnetic test of this hypothesis, showing that Miocene igneous rocks in the footwall of the South Mtn. detachment, a LANF in Arizona, record no evidence of rotation. Opponents argued their data did not definitively prove LANF slip; our work shows it did.

We report results of structural, paleomagnetic, and geochronologic analyses of pseudotachylyte veins lining LANFs (ave. dip = 14°) in the footwall damage zone of the South Mtn. detachment. A U-Pb CA-TIMS weighted mean zircon age shows mylonite cut by these veins formed from granodiorite intruded 20.574 ± 0.027 Ma. An UltraViolet Laser Ablation MicroProbe (UVLAMP) 40Ar/39Ar weighted mean age of K-feldspar from the host mylonite, 20.16 ± 0.32 Ma, demonstrates rapid cooling following mylonitization. Cross-cutting pseudotachylyte veins average 5% glass with less than 1 wt% K2O and 32 % K-feldspar crystallites with 15-17 wt % K2O. UVLAMP and step-heating analyses of pseudotachylyte glass yielded a range of ages between 18.88±0.18 Ma and 16.24 ± 0.23 Ma. Earthquake recurrence estimated from five ages distinct within uncertainty is 529 ± 117 ka. Alternating field and thermal demagnetization experiments on oriented pseudotachylyte samples reveal paleomagnetic recordings that were acquired in their current, low-angle configuration. Some veins show normal polarity, while others collected only meters away show reversed polarity (these directions pass a reversal test), indicating that the LANF was actively rupturing over an interval long enough to encompass several geomagnetic polarity chrons.