40AR/39AR DATING AND PALEOMAGNETIC ANALYSES OF PSEUDOTACHYLYTE REVEAL A MINIMUM OF 2.97 MILLION YEARS OF LOW-ANGLE NORMAL FAULT SEISMICITY

Low-angle normal faults are poorly oriented for failure, yet the geologic record generally supports slip at dips <30° and pseudotachylyte - an unequivocal indicator of seismic slip - has been documented in several low-angle normal fault settings. To evaluate the significance of this observation, we combined thermochronologic and paleomagnetic analyses of pseudotachylyte exposed in the footwall damage zone of the South Mountains metamorphic core complex of Arizona. A U-Pb CA-ID-TIMS zircon age indicates that South Mountains granodiorite - host rock to mylonite and pseudotachylyte - solidified at 20.574 ± 0.036 Ma. In situ UltraViolet Laser Ablation Microprobe (UVLAMP) ⁴⁰Ar/³⁹Ar ages of K-feldspar from mylonite adjacent to two pseudotachylyte fault veins are 20.41 ± 0.32 and 19.94 ± 0.16 Ma, respectively, recording rapid sub-solidus cooling in this extensional environment. Eight sets of in situ UVLAMP dates and two laser incremental heating analyses were obtained from fault veins collected from a single outcrop, yielding ages ranging from 18.82 ± 0.22 Ma to 15.37 ± 0.26 Ma. All fault vein ages overlap the weighted mean fission track age of 17.5 ± 2.0 Ma published by Fitzgerald et al. (1993). In the latter study, mean track lengths were all greater than 14 µm, indicating rapid cooling to 125°C. We isolated magnetizations associated with magnetite crystallized from these and six additional fault veins, compared their remanence to the expected Miocene paleopole, and conclude these faults experienced little to no reorientation since formation. Some veins record normal polarity; one of these is cut by a vein that records reversed polarity and passes a reversal test. Low-angle normal fault seismic activity thus spanned multiple geomagnetic polarity chrons over millions of years.